Conditional spinodal in the presence of tweed contrast and diffuse scattering in Ni-Al-Ti alloys

Metastable water-quenched bcc Cr40Ti60 is reported to undergo solid state amorphization during sub-eutectoid temperature anneals. Electron diffraction of the metastable bcc phase reveals complex patterns of diffuse scattering indicating the presence of deviation, topological, chemical or both, from the ideally random solid solution. The hypothesis that the microstructure causing the diffuse scattering plays a role in the amorphization process is supported by studies in which an electron- irradiated bcc-based Cu-Ti alloy shows similar diffuse scattering just prior to irradiation-induced amorphization.Diffuse scattering is observed in all major zone axes of the as-quenched alloy except for (111). Analysis of the spatial frequencies of diffuse scattering maxima indicates that in spite of a clear directional relationship between the bcc matrix scattering and the diffuse scattering, at least some diffuse intensity maxima have spatial frequencies which are incommensurate with the bcc matrix. For instance, the streaked diffuse form along <11> in the (110) zone axis pattern (Fig. 1, arrow), which has been determined to be due to kinematical scattering, does not correspond to any possible bcc superlattice spacing along the [11] direction. This suggests that the microstructure of the as-quenched phase includes regions in which static displacements from the bcc equilibrium sites are encountered.

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Anomalous X-ray diffuse scattering from nanoparticles in single-crystalline Ti alloys

Acta Crystallographica Section A Foundations and Advances ◽

10.1107/s2053273316093979 ◽

2016 ◽

Vol 72 (a1) ◽

pp. s413-s413

Author(s):

Václav Holý ◽

Jana Smilauerova ◽

Dominik Kriegner ◽

Petr Harcuba

Keyword(s):

Diffuse Scattering ◽

Single Crystalline ◽

X Ray ◽

Ti Alloys

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Structural analysis of ω-like diffuse scattering from quenched Cr-Ti alloys

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100120540 ◽

1992 ◽

Vol 50 (1) ◽

pp. 28-29

Author(s):

W. Sinkler ◽

D. E. Luzzi

Keyword(s):

Diffuse Scattering ◽

Reciprocal Lattice ◽

Equilibrium Phase ◽

Pure Titanium ◽

Peak Shift ◽

Cubic Phase ◽

Solute Content ◽

Β Phase ◽

Ti Alloys ◽

Cr System

Incommensurate diffuse scattering is seen in electron diffraction from the quenched-in body centered cubic phase (β phase) of alloys composed of titanium or zirconium with other early transition metals. Such diffuse scattering is related to the commensurate ω phase, a high pressure equilibrium phase of pure titanium, which also forms in dilute quenched alloys after annealing. The present study concerns ω-like diffuse scattering in the quenched (β phase of the Ti-Cr system. It is shown that the diffuse scattering in concentrated alloys consists of ω diffraction maxima which have undergone ordered displacements from commensurate ω reciprocal lattice positions. The displacement magnitude increases with solute content (see figure 1). A general expression in reciprocal space for the dependence of the diffuse peak shift on (hkl)ω was found. A model based on this expression correctly predicts the location of all diffuse peaks.

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Direct Observation of the Diffusionless β → β + ω Transformation in Titanium Alloys

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100064669 ◽

1971 ◽

Vol 29 ◽

pp. 122-123

Author(s):

N. E. Paton ◽

D. de Fontaine ◽

J. C. Williams

Keyword(s):

Electron Microscope ◽

Titanium Alloys ◽

Direct Observation ◽

Dark Field ◽

X Ray Diffraction ◽

Resistivity Measurements ◽

Selected Area Electron Diffraction ◽

Ti Alloys ◽

Thin Foils ◽

Field Device

The electron microscope has been used to study the diffusionless β → β + ω transformation occurring in certain titanium alloys at low temperatures. Evidence for such a transformation was obtained by Cometto et al by means of x-ray diffraction and resistivity measurements on a Ti-Nb alloy. The present work shows that this type of transformation can occur in several Ti alloys of suitable composition, and some of the details of the transformation are elucidated by means of direct observation in the electron microscope.Thin foils were examined in a Philips EM-300 electron microscope equipped with a uniaxial tilt, liquid nitrogen cooled, cold stage and a high resolution dark field device. Selected area electron diffraction was used to identify the phases present and the ω-phase was imaged in dark field by using a (101)ω reflection. Alloys were water quenched from 950°C, thinned, and mounted between copper grids to minimize temperature gradients in the foil.

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Calculation of structure images of crystalline defects

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100108556 ◽

1978 ◽

Vol 36 (1) ◽

pp. 282-283

Author(s):

M.A. O'Keefe ◽

Sumio Iijima

Keyword(s):

Diffuse Scattering ◽

Computing Time ◽

Continuous Distribution ◽

Sampling Interval ◽

Reciprocal Space ◽

Objective Lens ◽

Lattice Images ◽

Slice Method ◽

Space Cell ◽

Beam Lattice

We have extended the multi-slice method of computating many-beam lattice images of perfect crystals to calculations for imperfect crystals using the artificial superlattice approach. Electron waves scattered from faulted regions of crystals are distributed continuously in reciprocal space, and all these waves interact dynamically with each other to give diffuse scattering patterns.In the computation, this continuous distribution can be sampled only at a finite number of regularly spaced points in reciprocal space, and thus finer sampling gives an improved approximation. The larger cell also allows us to defocus the objective lens further before adjacent defect images overlap, producing spurious computational Fourier images. However, smaller cells allow us to sample the direct space cell more finely; since the two-dimensional arrays in our program are limited to 128X128 and the sampling interval shoud be less than 1/2Å (and preferably only 1/4Å), superlattice sizes are limited to 40 to 60Å. Apart from finding a compromis superlattice cell size, computing time must be conserved.

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Recent applications of TEM to the study of interfaces

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100174679 ◽

1990 ◽

Vol 48 (4) ◽

pp. 308-309

Author(s):

C. Barry Carter

Keyword(s):

High Resolution ◽

Sample Preparation ◽

Grain Boundaries ◽

Diffuse Scattering ◽

Imaging Techniques ◽

Amorphous Material ◽

Contrast Imaging ◽

Current State ◽

Actual Nature ◽

High Resolution Images

This paper will review the current state of understanding of interface structure and highlight some of the future needs and problems which must be overcome. The study of this subject can be separated into three different topics: 1) the fundamental electron microscopy aspects, 2) material-specific features of the study and 3) the characteristics of the particular interfaces. The two topics which are relevant to most studies are the choice of imaging techniques and sample preparation. The techniques used to study interfaces in the TEM include high-resolution imaging, conventional diffraction-contrast imaging, and phase-contrast imaging (Fresnel fringe images, diffuse scattering). The material studied affects not only the characteristics of the interfaces (through changes in bonding, etc.) but also the method used for sample preparation which may in turn have a significant affect on the resulting image. Finally, the actual nature and geometry of the interface must be considered. For example, it has become increasingly clear that the plane of the interface is particularly important whenever at least one of the adjoining grains is crystalline.A particularly productive approach to the study of interfaces is to combine different imaging techniques as illustrated in the study of grain boundaries in alumina. In this case, the conventional imaging approach showed that most grain boundaries in ion-thinned samples are grooved at the grain boundary although the extent of this grooving clearly depends on the crystallography of the surface. The use of diffuse scattering (from amorphous regions) gives invaluable information here since it can be used to confirm directly that surface grooving does occur and that the grooves can fill with amorphous material during sample preparation (see Fig. 1). Extensive use of image simulation has shown that, although information concerning the interface can be obtained from Fresnel-fringe images, the introduction of artifacts through sample preparation cannot be lightly ignored. The Fresnel-fringe simulation has been carried out using a commercial multislice program (TEMPAS) which was intended for simulation of high-resolution images.

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Superstructures and ordering phenomena in ceramic superconductors

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100166014 ◽

1996 ◽

Vol 54 ◽

pp. 710-711

Author(s):

R. Gronsky

Keyword(s):

Domain Growth ◽

Small Displacement ◽

Small Scale ◽

Ceramic Superconductors ◽

Oxygen Sublattices ◽

Tweed Contrast ◽

Diffraction Patterns ◽

Representative Image ◽

Static Lattice ◽

Modulation Wavelength

It is now well established that the phase transformation behavior of YBa2Cu3O6+δ is significantly influenced by matrix strain effects, as evidenced by the formation of accommodation twins, the occurrence of diffuse scattering in diffraction patterns, the appearance of tweed contrast in electron micrographs, and the generation of displacive modulation superstructures, all of which have been successfully modeled via simple Monte Carlo simulations. The model is based upon a static lattice formulation with two types of excitations, one of which is a change in oxygen occupancy, and the other a small displacement of both the copper and oxygen sublattices. Results of these simulations show that a displacive superstructure forms very rapidly in a morphology of finely textured domains, followed by domain growth and a more sharply defined modulation wavelength, ultimately evolving into a strong <110> tweed with 5 nm to 7 nm period. What is new about these findings is the revelation that both the small-scale deformation superstructures and coarser tweed morphologies can result from displacive modulations in ordered YBa2Cu3O6+δ and need not be restricted to domain coarsening of the disordered phase. Figures 1 and 2 show a representative image and diffraction pattern for fully-ordered (δ = 1) YBa2Cu3O6+δ associated with a long-period <110> modulation.

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