Phase Transformation and Shape Memory Effect in Ru-Based High Temperature Shape Memory Alloys

Among the different systems for high temperature shape memory alloys (SMA’s), equiatomic RuNb and RuTa alloys demonstrate both shape memory effect (SME) and MT temperatures above 800°C. For both systems, it is interesting to find a way to control the transformation temperatures while keeping the shape memory effect. One way to change the transformation temperatures is to change the composition in the binary alloys; another is to add a ternary element like Fe. The eight investigated alloys show two different space groups at room temperature. The monoclinic alloys undergo two successive displacive transformations on cooling, starting from the high temperature β phase field: β (B2) à β’ (tetragonal) à β’’ (monoclinic). The tetragonal alloys exhibit a single transition from cubic to tetragonal. A multiple twinned microstructure can be found in all alloys. Transformation temperatures decrease with lower Ru content and with the addition of Fe. The β’ à β transformation seems to be the main responsible for the SME. Compression tests performed in the martensitic phase give a quantitative result of the shape memory effect. In the binary alloys, the SME decreases with decreasing Ru content, which is in accordance with the evolution of the lattice parameters of martensites. A lower SME in the ternary alloys can also be linked to the lattice parameters and seems to be quite reliable to predict the evolution of the shape memory effect.