From deformation twinning to α'' martensitic transformation in deforming Ti–12Mo alloy with increasing grain size

The metastable β-Ti alloys have garnered a renewed interest in recent years due to their great advantages in achieving excellent mechanical properties. Their diverse deformation mechanisms, such as α'' martensite transformation, deformation twinning, dislocation slip, and their combinations, have been evidenced crucial in optimizing of the strength and ductility of metastable β-Ti alloys. The current work is therefore carried out to investigate the transition from deformation twinning to α'' martensite transformation during the deformation of Ti–12Mo alloy with increasing grain sizes. The corresponding mechanical properties and deformation products are systematically investigated by coupling mechanical evaluation methods and microstructure characterization techniques. It is found that the strength of this alloy decreases considerably with an inverse variation in the ductility as the grain size increases from 30 to 90 μm. A parallel transition from deformation twins to α'' martensites is detected in the primary products, indicating the variation of the predominant deformation mechanisms. Detailed crystallographic analysis reveals that this transition is due to the variation in the formation sequence of α'' martensites in the transformation path. In addition, α'' martensites are evidenced to be a more effective factor inducing excellent strain hardening compared to deformation twins, which is attributed to their dense dispersion in β matrix, high volume fraction, diverse crystallographic variants, and shear modulus different from β phase. These findings deepen our fundamental understanding of the deformation mechanisms of metastable β-Ti alloys and benefit the development of such alloys with excellent performance.