SEM-EDS and XRD study of a NiCrAlU alloy at temperatures from 800C to 1200C

Abstract
The oxidation behaviour of a nominally 74.5wt%Ni, 20wt%Cr, 5wt%Al and 0.5wt%U alloy was examined using Scanning Electron Microscopy with X-ray Energy Dispersive Spectroscopy (SEM-EDS) and X-ray Diffraction (XRD). The oxidation temperature ranged from 800C to 1200C in air, with oxidation times of up to 120 h. Analysis showed that during short term oxidation (40 h), at the lower temperatures, oxides of Cr, Ni and Al were formed. With increasing oxidation time, the Cr-rich oxide scale became predominant with only a few regions of Ni-rich oxide being observed. No uranium was detected, using EDS, within the outer scale. The scale formed was normally flat but with spallation occurring after 40 h oxidation at 900C, with more spallation and some convolution being observed with increasing temperature and time.

Cross-sectional investigations indicated the presence of an internal oxidation zone comprising an Al-rich oxide, with significant amounts of uranium, beneath the outer Cr-rich scale. The depth of this zone varied from 1 micron to 60 microns and the depth increased with increasing oxidation temperature and time. At higher temperatures (above 1000C), the internal oxidation zone ceased to grow with longer oxidation times, due to the internal oxides forming a complete layer of alumina which protected the metal substrate from further degradation. The compositions of the scales formed reflect the selective oxidation of chromium and aluminium as a result of their higher reactivity compared to nickel and the greater stability of Al₂O₃ and Cr₂O₃ compared to NiO.

This study showed that Al-rich oxide was not the predominant protective outer scale, but instead formed as an internal oxide. The outer scale was predominantly Cr-rich oxide and for most samples was flat and adherent, although a few regions were observed to convolute. This shows that uranium influences the growth morphology of the Cr-rich scales formed on NiCrAl and that it acts as a reactive element, even though uranium was only detected at the metal grain boundaries and within the internal oxidation zone. The concentrations of uranium required to change the scale morphology appear to be lower than can be detected using SEM/EDS.