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

Abstract

Composition dependence of crystallographic features of α″-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, η₃, became 0 at the critical Nb concentration of 23 mol% and the sign of η₃ 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 η₃ = 0, and that martensite with an internal twin of {111}_o type I was formed with a {443}_b habit plane for η₃ = +0.008. Subscript b and o indicate β and α″ lattice, respectively. The crystallography of these two α″-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 η₃ was not zero. It was found that there is a critical value of |η₃| for the formation of twinned martensite and that class B transformation, in which LIS is a {011}_o compound twin, hardly occurs in β-titanium alloys.

Acknowledgements

This work was partially supported by a Grant-in-Aid for Fundamental Scientific Research Kiban B (No. 17360334, 2005–2007), Wakate B (No. 18760522, 2006–2007), Scientific Research of Priority Areas 438 Next-Generation Actuators Leading Breakthroughs (No. 17040013, 2005–2006) and the 21st COE program from the Ministry of Education, Culture, Sports, Science and Technology, Japan. TI would like to thank Emeritus Professor T. Mori (Tokyo Institute of Technology) for his valuable advice for the infinitesimal deformation theory.