Processing-Induced Strain Glass Alloys in NiTi-based Shape Memory Alloy Material Systems

Shape memory alloys (SMAs) represent a revolutionary and innovative class of active materials which can provide potential solutions to many of today’s engineering problems due to their compact form, high energy densities, and multifunctional capabilities. Over the past several decades, many applications in the aerospace, automotive, biomedical, and civil engineering industries have been developed for Nickel-Titanium (NiTi) based SMAs. However, the effects of restricting the martensitic twins to nanodomains has not been thoroughly characterized or utilized in applications. The martensitic transformation present in near-equiatomic NiTi material systems can be replaced with a strain glass transition through compositional changes or residual plastic deformation. These resulting materials have been termed strain glass alloys (SGAs) due to the fact that they exhibit a glass-like, low temperature state. Recent efforts by the authors show that sufficient processing, e.g. cold work, can induce strain glass states in Nickel-lean NiTi SMAs. In the present study, processing-induced SGAs have been developed from both Nickel-rich and Nickel-lean NiTi SMAs. The Nickel-lean material system (Ni_49.5Ti_50.5 at. %) exhibits ferroelastic recovery well above room temperature (80°C), while the Nickel-rich material system (Ni_50.8Ti_49.2 at. %) exhibits a superelastic response at room temperature. Cold rolling and drawing were utilized to induce sufficient dislocations to suppress the martensitic transformation at zero stress. The resulting SGAs and the base materials from which they were developed were characterized via scanning/transmission electron microscopy (S/TEM), bulk thermomechanical testing with digital image correlation (DIC), differential scanning calorimetry (DSC), and synchrotron radiation x-ray diffraction (SR-XRD).