Low temperature aging behaviour of transformation temperatures in some Cu-based and NiTi SMA

Abstract
Effects of aging at intermediate temperatures (near 400°C) in SMA have been found to produce precipitation, which affects the transformation behaviour. In this work, we observe some dependences (recoverable or not) of transformation temperature with aging at temperatures near or under 100°C in beta phase. Two copper-based SMA alloys have been analyzed, CuZnAl and CuAlBe. Also, some analysis has been done on pseudo-elastic NiTi. By aging at temperatures near 100°C, the two copper alloys show changes in transformation temperatures, which can be related univocally to the changes in the temperature at which the alloy is kept. In both alloys, by staying long time at a given temperature, an asymptotic value of the transformation temperature is reached, and further coming to the same temperature produces the transformation temperature to evolve to that asymptotic value. The maximal amplitude of the changes is about 16% of the amplitude of the change in aging temperature. The changes are much faster in CuZnAl than in CuAlBe, and the sign of the changes is opposite in CuAlBe respect to that in CuZnAl. The process can be well modelled by a system of differential equations with one or two terms, giving a "tracking" of the transformation temperature respect the temperature at which the alloy is kept. The activation energies for the time evolution suggest atomic-order related processes as cause for the transformation temperature changes. For NiTi, the evolution is much slower, and seems to be monotonic with time, suggesting a precipitation-related mechanism as the origin of the transformation temperature changes, even at temperatures near 100°C. From the data, some conclusions may be drawn concerning long-time effects on the applications of the alloys