Directional and single-crystal solidification of Ni-base superalloys: Part II. Coincidence site lattice character of gra
- PDF / 1,341,748 Bytes
- 7 Pages / 612 x 792 pts (letter) Page_size
- 89 Downloads / 144 Views
I. INTRODUCTION
IT is recognized that not all high angle grain boundaries are equivalent. There exists a particular class of high angle boundaries, which for certain misorientations have a degree of lattice symmetry that is thought to be endowed with special properties. These particular boundaries, of which the low angle boundaries form a special set, are referred to as coincidence site lattice (CSL) boundaries; the name stems from considering the two crystals, separated by the boundary, to have inter penetrating lattices with a proportion of shared lattice sites.[1] By convention, the CSL character is represented by the reciprocal of the proportion of coincident sites, (. The special properties associated with these boundaries are well-documented.[2] The physical significance of the frequency of coincidence sites (1/() is generally considered to be the key factor influencing these special properties. In general, special properties have been associated with CSL boundaries characterized by low ( values, although significant energy cusps have been reported for some high ( boundaries.[3] In most experimental work, an upper bound of ( 5 29 is chosen as being significant.[4] The misorientation between two lattices is characterized by three independent angles, which can take the form of angular relationships between three crystallographic axes in each of the crystals (e.g., direction cosines for the cube directions in the cubic crystal system). Alternatively, the two lattices can be brought into coincidence by rotation about a single axis; two angles are required to define the rotation axis ^hkl& and a third to specify the angle of rotation. Coincidence site lattice boundaries are described by specific misorientation axes (MAx) and misorientation angles (MAn). Many processing routes produce strong crystallographic M.G. ARDAKANI, Research Associate, N. D’SOUZA, Postdoctoral Research Assistant, B.A. SHOLLOCK, Senior Lecturer, and M. McLEAN, Professor and Head of Department, 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
textures, which will influence the probability of CSL occurrence. In particular, during directional solidification of nickel-base superalloys, columnar crystals with ^001& axial orientations, but random transverse orientations, develop parallel to the solidification direction. In part I, we have shown that the sharpness of the ^001& axial texture is sensitive to both the solidification conditions and to characteristics of the alloy. For an ideally sharp axial texture, lattice coincidence is achieved by rotations about this axis, reducing the degrees of freedom from three to one that would result in a grain boundary misorientation distribution (GBMD) dominated by a MAx equivalent to the axial texture. A preferred GBMD would therefore be expected to have a bearing on the distribution of coincidence boundaries (CGBD).[2] Various numerical analyses have q
Data Loading...
