Aging hardening and precipitate behavior of a solution-treated Mg-6Zn-4Sn-1Mn (wt.%) wrought Mg alloy

Mg-Zn-Sn-Mn alloy is a new type of age hardening wrought Mg alloy. However, the precipitation behavior of the alloy is still not well understood. A solution-treated Mg-6Zn-4Sn-1Mn alloy is selected as the ex-perimental alloy in this paper. The hardening behavior, precipitate microstructure and precipitation se-quence of the solution-treated alloy during single aging and double aging at 180 degrees C are systematically studied by means of XRD, OM, SEM, TEM, EDS and microhardness. The results show that four types of precipitated phases are successively precipitated in the experimental alloy during the aging treatment, namely, G.P. zone parallel to the (0001)alpha base plane, rod-like 1 (MgZn2) phase perpendicular to the (0001)alpha base plane, disc-like 2 (MgZn2) phase parallel to the (0001)alpha base plane, and irregular beta' (Mg2Sn or Mg3Sn) phase. The habitual plane of most beta' phases is the (0001)alpha basal plane, and a small part of the beta' phase forms a certain angle with the (0001)alpha basal plane. A large number of finely dispersed G.P. zones can provide the heterogeneous nucleation sites for the 1 phase during the pre-aging stage, which promotes the nucleation of the precipitates during the second aging, so the peak aging hardness (87HV) of the double -aged alloy is much higher than that of the single-aged alloy (74HV), and the peak aging time (6 h) during second aging of the double aging is shorter than that (12 h) of single aging. The possible precipitation sequence of Mg-6Zn-4Sn-1Mn alloy during the isothermal aging treatment can be obtained, that is, alpha-Mn -> 1 (MgZn2)->beta (MgZn) in the Mn-rich zone, and SSSS -> G.P. zones -> 1 (MgZn2)-> 2 (MgZn2)->beta' (Mg2Sn or Mg3Sn)->(beta (MgZn) and beta (Mg2Sn)) in Zn-rich and Sn-rich zone. (c) 2021 Elsevier B.V. All rights reserved.