The shape memory effect in equiatomic TaRu and NbRu alloys

The shape recovery exhibited during reheating the three point bend test samples of TaRu and NbRu is the first demonstration of the shape memory effect in these two alloy systems. This shape memory effect is due to the high temperature, tetragonal-to-cubic (β1 to β) phase transition which occurs at 1120°C for Ta50Ru50 and 885°C for Nb50Ru50. The phase diagrams (Figure 1) show this transition to extend through a range of compositions from approximately 40% to 55% ruthenium, with transition temperatures extending from near room temperature to more than 1000°C. In addition, whereas commercial shape memory alloys are based on metastable phases, these ruthenium alloys are based on equilibrium phases and transitions. As such, they are not subject to the overaging effects caused by decomposition or precipitation which can degrade the performance of the shape memory alloy. Both TaRu and NbRu exhibited significant strain recoveries in these initial tests. Reheating samples deformed in bending produced a strain recovery on the tension surface of 5% for TaRu and 4% for NbRu. The lower (2%) strain recovery exhibited by the TaRu compression sample may have been due to the very large grain size of these initially-prepared samples. The average grain size of these samples varied considerably, but was typically larger than 1 mm. This large grain size may also contribute to the limited ductility observed in the bend tests.