Experimental study of the superplastic deformation mechanisms of high-strength aluminum-based alloy

The role of different deformation mechanisms and the contributing factors behind them needs to be clearly defined to develop materials exhibiting high strain rate superplasticity. It is still not fully understood to what extent the deformation conditions and microstructural parameters affect the mechanisms of superplastic deformation. A novel Al-3.9Zn-4.1Mg-0.8Cu-2.8Ni-0.25Zr alloy is employed to determine what effects the testing conditions, elemental and phase composition and evolution of grain and sub-grain structure make towards deformation mechanisms. To analyze the deformation behavior and microstructure evolution, uniaxial tensile tests are performed following two deformation regimes with the different constant strain rates and temperatures: (1) 2 x 10(-3) s(-1), 480 degrees C and (2) 2 x 10(-2) s(-1), 440 degrees C. The elongation to failure exceeds 650% and the strain rate sensitivity coefficient m is near 0.5 at both deformation regimes. Electron microscopy and focused ion beam techniques are employed to analyze mechanisms associated with grain boundary sliding and intragranular strain by using the samples in a stable flow. Surface grids indicated that superplastic flow is accompanied by the formation of striated regions on the surface, whereas grain boundary sliding involves grain neighbor switching and grain rotation. Intragranular deformation with increased dislocation density and dynamic recrystallization are also observed. Weaker intergranular and intense intragranular deformation are revealed at high strain rate deformation regime. Solute effect and the presence of two types of secondary phases, nanoprecipitates of L1(2)-Al3Zr and micron-sized eutectic Al3Ni, are discussed in comparison to conventional AA7475 alloy in different aspects of grain growth, dynamic recrystallization and strain-rate sensitivity.