Preparation Of Az91/sic Lamellar Composites Using Ice Templateand Analysis Of Infiltration Kinetics

Shell with a unique brick-and-mortar microstructure usually displays high strength and toughness. Based on the bionic ideas, MMCs with lamellar microstructure prepared by melt infiltration and freeze casting techniques have been extensively studied. Up to date, the preparation of the nacre-like metal-ceramic composites is limited to Al matrix, and most of the studies employed high-cost pressure infiltration technique due to the poor wettability between metal and ceramic. For the Mg matrix, there is no report. As known, Mg has strong chemical reactivity and high vapor pressure and thus is easy to ignite and evaporate at elevated temperatures, making the preparation of Mg-matrix composites difficult.In this work, we prepared the lamellar AZ91/SiC composites by using a two-step processing route involving ice templating and reactive infiltration. The strength of the SiC scaffolds was improved by adding sintering aids. Moreover, we analyzed reactive infiltration dynamics of the AZ91/SiC composites. In this study, the main results are as follows:(1) Adding Al2O3-Si O2-MgO-Y2O3-SrCO3 as sintering additives can effectively form low melting liquid strontium silicate(SrAl2Si2O8), which enhanced the strength of SiC scaffold. Under the condition of preoxidation for 2h at 1100 oC and sintering for 2h at 1500 oC, the porous SiC scaffolds with porosity of 73.3±0.8%、66.3±0.6%、54.6±1.1% showed a compressive strength of 18±3MPa、38±5MPa、55±7MPa.(2) The infiltration process of AZ91 alloy on the orientational porous SiC scaffolds can be divided into three stages: melting stage, incubation stage and rapid infiltration stage, respectively. The infiltration activation energy under different preoxidation conditions was also measured, showed 8.53kJ/mol at 1100oC-1h、7.74kJ/mol at 1100oC-2h and 7.56kJ/mol at 1200oC-1h, respectively. After infiltration, the main products between ceramic and alloy was Mg2 Si. The morphology of Mg2 Si is mainly determined by the infiltration temperature and preoxidation.(3) The compression and bending strength of lamellar composite is closely related to the peroxidation process of SiC scaffolad. The compression strength of AZ91/SiC composites under three different preoxidation process was measured, which showed 653±23MPa、743±16MPa、818±21MPa under the condition of 1100oC-1h、1100oC-2h and 1200oC-1h; The bending strength of AZ91/SiC composites under three different preoxidation process was also measured, which gave 403±22MPa、599±44MPa、460±20MPa under the condition of 1100oC-1h、1100oC-2h and 1200oC-1h.(4) The crack can continue extend in two ways: on the one hand, it propagated along the metal-ceramic interface; on the other hand, the cracks deflected in the metal layer and then extend to the next ceramic layer. This led to the extension of the crack propagation path and increase in the energy for the crack growth, thus enhancing the toughness of the composite.