Directional and single-crystal solidification of Ni-base superalloys: Part I. The role of curved isotherms on grain sele
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INTRODUCTION
THE development of directional and single-crystal casting technology for the production of turbine blades has been a key factor in the evolution of the gas turbine for aerospace applications.[1] Versnyder and Shank[1] pioneered these closely related processes, which led to improved ductility, creep rupture, and thermal fatigue resistance. Walton and Chalmers[2] reported the existence of a preferred crystallographic orientation in columnar-grained castings produced in a positive thermal gradient and attributed this preferred orientation to anisotropic growth rates of dendrites associated with grains of different orientations. In cubic alloys, grains with ^001& aligned parallel to the temperature gradient have the fastest growth rate and overgrow the slower growing orientations. This grain selection mechanism is of practical importance, since (1) superalloy directional and single-crystal castings are invariably produced with a dendritic growth front morphology; and (2) the natural ^001& axial texture coincides with the direction of minimum Young’s modulus leading to reduced thermal stresses and enhanced thermal fatigue resistance of gas turbine blades in service. There have been several proposals of the factors controlling solidification texture. Walton and Chalmers[2] have suggested that the ^001& fast growing directions in cubic crystals are a consequence of the anisotropy of the surface energy. Direct observations of ^001& dendritic growth in the organic analogue, succinonitrile, by Huang and Glicksman[3] supported this conclusion. Goss[4] suggested that dendrites grow in the direction having the highest thermal conductivity; N. D’SOUZA, Postdoctoral Research Assistant, M.G. ARDAKANI, Research Associate, M. McLEAN, Professor and Head of Department, and B.A. SHOLLOCK, Senior Lecturer, are with the Department of Materials, Imperial College of Science, Technology and Medicine, London SW7 2BP, United Kingdom. Manuscript submitted October 19, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
however, in cubic systems, such as superalloys, the thermal conductivity is isotropic.[5] Lee et al.[5] have investigated the fast growing directions in fcc (Al, Pb), bcc (Fe, Cr), hcp (Zn), and bct (Sn) systems, suggesting that the strain energy (thermal stresses) arising in the dendrite because of the orientation-dependent Young’s modulus is also an important factor. It is proposed that the ^111& dendritic growth in chromium, where the thermal conductivity is isotropic and the anisotropy of the surface energy criterion predicts the existence of the ^001& fast growing direction, results from such thermal stresses. Quested and McLean[6] have related the morphology of the growth front in directionally solidified (DS) superalloys to the G/V ratio and the scale of the dendrites to the solidification conditions. (G is the temperature gradient in the melt ahead of the solidifying front and V is the rate of solidification.) More recently, Gandin et al.[7,8] have characterized the evolution of grain orientations during the di
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