Thermodynamic analysis of chemical compatibility of ceramic reinforcement materials with niobium aluminides
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Niobium aluminide-based intermetallic matrix composites are currently being considered as potential high-temperature structural materials. One of the key factors in the selection of a reinforcement material is its chemical stability in the matrix. In this study, chemical interactions between two niobium aluminides, Nb3Al and Nb2Al, and several potential ceramic reinforcement materials, which include carbides, borides, nitrides, and oxides, were analyzed from thermodynamic considerations. Several thermodynamically stable reinforcement materials have been identified for these two matrices.
I. INTRODUCTION
There is considerable interest at present in developing fiber-reinforced intermetallic matrix composites for high temperature structural applications. Two of the niobium aluminides, Nb3Al with a melting point of 2233 K1 and Nb2Al with a melting point of 2143 K,1 are attractive for applications requiring temperatures greater than 1673 K. One of the key factors in selecting a reinforcement material for a given matrix is its chemical stability in the matrix. Prediction of the chemical stabilities of different reinforcement materials in a given matrix, based on thermodynamic considerations, can narrow down the choices of reinforcement materials, and thus reduce the experimental effort needed to identify suitable reinforcements. In this paper, the chemical stabilities of several ceramic reinforcement materials in Nb3Al and Nb2Al matrices are analyzed from thermodynamic considerations. The general approach is similar to that reported in an earlier paper by the present author2 that addressed the chemical compatibility of different reinforcement materials with iron aluminides. The ceramic reinforcement materials considered in this study include carbides, borides, nitrides, and oxides, and are listed in Table I. These ceramic materials were chosen primarily because of their high melting points.
and higher order compounds, only binary compounds will be considered in this study. Several A*Cy and BXC,, compounds can form due to the reaction of AB with the C element of the reinforcement material. However, only one C-containing compound would be stable in a given matrix. Similarly, only one D-containing compound would be stable in a given matrix. Once the stable C-containing and D-containing compounds in a given matrix are identified, the next step is to determine, from thermodynamic considerations, if these stable product compounds can form by the reaction of the intermetallic matrix with the reinforcement material. The reaction products can form either at unit activity or at reduced activities, i.e., at activities less than unity, by dissolving either in the matrix or in the reinforcement material. The possible reactions are:
A + CD = AD + C 2A + CD = AC + AD A + B + CD = AC + BD A + CD = AD + C A + CD = AD + C
(1) (2) (3) (4) (5) The underline in a given reaction denotes that the element or the compound is present at activities less than unity. TABLE I. Ceramic reinforcement materials considered in this study.
II. METHOD
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