Microstructure/mechanical behavior relationships in upset-forged powder-processed Al alloys containing icosahedral quasicrystalline dispersoids

A series of three Al–Cr–Mn–Co–Zr alloys with total alloy contents of 3.58, 4.41 and 4.90 at.% has been produced by blind-die-compaction of gas-atomized nano-composite powders. The alloy microstructures consist of a face-centered cubic (FCC) Al matrix with characteristic distributions of quasicrystalline I-phase dispersoids giving hardness values of up to 162 HV. The consolidated alloys were forged uniaxially to upsets of 30–90% at temperatures of 300–370 °C to investigate the effects of deformation processing on the microstructure and mechanical behavior. At low upsets the characteristic phase distribution was retained, whereas at high upsets a refined more homogeneous microstructure developed with a break-up of the coarser I-phase features. No I-phase decomposition was observed for any of the forging conditions studied. Tensile testing of samples cut perpendicular to the forging axis gave yield strengths of up to 480 MPa, and ultimate tensile strengths of up to 520 MPa, but have low ductilities at low upsets. At higher upsets, the samples exhibited moderately reduced strengths but much higher ductilities, with elongations to failure of up to 15%, and reductions in area of up to 50%. An analysis of the strengthening mechanisms indicates that solid solution strengthening by super-saturation of Cr and Mn in the FCC Al matrix may contribute significantly. The modest softening of the alloys at high upset thus probably corresponds to defect-mediated solute redistribution. The corresponding dramatic increase in ductility is attributed to closure of residual porosity from the consolidation process and to the elimination of weakly-bonded particle-particle interfaces that can act as sites for fracture initiation.