α″ martensite engineering: A strategy to achieve high yield strength and low yield ratio synergy for dual-phase titanium alloy

To breakthrough the long-term contradictory issue of high yield strength and low yield ratio for titanium alloys, and achieving the superior balance between these two key mechanical parameters. We have proposed an efficient “Quenching → Cold deformation → Recrystallization annealing” (QCR) strategy, which realizes the high yield strength (900 MPa) and low yield ratio (0.74) synergy for a model Ti6Al4V5.5Cu (wt.%) alloy which characterized with the unique multi-scale heterogeneous structure. The developed QCR processing route is based on the subtle utilization of orthorhombic α″ martensite, which plays the various roles in each processing step. By elaborately manipulating the recrystallization degree, chemical stability, effective domain of β phase and α_t nano-precipitates in β phase, the stress-induced α″ martensite transformation (SIM α″) can be controlled effectively and even postponed until after yielding induced by dislocation mechanisms, thus leading to substantially improvement of yield strength. After yielding, abundant proliferation of SIM α″which assisted by multi-scale α phases and their interactions are the fundamental reasons for achieving higher work-hardening ability, tensile strength (1215 MPa) and uniform elongation (11%). Thus, the core strategy to realize the excellent combination of high yield strength and low yield ratio is manipulating the activation sequence of the plastic deformation carriers.