PART V - Communications - Oxidation of Hf-Ta Alloys

MARNOCH has reported on the attractive high-temperature oxidation resistance of Hf-Ta alloys.' In view of current interest in these alloys, three Hf-Ta alloys were tested in the present study. These included a Hf-27Ta alloy and a Hf-20Ta-2Mo alloy which were supplied by Marquardt Corp. and a Hf-20Ta-Zr alloy provided by Illinois Institute of Technology Research Institute.' The microstructure of these alloys was heated inductively in a glass vacuum system; the oxidizing gas, helium carrier plus a known partial pressure of oxygen (35-102 Torr), was continually passed over the sample at temperature for a given time period. The concentration of oxygen in the gas stream was continuously measured by a differential thermal conductivity technique2 so that the rate of oxygen picked up by the sample could be obtained. The flow rates were 0.1 to 0.2 ft per sec in the vicinity of the specimen. The concentration of oxygen did not fall below 50 pct of the input level in any of the experiments. In several cases a single sample was exposed consecutively at a series of increasing temperature levels. In a second system at ManLabs, cylindrical specimens were exposed to flowing air in a z irconia tube which was heated by radiation from a carbon tube resistance furnace. The specimen was heated from room temperature to the test temperature in flowing argon, then oxidized in air flowing at 0.1 to 0.2 ft per sec. Following exposure, the air was displaced by argon and the specimen was cooled. Postoxidation metallography was performed on every sample. The depth of conversion of the metal matrix to oxide was measured as shown in Fig. 1. The conversion depth was obtained by measuring the oxygen-contaminated metallic zone (which is bounded by a dense outer oxide and a subscale oxide region), subtracting its dimensions from the original dimensions, and dividing by two. Preferential oxidation of