Sj. Andersen, QUANTIFICATION OF THE MG2SI-BETA'' AND BETA' PHASES IN ALMGSI ALLOYS BY TRANSMISSION ELECTRON-MICROSCOPY, Metallurgical and materials transactions. A, Physical metallurgy andmaterials science, 26(8), 1995, pp. 1931-1937
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.