Observing the Effect of Grain Refinement on Crystal Growth of Al and Mg Alloys during Solidification Using InSitu Neutron Diffraction

Abstract

The present research uses insitu neutron diffraction to examine the effect of grain refinement on grain growth during solidification of Al5 wt.% Cu and Mg5 wt.% Zn alloys. The alloys were grain refined through additions of Al5Ti1B and Zr, respectively. The insitu neutron diffraction experiments were carried out by heating the alloys to temperatures above the liquidus and subsequently cooling in 5 or 10 °C temperature steps to temperatures below solidus, while being irradiated by thermal neutrons. With the addition of grain refiners, grain size reductions of 92% were observed for both the Al5 wt.% Cu and Mg5 wt.% Zn alloys. The refined and unrefined Al5 wt.% Cu alloys contained αAl with Al2Cu along the grain boundary regions. Differences in Al2Cu morphology were observed in the grain refined alloys. The Mg5 wt.% Zn alloy contained MgZn intermetallic phases with primary Mg. The refined Mg5 wt.% Zn0.7 wt.% Zr alloy contained Mg, MgZn and Zn2Zr phases. Insitu neutron diffraction enabled quantification of individual plane solid fraction growth for the αAl and Al2Cu phases in the AlCu alloys, and for αMg in the Mg alloys. For the unrefined Al5 wt.% Cu, the coarse microstructure resulted in a rapid solid fraction rise at temperatures just below liquidus followed by a gradual increase in solid fraction until the sample was fully solid. The grainrefined Al5 wt.% Cu alloys showed a columnar to equiaxed microstructure transition and a more gradual growth in fraction solid throughout solidification. For the Mg5 wt.% Zn alloy, the more packed (0002) and (1011) αMg plane intensities grew at a slower rate than the (1010) plane intensity, resulting in an irregular grain structure. With the addition of the Zr grain refiner, the Mg5 wt.% Zn0.7 wt.% Zr alloy had (1010), (0002) and (1011) planes intensities all increasing at similar rates, especially at the early stages of solidification. FactSage™ (version 6.4, Montréal, QC, Canada) equilibrium solidification models followed the fraction solid curves developed by tracking the fastest growing planes of the Mg alloys.