Isothermal and athermal martensitic transformations in the B2–R–B19′ sequence in Ni–Ti shape memory alloys

The kinetics of accumulation of martensite/austenite during the direct/reverse two-stage B2–R–B19′ martensitic transformation in NiTi has been studied by means of resistance measurements during interruptions of cooling/heating scans. Experimental results indicate that the R–B19′ transformation is isothermal, whereas both direct and reverse B2–R and R–B2 transformations are perfectly athermal. The isothermal accumulation of the R-phase is also detected during B19′–R transformation. A good correspondence exists between the rate of the isothermal transformation and the transformation rate in an uninterrupted cooling cycle.