TENSION-COMPRESSION ASYMMETRY OF THE STRESS- STRAIN RESPONSE IN AGED SINGLE CRYSTAL AND POLYCRYSTALLINE NiTi

摘要

The purpose of this work is to thoroughly understand tension--compression asymmetry in preci- pitated 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 mod- eling, 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 orien- tation 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 co magnitudes t