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A Ni50.3Ti41.7Hf8 alloy was studied after two-step aging treatments consisting of 300 °C for 12 h followed by 550 °C for different times. An anomalous change in transformation temperatures was observed as the second aging time was increased from 7.5 to 13.5 h. Initially with increased aging time (0.5–7.5 h) at 550 °C, coherency strain fields about H-phase precipitates increased. The corresponding backstress favored martensite formation, hence an increase in transformation temperatures. However, a point was eventually reached where misfit dislocations relaxed those strain fields and the effect was reduced, resulting in a decrease in transformation temperatures.

Transmission electron microscopy and mechanical testing were used to determine the effect of pre-aging (300 °C for 12 h) on microstructure and mechanical behavior of a series of Ni50.3Ti49.7 − xHfx shape memory alloys (x = 6, 8, 8.5, 9 at.%) prior to normal aging at 550 °C for 3.5 h. Pre-aging was found to promote homogenous nucleation of nanosized H-phase precipitates, resulting in improved mechanical stability and strength and 4% recoverable compression strain without permanent deformation. The absence of pre-aging generally resulted in heterogeneous formations of larger H-phase precipitates, primarily clustered along grain boundaries, and correlated with poorer mechanical behavior.