Al‐B‐C Phase Development and Effects on Mechanical Properties of B4C/Al‐Derived Composites

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Abstract
B4C/A1 offers a family of engineering materials in which a range of properties can be developed by postdensiflcation heat treatment. In applications where hardness and high modulus are required, heat treatment above 600°C provides a multiphase ceramic material containing only a small amount of residual metal. Heat treatment between 600° and 700°C produces mainly A1B2; 700° and 900°C results in a mixture of A1B2 and A14BC; 900° and 980°C produces primarily A14BC; and 1000° to 1050°C results in A1B24C4 with small amounts of A14C3 if the heating does not exceed 5 h. Deleterious A14C3 is avoided by processing below 1000°C. All of these phases tend to form large clusters of grains and result in lower strength regardless of which phase forms. Toughness is also reduced; the least determinal phase is A1B2. The highest hardness (88 Rockwell A) and Young's modulus (310 GPa) are obtained in Al4BC‐rich samples. AlB2‐containing samples exhibit lower hardness and Young's modulus but higher fracture toughness. While the modulus, Poisson's ratio, and hardness of multiphase B4C/A1 composites containing 5–10 vol% free metal are comparable to ceramics, the unique advantage of this family of materials is low density (>2.7 g/cm3) and higher than 7 MPa‐m1/2 fracture toughness.

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