Phase transformation in NiTi shape memory alloy Under Thermomechanical conditions, pp. 317-338
Authors: Manjunatha Pattabi and K. Ramakrishna
Abstract: The effects of thermal cycling and mechanical operations like cutting and polishing on the phase transformation behavior of heat-treated NiTi shape memory alloy (SMA) have been investigated by means of Powder X-Ray Diffraction (XRD) and Differential Scanning Calorimetry (DSC). The results of studies carried out on the effect of thermal cycling on SMA heat-treated at 660 oC and effect of mechanical cutting and polishing on SMA heat-treated at 560 and 660 oC, are presented. In NiTi alloy heat-treated at 660 oC, the integrated peak area (integrated intensity) corresponding to (110)A plane of the austenitic phase increases and the area corresponding to martensitic phase decreases with increasing temperature. Austenitic start and finish temperatures change during the first few thermal cycles. The former decreases by 15 oC and the latter decreases by 35 oC during the first cycle, whereas martensitic start and finish temperatures do not get affected by thermal cycling. The transformation characteristics determined through XRD are compared with those obtained from DSC. The thermal width observed from XRD studies is larger than that from DSC measurements due to the better sensitivity of the former in detecting smaller volume fractions of the phases in the martensitic transformations as compared to the thermal methods like DSC. Mechanically cut and polished samples show broad thermal width and lesser heat flow associated with the M↔A transformation compared to chemically etched samples due to the internal stresses generated during mechanical operation. Among the samples heat-treated at 560 oC, the sample subjected to mechanical cutting and polishing (HT560MC) shows a single step M→A on reverse phase transformation and two step A→R→M on forward transformation whereas chemically etched sample (HT560CE) shows the intermediate R-phase both during forward and reverse transformations. In the case of HT560MC, the dislocations created by the mechanical cutting and polishing may be responsible for the non-appearance of R-phase in the reverse transformation even up to 150 cycles, possibly due to the entanglement of dislocations at high density of dislocations. Heat treatment at a higher temperature of 660 oC does not facilitate the formation of R-phase in the reverse transformation due to the reduced dislocation density. However, dislocations created due to mechanical operations (in HT660MC) help the formation of R-phase to a very small extent during forward transformation. Both the heat-treated samples show the effect of mechanical cutting and polishing even at 150th thermal cycle.