Texture induced anisotropic negative thermal expansion behavior of as-smelted Ti-rich Ti-Ni alloys with different Ni contents

Ti-Ni alloys have been found to exhibit unique negative thermal expansion (NTE) response, whereas there is a lack of deep and comprehensive understanding of the NTE behavior. In the present work, Ti-rich Ti-Ni alloy button-type ingots having Ni content of 38.0 to 50.0 at.% were prepared by vacuum arc melting with a water cooled copper crucible. Results of microstructural characterization show that texture was formed in the as-smelted samples, as a large temperature gradient exists between the top and the bottom of the samples, which are in contact with the arc and the crucible respectively during the smelting process. Coefficients of thermal expansion (CTEs) along the directions perpendicular and parallel to the temperature gradient, namely transverse and longitudinal directions respectively, of the alloy were measured by thermo-mechanical analyzer (TMA) between 283 K and 473 K at a heating/cooling rate of 3 K/min. TMA results show that with the increase of Ni content, c_Ni,  the absolute value of the longitudinal CTE between the NTE interval increases parabolically from 9.3×10^-6 K^-1 to 73.6×10^-6 K^-1; however, the absolute value of the transverse CTE increases from 13.8×10^-6 K^-1 to 27.0×10^-6 K^-1 when c_Ni≤46.0 at.%, and then decreases to 9.5×10^-6 K^-1 when c_Ni>46.0 at.%. The anisotropic NTE behavior of the alloy can be attributed to the formation of textures as characterized by the electron backscatter diffraction (EBSD).