Evolution of nt-microstructures
The Achilles heel of nanostructured metals has been the loss of ductility despite being able to achieve significant strengthening over their conventional coarse-grained counterparts. Various strategies have been proposed to mitigate this problem, and an attractive recourse is to engineer hierarchical micro-architectures that assist in strengthening by providing barriers to dislocation motion while concurrently retaining or introducing stabilizing mechanisms such as enhanced rate-sensitivity and strain-hardening. Hierarchical nanotwinned (nt) microstructures simultaneously exhibit impressive strengthening, hardening, and ductility with decreasing twin thickness (Lu and coworkers, Science (2004)) that is seldom observed in nano-grained microstructures obtained via grain refinement. Interestingly, these microstructures also exhibit a strengthening to softening transition at yield below a critical twin thickness (Lu and coworkers (2009)). Using experimental and molecular dynamics observations as the basis, we propose a length-scale dependent Discrete Twin Crystal Plasticity (DT-CP) framework for nt-metals. The novel feature of this work is that we retain the discreteness of twin lamellas, which provides unique avenues for investigating the mechanics of nt-microstructures.
The DT-CP approach allows probing local characteristics with higher resolution compared to some of the existing homogenized approaches. For example, plastic slip heterogeneities near twin boundaries are well-resolved, which are useful in addressing ductility and evolution of failure modes. Our current focus is on extending the DT-CP approach to modeling TB migration using mechanism-based kinetics. We have successfully integrated twin boundary migration kinetics into the finite element based crystal plasticity framework.