Effect of Zirconium on Cyclic-Oxidation of Ni-40Cr-3Re Alloys with α-Cr / β-NiAl Coatings

A coating with duplex structure of a outer β-NiAl and an inner α-Cr layer, was formed on a Ni-40Cr-3Re (in at%) alloy with or without Zr addition, and the coated alloys were oxidized under thermal cycling in air for up to 2300ks. The coated alloys containing Zr showed a two-step parabolic oxidation, the kp1st in the early stage of oxidation was 2.6~5.4×10-11 kg2
m-4
s-1 for four alloys tested, and kp 2nd for the longer oxidation increased with increasing Zr content from 2.6~5.4×10-11 for the alloy with 0.1at%Zr to 9.6×10-11 kg2
m-4
s-1 for the alloy with 1.0at%Zr. The rapid oxidation for the alloy with 1.0at%Zr is due to the formation of ZrO2 as an internal oxide. The oxide scale in the 1st stage consisted of both α- and θ- Al2O3 with whiskers, and with further oxidation the α-Al2O3 became the major product in the 2nd stage. After the oxidation for 2300ks the as-prepared, outer β-NiAl was changed into a mixture of β-NiAl and γ’-Ni3Al for the Ni-40Cr-3Re alloy containing Zr, while in the coated Ni-40Cr-3Re alloy the outer layer became a mixture of γ’-Ni3Al and γ-Ni(Al,Cr). It was concluded that the addition of Zr into the coated Ni-40Cr-3Re alloy helps maintain high Al contents in the outer Ni-aluminide layer by forming a protective Al2O3 layer.