Microstructure and mechanical properties of NZ30K alloy by semicontinuous direct chill and sand mould casting processes

The Mg-3.ONd-0.2Zn-0.4Zr(NZ30K)alloys were prepared by direct-chill casting(DCC)and sand mould casting(SMC)processes, respectively and their microstructures and mechanical properties were investigated. The results indicate that casting method plays a remarkable influence on the microstructure and mechanical properties of as-cast NZ30K alloy. The grain size increases from 35-40 pm in the billets made by the DCC to about 100-120pm in the billets by the SMC. The aggregation of Mg12Nd usually found at the triple joints of grain boundaries in the billets prepared by SMC while is not observable from the billets by DCC. The tensile strengths and elongations of the billets are 195.2 MPa and 15.5% by DCC, and 162.5 MPa and 3.2% by SMC, respectively. The tensile strength of the alloy by DCC is remarkably enhanced by T6 heat treatment, which reached 308.5 MPa. Fracture surfaces of NZ30K alloy have been characterized as intergranular fracture by SMC and quasi-cleavage fracture by DCC,respectively.