X-ray topographic studies of cadmium selenide

The perfection of single crystal platelets of cadmium selenide, as grown from the vapour phase, has been investigated by X-ray projection topography. Individual dislocations, stacking faults, fringe patterns, curvature of the lattice planes, and the effects of precipitates, have been detected. Dislocations have been identified having Burgers vectors 1/31 1¯20 and 0 0 01 with screw, edge, mixed and 60° configurations.     

The crystallization of the amorphous alloy Fe40Ni40P14B6

The isothermal transformation behaviour of the metallic glass Fe₄₀Ni₄₀P₁₄B₆ between 320 and 400° C is described. Crystallization occurs by a eutectic mechanism to form Fe-Ni austenite and a body-centred tetragonal phase which is isomorphous with Fe₃P and Ni₃P. The eutectic crystals have a barrel shape such that the c-axis of the tetragonal phase is parallel to the barrel axis. The orientation relationship between the two phases is 1 1 0T 1 1 0 and 0 0 1T 1 1 2. The austenite phase contains (1 1 1) twins.     

Electron diffraction study of superlattices and orientation variants in Ca3SrAl6SO16

Samples having the nominal composition of Ca₃SrAl₆SO₁₆ were sintered at 1380 °C and analysed by electron diffraction. The frequent appearances of forbidden and satellite reflections in this compound imply the presence of a number of basal and nonbasal superlattices so that the microstructure of this cement clinker was characterized by various superstructures including one-, two- and even three-dimensional superstructures along the 0 0 1, 1 1 0, 1 1 2, 1 1 4 or 2 2 1, 0 1 3 and 1 2 3 directions with repeat periods of two or three times of that of basic one, respectively, and intergrowth of these. Various domain structures with 90°, 120° and 48.2°, etc., orientation relationships were also detected in these superstructures and the total number of these orientation variants related to the symmetry elements lost in the process of phase transformation, can be predicted according to the conclusions of Van Tendeloo and Amerlinckx, or they are equal to the number of those unique planes in the matrix.     

Multi-length scale modeling of CVD of diamond Part II A combined atomic-scale/grain-scale analysis

Chemical vapor deposition of polycrystalline diamond films is studied by combining an atomic-scale kinetic Monte Carlo model with two one three-dimensional and one two-dimensional grain-scale models. The atomic-scale model is used to determine the growth rates of 111- and 100-oriented surface facets, the surface morphology of the facets and the extent of incorporation of the crystal defects. Using the atomic-scale modeling predicted growth rates for the 111- and 100-oriented facets, grain-scale modelling is carried out to determine the evolution of grain structure, surface morphology and crystallographic texture in the polycrystalline diamond films. It is found that depending on the relative growth rates of the 111- and 100-oriented facets, which can be controlled by selecting the CVD processing conditions, one can obtain either 110-textured films with a relatively smooth faceted surface or 100-textured films with a highly pronounced deep facets. In both cases, however, the film surface is composed entirely of the 111 facets. This findings are found to be fully consistent with the available experimental results.     

Photosensitive Centers in CdTe〈Ge〉, CdTe〈Sn〉, and CdTe〈Pb〉

The spectral and temperature dependences of photoconductivity in CdTePb crystals under band-gap and combined excitation were studied at photon energies in the range 0.53–1.7 eV and temperatures from 80 to 300 K. The high photosensitivity of the crystals and the observed IR and temperature quenching of photoconductivity indicate that, just as in CdTeGe and CdTeSn, the recombination processes in CdTePb are dominated by deep centers with drastically different capture cross sections for electrons and holes. Some parameters of the centers are determined. The results suggest that the likely defect species responsible for the high photosensitivity of CdTePb is an acceptor complex consisting of a Cd vacancy and a metal (Ge, Sn, Pb) ion on the Cd site: (V ^2– _CdM⁺)^–/0.     

Magnetic Properties of High-Resistivity CdTe〈In〉 and CdTe〈Cl〉 Crystals

The magnetic susceptibility of high-resistivity CdTeIn and CdTeCl crystals was measured between 4.2 and 300 K. The susceptibility was found to vary anomalously with temperature. Below 50 K, all the samples were paramagnetic. The observed anomalies are interpreted in terms of donor–acceptor pairs formed by native defects and dopant or uncontrolled impurity atoms. The effect of doping on the 300-K is related to the Van Vleck paramagnetic contribution resulting from the local electric fields of X_i–V _Cdand In_i–V _Cddefect complexes. In CdTeCl, this contribution is insignificant.     

The plastic behaviour of 〈100〉 and 〈111〉 textured polycrystalline metals during simultaneous torsion and extension

On the basis of computer-calculated yield functions, the work hardening of 100 textured Cu-0.64 at % Co-0.48 at % Si and 111 textured polycrystalline copper wires were studied by simultaneous torsion and extension and by pure extension. Representing the work hardening by resolved shear stress-resolved shear strain curves, the rate of hardening is significantly lower for torsion than for pure extension. This behaviour is explained by the operation of different slip systems activated in the two modes of deformation. In the 100 textured Cu-Co-Si wires, heterogeneous plastic deformation was observed beyond about 80% shear strain.     

Etching of MgO crystals in acids: surface micromorphology

Several etchants based on simple acids are proposed for revealing dislocations on {100}, {110} and {111} faces of MgO single crystals. Some acids under different etching conditions are shown to produce etch pyramids at decorated dislocations, while etch hillocks, spherulites and dendrites are formed under different conditions. With increasing concentration of H₂SO₄, 110 pits, circular pits, 100 pits, 110 pyramids as well as 100 pits, dendrites and spherulites, and hillocks are formed in that order. Etching characteristics of other acids also showed a more or less identical trend. The observations are interpreted with the assumption that with increasing concentration of acid, an effective undersaturation and later supersaturation is developed very close to the dissolving crystal face. The chemistry of the dissolution process is also discussed.     

Charge State of Indium and Point Defects in Indium-Doped Lead Telluride Crystals

Experimental data on the lattice parameter, thermoelectric power, and microhardness of PbTeIn crystals and the conversion fromp- to n-type with increasing indium content can be interpreted under the assumption that the indium in PbTeIn has variable valence: 2In²⁺ In⁺ + In³⁺. A crystal-quasi-chemical model is proposed for defect formation in PbTeIn: the incorporation of In⁺ into octahedral interstices and In³⁺ into tetrahedral interstices of the close packing of Te atoms, accompanied by In₂Te₃ precipitation.     

Size-strength effects in sapphire and silicon nitride whiskers at 20° C

Tensile tests at 20° C have been carried out on seventy-three sapphire whiskers and on seventeen silicon nitride whiskers. The sapphire whiskers were of 0001, 1¯120, 10¯10, and 10¯11 orientations, while the silicon nitride whiskers were 0001, 11¯20, and 10¯13. Tensile strengths were in the range 45 to 1500 kg/mm², and deformation was found to be purely elastic. The tensile strength data have been analysed and fitted to empirical equations describing the effect of size on strength for different orientations. These empirical equations have been used to deduce possible fracture nucleation mechanisms. It is concluded that, in the case of 0001 sapphire whiskers, fracture nucleation may be due to dislocation pile-ups or interactions, while in the other cases a Griffith flaw mechanism is probably applicable.     

Deformation and recrystallization textures of copper single crystals and bicrystals

Recrystallization textures in rolled copper specimens have been investigated so as to elucidate the mechanisms of cube texture formation. The specimens are single crystals with the orientations corresponding to the main components of the rolled textures, such as {112}111,{110}112, etc. and bicrystals consisting of such oriented crystals. The cube texture was not observed in any single crystal specimens, but observed in only two bicrystal specimens with {112}/111{100}001 and {110}112/{100}001 orientations. The formation of cube texture seems to require the existence of a cube oriented region in the deformed state, and the favourable oriented matrix to allow the growth of such nuclei.     

Structural and magnetic characterization of nanocrystalline Ni-20%Fe permalloy films

Samples of nanocrystalline Ni-20%Fe permalloy, obtained by electrodeposition, were studied using an X-ray diffraction technique and magnetic force microscopy. After deposition, the alloy exhibited strong fiber texture with the major component being 1 0 0 and a lesser contribution from 1 1 1. An increase in deposition current density led to a reduction in the strength of the 1 0 0 texture. Despite the small magnitude of the magnetic interaction, images of magnetic domains were obtained using magnetic force microscopy. The images were, however, unstable and changed significantly with the scanning time. Annealing of the permalloy above 573 K caused rapid grain growth and a transformation of the 1 0 0 fiber to 1 1 1 fiber. Numerical analysis of the texture data was conducted to assess the atomic structure of grain boundaries in the as-deposited and annealed states.     

Dislocations in extruded Co-49.3at% Al

Polycrystalline Co-49.3at% Al which had been extruded at 1505 K was examined using transmission electron microscopy. Diffraction contrast analysis showed that b = 100 as well as b = 111 dislocations contribute to elevated temperature deformation in CoAl. Therefore, it was concluded that sufficient slip systems exist in CoAl to allow for general plasticity in the absence of diffusional mechanisms. Since no examples of b = 110 dislocations were found, the importance of 110{1¯10} slip in CoAl at 1505 K is unclear. Dislocations of the type b = 001 were observed on both {110} and {100} planes while b = 111 dislocations were observed on {1¯10} planes.     

Dislocation structures in Zr3Al-based alloys

Transmission electron microscopy has been used to investigate dislocation structures in deformed binary and ternary Zr₃Al-based alloys. In the binary alloy deformed at temperatures between 293 and 673 K the dislocations in the Zr₃Al phase consisted of a/31 1 2-type partial dislocations bounding superlattice intrinsic stacking fault on {1 1 1} planes. The {111} a/31 1 ¯2 stacking fault energy was approximately 2mJ m^–2 at 673 K. In binary specimens deformed between 873 and 1073 K cube slip predominated. Dislocations consisted mainly of a/2 1 1 0 pairs separated by antiphase boundary. For this temperature range the {1 0 0} a/201 1 antiphase boundary energy was between 30 and 45 mJ m^–2. Alloying with niobium or titanium was found to increase the {111} a/31 1¯2 stacking fault energy and thus increase the propensity for antiphase boundary-type dissociation.     

Tensile properties and plastic deformation modes ofβ Zr-Nb alloys

Tensile properties and plastic deformation modes of zirconium-niobium alloys were investigated at 290 and 77 K in the wide composition range from metastable to stable phase. Three types of plastic deformation modes, {332}113 twinning, {112}111 twinning and slip, were observed depending on alloy composition and temperature. {332}113 twinning, which occurs in metastable zirconium alloys, is related to the stability of phase to decomposition and leads to low yield stress and large elongation. On the other hand, {112}111 twinning, which appears in stable zirconium alloys, results from high critical stress for slip due to solution hardening and high Peierls stress and does not affect tensile properties significantly. The results obtained for zirconium-alloys are similar to those for titanium alloys, strongly suggesting that {332}113 twinning is an important plastic deformation mode which is common to phase alloys containing athermal phase.     

The Debye-Waller factor for the noble gas solids

The mean-square nuclear displacements u ², the Debye 's associated with the u ², and the Debye 's associated with the temperature derivatives of u ² at constant volume have been calculated for the noble gas solids Ne, Ar, Kr, and Xe as a function of temperature and volume using self-consistent phonon theory. Instead of using the equation of state which gives volumes which are consistently smaller than the experimental volumes, the experimental lattice parameters at the saturated vapor pressure were used. In the case of neon, the theoretical u ² were also obtained as a function of temperature and the experimental lattice volume atT=0K. The interaction forces are described by a nearest neighbor (12–6) Mie–Lennard-Jones potential.     

Deformation behaviour of single crystals of titanium carbide

Single crystals of titanium carbide were deformed in compression over a wide range of temperature, and the operative slip systems were determined by etch-pitting and electron microscopy. Around the brittle-ductile transition temperature, the slip system undergoes a gradual change from {1 1 0} 1 ¯1 O to {1 1 1} 1 ¯1 0; this is interpreted to be the mechanism governing the brittle-ductile transition in titanium carbide.On leave from Defense Metallurgical Research Laboratories, Hyderabad 500258, India.     

Observation of dislocations and subboundaries in the optically active crystal Bi12GeO20 with reflection birefringence topography

The birefringence topography of a Bi₁₂GeO₂₀ single crystal which has a strong natural optical activity is presented. The natural optical activity of the crystal may be completely compensated by reflection polarised light microscopy so that the birefringence images of dislocations and subboundaries are observed. The Burgers' vectors of dislocations were found to be of the types 1/2 1 1 1, 1 0 0 and 1 1 0 and dislocations which form subboundaries are also of these types.     

Atom positions in the R-phase unit cell in TiNi shape memory alloy

A crystallographic analysis of the atom positions in the unit cell of the R-phase of the TiNi shape memory alloy has been performed. In addition to a homogeneous shape change the atoms are found to shuffle in the [111]_B2 and –211_B2 directions during the B2 to R-phase transformation. The origin of these shuffles is found to be a complex interaction between 1101–10 transverse acoustic and 111 longitudinal acoustic soft phonon modes.     

1 : 2 Long-range ordering and defect mechanism of WO3-doped perovskite Ba(Mg1/3Ta2/3)O3

The nature of the B-site cation ordering and the associated defect process necessary to stabilize the ordered domains were investigated using the WO₃-doped BaMg_1/3Ta_2/3O₃ BMT system as a typical example of BaB_1/3B_2/3O₃-type complex perovskites. It was shown that only the 1 : 2 long-range ordering of the B-site cation existed in both undoped and WO₃-doped BMT perovskites. The atomic defect mechanism associated with the stoichiometric 1 : 2 long-range ordering was systematically investigated. It is concluded that the substitution of W⁶⁺ for Ta⁵⁺ in the WO₃-doped BMT enhances the degree of the 1 : 2 long-range ordering and produces the positively charged W _Ta sites with a concomitant generation of tantalum vacancies V_Ta and mobile oxygen vacancies V _O for the ionic charge compensation.