Difference in Microstructure of Coarse Dendritic and Equiaxed Al-Cu-Ce-based Alloys after Extrusion and the Mechanisms of Room-Temperature Plasticity Improvement and Elevated-Temperature Strength Degradation

The effects of extrusion and annealing on the microstructure and mechanical properties of Al-Cu-Ce based alloys with coarse dendritic and equiaxed grains were systematically investigated. After deformation, both types of alloys exhibit a dual-phase heterogeneous lamellar microstructure: alternating layers of the Al-Al8CeCu4 eutectic region and the primary alpha-Al phase, and alternating layers of the fine-grained Al-Al8CeCu4 region and the coarse primary alpha-Al phase. The Al-Al8CeCu4 eutectic regions are distributed among the elongated equiaxed grains with different orientations, while multiple Al-Al8CeCu4 eutectic regions are embedded in a single coarse deformed primary alpha-Al dendrite. After annealing, the coarse deformed primary alpha-Al dendrites undergo complete recrystallization to form irregular coarse alpha-Al grains, which consume the fine-grained Al-Al8CeCu4 regions within them, whereas the deformed equiaxed grain microstructure remains almost unchanged. Notably, the fine intermetallic particles at the grain boundaries and L12-Al3(Sc,Zr) nanophases effectively inhibit recrystallization and dislocation recovery. Owing to the trade-off among the increments of various strengthening mechanisms, the yield strength of both alloys remains almost unchanged before and after deformation, while the strengthening effects of L12-Al3(Sc,Zr) nanophases and the refinement of primary precipitates induced by microalloying are still manifested. The fragmentation of coarse lamellar intermetallic phases alleviates stress concentration, modifies the orientation relative to the loading axis, delays crack initiation, and thus enhances the work-hardening capacity (UTS) and elongation of the alloys. However, the disruption of the continuous reticular intermetallic phase skeleton impairs its structural supporting effect on the material, increases the probability of grain boundary sliding, and causes premature failure of the alloys, thereby significantly degrading their elevated-temperature strength. This study clarifies the strategy for improving the elevated-temperature strength of heat-resistant Al alloys by adopting the microstructural template of a reticular intermetallic phase skeleton at grain boundaries and enhancing the thermal stability of theta'/theta"-Al2Cu phases via microalloying, which provides a clear direction for subsequent research.