Ultrafine-Grained Microstructures of Al-Cu Alloys with Hypoeutectic and Hypereutectic Composition Produced by Extrusion Combined with Reversible Torsion

In this study, binary as-cast Al-Cu alloys: Al25Cu (Al-25%Cu) and Al45Cu (Al-45%Cu) (in wt%) were severely plastically deformed by extrusion combined with a reversible torsion (KoBo) method to produce an ultrafine-grained structure (UFG). The binary Al-Cu alloys consist of alpha-Al and intermetallic Al2Cu phases. The morphology and volume fraction of alpha-Al and Al2Cu phases depend on the Cu content. The KoBo process was carried out using extrusion ratios of lambda = 30 and lambda = 98. The effect of phase refinement has been studied by means of scanning electron microscopy with electron backscattering diffraction and scanning transmission electron microscopy. The mechanical properties were assessed using compression tests. Detailed microstructural analysis shows that after the KoBo process, a large number fraction of high-angle boundaries (HABs) and a very fine grain structure (similar to 2-4 mu m) in both phases are created. An increase of lambda ratio during the KoBo processing leads to a decrease in average grain size of alpha-Al and Al2Cu phases and an increase in fraction of HABs. UFG microstructure and high fraction of HABs provide the grain boundary sliding mechanism during KoBo deformation. UFG microstructure contributes to the enhanced mechanical properties. Compressive strength (R-c) of Al25Cu alloy increases from 172 to 340 MPa with an increase of lambda. Compressive strain (S-c) for Al25Cu alloy increased from 35 to 67% with an increase of lambda. High fraction of intermetallic phase in Al45Cu alloy was responsible for room temperature strengthening of alloy and low compressive strain. The deformed Al45Cu alloy with lambda = 30 showed that R-c is 194 MPa and S-c is equal to 10%.