Nanostructured Titanium Nickelide: Realization of abnormally high recovery strain

Abstract

The paper focuses on the study of the shape memory effect (SME) and two-way SME (TWSME) in nanostructured Ti-50.7 at.%Ni alloy. Two different types of structure were studied: nano-subgrained structure (annealing after the moderate deformation with true strain e = 0.6) and nanocrystalline structure (annealing after the severe plastic deformation with true strain e = 1.55). A homogenizing annealing at 700°C for 20 minutes served as a reference heat treatment (recrystallized structure of austenite). The SME training procedure was carried out in bending under load using eight various isothermal and non-isothermal modes covering all phase states and their combinations. The maximum recovery strain (εr = 14.7%) in Ti-50.7%Ni alloy is provided by the nano-subgrained structure with the grain size ≤ 5 pm obtained as a result of 10 h-aging after deformation with true strain e = 0.6, loading in the R-phase state with subsequent cooling in the loaded condition through R→B19' transformation under loading strain εt = 15,7 %. The maximum TWSME value εtw = 3.5% is provided by the loading in the R-phase state and further unloading.