Phase transformation behavior of porous NiTi alloys fabricated by capsule-free hot isostatic pressing

Differential scanning calorimetry (DSC) was used to characterize the phase transformation of porous NiTi alloys. Results revealed that there existed a multi-stage martensitic transformation on cooling while either a single or two-stage transformation on heating. The DSC analysis indicated that the cold compaction pressure had great effect on the transformation temperatures. The transformation temperatures of NiTi with lower compaction pressure were higher than those of NiTi compacted at higher pressure. With increase in the annealing time, the transformation temperatures quickly increased for those alloys compacted at 150 MPa. However, there was only little change in the transformation temperatures for those compacted from 300 to 400 MPa. The change in the transformation temperature can be attributed to the combined effect of high dislocations density and precipitation of the second phase.