Strain plays an important role in many solid-state phase transformations and strain-related order parameters have recently been a topic of considerable research activity. Long range strain fields due to dislocations [1] and the strain around interstitial defects [2] have also come under study by electron diffraction and imaging methods. In this note we report the observation of weak, asymmetric diffuse size-effect scattering (SES) in a dilute partially-ordered binary alloy (Cu - 16.7 at. % Al) under dynamical diffraction conditions in a high-voltage electron microscope (HVEM) operating at around 305 kV. The asymmetry, which is of odd parity around the sharp peaks of the average lattice of the alloy crystal, is due to an elastic strain field associated with atoms displaced from their average positions in the lattice, as a consequence of their difference in atomic size.

Two different kaolinites were heated in the range 850–1000°C and subsequently analyzed using TEM to study the metakaolinite-to-mullite reaction sequence. Metakaolinite was found to exist as amorphous relics of the original kaolinites. Small particles of a spinel composition appeared and grew to a maximum size of ∼50Å at temperatures below the 980°C exothermic reaction. Mullite was detected at and above 980°C which accounts for the 980°C exothermic reaction of these kaolinites.

HoAlGa is hexagonal at room temperature. It undergoes two magnetic transitions succesively at TN = 32 K from a paramagnetic to a triangular antiferromagnetic state where the Ho moments lie in the basal plane and at Tt = 18 K in the course of which the moments rotate toward c giving rise to a colinear antiferromagnetic arrangement. X-ray experiments performed between 5 and 300 K allow to determine the crystal evolution through the two transitions. The hexagonal symmetry is not lowered through the transitions; this result is compatible with the observed magnetic groups. The thermal expansion curves show a very anisotropic behaviour of the lattice parameters. The “c” parameter shrinks below TN and this anomaly is to be related to the magnetic order. Along a, a positive thermal anomaly appears below 70 K and this can be interpreted by crystal field effects. Stability of magnetic structures is discussed with regard to exchange interactions and magnetocrystalline anisotropy.

Plasma sprayed pure alumina samples have been studied by Transmission Electron Microscopy and X–Ray diffraction. The main families of metastable forms (γ δ and θ) have been observed in as-sprayed or in annealed states. All derive from a spinel structure through a periodic antiphase boundaries mechanism, together with an ordering of vacant sites in the cationic sublattice.

Powdered samples and crystals of the antimony isomorphous MnSb2O4 and NiSb2O4 have been prepared using hydrothermal synthesis. The anisotropic elastic and magnetic properties of these tetragonal structures are compared, using low temperature diffraction techniques (X-ray and neutron) and Raman scattering experiments. Magnetostrictive effects in MnSb2O4 are interpreted. An anomalous frequency shift is observed below 70 K and the change of the Grüuneisen constant is interpreted.

Some systems show continuous and reversible phase transitions which are characterised by the appearance of irrational sunerlattice reflections with a position that shifts continuously and reversibly with temperature. This diffraction feature is not necessarily caused by a deformation modulation but can also originate from the reneated occurrence of planar defects with a variable “average” periodicity. The planar defects can be of different type (e.g. planes of different composition, antiphase boundaries, twin planes) as shown for the systems Ni3+xTe2, Cu3−xTe2, Cu2−S, mPbS-nBi2S3 ( > 2) and Cu 0.75VS2, using electron-microscopy and electron diffraction.

The cause of the stability of long period superstructures is still something of a mystery. Typically, two very different models have been proposed: according to model I, the period of the superstructure (or modulation) is determined by lowering of the electronic energy resulting from the formation of a new Brillouin zone. According to model II, competing short-range interactions tend to produce long-period structures, the wavelength of which is determined by configurational entropy considerations. Model I is exemplified by the Sato and Toth theory, apparently applicable to long-period superstructures in Cu-Au, for example. Model II is exemplified by the Axial Next Nearest Neighbor Ising Model, for which a low-temperature free energy expansion has recently been given by Fisher and Selke. The latter model appears to apply to long-period superstructures in Ag3Mg.

Changes in the ordered structures of the Au-Mn system, existing at temperatures below 400°C, with composition have been studied in the range of 20–28 at.% Mn by high-resolution electron microscopy. The superstructures, Au4Mn, Au22Mn6, Au31Mn9 and 2d-APS(I), exist in the range of 20–23 at.% Mn. The Au22Mn6 and Au31Mn9 are the one-dimensional (ld) and two-dimensional (2d) antiphase structure (APS), respectively, based on the Au4Mn, and the 2d-APS(I) is based on the DO22 structure and consists of parallelogram and lozenge shaped domains. When Mn content increases, the Mn-Mn nearest-neighbour pairs are formed across the antiphase boundaries of the 2d-APS(I), and the structure changes to orthorhombic 2d-Au3Mn at about 24 at.% Mn. When Mn content increases further, the Mn-Mn pairs align in the <120> direction and the structure transforms continuously to the monoclinic Au5Mn2.

Ordered structures of Cu-Zn-Au alloys were studied by electron diffraction. Two-dimensional antiphase structures exist at the compositions, Cu55Zn15Au30, Cu80-xZnxAu20 (10 ≤ x ≤ 35) and Cu85-xZnxAu15(25 ≤ x ≤ 35). The domain sizes M's measured in terms of the basic L12 cell along the two orthogonal directions are always the same, and they change from 3.4 to 1.6 as Zn content increases from 10 to 35 at.%. Based on the superstructure image of the Cu55Zn25Au20 with M=2.0 taken with high-resolution electron microscope, a structure model is derived, which explains intensities of all the superlattice spots on the diffraction pattern.

Long-period one-dimensional antiphase structures with average domain sizes of non-integral multiples (M=1.33 and M=1.50−1.91) of the basic L12 cell, existing in the Al3-xTi1+x(0<x>0.33) alloys, were studied by electron diffraction, high-resolution electron microscopy and X-ray diffraction. The mode of mixing of M=1 and M=2 domains was revealed and the nature of lattice modulation associated with the antiphase structure was clarified. A new type of superstructure Al5Ti3 was also studied by highresolution electron microscopy and the structure model proposed previously was verified.

Dark-field images of two-dimensional antiphase domain structures with stepped boundaries have been simulated for imaging conditions corresponding to some current high resolution electron microscopes. The simulations reveal that the positions of the dots in the dark-field images do not give a true representation of atomic structure; thus, in contrast to the brightfield superstructure images, it is very misleading to say that bright dots in the dark-field image correspond exactly to the positions of the minority atoms.

We examine the nature of all possible stationary states in an inhomogeneous ordered binary alloy. For this purpose, we use a development of the free energy in terms of the gradient of the non linear Euler equations is determined from the nature of its singular points. This method allows us to study these solutions for arbitrary expressions of the free energy of the homogeneous system as well as of the gradient coefficients. In general, periodic solutions which can be identified with long period superlattices are found. In specific cases, analytic solutions can be obtained. Fourier components are calculated and compared with experimental values determined for CuAu II.

The configuration of antiphase domains in a one-dimensional long period superstructure (LPS) was examined theoretically. As LPS is composed of two kinds of antiphase domains, the domain-arrangement is described by f(n) , where f(n) is the existence probability of n which is the number of the majority domains between two minority ones. The equations for f(n) were derived using statistical approaches or thermodynamical ones, and f(n) predicted from the equations were compared with those obtained experimentally.

Two kinds of process of ordering with phase separation, B2 or DO3 → (A2 + DO3), on annealing at a temperature of 873 K in an alloy of Fe-6 at%Al-9 at%Si were investigated by means of transmission electron microscopy and X-ray diffraction. The decomposition of DO3 phase proceeds by the development of periodic fluctuations along <100> directions in both B2 and DO3 types of degree of order. Newly born DO3 domains take the form of rectangular prism and align along <100> directions. In the transition from B2, on the contrary, there is a marked difference between the morphologies of the fluctuations in the two types of degree of order at the early stage of annealing. The degree of B2 type order fluctuates along <100> directions, while the fluctuation in the degree of DO3 type order does not form the <100> modulated structure sat the early stage of annealing. The morphology after a prolonged annealing is similar to that in the case of DO3 → (A2 + DO3).

The order-strengthening mechanism in the ternary alloy, Cu-Ni-Zn, is investigated by comparing stress-strain curves and electron micrographs of corresponding dislocation structure in disordered, partly ordered and ordered samples. Around the composition Cu2NiZn two different types of ordering are found experimentally: L10 and L12. The degree of ordering is determined by measuring the equilibrium distance between superlattice dislocations and by single crystal neutron diffraction. The long-range zinc order parameter thus obtained appeared to be in agreement with calculations using the cluster variation method (CVM) in the tetrahedron approximation. Electron microscopic observations show that even in the disordered phase, when short range order exists, superlattice dislocations are created by Frank-Read sources, rather than the generation of single dislocations.

The most interesting topics to be solved during the next years, in the field of short-range order and of ordering transitions is the accurate determination of pair interaction parameters, or more generally interactions in the case of higher order configurations. In fact, there are a number of papers in this conference devoted to this problem. Here, I would like to emphasize two aspects of this problem.

Using a method of linear inequalities similar to that introduced previously by Kanamori and others, we determine the ground state of the Ising model on the BCC lattice in the presence of first, second, thirdand fifth neighbour interactions. Thirteen new structures are found ; the C 11b (MoSi2)and B 11 (γ - TiCu) in particular are found to be stabilized by third and /for fifth neighbour interactions. Some regions of the phase diagram however are still not completely known.

Disordered systems have received a great deal of theoretical attention in the last fifteen years or so, and many advances in our understanding of such systems have been made. The greatest progress has been made in connection with substitutionally disordered alloys in which atoms of various species are randomly distributed over the sites of a regular lattice. The most satisfactory single-site theory for studying the properties of random substitutionally disordered alloys, in particular the one-particle properties such as the density of states (DOS), is the coherent potential approximation (CPA) [1,2]. In the CPA, one considers that the real disordered material is replaced by a self-consistently determined effective medium which is characterized by an energy-dependent site-diagonal selfenergy, and which preserves all symmetries of the lattice. The CPA yields unique and analytic results, i.e., yields DOS and spectral weight functions that are nonnegative and satisfy the fundamental sum rules. There exist several reviews of the CPA [3-5] both for systems describable by tight binding (TB) and by muffin-tin (MT) Hamiltonians.

Present calculations of the Cowley-Warren SRO parameters, using the Rasolt-Taylor-Dagens, first principles pair potential for Li-Mg alloys show that the higher order terms of Tahir-Keli's intermediate-temperature solution of the Ising, spin-1/2 ferromagnet modify the Clapp and Moss, hightemperature solution in a very significant way. The present results are compared to previous calculations and measurements on Li-10 at.% Mg and Li-50 at.%Mg alloys.

Recent experiments have proved the efficiency of T.E.P. measurements in phase transformation studies. The main features of the technique are presented and two particular examples of application are given (influence of plastic deformation on short range ordering (S.R.O.) and study of dissolution of the equilibrium Θ phase in Al-Cu alloys).

Since the first studies [1] only very few attempts have been made to use thermoelectrical power measurements (T.E.P.) to characterize the evolution of an alloy during phase transformation. However, in spite of the complexity of the theoretical interpretations, T.E.P. measurements can give valuable informations on the microstructure of the material, sometimes with more easiness or more precision than with other techniques or even informations which would be impossible to obtain by another way.