Compressive response of NiTi single crystals
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abstract
The deformation of NiTi shape memory single crystals are reported under compression loading for selected crystal orientations and two different Ti 3 Ni 4 precipitate sizes. For the [148] orientation, selected for highest recoverable strains, the peak aging treatment decreased the transformation stress from austenite to martensite. At the same time, peak aging raised the flow stress of both the austenite and martensite compared to the overaged case by increasing the resistance of the material to dislocation motion. The transformation proceeds beyond the stress plateau region and extends until martensite yielding occurs. This results in recoverable strain levels equivalent to the theoretical estimate of 6.4%. The [112] orientation was chosen to produce two variant formations and in this case, the transformation proceeded over an ascending stress-strain curve compared to the nearly plateau response for the [148] case. Since the austenite and martensite yield levels are reached at a smaller strain level in this case, the maximum recoverable strain was limited to 3.5% even though the theoretical estimates are near 5.1%. The theoretical estimates of transformation strains were established for Type I and Type II twinning cases to cover all possible habit plane and twin systems. TEM investigations support that slip in austenite occurs concomitant with increasing transformation strains. In the [001] orientation, the unfavorable slip systems for dislocation motion in the austenite inhibit slip and permit recoverable strains similar to the theoretical estimates (nearly 4.2%). The [001] orientation exhibits a continuous increase of flow stress with temperature beyond 360 K unlike any other orientation. The results point out that in order to optimize the material performance, close attention must be paid to the selection of the crystallographic orientation, and the precipitate size through heat treatment.
