Stress-induced α″ martensitic phase transformation and martensitic twinning in a metastable β titanium alloy

The stress-induced martensitic (SIM) β→α″ phase transformation and SIM α″ twinning in the fully-β Ti–7Mo–3Nb–3Cr–3Al (Ti-7333) alloy was investigated as a function of strain, following a surface-to-bulk methodology assisted by electron backscatter diffraction (EBSD), focused ion beam (FIB) milling and transmission electron microscopy (TEM). The results indicate that the Ti-7333 alloy is predominantly deformed by SIM and SIM α″ twinning. At the onset of deformation, the lattice correspondent SIM α″ martensite variant that produces the maximum transformation strain along the tensile direction is activated firstly, which follows the <-110>_β//<001>_α″ orientation relationship. As the strain increases, the SIM α″ laths deform by twinning, predominantly through the {130}<310>_α″ compound twinning and {111}_α″ type Ⅰ twinning modes. Their activation can be rationalized in terms of the magnitude of the shear and the complexity of the atomic shuffle using the Bilby-Crocker deformation twinning theory. The prevalence of {130}<310>_α″ compound twinning in this alloy is attributed to the comparatively small shear (0.1872) and the simple shuffle (q = 2, Δ_Ⅰa = 0.3257) mechanism involved. During the last stages of deformation, secondary microtwinning takes place within the primary twins due to the reduced mobility of the intervariant boundaries. The evolution of such hierarchical microstructure with strain accounts for the complex microstructural features that developed with deformation, responsible for the high work hardening displayed by this alloy.