Phase Stability In Ni-Rich Nitinol With Ternary Solute Additions

Two closely associated properties of NiTi alloys are the shape memory effect and superelasticity. In either case, most NiTi research has concentrated on near equiatomic compositions. However, prior work on a 55Ni-45Ti (at.%) alloy has shown an anomalously high hardness value (~645 VHN) that is comparable to tool steels and opens up an entirely new line of applications in the area of mechanical components such as bearings and gears. This high hardness is believed to be due to precipitate strengthening resulting from a high fraction of Ni₄Ti₃ precipitates. In the present work, the influence of 1 at.% Hf or Pd additions to these Ni-rich compositions has been undertaken. These solute additions were hypothesized to provide solid solution strengthening and/or modified precipitation behavior, which could further enhance the high hardness. In a 54Ni-45Ti-1Hf (at.%) alloy, a lower hardness (~525 VHN) was noted. Unlike the binary alloy, which retained the peak hardness after 300 hours of aging at 400 °C, the ternary version showed classic over-aging behavior after 72 hours at 400 °C. The peak hardness in the 54Ni-45Ti-1Hf alloy was associated with nanoscale Ni₄Ti₃ precipitates but did not contain the high strain contrast observed in the binary alloy. In the over-aged condition, a secondary precipitate, determined to be the H-phase, previously identified in Ni-Ti-Hf alloys, was found. This Ni-rich alloy also precipitated the R-phase after 72 hours but it was re-dissolved after further aging. This precipitation behavior will be compared to a 54Ni-45Ti-1Pd (at.%) alloy and the baseline 55Ni-45Ti composition.