Research On Microstructure And Properties Of Mg-9Al-xZn Alloys

By tensile test and using methods such as SEM, XRD and TEM, the tensile properties and microstructures of two kinds of Mg-9Al-xZn alloys (ZM5 and ZM5y, the latter has more Zn and a little Ag) are investigated, and the relations among the chemical composition, microstructure, and properties are analyzed, and the phase transformation (microstructure evolution) and alloying strengthening are discussed. It is indicated that Zn and Ag can well strengthen Mg-9Al-xZn alloy through solid solution (s.s.). Zn facilitates the precipitation of peralite-like secondary (Mg17Al12) in Mg-9Al-xZn alloy, and a new phase MgZn appears due to more addition of Zn. Ag has little effect on the microstructure of Mg-9Al-xZn alloy and no new phase related appears, but it segregates near the boundaries through artificial ageing and thus the corrosion resistance of the alloy is deteriorated. UTS (ultimate tensile strength) and elongation of Mg-9Al-xZn alloy increase through S.S., and YS (yield strength) and the value of YS/UTS increase through the followed artificial ageing with obvious loss of elongation. Artificial ageing has little effect on UTS of ZM5, but slightly decreased that of ZM5y due to the joint effects of more addition of Zn and artificial ageing. The morphology of Y phase can be obviously changed through s.s. and artificial ageing. It is also showed that the peralite-like secondary Y phase has remarkable effect on the mechanical properties of Mg-9Al-xZn alloy: with the increase of the amount and size of the secondary Y phase (its morphology also becomes more complicated), the elongation decreases while YS and the value of YS/UTS increase, but too large peralite-like secondary Y phase deteriorates UTS. It is deemed that the primary Y phase is resulted from composition segregation during un-equilibrium solidification, which is also partly facilitated by Zn. Boundaries are good sites for nuclear formation of the pearlite-like secondary Y phase under the condition of either as-cast or solid soluted or artificial aged. When as-cast, the boundary between the primary Y phase and the matrix grain is more preferential than other boundaries for nuclear formation of pearlite-like secondary Y phase, but composition segregation is not good for Y phase slices to grow up. The results in this paper can be useful to the heat treatment of Mg-9Al-xZn alloys. With reasonable control of thechemical composition and using appropriate process parameters, satisfactory properties can be got to meet the requirements in practice.