Microstructure evolution during mechanical working of three RS aluminium alloys: Al-4Cr-1Fe,Al-5Cr-2Zr and Al-6.43Cr-1.67Zr

Three Al-Cr alloys containing additions of Zr and Fe have been fabricated via cold compaction and hot extrusion. The decomposition of the powder microstructure and the subsequent coarsening during thermomechanical treatment have been studied. Detailed electron microscopy investigations were performed at different locations of partially extruded billets at 450 °C. The microstructure of the dead metal zone reflects the effect the induction heating exerts on the as-atomized powder microstructure. In the low Cr alloys, Al-4Cr-1 Fe and Al-5Cr-2Zr, decomposition of the rapidly solidified microstructure commences at the rich intercellular network, whereas the microstructure of the Al-6.43Cr-1.67Zr alloy remains relatively unaffected. Within the deformation zone the precipitation kinetics are affected by the shearing and the temperature rise. The cells and the powder particles are aligned along the extrusion direction. Precipitation is taking place within the primary segregation-free areas, observed in Al-4Cr-1 Fe and Al-5Cr-2Zr alloys, whereas in the Al-6.43Cr-1.67Zr alloy, decomposition of the powder microstructure starts at the Cr-rich intermetallic particles. The as-extruded microstructure is fibrous and heterogeneous. The heterogeneity of the as-extruded microstructure is a result of the microstructural variation observed within different size powder particles and within individual ones.