Preserving high work hardening without sacrificing yield strength in a metastable β Ti alloy via relay-like hardening mechanism

Metastable β-Ti alloys exhibiting transformation-induced plasticity (TRIP) + twinning-induced plasticity (TWIP) generally achieve a good combination of ultimate strength and ductility. However, these alloys often suffer from low yield strength and exhausted hardening capability at later strains due to poor sequential regulation of the TRIP and TWIP effects. Here, we report a TRIP + TWIP metastable β Ti, Ti-9.2Mo-2Al-2Fe (wt. %), exhibiting high yield/ultimate stress of ∼750/1200 MPa and large elongation of ∼35% under quasi-static loading (10−3/s). This alloy effectively avoids the premature exhaustion of available deformation mechanisms through a unique relay-like hardening mode that leads to high strain hardening (∼2 GPa) at a large strain (∼17%). Characterizations reveal that relay-like hardening behavior stems from the consecutive activation of predominant {332}β twinning followed by stress-induced α’’ martensite transformation with strains. A complex evolution map of microstructures is identified, involving primary {332}β twin → secondary/third {332}β twin with little {5 8 11}β twin → predominant α’’ martensite transformation → subsequent {110}α’’, {130}α’’, and {011}α’’ martensite twins. However, this deformation mode cannot be maintained upon elevating strain rate (10−1/s) because transformation becomes less active, which leads to poor ductility and low ultimate stress (∼20% and 1000 MPa). This demonstrates the importance of a relay-like hardening mode in mechanical performances.