Step-wise R phase transformation rendering high-stability two-way shape memory effect of a NiTiFe-Nb nanowire composite

In this work, we designed a NiTiFe-Nb nanowire composite to achieve a high-stability and small-hysteresis R phase two-way shape memory effect (TWSME). The R-phase TWSME was achieved in the composite in an annealed state without thermomechanical training due to the inherent internal stresses associated with the Nb nanowires, as demonstrated by in situ synchrotron high-energy X-ray diffraction. Transmission electron microscopic analysis also revealed some details of the R-phase transformation process, which render an explanation of the high cyclic stability of the R-phase TWSME. The R-phase transformation in this NiTiFe-Nb nanowire composite was found to proceed over several stages, including the formation of precursor nanodomains of partial lattice distortion for the R phase, the formation of R phase particles of coordinated orientations, the coalescence of the R phase particles into selected plate variants of preferential orientations for TWSME, and the continued evolution of the R phase crystallographic structure for further TWSME with zero hysteresis. Such step-wise process of the R-phase transformation divides the total TWSME strain into segments of smaller magnitudes in the macroscopic behavior, and more importantly reduces the discrete lattice distortion mismatch at the transformation interface on the microscopic scale. This drastically reduces the chances of generating dislocations during the transformation process, thus rendering the high cyclic stability of the R-phase TWSME. Such intricate behavior may be closely related to the unique NiTiFe matrix – Nb nanowire microstructure and its unique inherent internal stress condition.