Effects of composition on the mechanical properties and negative thermal expansion in martensitic TiNb alloys

Abstract

Certain martensitic alloys demonstrate significant crystalline anisotropy in coefficients of thermal expansion (CTEs), including very large negative (<−50 ppm) and positive (>100 ppm) CTEs along specific crystallographic orientations. In polycrystals, martensite detwinning during deformation imparts texture changes resulting in directional negative/positive thermal expansions. Several compositions of TiNb martensitic alloys were examined here for the compositional dependence of their degree of crystalline anisotropy in CTE and mechanical properties. Results indicated that increasing Nb content decreases detwinning stress and increases crystalline CTE anisotropy. An atomistic mechanism based on statistical thermodynamics was proposed to describe the reasons for anisotropic CTE in martensitic alloys.