Microstructure Stability And Hot Deformation Behavior Of Fine-drained Bimodal Size SiCp/AZ91Composites

In order to offset the defect of Mg alloys, e.g. low modulus and abrasionresistance, the fine-grained bimodal size (submicron+micron) SiCp/AZ91composites was fabricated by stir-casting technology combined with hotdeformation process. The effect of bimodal size SiC particles on microstructurestability of Mg matrix at elevated temperature was investigated by comparingwith AZ91alloys and single size SiCp/AZ91composites. Based on this point,the deformation microstructure and mechanical properties was investigated. Theeffect of bimodal size SiC particles on hot deformation behavior of fine-grainedSiCp/AZ91composites was revealed.The results show that the microstructure of SiCp/AZ91composites afteranneal treatment depends on temperature and time. When t≤320C, grain growthdominate the microstructure evolution of composite matrix; while t≥370C,recrystallization occurs around micron SiCp. The grains grow up with increasingannealing time. Both submicron and micron SiC particles can enhance themicrostructure stability of SiCp/AZ91composites. While, compared withmonolithic AZ91alloy and micron SiCp/AZ91composites, the obvious submicron particle dense zones are formed in the vicinity of micron SiCp inbimodal size SiCp/AZ91composite during thermal treatment, which results inthe small average grain size and improved microstructure stability.To describe the hot deformation behavior of fine-grained SiCp/AZ91composite more exactly, the stress calculated by three typical constructiveequations was compared with the measured value. The results show that thepower law is deduced to be the most suitable constructive relationship forfine-grained SiCp/AZ91composite which could describe the relationshipbetween flow stress and strain rate accurately at a wide stress range. Theexponential and hyperbolic law broke down at low and high stress level,respectively. The activation energy for deformation increase with increasingdeformation temperature and strain rate. By analyzing the activation energy andstress exponents, it was found that the deformation mechanism at0.001s-1isgrain boundary diffusion controlled dislocation climb; while it was dislocationclimb at0.01-1s-1. Comparison between fine-grained AZ91alloy, S-1, M-10andS-1+M-9composites suggests that the addition of single size SiCp could reducethe activation energy, while the addition of bimodal size SiCp could increase theactivation and transform the deformation mechanism from slide of dislocation todislocation climb.By analyzing the processing map combining with the observation ofmicrostructure and macro morphology of compressed specimens, the optimumprocess conditions was conclusive as270-320C/0.001-0.1s-1in which DRX may occur to modify microstructure, stabilize flow stress and improve theworkability. The flow instability zones mainly concentrated atlow-temperature-high-strain-rate zones (270-395C/>0.1s-1) andhigh-temperature-low-strain-rate zones (407-420C/0.001-0.01s-1), the mainlyflow instability mechanism is flow localization and macroscopic cracking.