Diffraction studies of an Al–Zn–Mg single crystal by synchrotron radiation

Abstract

Precipitation of phases from a supersaturated solid solution is an important process in materials technology. Precursors for the precipitates are local structures in the solid solution and partially ordered Guinier-Preston zones. Within the ternary alloy system Al-Zn-Mg, intermediate structures are recognized according to the precipitation sequence: Supersaturated solid solution-> zones-> '-precipitates->-MgZn2 The embedded '-precipitates are disc-shaped with a diameter in the order of 100 Å and a thickness of approximately 30 Å. The particles exhibit well-defined orientation relationships to the Al-matrix. Although their unit cell is known, their structure is still not finally determined. One experimental challenge is to measure the very strong Al-matrix reflections together with the much weaker diffracted intensities from the precipitates. This is however partly overcome by transforming the UB-matrix of the Allattice using the orientation relationships of the '-particles. Thus individual UB-matrices for every of the four principal precipitate orientations are obtained. Accurate intensity profiles may then be recorded using a point detector, in combination with CCD-data. A single crystal grain from a previously heat-treated polycrystalline sample of composition Al0.88Zn0.10Mg0.02 was investigated using the KM-6 single crystal diffractometer at the Swiss-Norwegian Beamlines (SNBL) at the ESRF. Using the method of UB-matrix transformations, four data sets from the ' particles have been successfully collected with a YAP-point detector. It was possible to obtain diffraction profiles up to 60 in 2θ using a wavelength of 0.7 Å. Owing to reflection overlap, each profile out of the 4216 must be individually treated in the subsequent reduction-and solution process.