In situ observation of twin-assisted grain growth in nanometer-scaled metal

For nanometer-scaled materials, grain boundary behaviors, such as grain coalescence and grain boundary migration, contribute significantly to the plasticity of materials. While mechanically driven grain growth has been observed in nanometer-scaled metals, its underlying mechanisms are still poorly understood especially for those correlated with twins. By using in situ aberration-corrected transmission electron microscopy and precession electron diffraction, we have directly revealed nanotwin assisted grain growth for low angle grain boundaries. The grains with low angle grain boundaries coalesce by firstly forming twin pairs, whose coherent twin boundaries then serve as fast lanes for dislocations, and thus dissolving the low angle grain boundaries. During this process, the constitute dislocations of the low angle grain boundaries decompose into Shockley partial dislocations, which subsequently move along the coherent twin boundaries. After all the constitute dislocations dissociate and move out, two grains will coalesce and twin pairs merge into complete twin lamellae. For high angle grain boundaries, portions of grain boundaries intersected with nanotwins show higher mobility for migration under external stress. This investigation provides insight to twin related grain boundary behaviors in nanometer-scaled metals.