2005 Volume 54 Issue 5 Pages 207-211
A coating consisting of duplex layer of β-NiAl and α-Cr layer was formed on Ni-40Cr-3X (X(at%)=Ta, Nb, Ti, Zr, V, Ru, Rh, Re, Ir, and Pt) alloys by electroplating of Ni (12μm thick) and high Al-pack diffusion, followed by heat treatment in air at 1373K for 86.4ks. The coated alloys were oxidized in air for up to 2332.8ks under thermal cycling between 1273K and room temperature. Oxidation amounts increased and then turned to decrease with increasing oxidation time. The critical times became longer in the order of Ta, Ti, Nb, Pt, Ir, V, Re, Rh, and Ru except for Zr which showed no scale spallation. The alloy containing Zr was oxidized in accordance with parabolic rate law showing the rate constant of 1.2×10-10kg2m-4S-1, while the alloy containing Re showed significant spallation of the scale. It was found from the result after oxidation at 1373K for 2338.8ks that the α-Cr layer became thinner from 30 to 10μm for the Ni-40Cr-3 at%Zr alloy, whereas it was almost unchanged to be 20μm for the Ni-40Cr-3 at%Re alloy. It seems to be the reason for α-Cr stability that the α-Cr formed on the Ni-40Cr-3 at%Re alloy contains 8-10 at%Re.
