869 Metal Matrix Composites

The success exhibited by organic matrix composites has led to parallel efforts to develop engineered metals including metal matrix composites (MMC's).

The CermeTi family of titanium alloy matrix composites, fabricated using the blended elemental approach, incorporates particulate ceramic (TiC or TiB2) or intermetallic (TiAl) as a reinforcement [19]. These composites exhibit minimal particle-matrix interaction while maintaining the integrity of the essentially 100% dense homogeneously dispersed particles within the matrix.

An innovative method (XD) for the production of in situ discontinuous titanium-based composites has resulted in interesting property combinations in alloys such as the intermetallic y [19], but to date there are no applications.

Reinforcement with continuous ceramic composites enhances the strength and modulus of terminal titanium alloys, such as Ti-6Al-4V [22, 23]. However, control of the reaction zone between the fiber and the matrix, inferior transverse properties, and very high cost remain major concerns. Innovative fabrication techniques such as plasma spray deposition and electron beam vapor deposition may help in controlling cost, while the design lessons learned with nonisotropic polymeric composites should be applicable in engineering metal composite structures.

The potential weight savings obtainable by replacing much heavier superalloys in both engine and airframes has resulted in considerable work being conducted on a2 and y MMCs [24]. Recently, increased attention has been given to the richer Nb varieties of Ti3Al-Nb known as the "orthorhombic" alloys (22-27 at % Nb) as matrix materials [7, 14].

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