Influence Of Precipitation In 50.3Ni-32.2Ti-17.5Zr (at.%) On Microstructure and Shape Memory Behavior

Ternary additions to slightly Ni-rich Nitinols have been shown to effect nanoscale precipitation in a manner that can result in significant improvement in various shape memory behaviors. In the present study, the addition of Zr has been explored. A 50.3Ni-32.2Ti-17.5Zr (at.%) alloy was solution annealed at 1050° C then water quenched followed by aging at 550°C for various times. The increase in hardness with aging correlated with the precipitation of a nanoscale, anisotropic shaped phase. At the peak aged condition (4 hours), no shape memory transformation response was measured down to -90 ^oC using differential scanning  calorimetry. In an over-aged condition (24 hours at 550°C), the austenite finish (A_f) temperature was approximately 75^oC and the martensite-start (M_s) was approximately -10^oC.  These transformation temperatures showed minor variations with thermal cycling. With further over-aging, the transformation temperatures became constant, during cycling, suggestive of a stabilizing influence of the coarsening precipitates.  The composition, size, and distribution of these precipitates were quantified using atom probe tomography at difference stages of aging.