Design Of Alloys For Reactive Melt Infiltration And The Structure And Properties Of SiC_f/SiC Composite Modified By The Alloys

SiC_f/SiC composites are considered to be one of the most promising thermal structural materials for aerospace applications due to their low density,high specific modulus,and excellent thermal stability properties at high temperatures.The reactive melt infiltration（RMI）process displays significant advantages of a short preparation time,a simple process,and the ability to prepare high densification SiC/SiC composites.However,too high preparation temperature tends to damage the SiC fibers and reduce the mechanical properties of the material;furthermore,the residual metal in the composite will have a large impact on the intrinsic oxidation resistance of the composite.In this study,low melting point alloys with excellent oxidation resistance were designed for the RMI process,and SiC_f/SiC composites were prepared by the RMI process.The microstructure,mechanical properties and oxidation properties of the composites were investigated.The main conclusions are as follows:（1）Three alloys with melting points of about 1400-1500°C,that is,Si-14.88B-7Mo,Si-14.88B-7Ti and Si-14.88B-7Cr,were designed and prepared according to the phase diagram.The Si-14.88B-7Mo alloy consists mainly of Si,Mo Si₂,borosilicate and a small amount of Mo₂B₅,the Si-14.88B-7Ti alloy is mainly composed of Si,Ti Si₂and Ti B₂,and the Si-14.88B-7Cr alloy consists of Si,Cr Si₂and Cr B₂.There are eutectic organizations of Si+Ti Si₂and Si+Cr Si₂in Si-14.88B-7Ti and Si-14.88B-7Cr alloys,respectively.（2）The Si-B-Mo alloy exhibits superior oxidation resistance compared with the other two alloys.The dense oxide layer and the low solubility of Mo ions in Si O₂inhibit the volatilization of Mo O₃and oxidation reaction,which reduces the oxidation rate of the alloy.The difference in thermal expansion coefficients of Si O₂and Ti O₂leads to penetrating cracks in the oxide layer of Si-B-Ti alloy during cooling,which reduces the oxidation resistance of the alloy.The Si-B-Cr alloy undergoes severe oxidation of the material due to the severe volatilization of the oxide film and the generation of molten holes within the alloy that destroy the surface oxide film.（3）SiC_f/SiC composites with a density of 2.53 g/cm³and porosity of5.17%were prepared by Si-B-Mo powder RMI at 1450°C.The SiC_f/SiC composites are mainly composed of SiC,Mo Si₂,residual Si and RC.The bending strength of the composites is about 320.24 Mpa.In addition,the residual Si and RC content in the material can be significantly reduced by the heat treatment.However,the heat treatment causes further damage to the SiC fibers,which is not conducive to the increase of the bending strength of the composites.The bending strength of the composites was reduced by 27.61%after 5 h of heat treatment.（4）The oxidation behaviors of SiC_f/SiC composites prepared by melt infiltration of pure Si and Si-B-Mo powder were investigated in the temperature range of 1200-1400℃.It was found that there were two stages of weight loss stage and weight gain stage during the oxidation process.The weight loss stage is caused by the preferential oxidation of the Py C interface and residual RC in the material.The weight loss and weight gain stages follow a linear and parabolic law,respectively.Based on the oxide film thickness and oxidation weight gain,the SiC_f/SiC composites prepared by Si-B-Mo powder have more excellent oxidation resistance than the composites prepared by pure Si.This is due to the oxidation of SiC_f/SiC composites prepared from Si-B-Mo powder that can form a low melting point borosilicate glass phase,which improves the self-healing ability of the surface oxide layer.