The oxidation of Nb-Re and Nb-W single crystals and polycrystals between 500 and 1100 °C

Abstract

Single crystals of Nb-1.5 at.% Re and Nb-3.5 at.% Re alloys have been oxidised in dry oxygen in the temperature range 500–1000 °C. Single crystals and polycrystals of Nb-1.5 at.% W and Nb-3.5 at.% W alloys have also been oxidised in dry oxygen in the temperature range 500–1100 °C. Over the whole temperature range studied W additions reduce the oxidation rate of pure niobium; rhenium additions also reduce the rate, but only at temperatures below 650 °C. In both cases the greater the addition of alloying element, the more marked is the decrease in rate. In the oxidation of pure Nb, suboxide platelets, anisotropic scale-failures and a transition to breakaway kinetics are only observed at temperatures below about 625 °C. Rhenium additions effectively reduce the time to breakaway by at least two orders of magnitude. Rhenium additions also appear to reduce the dynamic stability of suboxide platelets, and to produce unusual anisotropic scale-failures on oxidised single crystals. Rhenium seems to dissolve in Nb 2O 5 to produce a more “plastic” and adherent scale. At higher oxidation temperatures, rhenium is probably removed by the volatilisation of Re 2O 7 and the oxidation rates of the alloys approach that for pure Nb. Tungsten additions also reduce the tendency for breakaway oxidation, but at these alloying levels, the effect is not as marked as with Re additions. Below 640 °C, platelets are dynamically stable below {110} planes on Nb-W alloys but grow with habit selection, forming parallel to only one of the possible {100} planes in a given region of the specimen. Although the final morphology of breakaway oxidation is similar to that found in pure Nb, the formation of slightly smaller platelets and their non-intersection until oxidation has continued for some time provide an explanation for the longer induction periods before breakaway. At temperatures above 640 °C, platelets are also observed on Nb-W alloys after oxidation and in some cases scale features indicate that they may be isothermal; the evidence on this point is not as good as with platelet-induced porosity observed at lower temperatures.