Microstructural evolution and mechanical properties of ZrB2/6061Al nanocomposites processed by multi-pass friction stir processing

Microstructural evolution during multi-pass friction stir processing (FSP) of the as-cast ZrB2/6061Al nanocomposite and corresponding mechanical properties were investigated. The initial sample was characterized by primarily micrometer-sized clusters of ZrB2 nanoparticles within the coarse dendrites. Multi-pass FSP successively broke these clusters, refined the matrix grains and finally produced a thoroughly homogeneous microstructure in the stirred zone (except for the upper layer). The process of cluster redistribution was found to be governed by material flow around the pin, wherein the actual stir or mixing driven particle dispersion only occurred in the thread grooves. Meanwhile, the pinning effect of ZrB2 nanodispersoids retarded grain growth following recrystallization and led to a further reduction in grain size. On the other hand, compared with the initial state, both the microhardness and tensile strengths of the FSPed composites were gradually enhanced with consecutive passes. These increases can be attributed to the combination effect of grain refinement and dislocation strengthening induced by uniform distribution of ZrB2 nanoparticles. Quantitative analysis in the 4-pass FSPed composite indicated that Orowan strengthening contributed the most to the yield strength.