Computer Simulations & Experimental Observations of Dislocation Motion in Nanoscale Multilayer Films

A predominant attraction of nanoscale multilayer thin films is the ability to specify the phases and interfacial structure, so that dislocations are constrained to propagate in individual layers. This ability to constrain slip is cited as a primary reason why multilayer thin films display such extraordinary hardness. However, constrained slip is not always possible. In particular, many multilayer systems exhibit a critical layer thickness, below which the hardness either reaches a plateau or even decreases. This critical layer thickness occurs due to the inability of interfaces to contain dislocations as layer thickness is decreased. This presentation is three-fold. We will present transmission electron micrographs from in-situ straining tests to first identify important dislocation propagation and transmission modes. Next, we will discuss continuum and atomistic simulations of dislocation tranmission across interfaces, to identify the important features that control dislocation transmission across interfaces. Finally, we will incorporate the experimental observations from the microscopy and important features of the dislocation transmission studies into a 3D dislocation model. The 3D dislocation model is used to study the response of both threading and interfacial dislocation segments as a macroscopic biaxial tension is applied to the multilayer. The results shed light on how the choice of phases, layer thickness, and interfacial structure serve to contrain crystal slip and thus provide extraordinary plastic strength.