Effect of grain size on precipitation and microstrain of nanocrystalline NiTi alloys

The effect of aging treatment on the microstructure, phase transformation behavior and mechanical properties of nanocrystalline NiTi alloy was studied. The nanocrystalline NiTi alloys with different grain sizes were acquired by cold drawing followed by annealing at 350-500 degrees C for 10 min. The annealed samples were aged at 250-400 degrees C for 48 h. The Ti3Ni4 precipitates were found in aged nanocrystalline NiTi alloys. In the sample with smaller average nanograin size, the precipitates were found at the edge of grain boundaries and little lattice strain was shown in R phase matrix. In the sample with larger average grain size, the precipitates were found in the nanograins and exhibited a coherent interface with the matrix. The nanocrystalline R phase NiTi matrix exhibited a significant compressive stress at the end of the coherent precipitate. The coherent precipitates in sample aged at 250 degrees C after annealing at 500 degrees C suppress the stress-induced R -> B19 ' phase transformation and increased the upper plateau stress. The precipitation in sample aged at 250 degrees C after annealing at 350 degrees C unable to suppress the martensitic transformation effectively.