Effect of indentation temperature on nickel-titanium indentation-induced two-way shape-memory surfaces

Extended Cyclic Deformation Recovery of the Indentation‐Induced Two‐Way Shape‐Memory Effect in Nickel–Titanium

“…A handful of studies have expanded the understanding of the surface induced TWSME through investigations into both spherical indentation, and sharp indentation, as well as alternative methods of sub‐surface plastic deformations including laser shock peening, pulse electrochemical machining, and embossing . These investigations have typically quantified the amount of recovery as a function of the change in the depth relative to the low‐temperature depth.…”

“…These factors may also lead to the formation of stabilized martensite. Results from recent studies strongly suggest that the dislocation‐stabilized residual martensite remains at temperatures well into the austenite phase, which may assist in selectively guiding preferred martensitic variants during the reverse transformation …”

“…Previous studies have specifically illustrated the importance of the indentation temperature and the upper cycle temperature (UCT) for optimization of the indentation‐induced TWSME in the first few cycles. Results in Brinckmann et al display the importance of indenting the material fully in the martensitic phase to improve the TWSME recovery . This is a reflection of the Clausius‐Clapeyron relation, where at higher temperatures it becomes more difficult to activate martensite transformation and reorientation.…”

“…To date, only a limited number of studies have explicitly stated making observations of the surface TWSME over more than a handful of cycles, with 25 cycles being the largest number explicitly stated in the literature . In addition, these publications have not investigated the effects of indentation variables or over thermal cycling; even though results have shown over the short term both can produce diminished TWSME values when the material is indented at temperatures above the austenite finish temperature and recovered to temperatures around 100° above the austenitic finish temperature . Therefore, this study will independently vary the indentation temperature and then thermally cycle for up to 1000 cycles at systematically varying UCTs.…”

“…The goal is to provide insight into physical limitations for a reliable long‐term TWSME recovery. Because the amount of recovery observed is not only a function of the indentation constraints, but also the amount of material removed during the planarization process, all subsequent observations in the present study were based solely on indentations; thus limiting the number of variables. However, results of the experiments here should extend into cases followed by the subsequent planarization process as the underlying microstructural mechanisms remain the same.…”