Experimental Study And Numerical Simulation Of Hot Deformation Behaviors Of AZ91 Alloy

The plastic deformation behaviors of AZ91 magnesium alloy at high temperature are studied by experiments and computer simulation in this thesis. The influence of deformation conditions on dynamic recrystallization (DRX) is investigated and the electron back-scattered diffraction pattern (EBSP) technology is used to observe the microstructure evolution, change of misorientation of grain boundaries and the influence of precipitations on the nucleation of DRX during deformation. Based on the experimental analyses, a cellular automata model with DRX (DRX-CA model) is proposed to study recrystallization kinetics, change of size of recrystallized grains, distribution of precipitations and influence of precipitations on nucleation of DRX. In this model, the nucleation rate, critical dislocation density for nucleation, increase of dislocation density and driving force for grain growth, especially the influences of precipitations on the increases of dislocation density and migration of grain boundaries, are taken into account. The results show that:1. DRX occurs during the extrusion of AZ91 alloy, resulting in a marked refinement of grain size. At a given extrusion ratio, with the increase of extrusion temperature, DRX fraction increases and the average size of recrystallized grains increases. At a given extrusion temperature, with the increase of extrusion ratio, DRX fraction increases and the distribution of recrystallized grains tends to uniform. When DRX is completed, the average size of recrystallized grains decreases with the increase of extrusion ratio.2. Compared to as-cast AZ91 alloy, the tensile strength decreases and elongation increases after homogenized at 430℃for 12h, while both the tensile strength and elongation increase after extrusion. With the increase of extrusion ration or extrusion temperature, the tensile strength decreases and elongation increases. The tensile strength tested at 150℃is less than that tested at room temperature, while elongation at 150℃is higher than that at room temperature.3. With the increase of extrusion ratio or extrusion temperature, the fraction of precipitations decreases and the distribution tends to be uniform. The precipitation of the second phase is beneficial to the grain refinement and in turn to the promotion of the tensile strength.4. The nucleation mechanism of DRX is different at different deformation temperature ranges. At 250℃and 300℃, twins are observed and the very small recrystallized grains are observed at these twins boundaries and the original grain boundaries. At 350℃and 400℃, the small dynamically recrystallized grains nucleate at the original grain boundaries and then grow. The multi cycle DRX occurs during the deformation which accounts for the further refinement of grain even if the original grains have been consumed completely.5. The orientation image microscopy analyses show that the low angle grain boundaries gradually transfer into high angle grain boundaries with increasing strain during the deformation at high temperature. The fraction of low angle boundaries with misorientation ranging from 2o to 4o decreases and that with misorientation more than 4o increases. With the decrease of strain rate, the fraction and length of low angle grain boundaries increase at final microstructures.6. During the extrusion or hot compression of AZ91 alloy, a strong basal texture is prone to be formed, though the basal plane is parallel to extrusion direction in as-extruded alloy and the basal plane gradually rotates to about 90o to the compression axis.7. The peak stress in flow stress-strain curve is higher at lower temperature or higher strain rate. The strain at a peak stress (peak strain) tends to decrease with the increase of temperature and the peak strains under different deformation conditions (Zener-Hollomon parameter) can be given by an equationεp =2 .1×10?3Z0.124. The relationship between stress, strain rate and temperature can be predicted by a hyperbolic-sine constitutive law, A[ sinh(ασ)]n =εexp(QRT), with Q=175.8KJ/mol,α=0.013MPa-1 and n=5.6. The–( ?θ/ ?σ)-σcurve is not identical for different temperature regimes due to different deformation mechanism.8. A DRX-CA model is employed to simulate the deformation process and to provide necessary data for the analysis of recrystallization kinetics. In comparison with the peak stresses and peak strains in the flow curves, the simulated results agree reasonably well with the experimental findings, which demonstrate that the current CA model captures the main features of flow behavior during compression.9. Once the dislocation density in the primary grains reaches a critical value, dynamic recrystallization occurs. As a result, the average size of all grains decreases rapidly and then tends slowly to a steady value. At the same time, the average size of primary grains decreases linearly until all the primary grains are comsumed and the average size of recrystallized grains increases slowly to a peak value and then decreases to a steady value.10. An analysis of recrystallizaiton kinetics indicates that multi-cycle recrystallization is taking place during deformation, resulting in a single peak on the flow curve and a further grain refinement even though the primary grains have been consumed completely.11. The second phase particles precipitated during deformation are preferential for the nucleation of recrystallized grains, resulting in the grains adjacent to precipitations smaller than those long from precipitations. In the final microstructures, almost all the precipitations distribute on the grain boundaries and corners.