Formation of the gradient microstructure of a new Al alloy based on the Al-Zn-Mg-Fe-Ni system processed by radial-shear rolling

Abstract

The effect of high-temperature radial-shear rolling (RSR) on the structure formation and mechanical properties of a new high-strength aluminium alloy based on the Al-Zn-Mg-Ni-Fe system was investigated by using transmission electron microscopy, scanning electron microscopy, electron backscatter diffraction, microhardness measurements and tensile test. The processed rod with a diameter of 14 mm was obtained from RSR at 480 °C after four passes with the corresponding elongation ratios: 1.67, 2.78, 5.53 and 8.16 (total value). The microscopic study showed that RSR of the billet led to the formation of a gradient microstructure with a deformed coarse-grained interior and a recrystallized ultrafine-grained surface. The recrystallized structure led to the decrease in microhardness up to 105 HV, whereas the preservation of non-recrystallized structure in the central parts provided an increased microhardness up to 145 HV. The fine crystals of the Al9FeNi eutectic phase and secondary precipitates of the Al3Zr and η(MgZn2) phases provided stability of the formed grain structure due to their high pinning ability. The subsequent analysis revealed that such a combination of structural components provides attractive mechanical properties. In particular, the results of uniaxial tensile tests revealed that the mechanical properties after RSR processing are comparable to the industrial 7075 alloy after equal channel angular pressing for the new alloy with composition Al-7.0% Zn-2.5% Mg-0.6% Ni-0.4% Fe-0.2% Zr (UTS ~ 430 MPa, YS ~ 310 MPa and δ ~ 7%).