Oxidation behavior of a fine-grained rapidly solidified 18-8 stainless steel
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I.
INTRODUCTION
THE resistance of an alloy to oxidation at elevated temperatures is usually dependent on the formation of a thin scale of C r 2 0 3 , AI203, o r SiOz on its surface by reaction of Cr, A1, or Si dissolved in the alloy with oxygen or oxygen-bearing molecules in the gas phase. Continuous, pore-free layers of these oxides inhibit oxidation because diffusion of reactants through them is slow. Good resistance to oxidation in actual practice requires that protective oxide scales maintain good adherence to the alloy substrate during the application of external stresses and during thermal cycling, If a protective oxide scale spalls from the surface of an alloy, the alloy must be able to reform the protective oxide rapidly to avoid extensive oxidation of the bare alloy surface. The adherence of an oxide scale to an alloy may be increased considerably by the presence of a fine, uniform dispersion of intermetallic or nonmetallic precipitates in the alloy. 1,2 In general, it appears that the external protective scale tends to grow inward around the precipitates, thereby yielding intrusions of the scale into the alloy, which are apparently effective in mechanically keying the scale to the alloy substrate. The selective oxidation of an alloying element to form an external protective scale requires the formation of a critical volume fraction of the oxide to cause a transition from internal to external oxidation of the alloying element.3'4 A higher concentration and a higher diffusivity of the alloying element favors external oxidation, as does a lower solubility and diffusivity of oxygen in the alloy. The transition from internal to external oxidation of Ag-In alloys occurs at lower In concentrations when the surfaces of the alloys are mechanically polished rather than etched or electropolished. 4 Rapp suggested that this effect was due to mechanical deformation of the alloy surface, which increased the diffusivity of In or
decreased the solubility of oxygen. 4 Giggins and Pettit 5"6 demonstrated that protective Cr203 layers formed on binary Ni-Cr alloys at lower Cr concentrations when the surfaces of the alloys were grit blasted. In the latter case, a recrystallized layer of alloy, which was - 1 5 / x m thick and had an average grain size of - 1 0 / x m , formed at the surface of the alloy. Recently, Merz 7 and Baer and Merz 8 have demonstrated that the oxidation resistance of a fine-grained, sputter-deposited 304 stainless steel was more resistant to oxidation than a conventional, wrought 304 stainless steel that had a larger grain size. It appears that grain boundaries and dislocations that intersect the surface of an alloy promote the formation of a continuous layer of the most thermodynamically stable oxide on the surface. Very fine-grained alloys may be obtained by compaction of alloy powders produced by rapid solidification processing. 9'l~ In a number of cases, the fine grain sizes ( < 1 0 / x m ) remain stable at elevated temperatures. 9"~~The grains are apparently pinned by very fine nonmetallic inclus
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