Discrete Dislocation Mechanism On Submicro-crystal Plasticity

Crystal plasticity at submicron scales involves many new features, and becomesone of the hottest research topics in mechanics and material science in recent years.Dislocation is the base of plastic flow. At this scale, the discreteness of dislocationmotion emerges. Therefore, the classical plasticity theory based on continuummechanics is no longer valid. On the other hand, the atomic simulations at submicroscales have strong temporal and spatial limitations on computational efficiency. Todescribe the evolutions of dislocation microstructure and reveal the feature of the plasticbehavior, the discrete dislocation mechanisms on atypical plastic phenomenacharacterized by submicro-crystals are studied by means of three-dimensional discretedislocation dynamics (3D-DDD).Submicron scale covers the critical dimension for the formation of dislocation jamstructure inside the single-crystal at the room temperature. Below the critical dimension,the free surface effect is pronounced and the dislocation junctions exist in isolation.Spiral source predominates in dislocation multiplication. Due to the weak storage ofdislocations, the strain hardening process shows strong temporal and spatialintermittency. Above the critical dimension, robust dislocation jamming takes place andthe junctions are distributed in chain shapes. Inside the crystal, dislocation cell structureforms and the Frank-Read source becomes the main multiplication mechanism.Therefore, the forest hardening mechanism controls the plastic flow.At high temperatures, dislocation climb is an important means of motion. However,the existing simulation models for climbing have mostly treated climb as a glide-likemotion, which is believed to be out of physical principle. In the present thesis, therelationship between the gradient of vacancy concentration and the climb velocity isdeveloped based on the bulk-diffusion and pipe-diffusion theory. This relation is thencoupled with3D-DDD to realize the simulation process of dislocation climb. Thepresent methodology offers a powerful tool to study the evolution of dislocationmicrostructure and the plastic deformation of crystals at high temperatures. Thepredictions of some typical climb-dominated processes, e.g. the activation of aBardeen-Herring dislocation source as well as the shrinkage and annihilation of prismatic dislocation loop(s), show the high efficiency and accuracy of the presentmethod. Besides, it is found that pipe-diffusion, compared to bulk-diffusion, plays amore vital role in driving the climb. By studying the break-up of dislocation dipole, theenergy criteria for the break-up is also established.To overcome the deficiency of3D-DDD method in dealing with the somecomplicated problems such as shape variation of crystals, boundary condition responses,etc., a unified computational model for discrete dislocation plasticity"DD-FEM"isdeveloped by directly combining3D-DDD and finite element method (FEM). Usingvariable-transforming technique, the plastic strain induced by dislocation slip during theDDD simulation is brought into the constitutive relation and then the stress field in thecrystal is updated during the FEM simulation. On the other hand, the displacement fieldinside the crystal and the stress distribution on the free surface in the FEM simulationare both shared in the DDD simulation to guarantee the concurrency of the deformationand the accuracy of the calculation of the image force. A deeper study on the uniformcompression of copper micropillar indicates that the discreteness of dislocation structureinside the submicro-crystal is the essential reason for the deformation heterogeneity.Besides, the contact condition between the indenter and the top surface of the pillar andthe taper condition can also severely affect the plastic behavior.By combining the3D-DDD and the molecular dynamics (MD), a hierarchicalmultiscale dislocation-grain boundary (GB) interaction model is established by takingaccount of the four kinds of dislocation-GB interactions, i.e. transmission, absorption,re-emission and reflection. The simulation of a copper bi-crystal compression showsthat the GB can clearly enhance the ability of storing dislocations, which could improvethe deformation homogeneity and controllability based on stress-relaxation mechanism.