Design of metastable β-Ti alloys with enhanced mechanical properties by coupling αS precipitation strengthening and TRIP effect

A strain-transformable microstructure was successfully designed in a metastable beta Ti-7Mo-3Nb-3Cr-3Al alloy with enhanced mechanical properties, by introducing similar to 28% alpha precipitates coupled with the TRIP effect, over-coming the traditional trade-off dilemma between strength and ductility in most metastable beta-Ti alloys. The as-designed lamellar microstructure was predominantly deformed by stress-induced martensitic (SIM alpha") phase transformations, martensitic twinning and dislocation slip of the parent beta grains and alpha laths. The beta ->alpha" trans-formation followed the [113](beta)//[112](alpha)"//[-310](alpha)"//[1-21](alpha)" orientation relationship, with the {133}(beta) habit plane predicted by the Phenomenological Theory of Martensite Crystallography (PTMC). A novel 211 c+a pyramidal slip and shear of the alpha laths also contributed to the accommodation of internal stresses. Therefore, the origin of the enhanced tensile mechanical properties can be attributed to the combined effects of alpha precipitation strengthening coupled with the TRIP softening effect and the extra interaction stresses introduced by the alpha laths and other deformation products, validating the design concept. The current investigation may provide a novel strategy for designing new high-performance metastable beta-Ti alloys.