The oxidation of Nbb 5·5 at. % Mo single crystals and polycrystals in the temp. range 530°–1100°C

Abstract

Single crystals and polycrystals of a Nb-5·5 at.% Mo alloy have been oxidized in dry oxygen in the temp. range 530°–1100°C. Below 875°C after a period of slow oxidation there is a transition to breakaway kinetics. In this lower range the failures in the scale on oxidized single crystals are anisotropic and associated with the presence of a suboxide phase growing along {100} planes of the metal. This is in contrast to the oxidation of pure Nb where suboxide platelets, anisotropic scale failures and breakaway rates are only observed in oxidation at temps. below 625°C. The major reason for the breakaway rates for single crystal specimens is the rapid oxidation of metal fragments separated along {100} planes by cracking along platelets. Oxidation of polycrystalline specimens in this region leads to disintegration of the metal by cracking along platelets and also along grain boundaries. Breakaway oxidation of pure Nb, in contrast, has been associated with the formation of “root-like growths” when platelets oxidize rapidly along their narrow aspects. Above 875°C the rate diminishes abruptly to a minimum at 925°C; breakaway oxidation is no longer observed, oxidation is isotropic and the suboxide platelets are replaced by a feathery oxide penetrating into the metal, which may be NbO, and a compact layer, probably of NbO 2 underlying the pentoxide. Oxidation rates increase with increasing temp. to 1100°C where the scale morphology resembles that found on pure Nb above 900°C. This amount of Mo appears to shift the reaction mechanisms some 200°–300°C higher in temp. relative to pure Nb. There are, however, some important differences in these mechanisms.