The mechanism of silicon modification in aluminum-silicon alloys: Impurity induced twinning
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I.
INTRODUCTION
IN eutectic systems where one phase
solidifies in a faceted manner and the other(s) nonfaceted,',2 it is well known that the eutectic microstructures may change markedly with solidification conditions (cooling/growth rates) and also with minor additions of certain specific modifying agents. Apart from the complex structures of cast irons, the earliest example of this type of behavior was reported for the A1-Si system some 65 years ago, following an accidental introduction of sodium fluoride to a flux. 3 Since then, the problem of 'modification' in this system has been the subject of several hundred publications in the literature and a number of reviews (e.g., References 4 and 5). While there appear to be a variety of structural changes, 6 that which has received the most attention is the one where the minor silicon phase is modified from a flake to a fibrous morphology; this can be accomplished without specific modifying additions at rapid cooling rates, with equivalent growth rates ~ 1 mm s ~- - sometimes termed 'quench m o d i f i e d ' - - o r at much slower rates by minor additions of sodium (~-0.01 wt pct), strontium (=0.1 wt pct) and less certainly, with other alkali, alkaline earth, and rare earth metals. 4 There have been various suggestions as to how the impurity additions cause modification to occur, based on changes in phase equilibria or differing nucleation or growth kinetics for the minor silicon phase. In this paper it is assumed that the mechanism is primarily concerned with the growth of the silicon phase, for the following reasons: (a) Thermal analysis of impurity modified alloys shows depression of the eutectic and primary silicon arrests on cooling (by 5 to 10 K) but no such depression of the heating arrest for the former. Both the initial nucleation and horizontal parts of the eutectic cooling arrests are depressed; i.e., there is no recalescence to raise the growth temperature SHU-ZU LU, formerly Visiting Scholar at Michigan Technological University, is Instructor, Department of Foundry Technology. Beijing University of Iron and Steel Technology. Beijmg. People's Republic of China. A. HELLAWELL is Professor, Department of Metallurgical Engineering, Michigan Technological University, Houghton, MI 49931. Manuscript submitted December 8, 1986. METALLURGICAL TRANSACTIONS A
toward that of the eutectic isotherm. There is no significant effect upon the freezing point of the aluminum phase, v (b) The impurity (sodium, strontium, etc.) modified fibers are heavily twinned and are actually microfaceted, in contrast to the quench modified silicon which is essentially twin free and nonfaceted, s'9 This observation confirms an earlier report of twin formation in silicon, grown epitaxially from aluminum solutions in the presence of sodium. Jo (c) Sodium activity in an aluminum melt is sharply reduced by addition of silicon, H and scanning Auger spectroscopy indicates that sodium in a modified alloy is definitely associated with the silicon phase rather than in the aluminum matrix. ~2 In t
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