A phase-field model for simulating detwinning in nanotwinned materials

Nanotwins are nanostructures commonly observed in sputtered single-element and alloyed metals with low stacking fault energy. Nanotwins can improve several material and functional properties but are susceptible to detwinning under sufficiently elevated temperatures. In this work, we present an anisotropic multi-phase phase field model with fully variational evolution to treat the large boundary energy differences characteristic of nanotwinned structures in face centered cubic materials. This model formulation is first verified and validated against MD simulation, theory, and experiment. Using the model, we study the processes of grain boundary detachment and subsequent detwinning of nanotwins with thicknesses ranging from 1 nm to 15 nm under annealing temperatures at and below 700K. The simulations reveal that nanotwin migration velocity depends strongly on nanotwin thickness and annealing temperature, with thinner nanotwins and higher temperatures promoting faster migration. We further establish a connection between boundary thermodynamics and microstructural evolution. In particular, a tapered morphology for the nanotwin tip during detwinning emerges as a signature of high incoherent twin boundary energy, higher mobility, and lower thermal stability.