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.

We show that magnetic flux lattice interactions are important at fields above a few thousand gauss. These interactions interfere with thermally activated current-induced flux bundle hopping and reduce the superconductor's flux creep resistance below that estimated from the standard flux creep models (which assume completely independent hopping). We find that the flux bundles can hop independently at the low fields at which, e.g., antennas and SQUID detection coils would be used, but interact very strongly at fields typical of magnet applications.

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.

A high Tc superconducting (Bi, Pb)–Sr–Ca–Cu–O filament was prepared using a combined technique of suspension spinning and densification of the pyrolyzed filaments by pressing and sintering. The effects of the Jc of the filament obtained on the spinning and densifying conditions were examined. The Jc was dependent on the composition of the starting powder and the kind of suspension medium. The Jc was strongly affected by the pyrolyzed condition and densification time. The highest Jc value at 77 K, 0 T of 1900 A/cm2 was attained for the filament sintered under the optimum sintering condition. The grains in the filament were oriented with the c axis perpendicular to the filament surface, and the coupling of the grains in the perpendicular to the filament surface was weaker than that for the parallel direction in the filament surface.

Chemical compositions of superconductors of Bi1−xPbxSrCaCu1.8Oy (x = 0.1, 0.3, and 0.5) were investigated by analytical electron microscopy (AEM) supplemented by electron microprobe analysis. Samples were prepared by a solid state reaction method under oxygen partial pressures of 0.20 and 0.077 atm. The high-Tc phase appeared only in the samples of Bi1−xPbxSrCaCu1.8Oy (x = 0.1 and 0.3) prepared under an oxygen partial pressure of 0.077 atm. In the samples containing the high-Tc phase, stacking structures of 2, 3, and 4 perovskite layers were observed by transmission electron microscopy (TEM). From AEM analysis, it was shown that in order to make the high-Tc phase, the substitution ratio of Pb for Bi was about 0.2.

A series of simulations has been performed on grain boundaries in Ni and Ni3Al with and without boron doping using embedded atom-style potentials. A new procedure of obtaining “reference” data for boron related properties from electronic band structure calculations has been employed. Good agreement with existing experimental structural and energetic determinations was obtained. Boron is found to segregate more strongly to grain boundaries than to free surfaces. Adding boron to grain boundaries in Ni and Ni3Al increases their cohesive strength and the work required to pull apart the boundary. This effect is much more dramatic for Ni-rich boundaries than for stoichiometric or Al-rich boundaries. In some Ni-rich cases, adding boron increases the cohesive strength of the boundary to such an extent that the boundaries become stronger than the bulk. Bulk Ni3Al samples that are Ni-rich produce Ni-rich grain boundaries. The best cohesive properties of Ni3Al grain boundaries are obtained when the boundary is Ni saturated and also with boron present. Boron and nickel are found to cosegregate to the grain boundaries.

Threshold displacement energies for atomic displacements along 〈110〉, 〈100〉, and 〈111〉 directions, and formation enthalpies of several symmetric interstitial atom configurations were calculated for Ni3Al by computer simulation using “embedded atom method” potentials. The Ni–Ni (100) dumbbell in the plane containing only Ni atoms has the lowest interstitial-atom enthalpy although the enthalpies of other configurations are similar. Interstitial configurations involving Al atoms all have much higher enthalpies. The anisotropy of the threshold energies in Ni3Al is similar to pure metals and no significant difference in threshold energy was observed for 〈110〉 replacement chains in rows containing all Ni atoms or alternating Ni–Al atoms. Various metastable interstitial atom configurations were observed, including crowd-ions. In addition, the spontaneous recombination volume for some configurations can be much smaller than in pure metals. The consequences of these results for radiation induced segregation and amorphization are discussed.

The kinetics of crystallization of four amorphous (or partially amorphous) melt spun Nd–Fe–B alloys induced by thermal treatment is studied by means of differential scanning calorimetry and scanning electron microscopy, In the range of temperatures explored experimentally, the crystallization process is thermally activated and generally proceeds in various stages. The Curie temperature and the crystallization behavior have been measured. The apparent activation energy of crystallization of most of the crystallization stages has been determined for each melt spun alloy. The explicit form of the kinetic equation that best describes the first stage of crystallization has been found. It follows in general the Johnson-Mehl-Avrami-Erofe'ev model, but clear deviations to that model occur for one alloy. Scanning electron microscopy demonstrates that preferentially hetereogeneous nucleation occurs at the ribbon surface which was in contact with the wheel. From crystallization kinetics results the lower part of the experimental time-temperature-transformation curves for all studied alloys are deduced and extrapolated to the high temperature limit of their range of validity, also deduced.

The atomic configuration at the tip of a mode 1 crack in aluminum is modeled by means of molecular dynamics calculations using an embedded atom potential. This potential intrinsically incorporates many-body contributions. This paper is concerned with the characteristics of the atomic displacement fields in comparison to the linear elastic predictions and dislocation emission phenomena. Three crack/crystal orientations are examined in which the crack plane–crack propagation directions are (010)-[100], (10)-[110], and (10)-[111]. The first two models behaved in a brittle fashion as dislocation emission did not occur for reasons associated with the use of periodic boundary conditions parallel to the crack front. For the models which remained atomically sharp, the positions of the atoms near the crack tip in equilibrium configurations are different from the linear elastic predictions but, to first order, retain an r1/2 dependence, with smaller K, and with the origin displaced behind the physical crack tip. This near tip region is also observed to be elastically softer than in the far field. Dislocation emission readily proceeds in the (10)-[111] model by the sequential emission of partials with attendant nonzero uz displacements. The blunting is characterized by the creation of two corner defects that separate as emission occurs and relaxation of the strains in the region initially confronted by the crack tip. Additional features of the results are discussed.

Single crystal growth of CuO, synthetic tenorite, was accomplished in oxygen atmosphere by the chemical vapor transport technique. We present the first systematic study of the growth of CuO in various partial pressures of oxygen. Crystals were grown using trace, 0.1, and 0.5 atm partial pressures of oxygen. The growth temperatures used were 820 and 865 °C. Typical dimensions of the crystals obtained were 1 × 2 × 0.25 mm. We have shown that the stoichiometry of single crystal CuO can be controlled during the growth process. The crystals exhibited cation deficient stoichiometry, Cu1−xO, with x increasing from less than 0.005 with no added oxygen, up to 0.05 with 0.5 atm O2 when grown at 865 °C.

The effects of 193 and 308 nm excimer laser radiation on the filament-assisted chemical vapor deposition (CVD) of diamond were investigated, in attempts to reproduce and quantify the reported mechanism of laser-enhanced diamond deposition. The deposited materials were analyzed using optical microscopy, SEM, scanning Auger microprobe, and micro-Raman scattering. With fluence levels of >50 mJ · cm−2, UV laser irradiation was found to suppress rather than enhance the quantity of diamond deposition. The size, morphology, and Raman spectra of crystallites in the irradiated regions were nearly identical to those in adjacent unirradiated regions of the same sample. An additional laser-induced effect was a region of enhanced etching on the Si substrate, which appeared as a “shadowing” of the diamond crystallites. The results are interpreted in terms of a laser-induced depletion of diamond nucleation sites, and suggest a new method for patterning of CVD films.

The electronic mechanism behind the brittle fracture of trialuminide alloys is investigated using the full-potential linearized augmented plane-wave (FLAPW) total-energy method within the local density functional approach. To this end, the bulk phase stability, the elastic constants, the anti-phase boundary (APB) energy, the superlattice intrinsic stacking fault (SISF) energy, and the cleavage energy on different crystallographic planes have been determined. A small energy difference (=0.10 eV/unit formula) is found between the DO22 and L12 structures of Al3Ti. In general, the trialuminide alloys have large elastic modulus, small Poisson's ratio, and small shear modulus to bulk modulus ratio. An extremely high APB energy (=670 mJ/m2) on the (111) plane is found for Al3Sc, indicating that the separation between ½(110) partials of a (110)(111) superdislocation is small. Since the total superdislocation has to be nucleated essentially at the same time, a high critical stress factor for dislocation emission at the crack tip is suggested. The high APB energy on the (111) plane is attributed to the directional bonding of Sc(d-electron)-Al(p-electron). The same type of directional bonds is also found for Al3Ti. In addition, moderately high values of SISF energy (=265 mJ/m2) on the (111) plane and APB energy (=450 mJ/m2) on the (100) plane are found for Al3Sc. The brittle fracture of trialuminide alloys is attributed to the higher stacking fault energies and a lower cleavage strength compared to those of a ductile alloy (e.g., Ni3Al). While the (110) surface has the highest surface energy, the cleavage strength (=19 GPa) of Al3Sc is found to be essentially independent of the crystallographic planes. The directional Sc—Al bond becomes even stronger on the (110) surface, which may explain the preferred (110) type cleavage observed by experiment.

An excimer laser was used to mix Fe/Ti/C multilayered films on 304 stainless steel substrates. The samples were processed at both 1.1 and 1.7 J/cm2 with the number of pulses at each position varied between 1 and 10. Composition, microstructure, phase evolution, and tribological properties were observed to correlate with total laser fluence. Increases in Fe concentration from substrate interdiffusion and loss of C content were observed with increasing total laser fluence. The best tribological properties were observed in films possessing a combination of an amorphous or Fe3C phase plus fine grain TiC following processing at low and intermediate total laser fluence. At higher total fluences a combination of α-Fe, Fe3C, and fine-grain TiC was observed along with degradation in the wear and friction properties. Under optimum laser processing conditions the modified surface had a friction coefficient under dry sliding conditions reduced by a factor of 2 relative to uncoated 304 stainless steel and was significantly more wear resistant. These improvements in wear and friction appear to be related to the reduced chemical reactivity of the amorphous and carbide phases and to the influence of microstructure on improved mechanical properties.

A simple spring network model is utilized to investigate stress concentrations and toughness increases for a variety of traditional mechanics problems. The scaling of crack-tip stress with crack size and separation, the shielding due to a low modulus process zone, and the toughness increase from microcracking and transformation process zones are all in good agreement with available analytic or numerical mechanics results. Also, the, process zone growth and attendant R-curve behavior for microcracking and/or transformation toughening is simulated directly using the network model with only a few simple rules. The network model is thus expected to be useful for obtaining easy quantitative results for related problems which are not so easily solved by other means. In addition, the good agreement found here supports the use of the network model for studying problems involving distributed disorder, as discussed in the previous paper.

A study is made of the effects of annealing both La2−xSrxCuO4 (for x = 0, 0.1, and 0.15) and YBa2Cu3O7 in wet and dry O2 atmospheres at various temperatures between 200 and 930 °C. In the case of La2−xSrxSrCuO4, substantial degradation of resistive properties occurs during annealing in moist O2, the degradation being highest at 200 °C and decreasing as the treatment temperature increases. Since the Meissner effect remains unaffected, it is concluded that degradation is due to the formation of a hydroxide species at grain boundaries, which decomposes as the anneal temperature is increased to 930 °C. In the case of YBa2Cu3O7, on the other hand, moisture does not produce degradation for anneals at 200 °C and above, but severe degradation of resistive behavior does occur for dry O2 anneals, with a maximum effect at 500 °C. It is found that this effect results from a contaminant gas, possibly CO2, absorbed by the furnace when it is open to air. Again, the degradation is due to formation of a grain-boundary phase which decomposes by annealing at 930 °C.

XRD shows that doping BaBiO3 with alkali ions transforms the larger monoclinic parent lattice into a more compact cubic or nearly cubic lattice. This lattice contraction favors the transition from semiconductor to metal. It leads also to superconductivity. Doping BaBiO3 with both K and Rb raises the onset resistivity Tc above those obtained by doping with either K or Rb.

The atomic structure of liquid-quenched amorphous Al90FexCe10−x (x = 5,7) was studied by pulsed neutron and x-ray scattering. The atomic pair-density function determined by pulsed neutron diffraction indicates that a significant portion of Al–Fe distances is anomalously short, while some part of the Al–Al distances is anomalously long. Both neutron and x-ray scattering showed the presence of a prepeak in the structure factor. These results suggest that a strong interaction between A1 and Fe modifies the structure of this glass, leading to chemical and topological short-range ordering.

The chemical compatibility and tensile properties of a powder processed intermetallic matrix composite (IMC) were investigated in the temperature range 298–1373 K. The matrix alloy selected for this study was the ORNL developed advanced nickel-aluminide IC-221 (Ni-16Al-8Cr-1Zr-0.05B at. %). The composite contained 25 vol. % TiC particulate reinforcement. TiC/IC-221 compatibility samples were heat treated at 1373 K for up to 1000 h. A layer enriched in Zr and Ti formed at the TiC/IC-221 interface after heat treatment. In addition, Ti was observed to diffuse into the matrix. The composite exhibited higher yield strength and lower ductility than a similarly processed matrix alloy at all test temperatures. Heat treatment of the composite improved the tensile properties due to particulate/matrix interaction, resulting in improved load transfer. SEM fractography revealed that tensile failure occurred at the matrix/prticulate interface. Remnants of the matrix were observed on TiC particles on the fracture surface, suggesting good matrix/particulate bonding. The mechanical properties of the composite were very competitive with Ni-base superalloys.

The one-electron theory of metals is applied to the calculation of stacking fault energies in face-centered cubic metals. The extreme difficulties in calculating fault energies of the order of 0.01 eV/(interface unit-cell area) are overcome by applying the Force theorem and using the layer–Korringer–Kohn–Rostoker method to determine the charge density of isolated defects. A simple scheme is presented for accommodating deviations from charge neutrality inherent in this approach. The agreement between theoretical and experimental values for the stacking fault energy is generally good, with contributions localized to within three atomic planes of the fault, but suggest the quoted value for Rh is a significant overestimation.

A NiTi intermetallic compound was cold rolled at room temperature by 30% and 60% thickness reductions, and microstructures were studied by means of transmission electron microscopy (TEM). In the cold-rolled samples we observed both a phase of nanometer-sized crystals and an amorphous phase. A substantially high dislocation density, 1013 to 1014/cm2, was evident in the transition region between crystalline and amorphous phases. A simple estimate of the elastic energy arising from this dislocation density is of the same order as the crystallization energy, suggesting that dislocation accumulation is a major driving force for amorphization in cold-rolled NiTi.