The Modeling of Rapidly Solidified Powders
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Kear,
B.C.
Giessen,
and
M. Cohen,
editors
123
THE MODELING OF RAPIDLY SOLIDIFIED POWDERS
R. J. PATTERSON, II Pratt & Whitney Aircraft Group, Government Products Division, P.O. Box 2691, West Palm Beach, Florida, USA
ABSTRACT In the past several years, rapid solification technology has been extended from the realm of splats and foils to powders. This paper surveys several of the models that have been developed for such powders and considers their comparison with experimental results.
INTRODUCTION The vast body of literature, e.g., [1, 21, generated over the past twenty years in the area of rapid solidification has demonstrated the possibility of a number of alloy modifications which hold the promise of improved engineering properties. These modifications include a reduction in the degree of segregation, suppression of undesired phase formation, production of new non-equilibrium states, and extension of solid solubility limits, to name a few. Prior to the mid-1970's, the majority of the work had been done on thin flakes and foils prepared by solid-substrate quenching from the liquid state. Interest in powders is both practical and theoretical. The technology of powder metallurgy has demonstrated the ability to produce parts having large size and often complex shape; the addition of dynamic consolidation techniques [3, 4, 5] in recent years implies the possibility of retaining the as-produced state through consolidation. Theoretical interest stems not only from the powder processes themselves, but also from differences in the conditions of cooling and solidification between the powders and the solid-substrate quenching of flake and ribbon. The latter would include the rate and dimensionality of heat flow, contact with solid surfaces, and volumetric effects. In 1974, an analytical heat transfer study was undertaken at the Government Products Division of Pratt & Whitney Aircraft to determine whether one could achieve quench rates in gas-quenched droplets which would approach those reported for solid-substrate quenching. The results were favorable, and led to the P&WA "RSR" process by 1976 [6, 71. The variety of microstructural modifications subsequently observed (most recently, the amorphous state) has led to a series of increasingly sophisticated heat transfer models in an attempt to understand the nature of droplet cooling and solidification and the correlation with observed structures. Since the literature on the thermal and kinetic considerations is extensive, in this paper we will only attempt to provide a brief overview of those heat transfer models pertaining to rapidly solidified powders. MODELING AND STRUCTURE In the study of the solidification of atomized powders, many problems confront both the theorist and the experimentalist. With the possible exception of drop tower experiments, observation of discrete droplets during solidification is impossible. Except for submicron particles which are electron transparent, observation of resultant microstructures is performed on thin foils or particle cross se
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