Composition dependent crystallography of α"-martensite in Ti-Nb-based β-titanium alloy

Composition dependence of crystallographic features of alpha ''-martensite in Ti-Nb-based alloy was systematically examined in Ti-Xmol% Nb-3mol% Al alloy, where X is 10-30. One of the lattice deformation strains, eta(3), became 0 at the critical Nb concentration of 23 mol% and the sign of eta(3) changed across the composition. The phenomenological theory of martensite crystallography (PTMC) predicted that the lattice invariant shear (LIS) would vanish at the critical composition. Transmission electron microscopy revealed that non-twinned martensite was formed with a habit plane of {755}(b) when eta(3) = 0, and that martensite with an internal twin of {111}(o) type I was formed with a {443}(b) habit plane for eta(3) = +0.008. Subscript b and o indicate beta and alpha '' lattice, respectively. The crystallography of these two alpha ''-martensites were well explained by the PTMC. On the other hand, non-twinned martensite with {111}(b)-{775}(b) habit plane was formed even when eta(3) was not zero. It was found that there is a critical value of vertical bar eta(3)vertical bar for the formation of twinned martensite and that class B transformation, in which LIS is a {011}(o) compound twin, hardly occurs in beta-titanium alloys.