Hardening behavior and phase decomposition in very Ni-rich Nitinol alloys

A series of Ni-rich Nitinol compositions spanning from 53NiTi to 58NiTi (at%) have been solutionized at 1050 °C and aged at 400 °C, 625 °C, and 750 °C for varying times to determine the correlation between microstructure and hardenability. Compositions of 55NiT, 56NiTi, and 57NiTi displayed peak hardness values near 650 VHN upon quenching, which are on par with tool steels. Upon aging at 400 °C, all but the 55NiTi and 56NiTi compositions exhibited a decrease in hardness initiating, between 1 and 10 h for the lower Ni alloys and after 100 h for the highest Ni containing alloys. The high hardness, including the solution treated condition, was attributed to the precipitation of a large volume fraction of nanoscale Ni4Ti3 platelets that resulted in narrow B2 NiTi matrix channels. These channels provided a microstructure-driven strengthening morphology. Upon increasing Ni content to greater than 56NiTi and increasing the aging time and temperature, the Ni4Ti3 phase decomposed to either Ni3Ti2 and/or Ni3Ti with a subsequent loss in hardness. The decomposition morphology between the precipitates was characterized by serial sectioning showing that Ni4Ti3 platelets acted as a heterogeneous nucleation site and subsequent Ni-reservoir during the growth of globular Ni3Ti precipitates.