Stress-Induced Shape Changes and Shape Memory in the R and Martensite Transformations in Equiatomic NiTi

Abstract

Equiatomic NiTi wire was cooled below T_R, the critical temperature for theB R transition, and then stressed in situ in a custom-built X-ray diffraction stage to study the shape memory phenomenon. Tensile stressing the specimen causes a shifting of X-ray intensity from l_R to -l_R. This is rationalized in terms of domain growth under external stress, resulting in a preferred arrangement of domains in theR -phase, which dimensionally accommodates the applied force. A recoverable strain of ~1.37 pet is accommodated by theR-phase, 0.56 pct of which is recovered immediately after the stress is removed and the remaining 0.81 pct on heating to above T_R. This amount of strain is rationalized in terms of the difference in d-spacings and multiplicity factors between {111}_R and {-111}_R. The springback is primarily associated with the reversal of domain alignment while the shape memory on heating is primarily due to the return of the phase to cubic symmetry. Straining beyond 1.37 pct induces stress-assisted marteniste formation up to 5.47 pct Δl/l, the maximum strain achieved in this series of experiments. This results in a second stage of shape recovery on heating through theA, — A_f temperature range. Only 15 vol pct of stress-assisted martensite accounts for nearly all of the additional ~4 pct change in Δl/l. This emphasizes the important role of the martensitic transformation in achieving large changes in macroscopic length in the shape memory phenomenon.