Deformation-induced Bulk Invar and Elinvar Effects in Thermoelastic Shape-memory Alloys

Abstract

Thermal expansion control is key to further optimization of systems with large heat gradients or changes in temperature, and where extreme precision is required. Manipulation of this property in materials has traditionally been limited as thermal expansion responds mainly to significant composition changes. Various special exceptions including Invar alloys provide some ability to address specific requirements. Discovery of giant thermal expansion anisotropy is a significant new development, enabling alloys with tailorable thermal expansion properties. Measurements at the crystalline scale through x-ray diffractography has shown that this occurs in thermoelastic martensites in the NiTi, TiNb, and NiTiPd families. The directionality of the various expansion rates in the crystal structure can be predicted through the lattice mismatch between martensite and its parent phase. This anisotropic crystalline response can be harnessed by use of deformation to produce reorientation and net alignment (texture) of martensitic polycrystals.

Examination of bulk material has demonstrated directional linear thermal expansions ranging from -30 to +40 ppm in TiNb alloy, -15 to +30 ppm in NiTi alloy, and -6 to +13 ppm in NiTiPdTa alloy between 0 and 100 °C. The strength of expansion response in the bulk material is dependent on the degree of texturing; this allows for selection of specific expansion rates via mechanical processing. Further examination also shows a strong directional effect on the elastic properties in the deformed material, which is driven at least partly by these same mechanisms. Extension to nickel-rich NiTi alloy revealed that this same anisotropic behavior can be retained in a largely austenitic alloy, with the additional effect of generating a net Elinvar effect up to 300 °C. With development of the optimal design approaches for these alloys, the potential to revolutionize accommodation of thermal expansion and elastic properties in materials may be realized.