Achieving strength-ductility tradeoff in near alpha titanium alloy via multi-stage heat treatment-induced nano-martensite phase transformation
A multi-stage heat treatment (MSHT) strategy, comprising a high-temperature short-duration water quench (WQ) followed by low-temperature long-duration furnace cooling (FC), was applied to a near-alpha Ti-0.3Mo-0.8Ni-2Al-1.5Zr alloy to overcome the strength-ductility tradeoff. The WQ state produced lath nano-martensite alpha-prime, residual beta-prime, and equiaxed recrystallized alpha. Subsequent FC decomposition transformed alpha-prime/beta-prime into homogeneously dispersed nano-scale alpha+beta precipitates, while equiaxed alpha coarsened via grain boundary migration. The WQ condition exhibited an ultimate tensile strength (sigma_UTS) of 610 MPa and elongation to failure (epsilon_f) of 18.2%. The WQ+400FC condition achieved a peak sigma_UTS of 791.5 MPa with epsilon_f = 16.7%, yielding a strength-ductility product (sigma_UTS * epsilon_f) of 13.2 GPa*%, a 19% improvement over the WQ state. Texture analysis revealed a duplex texture in WQ: weak {0001}//Z0 and strong {0110}//Y0, inherited after FC. The 400FC sample showed the highest lattice strain inhomogeneity, with peak kernel average misorientation (KAM) of 1.5 degrees and grain orientation spread (GOS) of 0.96 degrees, correlating with the excellent sigma_UTS. Non-basal slip systems exhibited higher Schmid factor (SF) values after heat treatment, contributing to ductility. Burgers orientation relationship (BOR) reconstruction confirmed variant selection during beta to alpha-prime transformation, with only four predominant alpha-prime variants instead of the twelve theoretically possible.