Weak-beam Method Research Articles

Electron Microscopic Observation of Heterointerface in GaxIn1-xAsyP1-y/InP Grown by Liquid

Cross-sectional observation of the heterointerfaces of a GaxIn1-xAsyP1-yactive layer clad in InP in a laser diode wafer with x=0.3 and y=0.6 was carried out using an electron microscope. By comparing the image contrasts obtained by high resolution observation and weak beam observation, it is rerealed that if the heterointerface is smooth, high resolution observation can show the interface structure, but a weak beam method is still useful to characterize a heterointerface of relatively large unevenness.

Weak-beam observation on Shockley partials in a Ni3(Al, Ti) single crystal

Abstract Using the weak-beam method in transmission electron microscopy, this Letter reports for the first time that the superlattice dislocation in L12-type Ni3,(Al, Ti) alloy can be resolved following its fourfold dissociation, that is, the 1/2[101] superpartial pairs bounding the antiphase boundary (APB) dissociates further into Shockley partials as: where the CSF is the complex stacking fault.

Transmission electron microscopy study of the stacking-fault energy and dislocation structure in CuMn alloys

Abstract Single crystals of copper-based manganese alloys (Cu−1.05, 1.3, 3.3, 5.05 and 11.6 at.%Mn) were grown and deformed in tension at room temperature. The weakbeam method of electron microscopy has been used to resolve the partial dislocations and to determine the stacking-fault energy γ. Contrary to most other Cu alloys, the value of γ does not decrease with increasing solute content. It stays the same for all the investigated CuMn alloys and corresponds to the value of pure Cu (γ = 41 mJm−2). It is difficult to include the results in a γ against e/a plot since the valency value of Mn in Cu cannot be specified. In Cu−11.6 at.%Mn a pronounced planar dislocation structure is observed and it is suggested that this is caused by the occurrence of short-range order. The large values of the critical resolved shear stress of the investigated CuMn alloys cannot arise from chemical locking (the Suzuki effect) since γ does not change with solute concentration.

Transmission electron microscopy study of cross-slip and of Kear-Wilsdorf locks in L12ordered Ni3Fe

Abstract In L12 ordered alloys the cross-slip modes of the superlattice dislocations are of special interest since a sessile configuration (Kear-Wilsdorf (KW) lock) can be formed by cross-slip from {111} glide planes to {010} planes where the screws are assumed to be locked. Single crystals of ordered Ni3Fe were deformed at various temperatures below the critical order-disorder temperature; contrary to most Ll2 ordered alloys Ni3Fe does not show an increase of the flow stress with increasing temperature. At room temperature the density of screws is very low, indicating that they are annihilated by cross-slip. The superlattice screw dislocations show a large dissociation (antiphase-boundary (APB) fault) on (111) cross-slip planes but none on (010) planes. In specimens deformed at 600 K the weak-beam method of TEM was applied to analyse the superlattice dislocations. The four-fold dissociation of the superlattice dislocations with a character near edge orientation is clearly resolved. The screws are either dissociated on a (111) plane, on a (111) plane or on a (010) plane; the last leads to the formation of a KW lock. The observed KW locks show a large dissociation on the (010) planes. Since the density of the KW locks is rather low they have no effect on the mechanical properties. The reason for their low density is discussed, and it seems that the cross-slip behaviour is a complex function of both the anisotropy of the APB energy and the core structure of the dislocations. It is assumed that the cross-slipped configurations that are observed in Ni3Fe are formed mainly by the help of localized internal stresses.

Comparison of split dislocation images obtained by the weak-beam method and lattice-resolution techniques

Abstract The characteristic splitting values for screw and 60° dislocations in foils used in the weak-beam and lattice-resolution methods have been investigated theoretically. Theory and experiment are compared for dislocations in GaAs. Splitting of screw dislocations in foils 10 to 50 mm thick is shown to reduce considerably with decreasing foil thickness. Splitting of the 60° dislocations is found to be practically independent of foil thickness.

Fault energies in ordered and disordered Ni3Fe

Abstract The weak-beam method of electron microscopy has been used to study the dissociation and the fault energies in disordered and ordered Ni3Fe. For the ordered structure this is of particular interest since in this case the plastic behaviour is determined by the dissociation mode of the superlattice dislocations. Single crystals of Ni3Fe were quenched to suppress the formation of the long-range ordered phase and strained at room temperature. From the dissociation of the Shockley partials the stacking fault energy of the disordered f.c.c. phase γ = 95 ± 14 mJ m −2 was deduced. A section of the disordered and deformed crystal was subsequently fully ordered (Ll2 superlattice structure) and additionally deformed. The complex stacking fault energy γcsF = 123 ± 13 mJ m −2 was deduced from dissociated unit dislocations which were sessile since they had been introduced before ordering. The deformation of the ordered crystal proceeds by the formation of superlattice dislocations. These dislocations show a fou...

弱ビーム電顕法による双晶転位バーガースベクトルの決定

Burgers vectors of twin boundary dislocations in annealed Cu-Ni alloys were analysed by the weak beam method using HVEM in such a way that the number of extinction contours terminating at the end of a dislocation on the surface of a thin foil specimen is counted. By this method the size and sign of the Burgers vector was determined.The present method is applicable to general coincidence boundary dislocations, when electrons are diffracted simultaneously from two adjacent crystals.

Direct observations of carbon clusters in a high-carbon martensitic steel

The morphology and diffraction contrast of carbon clusters formed at room temperature in an Fe-7.9 wt.% Cr-1.1 wt.% alloy have been investigated using the weak-beam method of transmission electron microscopy. The contrast of the carbon clusters is strongly dependent on the diffraction condition and associated diffuse electron scattering. The (200) b reflection in the [010] b orientation and the (020) b reflection in the [100] b orientation are most suitable for imaging of the carbon clusters. Layers of carbon clusters are found on planes close to the four {012} b planes, deviating from the (001) b plane by an angle of about 27°. The layers have a uniform spacing of about 3–4 nm and a thickness of 1–2 nm. The present observations confirm previous results obtained for Fe-C alloys.

Weak-beam study of glide dislocations in h.c.p. cobalt

Abstract Abstract The weak-beam method of electron microscopy has been used to analyse the Shockley partial dislocations in a pure h.c.p. metal. Single crystals of cobalt were grown oriented for basal slip and deformed in compression. The dissociation of the basal glide dislocations into partials is resolved; tho arrangement of the magnetic domains does not influence the splitting width of the partials. From a careful analysis of the observed image peak separations, a room-temporature stacking-fault energy γ = 27 ± 4 mJ m−2 is doducod. The basal glide dislocations contain many jogs. Frequently tho sequence of the partials is reversed at a jog, which leads to the conclusion that the dislocation is jogging over an odd number of lattico planes and has a stacking fault of the same intrinsic type I2, on both sides of the jog. It should be pointed out that in the f.c.c. structure this behaviour has not been observed, since tho reversed sequence of the partials would lead to a glide dislocation containing an ext...

Direct observation of non-conservative motion of extended jogs on dissociated dislocations

Abstract A very low stacking-fault, energy f.c.c. alloy (Ag-15 at.% Al) has been irradiated at 423 K with 800 keV electrons in a HVEM. Observations of the behaviour of jog-containing dissociated dislocations in near-edge character during irradiation were made using the weak-beam method of electron microscopy. It is shown that extended jogs move non-conservatively along the dissociated edge dislocations by absorbing point defects, presumably an excess of interstitials, during electron irradiation.

Weak-beam studies of composite dislocations and dislocations gliding on (001) planes in silver

Abstract The weak-beam method of electron microscopy has been used to study the composite dislocations in a plastically deformed f.c.c. metal. In their sessile form (for example, as Lomer-Cottrell (LC) locks) these play a major role in all the work-hardening theories. Single crystals of pure Ag were strained into stage II and composite dislocations formed in a LC-type reaction were frequently observed. Their individual partial dislocations were resolved and all Burgers vectors analysed. The composite dislocations were found to occur in two typical configurations. First, regular networks of primary, secondary and LC dislocations were observed, which have widely extended nodes and were stable. Secondly, in (001) cuts, configurations frequently occurred which consisted of dissociated LC dislocations, undissociated dislocations gliding on (001) planes and ordinary glide dislocations dissociated into Shockley partials on the cross-slip plane. This experimental result is in agreement with the recent observations of glide on (001) planes in f.c.c. metals. To explain the observed dislocation reactions and their different nodes, a model is discussed. It shows that the constricted node of the composite dislocation can move either on the cube plane or on the appropriate cross-slip plane.

Gliding dissociated dislocations in hexagonal CdS

Abstract Basal dislocations in CdS have been studied using the weak-beam method of electron microscopy. Networks of dissociated nodes and long lengths of dissociated dislocations were observed, from which the stacking fault energy was determined to be γ = 85 ± 21 mJ m−2. Dissociated dislocations were observed gliding while dissociated, both partials being visible during the motion. This motion was recorded on video tape, and shows conclusively that the dislocations glide while dissociated.

Direct TEM determination of intrinsic and extrinsic stacking fault energies of silicon

Abstract The intrinsic and extrinsic stacking fault energies of silicon have been determined from images of double ribbons obtained using the weak–beam method of electron microscopy. The ribbons occurred in distorted regions of small–angle twist boundaries on {111} planes prepared by welding. The results are compared with values obtained from isolated dislocations in the screw and edge orientation in the same sample, which were found to be consistently lower than the values obtained from double ribbons. It is found that, contrary to other recent work, the ratio γMin γMex is actually only ∼ 14% greater than unity.

The geometry and formation of faulted dipoles in Cu-Al alloys

Faulted dipoles which occur in strained single crystals of Cu-5, 10 and 15 at.% Al are studied using the weak-beam method of electron microscopy. The geometry of all partial dislocations is determined in full and thus the height Y and the type of the faulted dipoles are evaluated. All the analysed ones are of Z type and Y ≈ 2 nm. Two mechanisms are important for the transformation of ordinary dipoles into faulted ones: 1. (i) cross-slip at the surface and 2. (ii) forest cutting by dislocations on the cross-slip plane. Detailed models are given explaining both of them. The formation of a dipole with a closed end and the splitting of the dislocation which lies on the cross-slip plane into a Frank and a Shockley partial are the important steps of this transformation. The observed maximum values of Y are much smaller than the ones which are theoretically possible. The reason for this is discussed by considering the details of the transformation mechanism and the influence of the friction stress.

Temperature dependence of the stacking-fault energy in pure silver

Abstract The weak-beam method of electron microscopy has been successfully applied to measurement of the variation of separations of Shockley partials of single dissociated dislocations in pure silver during in situ thermal cycling experiments in a HVEM. The temperature dependence of the stacking-fault energy γ has been determined unambiguously in the temperature range from room temperature to 500°C. It is shown that γ decreases with increasing temperature in a reversible manner; γ500°C is smaller by about 30 % than γR.T..

The study of dipoles in Cu—Al alloys using weak-beam electron microscopy

Abstract In f.o.c. metals and alloys, dipoles are the predominant dislocation structure during the early stages of plastic deformation. Single crystals of Cu-Al alloys, having low stacking fault energies, have been deformed in tension to the end of stage I of the work-hardening curve. The weak-beam method of electron microscopy has then been used to study the equilibrium configurations of dissociated dipoles having different character and glide-plane separation. By analysing the micrographs, the Burgers vectors of all four partial dislocations can be indexed, the various equilibrium positions identified, and the arrangement of the partial dislocations determined in full. The experimental results are compared with the calculated equilibrium configurations derived by Morton and Forwood (1974, private communication) using linear anisotropic elasticity theory. Various types of equilibrium positions predicted by the theory are observed experimentally and quantitative agreement is good. Deviations from the stre...

The formation and properties of faulted dipoles

Abstract The weak-beam method of electron microscopy has been used to study in detail faulted dipoles in several pure f.c.c. metals and alloys. Direct experimental evidence is presented for two mechanisms by which the defects can be formed: by transforming ordinary dipoles or pulling out faulted ones. Interactions between faulted dipoles and glide dislocations are discussed in detail with experimental evidence showing how such dipoles can be rotated or cut up into small sections. Constrictions on faulted dipoles have also been studied and their origin is discussed.

The formation and glide of jogs in low stacking-fault energy face-centred cubic materials

Abstract The formation and glide of jogs on dissociated dislocations in copper alloys have been observed directly for the first time. Using the weak-beam method of electron microscopy, constrictions caused by jogs in the dissociated dislocations can be identified, and their motion during in situ straining and annealing experiments can be followed. It is shown that it is also possible to study the intermediate stage of the formation of both jogs and kinks in these materials.

The stacking-fault energy of nickel

Abstract The weak-beam method of electron microscopy has been used to study dis sociated edge dislocations and faulted dipoles in nickel. Micrographs have been compared with many-beam computed image profiles to deduce values for the dimensions of the defects, and these results are then used in conjunction with anisotropic elasticity theory to obtain the stacking-fault energy, γ, or an upper limit, γmax. The interpretation of these results is discussed and it is concluded that the true value of γ is most likely to lie in the range γ ∼120–130 mJ m−2.

Constrictions in the stacking faults of dislocations in germanium

Abstract Under suitable imaging conditions using the weak-beam method of electron microscopy, constrictions in the stacking faults of dislocations in Ge become clearly visible. After deformation at relatively low temperatures they occur at an average separation of 950 Å. In short segments they reduce the width of the stacking fault.