On Meso Plastic Behavior And Constitutive Modeling Based On Slip And Twinning In HCP Metallic Crystal

The main plastic deformation mechanisms of Mg alloy with HCP structure include slip and twinning in air. It is necessary that the deformation slip is incorporated with twinning in crystal plasticity models for investigating the complicated plastic flow mechanism of Mg alloy. The theoy of crystal plasticity constitutive is extended to a wider range of polycrystalline metals. At present, Incorporation of deformation slip and twinning as a mode of plastic deformation has been an unsolved problem in extending crystal plasticity theoy to HCP metals. In the present paper, the crystal plasticity constitutive of Mg alloy and the evolution law of its microstructure are investigated in meso scale by testing, theoretical modeling, and simulating for the anisotropic plastic behavior and the asymmetric sigmoidal shaped hardening curve of wrought Mg alloy. The main works in this paper are as follws:1. Metallographic test, monotonic loading test, cycle loading test, and electron fractography were carried out for different extruded AZ31 Mg alloy in sequence. Tests results showed that the microstructures, such as grain size, and the amount, size and shape of precipitated phases and inclusions, are different among different Mg alloy. But the main deformation mechanisms of Mg alloy still are the slip and twinning deformation.2. The crystal plastic constitutive model including slip and twin deformation is established on crystal plasticity theory. The solving method of Newton-Raphson iteration for constitutive equations is developed with the stress components directly as the basic variables of iteration. Furthermore, the plastic deformation behaviors of single crystal AZ31 alloy were analyzed numerically under monotonic tension and compression, respectively, in four different strain paths (i.e. along<2-1-10>,<01-10>,<0001>,<01-11>) and the initial yield surfaces were calculated with this model. Numerical analysis shows that the activities of slip/twin system rely on the loading path. This model can predict the activities of slip/twin system, the number of dominant twin variants and the types of twin intersection. The initial yield surfaces show the obvious feature of kinematic hardening.3. The numerical simulation and meso analysis for the AZ31 Mg alloy under uniaxial loading were carried out by using the above constitutive relation with a representative volume element (RVE) consisting of Voronoi polycrystalline aggregation. The simulation results show that the macro stress-strain curve and the polycrystalline texture evolution of Mg alloy are in good accordance qualitatively with the test results. Meso analysis showed that (1) the macroscopic plastic behavior of Mg alloy depends on the competitive result of activation slip and twinning systems; (2) the tension twin deformation is the primary reason to the formation of typical (0001) basal texture in the polycrystalline; (3) the results of extremely inhomogeneous distribution led by the twin deformation for the orientation deflection and the twin volume fraction in the polycrystalline were gived.4. In consideration of the mechanism of twinning-detwinning, the cyclic plastic constitutive model of Mg alloy was developed including slip and twinning. The back stress of slip, twinning, and detwinning were introduced into the constitutive equation. Employing polycrystalline aggregate representative element, cycle loading of two low strain amplitudes were carried out the numerical simulation. The simulated results indicated that the sigmoidal shaped hysteresis loops with cycling is in agreement with experimental observations, the model correctly captures the particular shape of the hysteresis curve in the case of low strain amplitude. The meso analysis revealed that the deformation of slip, twinning, and detwinning play, in turn, the role of dominant deformation mechanism:above all, the alternative occurrence of twinning and detwinning resulted in asymmetric sigmoidal shaped hardening curve. Meanwhile. Non-uniform distribution and evolution of the stress and strain in polycrystal, twin volume fraction and stress triaxiality in grains were gived.5. Based on geometrically necessary dislocation (GND), the effect of particles on slip was described by additional resistance. The new hardening function of slip including the size and the volume fraction of particle in polycrystal was developed. Representative volume element (RVE) of polycrystalline aggregation embodying microstructure of MMCp material is established by utilizing Voronoi. It is simulated that the stress-strain response of four SiCp/Al composites of different particle diameter and volume fraction e.g. MMCp3.5-5、 MMCp3.5-10、MMCp10-5 and MMCp10-10 under the macroscopic homogeneous deformation with RVE. The results show the simulation curves of macro stress-strain response are in good agreement with the experimental ones. It is revealed that the new hardening function can effectively represent the effect of particles on slip. A new analysis concept would be privoded for studying the effect of precipitate phase on the other deformation mechanisms.