COMPOSITION AND SOLIDIFICATION MICROSTRUCTURE SELECTION IN THE INTERDENDRITIC MATRIX BETWEEN PRIMARY AL3FE DENDRITES IN HYPEREUTECTIC AL-FEALLOYS

The composition and constitution of matrix microstructure between plat e-like Al3Fe dendrites in Bridgman-grown hypereutectic Al-Fe alloys ha s been determined as a function of alloy concentration C-0 and growth velocity V in the ranges 2.5 < C-0 < 28.1 wt%Fe and 0.01 < V < 5.0 mm/ s. The transition at V = 0.1 mm/s from a fully eutectic matrix at C-0 = 3.5 wt%Fe to one containing alpha Al dendrites at C-0 greater than o r equal to 4.7 wt%Fe is attributed to growth temperatures of alpha Al dendrites that are higher than those of eutectic in a matrix of lower iron-content, which results from these conditions. The matrix eutectic changes from irregular alpha Al-Al3Fe to regular alpha Al-AlxFe with increasing V,the transition velocity increasing from 0.1 to 0.2 mm/s f or C-0 values of 9.5 and 14 wt%Fe up to 0.34-1.0 mm/s for C-0 values o f 18.7-28.1 wt%Fe. This increased transition velocity, compared with t hat for alpha Al-Al3Fe to alpha Al-Al6Fe at lower concentration, is in dicative of a lower formation temperature for the alpha Al-AlxFe than the alpha Al-Al6Fe eutectic. Copyright (C) 1996 Acta Metallurgica Inc.