Al-7 mass%Si, Al-4および6 mass%Cu合金における羽毛状晶のミクロ組織観察

Abstract

Feathery crystals formed in Al-7.0 mass%Si, Al-4.0 and 6.0 mass%Cu alloys which were poured into molds at 1163 K and water-cooled on and off during unidirectional solidification were observed microscopically to examine their morphology and etch pits for estimating crystal orientation. Results obtained were as follows:(1) The feathery dendrites in the 7.0 mass%Si alloy have an appearance of a V-shape in the early stage of growth, but later gradually changed into a Y-shape. Primary stalks of the feathery dendrites become very long in the late stage of growth.(2) The primary stalks of the feathery dendrites in the 4.0 and 6.0 mass%Cu alloys seem to be short in both the early and the late stages of growth, but they are connected slightly with other primary stalks or sidebranches.(3) The primary stalks of the feathery dendrites grow in the direction of 〈110〉, but the secondary branches grow in 〈100〉 or 〈112〉.(4) The secondary branches of the feathery dendrites in the early stage are sometimes observed to be growing long upward as columnar dendrites.(5) In the 4.0 and 6.0 mass%Cu alloys, the primary stalks in the feathery dendrites sometimes grew at angles between 5° and 10° from the surface trace of the growth twin plane, and in other cases the surface trace was parallel to the growth direction of the primary stalk, but deviated from the central line of the stalk.(6) In the feathery dendrites, ternary branches are easily generated from secondary branches, but distinguishing the primary stalks from the ternary ones is extremely difficult. This multiplication mechanism of the feathery dendrites may be a controlling mechanism of the primary arm spacing of the columnar dendrites.