Microsegregation of an Aluminum and Magnesium Alloy at High Solidification Rates
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MICROSEGREGATION OF AN ALUMINUM AND MAGNESIUM ALLOY AT HIGH SOLIDIFICATION RATES
L.J. MASUR, J.T. BURKE, T.Z. KATTAMIS, Massachusetts Institute of Technology,
M.C. FLEMINGS Cambridge, Massachusetts
ABSTRACT Results are presented of an on-going study of structure and solute redistribution in ribbon cast Al-Cu and Mg-Zn alloys. Regions found in these ribbons are (1) an apparently chemically homogeneous coarse-grained zone adjacent the chill, (2) a transition cellular region, and (3) a dendritic zone, usually equiaxed. In both alloys, structure in the equiaxed region becomes coarser with increasing distance from the chill. In Mg-Zn alloys, the amount of nonequilibrium second phase increases essentially linearly with distance in the cellular and dendritic regions. Scanning transmission electron microscopy can accurately measure solute distribution across rapidly solidified cells or dendrites, but the possibility of surface compositional changes during specimen preparation must be recognized.
INTRODUCTION A number of previous studies have been made of ribbon materials. The cast microstructure of chill block melt-spun ribbon of Nimonic 80A solidified at 8 rates of 105 to 10 K/s was investigated by Wood et al [1]. Three distinct solidification zones were observed, whose extent depended on the dwell time on the rotating substrate and on the surrounding atmosphere: a columnar zone next to the substrate, followed by an equiaxed zone and a large grain zone on the top ribbon surface.
The as-quenched microstructure was homogeneous,
apart
from segregation of titanium to the cell walls in the form of TiC and within the matrix in the form of compositional modulations. In another investigation [2], nickel-base superalloys, such as MAR M200, Inconel 718 and others were 4 melt-spun with cooling rates in the solidification range of 5x10 to 5xlO5 K/s, according to wheel speed. Close to the chill surface, small grains, up to 2pm, were subdivided into small cells of a size as small as 10-1am. Farther away from the chill, columnar dendrites appeared. Both intercellular and interdendritic spaces were delineated with MC carbides. In thick ribbons of certain alloys (>100pm), additional nucleation on the top surface was occasionally observed. Results are presented herein of an on-going investigation of microstructure and solute redistribution in melt-spun ribbons of Al-4.5wt% Cu and Mg-4.5 and 6.Owt% Zn alloys. EXPERIMENTAL PROCEDURE 4
In this work, ribbons of Al-4.5wt% Cu, Mg- .5 and 6wt% Zn were melt-spun on a 9-inch i.d. copper wheel. Charges of 50 to 100 gms were melted in A1203 crucibles for Al-Cu and in iron crucibles for I•g-Zn using a high frequency induction unit. The melts, superheated by about 100'C, were ejected through slots at the bottom of the crucible using helium under 7 atm pressure. The liquid alloy jet solidified by impinging on the inner surface of the wheel,
186 rotating at 24 to 40 m/s. Ribbons produced were about 1 cm wide and 20 to 120pm thick. They were studied by optical and SE microscopy. In addition
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