bluuebunny/crossref_metadata_2025_split huggingface dataset:
Microstructural Effects On Fatigue-Crack Growth Behavior In γ-TiAl/β-TiNb Intermetallic Composites
Authors: Rao K. T. Venkateswara , Ritchie R. O. 
Published: March 2011

ABSTRACTCyclic crack-propagation behavior is examined in a series of γ-TiAl intermetallic alloys reinforced with pancake-shaped, ductile β-TiNb particles as a function of microstructure and specimen orientation. In contrast to results under monotonic loading, TiNb reinforcements are found to be far less effective in impeding crack extension under cyclic loading due to their susceptibility to premature fatigue failure, and consequently to the diminished role of shielding from crack-bridging mechanisms. Modest improvements in fatigue-crack growth resistance are observed in TiAl/TiNb composites compared to monolithic γ-TiAl, provided the particle faces are oriented perpendicular to the crack plane; however, properties are compromised in orientations where the particle edges are stacked normal to the crack plane. Microstructural effects on cyclic crack growth are less prominent in the composites, with crack-growth rates exhibiting a strong dependence on the applied ΔK level; measured exponents for the da/dN-ΔK relationship range between 10 and 20, and are found to decrease with increasing ductile phase content, yet are independent of particle thickness.
Fracture Toughness of NiAlIn-SituEutectic Composites.
Authors: Heredia F. E. , Valencia J. J. 
Published: March 2011

ABSTRACTMechanical tests were performed on directionally solidified (DS) NiAlin-situeutectic composites in order to evaluate the effect of ductile reinforcements on the fracture resistance of the B2 ordered intermetallic compound NiAl. Reinforcements consisted of i) Mo fibers, ii) Cr fibers, and iii) Cr(Mo) solid solution plates. Near stoichiometric NiAl ingots were prepared by induction melting as reference material to compare with the eutectic composites. Resistance curves were obtained for the NiAl/Mo fibrous eutectic alloy as well as for the NiAl/Cr(Mo) layered material. The initiation fracture toughness of the DS NiAl/Mo and NiAl/Cr(Mo) eutectic composites is larger than that of the stoichiometric NiAl, with the layered material. producing the better properties. The mechanisms for such increase in fracture toughness are discussed.
Effect of Ductile Phase Reinforcement Morphology on Toughening of MoSi2
Authors: Alman D. E. , Stoloff N. S. 
Published: March 2011

ABSTRACTNiobium was added to MoSi2 in the form of particles, random short fibers and continuous aligned fibers. It was found that the morphology of Nb played a role in the toughening that occurred (as measured by the area under load displacement curves from room temperature three point bend tests and the examination of fracture surfaces). The Nb particles did not toughen MoSi2. The random short fibers appeared to toughen MoSi2 via crack deflection along the fiber matrix interface. Aligned fibers imparted the greatest toughness improvements, as toughening resulted from fiber deformation. However, larger diameter fibers displayed a greater ability to toughen MoSi2 than smaller diameter fibers. This was attributed to the constraint resulting from the interfacial layer between the MoSi2 matrix and the Nb fiber. Maximum toughness occurs when the fiber is able to separate from the matrix and freely deform.
Plasticity Enhancement of MoSi2 at Elevated Temperatures Through the Addition of TiC
Authors: Chang H. , Kung H. , Gibala R. 
Published: March 2011

ABSTRACTMonolithic MoSi2 and MoSi2-TiC particulate composites with 10 vol % and 15 vol % TiC were tested in compression between 950°C and 1200°C. The MoSi2-TiC composites can be deformed plastically at lower temperatures than MoSi2 can before brittle fracture occurs. The composites exhibit much lower strain hardening rates and attain zero strain hardening rate at much lower strains than monolithic MoSi2. The differences between the composites and monolithic MoSi2 in plasticity and in strain hardening behavior is attributed to efficient dislocation generation into the matrix from sources at the MoSi2-TiC interfaces.
In-Situ Growth of SiC in MoSi2 by Melt Processing
Authors: Tilly Daniel J. , Löfvander J. P. A. , Levi C. G. 
Published: March 2011

ABSTRACTThe growth of SiC during solidification of Mo-Si-C melts was investigated to explore the potential for developing in-situ refractory reinforcements for MoSi2 matrices. Volume fractions of up to 20% were readily incorporated by arc melting. Primary β-SiC grows as equiaxed particles, plates and hopper crystals, whereas secondary SiC grows with ribbon, thin flake or whisker morphologies. The dominant facets of both primary and secondary SiC are of the (111) and {002) type, which are also characteristic of the equilibrium crystal shape. There is a clear orientation relationship between the phases wherein the close packed planes and directions of the C11b and B3 structures are parallel.
Mechanics of Interfacial Failure during Thin-Slice Fiber Pushout Tests
Authors: Koss D. A. , Kallas M. N. , Hellmann J. R. 
Published: March 2011

ABSTRACTInterfacial failure along the fiber-matrix interface during fiber push-out tests is examined for conditions where (1) a “thin-slice” specimen geometry is used and (2) matrix plasticity is necessary for large scale fiber displacement. The influence of both test geometry and the combination of thermally and mechanically induced stresses on the potential failure process is discussed. Experimental results are presented which illustrate a range of interfacial failure processes as a consequence of different combinations of specimen geometries, thermally induced residual stresses, and mechanically applied stresses. It is concluded that mode I crack opening and growth induced by specimen bending can be a major contributor to the interfacial failure process. Accurate quantitative measurements of interfacial shear properties must therefore rely on test configurations in which specimen bending is eliminated.
Evaluation of A Ti-22ai-23nb “Orthorhombic” Alloy for use as the Matrix in A High Temperature Ti-Based Composite
Authors: Graves J. A. , Smith P. R. , Rhodes C. G. 
Published: March 2011

ABSTRACTThe production of titanium aluminide intermetallic compound foil represents a significant manufacturing challenge. Cold rolling, which imparts excellent thickness uniformity and surface finish characteristics that are of benefit in composite fabrication, is especially difficult with these alloys. However, recent modifications in Ti aluminide alloy compositions and advances in thermomechanical processing have made it possible to produce foil of thickness less than 100 μm, having the microstructure and mechanical property characteristics required for composite fabrication and improved performance. This paper describes the properties of a new Ti aluminide alloy, of nominal composition Ti-22AI-23Nb (at.%), comprising a three phase microstructure of α2 (Ti3Al), an ordered orthorhombic phase (Ti2AINb) and an ordered beta phase. The discussion emphasizes the processing of this alloy through cold rolling to foil, and the associated microstructures and mechanical property characteristics that are relevant to the use of this foil to form a composite matrix.
Statistical Analysis of the Properties of Advanced Fibers Designed for Titanium Alloy and Intermetallic Reinforcement
Authors: Porter John R. 
Published: March 2011

ABSTRACTThe mechanical properties of new ceramic reinforcing fibers need to be well characterized before their incorporation into composite materials. Critical fiber properties include strength and Weibull modulus, both off the spool and after matrix extraction, bundle strength, modulus and creep resistance. Important composite properties include thermochemical stability, interface debond energy and interfacial sliding resistance. Tailoring these interfacial properties invariably involves the use of a fiber coating that can, in turn, influence fiber properties. Methods of measuring strength related properties are addressed and the results of a computer simulation to assess the quality of measured data using statistical methods are presented. The simulation was developed to determine the errors associated with a strength/Weibull modulus determination based on a limited number of samples. Finally, an assessment of the effect of mixing of high and low quality fiber on bundle strength and composite properties is made.
Fiber Push-Out Testing of Intermetallic Matrix Composites at Elevated Temperatures
Authors: Eldridge Jeffrey I. 
Published: March 2011

ABSTRACTA newly developed apparatus for performing fiber push-out testing at elevated temperatures has been applied towards testing fiber-reinforced intermetallic and metal matrix composites. This new capability shows the effects of the relief of residual stresses and increased matrix ductility with increasing temperature on fiber debonding and sliding behavior.
Use of Shock Waves to Measure Adhesion at Interfaces
Authors: Nutt Gerald L. , King Wayne E. 
Published: March 2011

ABSTRACTThe central problem in the study of composite materials is the adhesive strength of the electronic bond between reinforcement and matrix. We have introduced a unique method of measuring the interface bond strength of a wide variety of engineering interfaces (e.g. metal/ceramic, semiconductor/metal, metal/polymer). Specimens are composed of a relatively thin overlayer on a thick substrate. The specimens are shocked using a magnetic hammer which accelerates a thin metal flyer onto the substrate. The shock, upon reflection at the free surface, is incident on the bonded interface as a tensile wave, spalling the overlayer. The method is unique in using free surface velocity measurements to determine the interface stress at the instant of separation. The debonding process is sufficiently rapid (on the order of 1.0 ns) that debonding occurs by the simultaneous breaking of atomic bonds, rather than by propagation of cracks nucleating at stress concentrations near existing flaws.
Strength and Toughness of Be12Nb, Be17 Nb2 and Two-Phase Be12Nb-Be17Nb2
Authors: Bruemmer Stephen M. , Arey Bruce W. , Henager Charles H. 
Published: March 2011

ABSTRACTBend strength, compression strength, and fracture toughness of niobium beryllide intermetallic compounds have been assessed at temperatures from ambient to 1200°C. Hot-isostatically-pressed (HIP) Be12Nb showed significantly improved lowand high-temperature mechanical properties over vacuum-hot-pressed (VHP) material. Strengths at 20°C were 250 MPa in bending and 2750 MPa in compression with a fracture toughness of ∼4 Mpa√m, much higher than previously measured for this compound. High-temperature (≥ 1000°C) mechanical properties were also improved with bend strengths of 250 MPa at 1200°C as compared to only 70 to 100 MPa for the VHP material. However, severe pest embrittlement was detected in the HIP material at temperatures between 650 and 1000°C.
Evaluation of an Scs-6/Ti-22al-23nb “Orthorhombic” Composite
Authors: Smith P. R. , Graves J. A. , Rhodes C. G. 
Published: March 2011

ABSTRACTstudy was undertaken to examine the attributes of utilizing a high niobiumcontaining titanium aluminide (orthorhombic) composition, specifically Ti-22AI-23Nb (a%), for use as a matrix for a continuously reinforced metal matrix composite. Both unreinforced “neat” panels and 35v% 4-ply unidirectional ([0]4) SiC (SCS-6) panels were fabricated by HIP'ing using a foil / fiber / foil approach. The microstructure of these panels were examined via optical, scanning electron and transmission electron microscopy. Reaction zone kinetics including both the primary reaction product and any beta-depleted zone growth were determined at 982°C. Analytical electron microscopy was employed to identify fiber / matrix interfacial compounds as well as local phases and their associated chemistries. Preliminary mechanical properties were obtained which included: longitudinal and transverse tensile, matrix thermal stability, thermal fatigue, thermal mechanical fatigue and transverse composite creep. The results were compared with panels fabricated from the baseline matrix composition, Ti-24AI-11 Nb.
Fracture and Fatigue Behavior in Nb3Al+ Nb Intermetallic Composites
Authors: Murugesh L. , Venkateswara K. T. , DeJonghe L. C. , Ritchie R. O. 
Published: March 2011

ABSTRACTModel high-melting point Nb3Al + Nb intermetallic composites have been fabricated in situ by vacuum hot pressing and reaction sintering elemental powders mixed in the ratio Nb + 7wt. % Al. In both cases, microstructures feature islands of ductile Nb solid solution (∼20 vol. %) in a brittle Nb3Al intermetallic matrix. Thermal treatment for 24 h at 1800°C results in a lamellar microstructure containing a uniform and fine distribution of filamentary Nb in a Nb3Al matrix following the massive peritectic transformation. In this paper, the fatigue and fracture resistance of these two microstructures are examined and compared to pure Nb3Al and Nb. Preliminary results suggest that the Nb phase can provide significant toughening to Nb3Al via crack bridging, plastic stretching and interfacial debonding mechanisms. Measured plane-strain fracture toughness values for the as hot-pressed and fully-aged microstructures are ∼6–8 Mpa√m compared to √fm for pure Nb3Al. However, under cyclic loading, the composites tend to show a strong dependence on applied stress-intensity level; fatigue thresholds range between 2–3 Mpa√m.
Laminate Orientation and Thickness Effects on An Scs-6/Ti-24ai-11 Nb Composite Under Thermal Fatigue
Authors: Revelos William C. , Roman Itzhak 
Published: March 2011

ABSTRACTA SiC/Ti-24AI-11 Nb (at. %) composite (30–35 vol. %) was thermally cycled in air and an inert environment between 150 °C and 815 °C for various cycle counts. Various hold times at maximum temperature were employed to determine timedependent effects on composite integrity. Laminate orientations investigated included: [0]4, [0]8, [90]4, [90]8 and [0/90]2S. Acoustic emission produced during thermal fatigue of selected specimens was employed to monitor damage progression. Post-cycling room temperature tension tests as well as optical and scanning electron microscopy were used to document damage, which was particularly acute when hold times at temperature were employed on tests performed in air. The roles of the environment, composite thickness, and off-axis fibers during thermal fatigue on the composite strength and integrity are discussed.
Interdiffusion and Phase Relations in Ti-Aluminide/NB/Ti-Aluminide Composite Structures
Authors: Ma Z. , Dayananda M. A. , Allen L. H. 
Published: March 2011

ABSTRACTWe have studied interdiffusion and phase relations in two titanium aluminide/Nb/titanium aluminide composite structures at 1100 °C using solid state diffusion approach. The composite structures were prepared with two practical intermetallic alloys, γ (Ti-48Al-2Nb-0.3Ta, in at.%) alloy and super-α2 (Ti-25Al-10Nb-3V-1Mo, in at.%) alloy, and pure Nb sheet by diffusion bonding technique. The interfacial microstructures evolved during thermal diffusion annealing were characterized using optical microscopy, SEM, TEM and X-ray diffraction. The effect of Nb on the phase stabilities of two intermetallic alloys and the phase relations among different phases formed in the diffusion zone due to interdiffusion were analyzed in terms of rules of diffusion paths on the basis of the observed diffusion structures and experimental diffusion paths.
Growth of and Critical Current Density of TL2BA2CACU2O8 Superconducting Single Crystals
Authors: Duan H. M. , Kaplan T. S. , Dlugosch B. , Hermann A. M. 
Published: March 2011

ABSTRACTLarge single crystals of Tl2Ba2CaCu2O8 superconductors were grown by a two step self-flux process. Crystals are plate-like. The in-plane crystal sizes are as large as 5mmX5mm and the thickness range from 0.01mm to 0.8mm. The critical currents were extracted from I-V curves measured at different temperatures. jc was about 2.4×104 A/cm2 at liquid nitrogen temperature.
Enhancement of Flux Pinning Force by Ion Beam Irradiation of Epitaxial Ba2Cu3O7-δ Films
Authors: Siegal M. P. , Phillips Julia M. , van Dover R. B. , Gyorgy E. M. , White A. E. , Marshall J. H. 
Published: March 2011

ABSTRACTEpitaxial Ba2YCu3O7-δ (BYCO) films grown by the ex situ BaF2 process are comparable to single crystals both in crystalline integrity (RBS/ion channeling χmin < 3%) and the value and temperature dependence of the critical current (Jc) in an applied magnetic field in the BYCO (001) direction of Ha = 0.9T. With the appropriate dose of either 2 MeV H+ or 135Xe+, we are able to enhance Jc by a factor of 2 in H = 0.9T with little effect on Tc. The ability to change Jc by such a large factor in these films is a prerequisite for isolation and study of the induced defects and study their properties.
In-Situ Preparation of Y-Ba-Cu-O Thin Films Using Mass-Spectrometer Rate Control and Atomic Oxygen
Authors: Ohlsén H. , Ottosson M. , Hudner J. , ÖStling M. , Stolt L. , Nordblad P. , Villegrer J. -C. , Moriceau H. , Weiss F. , Thomas O. 
Published: March 2011

ABSTRACTThin films of YBCO were prepared using a mass-spectrometer controlled coevapora-tion/MBE process with atomic oxygen. Under the conditions of low pressure, necessary for mass-spectrometer rate control, it is shown that an atomic oxygen beam source can be utilized in order to grow high quality thin films of YBCO as well as multi-layer structures involving YBCO and Y2O3. Values of Tc = 88.5 K and Jc = 6–106 A/cm2 at 77 K were determined for a strip with a width of 10 μm of YBCO deposited on a LaAlO3 substrate. Film structure is analyzed by XRD and rocking curve measurements. Magnetic characterization of films and multi-layer structures are reported.
Low-Field Dc Josephson Magnetization of Layered Superconductors
Authors: Wang Hsang-Jue , Tai Ming-Fong 
Published: March 2011

ABSTRACTTemperature dependences of DC magnetization with low magnetic applied field (< 160 G) have been studied on layered superconductors. Two-step behavior below Tc are seen in most of the bulk samples, when the field is smaller than a few tens of Gauss. A grain-cluster model which taking both inter- and intragrain regions into consideration as two components of the present system is used to interpret the observed phenomena. The intergranular component is conjectured as coming from Josephson tunneling supercurrent between grains. A two-critical-states model including the surface-critical-state is extracted to outline the transition behavior of the shielding currents. The two-step phenomenon of the “bulk” magnetization is well explained with appropriate selections of the coupling strength and phase for the intergrains and of those critical current densities for the intragrains.
Target Surface Modification and Particulates on YBa2Cu3O7−x Thin Film by Pulsed Laser Deposition
Authors: Suh Jeong Dae , Sung Gun Yong , Kang Kwang Yong 
Published: March 2011

ABSTRACTThe modifications of the laser ablated target surface morphology and the particulates on YBa2Cu3O7−x (YBCO) thin films have been investigated. As the number of laser pulse per site increases, the ablated target surface changes to conical structures. After the texturing of the target surface, the ablated surface morphology changed identically, independent of the initial surface roughness of the polished targets. The YBCO thin films deposited using the stationary target have an almost particulate-free surface. However, in the case of the YBCO films deposited using the rotating target, there is a considerable increase in the density of particulates on the film surface.
Superoonductivity in the R3CaBa3Cu7Oy System with R = Lanthanide
Authors: Kao H. -C. I. , Wang J. L. , Huang R. J. , Wang C. M. , Wu M. K. 
Published: March 2011

ABSTRACTA series of samples with a nominal composition of R3CaBa3Cu7Oy (abbreviated as R3137), where R = Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er and Tm, was prepared by a similar process as for an 80-K superconductor, La3CaBa3Cu7Oy. Most of the R3137 (except for R = Ce, Tb) had a tetragonal triple-perovskite phase like La3137 identified from the XRD patterns, and they were superconducting except Pr3137. For R3+ radius larger than Dy3+ (0.908 Å), a single phase R3137 was observed, and their Tc(zero)' probably related to the rCu-o distance in the copper-oxygen layer, was spread from 85 to 76 K with a maximum for the Eu3137. Oxygen stoichiometry of these R3137 was slightly decreased with decreasing lanthanide radii. The hole concentration, (Cu-O), was found between 0.47 and 0.38/unit cell. For Smaller R3+ ions, residual resistivity was found in the superconducting samples due to the existence of second phase. From the structural study of La3137, it was known that part of the lanthanide ions had to be fitted into the bigger Ba-site. Because ionic radii of the heavy lanthanides (R3+ < Ho3+ = 0.894 Å) were too small to be located at the Ba-site, it was rather difficult to prepare these single phase R3137.
Crystal Growth Modified by Pulsed Laser Irradiation on Growing Surface in Ba-Y-Cu-O Film Preparation by Laser Ablation
Authors: Morimoto Akiharu , Mizukami Shigeru , Shimizu Tatsuo , Mlnamikawa Toshiharu , Yonezawa Yasuto , Segawa Kazuhito , Otsubo Shigeru 
Published: March 2011

ABSTRACTBa2YCu3Ox superconducting films were prepared by Nd:YAG laser ablation, equipped with a second (ArF excimer) laser for irradiation onto the growing film surface. The irradiation onto the film during the deposition were delayed for various delay times against the ablation of Ba2YCu3Ox target. The experiment showed that the second laser irradiation within several tens us around the ablation event induces a change of the crystal orientation. This result suggests that the crystal growth for the laser ablation is determined mainly around this time scale.
Semi-Coherent Yttria Inclusions Occurring Spontaneously in YBa2Cu3O6+× Films Grown by Dc Magnetron Sputtering
Authors: Selinder T. I. , Han Z. , Helmersson U. , Magnusson J. , Norling P. , Svedlindh P. 
Published: March 2011

ABSTRACTSemi-coherent Y2O3 (yttria) inclusions have been observed in high quality, c-axis oriented YBa2Cu3O6+×,(YBCO) Alms. The inclusions were studied by transmission electron microscopy (TEM), and were found to be embedded in the YBCO matrix without disturbing its structure seriously. Their number density is estimated to higher than 2 × 1016 cm−3 in epitaxial YBCO films, having transport critical current densities in excess of 1 × 106 A cm−2 at 77 K. Magnetization measurements indicate a possible correlation between a high density of yttria inclusions and high pinning strength. The effects of the inclusions on film growth and microstructure is discussed.
Critical Scaling of LöW Frequency Ac Resistivity in Superconducting Y-Ba-Cu-O Single Crystals
Authors: Reed D. S. , Yeh N. -C. , Jiang W. , Kriplanp U. , Holtzberg F. 
Published: March 2011

ABSTRACTAC transport measurements show that the vortex dynamics in a twinned superconducting Y-Ba-Cu-O single crystal are consistant with a second order vortex-solid melting transition. Indentification of the universal functions and consistent measurements of the critical exponents v = 0.7 and z= 3.0 found in magnetic fields from IkOe to 90kOe as well as DC transport measurements offer strong evidence for this transition.
Fabrication of YBa2Cu37/Ag Sputtered Multilayers
Authors: Colino J. , Vicent J. L. 
Published: March 2011

ABSTRACTThin films of YBa2Cu3O7/Ag multilayered structures have been grown by dc magnetron sputtering on MgO(lOO) substrates. The samples have been characterized by X-ray diffraction, scanning tunneling microscopy and resistivity measurements. Multilayers with YBCO layer thickness above 40nm show superconducting transitions and zero resistances higher than 70K. These layers show c-axis growth perpendicular to the substrate and the silver layers have preferential (111) orientation normal to the substrate too. Finally STM images reveal screw dislocation spirals at the YBCO layer surface even when the superconducting layer is grown on silver (111) underlayers.