QUANTIFICATION OF THE MG2SI-BETA'' AND BETA' PHASES IN ALMGSI ALLOYS BY TRANSMISSION ELECTRON-MICROSCOPY

A method has been developed to determine the density of the Mg2Si beta'' and beta' phases in 6xxx aluminum alloys using transmission electron microscopy (TEM). Age-hardened extruded profiles of an AlMgSi alloy, having different coarseness in the precipitate structure, were examined. By darkfield (DF) imaging, beta'' needles aligned in a [100] viewing direction were brought into contrast. Thickness was determined within 10 pet using a spot contamination method. After subsequent corrections, the number density of precipitates could then be quantified, usually within 5 pct. As expected, the average number density of the beta'' precipitates correlates well with the material's tensile strength. A density of beta' was obtained by a similar method. The densities represent maximum values since precipitate-free areas are not considered. Tensile strength is quite sensitive to the ratio between the number densities of beta' and beta''. BY determining the volume of an average beta'' or beta' needle, corresponding volume fractions of the phases can be determined. Quantification of these phases could benefit calculations involving mechanical properties of such alloys.