Tension–compression asymmetry of the stress–strain response in aged single crystal and polycrystalline NiTi

Abstract

The purpose of this work is to thoroughly understand tension–compression asymmetry in precipitated NiTi using unique experimental results and micro-mechanical modeling. For the first time, tensile and compressive stress–strain behaviors were established on aged single crystals ([100], [110], and [111] orientations) and polycrystalline NiTi. The single crystal and polycrystalline Ti–50.8 at.% Ni materials were given both peak aged and over aged heat treatments. The drawn polycrystalline NiTi has a strong texture of the 〈111〉{110} type, thus it deformed in a manner consistent with the [111] single crystals. In contrast to the phenomenological theory of martensitic transformations (analogous to Schmid's law), the critical resolved shear stress required to trigger the transformation, τ crss, in the peak-aged single crystals was dependent on both the stress direction and crystallographic orientation. Using micro-mechanical modeling, the deviation from Schmid's law was attributed to the unique orientation relationship that exists between the Ti 3Ni 4 precipitates (their coherent stress fields) and the 24 martensite correspondence variant pairs. The over-aged single crystals generally obeyed Schmid's law within experimental error, consistent with the proposed micro-mechanical model. Qualitatively, the tension–compression asymmetry and orientation dependence of the recoverable strain level, ε 0, was consistent with the phenomenological theory for martensitic transformations. However, the peak- and over-aged single crystals generally both demonstrated smaller ε 0 magnitudes than predicted. The differences for both crystals were attributed to the inhibition of martensite detwinning coupled with several unique microstructural effects.