CHARACTERIZATION OF NI-25 WT.PERCENT CR ALLOYS CONTAINING REACTIVE ELEMENTS AND THE OXIDE SCALES GROWN ON THEM AT HIGH-TEMPERATURES - USING TRANSMISSION ELECTRON-MICROSCOPY

Nickel-base alloys of nominal composition Ni-25 Wt.%Cr-(0-0.6 Wt.%) RE (RE=Y, La, and Ce) were prepared by conventional arc-melting Ni, Cr, and Y metal. The microstructure of the alloys was characterized by using electron diffraction and X-ray energy dispersive spectroscopy (XEDS) to determine the structure, morphology, and distribution of second-phase particles. Selected alloys were oxidized at 900 degrees C and 1000 degrees C in 1-atm air, and the resultant oxide scales were characterized using the same analytical techniques. The experimentally determined electron-diffraction data were compared with a JCPDF-EDD database, and several compounds were matched. The observed phases were RE oxysulfide, cerium orthochromite-CeCrO3, yttria-Y2O3, yttrium orthochromite-YCrO3, and Ni5Y. The significance of these observations is discussed with respect to the current level of knowledge on the role of sulfur in the reactive-element effect. The ability of the reactive elements to getter sulfur is examined with respect to the thermodynamic stability of the oxysulfide species.