The Effect Of Alloying Elements On The Second Phases Of ZK60 Magnesium Alloy

It is the crucial factor determing the mechanical properties of magnesium alloys that the morphology, categories, amount, dimension and distribution of the alloy phases. However, the research of the alloy phases is not far matureer than that of metal materials, such as aluminum alloy, steel, and so on. In order to expand the application scope of ZK60 wrought magnesium alloy with high strength, it is necessary to conduct the basic research on its alloy phases. In this paper, the alloys phases were studied by using methodologies of metallurgical phase analysis, scanning electron microscopy (SEM) analysis, x-ray analysis, transmission electron microscopy (TEM) and microhardness mensuration in as-cast, solution-treated and as-aged Mg-5.6Zn-0.54Zr, Mg-6Zn-1.2Y, Mg-6Zn-0.6Zr-1.0Y magnesium alloy. The morphology, distribution of alloy phases and the effect of yttrium on the second phase in Mg-Zn, Mg-Zn-Zr alloys were ascertained initially. The research indicated that there was a great deal of eutectic with main components of α-Mg and MgZn. The eutectic had various morphologies and distributed in many ways. MgZn2 phase, in form of parallelogram, 200nm to 500nm in length, having no fixed orientation relationship with matrix, not sensitive to heat treatment, was found in as-cast, solution-treated and as-aged ZK60 alloy as well. MgZn phase, in length of 500nm, had fixed orientation relationship with matrix. Mg2Zn3 phase, in form of 100nm short rob, was the most fine phase in terms of size and the most even phase in terms of distribution in ZK60 alloy. The microstructure of Mg-6Zn-1.2Y, Mg-6Zn-0.6Zr-1.0Y alloys in as-cast were composed of α-Mg matrix and fish-bone-like eutectic structure and continuous reticulation structure at triangle crystal boundary or around the grain. Quantitative analysis of fish-bone-like structure showed that it consisted of Mg-Zn binary phase and Mg-Zn-Y ternary phase and the continuous reticulation structure was also Mg-Zn-Y ternary, which divorced from fish-bone-like structure. After the solid solution with 480℃×24h and diverse aging treatment, therehad some unmelting eutectic structure at the triangle crystal boundary in the Mg-6Zn-1.2Y and Mg-6Zn-0.6Zr-1.0Y alloy. The alloy phases had various morphologies in solution-treated and as-aged alloys. The experimental results indicated that the simultaneous addition of Zr and Y can refine the grain of Mg-Zn alloy notability. The melting temperature of eutectic structure in alloys containing yttrium is higher than that of Mg-5.6Zn-0.56Zr alloy, and the microhardness was improved.