Coherency strains of H-phase precipitates and their influence on functional properties of nickel-titanium-hafnium shape memory alloys

A Ni50.3Ti41.7Hf8 alloy was studied after two-step aging treatments consisting of 300°C for 12h 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 13.5h. Initially with increased aging time (0.5–7.5h) 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. [Display omitted]