IMPROVED OXIDATION RESISTANCE OF MOLYBDENUM BY ALLOYING WITH RARE EARTH AND OTHER SELECTED METALS.
Final rept., 8 June 55-31 Oct 61.
COLORADO SCHOOL OF MINES GOLDEN
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The choice of a suitable alloying element for selective oxidation in a molybdenum base alloy was narrowed to hafnium. A close second choice was yttrium, but on the basis of se-1ve oxi dation experiments it was ruled out in favor of hafnium which was superior in three principal respects. The principal investigations were carried out by preoxidizing the active component of the alloy in a hydrogen or helium atmosphere containing a partial pressure of oxygen which was below the decomposition pressure of molyb denum dioxide. This treatment resulted in in ternal oxidation of the active component. Two practical results have emerged from these in ternal oxidation experiments. First, an in ternally oxidized specimen can be exposed to air at 725 C resulting in volatilization of the MoO3 in the subscale to leave a coating of hafnium dioxide on the surface of the specimen. Sec ondly, the internal oxidation data can be used to calculate the diffusion coefficients of the active component in the alloy and of oxygen in the subscale. Knowledge of these parameters can then be used to predict the optimum conditions of temperature, partial pressure of oxygen, and alloy composition for external scale formation, a very desirable situation for achieving oxi dation resistance. Author