Microstructures of Rapidly Solidified Aluminum Alloys
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MICROSTRUCTURES OF RAPIDLY SOLIDIFIED ALUMINUM ALLOYS J.W. ZINDEL, J.T. STANLEY, R.D. FIELD AND H.L. FRASER Department of Metallurgy and the Materials Research Laboratory University of Illinois Urbana, IL 61801 ABSiRACT An investigation was performed to study the origin and stability of microstructures in rapidly solidified aluminum alloys. Al-Ni and Al-Fe base alloys were rapidly solidified by means of laser surface melting and melt spinning techniques. Microstructures were studied using optical and transmission electron microscopy. The effect of microstructure on mechanical properties was also studied using microhardness measurements. The origin of the observed microstructural constituents will be explained in terms of features of the metastable phase diagram. The effect of ternary additions on stability will also be considered. INTRODUCTION In the study of microstructures produced by rapid quenching from the melt it is important to understand both the origin of these microstructures and their stability upon thermal exposure. This paper describes two studies of rapidly solidified Al alloys which address these questions. The first is an investigation of solidification morphology as a function of composition in the Al-Ni system. The second is an investigation of the stability of the as-rapidly solidified microstructure of Al-Fe alloys and the effect of ternary additions on that stability. EXPERIMENTAL PROCEDURE Specimens for both of these investigations were prepared in two ways: melt spinning and laser surface melting. Only results from laser surface melting experiments will be presented here. Specimens for transmission electron microscopy (TEM) were prepared by first removing a slice of material parallel to the laser melted surface using a diamond wafering saw. The slice was then ground on both sides to reveal the melted region from which 3 mm disks were punched out. Specimens were thinned for TEM studies using conventional electrochemical techniques. These experiments were performed on a Philips 420 instrument equipped with an EDAX energy dispersive detector and software system for microchemical analysis. Microhardness measurements were made using a Knoop indentor with a load of 400 grams. RESULTS AND DISCUSSION Al-Ni Figure 1 shows the Al-Ni phase diagram with vertical lines drawn at the four compositions studied. The bars represent probable ranges of undercooling of the liquid met~l Sool which were estimated as described below. It has been postulated that for the case of an eutectic mixture involving a faceted phase (i.e. Al3 Ni), the coupled zone is asymmetric, shifting toward the faceted phase as undercooling is increased. This is Mat.
Res. Soc.
Symp. Proc.
Vol.
28 (1984)
Published by Elsevier Science Publishing Co.,
Inc.
318
FIG. I Al-Ni phase diagram showing alloy compositions investigated and metastable features (dashed line is extended a liquidus).
i
Compositlion Atomic%141
FIG. 2 Optical micrograph (transverse cross-section) of laser surface melted Al-8at%Ni alloy.
depicted on the phase dia
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