Observation of structures feathery of crystals formed during uni-directional solidification of Al-7.08% Mg, Al-0.68% and 1.5% Fe alloys

Abstract

The feathery crystals of Al-Mg (7.08%) and Al-Fe (0.68 & 1.5%) alloys were formed during upward unidirectional solidification in insulating (Isolite) mold with chill plate at the bottom. The crystals formed were studied by various methods such as macroscopic and microscopic observations, etching pit method for estimating crystal orientation, microscopic observations and surface inspection testings on decanted interface. The following conclusions were drawn.(1) The feathery grains consisted of thin lamination layer of crystals having twin plane of (111), and its growth direction was presumed to be <112>(2) It seemed that as a single feathery crystal began to grow, numerous lamellae of feathery crystals were successively formed in almost parallel direction, which showed laminated structure.(3) As the results of microscopic observations and surface inspection testings on structure of decanted solidliquid interfaces, it was concluded that feathery grains grew prior to dendrites.(4) An excess of iron segregation was recognized at a position, which was assumed to be an original source of feathery crystals in Al-Fe (0.68 & 1.5%) alloys. The segregation would be due to the local accumulation of rejected solute in Al-1.5%Fe alloy, while Al-0.68% Fe alloy caused banded segregation.(5) By the observations of several cross-sections, 3-dimensional growth behavior of feathery crystals was explained.