Crystallographic texturing effects on shape recovery strain and mechanical properties of polycrystalline nitinol

As the shape memory material Nitinol (55% Nickel - 45% Titanium alloy) emerges to find new applications in engineered products, understanding the effects of material processing becomes increasingly important. Nitinol's unique mechanical behavior is derived from the coordinated atomic movements manifesting in phase transformations from cubic austenite to monoclinic martensite. These phase transformations are solid-to-solid phase transformations that occur without diffusion or plasticity, making them reversible. They involve changes in the crystalline structure that can be induced by changes in either temperature or stress. In addition to phase transformations, Nitinol mechanical strength is strongly dependent on the alloy composition and the method in which the material is processed, i.e. rolled, drawn, extruded, or forged. The mechanical work, combined with their intermediate heat treatment steps, contribute to modify microstructure, transformation temperatures and mechanical properties. These manufacturing processing steps lead to texturing (crystallographic alignment) of the material.Nitinol's anisotropy results mainly from crystallographic texture due to the rotation of grains into preferred orientations. For single crystals, the recoverable transformations are strongly dependent on the orientation of the crystals. For the polycrystalline NiTi, a macroscopic anisotropy may results from the microscopic crystal anisotropies. This research found that the textured, polycrystalline NiTi material display the notable transformation strain anisotropy, with larger transformation strains in different textured directions. When compared to tensile stress-strain curves, the compressive stress-strain curves demonstrate smaller recoverable strain levels, steeper transformation stress-strain slopes and higher critical transformation stress levels. From our experiments it was gleaned that 5% compression material setting, results in the maximum expected shape recovery strain. In addition, the critical stress for stress-induced martensitic transformation was higher in compression than in tension. The recovery strain associated with the transformation, had an average value of 5.5 +/-0.25% in tension and 3.5 +/-0.25% in compression, giving a strain ratio of 1:57. Until now, there has been a dearth of information in the literature regarding the compressive deformation of polycrystalline NiTi and its martensite-austenite phase transformation strain and strength levels. The potential for strain recovery capabilities of compressed solid Nitinol products can enable the design of novel devices in a myriad of industries.