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.

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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.