The oxidation of tantalum-hafnium polycrystals and single crystals in the temperature range 750 °–1050 °C

Abstract

Polycrystalline coupons of Ta-4.04 at.% Hf have been oxidised in the temperature range 850 °–1000 °C in oxygen at approximately 760 Torr. Hafnium increases the oxidation rate throughout this range, and above 1000 °C the specimens ignite on admission of oxygen. The scale is finely laminated at all temperatures, and the decrease in lamination thickness is sufficient to mask any Wagner-Hauffe doping effect of the alloying element. There is some fine cracking in the scale formed at 950 °C and above, which might be related to the original metal grain boundaries, but there is no evidence of penetration of pentoxide into the metal grain boundaries or of separation of the metal grains. Single crystal cylinders of Ta-1.2 at.% Hf with 〈100〉 axes have been oxidised in the range 750 °–1050 °C. Fine suboxide platelets, which appear to have been present at reaction temperature, are present beneath the metal/oxide interface in all cases, but only at the lowest temperatures do they lead to pores and pore-related failures in the scale. At 950 °C the scale contains large aligned voids at about 60 ° over the {100} faces, and at higher temperatures there are cracks at the same angle. It has not been possible to determine the origin of these. The scale formed at 950 ° C exhibits a preferred orientation, but that formed at 1050 °C is random. Microhardness indentations in the scale are surrounded by extensive cracking, suggesting that hafnium increases the brittleness of the oxide.