Microstructure and mechanical properties of Ru-base ultra-high temperature SMAs

The need for ultra-high temperature shape memory alloys (SMAs), particularly higher than 500ºC, for aerospace and automotive industries has driven the study of Ru-based alloys, specifically Ru-Nb and Ru-Ta systems, which can exhibit the shape memory effect up to 1100ºC and 1400ºC, respectively. These systems undergo a multistep, reversible phase transformation path from the high temperature cubic phase (beta) to an intermediate tetragonal phase (beta’) and finally to a monoclinic phase (beta”), each contributing to the shape memory effect along its path. In this work, a set of stoichiometric binary alloys Ru-xNb, Ru-xTa (x=50-55at. %), and a set of ternary alloys 50Ru-xNb-(50-x)Ta (x=50, 40, 25, 10, 0) were produced via vacuum arc melting in argon. The ternary compositions were produced to examine how substitution between Nb and Ta affects the transformation temperatures and paths of each alloy and their ensuing properties for different actuator application in extreme-temperature environments.

Microstructural characterization using optical, SEM and TEM techniques revealed very distinct and well-defined straight fine twins within grains in the beta” phase, along with anti-phase boundaries in the twinned regions in between the twin boundaries. The beta” crystal structure was monoclinic with an orientation relationship with the tetragonal structure from which it transformed. Stress-free transformation temperatures and uniaxial constant force thermal cycling responses were measured across key temperature regimes. The results show that Ru-based SMAs exhibit stable high-temperature transformations and promising functional properties.