Rare Earth Elements On The Performance And Thermal Deformation Behavior Of Zk60 Alloy

1) It is shown in the results of metallographic and XRD analysis that elements Ce or Y have good effects of fining crystalline grains and purifying grain-boundaries. RE elements mainly aggregate at grain-boundaries. The compounds containing RE have good thermal stability, which can improve the heat resistant properties of magnesium alloy. After homogenizing treatment, the dendritic segregation dissolves into the matrix again.2) Obvious dynamic recrystallization occurs in tested alloys after hot-extrusion. The recrystal grains of the alloys containing RE elements are much finer than the master alloy. At the same time, they grow finer with increasing RE contents. It is indicated that the growth of recrystal grains is embarrassed by heat-resistant phases formed by Mg and RE. Fine grains are favorable for both tensile strength and elongation.3) RE elements are effective for improve the mechanical properties of alloys in different conditions, such as as-extruded state, T₅ state and T₆ state. The tensile test results of alloys in T₅ condition show that ZK60-1.5% Ce alloy has the best synthetical mechanical properties in the alloys added Ce, and ZK60-0.9% Y alloy has the best properties in the alloys containing Y. Mechanical properties of the former is σ_b = 338.6MPa and δ =15.6 %, and σ_b and 6 of the later are 355.9 MPa and 15.86%. It is indicated that the strengthening effects of element Y are better than Ce.4) T₅ and T₆ treatment were applied to ZK60-1.5Ce alloy and ZK60-0.9Y alloy in order to make a comparison of the two heat treat regime. The results of tensile tests show that the tensile strength of T₅ state is higher than that of the T₆ state, but the discipline of elongation is contrary to it.Because the metastructure propitious to the precipitation of second-phases disappears during the course of soaking period. When T5 treatment was applied to the two alloys, the dislocations and subgrains formed during the hot-extrusion make for the precipitating of second-phases. Proper tensile strength and plastic properties can be attained by T₅ heat treatment.5) During the course of high temperature compression experiment, true stress-strain curves present different forms. At the same strain rate, truestress decreases with elevatory temperature. The effects of work-hardening are evident in the phase of low strain capacities when the alloys are deformed at low temperature. The specimens crack after the rheologic stress reaching the peak value, otherwise the stress will get stable. At the same deformation temperature, the peak stress intensity gets higher with increasing strain rate. When the deformation temperature is lower than 473K, the structure properties of specimens are influenced by dynamic recovery and work-hardening. Dynamic recrystallization becomes dominant factor while the temperature exceeds 523K.6) Zener-Hollomon equation was applied to calculate activation energy of the tested alloys. The result is that the deformation activation energy gets higher with increasing temperature and strain rate.7) Rare E arth e lements are e ffective for i mproving t he a ctivation e nergy o f ZK60 alloy, which is due to the heat resisting phases formed by Mg and RE. The activation energy of ZK60-0.9Y is higher that of ZK6O-1.5Ce alloy. It is indicated that the improving effect of Y element is better than that of Ce.8) Influence of deformation degree and temperature on microstructure of ZK60-0.9Y and ZK60-1.5Ce alloy was studied, respectively. Part of the alloy dynamic recrystallizes at 523 K. But the actual deformation speed of hot-extrusion is much higher than the themal simulation, it isn't sufficient time slice for the dynamic recrystallization if the alloy is deformed at low temperature. As a result, deformation temperature should be properly increased. However, quantities of heat during the course of hot-extrusion are accordingly increased which is due to the high deformation speed, so. proper lower temperature is necessary for preventing the alloy from heat cracking. Aforementioned two factors should be considered sythetically when establish thermal deformation technology.