In-situ investigation of anisotropic crystalline and bulk negative thermal expansion in titanium alloys

Martensitically transforming polycrystalline Titanium-Niobium (TiNb) alloys exhibit tailorable coefficients of thermal expansions (CTEs) between large positive and negative values via deformation in martensite due to their anisotropic crystallographic CTE. However, the effects of Nb content, temperature, and deformation level on the crystal level and polycrystalline CTE anisotropy are not known, which limit the utilization of this unique feature in applications. In this study, TiNb alloys with 15 to 24 at.% Nb were systematically investigated using in-situ high energy synchrotron X-ray and iterative tension experiments to examine the evolution of crystalline CTE anisotropy, texture, and polycrystalline CTE. Analysis showed that crystalline CTE anisotropy and polycrystalline CTE values at a given temperature are the highest in Ti₇₆Nb₂₄ alloys, reaching +/-40 ppmK⁻¹ at 90 °C. Lower Nb content allows for higher maximum temperatures at which CTE anisotropy is observed, but at the expense of maximum CTE achieved. Similarly, increasing deformation level reduces crystalline CTE anisotropy. Ti₇₆Nb₂₄ experiences a swift and massive texture evolution with deformation, resulting in CTE change along tensile axis from 8 ppmK⁻¹ to -30 ppmK⁻¹ after only 5% strain. Lower Nb alloys exhibit lower CTEs and less pronounced texture evolution at a given strain, culminating in a failure to manifest any apparent change in CTE for Ti₇₅Nb₁₅ polycrystals. This was attributed to dislocation plasticity accompanying martensite reorientation in lower Nb alloys. The analysis of in-situ texture and tension experiments revealed that there is a direct correlation between applied strain level, texture evolution, and maximum negative CTE achieved in different TiNb alloys.