Effect of Be on the nucleation and growth of betau27 phase in aluminum-magnesium-silicon alloy.

摘要

An investigation on the effect of Be on $etaspprime$ phase precipitation in Al-0.7%Mg-0.4%Si alloy was carried out using microhardness and resistivity measurements, and optical and TEM observations. It is shown that addition of 0.4%Be to an Al-0.7%Mg-0.4%Si (base) alloy significantly enhances the age hardening response and increases the peak hardness levels of the alloy, which is associated with a refinement of the $metaspprime(Mgsb2Si)$ precipitate. Hardness and resistivity techniques used to study the precipitation kinetics show that the two methods are in good agreement for monitoring the transformation rates. A kinetic analysis shows that A precipitation of $etaspprime$ phase in both the base and the Be-containing alloy obeys the modified Avrami-Johnson-Mehl equation, and that the Be addition does not alter the growth parameter (n) or the activation energy $m (Qsb1),$ but increases 2-3 fold the nuclei density-dependent parameter $m (ksb1),$ which agrees well with the optical and TEM observations. The activation energy $m (Qsb1)$ obtained is in the accepted range for vacancy diffusion in Al alloys, indicating it is the transport mechanism controlling the rate of growth of the $etaspprime$ phase. Optical results show that the Be addition reduces the as-cast grain size, significantly inhibits grain growth during the aging, and increases the density of dislocation etch pits. The TEM results show that the increase in precipitate particle density in the aged Be-containing alloy, compared with the lower dislocation loops and higher density of dislocation lines (corresponding to a higher concentration of Be-trapped vacancies) for the quenched alloy, indicates that as aging proceeds at higher temperatures, the trapped vacancies are released and condense to form dislocation loops that act as sites for $etaspprime$ nucleation. A theoretical calculation of the activation energy $m (Delta Gsp* )$ for $etaspprime$ nucleation, which includes the effects of phase transformation, strain, and interfacial energies, shows $m a {sim}13%$ decrease for the Be-containing alloy, giving $m a {sim}1.7$-fold increase in the classical nucleation rate for $etaspprime .$ Source: Dissertation Abstracts International, Volume: 59-08, Section: B, page: 4387. Adviser: W. V. Youdelis. Thesis (Ph.D.)--University of Windsor (Canada), 1997.