Dislocation Density-based Crystal Plasticity Constitutive Model And Applications

Macroscopic mechanical behaviors of materials under high temperature and hydrogen environment are determined by their microscopic deformation mechanisms.How to relate these microscopic mechanisms to macroscopic mechanical properties is a key scientific problem.One of the most effective ways is to develop crystal plasticity models concerning various deformation mechanisms.Based on the literature reports on experiments and dynamics of discreate dislocations,the main features of dislocation climb and hydrogen enhanced localized plasticity,which are shown to be the crucial mechanisms under high temperature and hydrogen environment,are summarized respectively.Three different dislocation density-based crystal plasticity models concerning these two mechanisms are proposed for different scientific problems in this article,and their influences on mechanical behaviors are investigated by the finite element simulations in detailed.The main contents and innovations include:(1)A fully-implicit integration scheme for dislocation density-based crystal plasticity model is proposed,with plastic velocity gradient in the intermediate configuration and dislocation densities being the independent variables.The integration scheme is applied to all the models developed in this article,and showing good computational efficiency.(2)For plasticity and creep of crystalline materials,a dislocation density-based crystal plasticity model with explicit consideration of dislocation climb is developed.In this model,three contributions of dislocation climb to plastic deformation are involved: the kinematics,the climb-enhanced dislocation mobility and the climb-induced dislocation annihilation.The finite element calculations show that the present model can capture the temperature dependence of compression flow stress and the power law creep behavior of single crystalline aluminum;dislocation climb lowers the density of edge dislocations through climb-induced annihilation,and increases the density of screw dislocations by climb-enhanced dislocation mobility at the same time;kinematic contribution of dislocation climb is negligible compared with climb-enhanced dislocation mobility in most cases.(3)For microscale effect of crystalline material under elevated temperature,a scale-dependent crystal plasticity model is developed by introducing dynamics evolution of geometrically necessary dislocations(GNDs)densities,with special attention paid to the influence of dislocation climb on microscale effect under high temperature.Based on the experimental reports and the simulations of discreate dislocation dynamics,dislocation pile-ups against various interfaces are one of the main reasons for microscale effect.In the present model,the back stress of dislocation pile-ups at various interfaces is incorporated,and the relaxtion due to climb of piled up edge dislocations at elevated temperature is considered.The model is applied to the finite element calculations of micro-cantilevel beams,and the influences of dislocation climb on bending size effect is analysised in detailed.(4)For hydrogen-affected plasticity of polycrystalline face-centered cubic(FCC)metals,a dislocation density-based crystal plasticity model concerning hydrogen-enhanced localized plasticity(HELP)mechanism is proposed.In this model,the explicit consideration of dislocation line energy leads the way to the modelling of HELP mechanism according to the Gibbs theory of absorption isotherm.It is shown that the experimentally observed hydrogen-reduced activation volume and total activation free energy in FCC metals can be easily attributed to the reduction of dislocation line energy in hydrogen environment.Finite element calculations of tensile tests for polycrystalline Pd-H and Ni-H alloys capture several important hydrogen-affected plasticity behavior: hydrogen-increased flow stress,hydrogen-enhanced dislocation multiplication,hydrogen-promoted heterogeneity of plastic strain and hydrogen-delayed exhaustion of mobile dislocations,which can be all attributed to hydrogen reduced dislocation line energy in the present model.The present work is essential for further physically based modelling of hydrogen distribution in polycrystals and hydrogen induced damage and failure.