Ni-rich NiTi shape memory alloys (SMAs) are capable of attaining a wide range of transformation temperatures depending on the heat treatment conditions and superior thermo-mechanical cycling stability, which are desired for repeated solid-state actuation. High Ni-content Ni-rich SMAs have very low transformation temperatures in a solutionized condition due to the high Ni-content of the matrix. Slow cooling (furnacecooling) from solutionizing temperature and additional aging heat treatments result in the formation of Ni-rich precipitates such as Ni4Ti3, Ni3Ti2 and Ni3Ti and increase transformation temperatures above ambient by depleting excess Ni from the matrix. However, the precipitates do not undergo a martensitic phase transformation and they decrease the transformation strain by reducing the volume fraction of the material capable of transforming. Meanwhile, recent preliminary work shows that Ni3Ti precipitates dominate fatigue failure. The objectives of the present study are: (1) to eliminate Ni3Ti but still have Ni4Ti3 precipitates, which are responsible for the dimensional stability and increase transformation temperatures, (2) to investigate the effect of heat treatments on the transformation strain, and (3) to select single variant Ni4Ti3 precipitates through constrained aging for the formation of oriented internal stress and eventually obtain twoway shame memory effect (TWSME) and enhanced dimensional stability. Based on these objectives, the effect of aging heat treatment on transformation temperatures, microstructural evolution, and shape memory behavior were investigated for a Ni52Ti48 shape memory alloy (SMA) by using differential scanning calorimetry (DSC), optical microscopy, scanning electron microscopy (SEM), and thermo-mechanical testing, including isobaric heating-cooling experiments under various stress levels. It was observed that solutionizing at 900 degree C for 24 hours eliminated Ni3Ti type precipitates, but additional aging heat treatments are needed to form Ni4Ti3 precipitates to increase transformation temperatures. Furnace-cooling and additional aging heat treatment results in the multi-stage martensitic transformation due to chemical and stress inhomogeneities in the microstructure. Aging of the controlled furnace-cooled material at 400 degree C for 48 hours resulted in the highest transformation temperatures among all processing conditions investigated due to the combination of Ni3Ti precipitates and 27 percent volume fraction of the Ni4Ti3 precipitates, which led to the depletion of Ni from the transforming matrix. However, since overaging results in losing coherency of the precipitates, dimensional stability during isobaric thermal cycling was negatively impacted.
