3, Lawrence Livermore National Laboratory, Livermore, California, United States
2, Ames Laboratory, Ames, Iowa, United States
Strengthening metals through grain boundary (GB) and twin boundary (TB) interfaces into the nanoscale region is manifested by a maximum strength, a phenomenon that is known as Hall-Petch limit, followed by softening behavior. The underlying mechanisms of this observation have been studied extensively by atomistic simulations. Yet, experimental validations of these mechanisms have been challenging, primarily due to the technological difficulty in synthesizing nanotwinned FCC metals in the truly nanocrystalline regime (<100 nm) that could match grain sizes in atomistic models. In this talk, we present experimental and atomic simulation investigations on the effects of microalloying on microstructure stability and Hall-Petch strengthening in sputter-deposited nanotwinned Ag containing trace concentrations of Cu solute atoms (<1.0 wt. %). First, we discovered that annealing of nanotwinned Ag at a Cu concentration level of 0.81 wt. % results in grain size (d = 49 nm) and TB spacing (λ = 3.5 nm) that were well below those previously obtained in nanotwinned Ag, leading to a record hardness up to 39% above values reported for this metal without alloying. Second, large-scale hybrid Monte-Carlo and Molecular Dynamics simulations, and density-functional-theory-based calculations, were deployed to study the atomic-scale processes associated with Cu solute atom segregation, yielding and plastic flow in nanotwinned Ag, as a function of TB spacing. It was found that Cu atoms are segregated concurrently to GBs and intrinsic kink-like TB defects during thermal annealing, resulting in dramatic improvements of both twin stability and yield strength under applied stress in micro-alloyed nanocrystalline-nanotwinned Ag. Furthermore, we present the plastic deformation mechanisms underpinning the Hall-Petch strength limit in nanocrystalline-nanotwinned Ag models, for which the simulated microstructure is comparable to that of micro-alloyed experiments.