| Superplasticity was investigated in nanocrystalline nickel aluminide (Ni3Al) of composition Ni- 8.5Al- 7.8Cr- 0.8Zr- 0.02B (wt%), processed by high pressure torsion straining. A total strain of 2.25 was attained at a temperature of 923 k and a strain rate of 1 × 10−4 s−1. Grain size at the start of a 923 K test was about 85 nm, and the post deformation grain size was about 100 nm. A reduction of the superplastic temperature by 450 K compared to samples of the same alloy that had a grain size of 6 μm, was observed. The stress exponent was determined to be 2.3, and the inverse grain size exponent was estimated to be 3. The temperature dependence of superplastic flow was estimated to be between 150 kJ/mol and 190 kJ/mol, which correlated well to the activation energy for grain boundary diffusion. Superplastic flow was characterized by high flow stresses and strong strain hardening. The high flow stresses were attributed to the difficulty of generating dislocations within the nanostructure. Strong strain hardening was explained by a combination of grain growth and recoverable hardening processes, most likely internal strains accommodated by diffusion. Superplasticity was also investigated in electrodeposited nanocrystalline nickel (Ni). A total strain of 2.3 was observed at a temperature of 693 K and a strain rate of 1 × 10−3 s−1 . However, the superplastic behavior was dependent on the presence of sulfur (S) impurities in the deposits. When ultra-fine grained Ni was electrodeposited without significant S impurity, superplasticity was not observed. |
