Segregation and Precipitation Stabilizing an Ultrafine Grained Al-0.3%Cu Alloy

Understanding the coarsening mechanisms and the role of solute atoms during recovery annealing of ultrafine grained alloys produced by high strain deformation is crucial to tailor their microstructures and mechanical properties. In the present work, a lamellar structured Al-0.3%Cu alloy with a boundary spacing of 200 nm was prepared by cold rolling to a von Mises strain of 4.5 (a thickness reduction of 98%), featuring Cu segregation to high angle lamellar boundaries as determined by means of coupling misorientation measurement with elemental mapping followed by 3D atom probe detection. Recovery kinetics have been analyzed based on samples annealed isothermally at temperatures of 100, 125, 150 and 175 C covering a time span from 4 min to 4096 min. In situ observations of annealing in a transmission electron microscope revealed that the dominant coarsening process is the motion of Y-junctions formed by lamellar boundaries, which is subjected to various degrees of pinning from dislocations, dislocation boundaries and particles. Furthermore, it was found that the pinning can be reinforced with the increase of misorientation angles of the attached dislocation boundaries, the coarsening of Al2Cu particles and the combination of interconnecting boundaries and particles. The results underpinned the importance of alloy elements in stabilizing finely spaced lamellar structures during deformation and annealing, providing guidelines for tailoring stable ultrafine grained alloys.