Abstract
High speed high pressure torsion (HSHPT) is a novel severe plastic deformation technique that is used to produce bulk ultrafine-grained nickel-titanium shape memory alloys. In this study, the effect of grain refinement on phase transformation was investigated in a near equiatomic NiTi shape memory alloy subjected to processing by this technique. Phase transformations involving different degrees of deformation and stability of thermally-induced phase transformations were analyzed by differential scanning calorimetry (DSC). The measurements suggest that the martensitic transformation occurred even when the highest degree of deformation was applied. Optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) investigations bring to light that the true strain applied controls the evolution of the microstructure. The results are presented and discussed in detail in this paper.
Highlights
NiTi-based alloys have numerous characteristics that are in common with other shape memory alloys (SMAs), such as superelasticity, shape memory effect, two-way shape memory effect, etc., they have certain unique characteristics, such as low elastic anisotropy, one of the reasons for their incredibly high ductility [1], that are different from them
It induces refinement of grains size, the temperature in the sample rises from room temperature up to ~800°C
severe plastic deformation (SPD) processing by High speed high pressure torsion (HSHPT) technique leads to significant grain refinement in a nickel-rich Ni-Ti shape memory alloy
Summary
NiTi-based alloys have numerous characteristics that are in common with other shape memory alloys (SMAs), such as superelasticity, shape memory effect, two-way shape memory effect, etc., they have certain unique characteristics, such as low elastic anisotropy, one of the reasons for their incredibly high ductility [1], that are different from them. As the most popular functional material with excellent mechanical properties and functional fatigue resistance they have captivated the attentions of the researchers in recent times [2, 3] Due to their properties, such as fast response to temperature changes, narrow hysteresis, corrosion resistance and biocompatibility, near equiatomic NiTi alloys are a very promising candidate for myriad practical applications in medicine and engineering [4,5,6,7,8]. High pressure torsion (HPT) has received considerable attention from researchers considering that it is an SPD method that permits significant grain refinement, without the concomitant cracking involved in processing [16,17,18,19]. The reduction in grain size takes places accompanied by an increase in the density of dislocations disposed into equilibrium sub-grains or nonequilibrium grain boundaries [16, 20]
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