Grain texture evolution during the columnar growth of dendritic alloys
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INTRODUCTION
THE selection of columnar grains during the directional solidification of alloys is a phenomenon that has been known for many years. E~JIt plays a key role during the investment casting of directionally solidified (DS) or single-crystal (SC) components for aerospace applications/ 2J It is also a fundamental phenomenon intervening in most solidification processes, r31 including welding. Iaj Because the dendrites of cubic metals preferentially grow along (100) crystallographic directions, I3j they have to grow faster when they are misoriented with respect to the thermal gradient. Since the undercooling of the dendrite tips is an increasing function of the growth rate, the dendrites belonging to a misoriented grain lie slightly behind those that are better aligned with the heat-flow direction. This behavior results in a competition of the dendritic network propagation at grain interfaces tl,SJ leading to a selection of the grains that are best oriented in the thermal gradient. This selection mechanism has been the subject of many investigations. In particular, the pioneer work of Walton and Chalmers t~l already has established a link between the evolution of the density and texture of columnar grains with increasing distance from the mold wall. Two new tools, which are now available, enable this problem to be investigated in much greater depth. The first tool is a fully automatic indexing technique Ch.-A. GANDIN, Postdoctoral Fellow, and M. RAPPAZ, Professor, are with the Laboratoire de Mrtallurgie Physique, Ecole Polytechnique Frdrrale de Lausanne, MX-G, CH-1015 Lausanne, Switzerland. D. WEST, Graduate Student, and B.L. ADAMS, Professor, are with the Department of Manufacturing Engineering and Engineering Technology, Brigham Young University, 435 CTB, Provo, UT 84602. Manuscript submitted May 16, 1994.
METALLURGICAL AND MATERIALS TRANSACTIONS A
of electron backscattered diffraction patterns (EBSPs) that gives access to the crystallographic orientation of the grains, t6'7'81 Such a system can measure up to one crystallographic orientation per second at points regularly spaced on the surface of a specimen. Therefore, this unique tool combines the information that can be obtained from a standard texture analysis with that revealed by a Berg-Barrett topograph. Until the present study, this unique instrument had not been used to study solidification structures. The second tool is provided by simulation. Stochastic models for the prediction of grain structure formation during solidification have been developed recently for two-dimensional (2D) geometries, c9J Such models have been coupled with finite-element heat-flow computations tS~ in order to encompass nonuniform temperature situations. They have also been extended to three dimensions using a simplified growth algorithm that treats small specimens of nearly uniform temperature.l 1~ ~] Unlike Monte Carlo simulations, E~z'~31these stochastic models are based upon the basic mechanisms of nucleation and grain growth. In particular, they account for the
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