Microstructural evolution and strengthening mechanism of Al–Mg alloys with fine grains processed by accumulative continuous extrusion forming

Abstract

• The response of mechanical property to the microstructure variation was analyzed. • The grain refinement induced by CDRX and secondary phases was illustrated. • The strengthening mechanism caused by microstructure evolution was revealed. • A strategy to produce Al–Mg alloys with high mechanical performance was provided. In this work, the mechanical properties and strengthening mechanisms induced by microstructural evolution in a rheo-extruded 5087 alloy processed via accumulative continuous extrusion forming (ACEF) were investigated. Electron back-scattered diffraction (EBSD) and transmission electron microscopy (TEM) were utilized to characterize the microstructure of the alloy subjected to ACEF with various passes. The grain refinement caused by continuous dynamic recrystallization (CDRX) was discussed. The results demonstrated that after 3 passes of ACEF, there was a significant grain refinement effect on the alloy, and the average grain size decreased from 45.6 μm to 2.5 μm; the ultimate tensile strength (UTS) and yield strength (YS) of the alloy increased to 362.8 MPa and 234.6 MPa, respectively. Dislocation cells/walls generated during deformation promoted the formation of low angle grain boundaries (LAGBs). The accumulative strain accelerated the transformation of LAGBs to high angle grain boundaries (HAGBs). Dislocation pile-up enhanced the driving force of CDRX, and nano-sized Al 6 (Mn, Fe) phases at the grain boundaries inhibited the growth of grains due to the pinning effect. Based on the quantitative estimation, dislocation strengthening and grain boundary strengthening dominated the enhancement in YS of the ACEFed alloy.