Microstructure and mechanical properties of a high-Zn aluminum alloy prepared by melt spinning and extrusion

Abstract

High-Zn aluminum casting alloys usually contain coarse dendrites, network eutectoid structure and casting defects which deteriorate the mechanical properties seriously. In order to overcome these problems, the combination of melt spinning and extrusion was explored for microstructure modification in this work. A high-Zn aluminum alloy (Al–27Zn-1.5Mg-1.2Cu-0.08Zr) was prepared by melt spinning and extrusion. The results show that the melt spun alloy mainly consists of micro-sized network-like grain boundary (GB) η-MgZn2 structure (η phase) and disc-like precipitates (GP-zone, η′-phase and η-precursor) embedded into fine α-Al grains with high solute atomic concentration. After extrusion, the grain size of α-Al is further refined due to the recrystallization, and primary network GB η-phase structures are transformed to granular η particles. Moreover, the extrusion induces the precipitation of nano-sized η′-phase, η-precursor and Zn phases. As a result, the alloy exhibits a high tensile strength of 485 MPa and a reasonable elongation of 5.2%. The contributions of grain boundary, dislocation, solid solution and precipitate strengthening to the yield strength are calculated according to the microstructure analysis, and it is found that precipitation strengthening is the main strengthening mechanism in this alloy. Fracture analysis shows that micro-cracks preferentially occur at α-Al/η phase interfaces due to interfacial de-cohesion, and propagate along the GBs.