Effect of Sc microalloying on microstructure evolution and mechanical properties of extruded Al–Zn–Mg–Cu alloys

The effect of microalloying Sc on the microstructure and mechanical properties of Al–Zn–Mg–Cu alloys was experimentally investigated through X-ray diffraction, optical microscopy, scanning electron microscopy, electron backscatter diffraction, and transmission electron microscopy characterizations and tensile tests. The presence of Sc caused the formation of the primary Al3Sc phase and behaved a significant modifying effect on the cast microstructure with an equiaxed microstructure attained in the Sc-bearing alloys rather than the dendrites in Sc-free alloys. In the subsequent thermal extrusion processing, a remarkable grain refinement and retardation of recrystallization occurred, because the nano-sized Al3Sc dispersoids contributed to the pinning effect on the dislocation movement and grain boundary migration in the Sc-bearing alloys. The mechanical properties were significantly improved with the addition of Sc. The superior comprehensive mechanical properties in the Sc-bearing alloys were achieved via the optimal extrusion at 350 °C with the yield strength and ultimate tensile strength of 373.7 ± 3 MPa and 490.3 ± 5 MPa, respectively. The strong strengthening behavior of the Sc addition in the extruded alloys is mainly attributed to the grain refinement and the dispersoid strengthening from the Al3Sc particles. The corresponding contributions to the increase in yield strength derived from the grain refinement and the dispersoid strengthening were estimated, which indicated that the highest particle density attained by extrusion at 350 °C resulted in the excellent mechanical properties of the Sc-bearing alloys.