Influence of ECAP on precipitate distributions in a spray-cast aluminum alloy

Experiments were conducted to evaluate the influence of equal-channel angular pressing (ECAP) on the mechanical properties of a spray-cast aluminum 7034 alloy. In the unpressed condition, the grain size of the alloy was ∼2.1 μm and the microstructure contained an array of rod-like η-phase (MgZn2) precipitates plus metastable η′-phase and Al3Zr particles. It is shown that ECAP processing at a temperature of 473 K has three significant effects. First, it refines the grain size to ∼0.3 μm. Second, the high stresses imposed in ECAP lead to a fragmentation of the rod-like MgZn2 precipitates into smaller particles with significant fragmentation occurring in the first pass and additional breaking in subsequent passes. Third, there is a partial loss of the metastable η′-phase. Tensile testing at room temperature revealed a significant reduction in the ultimate tensile strength after ECAP due to the loss of the hardening η′-phase. Tensile testing at 673 K gave superplastic ductilities provided samples were pressed through a sufficient number of passes to achieve a reasonably large fraction of high-angle boundaries.