Identification of thermodynamically stable ceramic reinforcement materials for iron aluminide matrices
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I.
INTRODUCTION
I R O N aluminides are currently being considered as potential high-temperature structural materials. The ironaluminum system, shown in Figure 1,1~1 has an ordered a2 phase with a B2 crystal structure and wide solubility limits. The alloys of current interest are within this a2 phase with A1 concentrations ranging from 40 to 50 at. pct. The alloy Fe-40AI* transforms to a disordered a *All alloy compositions are given in atomic percent.
phase at about 1473 K. The melting temperature for the alloy Fe-50A1 is about 1488 K. Thus, the upper use temperature limit for these two alloys would probably be about 1273 K. These alloys, in addition to having lower densities compared to the superalloys, have excellent cyclic oxidation resistance t2j up to 1273 K. Furthermore, the alloy Fe-40A1 has exhibited good room-temperature ductility, t3~ with elongation on the order of 3 to 5 pct. One potential drawback for the iron aluminides is their loss of strength above 700 K.[41 This can be overcome by reinforcing the aluminides with high-strength, highmodulus fibers. These fibers are, then, expected to carry the major portion of the load. The reinforcement materials, besides having high strength and high modulus, must be chemically compatible with the matrices. For reinforcement materials that react excessively with the matrices, suitable coatings must be applied on the reinforcement materials. Prediction of chemical stabilities of different reinforcement materials in a given matrix, based on thermodynamic considerations, can narrow down the choices for reinforcement materials and thus reduce the experimental effort needed to identify potential reinforcement materials. In this paper, the chemical interactions between iron aluminides and several potential reinforcement materials are analyzed from thermodynamic considerations. The primary aim of these analyses is to identify chemically compatible reinforcement materials for iron aluminide matrices with A1 concentrations in the range of 40 to
AJAY K. MISRA, Senior Research Engineer, is with the NASA Lewis Research Center Group, Sverdrup Technology, Inc., 21000 Brookpark Road, Cleveland, OH 44135. Manuscript submitted May 8, 1989. METALLURGICAL TRANSACTIONS A
50 at. pct. Keeping in mind the application temperature for the iron aluminides, all of the thermodynamic analyses were performed at 1273 K. II.
REINFORCEMENT MATERIALS
The reinforcement materials considered in this study included carbides, borides, oxides, and nitrides. A list of specific reinforcement materials considered in the present study is given in Table I. The reinforcement materials were chosen primarily because of their high melting points. III. A C T I V I T I E S O F C O N S T I T U E N T E L E M E N T S IN THE INTERMETALLIC MATRIX Reaction between the alloy and the reinforcement material would depend upon the activity of Fe and A1 in the alloy. Radcliffe e t a l . ~51 have measured the activity of A1 (with respect to liquid AI) in Fe-A1 alloys in the temperature range of 1148 to 1273 K by electro
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