Processing, microstructural evolution and strength properties of in-situ magnesium matrix composites containing nano-sized polymer derived SiCNO particles

In-situ Mg matrix composites are fabricated by combining both liquid- and solid-state processing routes. Firstly, liquid polymer was injected into the molten Mg at a temperature of 800 degrees C to initiate pyrolysis. In-situ pyrolysis aids in the conversion of liquid polymer into sub-micron sized SiCNO particles (mean particle size in the range of 0.5-1 mu m) and Mg2Si particles. Most of the polymer derived SiCNO particles were pushed by the solidification front and as a result segregated at the grain boundaries of as-cast composites (mean grain size in range of 50-65 mu m) during subsequent solidification process. Formation of Mg2Si phase could be minimized by reducing the pyrolysis temperature from 800 to 700 degrees C. Single pass friction stir processing (FSP) of these as cast composites lead to improved homogeneity in the SiCNO particle distribution, particle refinement (mean particle size of about 200-300 nm) and grain refinement (mean grain size in range of 2.5-3.5 mu m). Mechanical properties (hardness, compressive yield stress, ultimate compressive stress, strain to failure and strain hardening exponent) of the FS processed composites were enhanced significantly as compared to their as cast counterparts. Strengthening mechanisms and numerical models are being evoked to explain the observed yield strength in these two stage processed composites.