A peierls model of dislocation transmission through coherent interfaces and embedded layers
The interfacial obstacle to dislocation transmission is an important factor controlling the strength of nanometer-scale multilayers. The critical shear stress (CSS) to transmit a screw dislocation across a coherent interface was studied using a Peierls framework. First a one-interface model was developed. A screw dislocation of Burgers vector b is represented by N model dislocations each of Burgers vector b/N, which can spread into a slipping interface upon energy relaxation during transmission. An envelope of E-X is constructed for the transmission with E being the line energy and X being the work conjugate to the Peach-Koehler force. CSS is taken as the maximum slope of the E-X curve. Several factors influencing CSS are studied. Interfacial spreading increases CSS with the effect more pronounced for smaller modulus mismatch. The slip plane inclination angle has little effect on CSS. Coherency stress increases CSS significantly for small modulus mismatch. Dislocation dissociation reduces CSS. Generally, CSS increases with DeltaE/DeltaX, where DeltaE is the change in dislocation line energy and DeltaX is the dislocation motion over which the energy transition occurs. There are three concepts to increase DeltaE/DeltaX: (1) increase the change in line energy for a dislocation on the outgoing versus incoming side; (2) decrease DeltaX; (3) create an energy trap by interfacial spreading. The elastic moduli, unstable stacking fault energies and antiphase boundary energies of the left phase, the interface, and the right phase are available to change DeltaE/DeltaX and therefore CSS. Later an embedded layer model was developed to study the effect of the layer thickness on CSS. The decrease of CSS is about 5-20% at a layer thickness of 4b relative to that at larger than 10^3b. Further a 3D model was developed to study the effect of thermal activation on CSS. The activation energy and corresponding temperature were estimated as functions of applied shear stress. The room temperature CSS for a full screw dislocation in Cu/Ni multilayer system is about 60% of that at 0K, while 75% for a partial dislocation. Finally the simulation is found comparable with experiments for Cu/Ni multilayers.
School:The Ohio State University
School Location:USA - Ohio
Source Type:Master's Thesis
Keywords:dislocation transmission critical shear stress peierls model coherent interface multilayer
Date of Publication:01/01/2004