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Phase transformation in NiTi shape memory alloy Under Thermomechanical conditions, pp. 317-338 $100.00
Authors:  Manjunatha Pattabi and K. Ramakrishna
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. 

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Phase transformation in NiTi shape memory alloy Under Thermomechanical conditions, pp. 317-338