Incoherent Twin Research Articles

The β→αTransformation in Poly crystalline SiC: III, The Thickening of α Plates

The final stages of the β→α polymorphic phase transformation in conventionally sintered and hot‐pressed SiC were studied by optical and transmission electron microscopy. The interface between the α and β regions of partially transformed grains invariably contains thin α lamellae adjacent and parallel to coherent β twins (or β stacking faults). These planar defects serve as nucleation sites for the transformation, with growth of α occurring by the motion of partial dislocations nucleated at the intersection of coherent and incoherent twin boundaries. Various structural phenomena in SiC, including faulting, mi‐crosyntaxy, and long‐period polytypism, are discussed in terms of this transformation mechanism.

Damage structure in nimonic PE16 alloy ion bombarded to high doses and gas levels

The Nimonic PE16 alloy in solution-treated-and-aged condition was bombarded simultaneously with nickel ions and α and deuteron beams at 625°C to doses of 80 to 313 dpa at He/dpa = 10 and D/dpa = 25. Microstructural changes consisted of the introduction of dislocations and of cavities, and the redistribution of γ precipitates to these defects. Cavitational swelling remained below 1%. Cavities were represented by several distinct size classes, the smaller ones believed to be gas bubbles, and some larger ones associated with preferred growth of precipitates. Formation of bubbles at grain boundaries, and large cavities at incoherent twins intensified the possibility of mechanical separation of interfaces under high-gas irradiation conditions.

Construction of an on-line system for FFT processing and analysis of atomic resolution microscopic images and its applications

The construction of an on-line image processing and analysis system and its applications to high resolution electron microscopy are described. The system is composed of a TV system, frame memory and microprocessor. Fast Fourier transform of atomic resolution electron microscopic images are demonstrated. Averaging the images obtained by superposing some numbers of the frames of TV pictures gives an image without random noise. After masking the area corresponding to the random noise in Fourier space, inverse Fourier transforms are carried out to get processed images without random noise. Fast Fourier transforms of the images of amorphous carbon films are analyzed to obtain the defocus values of the objective lens. It is demonstrated that the small selected area diffraction patterns can be obtained from the images of thin gold crystal containing an incoherent twin boundary and that Fourier transforms of the image of carbon film can be used for correcting the astigmatism of the objective lens.

Precipitation of M 23C 6 type carbide on twin boundaries in austenitic stainless steels

An electron microscopy investigation was made of M 23C 6 carbide precipitation in two kinds of austenitic stainless steels, A (AISI 316L type alloy) and B (DIN 4981 type alloy). The specimens were water quenched from high annealing temperatures and aged at 800°C. In particular, the precipitation process on twin boundaries was carefully examined. The M 23C 6 carbides were observed to form first on grain boundaries and on incoherent twin boundaries as small precipitates. The formation of lamellar M 23C 6 precipitates then proceeded on coherent twin boundaries and in the region adjacent to incoherent twin boundaries. The morphology of precipitates was found to depend strongly on the stress field. The growth of lamellar precipitates appears to be a consequence of planar distortion on {111} planes. The geometrical form of M 23C 6 precipitates is characterized by {111}- and {110}-type interfaces. In general, the {111} interfaces were predominant. One exceptional mode of precipitation was observed, however, in which the {110}-type lamellar precipitate formed around an undissolved residual NbC particle in Alloy B.

HVEM observations of [formula omitted] he-ion bombardments

in-situ helium ion irradiation experiments on type 316 stainless steel, a candidate first wall material for the fusion reactor program, have been conducted by means of a 200 kV light ion linear accelerator mechanical1y linked to the objective lens of a 500 kV high voltage electron microscope. An ion energy of 80 keV, Ion fluxes up to about 2 × 10 15 cm −2sec −1, doses up to about 5 × 10 18cm −2 and sample temperatures ranging from about 500°C to 650°C were employed. During irradiation of stainless steel, after a minimum dose was achieved, tiny dislocation loops were formed whose density increased with fluence. As dose was increased, these defects were apparently masked by the appearance and growth of gas containing voids hereafter referred to as “bubbles.” With additional irradiation, bubble formation was followed by the development of surface blistering and/or exfoliation. Bubble formation was observed to depend strongly on the microstructure of the sample. In particular, bubbles preferentially grew on grain boundaries, on incoherent twin boundaries and in regions of high plastic deformation. Irradiation of samples first produced bubbles preferentially on slip bands followed by bubble formation in other areas. Irradiations at high temperatures produced a larger ratio of bubble size on grain boundaries to that in the grain interior than the ratio found for irradiations conducted at low temperatures. By means of a video system, the rates of growth of microstructural features, such as bubbles, blisters, and exfoliated areas, have been recorded. For comparison purposes, in-situ helium irradiation experiments on molybdenum were carried out and essentially reproduced the transmission electron microscope in-situ results reported by Mazey and co-workers for 18–60 keV helium ion bombardment of molybdenum.

The Imaging Of Surface Step Structures By Dark Field Electron Microscopy

SUMMARYThe dark field imaging of monatomic surface steps is studied by weak beam electron microscopy using reflections in higher order Laue zones. Dynamical theory calculations of the step intensities and of the diffuse background intensity in various diffracting conditions are in moderately good agreement with experimental observation on (111) thin films of gold. From grooving effects visible at incoherent twin boundaries, the boundary energy is estimated to be about one‐third of the surface energy. The extension of the method to other crystal surfaces or other materials is discussed.

Effect of extrinsic grain-boundary dislocations on M23C6 precipitate nucleation in an austenitic stainless steel

From a comparison of the isothermal precipitation curve for M23C6 grain-boundary precipitates in an austenitic stainless steel and of the spreading times of extrinsic grain-boundary dislocations (EGBDs), it has been shown that M23C6 precipitates cannot directly nucleate on EGBD lines lying on random high-angle grain boundaries. This process can occur only on coherent and incoherent twin and other special boundaries.

The Role of Interfacial Dislocations in the Nucleation of Intergranular Creep Cavities

The literature concerning the nucleation of intergranular creep cavities is briefly reviewed together with evidence for a grain boundary dislocation mechanism for grain boundary sliding (gbs). Experimental observations have been made of cavities in a crept alpha brass by transmission electron microscopy at 100 kV. The cavities were often found to be associated with twin boundaries. Pile-ups of interfacial dislocations were observed in coherent and incoherent twin boundaries at their intersections with grain boundaries and also in grain boundaries at their intersections with twin boundaries. The occurrence of grain boundary dislocation pile-ups indicates that the passage of dislocations beyond the head of the pile-up requires an increased stress. If this is greater than the local cohesive strength, then local rupture and cavity nucleation can occur. Résumé On discute d'abord de la documentation disponible sur la nucléation de cavités aux joints de grains, en insistant sur tout ce qui supporterait l'existence d'un mécanisme de dislocation des joints de grains, lors du glissement des joints de grains (gbs). On a observé à l'aide d'un microscope à transmission électronique de 100 kV les cavités formées par la crique d'un bronze α. On a observé une accumulation de dislocations interfaciales aux joints cohérents ou incohérents de macles, à leur intersection avec un joint de grains, ou aux joints de grains, à leur intersection avec un joint de macles. La présence d'accumulationsde dislocations aux joints de grains signifie que celles-ci ne se détachent du sommet des accumulations qu'à une contrainte plus élevée. Si celle-ci est supérieure a l'adhérence locale, il peut y avoir rupture locale et nucléation d'une cavité.

In‐situ HVEM Investigations under the Influence of Ion Beams

AbstractA 200 kV ion accelerator was coupled to a 500 kV TEM for in‐situ studies of the 80 keV He+ irradiation of 316 stainless steel. Dynamic and static observations of bubble growth, exfoliation and blistering show that slip bands, incoherent twin boundaries and grain boundaries are preferred sites for helium bubble formation. Bubble growth is enhanced by plastic deformation during irradiation. Exfoliation and blistering occur in samples thinner than the projected range of 80 keV He+ ions. Crack propagation during tensile tests proceeds through the bubbles enlarging them in diameter by a factor of two. Bubble enlargement also accompanies the formation of a blister cap. Brittle intergranular failure, even in samples with a large bubble population on the grain boundaries, occurred only when tensile tests were performed at elevated temperatures.

The contrast from incoherent twin interfaces observed using the weak-beam technique

The contrast from incoherent twin interfaces observed using the weak-beam technique

Structure-dependent intergranular segregation of phosphorus in austenite in a Ni-Cr steel

The segregation of P to grain boundaries during austenitization of a low-alloy NiCr steel doped with P has been revealed by an etching solution which preferentially attacks grain boundaries containing P, coupled with observations by Auger electron spectroscopy. The correlation of grain boundary energy with segregation of P was examined by observing dihedral angles between boundaries which constitute triple points on a polished surface. Grain boundary compositions and the effect of austenitizing temperature on the degree of P segregation in the austenite phase were analyzed by a scanning Auger microprobe. It was found that the degree of P segregation can vary greatly from one boundary to another and along individual stepped or curved boundaries. Coherent twin boundaries apparently contained no P, but incoherent twin boundaries did. It was concluded that the degree of segregation is mainly dependent on the degree of grain boundary coherency, or on the free volume associated with the boundary. High energy boundaries generally absorbed more P than low energy boundaries; however, it is not considered that grain boundary energy, per se, is a separate controlling factor.

On the energy of slightly incoherent twins

This review describes the theoretical basis, experimental considerations, data analysis and applications of studies of grain misorientations in cubic materials. Recently, there have been considerable advances in experimental techniques, allowing huge sample populations of misorientations to be collected. To handle such amounts of data, and to balance sacrificing detail for convenience, poses challenges.

A general mechanism of martensitic nucleation: Part II. FCC → BCC and other martensitic transformations

The general mechanism of martensitic nucleation by faulting from groups of existing dislocations, as proposed in Part I, is applied to the fcc → bcc, bcc → fcc, bcc → hcp, and related transformations, including mechanical twinning. Where thermodynamic data are available, the conditions at the observedMs temperatures are consistent with nucleation from a defect composed of four or five properly spaced lattice dislocations. Examples of nucleation by faulting on the planes predicted are found in published electron microscopy. The faults are observed at the types of sites where the required dislocation groups are expected. These include grain boundaries, incoherent twin boundaries, and inclusion particle interfaces. Having defined the function of a nucleation site, mechanisms of strain induced nucleation and autocatalysis are then considered.

Rigid body translations at grain boundaries

Abstract Arguments are presented suggesting that the energy of a grain boundary may be minimized by the adoption of constant rigid body translations over facets of different inclination, continuously curved segments and/or adjacent structural units of different types within a given facet or curved segment. Consequently, a general high angle grain boundary should contain a network of dislocations bordering areas of constant rigid body translation. Observations by transmission electron microscopy of such translations in faceted and curved incoherent twin boundaries support the proposed ideas.

Interfacial energy of coherent and incoherent deformation twin boundaries in aluminum oxide

The interfacial energy of coherent and incoherent deformation twin boundaries in alpha aluminum oxide was measured relative to several crystallographic surfaces. Using thermal grooving and interferometer measurement techniques, the relative interfacial energy obtained for coherent twins was 0.31 (standard deviation = 0.03) with no significant measureable variation relative to different surface planes. The energy of incoherent twin boundaries relative to a {101̄0} plane was 0.58 (standard deviation = 0.03). Observations of crack nucleation from twin-twin intersections, migration of twin boundaries and the interaction of twin boundaries with sub-grain boundaries are reported.

Lattice resolution observations on the structure of twin boundaries, faults and dislocations in epitaxial silicon

Defects in an (011) silicon film grown epitaxially by an iodine transport process on a silicon substrate have been studied by lattice resolution electron microscopy. Use of the 2-beam tilted illumination technique, modified to admit a twin reflection to the image, enabled {111} fringes to be followed across microtwins. Defects studied included microtwins down to 3-plane thickness, intrinsic and extrinsic faults, loop defects and dislocations. Observations were made on both coherent and incoherent twin interfaces. A stepped twin interface was observed made up of coherent {111} segments and incoherent segments which tended to be {112}. The latter displayed highly regular periodic structures with a 3-plane (9.4 Å) repeat distance. One such structure appeared to consist of Shockley partial dislocations on consecutive planes, another of an array of Frank partials of alternating sign, a third may have been different in type. Incoherent boundary segments not parallel to {112} showed much less regularity. Loops and other geometrically shaped defects composed of twins and stacking faults were observed. The displacement field around an end-on dislocation, probably of the 60° type, was measured. The dislocation width was about 7.5 Å.

Motion of the boundaries of incoherent twins in cyclic deformation

Motion of the boundaries of incoherent twins in cyclic deformation

The Structure of an Incoherent Annealing Twin Boundary in FCC Alloys

Because of the simple crystallographic relationship between matrix and twin in FCC materials, an incoherent twin boundary can be thought of in terms of an array of 1/6 <112> Shockley partials or 1/3 <111> Frank partials such as that shown schematically in Figure 1. Here the interface is made up of alternately spaced Frank partial s of opposite sign; however, these could be replaced with appropriate Shockley partials such that no far-reaching strain field is associated with the interface.

Changes in twin structure during growth of continuous epitaxial (001) copper films on rocksalt

Abstract Abstract Microstructural changes in thick, continuous (001) films of copper have been studied by electron microscopy. Lamellar twins formed during the discontinuous growth stage were completely eliminated when the total deposit thickness had reached about 4000 A. It was found that motion of the twins was inhibited during the initial stages of growth of the continuous deposit by surface irregularities in the form of grooves. The results are interpreted in terms of the diffusion-controlled mobility of incoherent twin boundaries and it is shown that this interpretation is in good agreement with experiment.

The formation of ferrite and sigma-phase in some austenitic stainless steels

An austenitic steel of composition wt.% 24Ni, 25Cr, 0.5Ti has been solution treated at 1150°C., and aged at 750°C for periods of up to 2000 hr. The precipitation processes have been followed by transmission electron microscopy. Intragranular precipitation. Ferrite precipitates (with a Kurdjumov-Sachs orientation relationship with the matrix) nucleate on residual M 23C 6 and chromic oxide particles, and grow along 〈110〉 γ to form a Widmannstatten pattern of precipitates with two parallel {111 } γ interfaces and two curved interfaces along their length. Some ferrite particles undergo an in situ transformation to sigma phase, which then grow by the dissolution of neighbouring ferrite. Precipitation in grain, and incoherent twin boundaries. The growth of ferrite in these regions is accompanied by the dissolution of the M 23C 6 which formed there initially. The formation of sigma follows, and during its growth the ferrite is consumed. The role of ferrite in nucleating sigma. By studying a wider range of alloy compositions, and by comparing the data of other workers, it is concluded that sigma is precipitated directly from austenites of low (Ni + Cr) content, whereas in alloys of high (Ni + Cr) content sigma forms via the b.c.c. metastable ferrite phase.