Growth into Fibrous Preforms

The stiffness of a DMO composite of AI2O3-AI with 22% alloy and 4% porosity (231 GPa) was modeled successfully [60] by assuming the metal and the ceramic skeleton to deform equally in series and in parallel, that is, by taking a Reuss-Voigt average. The additional effect of isolated pores could be included by using empirical expressions derived from data on porous alumina. The elastic modulus 

DMO composites with particulate reinforcements of alumina and silicon carbide were found to possess moduli of 302 and 324 GPa, respectively [92]. In the former case, the total alumina content, including the reinforcement, was 81% while in the latter, the SiC volume fraction was 46% and the matrix alumina 37%. The alloy content and porosity were in the vicinity of 15 and 2 vol-%, respectively. A similar value of 313 GPa was reported for a SiC reinforced DMO composite whose composition was not disclosed [93], 

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Of the two most commonly used preform materials, one, alumina, is chemically inert during DMO while the other, silicon carbide, is reactive. We first examine the growth of a DMO composite into an alumina particulate, followed by that into SiC preforms, and finally the DMO infiltration of a suitably coated fibrous preform. Coatings of calcium sulfate with calcium silicate may be used to ensure that oxidative growth ceases at the external boundary of the preform, thereby ensuring near-net-shape fabrication. The mechanism by which these barriers poison the oxidation without impeding the flow of oxygen is not clear at this time. 

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