Effect of oxygen absorption on lattice parameter and specimen dilation in a tantalum- base alloy containing 8 Pct tungsten and 2 Pct hafnium

Abstract

Sheet and tube specimens of a tantalum-base alloy containing 8 Pct tungsten and 2 Pct hafnium (T-111) were exposed to low pressure oxygen at 1000° for the purpose of study-ing the effect of oxygen absorption on lattice parameter and specimen dilation. Oxygen absorption produced an increase in weight and dilation which was proportional to ex-posure time at 1000°. In the as-doped condition, dilation is roughly equal to the lattice expansion, suggesting that the absorbed oxygen is retained mainly in the bec base solu-tion. After isothermal aging at 1400°, the lattice parameter of the as-doped specimen decreases sharply due to the precipitation of HO₂ particles; however, the dilation is not affected. The solubility of oxygen in T-lll is calculated to be 64 ppm at 1400°, based on some thermodynamic considerations and lattice parameter measurements. The T-lll specimen dilated 1 Pct after only an 80 h exposure at 1000° at an oxygen pressure of 1.3 x 10_-3 Pa (1 × 10_-5 Torr). Comparison of creep rates in high vacuum with the dilation rate indicates that specimen dilation due to oxygen absorption is the main reason that causes a higher “creep rate” of refractory alloys in contaminated environ-ments, especially under the conditions of high test temperature and low applied stress.