Nanodomain structure and its effect on abnormal thermal expansion behavior of a Ti–23Nb–2Zr–0.7Ta–1.2O alloy

The microstructure and thermal expansion behavior of a Ti–23Nb–2Zr–0.7Ta–1.2O alloy (Gum metal) were investigated. A systematic diffraction pattern analysis using transmission electron microscopy revealed that a {11¯0}〈110〉-type transverse lattice modulation is present in the Ti–23Nb–2Zr–0.7Ta–1.2O alloy. Martensite-like nanodomains corresponding to {11¯0}〈110〉-type lattice modulations were observed in dark-field micrographs. Six variants of lattice modulation were confirmed to be distributed equivalently in the β phase in the annealed condition. Cold rolling resulted in preferential growth of a nanodomain variant which is most suited to releasing the applied stress although the long-range martensitic transformation was prevented by the local stress of the domain structure caused by oxygen atoms. It is suggested that the very small thermal expansion coefficient in the as-rolled specimen is caused by the preferential growth of the nanodomain variant due to residual stress.