Abstract
A study has been made of the initial stages of oxidation of yttrium ion implanted Ni20Cr in an attempt to shed some light on the so-called “reactive element effect” observed in chromia forming alloys. ReflEXAFS was used in order to observe the initial local atomic structural changes around the implanted ions, while glancing angle X-ray diffraction was used to monitor the longer range structural transformations taking place near the surface. The results indicated that the implantation amorphised the alloy surface. Following a short oxidation at 700°C Y2O3 nanocrystals formed throughout the implanted region. Simultaneously a highly oriented Cr2O3 layer formed on the sample surface. Preliminary measurements reported here on the thermal expansion of hexagonal chromium oxide indicate a pronounced anisotropy with respect to the crystallographic direction which could explain the superior adhesion to the substrate alloy of oriented scales under conditions of thermal cycling.
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