In situSEM studies of the transformation sequence of multistage martensitic transformations in aged Ti-50.8 at.% Ni alloys

The Influence of Thermal History on the Multistage Transformation of NiTi Shape-Memory Alloys

“…The microscopic description of the multistage MT together with the identification of the different phases have already been studied in previous works [12,15,23]. Very similar complex phase sequences, close to the one presented here, have been found in the works by G. Fan et al [19] and B. Karbakhsh Ravari et al [16,41]. In the latter study, up to a four-step transformation process was found in the forward MT of the samples after very similar thermal treatments, involving the same annealing temperature and time (950 • C, 1 h) and aging pattern (500 • C, 1 h), but no systematic thermal cycling seems to have been performed.…”

Effect of Thermal Cycling on Multistage Martensitic Transformation in Aged Ti–50.8 at% Ni Alloy

“…This result is consistent with previous reports. 16,18,25) M1* and M2* decrease slightly with increasing the number of thermal cycles. The decreases, ¦M1* and ¦M2*, amount to about 6 K, whereas ¦M3* is about 23 K after 100 thermal cycles.…”

“…There appear to be four exothermic peaks and three endothermic peaks in the DSC cooling and heating curves, respectively, even after 100 thermal cycles. Based on the partial DSC cycling experiments and in-situ SEM observations in the author's previous study, 16) the R* and AR* peaks are attributed to the R-phase transformation and its reversion. Similarly, the M1* and A1* peaks correspond to the first martensitic transformation and its reversion.…”

“…Details such as heat treatment atmosphere are described elsewhere. 6,11,15,16) Some specimens were aged at 773 K for 3.6 ks and then quenched into ice water. The solution-treated and aged specimens were subjected to thermal cycling in the temperature range between 77 and 373 K by alternately dipping them into liquid nitrogen and boiling water for 1 min at a time.…”

“…7,9,14) The present authors have previously reported the transformation sequence during the quadruple-stage transformation in Ti50.8 at% Ni, as determined by in-situ SEM. 16) In the quadruple-stage transformation upon cooling, the R-phase transformation occurred in the intermediate and grain boundary regions. The first martensitic transformation, which corresponded to the M1 peak in the DSC cooling curve, took place in the intermediate region of the grain via the R phase.…”