Effect of deformation of constituent phases on mechanical properties of Ti-6.5Al-3.5Mo-1.5Zr-0.3Si titanium alloy

Deformation of constituent phases determines the mechanical properties of titanium alloys. To address this relationship, the deformation of primary α (α_p), secondary α plates (α_s) and residual β phase (β_r) in Ti-6.5Al-3.5Mo-1.5Zr-0.3Si alloy was investigated based on the microstructure near fracture surface of uniaxial tensile samples. It is found that α_p is elongated and α_s mainly undergoes kinking deformation during tensile tests. The deformation of α_p is dominated by planar slipping, the deformation of α_s in colony is dominated by planar slipping and in basket weave is controlled by twin, planar slipping and dislocation tangling and the deformation of β_r is controlled by dislocation tangling. In the microstructure with high volume fraction of α_p (about 60%), the deformation amount of both α_p and α_s increases with the thickness of α_s, which increases the elongation of the alloy and changes the failure modes of α_p from fracture to boundary separation. In the microstructure with low volume fraction of α_p (19%) and long α_s, α_p has large deformation amount, while α_s with relative small deformation amount has large kinking region which increases the strength of the alloy. However, due to low volume fraction of α_p, its contribution to the ductility of alloy is limited and thus the elongation of the alloy is relatively small.