Morphological Stability of Electron Beam Melted Aluminum Alloys
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B.H. Kear,
B.C. Giessen,
and M. Cohen,
79
editors
MORPHOLOGICAL STABILITY OF ELECTRON BEAM MELTED ALUMINUM ALLOYS
R. J. SCHAEFER, S. R. CORIELL, R. MEHRABIAN, C. FENIMORE, F. S. BIANCANIELLO National Bureau of Standards, Washington, D.C. 20234
AND
ABSTRACT For constant velocity solidification, morphological stability theory delineates the temperature gradients required for plane front solidification of a specific alloy. Using electron beams, surface heating of metals can be carried out with sufficiently well characterized thermal input to permit reliable use of computer models of melting and solidification. From numerical calculations, the growth velocity and temperature gradients as a function of position during resolidification can be obtained; combining these results with (constant velocity) morphological stability theory indicates the resolidification regimes for which the plane front is unstable. Presumably, completely planar solidification may be attained by selecting heating modes such that the region of instability is totally avoided, but the expected interface morphology is more difficult to predict if the interface passes briefly through an unstable region and then re-enters a region of stability. Aluminum-silver and aluminum-manganese alloys were melted under an electron beam with particular emphasis on attaining solidification sufficiently rapidly to satisfy the gradient-independent absolute stability condition. It was found that the velocities required to produce stability were considerably larger than expected. INTRODUCTION Under the conditions normally encountered in conventional crystal growth techniques such as the Bridgman or Czochralski process, the theory of constitutional supercooling [1] gives good guidelines for the temperature gradients and growth velocities which are required for plane-front solidification of a dilute alloy. It predicts that steep gradients and slow growth velocities are required to prevent the formation of a destabilizing layer of supercooled liquid ahead of the planar interface. Morphological stability theory [2] is based on an analysis of the actual process by which a planar interface can transform to a non-planar one, and for the conventional crystal growth processes its predictions do not vary greatly from those of the constitutional supercooling theory. However, the theory of morphological stability can account for the effects of phenomena such as surface energy and interfacial kinetics, which under more severe crystal growth conditions can dominate over temperature gradient effects. The research described here is directed toward an understanding of the stability of planar solid/liquid interfaces which solidify at high velocities (above about 10- 2 m/s): for such growth rates morphological stability theory shows that an absolute stability regime exists in which plane-front solidification is stabilized by increasing growth velocity and is insensitive to temperature gradients. When velocities become very high, solute trapping is expected to occur, leading to a v
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