The Effect of Zirconium on the Cyclic Oxidation of NiCrAl Alloys

Abstract

Cyclic oxidation tests of Ni‐ (9–20) Cr‐(15–30) Al‐xZr alloys were carried out at 1100° and 1200°C in static air. In these alloys the concentration of zirconium varied from 0 to 0.63 atomic percent (a/o). The phases found in the cast alloys were γ, γ′, β, and α. The oxidized surfaces were characterized by metallography, XRD, and electron microscopy. Although the alloys were basically alumina/nickel aluminate formers, the zirconium‐containing alloys also formed small amounts of . The metallographic and electron microscopic examination of oxidized surfaces revealed significant aluminum oxide (s) penetration. The depth of attack and the degree of oxide penetration, however, strongly depended on the zirconium content of the alloy. The zirconium‐free alloy had massive penetration of aluminum oxide (s). Small amounts of zirconium led to minimal penetration. However, as the zirconium content increased, the oxide penetration again became more pronounced. All of the weight change data were fitted to the paralinear oxidation model: where represents the rate of scale formation and the rate of oxide spalling. The Zr‐containing alloys had relatively good cyclic oxidation resistance as a result of intraoxide spalling at low rates. The Zr‐free alloys had relatively poor cyclic oxidation resistance due to massive alumina spalling to bare metal, which resulted in a shift to oxidation and even greater rates of attack. The derived parameters (which equals ) and (metal consumption) which indicate the degree of total oxidation attack were also estimated as a function of zirconium content. All four parameters showed minimums in the range of 0.03 to about 0.20 a/o zirconium. Further addition of zirconium beyond 0.20 resulted in increased values of , , , and . Alloys with zirconium contents between 0.03 and 0.20 a/o seemed to have the best cyclic oxidation resistance over the range of compositions tested, while oxidation resistance was essentially independent of Cr and Al content over the compositional range evaluated for these alloys.