Detwinning in NiTi alloys

Abstract

This work focuses on the stress-induced transformation in solutionized and overaged single-crystal NiTi alloys. The potential role of detwinning on the recoverable strains was investigated both theoretically and also with temperature-cycling experiments. The detwinning is the growth of one variant within a martensite in expense of the other. It is shown that the experimental recoverable strains in tension (near 8.01 pct in the [123], 9.34 pct in the [111], and 7.8 pct in the [011] orientations) exceed the theoretical martensite (correspondent-variant pair (CVP) formation strains (6.49 pct in [123], 5.9 pct in [111], and 5.41 pct in [011]), lending further support that partial detwinning of martensite has occurred in both the solutionized and overaged specimens. In compression, the experimental recoverable strains are lower than the theoretical martensite (CVP) formation strain. In the compression cases, the detwinning strain contribution is calculated to be negligible in most orientations. The transformation strains observed in overaged NiTi are similar to the solutionalized NiTi, suggesting that incoherent precipitates do not restrict the detwinning of the martensite. For the [123] orientation, it is demonstrated that the thermal hysteresis is higher in solutionized NiTi compared to the overaged NiTi. The higher thermal hysteresis can be exploited in applications involving damping and shape stability, while the lower hysteresis is suited for actuators. I. BACKGROUNDning strains in compression [1] for selected orientations. The CVP is defined as the martensite plate with two correspon-SHAPE-MEMORY alloys have unique properties such dent variants. In this study, the focus is on determining both as recoverability of deformation upon heating and pseudothe CVP formation and detwinning strains in tension for all elastic stress-strain behavior. The materials-science backorientations. Transformation-strain contours are provided for ground has been documented for these alloys over the years both tension and compression within the stereographic trianin a variety of materials journals, but a number of issues gle demonstrating orientations favorable (and also unfavorare still under investigation, including detwinning of the able) to detwinning. In the current work, five different crystal martensite and the magnitude of recoverable strains. The orientations have been selected, with [123], [111] and [011] detwinning is the growth of one variant within a martensite exhibiting high degrees of detwinning in tension, [012] in expense of the other. The detwinning produces additional exhibiting moderate detwinning in tension, and [001] exhibrecoverable strain, especially in tension. It has been well iting no detwinning in tension. known that transformation strains in NiTi alloys in tensionThere have been previous attempts to demonstrate the are significantly larger than under compression. This prorole of detwinning in the NiTi alloys, which focused on duces differences in stress-strain response in tension vs comdeformation in the...

Compressive response of a single crystalline CoNiAl shape memory alloy

“…The lack of increase in SME strain with increasing applied stress level in Figure 3.b is also opposite to what have been observed in NiTi single crystals [21]. In NiTi, higher stress values above a critical level result in local plastic deformation and a decrease in the SME strain and increase in hysteresis.…”

“…The temperature hysteresis in both orientations decreases with increased applied stress. In conventional SMAs such as CuZnAl and NiTi, the opposite would be expected [18,[20][21]. In…”

“…NiTi single crystals in solutionized and overaged form, the SME strain and thermal hysteresis increase with applied stress [18,21]. Large hysteresis in NiTi originates from the partial accommodation of martensitic transformation with dislocations instead of internal twins and elastic distortion of the matrix [21,22].…”

“…Large hysteresis in NiTi originates from the partial accommodation of martensitic transformation with dislocations instead of internal twins and elastic distortion of the matrix [21,22]. When the stress levels are increased, dislocation formation becomes easier and thermal hysteresis increases.…”

Tensile deformation of NiTi wires

“…As such, detwinning is a process that most often occurs subsequent to SIM or MR, at least at the microstructural scale. 45 The above three inelastic strain mechanisms are grounded in crystallographic theory 46 and are necessary to fully explain the stress-strain response of NiTi. Of course, non-recoverable inelastic strain mechanisms such as dislocation generation and motion are also available to NiTi shape memory alloys under appropriate conditions of material microstructure, applied stress, and temperature.…”