Al-B-C phase development and effects on mechanical properties of B4C/Al-derived composites

B4C/Al offers a family of engineering materials in which a range of properties can be developed by post densification 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 AlB2; 700 and 900 C results in a mixture of AlB2 and Al4BC; 900 and 980 C produces primarily Al4BC; and 1,000 to 1,050 C results in AlB24C4 with small amounts of Al4C3 if the heating does not exceed 5 h. Deleterious Al4C3 is avoided by processing below 1,000 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 AlB2. 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/Al 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 high than 7 MPa·m1/2 fracture toughness