Transitions among martensitic phases during continuous deformation in metastable β Ti-10V-2Fe-3Al alloy

Abstract

Martensitic transitions as a function of cold rolling strain levels, in the range of 5 – 43 %, in the solution treated condition of metastable β Ti-10V-2Fe-3Al (Ti-1023) alloy has been systematically investigated. The formation of ω and α′′, α′ martensitic phases from β phase, for the different conditions of Ti-1023 samples, has been confirmed through X-ray diffraction (XRD), transmission electron microscope (TEM) and electron backscatter diffraction (EBSD) techniques. The lattice parameters obtained from XRD confirmed that (c/a)α" and [b/(a√3)]α" ratios approached the value of ∼ 1.587 and unity, respectively, along with consistent variations in principal lattice strains and relative volume changes indicating β→α′′→α′ phase transformations, with increase in cold rolling strain levels. The presence of ω-phase for the 5 % cold rolling sample is confirmed via slow scan XRD and TEM. It is further inferred that with the cold rolling strains the ω-phase act as preferential nucleating sites promoting α′ martensite formation by suppressing the stress-induced β→α′′ martensitic transformation. Moreover, to accommodate strains, additional deformation processes such as martensite compound twinning formation and strain-induced α′′→α′ martensitic transformation are confirmed, through TEM and EBSD, respectively. With further increase in cold rolling strains to 43 %, the transformation of β→α′ and strain-induced α′′→α′ martensites are pronounced, which is consistent with the formation of equilibrium hexagonal close packed structure in titanium alloys. Finally, the present investigation suggests that orthorhombic-α′′ martensites and strain-induced α′′→α′ martensitic transformations give Ti-1023 alloy high cold deformability up to ∼ 43 %.