Thermomechanical Behavior of NiTi-8Hf Low-Temperature Shape Memory Alloys

The actuation response of NiTi-8Hf alloys with low transformation temperatures was investigated. Four alloys with Ni-rich compositions of Ni50.3Ti41.7Hf8, Ni51Ti41Hf8, Ni51.5Ti40.5Hf8, and Ni52Ti40Hf8 (at.%) were melted, and subjected to several aging heat treatments between 450 and 550 degrees C. Transmission electron microscopy revealed the presence of H-phase precipitates, which varied in shape and size as a function of Ni content and aging condition. When aged at 550 degrees C for 3 h, the lowest Ni-containing alloy showed a preferential heterogeneous distribution of the H-phase precipitates with elongated lenticular shapes, while higher Ni resulted in a uniform distribution of less elongated particles. Uniaxial constant force thermal cycling experiments were used to reveal the shape memory properties of each alloy. In many aged conditions, no transformation peaks were observable in the DSC data. This absence of peaks in the DSC is attributed to the compounded effects of suppressed martensitic transformation due to higher off-stoichiometric Ni, and the strain fields around the precipitates that prevent transformation under stress-free cycling. However, these effects can be overcome by undercooling or thermally cycling under an applied stress. Thus, fully developed hysteresis loops were observed in the thermomechanical data for all aging conditions. The shape memory effect in compression showed maximum recoverable transformation strains of 3.7% in the lowest Ni containing alloy, which decreased with the addition of Ni to 1.7% in the 52Ni alloy. Inversely, the dimensional stability improved with added Ni, showing nearly zero residual strain in the higher Ni-containing alloys for stresses as high as 1 GPa.