Β-type Ti Research Articles

Design of new β-type Ti alloys with outstanding corrosion and wear resistance for dental application

β-type Ti alloys have drawn extensive attention as potential metallic implants. In this work, the microstructure, corrosion behavior, and wear performance of Ti−17.3Nb−1.8Fe (TNF) and Ti−22Nb−30Zr−4Cr (TNZC) biomedical alloys were experimentally investigated, and CP−Ti and Ti6Al4V alloy were also used for comparisons. TNF and TNZC alloys after solution treatment at 1273K showed a single β phase. It was found that TNF and TNZC alloys revealed better corrosion resistance than CP−Ti and Ti6Al4V alloy in different artificial saliva containing 0.15wt.% NaF with the pH values of 3.9 and 5.7, which could be ascribed to the formation of a double-layer passive film composed of multiple species oxides. Moreover, TNF and TNZC alloys exhibited higher wear resistance compared with CP−Ti and Ti6Al4V alloy, whose wear mechanism was adhesive wear and abrasive wear. The present results shows that TNF and TNZC alloys are nice candidates for dental application.

Superplastic behavior of a metastable β-type Ti alloy governed by grain size: Microstructure evolution and underlying deformation mechanism

The superplastic forming (SPF) technology has gained widespread adoption in the manufacturing of intricately shaped components made from Ti alloys with high dimensional accuracy due to its near net forming capabilities. It is crucial to systematically investigate the superplastic behavior in metastable β-type Ti alloys. However, the microstructure evolution and underlying superplastic deformation mechanism governed by grain size in these alloys remain unclear. In this study, three types of fine-grained (<10 μm) Ti-15V-3Cr-3Sn-3Al (Ti-15-3) alloys were obtained using friction stir processing (FSP), and tensile tests were conducted at temperature of 650 °C with strain rates ranging from 1 × 10−4 s−1 to 3 × 10−3 s−1. The deformation behavior of the coarse-grained (>3 μm) Ti-15-3 alloy can be described as quasi-superplastic rather than strictly superplastic. In this microstructure, plasticity is synergistically contributed by grain boundary sliding (GBS), continuous dynamic recrystallization (CDRX), and dynamic phase transformation (DPT). For the fine-grained (<3 μm) Ti-15-3 alloy, deformation enters into the superplastic region with GBS becoming the predominant deformation mechanism accompanied by DPT. Additionally, strain/stress promotes α phase precipitation from the β matrix, which inhibits grain growth and serves as a supplementary stress accommodation mechanism facilitating continuous operation of GBS and ultimately achieving exceptional superplasticity.

Effect of Mn content on the microstructure, mechanical properties and corrosion behavior of Ti–Nb–Zr–Mn alloys processed via spark plasma sintering

Herein, β-type Ti–Nb–Zr–Mn alloys were prepared by ball milling and spark plasma sintering. The effects of Mn addition and quenching treatment on the microstructure, mechanical properties, and corrosion behavior of the alloy were investigated. The Ti–Nb–Zr–Mn alloys are predominantly constituted of equiaxed β Ti grains with a small proportion of ω phase. An increase in Mn content inhibits the precipitation of the ω phase while promoting a more homogeneous solid solution. The tensile strength of the as-prepared alloys decreases and then increases, whereas the plasticity demonstrates the opposite trend. The Elastic modulus and hardness roughly show a trend of decreasing and then increasing. Additionally, the Ti–24Nb–4Zr–2Mn alloy, after undergoing a solid solution treatment at a temperature of 950 °C and subsequent water quenching (referred to as M2-950), demonstrates exceptional comprehensive performance, showing a yield strength of 956 ± 4 MPa, an ultimate strength of 995 ± 13 MPa, an elongation of 16.4 ± 1.6 %, a hardness of 312 ± 11 H V, and an Elastic modulus of 72.6 GPa. Moreover, the corrosion behavior in Ringer's solution was extensively investigated, and the M2-950 alloy has superior corrosion resistance.

Role of {332} twinning on adiabatic shear behavior during dynamic loading in β-type Ti–15Mo alloy

The microstructural evolution and formation mechanism of the adiabatic shear band (ASB) in {332}<113> twinning-induced plasticity (TWIP) Ti–15Mo alloy were investigated by applying stopper rings in a split Hopkinson pressure bar apparatus. Accompanied by activation of numerous {332}<113> twins, ASB consisting of elongated fine-grains initiated in the dynamically compressed sample with 0.28 true strain. Multiple twins adjacent to the ASB region were severely distorted and refined along the shear direction. Further deformation led to refinement of elongated twins into elongated subgrains, then some of them were subdivided into equiaxed fine-grains. The morphology of the fine-grains within the ASB underwent a complete transition from elongated to equiaxed as the strain increased to 0.33. The equiaxed fine-grains exhibited a random distribution of orientation, which was regarded as the result of dynamic recrystallization. The formation of equiaxed fine-grains was found to obey rotational dynamic recrystallization mechanism through the thermodynamic and kinetic calculations.

Tribocorrosion behaviour of additively manufactured β-type Ti–Nb alloy for implant applications

β-type Ti–Nb alloys are promising materials for load-bearing implant applications with improved mechanical biofunctionality and biocompatibility. In this work, the electrochemical and tribo-electrochemical behaviour of laser powder bed fusion (LPBF) produced β-type Ti–42Nb alloy processed via Gaussian and top hat laser was investigated and compared with commercial grade β-type Ti–45Nb and α+β-type Ti–6Al–4V ELI. Electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization experiments were performed in phosphate-buffered saline (PBS) for corrosion behaviour. Tribocorrosion behaviour was studied under open circuit potential (OCP) conditions in PBS by using a reciprocating pin-on-disk tribometer. The passivation nature of the LPBF alloys is more decisive than the microstructural particularities for electrochemical behaviour. The overall corrosion response is similar due to the protective nature of the passive films formed on Ti alloys. Ti–6Al–4V ELI exhibits the best corrosion performance among all tested alloys with lower corrosion and passivation current density values. However, LPBF-produced alloys exhibit less reactive surfaces with better passive film properties compared to Ti–45Nb. In addition, EIS results revealed that passive film resistance values are higher for LPBF-produced alloys than conventionally produced Ti–45Nb. LPBF-produced alloys exhibit better tribo-electrochemical behaviour compared to Ti–45Nb. The differences in volume loss are mainly attributed to the microhardness of the alloys and the volume loss is dominated by mechanical wear. The alloys produced with LPBF show promising corrosion and tribocorrosion performance to be a potential candidate for load-bearing implant applications.

Microstructure and mechanical behavior of rhombic dodecahedron-structured porous β-Ti composites fabricated via laser powder bed fusion

Porous β-Ti composites hold large promise for lightweight components due to their exceptional strength-ductility synergy. However, their microstructural behavior under both impact and compressive conditions have received limited attention. This study investigates the compressive and impact behavior of porous β-type Ti–25Nb (at.%) composites fabricated using laser powder bed fusion (LPBF). The composites feature a rhombic dodecahedron (RD) lattice structure and incorporate a central solid cylinder with varying diameters. Microstructural characterizations and finite element simulations were employed to analyze mechanical properties and stress distribution. Incorporation of 3 mm radius cylinders (TIII ∼45% porosity) significantly enhances compressive yield stress (88 ± 10 MPa), impact yield stress (153 ± 10 MPa), and impact energy absorption (41.2 J/cm³) while maintaining material plasticity. Compressive and impact loading initiate a martensitic transformation from β to α″ phase, strengthening the alloy through refined α″ formation with a finer grain size than β phase. Notably, a β→ α″ transformation and grain refinement occur in the node region that connects the solid cylinder and lattices, which undergoes significant deformation during impact. This observation suggests the impact resistance of porous β-Ti composites can be enhanced through tailored microstructures and strategic structural design.

Corrosion and wear performance of β-Ti based biomedical shape memory alloys

The corrosion performances and tribological behaviors of β-type Ti–V–Al based biomedical shape memory alloys were systematically investigated through electrochemical experiments and reciprocating friction and wear tests. The results revealed significant distinctions in corrosion performances between the Ti–V–Al–Zr and Ti–V–Al–Sn shape memory alloys, which can be attributed to the difference in the types of oxides formed during the corrosion process. In comparison, Ti–V–Al based shape memory alloys exhibited superior corrosion properties by adding an Sn alloying element. In addition, adding the moderate Sn alloying element could significantly improve the wear resistance compared with Ti–V–Al–Zr shape memory alloys. In summary, the present study provided a theoretical basis for the selection of Ti–V–Al based shape memory alloys applied for various biomedical fields.

Effect of CaB6 addition on the microstructure and mechanical properties of Ti–24Nb–4Zr-2.5Mn alloys fabricated by spark plasma sintering

Balancing the strength, ductility and Elastic modulus of β-Ti alloys is still a great challenge for medical implantation, extremely limiting its further applications. Herein, we prepared a fine-grain β-type Ti–24Nb–4Zr-2.5Mn alloy with high performance by adding CaB6 deoxidizer via mechanical alloying and spark plasma sintering. The effects of CaB6 addition on the microstructure and mechanical properties were systematically investigated, and the results show that adding a trace amount of CaB6 will form in-situ nano-sized CaTiO3 particles and micro-sized (Ti,Nb)B2 solid solution whiskers, which can significantly refine the grain size and inhibit the grain boundary movement and dislocation motion, thus improving the comprehensive mechanical properties of Ti–24Nb–4Zr-2.5Mn alloys. With a trace addition of 0.1 wt% CaB6, the Ti–24Nb–4Zr-2.5Mn alloy exhibits an excellent combination of high strength of ∼971 MPa, good ductility of ∼17.8% and low elastic modulus of ∼79.1 GPa. This work may provide a new methodology to prepare high-performance biomedical Ti alloys.

Promising composites for wear resistant load-bearing implant applications: Low elastic moduli of β Ti–Nb alloy reinforced with TiC particles and/or TiB whiskers

Despite β-type Ti alloys achieving a low elastic modulus, metallic implants generally exhibit poor wear resistance. To address this issue, in-situ Ti-based matrix composites (TMCs) are considered due to their strong bonding at the matrix/reinforcement interface. While TiC particles and TiB whiskers are common reinforcements for in-situ TMCs, limited research combines their effects with the low elastic modulus of Ti alloys. Here, we present a novel approach using arc-melting on Ti–40Nb alloy with 5 vol% additions of NbC, NbB2, and B4C. Respectively, three TMCs were obtained with the β-type Ti–Nb matrices reinforced individually with TiC particles, TiB whiskers, and hybridized with both. The produced TMCs exhibited the same passive behavior of the unreinforced Ti–40Nb alloy. However, tribocorrosion resistances were improved, as TMCs achieved nobler potentials during sliding than the unreinforced alloy (−0.5 and −0.9 V, respectively). Notably, the TiB-reinforced and the hybrid reinforced TMCs displayed minimal wear damage, while the TiC-reinforced TMC achieved a remarkable low elastic modulus of approximately 73 GPa. This study introduces promising alternatives for wear-resistant load-bearing implant applications.

A Review of Deformation Mechanisms, Compositional Design, and Development of Titanium Alloys with Transformation-Induced Plasticity and Twinning-Induced Plasticity Effects

Metastable β-type Ti alloys that undergo stress-induced martensitic transformation and/or deformation twinning mechanisms have the potential to simultaneously enhance strength and ductility through the transformation-induced plasticity effect (TRIP) and twinning-induced plasticity (TWIP) effect. These TRIP/TWIP Ti alloys represent a new generation of strain hardenable Ti alloys, holding great promise for structural applications. Nonetheless, the relatively low yield strength is the main factor limiting the practical applications of TRIP/TWIP Ti alloys. The intricate interplay among chemical compositions, deformation mechanisms, and mechanical properties in TRIP/TWIP Ti alloys poses a challenge for the development of new TRIP/TWIP Ti alloys. This review delves into the understanding of deformation mechanisms and strain hardening behavior of TRIP/TWIP Ti alloys and summarizes the role of β phase stability, α″ martensite, α′ martensite, and ω phase on the TRIP/TWIP effects. This is followed by the introduction of compositional design strategies that empower the precise design of new TRIP/TWIP Ti alloys through multi-element alloying. Then, the recent development of TRIP/TWIP Ti alloys and the strengthening strategies to enhance their yield strength while preserving high-strain hardening capability are summarized. Finally, future prospects and suggestions for the continued design and development of high-performance TRIP/TWIP Ti alloys are highlighted.

Finite element analysis of the mechanical performance of self-expanding endovascular stents made with new nickel-free superelastic β-titanium alloys

New Ni-free superelastic β-titanium alloys from the Ti–Zr–Nb–Sn system have been designed in this study to replace the NiTi alloy currently used for self-expanding endovascular stents. The simulation results, carried out by finite element analysis (FEA) on two β-type Ti–Zr–Nb–Sn alloys using a commonly used superelastic constitutive model, were in good agreement with the experimental uniaxial tension data. An ad-hoc self-expanding coronary stent was specifically designed for the present study. To assess the mechanical performance of the endovascular stents, a FEA framework of the stent deployed in the arterial system was established, and a simply cyclic bending loading was proposed. Six comparative simulations of three superelastic materials (including NiTi for comparison) and two arterial configurations were successfully conducted. The mechanical behaviours of the stents were analysed through stress localization, the increase in artery diameter, contact results, and distributions of mean and alternating strain. The simulation results show that the Ti–22Zr–11Nb–2Sn (at. %) alloy composition for the stent produces the largest contact area (9.92 mm2) and radial contact force (49.5 mN) on the inner surface of the plaque and a higher increase in the stenotic artery diameter (70 %) after three vascular bending cycles. Furthermore, the Ti–22Zr–11Nb–2Sn stent exhibited sufficient crimping capacity and reliable mechanical performance during deployment and cyclic bending, which could make it a suitable choice for self-expanding coronary stents. In this work, the implementation of finite element analysis has thus made it possible to propose a solid basis for the mechanical evaluation of these stents fabricated in new Ni-free superelastic β-Ti alloys.

Mechanical property of the shape memorable Ti–Zr–Nb–Sn alloy manufactured by in-situ alloying in directed energy deposition

Ni-free β-type Ti alloys have been developed to manufacture high-strength, low-elastic-modulus, shape-memorable medical, and non-toxic components. Because the machinability of these alloys is generally poor, processing via additive manufacturing is an important step in the development of order-made medical devices. Although many studies evaluated the mechanical properties of additively manufactured Ni-free β-type Ti alloys, investigation of their heterogeneous microstructure resulting from the rapid melting-solidification cycling has not been discussed yet. In this study, the role of a heterogeneous microstructure on the performance of an in-situ alloyed Ti–Zr–Nb–Sn alloy was investigated. The metastable and nanosized ω phase, which is initiated in the non-equilibrium environment, enhances the yield strength without severe ductility degradation. Additionally, deformation-induced martensitic transformation occurs near the unmelted particle/matrix interface, and this phase transformation not only contributes to transformation-induced plasticity but also provides a superelasticity to the present alloy. Although the remained partially melted particle induces a localized corrosion in the matrix, the present results show that the heterogeneous microstructure of the in-situ alloyed Ti–Zr–Nb–Sn alloy exhibits outstanding mechanical properties and superelasticity, which will be suitable to fabricate biomedical parts via additive manufacturing.

Minor additions of Sn suppress the omega phase formation in beta titanium alloys

A critical characteristic of β-Ti alloys is the inevitable formation of ω-precipitates during certain heat treatments which leads to embrittlement, or even to a complete loss of ductility. Therefore, alloy design with the goal to inhibit the elementary ω-formation process is of utmost importance. Here, we propose a design strategy for prototypical β-type Ti–Cr–(Mo) alloys to alleviate this problem using only minor additions of Sn. Upon addition of Sn, we observed an extensive deceleration or even suppression of the ω-formation kinetics during isothermal ageing. Furthermore, the internal friction response of the elementary formation process indicated a decisive reduction of potential ω-nucleation sites, while the activation energy of the process remained almost unchanged. The results show, that the addition of Sn can significantly increase the width of the time–temperature process window and the long-time ageing resistance of β-Ti alloys, opening up huge opportunities for advanced alloy design and manufacturing routes.

Effect of minor gallium addition on corrosion, passivity, and antibacterial behaviour of novel β-type Ti–Nb alloys

Metastable Ti–Nb alloys are promising bone-implant materials due to improved mechanical biofunctionality and biocompatibility. To overcome increasing bacterial infection risk, alloying with antibacterial elements is a promising strategy. This study investigates the effect of minor gallium (Ga) additions (4, 8 wt% Ga) to as-cast and solution-treated β-type Ti–45Nb-based alloy (96(Ti–45Nb)-4Ga, 92(Ti–45Nb)-8Ga (wt.%)) on corrosion and passive film properties, as well as cytocompatibility and antibacterial activity. The electrochemical properties were evaluated by potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), and Mott-Schottky analyses in phosphate-buffered saline (PBS). X-ray photoelectron spectroscopy (XPS) was performed to analyze the chemical composition of passive films. Early adhesion and viability of macrophages and Staphylococcus aureus were assessed by nucleocounting and colony-forming unit counting, respectively. The results showed that high corrosion resistance and passive film properties of Ti–45Nb are retained and even slightly improved with Ga. EIS results revealed that Ga addition improves the passive film resistance. XPS measurements of 92(Ti–45Nb)-8Ga show that the passive film contains Ti-, Nb- and Ga-based oxides, implying the formation of mixed (Ti–Nb-Ga) oxides. In addition, marginal Ga ion release rate was detected under free corrosion conditions. Therefore, it can be assumed that Ga species may contribute to passive film formation on Ga-containing alloys. The 92(Ti–45Nb)-8Ga elicited an antibacterial effect against S. aureus compared to cp-Ti at 4 h. Moreover, Ga-containing alloys showed good cytocompatibility with THP-1 macrophages at 24 h. In conclusion, it was demonstrated that Ga additions to Ti–45Nb are beneficial to corrosion resistance and showed promising initial host and bacterial interactions.

In-vitro corrosion and surface properties of PVD-coated β-type TNTZ alloys for potential usage as biomaterials: Investigating the hardness, adhesion, and antibacterial properties of TiN, ZrN, and CrN film

In this study, the corrosion and surface properties of non-toxic and non-allergenic β-type Ti–29Nb–13Ta–4.6Zr alloys (TNTZ) coated with TiN, ZrN, and CrN layers using the PVD technique were investigated under in-vitro conditions. The coatings' hardness, adhesion resistance, and antibacterial properties were also studied, with a focus on their potential usage as biomaterials. The corrosion tests were conducted using potentiodynamic polarization, the hardness tests were conducted using the Knoop method, and the adhesion tests were conducted using pull-off tests. The wetting properties of the coatings were examined through contact angle and 3D morphological analysis. The coatings were characterized using SEM-EDS and XRD. The results showed that the adhesion resistance of all coatings was significantly higher than the expected adhesion strength for biomaterial coatings, while their corrosion resistance was higher compared to the uncoated alloy. Besides, the study demonstrates the potential of TiN, ZrN, and CrN coatings on β-type TNTZ for use as a biomaterial with desirable surface and antibacterial properties.

The mechanism of multi-directional Invar effect in a cross-rolling TiNb alloy

In this work, a binary metastable β-type Ti-41 Nb (wt. %) with a multi-directional Invar effect (i.e., near-zero thermal expansion) was fabricated through the cross-rolling (Cro-R), with an aim to elucidate the relationship between martensitic phase transformation and multi-directional Invar effect. It was found that the Cro-R TiNb alloy shows the Invar effect along the transverse direction (TD) and rolling direction (RD) across a wide temperature range (∼180 K), and the coefficient of thermal expansion (CTE) along the TD and RD are αTD = +0.64 × 10−6 K−1 and αRD = −0.016 × 10−6 K−1, respectively. Further experimental results show that the randomly oriented β parent phase and α″ martensitic phase are distributed in the tiny grains with a high density of dislocations. This high density of dislocations can effectively impede martensitic phase transformation from β to α″ that appears during the subsequent cooling, resulting in the martensitic phase transformation occurring over a wide temperature range. In such cases, the multi-directional Invar effect was achieved via the compensation between the negative thermal expansion (NTE) caused by martensitic phase transformation and the positive thermal expansion (PTE) due to the TiNb matrix lattice vibration during cooling. These experimental results act as a foundation for the design and development of the metastable β-type Ti alloy with a multi-directional Invar effect.

Assessment of improved tribocorrosion in novel in-situ Ti and β Ti–40Nb alloy matrix composites produced with NbC addition during arc-melting for biomedical applications

Nontoxic and nonallergenic β-type Ti–Nb alloys are considered attractive metallic materials for long-term bone implant applications. However, metallic implants present poor wear resistance, and the degradation process can be intensified with the friction occurring in corrosive body fluids, such as joint prostheses. The present study aimed to improve the tribological behavior of Ti–Nb alloys by adding hard reinforcement. The applied strategy is based on in-situ conditions, in which the reinforcing phase can be synthesized during the fabrication of the composite. Thus, a strong interfacial bond can be achieved due to the high chemical compatibility between the matrix and the reinforcement. Therefore, two different in-situ composites were developed by adding NbC powder to Ti and β Ti–40Nb alloy during the arc-melting process. As-cast samples of Ti and β Ti–40Nb alloy without NbC were used as the control groups. Structural characterization was performed, along with corrosion and tribocorrosion tests in a phosphate-buffered solution at body temperature. Results demonstrated that in-situ reactions occurred during the arc-melting process and promoted the precipitation of TiC as the reinforcing phase surrounded by α Ti phase (when NbC was added to Ti), and by β Ti–Nb phase (when NbC was added to Ti–40Nb). Finally, both produced composites showed improved tribocorrosion behaviors with wear volumes less than half of that recorded by the unreinforced Ti and β Ti–40Nb alloy. Thus, this study presents promising alternatives for wear-resistant biomedical applications.

Nb modified Ti-5553 alloy: Effects of heating rate on mechanical properties, corrosion behavior, and crystallographic texture

Ti–5Al–5Mo–5V–3Cr (Ti-5553) is a metastable β-type Ti alloy with excellent hardenability and high mechanical strength designed for aerospace applications. Previous studies have shown that the replacement of V with Nb, resulting in a Ti–12Nb–5Al–5Mo–3Cr (Ti-12553) alloy, can enhance the mechanical strength and hardenability. This study focuses on exploring the effects of V replacement with Nb in this alloy, aiming to evaluate the effects of the aging heating rate during aging heat treatment on the microstructure and its effect on hardness, tensile and compression strengths, corrosion resistance, and crystallographic texture development. The Ti-5553 and Nb-modified Ti-5553 (Ti-12553) alloys aged at 600 °C for 2 h resulted in a microstructure formed by α precipitates in a β matrix, with larger precipitates at a faster heating rate. This effect was more pronounced for the Ti-5553 alloy. The coarsening microstructure affected the mechanical behavior by decreasing the mechanical strength (hardness, ultimate tensile strength, and yield strength) while improving ductility and intergranular fracture mode in combination with a smaller region containing dimples. A decrease in hardness was observed at a faster heating rate, but it was still superior to that of the solution heat-treated samples. The textures formed in both alloys were similar and lacked any strong components. The replacement of V with Nb and the microstructure produced after aging heat treatment did not affect the samples’ corrosion resistance in a 3.5% NaCl solution. A stable passive protective layer with noble corrosion potential and reduced corrosion current density was observed.

Effect of Niobium and Oxygen Contents on Microstructure and Mechanical Properties of α+β-Type Ti–(5–25)Nb–(0.5–1)O (mass%) Alloys for Biomedical Applications

Effect of Niobium and Oxygen Contents on Microstructure and Mechanical Properties of α+β-Type Ti–(5–25)Nb–(0.5–1)O (mass%) Alloys for Biomedical Applications

Influences of grain size on the deformation behavior of a twinning-induced plasticity metastable β titanium alloy

The influences of grain size on the deformation behavior of a metastable β-type Ti–12Mo (wt%) alloy has been investigated by using uniaxial tensile testing, scanning electron microscopy and electron backscatter diffraction. The results reveal that the twinning-induced plasticity (TWIP) effect mediated by {332}<113>β twins occurs in this alloy when its mean grain size (d0) ranges from 49 to 104 µm, leading to pronounced work hardening behavior and a large uniform elongation (> 25%). Furthermore, there is a Hall-Petch relationship between the yield strength and d0 for this d0 range, with a Hall-Petch coefficient higher than that expected for pure slip deformation. However, when this alloy is in a state with heterogeneous grain structures and smaller grain sizes (d0 ∼ 30 µm), only a very limited volume fraction (< 1.8%) of {332}<113>β twins occurs and these twins prefer to nucleate in large grains (> ∼40 µm), but they can propagate into rather small grains (< ∼25 µm). In such a state, the Ti−12Mo alloy has a high yield strength (1025 MPa) but does not exhibit any sign of work hardening, i.e., the TWIP effect is absent. Based on the α"-assisted {332}<113>β twinning mechanism, the effects of grain size on the {332}<113>β twinning and yield strength are discussed in terms of its role in affecting the stress-induced β→α" martensitic transformation.