Single Stacking Faults Research Articles

Developments in computer simulation of X-ray diffraction contrast images of stacking faults

Computer-simulated X-ray section topograph images of a specimen containing a single large-area stacking fault are compared with the experimental image to illustrate useful improvements in simulation realism with regard to both image geometry and stacking-fault fringe contrast. The computer-program developments include exact treatment of the X-ray polarization factor so that anomalous transmission and the periodic fadings of fringe visibility occurring when unpolarized or partially polarized sources are used can be correctly computed. A procedure is described for photographing a cathode-ray-tube display to obtain records with acceptable resolution, contrast and freedom from distortion.

TMA-offretite. Relationship between structural and catalytic properties

Previous studies reported that synthetic TMA-offretite is an open large-pore (12-membered oxygen ring opening) zeolite. Closer reexamination of the odd l lines using high intensity X-ray diffraction and additional catalytic diagnostic studies suggest that some of the open 12-ring channels in the synthetic sample may be blocked randomly by single stacking faults, such that the only other openings are 8-membered oxygen ring openings perpendicular to the large channels. These structural characteristics could explain the observed shape selective catalytic dewaxing activity and the preponderance of straight-chain cracked products.

The unit cell of talc

SummarySix specimens of talc studied by the electron microscope lattice-imaging and diffraction techniques were found to have the triclinic one-layer (1 Tr) structure. Since no evidence of a monoclinic two-layered structure was found in any specimen it is concluded that the 1 Tr structure is the common one for talc, and it may, moreover, be the only one. Extremely fine polysynthetic twinning and single intrinsic stacking faults were found in talc.

Diffraction from single and overlapping stacking faults in F.C.C. crystals

Single intrinsic stacking faults in f.c.c. crystals are observed to give rise to diffraction spots in h.c.p. positions and anomalous diffraction spots in twin positions. This, together with similar observations in diffraction patterns from overlapping faults, are discussed in relation to the problem of determining the extent to which overlapping faults generated by plastic deformation constitute nuclei of h.c.p. or twinned material. The results suggest that nuclei of twin or h.c.p. material develop by the filling in of a disordered array of faults rather than by the operation of polar mechanisms.

Structural Analysis of X-ray Diffraction Peaks of Thin Film Gallium Arsenide

AbstractA least-squares polynomial approximation of the Warren-Averbach Fourier coefficients line broadening analysis has shown flash-evaporated GaAs films to be characterized by De(lll) ≤ 400 Å and >εL2<½ ⋍ 0.002. Though twinning is the dominant faulting mechanism, a considerable amount of single and double deformation stacking faults are also present. Growth under a partial pressure of arsenic and tin has enhanced crystallite size by a factor of four.

Stacking-fault anomalies and the measurement of stacking-fault free energy in f.c.c. thin films

The stacking-fault energy of 304 stainless steel and Inconel 600 alloy was determined by electron microscopy techniques to be 21 and 28 ergs/cm 2, respectively. Electron microscopy studies also revealed anomalous, wide stacking faults in copper and nickel having a predominantly extrinsic nature. Bright- and dark- field electron microscopy techniques allowed the faults in nickel to be identified as essentially N-layer twins; leading to the conclusion that twin formation is energetically more favorable in high stacking-fault energy thin foils than single intrinsic stacking faults. The formation of N-layer twins in thin foils of high stacking-fault energy is further supported by direct observation of vapor-deposited foils of Ni, Cu, Au and Ag. In addition, it is demonstrated that for plane-strain, explosive shock-deformation where temperature effects are presumed to be negligible, stacking-fault free energy dominates the cross-slip of dislocations leading to coplanar dislocation arrays for λ SF < 30 ergs/cm 2, and dislocation cells for λ > 30 ergs/cm 2.

Electron microscopic investigation of lamellar mixtures of two close‐packed phases

AbstractMartensitic and massive phases containing fine lamellae of two different close‐packed structures have been studied by transmission electron microscopy. The stacking sequences involved in the lamellar mixtures are ABCBCACAB and ABC, or AB and ABC, or ABC and CBA. The fine lamellae produce fringe patterns which are similar to fringe patterns due to single stacking faults. However, by tilting the foil images can be obtained that are incompatible with single stacking faults.

Stacking faults in steam-oxidized silicon

Silicon wafers oriented along 〈111〉 and in which surface damage was removed were annealed in vacuum at 800, 1000 and 1200°C. After steam oxidation at 1150°C, only the wafers annealed at 1000 and 1200°C showed significant generation of stacking faults. The faults were observed through both chemical etching and transmission electron microscopy. The faults often showed accumulation of impurities believed to be silicon-oxygen complexes. Single-stacking faults were identified as extrinsic, and mechanism of generation of these faults is discussed.

The electron microscopy of deformation twinning

The incidence of deformation twinning in single crystals of f.c.c. Cu plus Al alloys has been studied by transmission electron microscopy. All the alloys have been found to twin under suitable conditions although in alloys with low stacking fault energy twinning is not associated with a plastic instability. In these alloys the growth of the twins can be followed from single stacking faults so that the dislocation reaction leading to the nucleation of twins may be studied experimentally.

Electron Microscopic Images of Single and Intersecting Stacking Faults in Thick Foils II. Intersecting Faults

AbstractThe dynamic theory of electron transmission microscopic images of intersecting stacking faults is developed and discussed. Intersecting faults of the same type in f.c.c. alloys are considered, the fault planes having opposite tilt. The expected images are compared with observations on thick Cu‐15% Ga foils.The aspects of the bright‐field images are different under different diffraction conditions: (i) “streaked” fringes parallel to the projection of the intersection line for large values of the excitation error s; (ii) for small values of s the images consist of two distinct parts: the central one and the outer one. The central part consists of a few nearly unmodulated straight fringes parallel to the intersection. The outer part is a more or less deformed superposition of two fringe patterns. A pure superposition of the fringe patterns due to each fault separately is found for a particular value of s and for the outer part of the image.The qualitative features of the image for small s in thick foils are due to anomalous absorption effects.

Fault Structures in Wurtzite

AbstractA more precise interpretation of the defect structure of wurtzite crystals as observed in the electron microscope is presented. It was found previously that the crystals contain faults in the basal planes (b‐faults) and in prism planes (p‐faults). Direct evidence is presented indicating that the transition wurtzite sphalerite is a martensitic transformation due to the movement of partial dislocations. The transformation leads to a faulted structure.Evidence is further presented for the occurrence of a domain structure. Possible models for this domain structure are proposed. According to the first, the domain boundaries result from the contact between two crystal regions for which the polar c‐axes have opposite sense. The origin of the contrast at such boundaries is discussed, and it is concluded that this type of boundary has presumably not yet been observed in wurtzite. According to the second model the boundaries result from the contact between two crystal regions which are displaced with respect to each other over a displacement vector which is identical to the Burgers vectors of a partial in the basal plane, or to the sum of such a vector with a vector (000 c/2)Evidence is presented to show that the domain structure results from the introduction during growth of single stacking faults. A generation mechanism for basal and prismatic faults is discussed.The contrast effects at single and double faults in the hexagonal closed packed structure are discussed in detail and a method is proposed to distinguish these two types of faults.

Electron Microscopic Images of Single and Intersecting Stacking Faults in Thick Foils. Part I: Single Faults

AbstractThe bright‐ and dark‐field images of single and intersecting stacking faults have been studied; including the effects of anomalous absorption. In this, the first part of the paper, single faults are discussed. It is shown that the expressions for the amplitudes of the transmitted and scattered waves can be simplified considerably by introduction of a simple approximation. The properties suggested by the numerical calculations of HASHIMOTO et al., can then be expressed and discussed in a more precise analytical way, and additional properties found. In particular, it is possible to show how the central part of the fringe pattern depends on the foil thickness. The practical method developed by ART et al. for the determination of the nature of a fault in f.c.c. crystals is re‐examined, and it is shown that sufficient data can be obtained from the dark‐field image alone. The above theory is in general agreement with experiment.

Deformation twinning in silver-gold alloys

Silver-gold alloy single crystals were studied with respect to their tendency to undergo deformation twinning in tensile tests at low temperatures. X-ray diffraction, metallography and stress-strain curves all indicated that deformation twinning occurs in crystals of suitable orientation throughout the entire range of alloy composition. Twinning takes place even at 0°C, with slow rates of testing, in favorably oriented crystals of pure silver; the upper temperature range for twinning decreases with increasing gold content, but is still above 77.3°K at 100 per cent gold. Three different propagation processes for the twinning are noted. The stress measurements indicate that there is a critical resolved shear stress for twinning which is independent of orientation and only weakly dependent on temperature; this stress in kg/mm 2, is found to rise from 6.48 ± 0.66 for 0 per cent Au to 9.84 ± 0.25 for 50 per cent Au and falls to 9.23 ± 0.39 for 90 per cent Au and 9.48 ± 0.40 for 100 per cent Au. The apparent restriction of twinning to certain orientations should be ascribed to the shear stress that is reached during the test with these orientations, not to the orientations themselves. Values of the stacking-fault energies can be computed from the twinning shear stresses by assuming a mechanism of twinning proposed by H. Suzuki, which involves a pair of twinning dislocations moving away from a Cottrell sessile dislocation. The double stacking fault trailing behind this pair has a computed energy of 21 erg/cm 2 for silver and 30 erg/cm 2 for gold, and it is suggested that these same values are approximately correct for a single deformation stacking fault in these metals.

Experimental and theoretical studies of the mica polymorphs

SummaryAn experimental and theoretical study has been made in order to determine the number and the structure of the possible polymorphs and to determine the structural relations between them. The simplest structures are 1M, 2M1, 2M2, 3T, 20, and 6H polymorphs, and more complicated types can be developed. Some of the previously described polymorphs were not contained in the theoretical list and were re-examined. The 6M structure was found to be a 2M2polymorph, the 6-layer triclinic type was found to be a 2M1polymorph, and the 3M structure was shown to be a 3T type. The 24-layer triclinic structure could be described on a simpler 8-layer cell. This type together with a new 12-layer monoclinic structure, as well as other structures of higher periodicity, presumably consists of complex stacking and results from spiral-growth mechanism. Two extreme types of layer-disordered crystals may be built and a disorder of individual ions may also occur. Single stacking faults result in twinned crystals. A new twin relation (180° rotation about the [100] axis) has been recognized. Twenty specimens from extreme geological environments have been examined in order to evaluate the control of environment on the stacking. The type of stacking could not be attributed solely to the influence of pressure and temperature. Composition appears to play a dominant role in the type of stacking, and semi-quantitative structural arguments appear to support this contention. The influence of growth mechanism is discussed. A scheme for the identification of the mica polymorphs by X-ray powder and single-crystal methods is given.