Next Article in Journal
Effect of Micro-Arc Oxidation Voltage on the Surface Morphology and Properties of Ceramic Coatings on 7075 Aluminum Alloy
Previous Article in Journal
Glass-Forming Ability and Crystallization Behavior of Mo-Added Fe82−xSi4B12Nb1MoxCu1 (x = 0–2) Nanocrystalline Alloy
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Metals
Volume 15
Issue 7
10.3390/met15070745
metals-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Effect of Oxygen and Zirconium on Oxidation and Mechanical Behavior of Fully γ Ti52AlxZr Alloys

by
Michal Kuris
1,*,
Maria Tsoutsouva
1,*,
Marc Thomas
1,
Thomas Vaubois
2,
Pierre Sallot
2,
Frederic Habiyaremye
3 and
Jean-Philippe Monchoux
3
1
DMAS, ONERA, Université Paris-Saclay, 92322 Châtillon, France
2
Safran Tech, Rue des Jeunes Bois, CS 80112, 78771 Magny-les-Hameaux, France
3
CEMES CNRS UPR 8011, 29 rue Jeanne Marvig, BP 94347, CEDEX 4, 31055 Toulouse, France
*
Authors to whom correspondence should be addressed.
Metals 2025, 15(7), 745; https://doi.org/10.3390/met15070745
Submission received: 28 May 2025 / Revised: 22 June 2025 / Accepted: 25 June 2025 / Published: 2 July 2025
(This article belongs to the Section Crystallography and Applications of Metallic Materials)
Browse Figures
Versions Notes

Abstract

This work provides a comprehensive investigation into the synergistic effects of zirconium and oxygen on the microstructural evolution, high-temperature oxidation resistance, and mechanical properties of γ-phase Ti52AlxZr alloys (x = 0, 0.5, 1, and 2 at.%) under systematically controlled oxygen concentrations. Unlike prior studies that have examined these alloying elements in isolation, this study uniquely decouples the contributions of interstitial (oxygen) and substitutional (zirconium) solutes by employing low (LOx) and high (HOx) oxygen levels. Alloys were synthesized via vacuum arc melting and subsequently subjected to homogenization annealing at 1250 °C for 100 h to ensure phase and microstructural stability. Characterization techniques including scanning electron microscopy (SEM), X-ray diffraction (XRD), and electron backscatter diffraction (EBSD) were employed to elucidate phase constitution and grain morphology. Zirconium addition was found to stabilize the γ-TiAl matrix, suppress α2-phase formation, and promote grain coarsening in LOx specimens. Conversely, elevated oxygen concentrations led to α2-phase precipitation along grain boundaries. Mechanical testing, comprising Vickers hardness and uniaxial compression at ambient and elevated temperatures (800 °C), revealed that both zirconium and oxygen significantly enhanced strength and hardness, with Ti52Al2Zr delivering optimal mechanical performance. Moreover, zirconium substantially improved oxidation resistance by promoting the formation of a thinner, adherent Al2O3 scale while simultaneously inhibiting TiO2 growth. Collectively, the findings demonstrate the critical role of zirconium in engineering advanced γ-TiAl-based intermetallics with superior high-temperature structural integrity and oxidation resistance.

1. Introduction

Gamma titanium aluminides (γ-TiAl) are increasingly being considered for structural applications in aerospace and automotive industries, where weight savings and high-temperature performance are critical [1,2]. Their low density (~4.0 g/cm3), high specific strength, good creep resistance, and relatively high oxidation resistance make them attractive candidates to replace heavier nickel-based superalloys in components such as low-pressure turbine blades and exhaust valves [3]. Among the different phases of TiAl alloys, the γ-TiAl (L10-ordered) phase is particularly appealing due to its excellent oxidation behavior and reasonable high-temperature mechanical properties [4,5,6]. However, practical application is limited by several persistent issues, including poor ductility at room temperature and oxidation degradation at service temperatures above 800 °C [7,8,9,10,11,12,13,14].
Most commercial TiAl alloys—such as Ti48Al2Cr2Nb or TNM alloys—are not single-phase but consist of complex, multiphase microstructures combining γ-TiAl, α2-Ti3Al, and β (beta/beta0) phases [15,16,17,18]. These microstructures (duplex, near-γ, or lamellar) are tailored to enhance strength and toughness through phase synergy. While effective from a mechanical design standpoint, such complexity poses significant challenges in isolating the individual effects of alloying elements on oxidation mechanisms and mechanical behavior. For this reason, model alloy systems based on a fully γ-phase microstructure offer a valuable platform for mechanistic investigations, allowing researchers to decouple and study elemental contributions within a well-defined crystallographic and thermodynamic framework.
In this context, zirconium (Zr) has attracted growing attention as an alloying addition in TiAl systems due to its ability to influence both microstructure and oxidation resistance. Zr is considered a stabilizer of the γ-phase and has been shown to suppress the formation of undesirable α2 and β phases when added in controlled amounts [19]. Zr additions can lead to grain refinement, reduced lamellar colony size, and improved mechanical homogeneity. More notably, Zr tends to segregate at grain boundaries and interfaces, where it can modify diffusion pathways and alter oxidation kinetics. Several studies have reported that Zr promotes the formation of a more adherent and protective Al2O3 layer by reducing Ti outward diffusion and forming Zr-rich interfacial oxides that inhibit spallation and growth of layers. Additionally, Zr may affect the adherence of the oxide layer by altering the stress state and accommodating growth strains at the metal/oxide interface. However, the role of Zr is complex and context-dependent, and its isolated effect in a fully γ matrix—absent of α2 or β phases—remains insufficiently explored [20,21,22,23].
In parallel, oxygen (O) plays a critical yet ambivalent role in TiAl alloys [16,24]. Due to Ti’s extremely high affinity for O, its uptake during processing (e.g., casting, hot isostatic pressing, or heat treatment) is almost unavoidable, even under vacuum or inert atmospheres. O dissolves readily into the TiAl matrix and often acts as an unintentional but impactful interstitial solute. In the γ phase, O occupies octahedral interstitial sites and contributes to solid solution strengthening by distorting the lattice and impeding dislocation motion. This can lead to improved yield strength and hardness, particularly in fine-grained γ structures [25,26]. However, excessive O content is also known to induce embrittlement, particularly via segregation to grain boundaries and crack initiation sites, and can reduce ductility and fracture toughness. Moreover, O is central to the high-temperature oxidation behavior of TiAl alloys. While the formation of a protective Al2O3 layer is generally desirable, competition with Ti and other reactive elements such as Zr can lead to the development of complex, multilayered oxide layers, including TiO2-rich outer layers and internal oxidation zones [27]. The early stages of oxidation involve rapid O ingress, and the layers’ composition and growth kinetics depend heavily on local element distribution, grain size, and phase constitution. The presence of O, therefore, not only modifies mechanical performance but also actively mediates long-term environmental stability under thermal exposure [28,29,30,31,32,33].
To date, the majority of research on TiAl-Zr-O systems has concentrated on multiphase alloys, wherein the individual contributions of alloying elements cannot be reliably decoupled due to complex phase interactions. There remains a lack of systematic investigations into single-phase γ-TiAl alloys, particularly with respect to the combined effects of zirconium and oxygen on microstructural evolution, oxidation resistance, and mechanical behavior. Model alloy systems with a fully γ-phase microstructure offer a simplified framework, free from microstructural heterogeneity, thus enabling a more precise assessment of fundamental elemental interactions. Accordingly, the present study targets γ-dominant Ti52AlxZr alloys with carefully controlled oxygen levels to isolate and elucidate the respective and synergistic influences of Zr and O within a pure γ matrix [34]. While such compositions may not directly correspond to service-grade industrial alloys, they provide critical insight into the role of these elements, which can inform the rational design and optimization of more complex, application-relevant multiphase TiAl-based materials for high-temperature structural applications.

2. Materials and Methods

Experimental ingots of the binary Ti52Al alloy with three different Zr contents (0.5, 1, and 2 at.%) and two O levels designated as LOx (low O content) and HOx (high O content) were prepared by vacuum arc melting (VAM) using a Cyberstar furnace (Cyberstar, Grenoble, France). To ensure homogeneous chemical composition, each ingot was re-melted eight times. The chemical composition of the eight analyzed samples was determined using ICP-OES Agilent 5800 (Agilent Technologies, Santa Clara, CA, USA) and Inductar ONH analysers (Elementar Analysensysteme GmbH, Langenselbold, Germany). The results are shown in Table 1. No significant loss of Al or Zr was observed. The binary Ti52Al alloy was selected as a reference material due to its favorable properties, particularly the theoretically zero α2-phase content (resulting from the shift into the single-phase γ region at Al concentrations above > 48 at.%), structural homogeneity, optimal grain size.
Segments of 15 mm size were cut from the central part of the ingots and subjected to heat treatment (HT) at 1250 °C for 100 h in an Ar atmosphere. Samples were encapsulated in Nb foil to prevent diffusion-related contamination. The heat treatment was carried out for 100 h to ensure the development of a fully γ-phase microstructure. As the aim was to work with model alloys exhibiting microstructures as close as possible to a single γ-phase, prolonged annealing was necessary. The heat treatment was carried out for 100 h to ensure the development of a fully γ-phase microstructure. As the aim was to work with model alloys exhibiting microstructures as close as possible to a single γ-phase, prolonged annealing was necessary. After HT, microstructural characterization focused on evaluating the influence of O and Zr content on grain size and phase distribution (α2 and γ phases) using optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD), and electron backscatter diffraction (EBSD). EBSD was used to observe α2-phase precipitation, while phase fraction quantification based on SEM images was carried out using ImageJ 1.53-f software. An increased α2-phase content near the surface was attributed to instability of the Ar atmosphere during HT. Average values were calculated from five images per alloy. To validate the accuracy of phase quantification, XRD analysis was performed.
XRD measurements were conducted using a PANalytical Empyrean diffractometer (Malvern PANalytical, Almelo, The Netherlands) in Bragg–Brentano (θ–θ) geometry with Cu Kα radiation. The scan range was set from 10° to 110° 2θ, with a step size of 0.016° and a scan time of 30 s per step. A fixed divergence slit of ½° and a 15 mm mask were used to control the incident beam. Phase identification and profile analysis were performed using the Pawley method in HighScore Plus 5.2 software, employing a pseudo-Voigt peak shape and the Caglioti function for FWHM refinement. Background modeling was based on a 7-term Chebyshev Type I function, and refinement included samples displacement, lattice parameters, and the Caglioti U, V, and W coefficients. Semi-quantitative phase analysis was performed using the Direct Derivation method. XRD evaluation was primarily conducted for LOx samples, as HOx ones exhibited grain inhomogeneity and significant signal noise.
Oxidation resistance was tested on samples with dimensions of 10 × 5 × 3 mm at 850 °C in ambient air for 100 h, with mass gain recorded at regular time intervals (1–100 h). After oxidation testing, the morphology, composition, and thickness of the oxide layers were characterized using OM ((IM-3MET, Zeiss, Oberkochen, Germany)), SEM ((MIRA3, TESCAN, Brno, Czech Republic)), energy-dispersive X-ray spectroscopy (EDX), and XRD for validation of oxide layers.
Mechanical properties were evaluated by hardness measurements and compression tests. Vickers hardness (HV1, 1 N load) was measured using a Buehler VH3300 tester (Buehler, Lake Bluff, IL, USA) with DiaMed software 04.08.11, including full-surface hardness mapping. Compression tests were performed on samples measuring 6 × 3 × 3 mm at room temperature (RTT, 20 °C) and elevated temperature (HTT, 800 °C; LOx samples only) to assess the effects of O and Zr content. The flow strength (Rp) was determined at plastic strain levels up to 1%.
Thermodynamic simulations were performed within the CALPHAD (CALculation of PHAse Diagrams) framework using the 2023a version of the Thermo-Calc 2023b software coupled with the TCTI1 and TCTI5 databases. CALPHAD is a thermodynamic simulation method based on Gibbs energy modeling to simulate phase diagrams. These calculations were used to predict the effect of O and Zr on phase fractions, under equilibrium conditions.

3. Results

3.1. Microstructural Analysis of LOx and HOx Ti52AlxZr Alloys

Initial observations of the reference Ti52Al alloy confirmed that the LOx samples exhibited a microstructure consisting exclusively of the γ-phase (Figure 1a). In the case of Ti52Al (LOx), a coarse-grained microstructure was observed with weak intragranular contrast, typical for the ordered tetragonal γ-phase (L10). Within the detection limit of the analytical method used, the presence of the α2-phase (D019) was not detected. The Ti52Al (HOx) (Figure 1b) alloy exhibited reduced microstructural homogeneity and an α2-phase volume fraction of approximately 1.1%. The unexpected presence of the α2 phase in this alloy, possibly influenced by the elevated O levels, will be further discussed in the discussion section of the manuscript.
A similar dominance of the γ-phase was also observed in the Ti52AlxZr (LOx) alloys, which contained approximately 500 ppm more O than the reference Ti52Al. Detailed analysis of grain boundaries revealed the presence of fine, isolated α2-phase colonies in both the reference Ti52Al alloy and the Zr-containing LOx alloys (Figure 2).
The Ti52AlxZr (LOx) alloys displayed a similarly coarse-grained microstructure as the reference Ti52Al, with grain size increasing with Zr content. In these cases, the α2-phase was likewise not observed (Figure 1(a2–a4)). In contrast, the HOx ternary alloys of the Ti52AlxZr alloys exhibited α2-phase precipitation predominantly along grain boundaries (Figure 1(b2–b4)). In the Ti52Al0.5Zr alloy, the α2-phase content reached 1.9%. A similar trend was found in the Ti52Al1Zr and Ti52Al2Zr alloys, which exhibited stable α2-phase fractions of 2.1%. This increase was likely related to the higher O content (Table 1) rather than to the Zr concentration, as Zr did not act as a primary α2-phase stabilizer.
Detailed EBSD analysis confirms the presence of precipitated α2-phase within a clearly dominant γ-phase matrix. No other secondary phases were detected, particularly those enriched in zirconium, such as Zr-based intermetallic compounds (Figure 3).
Although previous literature [21] reported that Zr tends to refine grain size, the present experimental results indicate the opposite effect. Assessment of the LOx samples confirmed a high degree of microstructural homogeneity. The Ti52Al (LOx) alloy maintained good stability and uniformity after HT (Figure 4).
Ti52AlxZr (LOx) alloys also exhibited a homogeneous microstructure despite the increased O content compared to the reference Ti52Al (LOx) alloy (approximately + 500 ppm). Following HT at 1250 °C, the reference Ti52Al alloy exhibited an average grain size of 667 μm. In the Ti52Al0.5Zr alloy, the grain size increased to 1068 μm, with a size distribution ranging from 700 to 1400 μm. The Ti52Al1Zr alloy showed an average grain size of 1086 μm, with the majority of grains exceeding 1000 μm. The largest grains were observed in the Ti52Al2Zr alloy, where the average grain size reached 1776 μm, and most grains were larger than 1000 μm (Figure 5).

3.2. Mechanical Analysis of LOx and HOx Ti52AlxZr Alloys

This section of the study focused on evaluating the hardness and mechanical response of the reference Ti52Al alloy and Ti52AlxZr ternary systems as a function of microstructural changes induced by chemical composition (O and Zr additions) after HT and test conditions during compression testing. The reference Ti52Al alloy in the LOx samples exhibited a hardness of 234 HV1 (Figure 6)
Increasing the O content resulted in a hardness of 254 HV1. The highest hardness within the LOx samples was recorded for the Ti52Al2Zr alloy, reaching 267 HV1. In the Ti52AlxZr HOx samples, a comparable hardness enhancement was observed in all compositions, with the Ti52Al2Zr alloy exhibiting the highest value.
RTT compression test (Figure 7) revealed that the Ti52Al (LOx) reference alloy exhibited the lowest strength but good plastic deformability without signs of fracture. The elevated O content increased the strength to levels comparable with the Ti52Al1Zr alloy. Alloys with Zr additions and high O content displayed enhanced resistance to deformation, with the Ti52Al2Zr alloy achieving the highest compressive strength. In contrast, the Ti52Al0.5Zr alloy demonstrated a slightly higher work-hardening rate, reflecting the classical trade-off between strength and plasticity. A reduced O content in Ti52AlxZr alloys contributed to improved ductility and fracture resistance. All tested samples exhibited elastic behavior up to ~0.02 mm/mm strain, followed by plastic deformation, during which significant differences between individual alloys became apparent.
The evolution of flow stress at different plastic strain levels (Rp0.1–Rp1, Figure 8) indicated that the lowest Rp values were recorded for the Ti52Al0.5Zr alloy (both LOx and HOx samples), suggesting limited resistance to plastic flow at low Zr content. In contrast, the highest Rp values were obtained in HOx samples, confirming the strengthening effect of O. The Ti52Al reference alloy also showed increased Rp values with elevated O levels, despite the generally known tendency of O to reduce ductility. These results confirm that the combined effect of Zr and O has a significant impact on the mechanical behavior of TiAl alloys. While higher O content increases strength, low Zr concentrations (0.5 at.%) without sufficient synergy with O do not contribute substantially to mechanical performance.
HTT further demonstrated the influence of chemical composition on the mechanical response of LOx samples. Despite the expected overall reduction in strength, which is typical for TiAl alloys at elevated temperatures [8], the strengthening effect of Zr remained evident, particularly at higher concentrations. The Ti52Al2Zr (LOx) alloy exhibited the highest compressive strength among all tested compositions. Its stress–strain curve revealed stable elastic behavior followed by plastic deformation, during which the alloy maintained high strength. This outcome confirms the role of Zr as a strengthening element (through solid solution strengthening) that enhances thermal stability and mechanical resistance in Ti52AlxZr alloys. While the Ti52Al1Zr and Ti52Al0.5Zr alloys showed lower maximum strengths compared to Ti52Al2Zr, their performance still exceeded that of the binary Ti52Al reference alloy. Ti52Al1Zr offered a balanced combination of strength and ductility, whereas Ti52Al0.5Zr achieved lower strength but superior plastic deformability. Conversely, the reference Ti52Al alloy exhibited the lowest strength at 800 °C, transitioning into plastic deformation at low strain levels, indicating limited thermal resistance under high-temperature loading (Figure 9).
The HTT results for LOx samples confirmed distinct differences in mechanical behavior between the reference Ti52Al alloy and its ternary modifications. All alloys showed increased flow strength (Rp) with rising strain; however, the rate of increase depended strongly on Zr content (Figure 9). The reference Ti52Al alloy exhibited the lowest strength at all deformation levels. The addition of 0.5 at.% Zr raised Rp, indicating an improvement in mechanical response. Further increasing the Zr content to 1 at.% resulted in a more pronounced rise in strength, while the best performance was achieved by the Ti52Al2Zr (LOx) alloy, which exhibited the highest strength at every deformation level. These findings highlight the pronounced strengthening effect of Zr at elevated temperatures, as well as the ability of the Ti52Al2Zr alloy to maintain microstructural stability and mechanical integrity under thermal and mechanical stress.

3.3. Oxidation Resistance of Ti52Al and Ti52AlxZr Alloys

Both LOx and HOx Ti52Al and Ti52AlxZr alloys were initially tested at 700 °C, where they exhibited only minimal mass gain due to oxidation. Such low levels of oxidation prevented reliable evaluation of the influence of O and Zr contents on oxide layers’ growth kinetics. More pronounced differences in oxidation behavior were observed at 850 °C after 100 h of exposure (Figure 10), allowing a more detailed analysis of the oxidation resistance of the reference Ti52Al alloy and Ti52AlxZr ternary alloys in both LOx and HOx conditions. First of all, Zr exhibits a positive effect on the oxidation resistance of fully γ TiAl alloys since lower mass gains are systematically obtained with increasing Zr content.
Another major trend is the Zr positive effect on the mass gain interval between the LOx and HOx samples. Indeed, a large difference is observed for binary LOx and HOx alloys whereas LOx and HOx curves are overlapped for ternary LOx and HOx Ti52Al2Zr alloys. The highest mass gain was recorded in the LOx samples. The Ti52Al (LOx) reference alloy showed the most significant increase, which developed into a near-linear growth regime after approximately 20 h. In contrast, the HOx sample exhibited a sharp decrease in mass around the same time, indicating disintegration or spallation of the oxide layer due to instability caused by internal stresses (Figure 10), a highly undesirable phenomenon in high-temperature environments. Among the Ti52AlxZr alloys, the Ti52Al1Zr composition showed the highest mass gain, suggesting that the effect of Zr on oxidation resistance became more apparent beyond a certain concentration threshold. Ti52Al2Zr demonstrated the lowest oxide mass gain, comparable to the HOx samples. For Zr concentrations < 1 at.%, the HOx samples generally showed lower mass gain than the LOx counterparts, indicating that a low Zr content may increase the oxidation rate. The mass gain observed in Ti52Al2Zr was lower than that of the Ti52Al reference, suggesting that even a minor Zr addition (0.5 at.%) contributed to a reduction in oxidation rate without compromising the oxide layers’ stability. Zr has a positive and strong effect on the initial O content (reference Ti52Al LOx and HOx are very different, while ternary Ti52Al2Zr and HOx overlap).
The morphology and thickness of the oxide layers were analyzed on samples exposed to 850 °C for 100 h. As illustrated in Figure 11, the oxide layer on the Ti52Al (LOx) reference alloy was uniform and indicated good oxidation resistance. In both LOx and HOx samples of the Ti52AlxZr alloys, a reduction in oxide layer thickness was observed compared to the reference Ti52Al alloy. The thinnest oxide layer, approximately 3 μm, was recorded for the Ti52Al2Zr alloy. All LOx samples exhibited high stability of the oxide layer, which was attributed to a reduced TiO2 sublayer thickness, leading to lower internal stresses and improved oxidation resistance. However, during cyclic oxidation testing, a contrasting behavior was observed between the LOx and HOx samples. The HOx sample of the Ti52Al alloy exhibited a sudden decrease in mass after 20 cycles, whereas the LOx sample showed a continuous mass gain. Despite the recorded mass loss, cross-sectional analysis revealed that the HOx sample developed a considerably thicker oxide layer. This apparent discrepancy is likely caused by spallation of the oxide layers in the HOx sample, suggesting that the mass reduction does not indicate lower oxidation activity, but rather mechanical degradation of the oxide layer during thermal cycling.
HOx samples displayed lower oxide layer homogeneity, especially in the Ti52Al and Ti52Al0.5Zr alloys, where pronounced structural inhomogeneity was observed (Figure 12). An increased Zr content enhanced the homogeneity of the oxide layer, confirming the positive effect of Zr on oxidation stability in the Ti52AlxZr system. These particles affected the morphology and chemical composition of the scale, resulting in significant differences compared to the Ti52Al and Ti52Al0.5Zr alloys.
This finding suggests that Zr played a key role in stabilizing and modifying the oxidation behavior of TiAl-based alloys. XRD analysis of the oxide layers confirmed a decrease in the TiO2/Al2O3 ratio, with values of 40/25 for Ti52Al0.5Zr, 40/30 for Ti52Al1Zr, and 25/25 for Ti52Al2Zr (Figure 13). This indicates a change in the oxidation mechanism and suggests a possible shift in the hierarchy of oxide formation. In the Ti52Al1Zr and Ti52Al2Zr HOx alloys, the precipitation of Zr-rich metallic particles was detected within the oxide layer.

4. Discussion

4.1. Alloys’ Thermodynamic Prediction

The unexpected formation of the α2 phase at elevated O content (HOx samples) required a more detailed analysis of the phase stability of Ti52AlxZr alloys. Since the α2 phase significantly affects the mechanical properties, thermodynamic simulations were performed using Thermo-Calc 2023 with the TCTI1 and TCTI5 databases in order to explain this phenomenon. The objective was to estimate the expected volume fraction of the α2 phase at both low (LOx) and high (HOx) O concentrations.
The TCTI1 database predicted an expansion of the α2 + γ-phase field and stabilization of the α phase already at ≥48 at.% Al (Figure 14). This trend is consistent with the known behavior of O, which modifies the primary solidification path by favoring α-phase formation, promotes peritectic transformation, affects Al and O segregation, and broadens the stability of the single-phase α region. The TCTI5 database provided a markedly different prediction, indicating a rapid expansion of the α2 + γ-, α + γ-, and γ + Al2O3-phase fields. The γ-phase region was predicted to extend up to 52 at.% Al at 4000 ppm O, suggesting a significant influence of O on phase boundaries. Comparison with experimental results showed that TCTI1 (up to 1 at.% Zr) provided relatively accurate predictions for LOx samples. The TCTI5 database proved to be more suitable for HOx samples. The simulation results were validated through contrast analysis of micrographs and XRD measurements, although the simulated values corresponded to experimental data only partially. This discrepancy is also addressed by Kulkova et al. [26], who reported that increased Al content enhances O diffusion in TiAl structures, which may affect the accuracy of thermodynamic calculations. These databases could be further improved with a more thorough assessment of the influence of O near the γ-phase composition range (Table 2).
When evaluating the influence of Zr (Figure 15) on the stability of the γ phase, significant differences were observed between databases. TCTI1 suggested only a weak stabilizing effect of Zr, whereas TCTI5 predicted an expansion of the γ- and γ + α-phase fields without a corresponding increase in the γ + α2 field, potentially indicating a stronger stabilizing effect of Zr on the γ phase. According to TCTI1, increasing Zr content leads to a reduction in the α2-phase fraction and the formation of AlZr intermetallic phases, which were also predicted by TCTI5. Such phase formation has been reported in other alloy systems, such as Al–Si, although in the Ti52AlxZr system, Zr-based intermetallics are not typically observed [27,28]. At 52 at.% Al, Zr behaves as a substitutional element, dissolving stably in the γ phase by replacing Ti atoms in the lattice. This substitution contributes to solid solution strengthening, reduces diffusion mobility, and may slightly affect phase transformation temperatures. However, unless a critical Zr concentration is exceeded, secondary Zr-rich phases do not form. In cases of local Al depletion, for instance due to oxidation or segregation, secondary Zr-based precipitates may appear [10].

4.2. Microstructure Analysis

In Ti52AlxZr alloys subjected to HT at 1250 °C for 100 h, it was confirmed that both Zr addition and O content influence phase stability and microstructure evolution. In the LOx samples, the α2 phase was absent, which corresponds to the behavior of binary Ti52Al, where the γ phase dominates. However, microstructural observations revealed small α2 colonies at grain boundaries, which are associated in the literature with increased strength and creep resistance, albeit at the expense of ductility [31]. In the HOx samples, such as Ti52Al0.5Zr, the α2-phase content increased to 1.9% compared to Ti52Al, likely due to the lower O content (3100 ppm). In Ti52Al1Zr and Ti52Al2Zr alloys, the α2 content further increased to 2.1%, indicating a limited effect of Zr in suppressing α2 formation at higher concentrations [31,32].
Significant grain growth was observed in both Ti52Al1Zr and Ti52Al2Zr, with grain sizes reaching 1086 μm and 1776 μm, respectively. Most grains exceeded 1000 μm, particularly in the latter alloy. It was confirmed that lower Zr concentrations contribute to γ-phase stabilization and microstructural homogeneity, while higher concentrations promote grain coarsening [24]. According to Bresler et al. [35], Zr enhances high-temperature microstructural stability; however, at higher concentrations, it leads to lamellar thickening. It can be assumed that Zr promotes the formation of the γ-phase while suppressing the formation of the α2-phase at grain boundaries and triple junctions, which impede grain boundary migration and consequently inhibit grain growth. It is assumed that oxygen promotes the formation of the α2-phase at grain boundaries and triple junctions [36,37,38,39]. Additionally, oxygen may segregate at the grain boundaries of the γ-phase, leading to reduced boundary mobility through the solute drag effect. These mechanisms contribute to a decrease in grain boundary mobility and, consequently, to the inhibition of grain growth [40,41,42,43,44].

4.3. Oxidation Resistance Analysis

The oxidation behavior of TiAl alloys has been extensively studied, and it has been shown that at 700 °C, the oxide layer grows at a very slow rate on the surface and the interpretation of oxidation kinetics under these conditions remains challenging [33,45]. This mechanism is consistent with the findings of Shaaban et al. [45], who showed that O atoms preferentially occupy Ti-rich octahedral sites, thereby limiting Ti diffusion and contributing to the formation of a protective oxide scale. These results collectively confirm the critical role of O in influencing not only phase stability and mechanical properties but also the oxidation resistance of TiAl alloys. At lower O concentrations, the oxide layers tend to be more homogeneous and continuous, whereas at higher O content, the mass gain rate decreases but internal stress accumulation leads to increased scale brittleness and reduced protective efficiency [42,43,44]. In addition to the effects discussed above, the following mechanisms associated with the presence of Zr also contribute to the enhancement of oxidation resistance:
  • Formation of stable ZrO2 oxide: Zr readily oxidizes at high temperatures to form ZrO2, a thermodynamically stable oxide with low O diffusivity. The presence of ZrO2 at the metal–oxide interface acts as an effective barrier that limits inward O ingress and outward diffusion of metal cations. This suppresses the growth of TiO2 and Al2O3 layers.
  • Improved oxide scale adherence: Zr tends to segregate at grain boundaries and at the metal–oxide interface, where it enhances adhesion between the oxide scale and the γ-TiAl substrate. It reduces scale spallation, particularly during thermal cycling, by relieving internal stresses and minimizing thermal expansion mismatch.
  • Modification of oxide growth kinetics: In the presence of Zr, the parabolic oxidation rate constant (kp) decreases, indicating slower, diffusion-controlled oxide growth. This effect is especially prominent in Ti-rich regions where rutile-type TiO2 typically dominates. A fine and homogeneously distributed microstructure containing Zr promotes uniform oxide growth and reduces overall scale thickening.
  • Suppression of volatile Ti–O species: At elevated temperatures, Ti tends to form volatile oxides such as TiO2. Zr helps stabilize the oxide layer and suppresses TiO2 volatilization, thereby indirectly protecting the underlying TiAl matrix from rapid oxidation.
  • Synergistic effects with other elements: In combination with elements such as yttrium (Y), Zr exhibits synergistic effects that further improve oxidation resistance. The joint addition of Y and Zr promotes grain refinement, inhibits cationic diffusion, and significantly increases oxide scale stability under cyclic oxidation conditions.
These mechanisms are supported by the recent literature. Li et al. [42] demonstrated that Zr in TiAl alloys promotes the formation of a protective mixed oxide layer enriched in Al2O3 and ZrO2. Similarly, Bacos et al. [43] confirmed improved scale adherence in Zr-modified γ-TiAl alloys under cyclic oxidation at 800 °C. Furthermore, a recent study by Liang et al. [45,46,47] showed that increasing Zr content leads to a finer and more thermally stable microstructure, which contributes not only to enhanced mechanical properties at elevated temperatures but also to improved oxidation resistance.

4.4. Mechanical Analysis

Hardness testing results showed that both Ti52Al and Ti52AlxZr alloys exhibited increased hardness with increasing O and Zr content. Among the tested alloys, Ti52Al2Zr showed the highest hardness (267 HV1), confirming the positive effect of Zr on microstructural strengthening. This result contrasts with the observations reported by Belova et al. [33], who noted a softening effect attributed to increased γ-phase grain globularization and mechanisms such as solid solution strengthening or Hall–Petch-type strengthening [25,46,47,48].
Ti52Al0.5Zr was the only composition that exhibited a decrease in strength with increasing O content under RTT conditions compared to the reference Ti52Al alloy. This behavior is attributed to microstructural changes involving the reduction in the α2 phase and the stabilization of the γ phase. In contrast, Ti52AlxZr alloys with Zr ≥ 1 at.% showed increased strength at higher plastic deformation levels, particularly in Rp0.1 and Rp0.2, confirming the strengthening effect of Zr in TiAl systems. RTT testing clearly demonstrated that the mechanical properties of TiAl alloys are significantly affected by the combined presence of O and Zr. Ti52Al2Zr (HOx) showed the highest strength, with Rp0.2 = 586 MPa, followed closely by Ti52Al1Zr (HOx). Increasing Zr content led to strength enhancement; in contrast, Ti52Al0.5Zr exhibited the highest work-hardening rate but the lowest strength among all tested alloys, regardless of O content [49,50,51,52,53,54].
With increased O, a strengthening effect was observed, as evidenced by the higher strength of Ti52Al in HOx compared to its LOx sample. These results confirm that the synergy between Zr and O has a critical influence on the mechanical behavior of TiAl alloys. While higher O levels increase strength, low Zr content (0.5 at.%) without sufficient synergistic effect leads to reduced mechanical performance. A positive influence of Zr becomes more apparent at ≥1 at.%, consistent with findings reported by Imayev et al. [20]. Additionally, Ti52Al1Zr and Ti52Al0.5Zr showed improved mechanical properties after HTT, but only Ti52Al2Zr achieved a balance of high strength and thermal stability [53,54,55,56,57]. In contrast, the reference Ti52Al alloy exhibited the lowest strength and premature yielding, indicating limited resistance to high-temperature loading [25]. These findings underscore the importance of Zr alloying for improving the mechanical integrity of TiAl alloys under thermal and mechanical stress [30,57]. In addition to the previously discussed effects, several strengthening mechanisms related to the presence of Zr contribute significantly to the enhanced mechanical performance of fully γ-TiAl alloys:
  • With increasing Zr content, solidification-induced segregation becomes more pronounced, promoting the formation and volumetric dominance of massive γ grains. This structural evolution is particularly relevant under high-temperature exposure, where Zr acts as a γ-phase stabilizer. Through solid solution strengthening, Zr increases resistance to plastic deformation, leading to a noticeable improvement in high-temperature strength.
  • Moreover, in fully γ-TiAl systems, the high solubility of Zr in the γ lattice facilitates the stabilization of a single-phase microstructure, though excessive Zr addition may induce microscale segregation. Such segregation can locally reduce ductility and negatively impact fracture toughness. Despite this trade-off, Zr additions up to moderate levels have been shown to refine microstructure and delay grain boundary diffusion, which is beneficial not only for creep resistance but also for long-term structural stability. In this context, Zr serves as an effective alternative to traditional alloying elements such as niobium (Nb) in β-free TiAl systems.
  • Furthermore, the role of O must be considered, as it acts as an interstitial solute solution strengthening in the γ phase. While low concentrations of O contribute to solid solution strengthening and enhanced hardness, they simultaneously reduce elongation and fracture toughness. The interaction between Zr and O is critical—at suboptimal Zr levels (<1 at.%), the strengthening effect of O is not effectively supported, resulting in an overall reduction in mechanical performance. Therefore, a synergistic balance between Zr and O content is essential for optimizing both strength and ductility in fully γ-TiAl alloys.

5. Conclusions

Based on the conducted analysis, the following conclusions can be drawn:
  • An increased O content does not significantly increase the α2-phase fraction in Ti52AlxZr alloys after high-temperature treatment at 1250 °C when compared to the reference Ti52Al alloy.
  • Increasing Zr content does not lead to α2-phase stabilization, suggesting that, unlike O, Zr does not act as a stabilizer of the α2 phase but may mitigate the negative structural effects caused by O.
  • The microstructure of LOx samples contains fine α2 colonies located at grain boundaries.
  • With increasing Zr content, a noticeable grain coarsening is observed in Ti52AlxZr alloys in comparison to the binary Ti52Al alloy.
  • The presence of both O and Zr, particularly at Zr concentrations above 2 at.%, reduces the oxidation layer growth rate, and alloys with higher O content show a comparable mass gain for both LOx and HOx samples.
  • Zr modifies the chemical composition of the oxide layers by decreasing the homogeneity of TiO2 and increasing the homogeneity of Al2O3, leading to changes in the characteristics of the oxide layers and a reduction in its overall thickness.
  • The combined effect of O and Zr increases hardness and strength. In Ti52AlxZr alloys, Zr contributes to mechanical stability under HTT conditions when compared to the reference Ti52Al alloy.
These results demonstrate that Ti52AlxZr alloys are promising candidates for use in extreme service conditions, particularly in the aerospace and automotive industries. From a mechanical and oxidation resistance standpoint, Ti52Al2Zr appears to be the most advantageous composition, offering an optimal combination of strength, hardness, and oxidation resistance. For practical applications, the following is recommended:
  • Optimize Zr content in the range of 1–2 at.% to improve both strength and oxidation resistance.
Elevated O levels (>4000 ppm), however, represent a promising direction for future research, especially for components exposed to lower mechanical loading, where higher strength may be prioritized over toughness. Furthermore, the results suggest that combining optimized Zr content with additional alloying elements which effectively binds O, or elements which act as a diffusion barrier to O, may further mitigate the negative effects of O. This approach supports the development of high-strength and thermally stable ternary (Ti–Al–X) or quaternary (Ti–Al–X–Y) alloys suitable for demanding industrial applications.

Author Contributions

Conceptualization: M.K., M.T. (Marc Thomas) and M.T. (Maria Tsoutsouva); data curation: M.K. and J.-P.M.; formal analysis: M.K., T.V. and M.T. (Maria Tsoutsouva); funding acquisition: M.T. (Marc Thomas), J.-P.M. and P.S.; investigation: M.K., M.T. (Maria Tsoutsouva) and F.H.; methodology: M.K. and M.T. (Maria Tsoutsouva); project administration: J.-P.M., M.T. (Marc Thomas) and P.S.; resources: J.-P.M., M.T. (Marc Thomas) and P.S.; software: T.V. and M.K.; supervision: M.T. (Marc Thomas), M.T. (Maria Tsoutsouva) and J.-P.M.; validation: M.T. (Maria Tsoutsouva), T.V. and F.H.; writing—original draft preparation: M.K.; writing—review and editing: M.K., M.T. (Marc Thomas) and M.T. (Maria Tsoutsouva). All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Agence Nationale de la Recherche (ANR), grant number (ANR-21-CE08-0040-03). The article processing charge (APC) was fully waived by MDPI.

Data Availability Statement

The data presented in this study are not publicly available due to restrictions associated with the DemenTiAl research project, conducted at the ONERA research institute in collaboration with the company Safran and the research institute CEMES. Access to the data may be granted upon request to the corresponding author, Michal Kuriš, or alternatively to the co-authors Marc Thomas, Maria Tsoutsouva, Pierre Sallot, and Jean-Philippe Monchoux. Each request will be reviewed in accordance with ONERA’s internal confidentiality policies and approval procedures, in consultation with Safran and CEMES.

Acknowledgments

The authors would like to thank the Agence Nationale de la Recherche (ANR) for funding the research activity within the DemenTiAl project, part of whose results are presented in this contribution. Sincere thanks are also extended to the company Safran and the CEMES research institute for the opportunity to collaborate on the addressed topic. The authors further acknowledge the technical support provided by ONERA staff from the DMAS-TECH, DMAS-SIAM, and DMAS-EPIC divisions for their assistance with the design, fabrication, and analysis of the evaluated samples. Appreciation is also expressed to the technical facilities of the CEMES research institute.

Conflicts of Interest

Author Dr. Michal Kuriš, was employed by the company ONERA. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Abbreviations

The following abbreviations are used in this manuscript:
α2Alpha-2 phase (Ti3Al), an ordered phase in titanium aluminides
β(B2)Beta phase with B2 crystal structure in titanium aluminides
γGamma phase of titanium aluminide (γ-TiAl)
γ-TiAlGamma titanium aluminides
Al2O3Aluminum oxide (alumina)
DFTDensity functional theory
HOxHigh oxygen content
HTTHigh-temperature testing
HTHeat treatment
ICP-OESInductively coupled plasma optical emission spectrometry
LOxLow oxygen content
OMOptical microscopy
RTTRoom temperature testing
TCTI1Thermo-Calc TiAl database, version 1
TCTI5Thermo-Calc TiAl database, version 5
Ti52AlSpecific designation of titanium aluminides where the value 52 corresponds to at.% Al
Ti52Al0.5ZrSpecific designation of titanium aluminides, where the value 52 corresponds to at.% Al and 0.5 to at.% Zr
TiO2Titanium dioxide
TNMTitanium aluminide alloy (43–47% Ti, 43–47% Al, 8–10% Nb, ~1% Mo)
VAMVacuum arc melting

References

  1. Mallikarjuna, B.; Reutzel, E.W. Reclamation of Intermetallic Titanium Aluminide Aero-Engine Components Using Directed Energy Deposition Technology. Manuf. Rev. 2022, 9, 27. [Google Scholar] [CrossRef]
  2. Knippscheer, S.; Frommeyer, G. Neu Entwickelte TiAl-Basislegierungen für den Leichtbau von Triebwerks- und Motorkomponenten—Eigenschaften, Herstellung, Anwendung. Mater. Werkst. Tech. 2006, 37, 724–730. [Google Scholar] [CrossRef]
  3. Ganji, D.K.; Gajjela, R. Selection of Elemental Composition of a Nickel-Based Super Alloy Through Neural Networks and Multi-Criteria Decision Making. Aust. J. Mech. Eng. 2023, 21, 1287–1299. [Google Scholar] [CrossRef]
  4. Gudivada, G.; Pandey, A.K. Recent Developments in Nickel-Based Superalloys for Gas Turbine Applications: Review. J. Alloys Compd. 2023, 963, 171128. [Google Scholar] [CrossRef]
  5. Caldwell, E.C.; Fela, F.J.; Fuchs, G.E. The Segregation of Elements in High-Refractory-Content Single-Crystal Nickel-Based Superalloys. JOM 2004, 56, 44–48. [Google Scholar] [CrossRef]
  6. Thomas, M.; Berteaux, O.; Popoff, F.; Bacos, M.-P.; Morel, A.; Passilly, B.; Ji, V. Effects of Exposure at 700 °C on RT Tensile Properties in a PM γ-TiAl Alloy. Intermetallics 2006, 14, 1143–1150. [Google Scholar] [CrossRef]
  7. Pyczak, F.; Appel, F.; Limberg, W.; Lorenz, U.; Oehring, M.; Paul, J.; Schimansky, F.-P. Novel Processing Techniques for γ-TiAl Alloys. Mater. Sci. Forum 2011, 690, 145–148. [Google Scholar] [CrossRef]
  8. Sallot, P.; Monchoux, J.-P.; Joulié, S.; Couret, A.; Thomas, M. Impact of β-Phase in TiAl Alloys on Mechanical Properties After High Temperature Air Exposure. Intermetallics 2020, 119, 106729. [Google Scholar] [CrossRef]
  9. Stangl, C.; Kollmannsberger, E.; Krüger, M.; Huber, O.; Klaus, H.; Saage, H. Reducing the Environmental Embrittlement Effect of TiAl Alloys Exposed to Air at High Temperatures by a Fine-Grained Surface Structure. Intermetallics 2024, 175, 108479. [Google Scholar] [CrossRef]
  10. Musi, M.; Graf, G.; Clemens, H.; Spoerk-Erdely, P. Alloying Elements in Intermetallic γ-TiAl-Based Alloys—A Review on Their Influence on Phase Equilibria and Phase Transformations. Adv. Eng. Mater. 2024, 26, 2300610. [Google Scholar] [CrossRef]
  11. Peters, M.; Kumpfert, J.; Ward, C.; Leyens, C. Titanium Alloys for Aerospace Applications. Adv. Eng. Mater. 2003, 5, 419–427. [Google Scholar] [CrossRef]
  12. Xu, X.; Liang, H.; Ding, H.; Davidson, K.P.; Ramanujan, R.V.; Chen, R.; Guo, J.; Fu, H. Oxidation Behavior and Mechanical Properties of a Directionally Solidified High Nb TiAl-Based Alloy Between 800 °C and 900 °C. Intermetallics 2025, 176, 108538. [Google Scholar] [CrossRef]
  13. Connétable, D.; Prillieux, A.; Thenot, C.; Monchoux, J.-P. Theoretical Study of Oxygen Insertion and Diffusivity in the γ-TiAl L10 System. J. Phys. Condens. Matter 2020, 32, 175702. [Google Scholar] [CrossRef] [PubMed]
  14. Zhang, K.; Zhang, L.; Li, J. The Effect of Refined Coherent Grain Boundaries on High-Temperature Oxidation Behavior of TiAl-Based Alloys Through Cyclic Heat Treatment. Metals 2024, 14, 521. [Google Scholar] [CrossRef]
  15. Distl, B.; Dehm, G.; Stein, F. Effect of Oxygen on High-Temperature Phase Equilibria in Ternary Ti–Al–Nb Alloys. J. Inorg. Gen. Chem. 2020, 646, 1151–1156. [Google Scholar] [CrossRef]
  16. Fröbel, U.; Stark, A.; Paul, J.; Pyczak, F. Assessment of the Capability of Zr, Y, La, Gd, Dy, C, and Si to Enhance the Creep Strength of Gamma Titanium Aluminide Alloys Based on Their Effect on Flow Stress and Thermal Activation Parameters. Intermetallics 2023, 162, 108018. [Google Scholar] [CrossRef]
  17. Voisin, T.; Monchoux, J.-P.; Thomas, M.; Deshayes, C.; Couret, A. Mechanical Properties of the TiAl IRIS Alloy. Metall. Mater. Trans. A 2016, 47, 6097–6108. [Google Scholar] [CrossRef]
  18. Tian, S.; Zhang, T.; Zeng, S.; Zhang, Y.; Song, D.; Chen, Y.; Kang, Q.; Jiang, H. Cyclic Oxidation Kinetics and Thermal Stress Evolution of TiAl Alloys at High Temperature. Metals 2024, 14, 28. [Google Scholar] [CrossRef]
  19. Liang, H.; Ding, H.; Xu, X.; Zhang, X.; Chen, R.; Guo, J.; Fu, H. Effect of Variation in Zr Content on Microstructure and High-Temperature Tensile Properties of a γ-TiAl Alloy. Mater. Sci. Eng. A 2024, 893, 1460. [Google Scholar] [CrossRef]
  20. Imayev, V.; Ganeev, A.; Trofimov, D.; Parkhimovich, N.; Imayev, R. The Effect of Initial Microstructure on Hot Deformation Behavior of TiAl–Nb Intermetallic Alloy. Mater. Sci. Eng. A 2021, 817, 141388. [Google Scholar] [CrossRef]
  21. Chen, R.; Zhao, X.; Yang, Y.; Guo, J.; Ding, H.; Su, Y.; Fu, H. Effect of Zr on Microstructure and Mechanical Properties of Binary TiAl Alloys. Trans. Nonferrous Met. Soc. China 2018, 28, 1724–1734. [Google Scholar] [CrossRef]
  22. Chepak-Gizbrekht, M.V.; Knyazeva, A.G. Oxidation of TiAl Alloy by Oxygen Grain Boundary Diffusion. Intermetallics 2023, 162, 107993. [Google Scholar] [CrossRef]
  23. Haußmann, L.; Bresler, J.; Neumeier, S.; Pyczak, F.; Göken, M. Interdiffusion Coefficients and Strengthening Effects of Nb, Ta, and Zr in the α2-Ti3Al Phase. J. Phase Equilib. Diffus. 2024, 45, 764–771. [Google Scholar] [CrossRef]
  24. Matsunaga, A.; Seriizawa, A.; Yamabe-Mitarai, Y. Effect of Zr on Microstructure and Oxidation Behavior of α and α + α2 Ti–Al–Nb Alloys. Mater. Trans. 2016, 57, 1902–1907. [Google Scholar] [CrossRef]
  25. Distl, B.; Dehm, G.; Pyczak, F. Phase Equilibria and Phase Transformations of Ti–Al–X (X = Nb, Mo, W) Alloys for High-Temperature Structural Applications Between 700 and 1300 °C. Dissertation Thesis, Ruhr University Bochum, Bochum, Germany, 2022. Available online: https://web.archive.org/web/20220827180640id_/https://hss-opus.ub.ruhr-uni-bochum.de/opus4/frontdoor/deliver/index/docId/9130/file/diss.pdf (accessed on 11 December 2024).
  26. Kulkova, S.E.; Bakulin, A.V.; Kulkov, S.S. First-Principles Calculations of Oxygen Diffusion in Ti–Al Alloys. Latv. J. Phys. Tech. Sci. 2018, 6, 20–29. [Google Scholar] [CrossRef]
  27. Bolibruchová, D.; Kuriš, M.; Matejka, M.; Kasińska, J. Study of the Influence of Zirconium, Titanium and Strontium on the Properties and Microstructure of AlSi7Mg0.3Cu0.5 Alloy. Materials 2022, 15, 3709. [Google Scholar] [CrossRef]
  28. Bolibruchová, D.; Matejka, M.; Brůna, M.; Kuriš, M.; Michalcová, A. Investigation of Microstructure and Mechanical Properties of AlSi7Mg0.3Cu0.5 Alloy with Addition of Zr, Ti and Sr. Int. J. Met. Cast. 2023, 17, 2584–2597. [Google Scholar] [CrossRef]
  29. Molénat, G.; Monchoux, J.-P.; Musi, M.; Spoerk-Erdely, P.; Clemens, H.; Couret, A. A TEM Study of a<001> Dislocations in the β₀ Phase of an Intermetallic TNM-TiAl Alloy. Scr. Mater. 2023, 226, 115247. [Google Scholar] [CrossRef]
  30. Murugesan, J.; Ping, D.; Yamabe-Mitarai, Y. Effect of Zr Addition on High Temperature Mechanical Properties of Near Alpha Ti–Al–Zr–Sn-Based Alloy. Mater. Sci. Forum 2014, 783–786, 580–583. [Google Scholar] [CrossRef]
  31. Kahrobaee, Z.; Palm, M. Critical Assessment of the Al–Ti–Zr System. J. Phase Equilib. Diffus. 2020, 41, 687–701. [Google Scholar] [CrossRef]
  32. Firstov, S.; Kulak, L.; Kuzmenko, M.; Shevchenko, O. Alloys of the Ti–Al–Zr–Si System Intended for Operation at High Temperatures. Mater. Sci. 2019, 54, 783–788. [Google Scholar] [CrossRef]
  33. Morris, D.G.; Naka, S.; Caron, P. High-Temperature Ordered Intermetallic Alloys; Wiley-VCH Verlag GmbH: Weinheim, Germany, 2000; ISBN 3-527-30192-5. [Google Scholar]
  34. Belov, N.A.; Akopyan, T.K.; Belov, V.D.; Gershman, J.S.; Gorshenkov, M.V. The Effect of Cr and Zr on the Structure and Phase Composition of TNM Gamma Titanium Aluminide Alloy. Intermetallics 2017, 84, 121–129. [Google Scholar] [CrossRef]
  35. Bresler, J.; Neumeier, S.; Ziener, M.; Pyczak, F.; Göken, M. The Influence of Niobium, Tantalum and Zirconium on the Microstructure and Creep Strength of Fully Lamellar γ/α2 Titanium Aluminides. Mater. Sci. Eng. A 2019, 744, 46–53. [Google Scholar] [CrossRef]
  36. Wang, Y.; Li, S.; Ma, T.; Wang, X.; Dong, D.; Zhu, D.; Fang, H.; Chen, R. Improving High Temperature Oxidation Resistance of TiAl Alloy via Hierarchical Ti₅Si3–Ti2AlC Precipitation Strategy. Corros. Sci. 2024, 228, 111834. [Google Scholar] [CrossRef]
  37. Yang, H.; Xu, S.; Lan, J.; Gao, J.; Cao, H.; Zheng, G.; Zhang, H.; Feng, W. Effect of Rhenium Addition on the High-Temperature Oxidation Resistance of Anodized γ-TiAl Alloy. J. Alloys Compd. 2025, 1010, 178087. [Google Scholar] [CrossRef]
  38. Feng, L.; Li, B.; Li, Q.; Gao, Y.; Pei, Z.; Liang, C. Enhancement of Mechanical Properties and Oxidation Resistance of TiAl Alloy with Addition of Nb and Mo Alloying Elements. Mater. Chem. Phys. 2024, 316, 129148. [Google Scholar] [CrossRef]
  39. Wang, H.; Liu, Z.; Liang, J. High-Temperature Oxidation Behavior of Ti–48Al–2Cr–2Nb Alloy by Laser Additive Manufacturing. Mater. Chem. Phys. 2025, 333, 130365. [Google Scholar] [CrossRef]
  40. Gao, Z.; Hu, R.; Zou, H.; Zhou, M.; Luo, X. Insight into the Ta Alloying Effects on the Oxidation Behavior and Mechanism of Cast TiAl Alloy. Mater. Des. 2024, 241, 112941. [Google Scholar] [CrossRef]
  41. Yang, Y.; Han, Y.; Ran, X.; Liu, Y. Improving the High-Temperature Ductility of γ-TiAl Matrix Composites by Incorporation of AlCoCrFeNi High Entropy Alloy Particles. J. Alloys Compd. 2025, 1012, 178515. [Google Scholar] [CrossRef]
  42. Li, L.; Zhao, Z.; Feng, Z.-X.; Sun, J.; Li, X.-Q. Microstructure and Shear Strength of γ-TiAl/GH536 Joints Brazed with Ti–Zr–Cu–Ni–Fe–Co–Mo Filler Alloy. Trans. Nonferrous Met. Soc. China 2020, 30, 2143–2155. [Google Scholar] [CrossRef]
  43. Bacos, M.-P.; Morel, A.; Naveos, S.; Bachelier-Locq, A.; Josso, P.; Thomas, M. The Effect of Long-Term Exposure in Oxidising and Corroding Environments on the Tensile Properties of Two Gamma-TiAl Alloys. Intermetallics 2006, 14, 102–113. [Google Scholar] [CrossRef]
  44. Liu, Z.; Xin, S.; Zhao, Y. Research Progress on the Creep Resistance of High-Temperature Titanium Alloys: A Review. Metals 2023, 13, 1975. [Google Scholar] [CrossRef]
  45. Shaaban, A.; Nakashima, H.; Ueda, M.; Takeyama, M. Effects of Oxygen Addition on the Microstructure, Phase Transformations, and Oxidation Resistance of TiAl Alloys. J. Alloys Compd. 2022, 918, 165716. [Google Scholar] [CrossRef]
  46. Neumeier, S.; Bresler, J.; Zenk, C.; Haußmann, L.; Stark, A.; Pyczak, F.; Göken, M. Partitioning Behavior of Nb, Ta, and Zr in Fully Lamellar γ/α2 Titanium Aluminides and Its Effect on the Lattice Misfit and Creep Behavior. Adv. Eng. Mater. 2021, 23, 2100156. [Google Scholar] [CrossRef]
  47. Mitoraj-Królikowska, M.; Drożdż, E. Some Aspects of Oxidation and Reduction Processes in Ti–Al and Ti–Al–Nb Systems. Materials 2022, 15, 1640. [Google Scholar] [CrossRef]
  48. Soyama, J.; Oehring, M.; Limberg, W.; Ebel, T.; Kainer, K.U.; Pyczak, F. The Effect of Zirconium Addition on Sintering Behaviour, Microstructure and Creep Resistance of the Powder Metallurgy Processed Alloy Ti–45Al–5Nb–0.2B–0.2C. Mater. Des. 2015, 84, 87–94. [Google Scholar] [CrossRef]
  49. Musi, M.; Kardos, S.; Hatzenbichler, L.; Holec, D.; Stark, A.; Allen, M.; Güther, V.; Clemens, H.; Spoerk-Erdely, P. The Effect of Zirconium on the Ti-(42–46 at.%)Al System. Acta Mater. 2022, 241, 118414. [Google Scholar] [CrossRef]
  50. Illarionov, A.G.; Stepanov, S.I.; Naschetnikova, I.A.; Popov, A.A.; Soundappan, P.; Raman, K.H.T.; Suwas, S. A Review—Additive Manufacturing of Intermetallic Alloys Based on Orthorhombic Titanium Aluminide Ti2AlNb. Materials 2023, 16, 991. [Google Scholar] [CrossRef]
  51. Lindwall, G.; Wang, P.; Kattner, U.; Campbell, C. The Effect of Oxygen on Phase Equilibria in the Ti–V System: Impacts on the AM Processing of Ti Alloys. JOM 2018, 70, 1692–1705. [Google Scholar] [CrossRef]
  52. Perdrix, F.; Trichet, M.-F.; Bonnentien, J.-L.; Cornet, M.; Bigot, J.-P. Variations in the α2/γ Lamellar Microstructure in Ti–48Al Alloys as a Function of Cooling Rate and Oxygen Content. J. Phys. IV France 2000, PR6, 15–20. [Google Scholar] [CrossRef]
  53. Lamirand, M.; Bonnentien, J.-L.; Ferrière, G.; Guérin, S.; Chevalier, J.-P. Influence of a Pre-Oxidation Treatment on the Oxidation Resistance of TiAl Alloys. Metall. Mater. Trans. A 2006, 37A, 2369–2376. [Google Scholar] [CrossRef]
  54. Perdrix, F.; Trichet, M.-F.; Bonnentien, J.-L.; Cornet, M.; Bigot, J. Influence of Oxygen on the Microstructure and Mechanical Properties of Ti–52Al–48 Alloys. Intermet. Superalloys 2000, 10, 289–293. [Google Scholar]
  55. Bacos, M.-P.; Thomas, M.; Raviart, J.-L.; Morel, A.; Navéos, S.; Josso, P. Influence of Surface Modification and Long-Term Exposure on Mechanical Creep Properties of γ-TiAl G4. Mater. High Temp. 2010, 27, 339–346. [Google Scholar] [CrossRef]
  56. Braun, J.; Ellner, M. Phase Equilibria Investigations on the Aluminum-Rich Part of the Binary System Ti–Al. Metall. Mater. Trans. A 2001, 32A, 1037–1047. [Google Scholar] [CrossRef]
  57. Couret, D.; Reyes, D.; Thomas, M.; Ratel-Ramond, N.; Deshayes, C.; Monchoux, J.-P. Effect of Ageing on the Properties of the W-Containing IRIS-TiAl Alloy. Acta Mater. 2020, 199, 169–180. [Google Scholar] [CrossRef]
Metals 15 00745 g001
Figure 1. SEM micrographs showing the microstructure of LOx (a) and HOx (b) samples of Ti52Al alloys (a1,b1), Ti52AlxZr alloys with 0.5 at.% Zr (a2,b2), 1 at.% Zr (a3,b3), and 2 at.% Zr (a4,b4) after HT at 1250 °C for 100 h.
Figure 1. SEM micrographs showing the microstructure of LOx (a) and HOx (b) samples of Ti52Al alloys (a1,b1), Ti52AlxZr alloys with 0.5 at.% Zr (a2,b2), 1 at.% Zr (a3,b3), and 2 at.% Zr (a4,b4) after HT at 1250 °C for 100 h.
Metals 15 00745 g001
Metals 15 00745 g002
Figure 2. SEM micrographs showing the precipitation of α2-phase along the grain boundaries of the ternary Ti52Al0.5Zr alloy after HT at 1250 °C for 100 h (the red square indicates the detail on the right).
Figure 2. SEM micrographs showing the precipitation of α2-phase along the grain boundaries of the ternary Ti52Al0.5Zr alloy after HT at 1250 °C for 100 h (the red square indicates the detail on the right).
Metals 15 00745 g002
Metals 15 00745 g003
Figure 3. EBSD phase map of the Ti52Al2Zr alloy (HOx sample) after HT at 1250 °C for 100 h (a); the microstructure reveals γ-phase matrix with α2-phase precipitation along grain boundaries (b).
Figure 3. EBSD phase map of the Ti52Al2Zr alloy (HOx sample) after HT at 1250 °C for 100 h (a); the microstructure reveals γ-phase matrix with α2-phase precipitation along grain boundaries (b).
Metals 15 00745 g003
Metals 15 00745 g004
Figure 4. OM macrographs showing grain homogeneity in LOx (a) and HOx (b) samples of the reference Ti52Al (a1,b1) and Ti52AlxZr alloys with 0.5 at.% Zr (a2,b2), 1 at.% Zr (a3,b3), and 2 at.% Zr (a4,b4) after heat treatment at 1250 °C for 100 h.
Figure 4. OM macrographs showing grain homogeneity in LOx (a) and HOx (b) samples of the reference Ti52Al (a1,b1) and Ti52AlxZr alloys with 0.5 at.% Zr (a2,b2), 1 at.% Zr (a3,b3), and 2 at.% Zr (a4,b4) after heat treatment at 1250 °C for 100 h.
Metals 15 00745 g004
Metals 15 00745 g005
Figure 5. Effect of Zr addition on the average grain size and grain size distribution in LOx samples of the reference Ti52Al and Ti52AlxZr alloys after heat treatment at 1250 °C for 100 h, suggests that the presence of Zr may contribute to solid solution strengthening while also influencing grain coarsening.
Figure 5. Effect of Zr addition on the average grain size and grain size distribution in LOx samples of the reference Ti52Al and Ti52AlxZr alloys after heat treatment at 1250 °C for 100 h, suggests that the presence of Zr may contribute to solid solution strengthening while also influencing grain coarsening.
Metals 15 00745 g005
Metals 15 00745 g006
Figure 6. Characterization of hardness for the reference Ti52Al (1), Ti52Al0.5Zr (2), and Ti52Al2Zr (3) alloys in Lox (a) and HOx (b) samples after cyclic oxidation at 850 °C for 100 h. The upper part shows the hardness map of the evaluated area of the sample after HT, while the lower part presents the average HV 1 hardness value in the same area.
Figure 6. Characterization of hardness for the reference Ti52Al (1), Ti52Al0.5Zr (2), and Ti52Al2Zr (3) alloys in Lox (a) and HOx (b) samples after cyclic oxidation at 850 °C for 100 h. The upper part shows the hardness map of the evaluated area of the sample after HT, while the lower part presents the average HV 1 hardness value in the same area.
Metals 15 00745 g006
Metals 15 00745 g007
Figure 7. Stress–strain behavior during room temperature compression testing (RTT) of Ti52Al and Ti52AlxZr alloys.
Figure 7. Stress–strain behavior during room temperature compression testing (RTT) of Ti52Al and Ti52AlxZr alloys.
Metals 15 00745 g007
Metals 15 00745 g008
Figure 8. Flow-stress behavior during room temperature compression testing (RTT) of Ti52Al and Ti52AlxZr alloys.
Figure 8. Flow-stress behavior during room temperature compression testing (RTT) of Ti52Al and Ti52AlxZr alloys.
Metals 15 00745 g008
Metals 15 00745 g009
Figure 9. (a) Stress–strain behavior during high-temperature compression testing (HTT) of Ti52Al and Ti52AlxZr LOx alloys; (b) Flow-stress behavior during high-temperature compression testing (HTT) of Ti52Al and Ti52AlxZr LOx alloys.
Figure 9. (a) Stress–strain behavior during high-temperature compression testing (HTT) of Ti52Al and Ti52AlxZr LOx alloys; (b) Flow-stress behavior during high-temperature compression testing (HTT) of Ti52Al and Ti52AlxZr LOx alloys.
Metals 15 00745 g009
Metals 15 00745 g010
Figure 10. Cyclic oxidation mass gain of the reference Ti52Aland Ti52AlxZr alloys LOx and HOx after 100 h at 850 °C in air.
Figure 10. Cyclic oxidation mass gain of the reference Ti52Aland Ti52AlxZr alloys LOx and HOx after 100 h at 850 °C in air.
Metals 15 00745 g010
Metals 15 00745 g011
Figure 11. Surface OM macrographs (a) and cross-sectional SEM micrographs (b) of the oxide layers formed on Ti52Al LOx (a2,b2) and HOx (a1,b1) samples after cyclic oxidation at 850 °C for 100 h.
Figure 11. Surface OM macrographs (a) and cross-sectional SEM micrographs (b) of the oxide layers formed on Ti52Al LOx (a2,b2) and HOx (a1,b1) samples after cyclic oxidation at 850 °C for 100 h.
Metals 15 00745 g011
Metals 15 00745 g012
Figure 12. Surface OM macrographs (a,e) and cross-sectional SEM micrographs (b,f) of oxide layers formed on LOx [Ti52AlxZr alloys with 0.5 at.% Zr (e1,f1), 1 at.% Zr (e2,f2), and 2 at.% Zr (e3,f3)] and HOx [Ti52AlxZr alloys with 0.5 at.% Zr (a1,b1), 1 at.% Zr (a2,b2), and 2 at.% Zr (a3,b3)] samples after cyclic oxidation at 850 °C for 100 h. Notably, despite being thinner, the Al2O3 layer in the Ti52Al0.5Zr alloys was considerably more homogeneous than in the Ti52Al reference alloy (Figure 13).
Figure 12. Surface OM macrographs (a,e) and cross-sectional SEM micrographs (b,f) of oxide layers formed on LOx [Ti52AlxZr alloys with 0.5 at.% Zr (e1,f1), 1 at.% Zr (e2,f2), and 2 at.% Zr (e3,f3)] and HOx [Ti52AlxZr alloys with 0.5 at.% Zr (a1,b1), 1 at.% Zr (a2,b2), and 2 at.% Zr (a3,b3)] samples after cyclic oxidation at 850 °C for 100 h. Notably, despite being thinner, the Al2O3 layer in the Ti52Al0.5Zr alloys was considerably more homogeneous than in the Ti52Al reference alloy (Figure 13).
Metals 15 00745 g012
Metals 15 00745 g013
Figure 13. EDS elemental maps of oxide layers cross-sections in the reference Ti52Al alloy (a) and Ti52AlxZr alloys with 0.5 at.% Zr (b), 1 at.% Zr (c), and 2 at.% Zr (d) after cyclic oxidation at 850 °C for 100 h.
Figure 13. EDS elemental maps of oxide layers cross-sections in the reference Ti52Al alloy (a) and Ti52AlxZr alloys with 0.5 at.% Zr (b), 1 at.% Zr (c), and 2 at.% Zr (d) after cyclic oxidation at 850 °C for 100 h.
Metals 15 00745 g013
Metals 15 00745 g014
Figure 14. Pseudo-binary diagrams of TiAl with stable O addition (4000 ppm) generated by Thermo-Calc 2023: (a) TCTI1; (b) TCTI5. Ideal Ti-Al binary diagram—dashed line.
Figure 14. Pseudo-binary diagrams of TiAl with stable O addition (4000 ppm) generated by Thermo-Calc 2023: (a) TCTI1; (b) TCTI5. Ideal Ti-Al binary diagram—dashed line.
Metals 15 00745 g014
Metals 15 00745 g015
Figure 15. Pseudo-binary diagrams of TiAl with stable Zr addition (2 at.%) generated by Thermo-Calc 2023: (a) TCTI1; (b) TCTI5. Ideal Ti-Al binary diagram—dashed line.
Figure 15. Pseudo-binary diagrams of TiAl with stable Zr addition (2 at.%) generated by Thermo-Calc 2023: (a) TCTI1; (b) TCTI5. Ideal Ti-Al binary diagram—dashed line.
Metals 15 00745 g015
Table 1. Chemical composition of reference Ti52Al and Ti52AlxZr alloys with low (LOx) and high (HOx) O content.
Table 1. Chemical composition of reference Ti52Al and Ti52AlxZr alloys with low (LOx) and high (HOx) O content.
Experimental AlloysChemical Composition (at.%)
AlZrO
LOx samples
Ti52Al0.5Zr51.920.500.137
Ti52Al1Zr52.070.970.137
Ti52Al2Zr51.902.030.148
Ti52Al51.98-0.079
HOx samples
Ti52Al0.5Zr51.510.500.488
Ti52Al1Zr51.791.000.446
Ti52Al2Zr51.652.000.425
Ti52Al51.86-0.313
Table 2. Comparison of theoretical and experimental α2-phase content in reference Ti52Al and Ti52AlxZr alloys.
Table 2. Comparison of theoretical and experimental α2-phase content in reference Ti52Al and Ti52AlxZr alloys.
Evaluation MethodTi52Al
1250 °C		Ti52Al0.5Zr
1250 °C		Ti52Al1Zr
1250 °C		Ti52Al2Zr
1250 °C
LOx samples
TCTI1 LOx0.00.01.21.6
TCTI5 LOx0.61.01.21.3
Real image analyses LOx0.00.00.00.0
XRD analyses LOx0.00.00.00.0
HOx samples
TCTI1 HOx0.00.01.14.7
TCTI5 HOx2.04.13.63.9
Real image analyses HOx1.11.92.12.1
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Kuris, M.; Tsoutsouva, M.; Thomas, M.; Vaubois, T.; Sallot, P.; Habiyaremye, F.; Monchoux, J.-P. Effect of Oxygen and Zirconium on Oxidation and Mechanical Behavior of Fully γ Ti52AlxZr Alloys. Metals 2025, 15, 745. https://doi.org/10.3390/met15070745

AMA Style

Kuris M, Tsoutsouva M, Thomas M, Vaubois T, Sallot P, Habiyaremye F, Monchoux J-P. Effect of Oxygen and Zirconium on Oxidation and Mechanical Behavior of Fully γ Ti52AlxZr Alloys. Metals. 2025; 15(7):745. https://doi.org/10.3390/met15070745

Chicago/Turabian Style

Kuris, Michal, Maria Tsoutsouva, Marc Thomas, Thomas Vaubois, Pierre Sallot, Frederic Habiyaremye, and Jean-Philippe Monchoux. 2025. "Effect of Oxygen and Zirconium on Oxidation and Mechanical Behavior of Fully γ Ti52AlxZr Alloys" Metals 15, no. 7: 745. https://doi.org/10.3390/met15070745

APA Style

Kuris, M., Tsoutsouva, M., Thomas, M., Vaubois, T., Sallot, P., Habiyaremye, F., & Monchoux, J.-P. (2025). Effect of Oxygen and Zirconium on Oxidation and Mechanical Behavior of Fully γ Ti52AlxZr Alloys. Metals, 15(7), 745. https://doi.org/10.3390/met15070745

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop