Effect of Ni on the Contributions of Superplastic Deformation Mechanisms in an Al–Zn–Mg–Cr Alloy

The influence of nickel on superplasticity characteristics, microstructure evolution and the contribution of acting superplastic deformation mechanisms in the Al–Zn–Mg–Cr based alloys has been studied. In the Al–Zn–Mg–Cr alloy, dispersoids with an average size of 140 nm containing aluminum, chromium, magnesium, and a small amount of zinc are precipitated. In the Al–Zn–Mg–Cr–Ni alloy, additionally an Al₃Ni phase has formed. Nickel aluminide provides a more homogeneous and stable grain structure at elevated annealing temperatures and during superplastic deformation at 440°C. An alloying with Ni reduced the average grain size from 7.7 to 7.3 μm before deformation and from 10 to 8.6 μm after straining to 0.69. An increased dislocation density has been found near the Al₃Ni particles after deformation. At comparable values of the strain rate sensitivity coefficient (m ≈ 0.6), the presence of Al₃Ni particles results in a higher contribution of GBS, and a lower contribution of intragranular dislocation slip, as compared to the alloy without these particles. The alloying with Ni provided a more equiaxed fine-grained structure and an increase in elongations-to-failure.