Al‐B‐C Phase Development and Effects on Mechanical Properties of B ₄ C/Al‐Derived Composites

Abstract

B 4 C/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 A1B 2 ; 700° and 900°C results in a mixture of A1B 2 and A1 4 BC; 900° and 980°C produces primarily A1 4 BC; and 1000° to 1050°C results in A1B 24 C 4 with small amounts of A1 4 C 3 if the heating does not exceed 5 h. Deleterious A1 4 C 3 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 A1B 2 . The highest hardness (88 Rockwell A) and Young's modulus (310 GPa) are obtained in Al 4 BC‐rich samples. AlB 2 ‐containing samples exhibit lower hardness and Young's modulus but higher fracture toughness. While the modulus, Poisson's ratio, and hardness of multiphase B 4 C/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/cm 3 ) and higher than 7 MPa‐m 1/2 fracture toughness.