Structure and mechanical properties of internally oxidized Ta-8 Pct W-2 Pct Hf (T-111) alloy

The deformation and fracture behavior of the Ta-8 wt pct W-2 wt pct Hf (T-111) alloy internally oxidized at 1273 K was studied at test temperatures up to 1589 K (2400°F). The results are discussed in terms of oxygen content, and the morphology of the oxide. Thermodynamic calculations based on lattice parameter measurements indicate that the oxygen solubility in T-111 is low, about 72 ppm at 1973 K. Beyond the solubility limit, oxygen reacts with hafnium to form hafnium oxide, which causes a continuous decrease in lattice parameter until all the hafnium has been removed from the solid solution. A homogeneous dispersion of fine, coherent hafnium-oxygen zones was observed in as-oxidized specimens. These zones coarsened upon annealing at or above 1673 K and eventually precipitated as rectangular HfO₂ particles. Alteration of the oxygen distribution and oxide morphology by heat treatment resulted in large changes in the mechanical properties. For example, specimens aged 15 min at 1973 K showed little change up to 3500 ppm O, but above this level the yield strength increased and ductility decreased abruptly. Ductility was completely lost at 4200 ppm O, independent of test temperature. Specimens with no aging treatment other than oxygen addition lost all ductility at 800 ppm O. The structure and mechanical properties are correlated.