Microstructure and mechanical properties of NZ30K alloy by semi-continuous direct chill and sand mould casting processes

The Mg-3.0Nd-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 35-40μm in the billets made by the DCC to about 100-120μm 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.

Prediction of flow stress of Mg-Nd-Zn-Zr alloy during hot compression

Isothermal hot compression tests were carried out on Mg-3.0Nd-0.2Zn-0.4Zr （mass fraction, %, NZ30K） alloy using a Gleeble-3500 thermo-simulation machine at temperatures ranging from 350 to 500 ℃and strain rates from 0.001 to 1 s^-1. A correction of flow stress for deformation heating at a high strain rate was carried out. Based on the corrected data for deformation heating, a hyperbolic sine constitutive equation was established. The constants in the constitutive equation of the hyperbolic sine form were determined as a function of strain. The flow stresses predicted by the developed equation agree well with the experimental results, which confirms that the developed constitutive equations can be used to predict the flow stress of NZ30K alloy during hot deformation.

Electrodeposition of Cu coating with high corrosion resistance on Mg-3.0Nd-0.2Zn-0.4Zr magnesium alloy

To improve the corrosion resistance, electrodeposition of Cu coating on Mg-3.0Nd-0.2Zn-0.4Zr （mass fraction, %NZ30K） magnesium alloy via an appropriate pretreatment was investigated. The surface morphologies, compositions and microstructures of the pretreated films and Cu coating were characterized in detail. The results show that the activation film consists of fluoride and phosphates and Zn immersion film forms preferentially on the eutectic compound Mg12Nd phase region. A smooth, uniform and dense Cu coating is successfully obtained. Potentiodynamic polarization tests reveal that Cu coating can greatly improve the corrosion resistance of NZ30K magnesium alloy. Open circuit potential （OCP） and electrochemical impedance spectroscopy （EIS） tests during long-term immersion further demonstrate that Cu coating can provide an effective protection for NZ30K magnesium alloy from corrosion up to ~60 h, due to its dense structure and a stable passive film formed. In addition, Cu coating exhibits good adhesion to substrate as confirmed by thermal shock test.