Resistance of Ni-Cr-Al alloys to cyclic oxidation at 1100 and 1200�C

Abstract

A series of Ni-rich alloys in the Ni-Cr-Al system were cyclically oxidized in still air for 500 1 -hr heating cycles at 1100°C and 200 1 -hr heating cycles at 1200° C. The specific sample weight-change data for each sample were then used to determine both a scaling constant k1 and a spalling constant k2 for each alloy, using the regression equation Δw/A=k 1 1/2 t1/2 − k2t±σ.These in turn were combined to form an oxidation attack parameter Ka,where Ka= (k 1 1/2 + 10 k2).Log Ka was then fitted to a fourth-order regression equation as a function of the Cr and Al content at the two test temperatures. The derived estimating equations for log Ka were presented graphically as iso-attack contour lines on ternary phase diagrams at each temperature. At 1100°C compositions estimated to have the best cyclic oxidation resistance were Ni-45 at. % Al and Ni-30 at. % Cr-20 at. % Al, while at 1200°C compositions estimated to have the best cyclic oxidation resistance were Ni-45 at. % Al and Ni-35 at. % Cr-15 at. % Al. In general, good cyclic oxidation resistance is associated with Al2O3 and/or NiAl2O4 formation. The analysis also indicated that alloys prepared by zirconia crucible melting, compared to other types of melting, had tramp Zr pickup, which significantly improved the cyclic oxidation resistance. The nature of the improvement in oxidation due to tramp Zr pickup, however, is not yet understood.