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

The present research uses in-situ neutron diffraction to examine the effect of grain refinement on grain growth during solidification of Al-5 wt.% Cu and Mg-5 wt.% Zn alloys. The alloys were grain refined through additions of Al-5Ti-1B and Zr, respectively. The in-situ 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 Al-5 wt.% Cu and Mg-5 wt.% Zn alloys. The refined and unrefined Al-5 wt.% Cu alloys contained α-Al with Al2Cu along the grain boundary regions. Differences in Al2Cu morphology were observed in the grain refined alloys. The Mg-5 wt.% Zn alloy contained MgZn intermetallic phases with primary Mg. The refined Mg-5 wt.% Zn-0.7 wt.% Zr alloy contained Mg, MgZn and Zn2Zr phases. In-situ neutron diffraction enabled quantification of individual plane solid fraction growth for the α-Al and Al2Cu phases in the Al-Cu alloys, and for α-Mg in the Mg alloys. For the unrefined Al-5 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 grain-refined Al-5 wt.% Cu alloys showed a columnar to equiaxed microstructure transition and a more gradual growth in fraction solid throughout solidification. For the Mg-5 wt.% Zn alloy, the more packed (0002) and (101¯1) α-Mg plane intensities grew at a slower rate than the (101¯0) plane intensity, resulting in an irregular grain structure. With the addition of the Zr grain refiner, the Mg-5 wt.% Zn-0.7 wt.% Zr alloy had (101¯0), (0002) and (101¯1) 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.

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