Aging behavior and microstructure of aged excess Mg type Al-Mg-Si alloys after HPT processing

Abstract

The aging behavior of excess Mg type Al-Mg-Si alloy processed by high pressure torsion (HPT) and effect of Cu addition to this alloy have been investigated by hardness test and microstructure observation. The samples were solution heated, quenched, processed by HPT and aged. The hardness of HPT-processed samples increased about 100 HV rather than samples without HPT, and they showed hardening with peak hardness during aging. The level of hardness on alloys was increased by increasing with amount of Cu-addition. The mean grain size in HPT-processed samples was about 200-250 nm for each alloy. All HPT-processed alloys showed peak hardness during aging, and two Cu added alloys show positive value of age-hardening ability for the aging condition in this study. Precipitates in HPT-processed and aged samples existed on and near the grain boundaries, and a few small precipitates are found at some dislocations. According to the analysis of selected area diffraction (SAED) patterns, aluminum, Mg2Si and Q-phase are found in the HPT-processed and aged Cu-added alloys. The hardening of HPT-processed samples is probably caused by precipitation at the homogeneous grain boundary networks of 200-250 nm in the sample which acts as heterogeneous nucleation sites for alloys. © 2013 The Japan Institute of Light Matals.