A Microstructural Investigation of a Duplex Chill Cast Brass Grain Refined with Aluminium and Boron.

A 0. 8wt% addition of aluminium masteralloy containing 4wt% boron was found to grain refine duplex brass cast into graphite moulds. The grain refined brass had three distinct grain zones: an outer zone of fine chill crystals, an inner zone of very fine equiaxed crystals and a zone of coarser equiaxed crystals between the two. An identical addition to a fully alpha brass showed no refining action while fully beta brass was moderately refined. Using SEM and extraction replica techniques in the TEM the microstructure of all three brass compositions has been investigated. The marked grain refinement in the duplex brass was associated with the presence of a highly branched amorphous precipitate situated at the original beta grain boundaries. Crystalline second phase particles were observed in the fully alpha or fully beta alloys. The composition of the amorphous and crystalline products has been investigated using WEDX and EELS analysis in the TEM. By combination of these techniques which are sensitive to light elements, an accurate analysis of the products was possible which proved them to be rich in boron and containing varying quantities of aluminium and oxygen. The boron glass was found to consist of; 88. 7 wt% B, 6. 3 wt% 0, 2. 8 wt% Al with minor contents of Zn, Fe, S, Cu and P. By contrast the crystalline boron contained relatively more aluminium and less oxygen. An explanation of crystallinity of the boron products is made according to the composition. Based upon this analysis and other considerations a novel grain refining mechanism is proposed which relies on the redistribution of boron solute ahead of the solidification front restricting the crystal growth