D. Broddin

“Tweed” structure of Fe-doped YBa2Cu3O7−δ

Abstract Fe-doped YBa 2 Cu 3 O 7−δ single crystals have been prepared by a flux method from an Y-Ba-Cu-O precursor with copper partially substituted by iron. Electron diffraction and microscopy were applied to study the domain structure of the material. Superposed on a broad twin band texture, as also observed in undoped material, all samples reveal a finer “tweed” structure of overlapping lenticular domains oriented along [120] and [1 1 0]. This suggests a two-step formation process of the domain structure. The displacement field, corresponding with the tweed structure, resembles that of microtwinning in undoped, quenched YBa 2 Cu7 3 O 7−δ samples. The randomly dispersed Fe-ions act as pinning centers or fragment the structure by the induction of twin interfaces. Monte Carlo simulations support this idea.

Detwinning mechanism, twinning dislocations and planar defects in YBa2Cu3O7−δ

Abstract The detailed geometry of twin boundaries in YBa 2 Cu 3 O 7− δ is discussed on the basis of high-resolution electron microscopy observations. “Sharp” as well as “diffuse” twin boundaries are observed, depending on the composition and thermal history of the sample. It is shown by means of Monte Carlo simulations based on a two-dimensional anisotropic Ising model that the concentration of oxygen ions in the CuO 1−δ layers exhibits a minimum in the region adjacent to the twin interface. The vacancy enriched region is more pronounced at low temperature and disappears at high temperature. A model is proposed for twinning dislocations in YBa 2 Cu 3 O 7−δ and it is shown that the ordering stresses created by the tetragonal-orthorhombic phase transition induce the “synchro-shear” motion of such twinning dislocations, leading to the lateral displacement of twin interfaces. It is shown that this atomic model for twinning dislocations provides an atomic mechanism for the “detwinning” of crystals under uniaxial compression. Crystallographic shear planes accommodating an excess of CuO are observed; Frank dislocations bordering such shear planes have been analysed as well.

Electron microscopy study of the coherent two-phase mixtures L10+ L12, in Co–Pt alloys

Abstract Conventional and high resolution electron microscopy have been used to investigate the L12-L10, two-phase mixtures in the Co-Pt system. The domain of coexistence of the phases L12 (Copt3) and L10 (Copt) has been determined. Three different processes of formation of the two phase mixture from the high temperature disordered state (A1) have been studied as a function of the aging temperature: (1) A1 → L10 → L10 + L12, (2) A1 → L12 → L1 0 + L12, (3) A1 → L10 + L12. In the final state, the microstructures present the same main topologies. Processes (1) and (2) involve two steps, the first one consists in ordering one of the single phases. The second step is shown to involve an interfacial wetting of the domains walls of the ordered structure formed during the first step. When the phases L10 and L12 simultaneously appear within the disordered matrix (process (3)), an archetypal microstructure is formed: it consists of a regular tiling of domains according to a precise and fascinating topology driven b...

Long Period Superstructures in Cu3+xPd

Electron microscopy and electron diffraction results on the one-dimensional long period superstructures (1D-LPS) in CU3+xPd are presented. The observations strongly support the models (like the ANNNI model) in which the profile of the modulating function smoothens with rising temperature. Near the order-disorder transition temperature, the low PD content LPS have an incommensurate character.

Order-disorder transformation in Co30Pt70 alloy: evidence of wetting from the antiphase boundaries

A study of the order-disorder L12 to A1 transformation near a congruent point of the Co-Pt phase diagram is presented. This work was performed by combining in situ transmission electron microscopy observations, resistivity measurements carried out in conditions near thermodynamic equilibrium and high-resolution imaging on quenched samples. Resistivity measurements display three characteristic temperatures: 994 K, 1020 K and 1032 K. The evolution of the microstructure with temperature at different heating rates was followed in dark field images, until the order-disorder temperature was reached. It was found that at temperatures above 994 K ( approximately=40 K below Tc), the antiphase boundaries (APB) undergo a structural change; the phenomenon becomes more pronounced as the temperature increases. High-resolution images of modified APB show thin layers of disordered phase in the core of these APB. This can be described as a wetting of the antiphase boundaries by the disordered phase. Above 1020 K, up to 1032 K, a second mechanism is superimposed; the disordering occurs through a nucleation, growth and coarsening of disordered regions inside the ordered domains. This nucleation and growth process starts at the beginning of the two-phase region (1020 K). These results are in excellent agreement with those from cluster variation method calculations performed by Kikuchi and Cahn (1979) and Finel and co-workers (1990).

Chaotic and uniform regimes in incommensurate antiphase boundary modulated Cu3Pd alloys (18–21 at.%Pd)

Abstract High-resolution electron microscopy, electron diffraction and optical diffraction have been used to analyse in detail the antiphase boundaries (APBs) constituting the long-period structure (LPS) in low-Pd Cu-Pd alloys. The evolution of APB density, periodicity and sharpness was studied as a function of temperature in view of the correlation of the observations with statistical models such as the ANNNI-model and the phason-soliton descriptions. The measurement of defect widths from high-resolution images is critically assessed, and a way of quantifying the degree of wavyness and periodicity of the APBs is proposed. At high temperature, close to Tc, the APB can be characterized as wavy and with a substantial width, and thus to have a mean position which is not directly related to the basic structure (incommensurate M-values). At low temperature, near the L12- LPS transition, the APBs are sharp and pinned to the lattice. The APB width increases with increasisng annealing temperature. These results a...

The long-period antiphase-boundary-modulated structures in Cu3+xAl1−x

Abstract Observations by means of high-resolution electron microscopy (HREM) and electron diffraction (ED) of long-time annealed Cu3+xAl1−x martensite and α-Cu-Al precipitates reveal a large but limited number of commensurate uniform L12-based long-period antiphase-boundary-(LPAPB-)modulated structures (M=7/5, 4/3, 13/10, 9/7, 14/11 and 5/4). The satellite configurations in ED and the very regular antiphase boundary (APB) sequences in HREM support Fujiwaras model of a (commensurate) square-wave modulating function. The observation of the coexistence of different simple commensurate structures excludes the stability of more complex interpolating phases and suggests that the period of the LPAPB structures varies in a discontinuous way with external parameters such as the annealing temperature and the composition (via the electron-to-atom ratio). Also defective non-equilibrium antiphase boundary (APB) configurations, such as node-like junctions of minority domains, are analysed by means of HREM.

Two-dimensional long period structures in Cu-Pd. A study of the mechanism of the transition from a one-dimensional LPS to a two-dimensional LPS

Abstract The nucleation and formation mechanisms by which two-dimensional long period structures (2D-LPS) are generated from one-dimensional long period structures (1D-LPS) in Cu3Pd alloys are analysed. A series of alloys with very specific heat treatments was prepared to permit an analysis of different static steps in the transition from 1D-LPS to 2D-LPS. This analysis is based on studies by high-resolution electron microscopy and electron diffraction, which were essential to substantiate a crystallographic model for the transformation mechanism. The proposed mechanism ascribes the formation of the 2D-LPS to the propagation of parallel sets of hairpin-shaped configurations of non-conservative antiphase boundaries. Their progression in a direction perpendicular to the 1D-LPS boundaries gives rise to the 2D-LPS. Displacement vector analysis, and detailed observations of boundary geometries, support the proposed crystallographic model. The nucleation of the transformation proceeds in two ways, both related ...

The long-period superstructures in binary Au3+xZn1-x alloys: stability and off-stoichiometry effects

A detailed analysis by means of high-resolution electron microscopy (HREM) of the long-period anti-phase boundary structures in Au3+xZn1-x alloys is presented. The initial complex one-dimensional superstructures, consisting of a uniform mixture of anti-phase domains of two and three L12 unit cells width, are found to be metastable. Comparison between observed and calculated HREM images along two distinct zone axes indicates that the anomalous contrast in the imaging of the minority (3) domains in the off-stoichiometric alloys can be attributed to a combined decrease of the order parameter and a localised accommodation of excess Au. Monte Carlo simulations in the context of the three-dimensional face-centred cubic Ising model are presented, which show a similar localised decrease of the order parameter and the localisation of excess Au at an isolated (3) wall in a (2) structure, the first effect being dominant at high temperatures whereas the latter effect is more important at low temperatures. The localisation of excess Au at the (3) walls is shown to play an important role in the decomposition process, which causes a compositional segregation after prolonged annealing, with the commensurate (2) structure as one of the end-products. The electron microscope observations are interpreted in terms of a possible microscopic model for the segregation mechanism.

Electron microscopy observations of pretransition effects in alloys

Abstract A number of alloy systems show structural modifications upon approaching a phase transition. This can be an ordering (or disordering) effect or a deformation modulation. High-resolution electron microscopy allows one to deduce significant information on the transformation process. Examples considered are Co-Pt and Cu3Pd for the ordering alloys and Ni—Al where ‘tweed’ shows a prefiguration of the martensite product.