Plastic strain-induced grain refinement at the nanometer scale in copper

Microstructural evolution and grain refinement in pure Cu subjected to surface mechanical attrition treatment (SMAT) were investigated by means of systematic transmission electron microscope observations. Two different mechanisms for plastic strain-induced grain refinement in Cu were identified, corresponding to different levels of strain rate. In the subsurface layer of the SMAT Cu samples with low strain rates, grains are refined via formation of dislocation cells (DCs), transformation of DC walls into sub-boundaries with small misorientations, and evolution of sub-boundaries into highly misoriented grain boundaries. The minimum size of refined grains via this process is about 100 nm. In the top surface layer (thickness <25 μm) with a high strain rate, the grain refinement includes: (i) formation of high-density, nanometer-thick twins dividing the original coarse grains into twin-matrix (T-M) lamellae; (ii) development of dislocation walls that further subdivide the T-M lamellae into equiaxed nano-sized blocks; (iii) evolution of these preferentially oriented blocks into randomly oriented nanosized grains. The minimum size of such refined grains is about 10 nm. The present study demonstrates the critical role of strain rate on the plastic strain-induced grain refinement processes and on the minimum grain size obtainable via plastic deformation.