Comparative Analysis of Process-Induced Strain Glass States in Austenitic and Martensitic NiTi Shape Memory Alloy Plates

Strain glass alloys (SGAs) are metallic alloys with martensitic nano-domains within a crystalline material, which occur from process- or compositionally-induced strain. SGAs originate from shape memory alloys (SMAs), and exhibit similar shape memory properties and high actuation densities. The transition from SMA to SGA is relatively unexplored, and there is much to discover. This transition occurs by interrupting the long range martensitic order, which in turn disrupts the martensitic transformation, resulting in short range martensitic order. A glassy martensitic phase is produced that has been shown to exhibit enhanced structural and load bearing abilities, functional stresses, and recoverability.

In this study, the transformation from SMA to SGA is explored in Ni_49.5Ti_50.5 and Ni_50.8Ti_49.2 (at. %) SMA plates to compare martensitic versus austenitic SGAs, respectively. SMA samples were cold worked in 5% increments until a strain glass transition occurred. Characterization of the samples were examined via differential scanning calorimetry (DSC), Vickers hardness, transmission election microscopy (TEM), and synchrotron radiation x-ray diffraction (SR-XRD). Some of the prominent characteristics such as enthalpy peaks, twin size reduction, and crystallographic structure between the two plates were examined and compared to improve the understanding of the SMA to SGA transition.