Achievement of Room Temperature Superelasticity in Ti-Mo-Al Alloy System via Manipulation of ω Phase Stability

Abstract

The achievement of room-temperature (RT) superelasticity in a Ti-Mo-Al ternary alloy system through the addition of a relatively high concentration of Al to manipulate the phase stability of the ω phase is realized in this study. The composition of the Ti-6 mol% Mo (Ti-11.34 mass% Mo) alloy was designated as the starting alloy, while 5 mol% Al (=2.71 mass% Al) and 10 mol% Al (=5.54 mass% Al) were introduced to promote their superelastic behavior. Among the alloys, Ti-6 mol% Mo-10 mol% Al alloy, which was investigated for the very first time in this work, performed the best in terms of superelasticity. On the other hand, Ti-6 mol% Mo and Ti-6 mol% Mo-5 mol% Al alloys exhibited a shape memory effect upon heating. It is worth mentioning that in the transmission electron microscopy observation, ω phase, which appeared along with β-parent phase, was significantly suppressed as Al concentration was elevated up to 10 mol%. Therefore, the conventional difficulties of the inhibited RT superelasticity were successfully revealed by regulating the number density of the ω phase below a threshold.

Effects of 3d Transition Metal Substitutions on the Phase Stability and Mechanical Properties of Ti–5.5Al–11.8[Mo]eq Alloys

“…Along with the phase identification results from Figure 2 , it could thus be determined that the alloys (b) Ti–Al–Cr, (c) Ti–Al–Co, and (d) Ti–Al–Ni were composed of the single parent β phase. Additionally, these microstructures are similar to that of the Ti–Al–Mo alloy in our previous result [ 25 ], which is cited in Figure 3 e for comparison. On the other hand, in Figure 3 a, obvious precipitates, which are pointed out by arrows, can be observed.…”

“…In the continuous tensile tests, it was found that no obvious two-stage yielding behavior could be found in the Ti–Al–X (X = (a) Cr, (b) Co, and (c) Ni) alloys. On the other hand, it was found that the Ti–Al–Mo [ 25 ] alloy shows slight two-stage yielding in the literature. The first-stage yielding indicates the stress-induced martensite transformation (SIMT), while the second yielding suggests the plastic deformation (i.e., slip of dislocations).…”

“…In the cyclic loading–unloading tensile tests ( Figure 6 ), the (c-ii) Ti–Al–Ni alloy showed a superelasticity similar to that of the (d) Ti–Al–Mo alloy [ 25 ]. The (d) Ti–Al–Mo alloy performed the best elongation with a total strain of about 7%, while the elongation of the (c-ii) Ti–Al–Ni alloy was approximately 4%.…”

“…To evaluate the superelastic properties of the (c) Ti–Al–Ni and (d) Ti–Al–Mo [ 25 ] alloys that exhibited RT superelasticity, the overall shape recovery strain ε sr (including elastic recovery strain), superelastic recovery strain ε se (excluding elastic recovery strain), and residual plastic strain ε r of each cycle read from the cyclic stress–strain curves ( Figure 6 ) are plotted as a functions of applied strain ε ap in Figure 7 a. Because of the poor ductility of the Ti–Al–Ni alloy, plots present up to 4% of the applied strain.…”

“…It has been reported that the addition of a relatively high concentration of tin (Sn) and aluminum (Al) could be an effective strategy to suppress the generation of the athermal ω (ω ath ) phase, resulting in a cause of MT-inhibition and the isothermal ω (ω iso ) phase corresponding to the ω embrittlement [ 20 , 21 , 22 , 23 , 24 ]. In addition, our research group also reported that the Ti–Al–Mo ternary system with the nominal chemical composition of Ti–5.5Al–11.8Mo (mass%) achieved a superelastic behavior at room temperature (RT; i.e., 297 K) by introducing a relatively high concentration of Al to suppress the formation of the ω ath phase [ 25 ]. This enhancement, namely, achieving superelasticity at RT, was attributed to the suppression of the generation of the undesired ω ath phase via an elevated Al addition concentration.…”