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Article

Mechanical and Corrosion Properties of Ultrafine-Grained TC4-0.55Fe Alloy Processed by Equal-Channel Angular Pressing

by
Yumeng Guo
1,
Yu Lu
1,
Miaoxia He
1,2,
Yu Wang
1,
Yuecheng Dong
1,* and
Igor V. Alexandrov
2
1
College of Materials Science and Engineering, Tech Institute for Advanced Materials, Nanjing Tech University, Nanjing 211816, China
2
Department of Materials Science and Physics of Metals, Ufa University of Science and Technology, Ufa 450008, Russia
*
Author to whom correspondence should be addressed.
Crystals 2025, 15(9), 795; https://doi.org/10.3390/cryst15090795
Submission received: 11 August 2025 / Revised: 4 September 2025 / Accepted: 5 September 2025 / Published: 8 September 2025
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Abstract

This study investigates the effects of multi-pass Equal-Channel Angular Pressing (ECAP) on the mechanical and corrosion properties of TC4-0.55Fe alloy through room-temperature tensile tests, electrochemical experiments, SEM, and EBSD characterization. The results demonstrate that, with increasing ECAP passes, the average grain size is progressively refined from the initial 3.8 μm to 1.8 μm after four passes. After four passes, the yield strength and ultimate tensile strength increase from initial values of 906 MPa and 939 MPa to 995 MPa and 1022 MPa, respectively, while the elongation at fracture slightly decreases to 12.0%. Electrochemical corrosion results reveal that ECAP processing significantly enhances the corrosion resistance of the TC4-0.55Fe alloy. Specifically, the two-pass specimen exhibits nearly an order-of-magnitude reduction in both corrosion rate and self-corrosion current density compared to the initial state. The simultaneous improvement in strength and corrosion resistance is primarily attributed to the synergistic effects of grain refinement, increased dislocation density, and the evolution of basal texture.

1. Introduction

Titanium and its alloys are essential in aerospace, marine engineering, and biomedical fields due to their high specific strength, excellent corrosion resistance, and good biocompatibility [1,2,3,4]. However, the expansion of marine engineering equipment applications into extreme environments such as deep-sea and polar regions imposes more stringent demands on the strength, toughness, and corrosion resistance of titanium alloys and their structural components used in marine engineering. In complex marine conditions, high concentrations of Cl− can attack persistently. This long-term service can lead to the localized breakdown of the passive film. It causes pitting and intergranular corrosion [5,6]. The newly developed TC4-0.55Fe alloy is produced by adding 0.55 wt.% Fe to the Ti-6Al-4V (TC4) alloy [7,8,9]. As a eutectoid β-stabilizing element, Fe confers several benefits, including grain refinement [10], a reduction in flow stress and β-transus temperature [8], and an enhancement of the overall mechanical properties. Our preliminary investigations into TC4-xFe (x = 0–0.9 wt.%) alloys [11] have revealed that an Fe addition of 0.55 wt.% results in an alloy with mechanical and corrosion properties comparable to those of TC4 [12], while also exhibiting superior fatigue resistance [13] and fracture toughness [7].
Grain refinement is a potent strategy for the simultaneous enhancement of both the strength and toughness of metallic materials. Severe plastic deformation (SPD) techniques refine grains by imposing significant plastic strain without altering the overall dimensions of the material, thereby endowing the materials with superior physical and chemical properties. As such, SPD has been widely adopted as a prevalent grain refinement approach [14]. Currently, various SPD processes have been developed, including Equal-Channel Angular Pressing (ECAP) [15], high-pressure torsion (HPT) [16], and accumulative roll bonding (ARB) [17]. Among these, ECAP stands out as one of the most promising methods for industrial-scale production due to its unique pure shear deformation mechanism, which allows for continuous processing while achieving grain refinement [18]. SPD techniques greatly improve the mechanical properties of titanium alloys by significantly refining their grain size. Zhao et al. [19] illustrated that the compressive yield strength of the Ti-6Al-4V alloy can be enhanced from 1296 MPa to 1432 MPa through the application of multi-pass ECAP. Radnia et al. [20] conducted a comprehensive investigation into the effects of ECAP followed by subsequent annealing treatment on the microstructure, mechanical properties, wear performance, and corrosion behavior of the Ti-6Al-4V alloy. Their research indicated that the samples subjected to both ECAP and annealing exhibited superior wear resistance and corrosion wear resistance. Liu et al. [21] found that multi-pass heavy rolling at 720 °C, combined with oxidation prevention treatment, could increase the tensile strength of Ti-6Al-4V (TC4) alloy to 1325 MPa.
However, despite the remarkable success of SPD in improving mechanical properties, research on its effects on corrosion behavior remains limited. Hoseini et al. [22] made an observation on ultrafine-grained commercially pure titanium processed by ECAP. They found that grain size did not significantly affect corrosion resistance in a 0.16 mol/L NaCl solution. Instead, the basal texture played a more dominant role. Li et al. [23] reported that ultrafine-grained TC4 ELI prepared via multi-pass ECAP showed only a slight increase in corrosion current density compared to its coarse-grained counterpart in simulated body fluids. Hu et al. [24] successfully prepared three different morphologies of uniform anodized nanotube layers on an ultrafine-grained Ti-6Al-4V alloy substrate using HPT technology and electrochemical methods. These layers not only exhibited significantly enhanced interfacial bonding strength, but also superior corrosion resistance. Currently, although some studies have been conducted on the corrosion performance of a TC4 alloy prepared using SPD technology, there are still certain limitations, and systematic exploration is lacking. Therefore, this study takes the improved TC4-0.55Fe alloy as the research object to investigate the mechanism of how the ECAP process affects mechanical and corrosion properties, which can further expand the research foundation in this field and provide theoretical guidance.
In this study, TC4-0.55Fe alloys with varying grain sizes and microstructures were fabricated through multi-pass ECAP. The microstructural evolution at different pressing passes was systematically characterized using electron backscatter diffraction (EBSD). The mechanical properties were evaluated through hardness testing and room-temperature tensile tests, while the corrosion resistance in simulated seawater environments was assessed by electrochemical measurements. This work investigates the mechanisms by which grain refinement and microstructural evolution induced by ECAP processing influence both mechanical performance and corrosion behavior.

2. Materials and Methods

The experimental material was forged TC4-0.55Fe titanium alloy, which underwent double annealing heat treatment before ECAP processing to achieve a bimodal microstructure. Grain refinement was accomplished using the ECAP technique. The ECAP die featured a channel diameter of 20 mm, an internal angle (ϕ) of 120°, and an outer arc angle (Ψ) of 20°. The pressing followed route Bc, characterized by a 90° rotation of the sample around its longitudinal axis between successive passes, as depicted in Figure 1a. Considering that the β-transus temperature (Tβ) of the TC4-0.55Fe alloy is 950 °C, ECAP processing was executed at 700 °C within the α + β phase field to ensure optimal workability, enhanced material homogeneity, and superior extrusion quality. The pressing was conducted for 1, 2, and 4 passes to produce samples with varying grain sizes. The resulting test specimens are referred to as ECAP 1P, ECAP 2P, and ECAP 4P, respectively, while the original material without ECAP processing is denoted as ECAP 0P.
Figure 1b shows the sampling locations for tensile specimens and metallographic specimens. Specimens with dimensions of 6 mm × 6 mm × 3 mm were sectioned from the mid-radius region along the longitudinal direction (parallel to the final extrusion flow direction) of the ECAP-processed TC4-0.55Fe alloy cylinders using wire electrical discharge machining. The specimens were ground sequentially with SiC abrasive papers from 80# to 3000# grit, with a 90° rotation between each grit change to ensure uniform material removal until no visible scratches remained. Subsequently, the specimens were polished with SiO2 suspension on a metallographic polishing machine and then cleaned ultrasonically in anhydrous ethanol for 5 min.
Phase analysis was performed using a X-ray diffractometer (XRD) (Bruker D2 PHASER, Germany), with a scanning range of 30–80° and a scan rate of 5°/min. Microstructural characterization was carried out using a JSM-6700F field-emission scanning electron microscope, equipped with an electron backscatter diffraction (EBSD) system. Electrolytic polishing was conducted in an electrolyte solution containing 5% perchloric acid, 60% methanol, and 35% n-butanol (by volume), with an aluminum sheet as the cathode and the specimen as the anode, under a constant voltage of 38 V and a current of 300 mA for 90 s, followed by immediate cleaning in anhydrous ethanol. Grain size, texture, dislocation density, and the proportion of low-angle to high-angle grain boundaries were analyzed using Aztec Crystal2.1.2 software.
Microhardness was measured using the HV-1000 instrument (Suzhou MEGA Test Equipment Co., Ltd, Suzhou, China) via a point selection method. Metallographic specimens were prepared from the cross-section of the sample, with measurements taken at intervals of 1 mm along the radial direction toward the center. At least 50 points were tested and averaged to ensure the accuracy of the results, under an applied load of 200 gf and a dwell time of 15 s. Room-temperature tensile tests were conducted on an Instron 4507 universal testing machine with a contact extensometer (Model: MTS 634.31F-24, Shanghai, China, Gauge Length: 3 mm) at a strain rate of 1 × 10−3 s−1. Three parallel samples were tested for each ECAP pass condition, and the average values were reported as the experimental results.
Electrochemical tests were performed in a 3.5 wt.% NaCl solution (simulating seawater) using a CHI660E electrochemical workstation with a three-electrode system: the working electrode was the test specimen, a platinum sheet served as the counter electrode, and a saturated Ag/AgCl electrode was used as the reference electrode. Before testing, the specimens were sequentially ground with SiC abrasive papers up to 3000 grit, polished to a mirror finish using SiO2 suspension, ultrasonically cleaned in anhydrous ethanol, and dried. A nickel wire was spot-welded to the side of each specimen, and non-testing surfaces were insulated with epoxy resin. In the experiments, a 3.5 wt.% NaCl solution was employed as the electrolyte to simulate a seawater environment. This solution was prepared using 99.9% analytical-grade NaCl reagent (Sinopharm Chemical Reagent Co., Ltd.) and deionized water. All electrochemical tests were performed at a controlled room temperature of 25 °C, maintained by a constant-temperature water bath. The initial pH of the electrolyte was determined using a precision pH meter (SAN XIN PHS-3C, Shanghai, China), and the pH was consistently maintained at 7.8 ± 0.3 throughout the testing process.
The electrochemical testing protocol was carried out sequentially as follows: (1) monitoring the open circuit potential (OCP) until it stabilized (approximately 30 min); (2) performing electrochemical impedance spectroscopy (EIS) measurements with a frequency range from 0.01 Hz to 100 kHz and an AC perturbation amplitude of 10 mV; and (3) conducting potentiodynamic polarization scans from −800 mV to +1500 mV versus OCP at a scan rate of 2 mV/s. Notably, EIS measurements were prioritized over polarization tests to prevent surface changes caused by polarization that could compromise the accuracy of impedance measurements.

3. Results

3.1. Microstructure

Figure 2 shows the XRD patterns of the specimens after different ECAP passes. All specimens were predominantly composed of the α-Ti phase, while the diffraction peaks of the β-Ti phase show relatively low intensity. Comparison with the standard α-Ti diffraction card revealed the systematic shift of all diffraction peaks towards higher angles. This phenomenon was primarily attributed to the smaller atomic radii of the alloying elements (Al: 0.143 nm, V: 0.132 nm, Fe: 0.126 nm) compared to Ti (0.146 nm), which resulted in lattice contraction and reduced interplanar spacing in the Ti matrix. According to Bragg’s law (2dsinθ = nλ), the reduction in interplanar spacing (d) directly leads to an increase in the diffraction angle (2θ), consistent with the observed peak shift towards higher angles. Furthermore, the diffraction peaks exhibited slight broadening with increasing ECAP passes, which was attributed to grain refinement and increased microstrain.
Figure 3 shows electron backscatter diffraction (EBSD) maps of the TC4-0.55Fe alloy microstructure after different numbers of ECAP passes. Significant differences in the microstructure can be observed with increasing ECAP deformation. As evidenced by the inverse pole figure (IPF) maps (Figure 3a–d) and the grain size distribution maps (Figure 3e–h), the as-received ECAP 0P sample (Figure 3a) showed a characteristic bimodal microstructure, with maximum grain sizes reaching several tens of micrometers and minimum grain sizes on the nanometer scale, resulting in an average grain size of approximately 3.8 μm. After one ECAP pass (Figure 3b), significant microstructural homogenization and remarkable grain refinement occurred, reducing the average grain size to 2.5 μm. Numerous fine dynamically recrystallized (DRX) grains appeared along the boundaries of larger grains, indicating the occurrence of discontinuous dynamic recrystallization (DDRX) during processing [25]. After two ECAP passes (Figure 3c), the population of DRX grains increased further, and the average grain size decreased to 2.2 μm. After four ECAP passes (Figure 3d), the microstructure evolved into uniformly distributed fine equiaxed grains with an average size of 1.8 μm, demonstrating preferred orientations closely correlated with the shear strain distribution during ECAP [26]. This observation is consistent with the findings reported by Radnia [20] and Partha Sarathi et al. [27] regarding the grain refinement of TC4 alloys processed by ECAP. Notably, the rate of grain refinement gradually decreased with increasing ECAP passes, suggesting a progressive balance between work hardening and recrystallization softening.
Quantitative analysis (Table 1) of grain type was conducted based on the grain orientation spread (GOS) values obtained from EBSD measurements (Table 1). In the GOS map, grains with a GOS value below 2° are classified as recrystallized grains and colored blue; those with GOS values between 2° and 7° are defined as substructured grains and marked yellow; and grains exhibiting GOS values exceeding 7° are identified as deformed grains and colored red [28]. In the as-received state (Figure 3i), substructure grains dominate, accounting for 66.3%, while deformed grains and recrystallized grains account for 37.2% and 3.5%, respectively. After one pass of ECAP (Figure 3j), the fraction of deformed grains increased significantly to 53.3%, with substructure grains decreasing to 41.0% and recrystallized grains increased to 5.7%.
As the number of ECAP passes increased to four, the fraction of deformed grains continued to rise to a peak of 57.9%, while the substructure grain fraction decreased monotonically, reaching 28.1%, and the recrystallized grain fraction exhibited a consistent upward trend, ultimately reaching 14.0%. Quantitative analysis demonstrated that, during ECAP processing, (1) the deformed grain fraction exhibited a continuous increase, (2) the substructure grain fraction decreased correspondingly, and (3) the recrystallized grain fraction showed a steady increase, reflecting the strain-accumulation-dominated microstructural evolution under severe plastic deformation. This microstructural evolution can be attributed to the intense shear deformation and accumulation of high-density dislocations under the present ECAP parameters (700 °C), which provided the driving force for dynamic recovery (DRV) and dynamic recrystallization (DRX). As revealed by the EBSD GOS analysis in Figure 3, the microstructure is typically characterized by the coexistence of deformed grains, substructures, and newly formed fine recrystallized grains. These recrystallized grains are not uniformly distributed but preferentially nucleate and grow at original grain boundaries. Given that this process occurs synchronously with high-temperature deformation, static recrystallization (SRX) can be ruled out. The grain boundary nucleation feature, exhibiting a chain-like distribution, indicates that discontinuous dynamic recrystallization (DDRX) is an important recrystallization mechanism for this material under the present processing conditions. Furthermore, the β-phase content in the TC4-0.55Fe alloy increased progressively during ECAP processing, rising from 0.4% in the as-received state to 0.8% after one pass, reaching 2.0% after two passes, and further increasing to 2.2% after four passes.

3.2. Mechanical Properties

Following ECAP, the mechanical properties of the TC4 alloy typically demonstrate enhanced strength characteristics. Radnia [20] et al. observed that, after two ECAP passes at 520 °C, the grain size of TC4 was refined from 970 nm to 400 nm, resulting in a 30% increase in yield strength to 1140 MPa and a 6.9% increase in tensile strength to 1768 MPa. Similarly, Arabi [29] et al. reported that, after eight ECAP passes at 650 °C, the size of the α phase decreased from 4 μm to 3.1 μm after two passes but increased to 3.6 μm after eight passes; meanwhile, the yield strength increased by 11% to 980 MPa and the tensile strength increased by 12.1% to 1039 MPa. In another investigation, Partha Sarathi [27] et al. conducted one ECAP pass on TC4 sheet at 650 °C, refining the grain size from 906 nm to 359 nm, and observed a 6.9% increase in yield strength to 960 MPa and a 14.9% increase in tensile strength to 1103 MPa.
However, these specific values can vary depending on the initial material condition and processing parameters. The variations in mechanical properties are primarily influenced by the number of passes, deformation temperature, and initial microstructure. These factors reflect the combined effects of grain refinement, dislocation strengthening, and phase transformation behavior during ECAP under different thermomechanical conditions on the macroscopic mechanical properties.
Figure 4 displays the room-temperature tensile stress–strain curves and the variations in yield strength and elongation at fracture of the TC4-0.55Fe alloy processed with different numbers of ECAP passes. It can be observed that the stress–strain curves exhibit similar elastoplastic response characteristics. A noticeable difference existed between the yield strength and ultimate tensile strength of the alloy, indicating significant strain hardening capability during plastic deformation.
As clearly evidenced by the data in Figure 4 and Table 2, the strength of the alloy increased after ECAP processing, while the elongation to fracture decreased. The initial ECAP 0P sample exhibited the highest elongation at fracture of 13.5%, with yield strength, ultimate tensile strength, and microhardness values of 906 MPa, 939 MPa, and 325 HV, respectively. After one pass of ECAP processing, the yield strength, ultimate tensile strength, and hardness increased to 919 MPa, 988 MPa, and 326 HV, respectively, while the elongation at fracture decreased to 13.3%. After two passes of processing, the TC4-0.55Fe alloy exhibited a yield strength of 934 MPa, a maximum ultimate tensile strength of 1024 MPa, and a fracture elongation at fracture of 12.6%. Following further processing of up to four passes, the yield strength increased to a maximum value of 995 MPa, while the ultimate tensile strength slightly decreased to 1022 MPa, with the fracture elongation at fracture decreasing to 12.0%. Concurrently, the hardness reached 330 HV. These results demonstrated that ECAP processing enhanced the strength of the TC4-0.55Fe alloy with a slight reduction in elongation at fracture. The aforementioned changes can be attributed to the interplay between dynamic recrystallization and dislocation multiplication during plastic deformation.
Figure 5 presents the tensile fracture morphologies of the TC4-0.55Fe alloy in the as-received state and after different numbers of ECAP passes. In the as-received state (Figure 5a), the fracture surface exhibits typical characteristics of ductile fracture. A large number of uniformly distributed and relatively large and deep dimples are observed, indicating that the specimen underwent substantial plastic deformation before fracture, demonstrating good ductility. After one pass of ECAP (Figure 5b), the fracture surface is still predominantly characterized by dimples. However, the dimple size is slightly reduced compared to the as-received state, and their distribution becomes somewhat less uniform. This suggests that, with grain refinement, the strength of the alloy increases, while the ductility decreases slightly; nevertheless, the overall fracture mechanism remains dimple-dominated. After two passes of ECAP (Figure 5c), the fracture surface still retains obvious dimple morphology. However, the number of dimples decreases, and they become shallower. This characteristic indicates that the fracture process still primarily relies on dimple formation, but a slight decreasing trend in ductility may occur. When the number of passes increases to four (Figure 5d), the number of dimples on the fracture surface further decreases, and the dimples become finer and shallower. This indicates that the strength of the material is further enhanced after multi-pass ECAP, while the ductility decreases, albeit not significantly, and the fracture mechanism remains primarily ductile. In summary, with increasing ECAP passes, the fracture morphology of the TC4-0.55Fe alloy evolves from large and deep dimples towards finer and shallower dimples. This evolution corresponds to the changes in mechanical properties: the strength of the alloy increases with the number of passes, while the ductility slightly decreases, and yet, overall, remains at an acceptable level.

3.3. Corrosion Properties

Figure 6 displays the potentiodynamic polarization curves of specimens with different ECAP passes in a 3.5 wt.% NaCl solution, illustrating the variation in corrosion current density with increasing electrode potential along the vertical axis. In the Tafel cathodic region of the polarization curves, the predominant reaction is oxygen reduction, O2 + 4H+ + 4e− → 2H2O, while, in the Tafel anodic region, the primary reaction is titanium matrix oxidation: Ti − 4e− → Ti4+. Linear fitting was performed on both the cathodic and anodic Tafel regions of the polarization curves by drawing tangents to these segments. The intersection of these tangents gave the corrosion current density (icorr) and corrosion potential (Ecorr), with the horizontal and vertical coordinates corresponding to these parameters, respectively. For a given material, the corrosion rate is proportional to the corrosion current density and can be calculated using Equation (1):
R   =   3.27   ×   10 − 3   ( A   ×   i corr ) / n ρ
where R represents the corrosion rate (mm/a), A is the relative atomic mass, icorr denotes the corrosion current density (μA/cm2), n stands for the valence state of the metal after corrosion, and ρ is the density (g/cm3). A higher R value indicates the poorer corrosion resistance of the specimen.
The electrochemical corrosion performance of the TC4-0.55Fe alloy processed with different ECAP passes is summarized in Table 3. The electrochemical parameters in Table 3 show that Ecorr values reflected thermodynamic differences in corrosion tendency, i.e., the difficulty of corrosion initiation [30]. The ECAP 2P specimen exhibited the highest Ecorr value (−0.236 V), suggesting a higher activation energy barrier for corrosion reactions and lower corrosion susceptibility, while ECAP 0P and ECAP 1P specimens showed the lowest Ecorr values (−0.279 V), indicating a significantly higher thermodynamic corrosion tendency and greater susceptibility. However, Ecorr alone cannot fully characterize corrosion kinetics, and, thus, a comprehensive evaluation that includes corrosion current density (icorr) and corrosion rate (R) is necessary [31]. A detailed analysis demonstrated that ECAP processing significantly improved the corrosion resistance of the TC4-0.55Fe alloy. The ECAP 2P specimen exhibited the best performance (icorr = 0.0961 μA·cm−2, R = 0.832 × 10−3 mm·a−1), with ECAP 0P showing 9.3 times the corrosion rate of ECAP 2P. Notably, corrosion resistance did not increase monotonically with the number of ECAP passes. Although the ECAP 4P specimen had the smallest grain size (1.8 μm), its corrosion resistance was slightly inferior to that of ECAP 2P (grain size 2.2 μm). When titanium alloys enter the passive region, the current density remains constant with increasing applied potential due to the protection provided by passive films, with a lower passive current density (ipass) indicating a slower dissolution rate of the titanium matrix. The ECAP 4P specimen demonstrated the lowest passive current density (0.402 μA·cm−2), while ECAP 0P showed the highest (1.23 μA·cm−2), confirming that ECAP processing reduces matrix dissolution. Polarization curve analysis revealed that all ECAP-processed specimens exhibited distinct passive behavior, with significant differences in passive region width. ECAP 0P had a passive region of 0.2007 V (−0.279 V to −0.0783 V), whereas ECAP 4P showed an expanded passive region of 0.471 V, along with a marked improvement in pitting potential (Epit). These results confirmed that ECAP processing not only enhanced the overall corrosion resistance of the TC4-0.55Fe alloy, but also significantly improved its pitting resistance and passive film stability.
Figure 7 presents the Nyquist plots of specimens with different ECAP passes in 3.5 wt.% NaCl solution. The data points represent experimental measurements, and the solid lines correspond to fitting curves based on the equivalent circuit shown in the upper-right inset. All Nyquist plots exhibited characteristic semicircular features, with the arc radius positively correlated with corrosion resistance. Larger radii indicate stronger impedance to electron transfer [32]. From the Bode plot (Figure 7b), it could also be observed that the slope of the relationship between the impedance modulus |Z| and frequency approached zero in the high-frequency region, while, in the low-frequency region (frequency < 103 Hz), the slope tended toward −1, indicating favorable capacitive characteristics of the material. Furthermore, from the relationship between the negative phase angle (−θ) and frequency, it was noted that, in the low-frequency range (frequency < 10−1 Hz), the ECAP 2P specimen exhibited the highest phase angle value and impedance modulus |Z|. At this stage, the electrolyte encounters greater difficulty in permeating the material, suggesting superior corrosion resistance.
In the equivalent circuit, Rs represents the solution resistance (3.5 wt.% NaCl), CPE1 and R1 characterize the electric double layer behavior and charge transfer resistance at the electrode/solution interface, and CPE2 and R2 describe the dielectric response and film resistance at the passive film/solution interface [33]. As shown in Table 4, the Rs values of all specimens were similar (3.952–4.645 Ω·cm2), confirming stable testing conditions. Regarding electric double layer properties, when n1 approaches 1, CPE1 can be approximated as an ideal capacitor, indicating high interface homogeneity. The test results showed that all specimens had n1 values exceeding 0.85, suggesting nearly ideal capacitive behavior of the electric double layer with uniform interfaces and negligible defects, with ECAP 2P demonstrating the strongest charge storage capacity. For passive film characteristics, the n2 parameter of CPE2 showed that ECAP 4P (n2 = 0.998) possessed a dense and uniform passive film, while other specimens (n2 = 0.8–0.9) exhibited mild inhomogeneity, consistent with the previously mentioned pitting potential results. The polarization resistance Rp (R1 + R2), a critical parameter, directly reflects the material’s impedance to charge transfer. All ECAP-processed specimens show significantly higher Rp values than ECAP 0P, with ECAP 2P achieving the maximum Rp of 1.784 × 105 Ω·cm2. These results align perfectly with the potentiodynamic polarization tests, further confirming that ECAP processing significantly enhances the corrosion resistance of the TC4-0.55Fe alloy, with ECAP 2P exhibiting optimal performance.

4. Discussion

Based on the aforementioned experimental results, ECAP processing significantly enhanced both the mechanical properties and corrosion resistance of the TC4-0.55Fe alloy, with the specimen exhibiting the optimal overall performance after two passes. To gain deeper insights into the underlying mechanisms responsible for these improvements, the following section will systematically analyze the influence of grain refinement strengthening, texture evolution, and grain boundary structure on both mechanical properties and corrosion behavior in correlation with the microstructural evolution.
Compared with the existing TC4 [34,35,36] and other commercial titanium alloys [37,38], the TC4-0.55Fe alloy exhibited superior overall properties. In terms of mechanical performance, tensile strength increased with additional ECAP passes, reaching 1024 MPa after two passes, which was a 9% improvement over the initial value (939 MPa). The observed improvement was primarily ascribed to the grain refinement induced by the continuous shear strain introduced through the ECAP process. The synergistic effects of grain boundary strengthening, as described by the Hall–Petch relationship, due to grain refinement, and dislocation strengthening, resulting from an increased dislocation density, collectively contribute to the enhancement of material strength. This Hall–Petch-type relationship is clearly illustrated in Figure 8, which shows the simultaneous decrease in grain size and increase in yield strength with increasing ECAP passes. Nevertheless, this increase in strength was generally accompanied by a reduction in elongation at fracture. The initial elongation at fracture of the material was 13.5%, which slightly decreased to 12.0% after four passes. This phenomenon could be attributed to the impediment of dislocation motion and the increased difficulty in activating slip systems, both consequences of grain refinement and elevated dislocation density, ultimately leading to a reduction in plasticity.
However, after four passes of processing, the grain size was further refined to 1.8 μm, and the yield strength significantly increased from 933 MPa to 995 MPa. In contrast, the ultimate tensile strength slightly decreased to 1022 MPa. To gain deeper insight into the microstructural mechanisms underlying the changes in mechanical properties after four passes, kernel average misorientation (KAM) and grain boundary distribution analyses were further employed. Figure 9 presents the kernel average misorientation (KAM) maps of TC4-0.55Fe, showing the evolution of average KAM values with successive ECAP passes: 0.36° → 1.16° → 1.10° → 1.08°. This evolution reflects initial shear strain, which induces dislocation multiplication, while subsequent dynamic recrystallization and recovery partially absorb dislocations. Figure 10 shows the grain boundary distribution maps and misorientation angle distribution of TC4-0.55Fe, where black lines represent high-angle grain boundaries (HAGBs) and red lines denote low-angle grain boundaries (LAGBs). ECAP processing significantly altered the grain boundary structure: the proportion of LAGBs in ECAP 2P increased from 54.5% to 69.6%, then slightly decreased to 62.0% after four ECAP passes. This evolution reflected the kinetic characteristics of dislocation rearrangement and subgrain boundary transformation. After four ECAP passes, the decreased average KAM value indicates a reduction in dislocation density, accompanied by a decline in the proportion of low-angle grain boundaries (LAGBs), which is closely associated with the mechanisms of dislocation rearrangement and annihilation during recovery. The continued notable increase in yield strength is primarily attributed to Hall–Petch strengthening resulting from the increased proportion of high-angle grain boundaries, whose beneficial effect outweighs the strength loss caused by reduced dislocation density. Conversely, the decrease in tensile strength is ascribed to the reduction in low-angle grain boundaries (dislocation structures), which significantly diminishes the strain hardening capacity, leading to rapid instability and necking shortly after yielding.
The dense TiO2 passive film formed on titanium and its alloys provides excellent corrosion resistance. However, in high-concentration Cl− environments, Cl− ions may penetrate grain boundary defects in the passive film to reach the film/substrate interface. This leads to the formation of a galvanic couple between the substrate and the passive layer, which accelerates local electrochemical dissolution and initiates pitting corrosion [39]. This corrosion process involves three critical stages: (1) Cl− adsorption and passive film activation: Cl− preferentially adsorbs at oxygen vacancies or grain boundaries on the TiO2 film surface, replacing O2− in the passive film and forming soluble chlorides (e.g., TiCl4), which locally disrupt film continuity; (2) galvanic corrosion formation: the exposed substrate and surrounding passive film regions form micro-galvanic couples, with the substrate acting as the anode to accelerate dissolution; and (3) autocatalytic acidification: Cl− accumulation within pits causes a significant pH reduction, further inhibiting repassivation [40]. The grain refinement induced by ECAP enhanced the stability of the passive film through microstructural regulation, effectively improving Cl− resistance via three primary mechanisms: Firstly, crystalline defects like dislocations provided highly active sites that promoted the formation of protective oxides, optimizing the passive film’s coverage and densification within grains. Secondly, the SPD modified the initial crystallographic texture, potentially promoting the formation of thermodynamically more stable oxide film orientations on the surface. This modification intrinsically enhanced the resistance of the passive film to Cl− attack. Lastly, the high density of grain boundaries provided numerous homogeneous nucleation sites for the passive film, promoting the rapid and continuous formation of a titanium dioxide (TiO2) layer, which effectively impeded the lateral penetration of Cl− ions.
The corrosion resistance improved significantly with increasing ECAP passes but not monotonically, showing an initial increase followed by a slight decrease after the optimum. This non-monotonic trend was closely related to the variation in dislocation density (Figure 9). These microstructural modifications produced dual effects: the fine grains generated through dynamic recrystallization reduced grain boundary discontinuity in passive films, optimizing film continuity, while high-density dislocation networks provided abundant active sites for passive film formation, promoting uniform growth both within grains and along grain boundaries, thereby enhancing passive film densification. This combined effect substantially impeded Cl− penetration through surface passive films. Sotniczuk et al. [41] also confirmed that moderate increases in the surface roughness of Ti improve passive film stability, while increased microstructural defects (e.g., dislocations, subgrain boundaries) further optimize the structural density of passive films. Such synergistic effects effectively inhibited pit channel formation and significantly enhanced the material’s corrosion resistance.
Through IPF mapping, Figure 11 illustrates the crystallographic texture evolution of specimens processed with different ECAP passes along the direction perpendicular to extrusion. Figure 11a shows a random orientation distribution in ECAP 0P, with primary crystallographic orientations scattered between (0 1 ¯ 10) and ( 1 ¯ 2 1 ¯ 0). Figure 11b shows that the texture of ECAP 1P begins to concentrate around the (01 1 ¯ 0) orientation. Figure 11c shows that ECAP 2P developed the strongest basal texture, in contrast to the weak textures of 0P and 1P, with the highest proportion of grains having (0001) planes parallel to the surface. This strong basal texture directly accounted for the superior corrosion resistance [42,43], as these densely packed planes have the highest atomic density, lowest surface energy, and hydrophobic properties that effectively inhibit the adhesion of corrosive media and products.
Previous studies highlight the beneficial role of LAGBs in enhancing corrosion resistance [44]. As shown in Figure 10, the proportion of high-angle grain boundaries (HAGBs) increased with successive ECAP passes but decreased after four passes, while the variation trend of low-angle grain boundaries showed an opposite pattern. Corrosion tends to initiate at high-angle grain boundaries (HAGBs) due to their high interfacial energy and structural disorder, which provide fast diffusion paths for corrosive media [45]. In contrast, low-angle grain boundaries (LAGBs) inhibit corrosion through two mechanisms: on the one hand, their ordered dislocation arrays with dense structure and low interfacial energy significantly suppress Cl− penetration; on the other hand, as preferential nucleation sites, the strain fields of dislocations in LAGBs promote oxygen diffusion, accelerating the formation of denser and more uniform TiO2 passive films. In summary, ECAP concurrently refined the grain structure and altered three key microstructural factors: dislocation density, crystallographic texture, and grain boundary character. The corrosion resistance was synergistically enhanced by these factors, with the ECAP 2P specimen achieving an optimal balance and, thus, the best performance.
In terms of corrosion resistance, the TC4-0.55Fe alloy exhibited excellent performance, with the ECAP 0P specimen showing an exceptionally low icorr of only 0.890 µA·cm−2, significantly superior to many conventional titanium alloys and commercial pure titanium. For instance, TC4 typically demonstrates icorr values of around 3.850 µA·cm−2 and 3.008 µA·cm−2 [36], while industrial pure titanium grades TA1 [37] and TA2 [35] exhibit icorr values of 1.240 µA·cm−2 and 1.565 µA·cm−2, respectively. After ECAP processing, particularly after two passes, icorr dramatically decreased to 0.096 µA·cm−2, representing nearly an order-of-magnitude reduction compared to the initial value, far surpassing other TC4 and commercial titanium alloys. The enhancement in corrosion resistance shared common microstructural origins with the optimization of mechanical properties: grain refinement increases grain boundary density, which both impedes dislocation motion (enhancing mechanical properties) and blocks corrosion propagation paths (improving corrosion resistance); The dislocation density undergoes dynamic evolution, where the initial high dislocation density promotes passive film nucleation, while subsequent dynamic recrystallization reduces stress concentration; intensified (0001) basal texture increases surface atomic density and decreases slip system activation difficulty; and an increased fraction of low-angle grain boundaries (up to 69.6%) coordinates plastic deformation while suppressing intergranular corrosion.
In summary, the ECAP processing successfully produced TC4-0.55Fe alloy with enhanced strength, toughness, and corrosion resistance. The results showed that, after two-pass ECAP treatment, the alloy achieved a tensile strength exceeding 1000 MPa, along with exceptional corrosion resistance (icorr < 0.1 µA·cm−2). This makes it suitable for aerospace and biomedical applications. In aerospace, its strength and corrosion resistance suit engine components and structural parts, balancing weight and durability. In biomedical uses, its low corrosion rate ensures reduced metal ion release, enhancing biocompatibility and stability. Based on these advantages, our research team will subsequently focus on performance evaluation and component validation studies of this material in specific application scenarios to facilitate its engineering application.

5. Conclusions

This study systematically investigates the effects of ECAP on the mechanical properties and corrosion resistance of TC4-0.55Fe alloy, elucidating the intrinsic relationships between microstructural evolution and the alloy’s strength–toughness–corrosion performance. The main conclusions are as follows:
  • The TC4-0.55Fe alloy was processed by ECAP for 0, 1, 2, and 4 passes, resulting in grain sizes of 3.8 μm, 2.5 μm, 2.2 μm, and 1.8 μm, respectively. During ECAP, the fraction of dynamic recrystallization increased from 3.5% to 14.0%, while the dislocation density exhibited an overall increasing trend. Concurrently, the proportion of LAGBs increased from 54.5% to 69.6%, followed by a decrease to 62.0%.
  • Room-temperature tensile tests were conducted on TC4-0.55Fe titanium alloys with different grain sizes. With an increasing number of ECAP passes, both the yield strength and ultimate tensile strength of TC4-0.55Fe improved, rising from 906 MPa and 939 MPa (ECAP 0P) to 995 MPa and 1022 MPa (ECAP 4P), respectively. In contrast, the ductility exhibited an opposite trend, decreasing from 13.5% (ECAP 0P) to 12.0% (ECAP 4P).
  • ECAP treatment significantly improved the alloy’s corrosion resistance in a simulated seawater environment, with two-pass-processed specimens exhibiting optimal performance. The corrosion current density (icorr) decreases to 0.0961 μA·cm−2, representing a 9.3-fold reduction in corrosion rate compared to untreated samples, along with significantly enhanced passive film stability and pitting resistance.
  • The enhanced corrosion resistance was attributed to microstructural changes from grain refinement, including increased dislocation density, stronger basal texture, and improved grain boundary characteristics. The ECAP 2P specimen had the best corrosion resistance because it has a relatively high dislocation density, the strongest basal texture, and the most low-angle grain boundaries (69.6%). Conversely, the corrosion resistance of the ECAP 4P specimen was slightly inferior to that of the 2P specimen due to its somewhat weaker texture and a lower proportion of low-angle grain boundaries.

Author Contributions

Conceptualization, Y.G. and Y.D.; methodology, Y.D. and Y.G.; validation, Y.D. and I.V.A.; formal analysis, Y.G.; investigation, Y.G. and Y.D.; data curation, Y.G.; writing—original draft preparation, Y.G.; writing—review and editing, Y.G. and Y.D.; supervision, Y.L., M.H., Y.W., Y.D. and I.V.A.; project administration, Y.D. and I.V.A.; funding acquisition, Y.D. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Key Research and Development Program of China (No. 2021YFB3700802), the Russian Science Foundation (Project 23-43-00041), and the A Project, funded by the Priority Academic Program Development (PAPD) of Jiangsu Higher Education Institutions.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Crystals 15 00795 g001
Figure 1. (a) Machining flow chart and (b) sampling diagram.
Figure 1. (a) Machining flow chart and (b) sampling diagram.
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Figure 2. XRD patterns of TC4-0.55Fe alloy with different ECAP passes. The reference diffraction peaks for α-Ti (hexagonal, hcp) and β-Ti (cubic, bcc) are marked with red and green vertical lines, respectively. The α-phase peaks are indexed based on PDF89-5009, while the β-phase peaks correspond to PDF89-4913.
Figure 2. XRD patterns of TC4-0.55Fe alloy with different ECAP passes. The reference diffraction peaks for α-Ti (hexagonal, hcp) and β-Ti (cubic, bcc) are marked with red and green vertical lines, respectively. The α-phase peaks are indexed based on PDF89-5009, while the β-phase peaks correspond to PDF89-4913.
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Figure 3. IPF diagrams (a–d), grain size distribution maps (e–h), and grain orientation spread maps (i–l) of TC4-0.55Fe alloy under different ECAP passes: (a,e,i) ECAP 0P; (b,f,j) ECAP 1P; (c,g,k) ECAP 2P; (d,h,l) ECAP 4P.
Figure 3. IPF diagrams (a–d), grain size distribution maps (e–h), and grain orientation spread maps (i–l) of TC4-0.55Fe alloy under different ECAP passes: (a,e,i) ECAP 0P; (b,f,j) ECAP 1P; (c,g,k) ECAP 2P; (d,h,l) ECAP 4P.
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Figure 4. Room-temperature tensile test of TC4-0.55Fe alloy processed by ECAP with different passes. (a) Strain–stress curves. (b) Histogram of yield strength and elongation at fracture variations.
Figure 4. Room-temperature tensile test of TC4-0.55Fe alloy processed by ECAP with different passes. (a) Strain–stress curves. (b) Histogram of yield strength and elongation at fracture variations.
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Figure 5. Scanning electron microscope (SEM) showing the fracture surfaces of TC4-0.55Fe alloy with different ECAP passes: (a) ECAP 0P; (b) ECAP 1P; (c) ECAP 2P; (d) ECAP 4P.
Figure 5. Scanning electron microscope (SEM) showing the fracture surfaces of TC4-0.55Fe alloy with different ECAP passes: (a) ECAP 0P; (b) ECAP 1P; (c) ECAP 2P; (d) ECAP 4P.
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Figure 6. Potentiodynamic polarization curves of TC4-0.55Fe alloy processed by ECAP with different passes.
Figure 6. Potentiodynamic polarization curves of TC4-0.55Fe alloy processed by ECAP with different passes.
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Figure 7. Electrochemical impedance spectra of TC4-0.55Fe alloy processed by ECAP with different passes in 3.5% NaCl solution. (a) Nyquist plots with an equivalent circuit diagram shown in the upper right corner; (b) Bode plots.
Figure 7. Electrochemical impedance spectra of TC4-0.55Fe alloy processed by ECAP with different passes in 3.5% NaCl solution. (a) Nyquist plots with an equivalent circuit diagram shown in the upper right corner; (b) Bode plots.
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Figure 8. The variations in grain size and yield strength of the TC4-0.55Fe alloy processed with different numbers of ECAP passes.
Figure 8. The variations in grain size and yield strength of the TC4-0.55Fe alloy processed with different numbers of ECAP passes.
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Figure 9. KAM maps of TC4-0.55Fe alloy under different ECAP passes: (a) ECAP 0P; (b) ECAP 1P; (c) ECAP 2P; (d) ECAP 4P.
Figure 9. KAM maps of TC4-0.55Fe alloy under different ECAP passes: (a) ECAP 0P; (b) ECAP 1P; (c) ECAP 2P; (d) ECAP 4P.
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Figure 10. Grain boundary distribution and misorientation distribution of TC4-0.55Fe alloy under different ECAP passes. High-angle grain boundaries (HAGBs, θ ≥ 15°) are marked by black lines, and low-angle grain boundaries (LAGBs, 2° ≤ θ < 15°) are marked by red lines. (a,b) ECAP 0P; (c,d) ECAP 1P; (e,f) ECAP 2P; (g,h) ECAP 4P.
Figure 10. Grain boundary distribution and misorientation distribution of TC4-0.55Fe alloy under different ECAP passes. High-angle grain boundaries (HAGBs, θ ≥ 15°) are marked by black lines, and low-angle grain boundaries (LAGBs, 2° ≤ θ < 15°) are marked by red lines. (a,b) ECAP 0P; (c,d) ECAP 1P; (e,f) ECAP 2P; (g,h) ECAP 4P.
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Figure 11. Inverse pole of TC4-0.55Fe alloy with different ECAP passes: (a) ECAP 0P; (b) ECAP 1P; (c) ECAP 2P; (d) ECAP 4P.
Figure 11. Inverse pole of TC4-0.55Fe alloy with different ECAP passes: (a) ECAP 0P; (b) ECAP 1P; (c) ECAP 2P; (d) ECAP 4P.
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Table 1. Average grain size, grain type (recrystallized grain, substructure grain, deformed grain), and β phase content of TC4-0.55Fe alloy with different ECAP passes.
Table 1. Average grain size, grain type (recrystallized grain, substructure grain, deformed grain), and β phase content of TC4-0.55Fe alloy with different ECAP passes.
SamplesAverage Grain Size (μm)Recrystallized Grains (%)Substructure Grains (%)Deformed Grains (%)β-Phase (%)
ECAP 0P3.83.566.337.20.4
ECAP 1P2.55.741.053.30.8
ECAP 2P2.210.135.154.82.0
ECAP 4P1.814.028.157.92.2
Table 2. Room-temperature tensile data and hardness test data of TC4-0.55Fe alloy under different ECAP passes.
Table 2. Room-temperature tensile data and hardness test data of TC4-0.55Fe alloy under different ECAP passes.
SampleYield Strength (MPa)Ultimate Tensile Strength (MPa)Elongation at Fracture (%)Hardness (HV)
ECAP 0P906 ± 15939 ± 913.5 ± 0.6325 ± 6.1
ECAP 1P919 ± 13988 ± 1013.3 ± 0.6326 ± 8.6
ECAP 2P934 ± 201024 ± 912.6 ± 0.4329 ± 5.9
ECAP 4P995 ± 181022 ± 712.0 ± 0.5330 ± 7.2
Table 3. Electrochemical parameters corresponding to polarization curves of TC4-0.55Fe alloy processed by ECAP with different passes.
Table 3. Electrochemical parameters corresponding to polarization curves of TC4-0.55Fe alloy processed by ECAP with different passes.
SampleEcorr
(V)		Epit
(V)		icorr
(µA·cm−2)		ipass
(µA·cm−2)		R
(10−3 mm·a−1)
ECAP 0P−0.279−0.07830.8901.237.73
ECAP 1P−0.2790.08120.5121.674.45
ECAP 2P−0.2360.1140.09610.4680.832
ECAP 4P−0.2560.2150.1080.4020.946
Table 4. Equivalent circuit fitting parameters for TC4-0.55Fe alloy processed by ECAP with different passes.
Table 4. Equivalent circuit fitting parameters for TC4-0.55Fe alloy processed by ECAP with different passes.
SampleRS
(Ω·cm2)		CPE1
(10−5 S·sn·cm2)		n1R1
(105 Ω·cm2)		CPE2
(10−5 S·sn·cm2)		n2R2
(105 Ω·cm2)		Rp
(105 Ω·cm2)
ECAP 0P4.3254.7050.8960.018110.750.8040.3750.3931
ECAP 1P4.6452.8570.9130.2223.8150.8100.7961.018
ECAP 2P3.9527.6520.8560.07838.4980.8591.711.784
ECAP 4P4.6425.1150.8580.24410.310.9981.151.396
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Guo, Y.; Lu, Y.; He, M.; Wang, Y.; Dong, Y.; Alexandrov, I.V. Mechanical and Corrosion Properties of Ultrafine-Grained TC4-0.55Fe Alloy Processed by Equal-Channel Angular Pressing. Crystals 2025, 15, 795. https://doi.org/10.3390/cryst15090795

AMA Style

Guo Y, Lu Y, He M, Wang Y, Dong Y, Alexandrov IV. Mechanical and Corrosion Properties of Ultrafine-Grained TC4-0.55Fe Alloy Processed by Equal-Channel Angular Pressing. Crystals. 2025; 15(9):795. https://doi.org/10.3390/cryst15090795

Chicago/Turabian Style

Guo, Yumeng, Yu Lu, Miaoxia He, Yu Wang, Yuecheng Dong, and Igor V. Alexandrov. 2025. "Mechanical and Corrosion Properties of Ultrafine-Grained TC4-0.55Fe Alloy Processed by Equal-Channel Angular Pressing" Crystals 15, no. 9: 795. https://doi.org/10.3390/cryst15090795

APA Style

Guo, Y., Lu, Y., He, M., Wang, Y., Dong, Y., & Alexandrov, I. V. (2025). Mechanical and Corrosion Properties of Ultrafine-Grained TC4-0.55Fe Alloy Processed by Equal-Channel Angular Pressing. Crystals, 15(9), 795. https://doi.org/10.3390/cryst15090795

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