Microstructure and mechanical properties of friction stir processed Al-Mg-Si alloys dispersion-strengthened by nanosized TiB2 particles

In this paper, we reported microstructure and mechanical properties of the in-situ TiB2/Al-Mg-Si composites processed by friction stir processing (FSP). Compared with the initial state, the proper FSP conditions can enhance both the ultimate strength and the elongation of the composite samples obtained from the nugget zone. Detailed microstructure investigation has been performed by synchrotron X-ray diffraction, scanning and transmission electron microscopy and associated electron backscattered diffraction in order to reveal the mechanisms being responsible for the unusual mechanical behaviors. The results show that the initial composite has a grain size of 50-100 mu m and the synthesized nanosized TiB2 particles are almost agglomerated to form micrometer sized clusters at grain boundaries. Comparatively, after FSP, the nugget zone is characterized by fine and equiaxed recrystallized grains (1-5 mu m in average grain size). The initial clusters are also broken up, while the nanosized TiB2 particles are distributed much more uniformly in the matrix and act as effective pins to interact with dislocations. Hence, the significantly refined grains and the uniform distribution of the nanosized TiB2 particles mainly contribute to the increase of both strength and ductility of the FSPed composites. The strengthening mechanisms are also discussed. (C) 2014 Elsevier B.V. All rights reserved.