Nano-plasticity in BCC and HCP metals

Abstract

Nano-plastic deformations attributable to diffusion of point defects, and motion of dislocations in bcc and hcp metals are studied in this thesis. Tungsten and zirconium are chosen as typical bcc (body-centered-cubic) and hcp (hexagonal-close-packed) metals, respectively. Standard and modified conjugate gradient methods are used to locate the ground state and saddle point configurations of point defects, namely, vacancies and interstitials. The nano-plasticity originated from dislocation motion is also investigated using quasi-dynamic simulation-a combination of molecular dynamic calculation and Green's function flexible boundary condition. In the first part of this thesis, the formation energy, migration energy, migration mechanisms, ground state and saddle point configurations and the associated elastic dipole tensors are determined. Two migration mechanisms of vacancies and four mechanisms (in the form of <1120> crowdion) of interstitials are identified in hcp zirconium. For bcc tungsten, one migration mechanism of the vacancy and two mechanisms for the interstitial (in the form of <111> crowdion) are found. The elasto-diffusion tensors are derived for both zirconium and tungsten based on the dipole tensors and diffusion mechanisms. An application of the elasto-diffusion tensor is carried out in the last part of section 3.1 to demonstrate the contribution of diffusional anisotropic difference (DAD) of point defects to irradiation-induced deformations in hcp zirconium. Then, we simulate the glide of edge dislocations and their interactions with voids using the quasi-dynamic simulation method in bcc tungsten. Core structure, energetics and glide mechanisms of edge dislocations are investigated to illustrate the direct relation between the nano-plastic deformation and the initiation of dislocation motion. Furthermore, the interactions between edge dislocations and voids in bcc tungsten, the bow-out of a straight dislocation under an external applied stress against a field of defects, are carried out to elucidate the relation between the nano-plastic deformation and dislocation evolution in the presence of existing obstacles. The core structure of an edge dislocation is symmetrical on {110} planes, but asymmetrical on {112} plane in bcc tungsten. Consequently, the Peierls stress is independent of shear direction on {110} planes, but depends on shear direction on {112} planes. The Peierls stress is on the order of 10-4 u for edge dislocations gliding on both kinds of planes, where u is the shear modulus. For the dislocation on {110} planes, three individual displacements are required for the glide of one Burgers vector; and two for the dislocation on {112} planes. Against voids, the edge dislocation bows out under an external applied stress, all the necessary parameters describing the void strengthening effect are determined. The simulation results validate the continuum model, assuming an effect radius of 1.78b, where b is the magnitude of Burgers vector. Dislocation climb is seen when the dislocation moves through a sufficiently large void. Finally, the interactions between a screw dislocation and vacancy clusters are investigated using a combination of standard conjugate gradient method and modified Green's function flexible boundary condition. The vacancy clusters, in the form of platelets, collapse into staking-faults in the presence of a screw dislocation, while in the absence of the screw dislocation, they keep their un-collapsed form. The generation of faulted loops is very important to the further evolution of microstructure. A faulted vacancy loop may continuously expand in the basal plane until it meets other obstacles existing in crystal. In contrast, an un-faulted vacancy cluster will expand into cavity and eventually annihilate by mutual combination of vacancies and interstitials due to the difference of diffusion mechanisms of interstitials and vacancies in hcp metals.