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 electro n microscopy (TEM). Age-hardened extruded profiles of an AlMgSi alloy, having different coarseness in the precipitate structure, were examin ed. By darkfield (DF) imaging, beta'' needles aligned in a [100] viewi ng direction were brought into contrast. Thickness was determined with in 10 pet using a spot contamination method. After subsequent correcti ons, the number density of precipitates could then be quantified, usua lly 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 repr esent maximum values since precipitate-free areas are not considered. Tensile strength is quite sensitive to the ratio between the number de nsities of beta' and beta''. BY determining the volume of an average b eta'' or beta' needle, corresponding volume fractions of the phases ca n be determined. Quantification of these phases could benefit calculat ions involving mechanical properties of such alloys.