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 (8and 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, decreasing strength with increasing temperature, brittle behavior, and transgranular fractures. This two-phase material with a grain size of 25 micrometers was, however, stronger than the 24-micrometer grain size single -phase material, which indicates a strengthening 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 apparently 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.
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