| Rare Earth heat-resistant magnesium alloy can apply to the high temperature working condition because of its high strength and good creep resistance ability while it has poor plasticity, therefore, the application was hampered. Mg-8Gd-3Y-lNd-0.5Zr alloy, like other Rare Earth heat-resistant magnesium alloy, also have all the unique advantages which were mentioned above. However, its poor formability was the disadvantage that lead it limited use. Firstly, on the basis of the early research in our laboratory, the paper improved the existing homogenization annealing process, and formulate the better scheme of step homogenization to get higher strength and elongation. By setting the thermal simulation experiment (high-temperature uniaxial compression), established high temperature flow stress constitutive equation model of the alloy, and based on recrystallization kinetics of Avrami equation modified model, research and predict the phenomenon of dynamic recrystallization during hot deformation, The accuracy of the kinetics model was analyzed and verified by the microstructure under different deformation conditions.In the first stage of homogenization process, a large number of black needle shaped phase were precipitated near the eutectic phase, the precipitation phase would more benefit for the solution of eutectic phase in the second stage of homogenization process. Further more, under the condition of 325℃×12h+530℃×12h, air cooling was the better scheme in all homogenization results, after this process the alloy has achieved ultimate tensile strength 197.30Mpa and elongation rate 3.42% respectively, and benefit for the Dynamic recrystallization.The instable phenomenon of some samples in the range of experiment setting appeared under experimental conditions. At 350℃, 1s-1, ε= 0.418 and 350℃,0.1s-1, ε= 0.693, the tested samples were also instable and the flow stress suddenly dropped. According to the hyperbolic sine of Arrhenius equation and through linear regression analysis calculation of the flow stress peak, the value of average activation energy was Q=253.14KJ/mol.The dynamic recrystallization kinetics model for the experimental alloy was XDRX= and the parameters were ε*= 0.1889· (Z/A)0.1561, εc= 0.3334· (Z/A)0.0172, respectively. This model suggested that the higher temperature, larger strain and lower strain rate were all favorable to the occurrence of dynamic recrystallization. The alloy also have two step during the hot deformation progress including nucleation and grain growth.The twin dynamic recrystallization(TDRX) was the mainly DRX mechanism at 350℃,and the microstructure were composited by un-DRX region,local deformation bands, twins and micro cavity. Micro cavity had a trend to continuous expand to the micro-crack, and this was the reason for the instable phenomenon happened in some experiment condition. A large number of precipitation phases were distributed in the original grain boundaries at 400℃ and 450 ℃ and hindered the growth of new DRX grains, and the dynamic recrystallization mechanism was dominated by continuous dynamic recrystallization (CDRX). During the deformation process of 500℃, the grain boundary arch appeared, the dynamic recrystallization was dominated by discontinuous dynamic recrystallization (DDRX) with a small amount of continuous dynamic recrystallization (CDRX). Due to the high temperature, grain boundary migration ability is enhanced, grains would grow and even some would abnormally grow. |
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