Abstract
Microstructural evolution of NiTi shape memory alloy (SMA) with a nominal composition of Ni50.9Ti49.1 (at %) is investigated on the basis of heat treatment and severe plastic deformation (SPD). As for as-rolled NiTi SMA samples subjected to aging, plenty of R phases appear in the austenite matrix. In terms of as-rolled NiTi SMA samples undergoing solution treatment and aging, Ni4Ti3 precipitates arise in the austenite matrix. In the case of as-rolled NiTi SMA samples subjected to SPD and aging, martensitic twins are observed in the matrix of NiTi SMA. With respect to as-rolled NiTi SMA samples subjected to solution treatment, SPD, and aging, neither R phases nor Ni4Ti3 precipitates are observed in the matrix of NiTi SMA. The dislocation networks play an important role in the formation of the R phase. SPD leads to amorphization of NiTi SMA, and in the case of annealing, amorphous NiTi SMA samples are subjected to crystallization. This contributes to suppressing the occurrence of R phase and Ni4Ti3 precipitate in NiTi SMA.
Highlights
NiTi shape memory alloys (SMAs) have been widely used in aviation, medical, dental, and automotive fields because of their excellent abrasion resistance, good functional properties, and high mechanical properties [1,2]
It has been proposed that the existence of Ni4 Ti3 precipitates contribute to the emergence of the R phase, which plays a significant role in multiple-stage transformation of SMAs
The grain boundary energy plays an important role in accelerating the nucleation of Ni4 Ti3 precipitates at the grain boundary
Summary
NiTi shape memory alloys (SMAs) have been widely used in aviation, medical, dental, and automotive fields because of their excellent abrasion resistance, good functional properties, and high mechanical properties [1,2]. Solution treatment along with aging results in the precipitation of Ni4 Ti3 phase, which further contributes to the occurrence of two-stage transformation, three-stage transformation, or even four-stage transformation of NiTi SMAs [6,7,8,9]. It has been proposed that the existence of Ni4 Ti3 precipitates contribute to the emergence of the R phase, which plays a significant role in multiple-stage transformation of SMAs. Severe plastic deformation (SPD) methods, such as high pressure torsion (HPT) [10,11], cold drawing [12,13], cold rolling [14], surface mechanical attrition treatment (SMAT) [15], and local canning compression [16], can lead to amorphization of NiTi
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