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 postdensification heat treatment. In applications where hardness and high modulus are required, heat treatment above 600 degrees C provides a multiphase ceramic material containing only a small amount of residual metal. Heat treatment between 600 degrees and 700 degrees C produces mainly AlB2; 700 degrees and 900 degrees C results in a mixture of AlB2 and Al4BC; 900 degrees and 980 degrees C produces primarily Al4BC; and 1000 degrees to 1050 degrees C results in AlB24C4 with small amounts of Al4C3 if the heating does not exceed 5 h. Deleterious Al4C3 is avoided by processing below 1000 degrees 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/cm(3)) and higher than 7 MPa.m(1/2) fracture toughness.