Superstrength of Nanostructured Ti Grade 4 with Grain Boundary Segregations

Abstract

Severe plastic deformation and subsequent heat treatments yield nanostructured commercially pure (CP) titanium Grade 4 with average grain size of about 100 nm and exceptional strength. To elucidate the underlying strengthening mechanisms in this nanotitanium (nanoTi), this study uses atom probe tomography (APT) to analyze the atomic structure of grain boundaries and assess impurity segregation. Results reveal the formation of grain boundary segregations, primarily composed of iron (Fe) atoms, reaching concentrations up to 3.3 ± 0.2 at% in localized regions. The average width of these segregation layers is 6.13 ± 0.45 nm. The paper considers a mechanism for forming these segregations and discusses relevant theoretical models describing their contribution to the material’s enhanced strength.

1. Introduction

The development of ultra-strong materials remains a significant challenge in materials science. Recently, high-strength metallic biomaterials have attracted considerable attention for biomedical applications [1,2], particularly the enhancement of commercially pure (CP) titanium’s strength for biomedical implants with improved design and enhanced osseointegration [3,4]. Nanostructuring via severe plastic deformation (SPD) techniques, such as high-pressure torsion (HPT) followed by annealing, has been shown to produce ultra-strong titanium [4,5]. For example, this approach yielded a nanostructured Ti Grade 4 with an average grain size of 100 ± 10 nm, exhibiting a yield strength of 1340 ± 20 MPa and an ultimate tensile strength of 1510 ± 30 MPa.

To understand this high-strength state, the contribution of the different strengthening mechanisms to the overall yield strength (σ_y) was calculated in [5] using an additive approach that considered the contributions of different strengthening mechanisms, including grain boundary (σ_gb), dislocation (σ_dis), solid solution (σ_ss), dispersion (σ_Or), and shear lag (σ_sl) [6,7,8]:

$${\sigma }_y={\sigma }_0+{\sigma }_{gb}+{\sigma }_{dis}+{\sigma }_{ss}+{\sigma }_{Or}+{\sigma }_{sl},$$

(1)

where y₀ ≈ 80 MPa is the frictional stress of the crystal lattice in Ti [9].

Analysis of the strengthening mechanism contributions indicates that the exceptional mechanical properties result not only from the ultrafine-grained (UFG) structure (d ≈ 100 nm) but also from the presence of β-phase and intermetallic Ti₂Fe nanosized particles [5]. However, a discrepancy in 500 MPa remains between the calculated and experimentally observed strength. This study hypothesizes that incorporating grain boundary strengthening, specifically the effect of grain boundary segregation, is necessary to resolve this discrepancy.

Previous studies [10,11,12,13] have demonstrated that grain boundary segregations markedly enhance the strength of UFG materials by suppressing dislocation nucleation at grain boundaries. The present study aims to investigate by atom probe tomography the grain boundary atomic-level chemical composition of high-strength commercially pure (CP) Ti Grade 4 with UFG structure produced previously in [4,5]. This work is performed in order to identify, characterize (morphology and concentration), and elucidate the role of grain boundary segregations to the material’s strength.

2. Materials and Methods

In this work, the experiments were performed on CP Ti Grade 4 produced by VSMPO-AVISMA Corporation (located in Verkhnaya Salda, Russia). The chemical composition of this material in wt.%: Fe—0.37, N < 0.05, C—0.005–0.008, H—0.0001–0.0006; O—0.32, rest—Ti).

UFG Ti samples in the form of the discs with Ø20 mm were produced using HPT, a highly effective severe plastic deformation (SPD) technique for grain refinement [13,14,15,16,17,18]. HPT typically results in the formation of UFG structure with average grains size of 100 nm and less, and also produces an effect on phase transformation, leading to the appearance of nanoclusters and/or highly dispersed second phase precipitates [19,20].

The nanostructured state in titanium was obtained by a combined treatment, including multi-stage HPT and annealing (HPT, annealing at 700 °C, further HPT, final annealing at 350 °C). HPT was implemented at room temperature with a number of turns n = 5 under a pressure of 6 GPa and rotation rate of 0.2 rpm. The choice of annealing temperatures was aimed at obtaining the highest strength while providing significant ductility in nanotitanium [21].

The microstructure of the samples was analyzed using a transmission electron microscope (TEM) JEOL JEM-2100 (JEOL, Tokyo, Japan) operating at an accelerating voltage of 200 kV. TEM thin foils with a diameter of 3 mm were prepared by mechanical thinning of the disks on sanding paper to a thickness of 0.1–0.2 mm, followed by electrolytic polishing on a Tenupol-5 polisher (Struers, Ballerup, Denmark) in an electrolyte containing 300 mL methanol (CH₃OH), 175 mL butanol (C₄H₉OH), and 30 mL perchloric acid (HClO₄).

In order to conduct a detailed examination of the chemical composition at the atomic level of the samples, atom probe tomography (APT) was employed utilizing a local electrode atom probe CAMECA LEAP 4000X Si (Madison, WI, USA), operating in the voltage range of 2–10 kV. Samples for APT characterization were prepared using a dual-beam focused ion beam/scanning electron microscope (Zeiss Auriga FIB/SEM, Carl Zeiss, Oberkochen, Germany), with standard FIB lift-out procedures [22]. The data was acquired under a high vacuum of 2 × 10⁻¹¹ torr, at a specimen temperature of 40 K, a pulse repetition rate of 200 kHz, and UV laser energy of 40 pJ. The APT data were reconstructed using a CAMECA integrated visualization and analysis software (IVAS 3.8.2) (CAMECA Scientific Instruments, Madison, WI, USA), with a tip profile method.

The samples for TEM and APT studies were traditionally cut from the areas at distances over 5 mm from the sample center [5,13].

3. Results and Discussion

Figure 1 presents a typical TEM image of the studied CP Ti Grade 4 with UFG structure, which was obtained by HPT processing followed by annealing [5]. It is evident that this treatment leads to the formation of a fairly homogeneous microstructure. The grain size evaluation, which was carried out using the dark-field image (Figure 1b), showed that its average size is 100 ± 10 nm. Such a structure was typical for the entire sample after HPT treatment, with the exception of the central part, which had a diameter of approximately 5 mm. As noted above, this region was excluded from the evaluation of grain sizes and mechanical properties. As illustrated in Figure 1a, the bright-field image reveals that the grain boundaries are predominantly curved and wavy. Concurrently, certain boundaries are characterized by a lack of clarity, a phenomenon attributable to inhomogeneous contrast. The contrast inside the grains is uneven and often changes in a complex manner. The observed phenomena suggest that the grain boundaries are in a nonequilibrium state due to the presence of grain boundary dislocations. As is known, the observed nonequilibrium state of grain boundaries is typical for metallic materials subjected to SPD processing [13,18,23], which was clearly demonstrated in the high-resolution electron microscopy (HREM) studies. Also, the appearance of the electron diffraction pattern with a large number of reflections indicates the presence of predominant grain boundaries with high-angle misorientation (Figure 1a).

Atom maps of each alloying element in the reconstructed volume of an APT dataset of the high-strength Ti Grade 4 alloy after HPT + 700 °C for 30 min. + HPT + 350 °C for 30 min. treatment are presented in Figure 2.

The analyzed volume of the nanostructured Ti Grade 4 sample contained grain boundaries (GBs) with clear segregation of Fe and negligible segregation of Ni, as seen in Fe and Ni atom maps, respectively. Other alloying elements exhibited no discernible segregation at these GBs. Figure 3 presents a 3D-reconstruction with Ti atoms and iso-concentration surfaces at 2.0 at% Fe to show distribution of Fe-rich regions including GBs and Fe-rich clusters in the nanostructured Ti Grade 4.

The 3D-reconstruction with Fe iso-surfaces (Figure 3a) clearly demonstrates the Fe segregations at the grain boundaries in the HPT + annealing at 700 °C for 30 min + HPT + annealing at 350 °C for 30 min. treated sample. The grain size in the observed area constitutes 60–80 nm. A relatively uniform distribution of segregations along the grain boundaries is observed. Elemental profiles across grain boundaries (Figure 3b) reveal that Fe concentration in these segregations reaches 3.3 ± 0.2 at.% in some grain boundaries, which is ten times higher than the bulk concentration (0.26 ± 0.03 at.%). The mean width of these segregation layers is 6.13 ± 0.45 nm. Also noteworthy is the presence of nanoscale segregations of Fe atoms in the inner volume of grains, with an average size of 2.08 ± 0.50 nm. The volume fraction of these nanoclusters is ~0.25%, and their Fe concentration averages 3%. In order to evaluate the contribution of the observed nanoclusters to the strength of the material, the Labusch model for solid solution hardening [24,25,26] was applied. The evaluation showed that the contribution to the strength of these nanoclusters is quite insignificant and does not exceed 10 MPa.

The observed width of the grain boundary segregations of Fe in nanostructured Ti Grade 4 is quite unusual, considering the standard grain boundary width of about 1 nm [27], and there is great interest in why this occurs. As is known, the local magnification effect in the APT dataset could lead to some overestimation of the width of the Fe enrichment region across the GB, but it does not exceed 1 nm [28]. At the same time, it is evident that this is predominantly influenced by the state of grain boundaries in the material under study. Extensive research in this field, as evidenced by numerous publications, including [13,23], has demonstrated that SPD processing facilitates the refinement of grain structures in various metallic materials to nanoscale, resulting in the formation of UFG structures with high-angle grain boundaries characterized by a non-equilibrium dislocation-disturbed structure. These nonequilibrium boundaries are distinguished by substantial distortions of the crystal lattice near the boundaries and increased energy due to the presence of extrinsic grain boundary dislocations (EGBDs). Consequently, the nonequilibrium state of the boundaries may collect a high concentration of impurity atoms and lead to an increased width of their segregations at such GBs. It is noteworthy that a substantial broadening of the segregation zone has been observed through the APT method and in various UFG Al alloys obtained by SPD techniques, where the concentration of alloying element segregations at the boundaries was significantly higher than within the grains [23,29].

In order to further characterize the detected segregations, GB solute excesses of Fe at GBs have also been calculated for nano-Ti for all four grain boundaries (Figure 3). The estimation of GB solute excesses of Fe was conducted in accordance with the methodology outlined in [30,31]. The results obtained are summarized in Figure 4 and

Table 1. Summary table with characterization of grain boundary segregations of Fe in nanostructured Ti Grade 4.

Grain Boundary	Width of Fe Grain Boundary Segregations, nm	Fe Concentration at GB, at.%	GB Solute Excesses of Fe, Atom/nm2
GB1	5.5 ± 0.25	3.30 ± 0.20	4.50
GB2	6.5 ± 0.25	2.39 ± 0.18	4.20
GB3	6.0 ± 0.25	2.19 ± 0.19	2.48
GB4	6.5 ± 0.25	3.01 ± 0.12	5.46
Average	6.13 ± 0.45	2.72 ± 0.47	4.16

, where the segregation widths, Fe concentration values, and solute excesses for all four grain boundaries are presented. The outcomes demonstrated that the measured Fe excess values were considerably higher than the values reported in literature, which for equilibrium grain boundaries in Ti is 1.25 atom/nm² [30].

It is also important to note that, recently, the structural features of Ti grain boundaries upon the addition of Fe atoms were investigated in the works [30,32]. With the use of high-resolution electron microscopy and modelling, it was demonstrated that the incorporation of Fe atoms results in the reconstruction of GBs, accompanied by the formation of icosahedral Ti-Fe clusters [32]. This process also leads to the broadening of GBs and, consequently, of the Fe segregation layer, although its value did not exceed 2–3 nm.

As is evident from the provided results, the variation in the width of Fe segregation zones in titanium is attributable to the different states of grain boundaries in the referenced works and the present study. Specifically, the former works examined special and equilibrium grain boundaries, as reported in [30,32], while the present study focused on random nonequilibrium GBs.

For visual comparison, it is also interesting that, as demonstrated by model calculations [33], the free energy of Fe segregations in proximity to grain boundaries exhibits a range of −0.06 to −1.2 eV, which is markedly lower than that observed in the grain bulk (0.53 eV). Furthermore, the calculations indicate that the interstitial position of Fe atoms at grain boundaries is also more favorable than their positions in substitution positions in the grain bulk, due to the large volume of voids at the boundaries. It can therefore be assumed that the diffusion of Fe atoms is due to the energetically more favorable position at the grain boundaries. The authors also demonstrated that Fe atoms prefer to occupy positions with the largest Voronoi volumes despite different local coordination. Thus, it was concluded that the principal driving force behind the interstitial segregations of Fe at grain boundaries is the crystal lattice free volume present there, with the coordinates of these volumes in the boundary structure being of lesser significance [33].

As previously indicated, the identified segregations can impart a remarkable level of strength to the resulting nanostructured CP Ti Grade 4. There are a number of potential explanations for this phenomenon. First, as evidenced by the results of computer modelling [33], the formation of impurity segregations at grain boundaries results in a notable reduction in the energy of these boundaries. This enhances the overall stability of the grain boundary structure with regard to transformations associated with intergranular sliding, grain boundary migration, and the formation of nanocracks at these locations. Furthermore, it can be anticipated that the saturation of grain boundaries with atoms of specific alloying elements, in this case Fe, will result in an enhancement of the Hall–Petch coefficient k_HP [34]. This can be calculated using the following formula [35].

$$k_{HP}=M\sqrt{{\displaystyle \frac{{\tau }_{c}4Gb}{(1-\nu )\pi }}},$$

(2)

where M is the Taylor factor, τ_c is the critical stress for the onset of dislocation slip across a grain boundary, G is the shear modulus, b is the Burgers vector value, and ν is the Poisson ratio. The various studies [36,37] have demonstrated, through the use of molecular dynamics, that a reduction in the free energy of grain boundaries results in an enhancement of the value of τ_c. Accordingly, based on the aforementioned equality (2), it can be assumed that the formation of grain boundary segregations of Fe in nanostructured Ti Grade 4 results in an increase in the k_HP coefficient, which, in turn, leads to an increase in the yield strength of the material in accordance with the well-established Hall–Petch law ${\sigma }_{gb}~k_{HP}{d}^{-1/2}$, where d is the average grain size.

Second, in the case of a non-uniform structure of grain boundary segregations, when the impurity-enriched areas (which are specific enriched zones) of a grain boundary can be interpreted as precipitates of the second phase, the results of previous studies [10,11,12] can be utilized to hypothesize that these precipitates may inhibit the emission of dislocations from the boundaries, thereby increasing the yield strength of the material. The model can also then be used to determine the contribution to material strength of the Fe atoms nanoscale segregations inside the grain volume, as observed in this study.

It is also interesting that a theoretical model has recently been proposed to describe the effect of Mg grain boundary segregations on the yield strength of UFG Al-Mg-Zr alloys [38]. The model is illustrated schematically in Figure 5. The model’s key advantages include its consideration of the non-equilibrium structure of grain boundaries in SPD UFG materials and its suitability for describing the impact of segregations distributed uniformly along grain boundaries. The model represents a further development of the earlier model [39], which was aimed for CP UFG Al. This model assumed that plastic deformation under applied stress τ₁ occurs primarily through the emission of lattice dislocations from triple junctions of grain boundaries containing clusters of EGBDs pressed towards these triple junctions and forming pile-ups (EGBD pile-ups) there (Figure 5a). The model [38] proposes that the formation of Mg segregations at non-equilibrium grain boundaries in the UFG Al-Mg-Zr alloy results in pinning the EGBDs in their original positions (Figure 5b), effectively preventing the formation of dislocation pile-ups and causing them to form a relatively uniform row (EGBD row). In the framework of the model, there is no special consideration of the micromechanisms providing the dislocation pinning. This could relate to either standard Cottrell’s atmospheres for the case of perfect EGBDs or Suzuki’s atmospheres for the case of extended EGBDs that would form through either pipe or grain-boundary diffusion. In the absence of such pinning, dislocation pile-ups would otherwise be concentrated at the triple junctions, acting as strong concentrators for the applied stress. Consequently, in order for these triple junctions to effectively emit lattice dislocations, it is essential to significantly increase the applied stress (τ₂ > τ₁). In [38], the yield strengths of the UFG Al-Mg-Zr alloy after HPT treatment were calculated, and it was demonstrated that the presence of Mg atom segregations at grain boundaries results in a considerable additional strengthening effect of 180 MPa (approximately 82%), which was in good agreement with the available experimental data. This considerable effect allows us to posit that a comparable strengthening mechanism may operate in the case of Fe segregations at grain boundaries in UFG Ti, directly related to the alteration in the defect structure of non-equilibrium grain boundaries. In order to calculate the magnitude of this effect in this case, it is necessary to adapt the model [38] and conduct a specific study. In addition, when adapting the proposed model with respect to Ti, it is important to take into account the possible formation of icosahedral Ti-Fe clusters [32]. This may serve as an additional reason for the inhibition of EGBDs, as well as for preventing the formation of clusters pressed to the triple junctions of the GBs.

4. Conclusions

The present study demonstrates that the nanostructuring of CP Ti Grade 4 through a combined treatment, including HPT and heat treatment, results in segregation of Fe atoms located along the grain boundaries in the material. The use of APT has demonstrated that the concentration of these grain boundary segregations in certain regions reaches 3.3 ± 0.2%, which is 10 times higher than their concentration in the grain volume. In this instance, the mean width of the segregation layers is 6.13 ± 0.45 nm. Obviously, such width of grain boundary segregations of Fe in nanostructured Ti Grade 4 is provided by the non-equilibrium state of grain boundaries.

It is hypothesized that the formation of grain boundary segregations is caused by the diffusion of Fe atoms due to their position at the grain boundaries, which is energetically more favorable. In this instance, it is evident that the identified grain boundary segregations play a pivotal role in the high-strength state of nanostructured Ti Grade 4. At present, there are numerous theoretical models that seek to elucidate the mechanisms by which grain boundary segregations can enhance the strength of a material. In the case of nanostructured Ti Grade 4, the most suitable theoretical model is that which assumes the fixation of extrinsic grain boundary dislocations by segregations, thereby preventing the formation of dislocation pile-ups pressed to the triple junctions. Consequently, the emission of lattice dislocations by these triple junctions necessitates a considerable increase in the applied stress. Nevertheless, in order to ascertain the extent to which this phenomenon contributes to the strength of nanostructured Ti Grade 4, a dedicated study is necessary.