Crystal Plasticity Modeling Of Deformations In Magnesium Based On Slip And Twinning Mechanisms

Magnesium(Mg)and its alloys are attractive in the lightweight structure designing of aircraft and vehicles due to their low density,high specific strength and high specific stiffness.The plastic behavior of Mg arises from the variant underlying mechanisms and their interactions.Thus,the study of physics-based constitutive model will contribute to predict and analyze the macroscopic behavior of materials.In this paper,we have developed a single crystal plasticity model for Mg with the consideration of the following mechanisms: slip,twinning,and twinning-induced lattice reorientation.The constitutive equations have been implemented in the finite-element program ABAQUS by writing a user-material subroutine(UMAT).In order to validate the proposed model,plane-strain compression tests of single-crystal Mg are simulated,and the results are compared with the experimental observations.Then,we further develop a strain-rate and temperature-dependent constitutive model to study the strain rate and temperature sensitivities of Mg.The macroscopic behavior of Mg is well explained and understood through the detailed analyzing of the evolution of microscopic mechanisms.This work will certainly create guidelines for the improvement of plastic forming processes of Mg and geometric optimizations of Mg components.The main work and conclusions are as follows:1.We present a single crystal plasticity model for Mg based on the framework of conventional crystal plasticity,but incorporate the following mechanism: slip,twinning and twinning-induced lattice reorientation.The interactions of slip-slip,slip-twin and twin-twin are considered.The hardening laws for tension twinning and compression twinning are distinguished to better represent their roles in strain hardening.We also provide a simplified method to consider the twinning-induced lattice reorientation effects.2.The UMAT of ABAQUS has been programmed based on the proposed constitutive model.Then the plane-strain compression tests of Mg single crystals are simulated.The simulation results compare well with the experimental data,which validates of the proposed model.We also elaborate the evolution of slip and twinning to better interpret the macroscopic response of Mg.The study shows that the compressive response of Mg exhibits strong anisotropy,which is due to the fact that the activities of slip and twinning are considerably dependent on the lattice orientation.In addition,under some orientations tension twins can largely evolve and when their total volume fraction reach the threshold value,the twinning-induced lattice reorientation has a strong influence on the following mechanical behavior.3.Finally,we further investigate the strain rate and temperature sensitivities in Mg.In doing so,we consider the differing strain rate and temperature dependences of different slip and twinning systems,which are incorporated into the Johnson-Cook type hardening laws.Based on experimental evidence,here we assume that non-basal slip is strain rate and temperature dependent,and the CT temperature sensitivity is set to arise only within a certain temperature range,i.e.?150 ?.Besides,we assume that the strain rate sensitivity parameter is linear function of temperature.The model shows great capability to predict mechanical responses of Mg under different strain rates and temperatures.The results indicate that the flow stress increases with increasing strain rate and decreases with increasing temperature.Non-basal slip activities are enhanced at lower strain rates and higher temperatures.Compression twinning can be activated easily under c-axis compression of Mg crystals as the temperature is increased beyond 150 ?.The strain rate sensitivity of Mg becomes more evident with increasing temperature.The results coincide well with experimental observations in the literature.