Processing, structure, and property relationships in nickel-titanium shape-memory alloys at multiple length scales

The objective of this study is to examine fundamental processing-structure-property relationships in polycrystalline NiTi bars. Three different polycrystalline Ti-50.9 at.%Ni materials were examined: (1) cast, (2) cast then hot rolled, and (3) cast, hot rolled, then cold drawn. The structure of the materials was investigated at various scales ranging from nanometers to micrometers. The cast materials contained random crystallographic textures along the loading axis of the extracted samples. The hot rolled and cold drawn materials contained a strong <111> texture parallel to the deformation processing direction. 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 primarily on the hot-rolled, as well as, hot rolled then cold drawn materials at various temperatures for 1.5 hours. Transmission Electron Microscopy observations revealed that Ti3Ni 4 precipitates progressively increased in size, and changed their interface with the matrix from being coherent to incoherent with increasing heat treatment temperature. Accompanying the changes in precipitate size and interface coherency, transformation temperatures were observed to systematically shift, leading to the occurrence of the R-phase and multiple-stage transformations. Room temperature stress-strain tests illustrated a variety of mechanical responses for the various heat treatments, from pseudoelasticity to shape memory. The results confirm that Ti3Ni4 precipitates can be used to elicit a desired isothermal stress-strain behavior in polycrystalline NiTi.; Instrumented microindention tests revealed that Martens (Universal) Hardness values are more dependent on the resistance to dislocation motion than measured uniaxial pseudoelastic or shape memory response. Measuring indentation depth before and after heating more distinctly confirmed shape memory or pseudoelastic behavior. In addition, instrumented nanoindentation experimental results presented in this study are the first to illustrate that a stress-induced martensitic transformation can occur continuously, or discretely in nanometer scaled volumes of material. It is also demonstrated that the local material structure can be utilized to modify transformation behavior at nanometer scales, providing fundamental insight into martensitic phase transformations at small length scales.