Shape Memory Behavior of Ni49.5Ti50.5 Processing-Induced Strain Glass Alloys

Due to their high actuation energy density and ability to sustain large deformations, shape memory alloys (SMAs) have many potential engineering applications, such as solid-state actuation, energy damping, and shape recovery upon deployment. While SMAs have been studied extensively since the discovery of NiTiNOL in 1959, the effects of restricting the reversible martensitic transformation to nanodomains is not well-understood and is yet to be utilized in engineering applications, despite showing promising multifunctional properties. In binary NiTi, the martensitic transformation, characterized by long-range strain ordering, can be replaced with a strain glass transition. Such alloys have been named strain glass alloys (SGAs) due to the fact that they exhibit a glass-like state which is caused by compositionally- or processing-induced strain. In the present study, cold work is used to produce a strain glass phase in bulk, Ti-rich NiTi specimens, and their bulk shape recovery properties are analyzed. In addition to showing reduced dimensional instability as well as increased energy density, the processing-induced SGA shows an order of magnitude less strain localization relative to the SMA material system from which it was developed. For this research, bulk thermomechanical testing, digital image correlation (DIC), differential scanning calorimetry (DSC), synchrotron X-ray diffraction (SR-XRD), and transmission electron microscopy (TEM) were utilized to characterize a processing-induced SGA which was developed from commercially available Ni49.5Ti50.5 (at.%).