THE CYCLIC OXIDATION BEHAVIOR OF ALPHA-CR-NIAL ALLOYS WITH AND WITHOUT TRACE ZR ADDITION(BETA)

This study reports the cyclic oxidation behaviour of a series of Ni-Cr -Al alloys in still air at 1100 degrees C for up to 250 one-hour cycle s. Each of the alloys had a beta-NiAl+alpha-Cr microstructure with a s imilar alpha-Cr volume fraction, but different phase compositions. All of the alloys exhibited poor cyclic oxidation resistance, undergoing extensive scale spallation and internal oxidation. The scale morpholog y, weight-change kinetics, and maximum depth of internal oxide penetra tion were all very dependent on the aluminium content in the alloy. Fu rther, the Al-rich, beta-NiAl+alpha-Cr alloys were harder and more bri ttle than the Ni-rich alloys, thus causing the former to be more susce ptible to cracking and, hence, localised internal oxide penetration. A ll of the alloys were found to undergo diffusion-induced phase transfo rmations during cyclic oxidation. The selective oxidation of aluminium , together with the scale spallation, resulted in the eventual transfo rmation of the original beta-NiAl+alpha-Cr structure to either gamma-N i in the Ni-rich alloys or beta-NiAl(Cr) in the Al-rich alloy. In the Ni-rich alloys, the interdiffusion of nickel from the gamma-Ni layer r esulted in an enrichment of nickel in the alpha+beta base-alloy struct ure which, in turn, resulted in a alpha+beta-->gamma+beta phase transf ormation. In the case of the Al-rich alloy, the enrichment of nickel i n the beta-NiAl(Cr) structure resulted in its transformation to a gamm a+beta structure. The addition of 0.1 at.% zirconium to a Ni-rich, alp ha+beta alloy was found to have little effect on its cyclic oxidation resistance. (C) 1997 Elsevier Science.