Hot Compressive Deformation Behaviors And Microcosmic Mechanisms Of Al₁₈B₄O₃₃w/Mg Composite

Al₁₈B4_O33w/ZK60 composites were prepared successfully by squeeze casting. Hot compressive tests of squeeze cast ZK60 alloy and Al₁₈B4_O33w/ZK60 composite were performed at a strain of 0.6, various temperatures (523-723K) and strain rates (0.001-10s-1). The flow stress behaviors of the alloy and the magnesium matrix composite at different deformation conditions were investigated. The processing maps of the alloy and the composite were obtained and validated by making use of Dynamic Material Modeling (DMM) calculation method. Optimum hot deformation parameters of the alloy and composite were obtained according to processing map theory and microstructure observations. The extrusion deformation of the alloy and the composite were performed at the optimum parameters conditions. The tensile properties at ambient temperature and microstructure of the as-extruded materials were studied. Microstructure evolution of ZK60 alloy and Al₁₈B4_O33w/ZK60 composite during compression was investigated by optical microscopy (OM), scanning electronic microscopy (SEM) and transmission electronic microscopy (TEM). Compressive deformation mechanisms of the alloy and the composite at elevated temperature were discussed.The flow behaviors of the compression at elevated temperature of as-cast ZK60 alloy and Al₁₈B4_O33w/ZK60 composite were studied. The results showed that the flow stress decreased with increasing temperature at the same strain rate, and the flow stress of Al₁₈B4_O33w/ZK60 composite was higher than those of the ZK60 alloy. The flow softening effect of the composite decreased with increasing temperature. The flow stress increased with increasing strain rate at the same temperatures, and the flow softening effect of the composite were distinctively enhanced with increasing strain rate.The processing maps of the alloy and the composite were obtained on the base of the processing map theory, irreversible thermodynamics principle and the calculation method of DMM. The power dissipation efficiency, stable and unstable deformation of the alloy and the composite at different temperature and strain rate were analyzed basing on the power dissipation efficiency maps and flow instability criterion maps, respectively.The correctness and validity of the processing maps of the materials were manifested by the observation of microstructures. The optimum hot deformation parameters of the alloy and the composite obtained combining the processing maps and microstructures. The optimum temperature and strain rate combination for squeeze cast ZK60 magnesium alloy are T=648K andε& =0.001s-1, and the corresponding power dissipation efficiency (η) was 39%. The optimum processing parameters (temperature, strain rate) for squeeze cast Al₁₈B4_O33w/ZK60 are T=673K andε& =0.1s-1, and the corresponding power dissipation efficiency (η) was 31%.Based on the above investigations, ZK60 alloy and Al₁₈B4_O33w/ZK60 composite rods with good surface quality and excellent properties were extruded successfully at the optimum deformation parameters suggested by processing maps. This indicates that the processing maps are effective path to optimize deformation parameters of the alloy and the composite. After extrusion, the mechanical properties of ZK60 alloy and Al₁₈B4_O33w/ZK60 composite were increased significantly. The ultimate tensile stress, yield strength, elongation and elastic modulus of the alloy were increased 39.5%, 89.4%, 25% and 6.7%, respectively. As for Al₁₈B4_O33w/ZK60 composite, the ultimate tensile stress, yield strength, elongation and elastic modulus were increased 29.3%, 32.2%, 16.7% and 17.1%, respectively. The results also indicated that the mechanical properties of both the ZK60 alloy and Al₁₈B4_O33w/ZK60 composite were improved after hot extrusion.Microstructures of the alloy and the composite varied with compressive temperature and strain rate. At the certain strain rate, the number of twinning decreased with increasing temperature, while the number of dynamic recrystallization grain increased with increasing temperature. The sizes of twinning and recrystallization grain became larger, and dislocation density reduced with increasing temperature. At the fixed temperature, dislocation density increased with increasing strain rate. The sizes of twining and recrystallization grain became smaller, and the number of recrystallization grain was reduced with increasing strain rate.Microstructure observations of the squeeze cast ZK60 alloy during compression indicated that twining occurred at the stress concentration grain boundaries. The interaction of twins/twins and twins/dislocations enhanced the new DRX grain nucleation. The DRX nucleus formed prior to nearby the grain boundaries. Microstructure observations of the squeeze cast Al₁₈B4_O33w/ZK60 composite during compression indicated that whiskers were rotated and broken in order to accommodate the plastic deformation of the matrix alloy. However, the Al18B4O33 whisker enhanced the DRX nucleation, with DRX grains nucleating at matrix in the vicinity of the whiskers. At the same time, the whisker suppressed the growth of the DRX grains.There are two deformation mechanisms in the alloy and the matrix of the composite. One was twining dynamic recrystallization (TDRX). Another one was continuous dynamic recrystallization (CDRX). The main deformation mechanisms of ZK60 alloy and the matrix of Al₁₈B4_O33w/ZK60 composite were twinning and dislocation sliding during the plastic deformation.