High-resolution transmission electron microscopy (HREM) of { 111 } precipitates in an A1-Cu-Mg-Ag alloy has been used to confirm by direct observation down (110) and (211) A1 matrix zone axes that the structure of these precipitates in peak-and over-aged material is consistent with the monoclinic structure proposed by Auld [Acta Cryst. (1972), A28, $98] of a =b= 4.96, c= 8.48 A, y= 120 ° , rather than the hexagonal structure with a=4.96, c= 7.01 A proposed by Kerry & Scott [Met. Sci. (1984), 18, 289-294]. Reexamination of the monoclinic structure suggested by Auld shows that the structure he proposes is in fact orthorhombic (a=4.96, b=8.59, c= 8.48 A), and is best regarded as a distortion of the structure of tetragonal 0-AI2Cu precipitates found in over-aged A1-Cu alloys. A detailed reanalysis of electron diffraction patterns from this alloy in the light of HREM observations confirms that this structure and the relative thinness of these preciptates perpendicular to the {111} planes can indeed together satisfactorily account for the extra spots and streaks in the patterns.
The martensitic phase of the shape-memory alloy equiatomic Ni-Ti has been examined by the high-resolution lattice-fringe imaging technique of transmission electron microscopy using axial illumination. This has, for the first time, shown evidence for the step and ledge structure of Type I1 twin interfaces, with the ledges being the nearest low-index rational plane to the irrational twin plane, as predicted in theoretical models of such interfaces. I n addition, small areas with (001) martensite twins have been found which show features consistent with faulting on (001) planes and also occasionally features consistent with steps corresponding to twinning dislocations at the twin planes. However, simulations of the experimental two-dimensional lattice fringe images of Ni-Ti using a multislice approach to the dynamical theory of electron diffraction have demonstrated how difficult it is to correlate these images with any detailed picture at an atomistic level of either the twin interfaces or even, in this case, of the atomic positions in Ni-Ti martensite.
Basal plane stacking disorder, delamination cracks, misorientation bands and low angle (0001) twist boundaries are observed by high resolution transmission electron microscopy in small, highly defective, boron nitride inclusions introduced unintentionally in silicon nitridesilicon carbide composites during the fabrication process. The delamination cracks are produced as a consequence of the magnitude of the thermal stresses introduced during cooling from the consolidation temperature and the layered nature of the hexagonal boron nitride crystal structure. Detailed consideration of the misorientation bands shows that, while the majority can be safely described as kink bands, there are a number of examples where the geometry is strikingly similar to the geometry of deformation twinning in hexagonal boron nitride.
The ability of scanning electron acoustic microscopy (SEAM) to characterize ceramic materials is assessed. SEAM images of Vickers indentations in SiC whisker-reinforced alumina clearly reveal not only the radial cracks, the length of which can be used to estimate the fracture toughness of the material, but also reveal strong contrast, interpreted as arising from the combined effects of lateral cracks and the residual stress field left in the SiC whisker-reinforced alumina by the indenter. The strong contrast is removed after the material is heat treated at 1000 °C to relieve the residual stresses around the indentations. A comparison of these observations with SEAM and reflected polarized light observations of Vickers indentations in soda-lime glass both before and after heat treatment confirms our interpretation of the strong contrast.
The formal theory of surface dislocations has been applied to the f.c.c.-b.c.c, interfaces defined by (111) F II (110)B. With the Bain correspondence between the two lattices, various theoretical models and experimental results on these interfaces have been analyzed. The results of the analysis suggest that preferred interface orientations can be explained on the basis that they are those of minimum or near-minimum Burgersvector contents. This concept leads to an improved criterion for comparing the elastic component of interfacial energies. The limitations of geometrical models for predicting low-energy interfaces are discussed.
IntroductionIn this paper, we describe f.c.c.-b.c.c, boundaries in terms of the formal geometrical theory of surface dislocations (Bilby, Bullough & de Grinberg, 1964), of which the 0-lattice theory (Bollmann, 1970) may be considered to be a quantized version (Christian, 1976). We also discuss the extent to which criteria such as 'best fit' are successful in predicting observed interface orientations. Particular emphasis is given to experimental results from the copper-chromium agehardening alloy system (Hall, Aaronson & Kinsman, 1972;Weatherly, Humble & Borland, 1979) for which the theory of Bollmann (1974)
c, interfaceThe Burgers vector content B of an interface between two phases designated by the subscripts + and -can be defined through the formulawhere p is a vector in the interface and S+ and 5_ are the deformations carrying the reference lattice, in which B and p are expressed in the final orientations of the (+) and (-) lattices respectively. If we choose the (+) lattice to be the reference lattice, which is transformed into the (-) lattice by the deformation $, the formula becomes
B = (I--S-m) p. (2)If we suppose that the misfit in the interface defined by (2) i where vx~,and v is a unit vector normal to the boundary. The form of (2) demonstrates that if p is fixed in length and none of the three eigenvalues of S is equal to unity, then the locus of all points B defined through the equation is the surface of an ellipsoid. The principal axes of the ellipsoid can be determined by application of Lagrange multipliers to the function f(p)= ~ BiB ii
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