The utilization of Ti alloys as high-strength components, such as landing gears, calls for high-strength metastable β-Ti alloys with strength exceeding 1200 MPa. This level of strength is primarily attained through α precipitation strengthening and solid solution strengthening mechanisms ^[1][2][3][1,2,3]. However, the successful development of high-strength Ti alloys inevitably comes at the cost of decreased ductility and strain hardening capability owing to the restricted dislocation activity, which is typically faced in the materials’ development. For example, the 0.2% yield strength of the most widely used Ti-6Al-4V (weight percentage hereafter unless otherwise specified) can be as high as ~1000 MPa, but its fracture elongation is only 12%, and its strain hardening rate is rather limited [4]. In comparison, a high ductility of 35% and high strain hardening ability (ultimate tensile strength minus yield strength) of about 730 MPa were obtained in a TRIP/TWIP Ti-8Cr-1.5Sn alloy [4]. In addition to high strength, especially the high yield strength required to prevent the alloy from plastic yielding during stress loading, combined high ductility and high strain hardening are also crucial for Ti alloys to increase the absorbable work before abrupt material fracture, thereby improving the overall service reliability [5].

A feasible way to mitigate the low ductility and limited work hardening observed in high-strength Ti alloys is to introduce additional deformation modes that can accommodate the plastic strain during dynamic deformation. These deformation mechanisms include stress-induced martensite transformation and deformation twinning, which can be achieved by tailoring the phase stability of the body-centered cubic β phase ^[6][7][8][6,7,8]. As historically observed in steels, the dynamic phase transformation and twinning during deformation lead to simultaneous improvement in strength, ductility, and strain hardening, a mechanism known as the transformation-induced plasticity (TRIP) effect and twinning-induced plasticity (TWIP) effect ^[9][10][11][9,10,11].

Since the first report of Ti-12Mo alloy combining TRIP and TWIP effects in 2012 [12], TRIP/TWIP Ti alloys have attracted increasing interest in the field of Ti science. They constitute an important class of Ti alloy due to their ability to achieve an outstanding combination of high strength, high ductility, and high strain hardening capability. Ongoing efforts are dedicated to designing new TRIP/TWIP Ti alloys with improved yield strength, ultimate tensile strength, and total elongation by manipulating the activation sequence of the various deformation modes ^{[4][8][13][14][15]}[4,8,13,14,15]. Understanding the intrinsic correlation between the compositional design strategy (specifically, β phase stability), deformation mechanisms, and mechanical properties is essential for the design and development of new TRIP/TWIP Ti alloys that can simultaneously offer high yield strength, high strain hardening capability, and high ductility to meet the ever-growing demands for high-strength Ti alloy in advanced structural applications.

Since the first publication of Ti-12Mo TRIP/TWIP Ti alloy by Marteleur et al. in 2012 [12], which was designed using d-electron theory, there has been growing interest in these alloys over the past decade due to their remarkable combination of high strength, high ductility, and high strain hardening rate. Inspired by the compositional design method and complex combination of deformation mechanisms observed in Ti-12Mo alloy, including stress-induced β-to-α′ martensitic transformation, stress-induced β-to-α″ martensitic transformation, and {332}_β deformation twinning ^[12][16][17][12,26,67], a variety of new TRIP/TWIP Ti alloys with promising mechanical properties were designed using d-electron theory. Examples of these alloys include Ti-9Mo-6W ^[18][57], Ti-8Cr-1.5Sn [4], and Ti-12Mo-5Zr ^[19][106]. These alloys have demonstrated simultaneously enhanced strength and ductility as a result of stress-induced martensitic phase transformation (TRIP) and deformation twinning (TWIP). For instance, the TRIP/TWIP Ti-8.5Cr-1.5Sn exhibited 3–4 times greater ductility than Ti-6Al-4V and a 50% higher yield strength than Fe-22Mn-0.6C TWIP steel [4]. Nevertheless, the yield strength of Ti-8.5Cr-1.5Sn is about 520 MPa, and it is about 480 MPa for Ti-12Mo [4]. The design and development of new TRIP/TWIP Ti alloys have recently been a significant research focus in the Ti community. In addition to ternary alloys, Ti-Mo ^[20][21][22][17,91,124], Ti-Nb ^[8][23][8,125], Ti-V ^[13][24][25][13,96,105], and Ti-Cr ^[20][26][17,68] based multi-component alloys have been developed, such as Ti-6Cr-4Mo-2Al-2Sn-1Zr (Ti-64221) [14], Ti-3Al-5Mo-7V-3Cr (Ti-3573) [13], and Ti-3Mo-3Cr-2Fe-2Al ^[20][17]. More detailed information about the chemical compositions, deformation mechanisms, and mechanical properties of these developed TRIP/TWIP Ti alloys can be found in References ^[27][28][29][41,43,50]. Table 1 summarizes some recent findings on TRIP/TWIP Ti alloys and their mechanical properties. Researchers are committed to discovering new TRIP/TWIP Ti alloys through compositional design and studying their deformation mechanisms and mechanical properties. Despite the excellent combination of high strength, high ductility, and high strain hardening capability, one of the limitations of the TRIP/TWIP Ti alloys is the inherently soft nature of the parent β phase, which leads to low yield strength (Table 1) owing to the low trigger stress required to initiate stress-induced martensitic transformation and deformation twinning. TRIP/TWIP Ti alloys typically tend to exhibit a yield strength below 600 MPa, for example, Ti-12Mo (~480 MPa) [12], Ti-10V-4Cr-1Al (~420 MPa) ^[25][105], and Ti-8.5Cr-1.5Sn (520 MPa) [4], as stress-induced β-to-α″ martensitic transformation is activated at low applied stress. Therefore, the primary challenge lies in enhancing the yield strength of TRIP/TWIP Ti alloys while preserving large ductility and high strain hardenability. Several strategies are proposed to address this challenge and improve the yield strength of TRIP/TWIP Ti alloys. These strategies include solid solution strengthening ^[7][13][14][7,13,14], the precipitation of secondary phases ^{[15][30][31][32]}[15,18,88,99], oxygen interstitial strengthening ^{[16][33][34][35][36]}[26,126,127,128,129], and grain refinement ^[30][37][18,130].

Alloy (wt.%)	Year	Researchers	Research Findings	YS	UTS	uEL	Ref.
Ti-12Mo (TRIP/TWIP)	2012	M. Marteleur, et al.	Developing a new family of TRIP/TWIP Ti alloys based on d-electron alloy design	485	661	0.35	[12]
Ti-12Mo (TRIP/TWIP)	2013	F. Sun, et al.	Unveiling the deformation mechanisms at the early deformation stage in TRIP/TWIP Ti alloy	-	-	0.4	[6]
Ti-15Mo (TWIP)	2013	X. Min, et al.	Quantitative evaluation of {332}β twinning at various tensile strains	504	765	0.24	[38]
Ti-10Mo-0.2O (TRIP/TWIP)	2014	X. Min, et al.	Strengthening TRIP/TWIP Ti alloy by oxygen interstitials	800	852	-	[39][131]
Ti-15Mo (TWIP)	2015	X. Min, et al.	Strengthening TWIP Ti alloy by pre-strain-induced twins and ωiso	760	-	0.15	[40][132]
Ti-9Mo-6W (TRIP/TWIP)	2015	F. Sun, et al.	Outstanding work hardening and uniform elongation by stress-induced β-to-α″ and β-to-ω transformation and {332}β twinning	528	791	0.33	[18][57]
Ti-9Cr-0.2O (TRIP)	2015	H. Liu, et al.	TRIP by stress-induced β-to-ω transformation	850	1025	0.2	[41][133]
Ti-27Nb (at.%) (TRIP/TWIP)	2016	P. Castany, et al.	Origin of {332}β as a result of reversion of a parent {130}<310>α″ twinning	-	-	-	[42][60]
Ti-12Mo (TRIP/TWIP)	2017	F. Sun, et al.	Strengthening TRIP/TWIP Ti alloy through low-temperature aging	730	793	0.38	[32][99]
Ti-6Cr-4Mo-2Al-2Sn-1Zr (TWIP)	2018	L. Ren, et al.	Ultrahigh product of strength and elongation (42.6 GPa%) by {332}β and {112}β twinning and reverse ω transformation	670	820	0.31	[14]
Ti-12Mo-5Zr (TRIP/TWIP)	2018	J. Zhang, et al.	Improving yield strength by increasing critical resolved shear stress (CRSS) of stress-induced β-to-α″ transformation	656	733	0.31	[19][106]
Ti-3Al-5Mo-7V-3Cr (TRIP/TWIP)	2018	S. Sadeghpour et al.	Increasing yield strength by solid-solution strengthening	750	1100	0.19	[13]
Ti-6Mo-4Zr (at.%)	2018	C. Wang, et.al	Introducing a semi-empirical approach based on the average electron-to-atom ratio (𝑒/𝑎�/�¯) and atomic radius difference (Δ𝑟��¯) to predict the deformation behaviors of metastable β-Ti alloys	475	-	-	[43][108]
Ti-10V-2Fe-3Al (TRIP/TWIP)	2019	Y. Danard, et al.	Developing design strategy to reach (α + β) dual-phase TRIP/TWIP Ti alloy	670	-	0.30	[15]
Ti-18Mo-13Zr (TWIP)	2019	J. Zhang et al.	Multimodal twinning by microscale {332}β, nanoscale {112}β, and novel {5811}<135>β	800	-	0.18	[44][40]
Ti-4Al-4Fe-0.25Si-0.1O (TRIP)	2019	S. Lee, et al.	Stress-induced β-to-α′ transformation mediated by the O′ phase resulted in an excellent combination of strength and ductility	600	1352	0.3	[45][46][19,20]
Ti-11Mo-5Sn-5Nb (TWIP)	2019	G. Zhao, et al.	Building a multiscale dislocation-based model to describe microstructural evolution and strain-hardening of {332}β TWIP Ti alloy	490	788	0.24	[27][41]
Ti-16Nb-8Mo (TRIP/TWIP)	2020	D.M. Gordin, et al.	Designing strain transformable Ti alloy for biomedical applications	420	650	-	[47][107]
Ti-4Mo-3Cr-1Fe (TWIP)	2020	L. Ren, et al.	Ultrahigh yield strength and ductility harnessed by a stress-induced nano-scale hierarchical twin structure and ωath-to-β reversion	870	1092	0.27	[7]
Ti-15Nb-5Zr-4Sn-1Fe (TRIP)	2020	Y. Fu, et al.	Designing TRIP Ti alloy with stress-induced β-to-α′ transformation	546	939	0.17	[8]
Ti-10V-2Fe-3Al (TRIP)	2021	B. Ellyson, et al.	β + ω TRIP Ti	-	-	-	[5]
Ti-12Mo (TRIP/TWIP)	2021	B. Qian, et al.	Determining transformation pathways in TRIP/TWIP Ti alloy	-	-	-	[48][134]
Ti-6Mo-3.5Cr-1Zr (TRIP)	2022	K. Chen, et al.	Designing TRIP Ti alloy with stress-induced β-to-ω transformation	698	1242	0.32	[49][58]
Ti-12Mo (TRIP/TWIP)	2022	B. Qian, et al.	Strengthening TRIP/TWIP Ti alloy by grain refinement and ωiso	865	-	0.35	[30][18]
Ti-15Nb-5Zr-4Sn-1Fe (TRIP)	2023	Y. Fu, et al.	Natural aging in TRIP Ti alloy led to simultaneously enhanced yield strength and uniform elongation	683	987	0.17	[50][51]
Ti-12Mo-0.3O (TRIP/TWIP)	2023	Y. Chong, et al.	Strengthening TRIP/TWIP Ti alloy by grain refinement and oxygen interstitials	826	1064	0.24	[51][135]

2. Solid Solution Strengthening

Solid-solution strengthening by adding substitutional and interstitial elements is a feasible way to improve the yield strength of TRIP/TWIP Ti alloys. The advantages of solid solution strengthening are twofold. The increased yield was obtained in Ti-12Mo-5Zr (656 MPa) ^[19][106] and Ti-12Mo-0.18O (623 MPa) ^[16][26], compared to baseline Ti-12Mo (480 MPa) due to the solid-solution strengthening effect of Zr and O, respectively. A multi-element Ti-3Al-5Mo-7V-3Cr TRIP/TWIP alloy with an enhanced yield strength of up to 750 MPa was designed based on considering the solid-solution strengthening effect of alloying elements [13]. Furthermore, a TWIP Ti-4Mo-3Cr-1Fe alloy, boasting a high yield strength of 870 MPa, which is the highest among the currently developed TRIP/TWIP Ti alloys ^[52][72], was designed by Ren et al. by combined addition of strong solid solution strengthening elements Mo + Cr + Fe [7]. The enhanced yield strength by solid-solution strengthening is also related to the reduced probability or even fully suppressed stress-induced martensitic transformation as a result of improved β phase stability ^[53][49]. Therefore, the addition of strengthening elements should be carefully tailored utilizing d-electron theory and [Mo]_eq to ensure the occurrence of TRIP and/or TWIP effects. A small amount of oxygen interstitial doping can effectively enhance the yield strength by suppressing stress-induced martensitic transformation and deformation twinning in metastable β-Ti alloys ^[33][39][54][126,131,136], i.e., the interstitial oxygen acts as a strong β-stabilizing element in metastable β-Ti alloys. Doping 0.3 wt.% oxygen combined with grain refinement was successfully employed by Chong et al. to substantially improve the yield strength of Ti-12Mo-0.3O to 826 MPa while maintaining a high strain hardening ability and high uniform elongation ^[51][135]. The beneficial role of O interstitial strengthening is also reported in Ti-32Nb ^[55][118], Ti-38Nb ^[36][55][118,129], and Ti-20V ^[34][127]. However, oxygen is excluded by the calculation of electron parameters $\stackrel{}{\mathit{Bo}\stackrel{}{¯}}$ , $\stackrel{}{\mathit{Md}\stackrel{}{¯}}$ , and $\stackrel{}{e/a\stackrel{}{¯}}$ , while it is treated as an α-stabilizer in calculating the [Mo]_eq, meaning that the β-stabilizing effect of interstitial oxygen is not considered in the current semi-empirical alloy design approaches. So do the neutral elements Zr and Sn. Consequently, the current d-electron theory, [Mo]_eq, and electron-to-atom ratio approach need to be modified to fully harness the benefits of the β-stabilizing effect of oxygen, Zr, and Sn, and their interaction with other β-stabilizers.

3. Precipitation Strengthening

3.1. α Precipitation Strengthening

The yield strength can also be enhanced by developing TRIP/TWIP Ti alloys from the single β phase alloys to β + α and β + ω dual-phase alloys, which can be achieved by utilizing equiaxed α phase or nanoscale ω phase precipitation to strengthen the least stable β matrix. The yield strength (760 MPa) of 20% α strengthened Ti-7Cr-1.5Sn dual-phase alloy is 200 MPa higher than that of the Ti-8.5Cr-1.5Sn single β phase alloy while maintaining the TRIP/TWIP effects ^[56][59]. A transition from TRIP to TRIP/TWIP in Ti-10V-2Fe-3Al accompanied by remarkably enhanced yield strength was achieved by precipitating the 20% α phase [15]. Simultaneously enhanced yield strength, ultimate tensile strength, and ductility were observed in a 4 vol.% α containing α + β dual-phase Ti-3Mo-3Cr-2Fe-2Al TRIP/TWIP alloy as compared to its single-phase counterpart ^[20][17]. The precipitation of the α phase leads to solute-atom partitioning, enriching the β matrix with β-stabilizers and thus enhancing its phase stability, which results in a transition of deformation mechanism from TRIP to TWIP or even the complete suppression of the TRIP/TWIP effect. It appears that a small fraction of no more than 20 vol.% α should be ensured to maintain the TRIP/TWIP effects in α + β dual-phase Ti alloys ^{[46][57][58][59]}[20,98,137,138]. The volume fraction of α precipitates and the resultant transition of deformation mode in α + β dual-phase TRIP/TWIP Ti alloys can be guided by coupling the Calphad calculation and $\stackrel{}{\mathit{Bo}\stackrel{}{¯}}$ - $\stackrel{}{\mathit{Md}\stackrel{}{¯}}$  diagram ^[15][56][15,59]. The α precipitation strengthening opens a new branch of TRIP/TWIP Ti alloy development from single β phase alloys to α + β dual phase alloys.

3.2. ω Precipitation Strengthening

Controlling the development of ω phase has also been shown to be quite effective in increasing the yield strength of TRIP/TWIP Ti alloys while reserving their promising strain hardening, leading to the development of ω strengthened β + ω dual-phase TRIP/TWIP Ti alloys ^[30][32][18,99]. Sun et al. ^[32][99] conducted low-temperature short-time aging (150 °C for 60 s) to control the development of the isothermal ω phase in Ti-12Mo without causing any discernible compositional partitioning. This method takes full advantage of the precipitation hardening of the ω phase while minimizing its harmful effect on suppressing stress-induced martensitic transformation and deformation twinning. The result was a substantial increase in yield strength from 480 MPa to 730 MPa while maintaining the original high ductility. Low-temperature short-time aging (200 °C for 60 s) combined with grain refinement was employed by Qian et al. ^[30][18] to achieve high yield strength as high as 990 MPa in Ti-12Mo without significantly sacrificing the ductility. Enhanced uniform elongation in the Ti-15Mo TWIP alloy was achieved by Min et al. through coupling pre-strain-induced twins and isothermal ω (ω_iso) precipitation, as compared to the counterpart with only ω_iso precipitation ^[40][60][61][132,139,140]. These works have highlighted the potential of tuning the ω phase precipitation to enhance the mechanical properties of TRIP/TWIP Ti alloys. It is worth mentioning that the athermal ω (ω_ath) phase formed during quenching is not harmful to the mechanical properties of TRIP/TWIP Ti alloys, contrary to the isothermal ω phase. It is reported that the ω_ath disappeared after the formation of stress-induced martensite and deformation twinning, which is believed to be reversed back to the parent β phase during deformation ^[7][8][62][7,8,93]. The excellent combination of high yield strength of 870 MPa, ultimate tensile strength of 1092 MPa, and excellent ductility with a fracture elongation of 41% observed in Ti-4Mo-3Cr-1Fe is likely related to the dynamic ω_ath-to-β reversion during tensile deformation, which promotes the formation of stress-induced nano-scale hierarchical twinning structures [7]. The ω_ath-to-β reversion ability during deformation makes it possible to tune the deformation mechanisms and mechanical properties of TRIP/TWIP Ti alloys by regulating the development of ω_ath. This can be realized by low-temperature short-time aging ^[5][30][32][5,18,99] and room-temperature aging ^[5][50][5,51]. Accordingly, substantially increased yield strength without obviously sacrificing ductility or even enhanced ductility was achieved in Ti-12Mo, Ti-10V-2Fe-3Al, and Ti-15Nb-5Zr-4Sn-1Fe ^{[5][30][32][50]}[5,18,51,99]. Especially, a high yield strength of 683 MPa, which is comparable to most of the TRIP/TWIP Ti alloys, was reported in a TRIP Ti-15Nb-5Zr-4Sn-1Fe TRIP Ti alloy after regulating the ω_ath by nature aging ^[50][51]. These results demonstrate a new opportunity to enhance the mechanical properties of TRIP/TWIP Ti alloys via engineering the ω_ath. Future efforts may be dedicated to revealing the evolution of ω_ath during aging and its influence on the deformation mechanisms and mechanical properties of TRIP/TWIP Ti alloys.

4. Grain Refinement Strengthening

The role of grain refinement strengthening in increasing the yield strength of TRIP/TWIP Ti alloys has received little attention. This might be attributed to the fact that the critical resolved shear stress for stress-induced martensitic phase transformation and deformation twinning is not very sensitive to changes in grain size ^[63][64][65][141,142,143]. A higher yield strength was observed in coarser-grained TRIP Ti alloys ^[66][67][68][144,145,146]. Accordingly, the impact of grain refinement on the yield strength of TRIP/TWIP Ti alloys is limited. It is challenging to refine the grain size of metastable β-Ti alloys to the ultrafine grain scale after undergoing severe plastic deformation. This difficulty is related to the rapid recrystallization and grain growth kinetics during the annealing of Ti alloys above the β transus temperatures ^[51][64][135,142]. Chong et al. ^[51][135] demonstrate that significantly refining the grain size from 50 μm to 4.5 μm in Ti-12Mo only slightly increased the yield strength by 43 MPa. A similar result has also been reported in Ti-13.3Nb-4.6Mo (at.%) ^[37][130]. This is because substantial grain refinement promotes the occurrence of stress-induced martensitic phase transformation, leading to a reduction in the ultimate tensile strength and ductility. However, combining grain refinement with other strengthening methods, such as α phase precipitation ^[20][17], ω phase precipitation ^[30][18], and oxygen addition ^[16][26], has shown the potential to significantly increase the yield strength while maintaining high ductility and a high work-hardening rate. Therefore, the combination of grain refinement with other strengthening methods is worthy of further research and exploration.