摘要
Recently the isothermal martensitic transformation in shape memory alloys(SMAs)has been reported in many literatures,and several models have been proposed to interpret the isothermal and athermal kinet-ics.However,the underlying mechanisms remain inadequately understood.In this work,the isothermal transformation from B2 to B19 is confirmed in Ti-Ni-Cu-Co melt-spun ribbons at the temperature range between M s and M f.It reaches a saturation point at every isothermal temperature T iso,and the saturation points correspond to the f−T curve at the cooling rate of 0.5 K/min.The experimental results indicate that the isothermal accumulation of martensite is a relaxation process from the transient state to the thermoelastic balance one.A thermally activated kinetic model is developed in this study to characterize the isothermal and athermal kinetics.The model is able to estimate the evolution of martensite volume fraction under any temperature path T(t)and it agrees with the experimental results well.According to the model,the effects of elastic energy,nucleation density,and activation energy on the kinetics are in-vestigated.Among those,a small nucleation density n i as well as a large activation energy Qwill result in a significant isothermal transition.In this work,the slighter isothermal effects originate from the higher value of n i.As for the non-stoichiometric SMAs,the higher value of Qis responsible for the accumula-tion of martensite at the isothermal process.Accordingly,the present work provides a novel view from a kinetic model to understand the isothermal martensitic transformation in SMAs.
基金
the National Natural Science Foun-dation of China(Nos.51931004 and 51871080).
