Transverse rupture tests were conducted on single-phase hafnium carbide and hafnium carbide containing 13- volume- percent hafnium diboride as a distinct second phase. These tests were performed between room temperature and 4755° F (2625°c).The test materials were hot pressed to densities in the 95 to 97 percent of theoretical density range.
　　For the single-phase material at temperatures below about 4000°F (2205° c),strength decreases with increasing temperature, fractures are completely brittle, and
the fracture path is transgranular. Above 4000°F (2205°c),strength increases with temperature, some plasticity is present, and fractures are intergranular. Single-phase material of two different grain sizes (8 and 24 μm) both exhibited these general features; however, the smaller grain material was stronger at all temperatures.
　　Below 4000°F(2205° c), the material containg hafnium diboride as second phase exhibited the same general behavior as did the single -phase material, that is, decreas-ing strength with increasing temperature, brittle behavior, and transgranular fractures.This two-phase material with a grain size of 25 micrometers was, however, strongerthan the 24-micrometer grain size single -phase material, which indicates a strengthen-ing brought about by the hafnium diboride. Above 4000° F (2205° C) the two-phase material exhibited plastic behavior and intergranular fractures similar to single -phase hafnium carbide, however, strength decreases rapidly above 4000°F (220 5° C) in sharp contrast to single -phase hafnium carbide.
　　Metallographic evidence suggests that the plasticity observed in both materials is the result of grain-boundary sliding. In the single-phase material this sliding appar-ently acts as a stress relieving mechanism preventing premature failures, and, thus, strength increases after 4000° F (2205° c) is reached at which grain-boundary sliding
becomes operable. On the other hand, in the two-phase materials, it appears that the portion of the hafnium diboride present in the grain boundaries renders grain-boundary sliding ineffective as a strengthening mechanism, and leads to a rapid loss of strength at temperatures where grain-boundary sliding occurs.