Improvement of NiTi Shape Memory Actuator Performance Through Ultra‐Fine Grained and Nanocrystalline Microstructures

Abstract

Due to their combination of good thermomechanical properties, corrosion resistance, and biocompatibility, nearstoichiometric NiTi alloys are the shape memory alloys (SMAs) that have had the greatest commercial success. [1][2][3][4][5] Their shape memory effect is derived from a diffusionless transformation in the solid state known as the martensitic transformation. On cooling from high temperatures, the material undergoes a transformation from the austenite phase to the martensite phase that begins at the martensite start temperature (M S ) and is completed at the martensite finish temperature (M F ). Upon reheating, the reverse transformation occurs between the austenite start temperature (A S ) and the austenite finish temperature (A F ). Martensitic transformations are typically associated with a thermal and/or mechanical hysteresis: [6][7][8][9] The forward transformation starts at a lower temperature (in the case of a thermal transformation) or at a higher stress (in the case of a stress-induced transformation) than the reverse transformation. Upon heating or unloading, the martensite begins to transform back to austenite at a relatively higher temperature than was required to induce the transformation upon cooling or mechanical loading of the material. [6][7][8][9] Ball and James [6,7] and others [8,9] have shown that the width of the hysteresis depends on the crystallographic compat- COMMUNICATION[*] Dr.

On the widths of the hysteresis of mechanically and thermally induced martensitic transformations in Ni–Ti-based shape memory alloys

“…After melting and casting the materials were subjected to thermo-mechanical treatments (wire drawing, rolling). All details of our ingot metallurgy processing route have been described in the literature [24,28,31,32]. The compositions of the materials which were investigated in the present study are summarized in Table 1.…”

Thermomechanical fatigue of post-weld heat treated NiTi shape memory alloy wires

“…[17]. The large reduction in grain boundary area, dislocations, precipitates and other structures that limit transformation induced plasticity make these coarse grained structures less stable than the ideal nanocrystalline NiTi base metals during thermomechanical cycling [21]. While being a small region of the wire, the substantially different properties of the welded region may have a significant effect on the performance of the welded assembly.…”

“…The sole investigation on a small number of thermal cycles of a welded SMA involved a two-way shape memory effect actuator, which concluded that the effect on the actuator performance was minimal [19]. The same cannot be concluded for the cycle life, which has been shown to be greatly affected by coarse grained microstructures like those in the heat affected and fusion zones of the welded wires [16,17,[20][21][22].…”

In Situ Neutron Diffraction Studies of Increasing Tension Strains of Superelastic Nitinol

“…The effects of pre-strain or ''training'' has long been recognized in the shape-memory actuator field [9][10][11][12][13][14][15]. These references indicate that there are optimal combinations of thermomechanical processing with subsequent prestraining (straining over that of the application) that tends to stabilize the microstructures for either stress or strain cycling.…”