Nickel-titanium (NiTi) shape memory alloys have been used in various small-scale applications such as thin films or MEMS devices owing to their ability to recover large strains and high actuator work density. Because determining and tailoring mechanical properties of NiTi at a small-scale is relatively difficult, it is necessary to find a means to influence and characterize its mechanical properties. The objective of this study is to examine the effect of heat treatment on polycrystalline Ti-50.9 at.%Ni on hot rolled and additionally cold drawn NiTi. The high temperature hot rolling process facilitated recrystallization, recovery and curtailed precipitate formation, leaving the hot rolled and cold drawn materials in near solutionized states. The cold drawn material contained a high density of dislocations and martensite. Heat treatments were carried out at 350 oC, 450 oC, 550 oC and 600 oC for a duration of 1.5 hours to induce Ti3Ni4 precipitates, and in the case of the cold drawn NiTi cause partial dislocation recovery. Based on transmission electron microscopy (TEM) observations these precipitates progressively increased in size from coherent to incoherent, and ultimately reversed to near solutionized. In addition, hardness was determined via instrumented micro-indention tests demonstrating a relative maximum at peak aging for the 350 oC heat treatment. Atomic Force Microscopy was used to measure indentation depth before and after heating to confirm shape memory or pseudoelastic behavior. Isothermal compressive stress-strain tests illustrated a variety of mechanical responses for the various heat treatments from pseudoelastic to shape memory.
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