Tension-compression asymmetry of the stress-strain response in aged singlecrystal and polycrystalline NiTi

The purpose of this work is to thoroughly understand tension-compression as ymmetry in precipitated NiTi using unique experimental results and micro-me chanical modeling. For the first time, tensile and compressive stress-strai n behaviors were established on aged single crystals ([100], [110], and [11 1] orientations) and polycrystalline NiTi. The single crystal and polycryst alline Ti-50.8 at.% Ni materials were given both peak aged and over aged he at treatments. The drawn polycrystalline NiTi has a strong texture of the [ 111] {110} type, thus it eformed in a manner consistent with the [111] sing le crystals. In contrast to the phenomenological theory of martensitic tran sformations (analogous to Schmid's law), the critical resolved shear stress required to trigger the transformation, tau(crss), in the peak-aged single crystals was dependent on both the stress direction and crystallographic o rientation. Using micro-mechanical modeling, the deviation from Schmid's la w was attributed to the unique orientation relationship that exists between the Ti3Ni4 precipitates (their coherent stress fields) and the 24 martensi te correspondence variant pairs. The over-aged single crystals generally ob eyed Schmid's law within experimental error, consistent with the proposed m icro-mechanical model. Qualitatively, the tension-compression asymmetry and orientation dependence of the recoverable strain level, epsilon(0), was co nsistent with the phenomenological theory for martensitic transformations. However, the peak- and over-aged single crystals generally both demonstrate d smaller epsilon(0) magnitudes than predicted. The differences for both cr ystals were attributed to the inhibition of martensite detwinning coupled w ith several unique microstructural effects. (C) 1999 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.