Samples of Y-Ba-Cu-O materials with the formulation Y:Ba:Cu = x:2:3 and values of x ranging between 1 and 3 were prepared by partial melting at a maximum temperature of 1045 °C. Measurements of magnetic susceptibility and maximum (low-field) as well as remanent magnetization show highest values for x = 2. XRD and SEM/EDX analyses show that the corresponding structure involves numerous small crystals of Y2BaCuO5 (211) embedded in highly ordered assemblages of continuous YBa2Cu3O7−y (123) layers. The presence of these impurity sites is correlated with flux pinning capacity. Other impurity phases include CuO, a minor phase which shows an increase in amount when x increases, BaCuO3, a possibly detrimental minor phase which vanishes when x is raised from 1 to 2, and zones of intermediate composition between the 211 grains and the 123 layers, which are formed from the residual liquid upon cooling. These intermediate regions, like the 211 grains themselves, become gradually more important when x is increased from 1 to 3.

Elastic moduli of both as-hot-pressed and oxygenated YBa2Cu3Ox were measured, under isostatic pressures up to 1 GPa, using a combination of strain gage and ultrasonic techniques. Pressurization of the as-hot-pressed samples caused no significant change in moduli; however, pressurization of the oxygenated sample produced a 5-fold increase in bulk modulus. The results show that the significant decreases in moduli between the as-hot-pressed and the oxygenated state were due to extensive microcracking, of an extremely fine nature, that occurred during the oxygenating sequence.

The effect of the γ′ phase on the deformation behavior fracture resistance of melt-spun ribbons and consolidated bulk specimens of a series of NiAl-based alloys with Co and Hf additions has been examined. The morphology, location, and volume fraction of the γ′ phase are significant factors in enhancing the fracture resistance of the normally brittle NiAl-based alloys. In particular, the results indicate that a continuous grain boundary film of γ′ can impart limited room temperature ductility regardless of whether B2 or L10 NiAl is present. Guidelines for microstructure control in multi-phase NiAl-based alloys are also presented.

Transformation of boehmitc-derived alumina gel to α–Al2O3 in unseeded gels is strongly influenced by the parent phase, θ-Al2O3. In seeded gels the perfect structure of the epitaxial substrate (α–Al2O3, 0.3 wt. %) influences the surrounding defect θ-phase resulting in (a) fewer imperfections due to lower transformation enthalpy to α–Al2O3, and (b) multiple nucleation sites of α–Al2O3 at the α–Al2O3 seed surface. The rate of transformation expressed for the same number of α–Al2O3 nuclei in unseeded and seeded gels indicates that it is faster in the unseeded gel than in the seeded gel.

Stoichiometric deviations of up to ±5% in Ba2YCu3O7−δ, thin films grown by coevaporation on LaAlO3(100) substrates are found to cause (1) a decrease of the critical current density (Jc) of up to an order of magnitude, (2) a depression of the critical temperature (Tc) and a broadening of the superconducting transition width (ΔT), (3) a deterioration of the surface morphology, and (4) a decrease in the crystallinity of the films. The data indicate that composition deviations of greater than ±1% result in degradation of film quality. These findings have significant implications for the degree of composition control required during deposition to produce films with optimized properties.

We report a first observation, of the behavior of clusters of deuterium atoms confined in a lattice vacancy in a Pd crystal, either at equilibrium or under the influence of externally applied lattice strains. Molecular dynamics simulations have been performed on both Pd perfect crystals and Pd crystals with a single vacancy, showing the energetic behavior and the dynamical values of the interatomic distance among deuterium atoms belonging to clusters of different sizes confined within the vacancy. At low temperatures, the smallest interatomic D-D distances are of the order of the D2 molecular bond length and are obtained during the transitory regimes induced by hydrostatic strains applied to the lattice.

The compound Bi2Sr2(GdxCa1−x)0.8Cu2O8+δ is examined by electron diffraction for several values of x. The incommensurate modulation which has been reported for x - 0 persists for all values of x with the wave vector increasing as x increases. A secondary modulation with a wave vector equal to one-half that of the primary is observed for x  0.85. This modulation manifests itself through satellite reflections that are highly streaked normal to the (001) planes and that flank certain select positions in reciprocal space. The appearance of this secondary modulation is accompanied by a change in the average crystal structure from B-centered to F-centered orthorhombic.

A Bi-Ca-Sr-Cu oxide composition (2:4:2:5) was rapidly solidified from the melt, and the crystallization behavior examined on heat-treatment. Annealing conditions were 865°C for up to 11 days. The high Tc 2223 phase (105 K) evolved from the 2122 phase (80 K), which in turn developed from the 2021 phase (12 K). The high Tc phase developed only in the presence of a liquid phase at 865 °C. Lattice imaging was used to follow the conversion of 2122 phase to 2223. Data are reported for syntactic intergrowths, which became less frequent with time at temperature. EDS results are consistent with the conversion of 2122 to 2223. Crystals of 2223 could not be grown from the melt, nor crystallized from the solid at temperatures below 820 °C. The presence of a Cu- and Ca-rich liquid was essential for the development of 2223 at 865 °C. A tentative model for the formation of 2223 via a liquid mediated reaction is proposed. EDS confirmed the liquid was rich in Ca and Cu near the solid-liquid interface, and precipitates of secondary phases were identified by SEM, TEM, and XRD methods. The presence of CuO and (Ca,Sr)2CuO3 verified the enrichment of Cu and Ca at the solid-liquid interface. The results are consistent with the evolution of structure of a 2223 from a 2425 starting composition.

The susceptibility of a hot isostatically pressed Ni3AI, Cr, Zr alloy to hydrogen embrittlement has been studied. The base alloy and a second alloy containing 5 vol. % Y2O3 particles were tested by cathodically charging with hydrogen prior to or simultaneously with tensile testing. Embrittlement of both alloys was noted under both charging conditions, but was much more severe for simultaneous charging. Intergranular fracture due to hydrogen was noted in the base alloy, while the dispersoid-containing alloy failed along prior particle boundaries. The results are explained by a dislocation transport mechanism in which hydrogen is delivered to interior fracture sites by mobile dislocations. Much greater penetration of hydrogen is achieved under simultaneous charging conditions.

The large number of constituent elements and high symmetry of the ordered Fe–Co−V alloys make a complete solution of its structure impossible from a single diffraction experiment. We have carried out a Rietveld refinement of the site occupancies for a partially ordered Fe–Co–V alloy using both neutron and anomalous dispersion x-ray powder diffraction data in order to obtain a complete crystallographic solution to its structure. Contrary to previous assumptions that V occupies both sites randomly or magnetic and Mössbauer studies that suggest that V has a site preference for the Fe sublattice, we find that the V dopant preferentially occupies the Co sites.

A thermodynamics-based description, in the form of an extended phase diagram, of melting and solid-state amorphization is proposed which brings out the parallels between these two phenomena and suggests that their underlying causes are apparently the same. Through molecular dynamics simulations we demonstrate that every crystal, in principle, can undergo two different types of melting transitions with characteristic features that are also observed in radiation- and hydrogenation-induced amorphization experiments on ordered alloys. The first type, defined in terms of free energies, is shown to involve the heterogeneous nucleation of the liquid or amorphous phase at extended lattice defects (such as grain boundaries, free surfaces, voids, or dislocations) and subsequent thermally-activated propagation of solid-liquid/amorphous interfaces through the crystal. The second type, arising from a mechanical instability limit described by Born, is homogeneous and does not require thermally-activated atom mobility. It is suggested that the role of chemical and structural disordering, a prerequisite for irradiation- but not hydrogenation-induced solid-state amorphization, is merely to drive the crystal lattice to a critical combination of volume and temperature at which the amorphous phase can form either heterogeneously or homogeneously.

This paper presents an atomic calculation of the wedging effect which occurs in a brittle crack when molecules of a chemisorbing species of molecules of sufficient size enter the crack mouth. A surface tension develops at the tip of the wedge caused by the difference between the covered and vacuum surface energies. This force draws the chemisorbing molecules toward the crack tip and distorts the crack faces, causing, in turn, a compensating elastic force on the molecules which tends to eject the molecules. We calculate the equilibrium penetration of the wedging molecules and the configuration of the crack and wedge by an atomistic calculation. We simulate mica/water chemistry by means of a simplification of the mica lattice and calculate interactions between the water and mica on the basis of Born–Mayer. Water is found to form a wedge tongue of two or three molecular thicknesses and a length of about 20 molecular distances, which penetrates into the crack tip cohesive zone. When strong wedging action occurs at a crack tip, crack advance near threshold loadings will be limited by molecular diffusion through the wedge tongue.

Laser-heated float zone growth was used to study the directional solidification behavior of Bi–Sr–Ca–Cu–O superconductors. The phases that solidify from the melt, their morphology, and their composition are altered by growth rate. Highly textured microstructures are achieved by directional solidification at all growth rates. The superconducting phase is found always to have the composition Bi2.5Sr2CaCu2.2Oy when grown from boules with composition 2:2:1:2 (BiO1.5:SrO:CaO:CuO). Planar growth fronts of Bi2.5Sr2CaCu2.2Oy are observed when the temperature gradient divided by the growth rate (G/R) is larger than 3 ⊠ 1011 K-s/m2 in 2.75 atm oxygen. Thus, the 2212 compound was observed to solidify directly from the melt at the slowest growth rates used in this study. Measurement of the steady-state liquid zone composition indicates that it becomes bismuth-rich as the growth rate decreases. Dendrites of the primary solidification phase, (Sr1−xCax)14Cu24Oy, form in a matrix of Bi2.5Sr2CaCu2.2Oy when G/R is somewhat less than 3 ⊠ 1011 K-s/m2. Observed microstructures are consistent with a peritectic relationship among Bi2.5Sr2CaCu2.2Oy, (Sr1−xCax)14Cu24Oy (x = 0.4), and a liquid rich in bismuth at elevated oxygen pressure. At lower values of G/R, Sr3Ca2Cu5Oy is the primary solidification phase and negligible Bi2.5Sr2CaCu2.2Oy forms in the matrix.

Using both x-ray and electron diffraction, we have studied the incommensurate modulation structure in single crystal Bi2Sr2CaCu2Ox. Electron diffraction clearly indicates a small a∗-axis component of the modulation wave vector k, along with the more familiar b∗-axis component (0.004a∗ + 0.212b∗). This tilt of the k-vector breaks the mirror symmetry perpendicular to a- and b-axes as well as the twofold symmetry about a- and b-axes. The modulated structure thereby has only a mirror plane normal to the c-axis. Intense x-ray streaking along the c-axis at several satcllite reflections was considerably more pronounced than at the fundamental reflections, indicating a stacking disorder along the c-axis predominantly in the modulated structure.

A study of diamond synthesis in an atmospheric pressure inductively coupled argon-hydrogen-methane plasma is presented. The plasma generated has an active area of 20 cm2 and a free stream temperature of approximately 5000 K. Deposition experiments lasting 1 h in duration have been performed in both stagnation flow and flat plate parallel flow geometries. The diamond film deposited in both configurations are nonuniform yet fairly reproducible. The variation in the growth rates at various regions of the substrate is attributed to the variation in the surface atomic hydrogen flux. Growth rates are as high as 50 μm/h, in regions of the substrate where the atomic hydrogen flux is expected to be large. Little or no growth is observed in regions where the atomic hydrogen is expected to recombine within the thermal boundary layer before arriving at the surface. Individual particles are analyzed by micro-Raman spectroscopy. Large (50 μm) size well-faceted particles show little evidence of non-diamond carbon content and are found to be under a state of compression, displaying shifts in the principal phonon mode as great as 3 cm−1.

The phase relations of equilibrium compounds in the pseudoternary system Bi2O3–(Ca,Sr)O–CuO at 850 and 900°C were studied. The ratio of Ca : Sr was fixed at 1:2. Starting materials of Bi2O3, CaCO3, SrCO3, and CuO with various ratios were mixed, pressed into pellets, and heated at or above and then brought back to 850 or 900°C for different durations to ensure that equilibrium had been reached. The products were cooled in air or quenched in liquid nitrogen and then identified by x-ray powder diffraction. At 850°C, only the superconducting phase, Bi2CaSr2Cu2Ox (2122), was observed inside the triangle. The other stable phases were all positioned on the boundary lines, and included CuO·⅗MO, CuO·MO, CuO·2MO, 1.½Bi2O3·0.9MO, Bi2O3·4MO, Bi2O3·9MO, and a solid solution, Bi2O3·xMO, where 0.16  x  0.82 and MO represents ⅓(CaO·2SrO). At 900°C, the above boundary line phases remained stable but the 2122 phase was not observed. The tie lines among the stable phases in the two isotherms were established.

Highly textured diamond thin films with (111) orientation have been successfully grown on the single crystal silicon (100) substrates by the flame method using the premixed acctylene and oxygen gas. Scanning electron microscopy (SEM), x-ray diffraction, and Raman spectroscopy studies have shown the formation of uniform diamond films with the full coverage of the deposition area on the Si substrates. An approach which leads to growing diamond films with very low graphite and/or amorphous carbon contaminations is described.

The compound Sr2Bi2CuO6 should nominally be the phase with n = 1 of the high Tc superconducting series Sr2Bi2CanO4+2n. However, the superconducting phase with n = 1 (with no CaO) occurs only with a gross deficiency in SrO content. Instead, at the composition Sr2Bi2CuO6, a different phase is formed with an x-ray diffraction pattern considerably different from that expected for the n −1 member of the series. This phase has been found, by a combination of electron diffraction and single crystal and powder x-ray diffraction, to have a commensurate lattice with monoclinic symmetry, space group C2/m or Cm, a = 24.473 (2), b = 5.4223 (5), c = 21.959 (2)A, and β = 105.40 (1)°. The actual composition of this phase may be deficient in CuO by as much as 1.0 mole %.

Three trialuminide alloys, binary Al–25Sc, ternary Al–25Zr–6Fe, and quaternary Al–23Ti–6Fe–5V, all having the cubic L12 structure, were investigated. All three alloys fracture in a brittle manner (fracture toughness, 2–3 MPa m½), predominantly by transgranular cleavage. Of nineteen cleavage facets examined in binary Al3Sc, seventeen were of the {110} type and only two were of the {100} type, consistent with our earlier work which showed that the cleavage plane occurring most frequently in quaternary Al–23Ti–6Fe–5V is also {110}. The room-temperature hardnesses and yield strengths (100–200 DPH and 100–270 MPa, respectively) of all three alloys are quite low (comparable to ductile L12 alloys like Ni3Al), indicating that there is significant dislocation activity in these materials. Consistent with this, transmission electron microscopy identified several APB-coupled dislocations with b - a/2〈110〉 gliding on the {111} planes in both binary Al–25Sc and quaternary Al–23Ti–6Fe–5V. The separations between the superpartials in Al–25Sc and Al–23Ti–6Fe–5V were measured to be 3.7 and 4 nm, respectively, giving APB energies of 313 and 274 mJ/m2, respectively. Auger analyses failed to detect any impurities on the cleavage facets themselves, or on second phase particles (or other potential cleavage crack nucleation sites). It is therefore concluded that brittle fracture in these alloys is not impurity-induced. Based on all the results obtained to date we conclude that the unusual brittleness of L12 trialuminides is related to their intrinsically low cleavage strength. Possible reasons for their low cleavage strength are discussed.

A model for investigating the influence of distributed disorder on the failure of brittle materials is introduced. The model assumes that microstructural features of a material can be represented by simple linear springs with a failure threshold, and that the entire material can be represented by a connected network of such springs. Distributed disorder is introduced by allowing spring-to-spring variations in spring characteristics such as the modulus and the failure strain. The conditions under which such a spring network model is valid for studying failure are discussed. The consequences of distributed residual stress disorder on macroscopic mechanical behavior are then studied using the network model, and a brittle to ductile-like transition in the stress-strain behavior is observed with increasing disorder. All the qualitative features of the network results can be described theoretically by a statistical analysis of this problem. Finally, notch tests are performed to evaluate the strength and toughness of the ductile-like materials as compared to the uniform (no disorder) material, and the ductile-like material is found to have (i) a larger work of fracture, (ii) comparable strength in the presence of processing flaws, and (iii) the possibility of larger toughness. Based on these results, the possibility of observing such ductile-like behavior in real composite materials is discussed.