Strain Contrast Research Articles

Large 〈110〉-segmented helical dislocations in natural diamond

Abstract Helical dislocations composed of very straight 〈110〉 line segments (helical shape due to climb from original screw orientation) have been observed by transmission electron microscopy in a region of diamond where growth had occurred on non-faceted, near-{100}-orientation surfaces. Of the six 〈110〉 segments forming each helix turn, the two that are pure edge segments normal to the helix axial Burgers vector are about ten times longer than the other segments, thereby producing strong flattening of the helix in the cube plane containing the helix axis. One of 30 such helices counted in a thinned specimen area of 3 × 104 μm2 was studied in detail. Its major and minor diameters were 5 and 0·6 μm respectively. The helix axis was marked by a line of vesicles, diameters from about 20 to about 130 nm, producing matrix strain contrast attributed to fluid under pressure within the vesicles. Diffraction contrast analysis showed the helix to be of interstitial character, and the vacancy volume emitted to form ...

Initial stage Fe-clustering in the Au–Fe spin-glass system

Aging experiments were conducted to study initial state Fe-clustering in Au–Fe alloys with Fe content from 10.7 to 33%. Information derived from experiments using transmission electron microscopy, electron diffraction, and high resolution electron microscopy suggests the coexistence of short-range-order and Fe-clusters. At early stages of aging, lobe-like and/or rod-shaped strain contrast images, identified as clusters, were revealed after specimens were further cleaned by an ion-miller. The (11/20) special point diffuse reflections were prominent in the as-quenched condition and/or early stage aging, and the intensities decrease gradually as aging proceeds. After a certain period, (11/20) diffuse reflections disappear but strain contrast images still remain similar. This suggests that the strain contrast images are not related to the (11/20) diffuse reflections. The cluster characteristics of Au–Fe alloys are differentiated from those of Al–Cu and Cu–Be based upon theoretical calculations.

Influence of Deformation on Stability of TiC Precipitates in .ALPHA.-Fe.

To investigate the influence of deformation on the stability of TiC precipitates in the α-Fe region, the temperature of precipitation treatment and the strain rate of deformation prior to final soaking at 773 K were changed with an Fe-0.05mass%Ti-0.002mass%C alloy. Precipitation treatment at 973 K for a short time generated very fine TiC particles (average radius: 2 nm) which were presumed to be coherent or semi-coherent because of a strong strain contrast in TEM images. Deformation with a high strain rate (1226 s-1) just after precipitation treatment of 973 K for a short time resulted in a distinct increase in the solute C content, suggesting that some of the coherent TiC precipitates had dissolved. The elastic interaction between the coherent TiC particles and a high density of dislocation induced by the high strain rate deformation is considered to be the most conceivable explanation of several mechanisms including the temperature increase due to deformation and binding between dislocations and C at grain boundaries. Based on the micromechanics of an anisotropic and inhomogeneous system, the interaction energy was calculated from a simple case consisting of an isolated coherent TiC particle and a straight-edge dislocation. The calculation of the change in total free energy supported the possibility of TiC dissolution due to the elastic interaction between coherent TiC precipitates and a high density of dislocations induced by the high strain rate deformation.

Behavior of defects in dual-implanted AlGaAs superlattices

The behavior of GaAs AlAs superlattices implanted with Ge and Be has been studied. As-implanted specimens showed the presence of microtwins and strain contrast. These arise due to the effect of dynamic annealing on the high concentrations of self-interstitials and of implanted atoms present in the superlattice. Dislocation loops observed in annealed samples were present on {111} or {110} planes, these being the low-energy planes in GaAs. Co-implanted samples showed a difference in behavior of damage from singly implanted samples. The difference is explained in terms of an interaction between point defects created by one implant with dopant atoms of the other implant. Voids were seen in near-surface layers of co-implanted superlattices. The origin of the voids is explained in terms of coalescence of vacancies that result from the implant.

On strain contrast from B2 antiphase domain boundaries in rapidly solidified Fe32at%Al15at%Mo alloy

On strain contrast from B2 antiphase domain boundaries in rapidly solidified Fe32at%Al15at%Mo alloy

Strain field measurement of plate-like defect in silicon

Strain contrast around the precipitate in crystalline solid has been interpreted as due to the elastic distortion of the matrix around the precipitate. Adding the displacements into the dynamical equations of diffraction contrast in the 2-beam form developed by Howie and Whelan, and using the column approximation to calculate the intensities point by point, it is possible to obtain the intensity map of the strain contrast for a particular strain parameter ε. Ashby and Brown showed that the strain-measuring methods for the simplest case of spherical precipitate can be extended to determine the strain of the plate-like defect (PLD) normal to the plane of the plate. Most of the precipitates in silicon induce strain fields in the surrounding matrix with PLD the largest. In this abstract, we are trying to measure the strain field ε of PLD in silicon by matching the experimental and simulated images.

Ion bombardment of Zr 3Fe

Ion bombardments, transmission electron muscopy (TEM) and analytical electron muscopy (AEM) techniques were used to obtain information on the effect of irradiation on the phase stability of Zr 3Fe, which has an orthorhombic crystal structure. 40Ar ion bombardments were performed at 35–773 K. using 1.5, 1.0 and 0.5 MeV ions, successively. The resulting irradiation damage was distributed over a depth of ∼ l μm. At low ion fluences (10 11−10 12 ions cm −2), the damaged regions appeared as small defects (1.0–5.0 nm diameter) exhibiting strain contrast features. At higher ion fluences (10 13−10 15 ions cm −2), there was an appreciable overlap of the strain-contrast regions, thus resulting in a more complex appearance. In the fluence regime 1 × 10 14−1 × 10 16 ions cm −2, there was a gradual transition from the crystalline to amorphous state at temperatures of 35–473 K. The fluence required to amorphize a given fraction of the irradiated volume was quite dependent upon the amount of annealing that occurred within the collision cascade at the temperature of the bombardment. At temperatures above ∼ 623 K, Zr 3Fe remained crystalline during ion bombardments. Post-irradiation annealing experiments performed at 500–623 K, on Zr 3Fe which had been rendered amorphous during ion bombardment, revealed the nucleation and growth of crystalline Zr 3Fe regions within the amorphous matrix.

Misfit accommodation by steps in cubic materials

Most previous studies on misfitting crystal interfaces have been aimed at understanding the accommodation of misfit at fairly plane interfaces in terms of minimum (equilibrium) energy principles. At structural ledge interfaces, terrace patches (widthl x) of opposing interfaces are separated by atomic steps at the ledges. The mismatch which builds up along each terrace pair patch is compensated for by a relative displacement of the atomic patterns of the two terraces (pattern advance) at the next step. Also, since the atomic plane spacings parallel to the terraces are different on the two sides of the interface, a misfit exists in a direction normal to the terraces, which is accommodated by a misfit dislocation (vector perpendicular to the terraces). In considering rhombic symmetries, {111}fcc//{110}bcc, we show that the misfits, as well as the pattern advances, are interrelated through the ratior =b/a of nearest-neighbor distances in the crystals. Calculations show that there exists a critical ratior c above which planar interfaces are more stable than stepped interfaces. The resolved shear stress at the step is shown to possibly facilitate the formation of dislocations in the presence of a subcritical applied stress. The corresponding displacements (bending) of atomic planes are probably most easily observable with strain contrast electron microscopy techniques.

Structural ledges in interphase boundaries

This article addresses the properties of stepped misfitting interfaces and their energetic preference to planar misfitting interfaces. It highlights: (a) the purely geometrical or rigidlike, (b) the rigid (unrelaxed) energetic, and (c) the relaxed energetic properties of stepped interfaces. In (a), we address (1) the accommodation of misfit by the step or ledge mode through the cancellation of the mismatch, that builds up along a terrace, by the forwardpattern advance effected by a step,i.e., the relative displacement of atomic patterns on either side of the interface as observed in crossing a structural ledge along the interface, (2) the sideways (shear) pattern advance which seems to be energetically undesirable, (3) the need for tilt-type misfit dislocations to accommodate the misfit normal to the interface, and (4) the fact that at {III}fcc(face-centered cubic)/{110}bcc(body-centered cubic) interfaces with rhombic symmetries, the misfits, as well as the pattern advances, are interrelated through the ratior = b/a of nearest-neighbor distances in the crystals. In (b), we exploit the rigid model approach that (1) yields ideality criteria for minimum energy and provides energetic justification for the step mode of misfit accommodation, (2) confirms that the average terrace widthl[inx defined by this mode also meets the condition for positive energy gain, and (3) defines the upper and lower energy bounds to provide a perspective of the system energetics. In (c), the foregoing considerations are refined by a transition to the harmonic (elastic) model to yield (1) the dependence of the mean energy per atom of a stepped interface on interfacial misfit and pattern advance, as well as the dependence of the mean energy per atom of a planar interface on misfit, (2) expressions for the stresses related to the atomic interaction between opposing terraces, (3) atomic displacements that might be probed by modern analytical techniques, and (4) resolved shear stresses and normal stresses that may facilitate the formation of glide dislocations in the presence of applied stresses. The boundary in a two-dimensional space—spanned by misfit and pattern advance—between regions where stepped interfaces are more stable than planar ones has been determined, suggesting that a critical misfit exists above which only planar interfaces are stable. Whereas the resolved shear stress related to the formation of structural ledges may facilitate the formation of dislocations in the presence of a subcritical applied stress, the corresponding displacements (bending) of atomic planes are probably observable only with strain contrast electron microscopy techniques.

Transmission Electron Microscopic Study of the Kaolinitization of Muscovite

AbstractHydrothermally kaolinitized muscovite from the Otago schist of Brighton, New Zealand, has been studied by transmission and analytical electron microscopy (TEM and AEM) to determine the mechanism of alteration and to compare reactant-product relations for di- and trioctahedral micas. The muscovite is a primary metamorphic phase having a phengitic composition. It occurs as well-ordered, two- and three-layer polytypes, in grains as thick as 30 μm. Kaolinite occurs as packets of layers, each about 100–600 Å thick, which alternate with packets of muscovite or smectite-like layers. Most of the kaolinite is highly disordered in stacking sequence, although a one-layer polytype is also present, occurring as relatively thick sequences of layers. Phase boundaries between kaolinite and muscovite are invariably parallel to the 001 lattice fringes with no strain contrast; i.e., no transitions exist along layers. Parallelism of 00l and 11l reflection rows of both kaolinite and muscovite implies a topotaxial intergrowth. A smectitelike phase is also present, occurring as packets of wavy layers, which locally have periodic contrast that may reflect R1 ordering of illite/smectite. This material appears to be a direct, “along-layer” alteration product of muscovite. Electron diffraction data and lattice-fringe images imply that kaolinite alternates with micaceous phase(s) with some regularity; i.e., micaceous layers are separated by approximately equal numbers of kaolinite layers. Similar long-range periodicity occurs in contrast variations within packets of kaolinite layers.The data collectively suggest that the alteration interface was self-perpetuating and that alteration proceeded rapidly along layers once it initiated in 2:1 layers at crystal edges or strained areas, with no observable component normal to the layers. They also suggest that smectite may have formed as an intermediate phase during the hydrothermal kaolinitization of muscovite. In the previous study of alteration of biotite in the same sample, “along-layer” transition boundaries were commonly observed, and a second, intermediate product phase was not detected, implying a relation between the alteration mechanisms and the chemical differences between reactants and products.

Mechanism of Al2CuLi (T 1) nucleation and growth

Conventional strain contrast transmission electron microscopy (CTEM) and high-resolution transmission electron microscopy (HRTEM) were performed to establish the nucleation and growth mechanism of Al2CuLi (T1) precipitates in an Al-Li-Cu alloy. It is shown that the growth mechanism ofT 1 precipitate plates occurs by the diffusional glide of growth ledges composed of b = 1/6〈112〉 partial dislocations on 111 matrix planes and that the growth ledges migrate by the ledge-kink mechanism, as previously suggested by Cassadaet al. 1 for this system.T 1 plate nucleation is modeled as the dissociation of a perfect b = 1/2〈110〉 matrix dislocation in the vicinity of a dislocation jog. The coordinated dissociation of the dislocation line segments on each side of the sessile jog provides the displacement necessary for the formation of a new hexagonal plate or plate ledge. Strain contrast analysis of the Burgers vector of plate edges and the edges of growth ledges indicates the stacking of partial dislocations is of mixed displacement. Formerly Graduate Student, Department of Materials Science, University of Virginia,

Strain contrast of coherent precipitates in bright-field images under zone axis incidence

Abstract It has been proposed that bright-field imaging of coherent spherical precipitates under zone axis incidence would give improved resolution in the strain contrast compared with the weak-beam dark-field imaging. We observed the strain contrast from coherent Co-precipitates in Cu-Co alloys, and simulated the images on the basis of many-beam dynamical electron diffraction, taking into account the elastic anisotropy. Zone axis incidence such as [001] or [001] considerably reduces the effective extinction distance of the transmitted beam, and suppresses the intensity oscillation with foil thickness, in comparison with a conventional two-beam condition. The contrast in the bright-field images is straightforwardly related to the precipitate geometry. Individual precipitates are well resolved in the image when the number density of precipitates is fairly high. It has been demonstrated that the imaging under zone axis incidence is quite useful to understanding of microstructures in phase transformation in ...

The formation of multi-domain precipitates in a Ni [sbnd]W alloy

The formation of twinned Ni 4W precipitates in a Ni W alloy has been studied. On prolonged aging the dominant morphology found is one where two perpendicular twin related variants of Ni 4W form plate-shaped precipitates with a near (100) Ni habit plane. The orientation relationship between the Ni 4W variants and the Ni matrix leads to a macroscopic shape change with the (100) habit plane interface nearly invariant. In addition each domain has a simple shear associated with it, which can be accomodated either elasticity or by arrays of screw dislocations. Elastic accommodation leads to a strain contrast effect which is similar to that observed at screw dislocations lying normal to the foil surface.

Quantitative stress determination in SiC reinforced Al2O3

Whisker and fiber reinforced ceramic composites offer the potential for increased fracture toughness and fracture strength. However, differences in thermal expansion between whisker and matrix generate residual stresses which may affect the mechanical properties of such composites. The microscopic stress can be determined by analyzing the strain contrast around SiC whiskers quantitatively, similar to the strain analysis performed on ZrO2 inclusions in AI2O3. Previously obtained experimental TEM results on strain contrast around SiC whiskers embedded in Al2O3 were only qualitatively. In the present study the experimental observations are quantitatively evaluated.Contrast simulation requires the knowledge of the matrix displacement field in the vicinity of the whisker. Due to two force-free surfaces adjacent to the inclusion, Eshelby’s method for an inclusion in an infinite matrix can not be used for the evaluation of the elastic field in a TEM foil. Therefore, the Green’s function method was applied to fulfill the elastic boundary conditions for both surfaces neglecting only the elastic interaction between the surfaces.

Twinning and oxygen ordering in Ba2YCu3O7−x crystals

Formation of twins in Or-BYCO crystals was studied by transforming them to T-phase by electron beam irradiation in the microscope. It is known that during the transformation the crystals developed twinning (STW), Ortho-II phase (1×2 superstructure), tweed structure. These structures occur in that order by the irradiation and the structural changes are caused by oxygen deficiency and oxygen atoms ordering. This work is aimed at elucidation of oxygen atom ordering by examining the structural evolution in terms of morphologies of the twins.Atypical evolution of an Or-BYCO crystal is shown in a series of micrographs in Figs. 1 to 5. As-grown twin (FTW) boundaries have a characteristic lens shape extending in the < 110> direction and terminate at a twin boundary in horizontal direction (Fig.1). These boundaries don't accompany strain contrast which is developed in the crystal after a brief irradiation (Fig.2). The strain contrasts, which are differentiated easily from the thickness contrast, appear parallel to the boundaries and change slowly across the boundaries.

A transmission electron microscopy study of the (Sm,Pr)5Co19 phase

Transmission electron microscopy has been used to characterize the rhombohedral 5:19 phase (R3̄m) in a stoichiometric (Sm0.75Pr0.25)5Co19 alloy. In some regions, strain-induced contrast was observed in the 5:19 phase. By contrast analysis, this strain contrast was associated with a displacement vector of the type (1)/(3) [011̄0]. In other regions where the strain-induced contrast was significantly reduced, faults with the crystal structure of hexagonal (Sm,Pr)2Co7(2:7H) phase (P63/mmc) were observed. The formation of the faults is believed to release elastic energy within the 5:19 phase. In the interface regions between the 5:19 phase and the minor component hexagonal 2:7H phase, it was found by lattice imaging that the 5:19 and 2:7H plates are superimposed with a common c direction. The orientation relationship between the 5:19 and 2:7H phases is given by [0002]2:7H∥[0003]5:19R and [112̄0]2:7H∥[112̄0]5:19R.

The martensitic transformation in silicon—I experimental observations

Ribbons of hexagonal material are produced when silicon is indented in the temperature range 400–650°C. This series of three papers, based on two previous preliminary reports [P. Pirouz, R. Chaim and J. Samuels, Proc. 5th Int. Congr. Structure and Properties of Dislocations in Semiconductors, Moscow, U.S.S.R., 1986. Izvestiya Nauk Akademiya S.S.S.R. 51, 753 (1987); P. Pirouz, R. Chaim and U. Dahmen, Mater. Res. Soc. Symp. Proc. 104, 133 (1988)], is a detailed account of an investigation of this phase. In the first paper, the details of the experimental investigation are described. Vickers microindentations have been made in float-zone silicon in the temperature range of 400–500°C. The ribbons of hexagonal silicon are then studied by different techniques of transmission electron microscopy (TEM). Conventional microscopy using strain contrast, selected area diffraction technique, and high resolution electron microscopy (HREM) have been employed to investigate the structure of the hexagonal phase. Using the latter technique, the ribbons are investigated to within a resolution of 0.18 nm and HREM images are compared with simulated ones. HREM is specially useful for studying the faulting within the hexagonal phase and the cubic/hexagonal interface. Analytical microscopy using Electron Energy Loss Spectroscopy (EELS) has been used to show that no oxygen is present in the ribbons within the resolution of this technique. Throughout the paper, it has been attempted to show that the present hexagonal Si is not a thermodynamically stable high pressure phase, but is related to stress relief when twins interact in silicon. In Part II of this series [U. Dahmen, K. H. Westmacott, P. Pirouz and R. Chaim, Acta metall. mater. 38 323 (1990)], a mechanism for the formation of hexagonal silicon by double twinning is presented through a crystallographic analysis. Finally, in Part III [P. Pirouz, U. Dahmen, K. H. Westmacott and R. Chaim, Acta metall. mater. 38, 329 (1990)], a discussion of the results in the context of martensitic transformations and dislocation mechanisms is given and other recent reports of this phase in silicon which has undergone different treatments are discussed.

Collision Cascades in Zr3Al

AbstractZr3Al is an ordered f.c.c. alloy of the L12 type, which under various irradiation conditions can be disordered and then eventually amorphized. In the present transmission-electron-microscopy (TEM) investigation, the nature of the damaged regions formed in individual collision cascades was investigated using low fluence (< 1 × 1011 ions cm−2) bombardments at 290 K with 15 − 120 keV 63Cu ions.Imaging under dynamical conditions with the fundamental 111, 200, 220 and 113 reflections revealed damaged regions (5.1 - 8.1 nm average diameter) that exhibited black-white strain contrast features. These damaged regions have essentially a spherically symmetrical strain field and probably are three-dimensional clusters of point defects. Imaging with 110 superlattice reflections revealed additional damaged regions (2.4 - 3.7 nm average diameter), which appeared as black spots, that represent disordered regions in the ordered lattice. For both the strain-contrast regions and the disordered regions, as the incident ion energy increased, the number of damaged regions produced per incident ion increased and the fraction of the theoretical collision cascade volume occupied by these regions decreased.

Short-Range-Ordering and Fe-Clustering in Au-Fe Alloys

AbstractAs an “archtypal” spin-glass system, the local atomic arrangement in Au-Fe alloys is still a controversial subject. In the present experiment, Au-Fe alloys with Fe content from 10.7 to 33% were studied as to microstructure evolution, through various transmission electron microscopic techniques. Information derived from aging experiments using real and reciprocal spaces appears to suggest the coexistence of short-range-order and Fe-clusters. At early stages of aging, lobe-like and/or rod-shaped strain contrast images, identified as clusters, were revealed after specimens were further cleaned by an ion-miller. The (1 1/2 0) special point diffuse reflections were prominent in the as-quenched condition and/or early stage aging, and the intensifies decrease gradually as aging proceeds. After a certain period, (1 1/2 0) diffuse reflections disappear but strain contrast images still remain similar. This suggests that the strain contrast images are not related to the (1 1/2 0) diffuse reflections. High resolution electron microscopy was also employed. The results are consistent with the arguments derived from aging experiments.

Guinier-Preston zones in AuFe alloys

Transmission electron microscopy was employed to study aging effects in the AuFe system. Specimens containing 10.7 18, 25 and 35 at.% Fe in both as-quenched and aged conditions (from room temperature up to 500 °C for as long as 2 months) were investigated. Rod-shaped and/or lobe-like strain contrast images whose visibility was sensitive to specimen preparation and diffraction conditions were seen after aged specimens were further cleaned using an ion miller, whereas a dynamical flickering “streaming effect” was observed in as-electropolished specimens. Trace analysis as well as other evidence indicates that the images correspond to small precipitates rather than defects generated by ion milling. Dark-field and stereomicroscopy analyses show that the nature of the strain field is inwards. A model of Guinier-Preston (G-P) zone precipitation is proposed to interpret the various types of diffuse streaks present in the diffraction patterns. Theoretical calculations indicate that the habit of the iron-rich G-P zones might lie on planes with normal n 0 = <102〉 or thereabouts. The present results indicate that a G-P zone model appears to be plausible, but are also consistent with the static concentration wave concept. This will be discussed in a subsequent paper.