Strengthening of sheath-rolled aluminum based MMC by the ARB process

The Accumulative-Roll Bonding (ARB) process was applied to an aluminum based MMC fabricated by a sheath rolling method, in order to improve the mechanical properties. The 8 cycles of ARE were performed at ambient temperature under unlubricated conditions. The ARE process was also performed on the unreinforced material, for comparison with the composite sample. The tensile strength of the composite increases with the number of cycles up to 5 cycles of ARE, above which it decreases slightly. The tensile strength of the unreinforced material increases up to 2 cycles of ARE, above which it does not change. The strengthening in the composite by the ARE process is caused by the improvement in homogeneity of distribution of reinforcement particles with the number of cycles as well as work hardening of the matrix material, while the strengthening in the unreinforced material is only caused by work hardening. The drop in strength for both materials is caused by recovery or recrystallization. On the other hand, the elongation tends to greatly decrease after 1 cycle, but from 2 cycles it increases with the number of cycles for both materials. The unreinforced material shows an inhomogeneous microstructure along the thickness direction at lower cycles of ARE; having a finer grained structure near surface and bonded interfaces than the other area. However, the inhomogenity of microstructure vanishes gradually as the number of cycles increases. As a result, the microstructure after 8 cycles consists almost entirely of ultra-fine grains under 1 mu m in diameter along the thickness direction. The ultra-fine grain initiate to develop at lower ARE cycle in composite than in unreinforced material. This is probably due to enhanced strain induced by locally accumulated deformation around reinforcement particles.