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Influence of Oxygen Supply on Oxide Coarsening During the Internal Oxidation of Two-Phase Fe-Y Alloys

“…In a study on oxidation of Fe-15 wt pct Y in an H 2 -CO 2 gas (p O2 = 10 À20 atm, flowrate not reported), a similar transition was noted and the rate constants reported in that study are also listed in Table I. [6] Finally, all measured rates are significantly faster than the rate predicted assuming bulk diffusion of oxygen controls the internal oxidation rate. Enhanced diffusion at interfaces is important, but the kinetics are sensitive to alloy microstructure and oxidizing conditions as discussed below.…”

“…The solute has low solubility and low diffusivity in the matrix, and under conditions that lead to selective oxidation of the solute, diffusionless or in situ internal oxidation occurs and the solute-rich intermetallic internally oxidizes in place, i.e., the oxidized phase retains its initial distribution and morphology while the matrix phase is left unaltered. [1,5] Alloys which have exhibited this behavior include Fe-Y, [6] Fe-Cu, [7] Ni-Zr-Y, [8] Fe-Nb, Co-Nb, and Ni-Nb. [9] The classic Wagner theory of internal oxidation [10,11] was derived using the assumption of a single phase alloy with the oxidizing component in solution.…”

“…This communication focuses on the observation of an effect of gas flowrate on Table I. Summary of Rate Constants from Each Flow Rate in Region I (k I ) and Region II (k II ) As Well As Experimental Results from Similar Experiments Done by Niu et al [6] internal oxidation behavior. The work presented here shows that gas phase mass transfer processes affect both internal oxidation kinetics and microstructures.…”

“…1.5 9 9 10 À6 1 9 10 À8 9 9 10 À6 2 9 10 À07 3 2 9 10 À5 1 9 10 À7 1 9 10 À6 -6 8 9 10 À5 2 9 10 À6 3 9 10 À6 -12 2 9 10 À5 1 9 10 À6 2 9 10 À5 -15 9 9 10 À5 1 9 10 À6 3 9 10 À5 -15 H 2 -CO 2 1073 K (800°C), 10 À20 atm [6] 5.7 9 10 À4 1.2 9 10 À4 -- Oxides are coarser at this front in the sample exposed at the lower flowrate (b). Select larger oxides are circled for clarity.…”

On the mechanism of Ag exudation during internal oxidation

“…Exudation of the noble alloy component is not limited to Ag-based alloys. A pure metal layer has also been discovered on the surface after internal oxidation in various other alloy systems, such as Pd [1,12], Ni [13,14], Co [15,16] and Fe [17] base alloys, and also after internal nitridation in Ni-based alloys [3,18]. It is worth noting that some alloy systems, such as Ag-Cd, do not develop an exuded Ag layer.…”