Shape recovery in high temperature shape memory alloys based on the Ru-Nb and Ru-Ta systems

Abstract
This work investigates the high-temperature behaviour of three RuTa alloys and three RuNb alloys. The first step was to determine how Ta or Nb content affects the MT temperatures. The monoclinic Ru₅₀Ta₅₀ and Ru₅₀Nb₅₀ alloys undergo two successive displacive transformations from the high temperature β phase field: β (B2) → β’ (tetragonal) → β” (monoclinic) whereas Ru₄₅ Nb₅₅ , Ru₄₅Ta₅₅ , Ru₄₃Nb₅₇ , and Ru₄₃Ta₅₇ exhibit a single transition from cubic to tetragonal on cooling. All alloys exhibit a highly twinned microstructure with a (011) compound twinning mode. The main feature of the β’ → β” transformation is the formation of domains boundaries separating translation variants instead of formation of new twin separating orientation variants. The shape memory effect was studied through compression tests performed in the β’ or β” phase. The total shape recovery is mainly due to the β’ → β transformation and appears to decrease from about 3% for the monoclinic alloys to about 0.1% for alloys with 43% Ru in accordance with the evolution of the lattice parameters of martensites.