Correlation between grain boundary evolution and mechanical properties of ultrafine-grained metals

We put stiffness and strength of ultrafine-grained metals under scrutiny by means of experimental and analytical mechanics. Samples of Al 1050 were processed by Accumulative Roll Bonding (ARB) at different cycles to give ultrafine-grained microstructures. The microstructure, strength, and stiffness of the processed Al were analyzed by XRD and SEM. An analytical micromechanical model was developed to explain the predicted changes in the mechanical properties of the ultrafine-grained Al. It was revealed that an ultrafine-grained pure metal (e.g., Al) possesses a heterogeneous microstructure with the grain boundary stiffness lower than the grain/subgrain stiffness. This heterogeneous microstructure gave a decrease-after-increase behavior of the overall stiffness of Al during ARB. The grain boundary fraction of the microstructure increased from 1.9% after 2 cycles to 13.4% after 9 cycles of ARB. The stiffness decreased to 0.73 of its maximum value obtained after 2 cycles. With the aim of changing the stiffness and strength of the grain boundary, other samples were processed such that 4% SiC particles were added to Al sheets during the 1st-ARB cycle. The addition of SiC particles gave stiffer and stronger grain boundaries, which enhanced the overall stiffness and strength during ARB. Both the stiffness and strength were observed continuously increasing during ARB up to 484% and 793% after 9 cycles compared to the unprocessed Al, respectively.