High Strain Rate Deformation and Microstructure Of Austenitic NiTi & Nitife Shape Memory Alloys

Shape memory alloys (SMAs) are used widely in the aerospace, automotive, and medical industries. While the quasi-static compressive mechanical properties of SMAs have been thoroughly examined, little research has been focused on dynamic compressive mechanical behavior. Recently, there has been growing interest in using shape memory alloys for applications involving high strain rate deformation. Unlike the behavior of conventional materials used for high-rate applications, pseudoelasticity observed in SMAs allows for much larger recoverable strains (~ 6%) due to a unique stress-induced martensitic phase transformation. In the case of dynamic compressive loading, the transformation stresses are more sensitive to the latent heat of transformation and heat of deformation, since this energy cannot be dissipated in such a short time interval (i.e. the typical loading duration for high strain rate experiment is 50-1000 microseconds). Furthermore, plasticity or work hardening must also be considered during dynamic compressive loading. While some high strain rate studies on NiTi-based SMAs have been performed, there has not been a systematic study of the relationship between microstructural evolution and high strain rate during compressive loading and unloading. In this paper, we present a study on 0.25” diameter rod of austenitic pseudoelastic NiTi and NiTiFe SMAs at both quasi-static and dynamic compressive loading and unloading. The specimen deformation under high-strain rate loading is precisely controlled using a single-loading momentum trap technique. The microstructure of the specimens deformed (and then recovered) at different strain rates and strain levels are analyzed to find the observed mechanical behavior and microstructure.