Several Fe-Cr-Al and Cu-Zn-Al alloys with different compositions have been oxidized in 1 atm O2 at high temperature to investigate the third-element effect due to Cr and Zn. Cr/Zn act as third elements in these two systems and improve the high temperature oxidation resistance of the ternary alloys by promoting the transition to the formation of external scales of the most reactive alloy component. In order to compare the oxidation behavior of these ternary alloys with that of the corresponding binary alloys, it was necessary to know the critical contents of active components required for the transitions between different oxidation modes in binary Fe-Cr, Fe-Al, Cu-Zn and Cu-Al alloys. Since there are few investigations concerning these transitions for Cu-Zn and Cu-Al alloys, five binary Cu-Al alloys with Al contents ranging approximately from 1 to 7 at.% and five Cu-Zn alloys with Zn contents ranging approximately from 5 to 20 at.% have been prepared and oxidized at high temperature in 1 atm O2.
The progressive addition of Cr improved gradually the oxidation resistance of a Fe-3Al alloy at 1000oC in 1 atm O2 for 24 h, promoting first a transition from internal oxidation of Al to the mixed external oxidation of all the three alloy components, with an addition of 3 at.% Cr, and then allowing to establish an exclusive external Al2O3 scale for Cr additions above 10 at.%.
The increase of the concentration of Zn and Al in binary Cu-Zn and Cu-Al alloys transformed their oxidation mode at 800oC in 1 atm O2 for 24 h first from internal oxidation of the active elements to a mixed external oxidation of both alloy components and later to the selective external oxidation of Zn and Al. The critical Al contents of Cu-Al alloys for the two transitions are about 2.5 at.% and 4at.%, while the corresponding Zn contents of Cu-Zn alloys are about 12 at.% and 14 at.%, respectively. These critical concentrations have also been calculated by Wagner’s criterion for the transition between internal and external oxidation of the active component in binary alloys and the criterion for the selective external oxidation of the active component. All the calculated values have been compared with the experimental results, and possible reasons for the discrepancies observed between them have been analyzed.
The oxidation rates at high temperature of ternary Cu-Zn-Al alloys with a constant Al content reduced gradually with a progressive increase of their Zn content. The addition of 5 at.% Zn to a Cu-2Al alloy can inhibit the internal oxidation completely, but even the addition of 10 at.% Zn is still not sufficient to induce the formation of a protective Al2O3 scale, which requires the addition of at least 15 at.% Zn. The effect of the Zn additions to a Cu-4Al alloy is only to shorten the time required for the establishment of the protective Al2O3 scale and to inhibit the formation of external CuxO scales during the initial oxidation stage.
The oxidation maps of the quaternary Fe-Cr-Al-O and Cu-Zn-Al-O systems have been drawn by combining the results from both theoretical calculations and experimental measurements. The possible mechanisms of the third-element effect due to Cr in Fe-Cr-Al and to Zn in Cu-Zn-Al alloys have been interpreted with the help of these oxidation maps.
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