Abstract

The size distribution and volume fraction of Cr$\sb{23}$C$\sb6$ have been isolated from the distributions of all other precipitates in aged samples of a ferritic alloy, Modified Fe9Cr1Mo steel, by the technique of anomalous small-angle X-ray scattering (ASAXS), in what is believed to be the first application of this technique to precipitation in an engineering alloy. The steel has been proposed for use at elevated temperatures for long times in power generation equipment and the stability of the microstructure must be verified. Six samples were aged for 5000 hours at either room temperature, 482, 538, 593, 649, or 704$\sp\circ$C to simulate a typical in-service condition. Synchrotron radiation was used as a variable-wavelength source of X-rays. Three X-ray wavelengths near the Cr K absorption edge were used to vary the scattering contrast of Cr$\sb{23}$C$\sb6$ while leaving that of the other precipitates fixed. A double-crystal diffractometer and a silicon photodiode X-ray detector were specially designed for use at the synchrotron to measure the scattered radiation. The three small-angle scattering curves from each sample were analyzed by a maximum entropy technique to obtain three scattering contrast-weighted size distributions of all the precipitates that give rise to the observed scattering. A scattering contrast gradient analysis combined the three experiments to isolate the Cr$\sb{23}$C$\sb6$ volume fraction distributions. The mean diameter of Cr$\sb{23}$C$\sb6$ particles was found to increase with temperature for 5000 hour aging between 538 and 704$\sp\circ$C, consistent with prior transmission electron microscopy results.

The ultra-high strength steel alloy AF1410, currently used for arresting hooks on carrier-based aircraft, derives its desirable properties by a delicate heat treatment that carefully balances the formation of one carbide with the depletion of another. The lack of ASAXS near the Cr and Fe K absorption edges indicates that the distributions of precipitates observed (presumably M$\sb3$C and austenite) are iron-enriched and chromium-deficient.