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 pr operties can be developed by postdensification heat treatment. In appl ications 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 degr ees and 700 degrees C produces mainly AlB2; 700 degrees and 900 degree s C results in a mixture of AlB2 and Al4BC; 900 degrees and 980 degree s C produces primarily Al4BC; and 1000 degrees to 1050 degrees C resul ts in AlB24C4 with small amounts of Al4C3 if the heating does not exce ed 5 h. Deleterious Al4C3 is avoided by processing below 1000 degrees C. All of these phases tend to form large clusters of grains and resul t 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 mo dulus but higher fracture toughness. While the modulus, Poisson's rati o, and hardness of multiphase B4C/Al composites containing 5-10 vol% f ree metal are comparable to ceramics, the unique advantage of this fam ily of materials is low density (<2.7 g/cm(3)) and higher than 7 MPa.m (1/2) fracture toughness.