Phenomenological analysis of martensitic transformation in cold-rolled TiNi-base shape memory alloys

The martensitic transformation in cold-rolled TiNi-base shape memory alloys has shown that the first reverse martensitic transformation temperature (A1∗) increases with an increasing amount of cold rolling due to the increment of the transformation energy barrier introduced by cold-rolling induced dislocations. Meanwhile, the increment of A1∗ temperature increases linearly with an increase in the specimen’s solution-treated hardness. A phenomenological analysis from the thermodynamic viewpoint of cold-rolling induced dislocations can elucidate these features successfully. After the first reverse transformation, the strengthening effects of retained dislocations on martensitic transformation temperatures follow the equation M∗=TO−KΔσys. The K value is found to be proportional to the specimen’s solution-treated hardness, because the harder specimen has the stronger interaction of martensite plates and dislocations induced by cold rolling. The thermal hysteresis of A2∗−M∗, associated with the frictional work during the transformation, does not obviously change in the cold-rolled TiNi-base shape memory alloys.