Add Yb And Ca Zk60 Magnesium Alloy Microstructure And Properties

The effects of rare earth element Yb and alkaline earth element Ca additions on microstructure and mechanical property of ZK60 magnesium alloy have been investigated. The microstructure and mechanical property of ZK60, ZK60-0.5Ca, ZK60-1.5Yb and ZK60-1.5Yb-0.5Ca alloys under the condition of cast, hot rolling, solution treatment, anneal and aging have been studied by use of optical micrscope(OM), scanning electron microscope (SEM), energy dispersive(EDS), X-ray diffractometer(XRD), differential scanning calorimeter(DSC) and microhardness etc. The main research results are demonstrated as follow:(1) The microstructure of ZK60 alloys were changed obviously due to the addition of Yb and Ca. By the addition of trace Ca, the grain of ZK60 alloy was refined and partial dendrite grains were transformed to equiaxed grains. The effect of grain refinement of Yb addition on ZK60 alloy was better than that of Ca addition and dendrite grains fully changed to equiaxed grains. The grain refinement effect of compound addition of Yb and Ca on ZK60 alloy was superior to that of single additions of Yb and Ca, but more second phases were formed and distributed continuously along grain boundaries.(2) In this study, the optimized solution treatment regime of ZK60-0.5Ca, ZK60-1.5Yb and ZK60-1.5Yb-0.5Ca alloys were established through experiments. After solution treatment of 370℃×24h+420℃×24h, The second phases in as-cast ZK60 alloy with addition of Ca were fully dissolved, but a few undissolved phase remained in the alloy with Yb addition and a large numbers of second phase accumulated on the grain boundary in as-cast ZK60-1.5Yb-0.5Ca alloy. These undissolved phases could be eliminated after solution treatment with 370℃×24h+430℃×30h regime. But the hardness value have not been improved obviously when as-cast ZK60-0.5Ca, ZK60-1.5Yb and ZK60-1.5Yb-0.5Ca alloys were carried directly on solution and artificial aging at 150℃and 200℃.(3) Hot rolling process of ZK60-0.5Ca,ZK60-1.5Yb and ZK60-1.5Yb-0.5Ca alloys were conducted. It is found that the larger the rolling reduction, the more obvious effect of refinement of the three experimental alloys. The recrystallization grain size of ZK60-1.5Yb-0.5Ca alloy under 93% rolling reduction after anneal is only about 6μm.(4) With artificial aging at both 150℃and 200℃after rolling, the hardness value of these experimental alloys enhanced remarkably, the effect of 150℃aging is better than that of 200℃. The peak hardness values of three alloys ageing at 150℃are 76Hv,81Hv and 87.5Hv, respectively.(5) There exist a Mg-Zn-Yb ternary stable phase in the ZK60 alloy with Yb addition in both as-cast and as-rolled states. It presents different shapes in ZK60-1.5Yb-0.5Ca alloy, such as reticular, chain-like, rod-like and so on. The melting point of the phase is about 389℃and the Zn/Yb atomic ratio is round 4.