On phase transition velocities of NiTi shape memory alloys

Two important velocities of austenitemartensite phase transformation in NiTi shape memory alloys (SMAs) have been experimentally investigated in this work: the velocity of stress-induced martensitic transformation upon mechanical loading and the recovery velocity for thermally driven and mechanically detwinned martensitic phase to transform back to austenite upon heating. To measure the velocity for stress-induced martensitic transition, a series of uniaxial compressive loading tests have been performed for both quasi-static and dynamic loading conditions, using an Instron servohydraulic testing machine and enhanced Hopkinson bar facilities, respectively. A pulse shaper has been employed during the Hopkinson bar dynamic loading test to obtain constant loading and unloading strain rates. According to the testing results, the velocity limit for a stress-induced martensitic transition in samples NiTi-A can be measured as the highest observed deforming velocity that allows pseudoelastic behaviors. It is also found that martensitic transformation stress,δ_tr, tends to approach to a constant value when strain rate is above 0.1/s. To understand the thermally driven recovery velocity for reverse martensitic transformation, however, samples of NiTi-B, in martensitic twins, are firstly detwinned by uniaxial loading at room temperature, and then heated to recover their austenite phase. Multi-point temperature increase tests and fixed temperature tests are performed. It is observed that the recovery velocity is approaching to an upper bound of 0.014 mm/s. A few other noteworthy observations have been obtained as well.