Aging behavior of a fine-grained Mg-10.6Gd-2Ag alloy processed by ECAP

Abstract

Multipass (up to 16 passes) ECAP and heat treatment were performed on a Mg-10.6Gd-2Ag (wt%) alloy to achieve a combination of refinement strengthening and precipitation strengthening. The age-hardness response, mechanical properties and multiphase precipitate microstructure of the samples processed by ECAP were systematically investigated by varying the number of ECAP passes from 8 to 16. The results indicated that the multipass ECAP processing induced significantly refined grains and submicron β precipitates. After peak aging, a multiphase precipitate microstructure was observed, which consisted of the high-density γ″ nanoprecipitates and some β′ nanoprecipitates within grain interior, as well as the β nanoprecipitates along grain boundaries. The precipitate microstructure significantly changed with the ECAP passes, while there was no visible discrepancy in the grain size. More ECAP passes brought in more submicron and nanoscale β precipitates but less γ″ and β′ nanoprecipitates so that the precipitate-free regions were presented near the grain boundaries in the 16 passes ECAPed alloy in peak aging status. Due to the above microstructure modification, the strength and ductility of this alloy were simultaneously improved via the multipass ECAP compared to the as-cast alloy, and a further improvement in strength was obtained by subsequent aging. The increased ECAP passes degraded the age-hardening response of the ECAPed alloy compared to the 8 passes ECAPed alloy, as manifested by the lower hardness during the whole aging process and longer peak-aging time. The increased ECAP passes also brought in substantially decreased strength both in deformed status and peak aging status.