bluuebunny/crossref_metadata_2025_split huggingface dataset:
An Atomistic Study of Dislocations Controlling the Deformation Behavior of TiAl
Authors: Girschick A. , Vitek V 
Published: April 2011

AbstractAtomistic studies of dislocation cores in a model TiAl compound (L10) carried out using Finnis-Sinclair type many body potentials are presented. Two different core configurations, one planar and the other non-planar, were found for both 1/2〈110] ordinary dislocations and 〈101] superdislocations while a core containing a microtwin was found in the case of the l/2[112] dislocation. The impact of these core structures upon the plastic behavior or PST and single-phase TiAl crystals is discussed.
The Creep Deformation and Elevated Temperature Microstructural Stability of a Two-Phase TiAl/Ti3Al Lamellar Alloy.
Authors: Bartholomeusz M. F. , Wert J. A. 
Published: April 2011

AbstractEnhanced work hardening of the phases in the lamellar microstructure has been cited as an explanation for the lower minimum creep rates of a two-phase TiAl/Ti3Al lamellar alloy compared with the minimum creep rates of the individual TiAl and Ti3Al single-phase alloys tested between 980 K and 1130 K. This proposition is confirmed by TEM observations. Thermal and thermomechanical exposure result in the microstructural evolution, which increases the minimum creep rate (εmin) of the lamellar alloy. The effect of microstructural evolution on εmin will be discussed in the present paper.
In Situ Deformation in the T.E.M. on the two Phase Alloy Ti-24A1-9Nb First Results at Room Temperature
Authors: Legros Marc , Caillard Daniel , Couret Alain 
Published: April 2011

AbstractIn situ deformation experiments have been performed on a two-phase Ti-23.7Al-9.4Nb (%at) alloy at room temperature. Dislocation glide processes in each phase are studied and discussed. An exemple of dislocation transmission through an interface is analysed in detail.
Atomistic Simulation of Grain Boundaries of the Twin Limited Structure in Ni3Al
Authors: Turi Maria-Lynn , Zugic R. , Szpunar B. , Erb U. , Palumbo G. , Krstic V. 
Published: April 2011

AbstractEmbedded atom method molecular dynamics simulations of low Σ grain boundaries in Ni3Al are presented. The results show that the grain boundary plane has a larger effect on grain boundary energy than the Σ value, rigid body translations and stoichiometry. Assessment of the energies of Σ3n (n ≥ 1) grain boundaries in Ni3Al for various grain boundary planes indicates that only the Σ3 grain boundary is energetically preferred. The implications of this result for the development of the twin limited structure based on energetic considerations are discussed.
Diffusion Bonding Of Ti-48A1-2Mn-2Nb (at. %)
Authors: Godfrey S. P. , Threadgill P. L. , Strangwood M. 
Published: April 2011

AbstractThe diffusion bonding behaviour of Ti-48at.%Al-2at.%Mn-2at.%Nb has been studied as a function of temperature (in the range 1200–1350°C), time (15–45 minutes) and starting microstructure (lamellar, duplex and near γ) at constant bonding pressure of 10 MPa. It was found, that under the above conditions, small twin related γ grains, approximately 10–20 μm in size, nucleated at the original interface and grew into the matrix forming a double necklace grain structure. Particles of α2 were observed around the interface, the formation of α2 particles was believed to be related to oxygen partitioning and stabilisation effects from dissolved oxide films during the bonding process. Evidence for this mechanism was obtained from parallel electron energy loss spectroscopy (PEELS), which identified oxygen partitioning in the α2 particles. For the fully lamellar structure bonded at 1250°C for 45 minutes the failure strength of the bond was found to be 250 MPa, approximately 50 MPa lower than the failure strength of the base material.
Soft and Hard Modes of Deformation in Mosi2 Single Crystals
Authors: Ito K. , Yano T. , Inui H. , Yamaguchi M. 
Published: April 2011

AbstractThe deformation behavior of MoSi2 single crystals has been studied in the temperature range of -196<1500°C. While [001]-oriented crystals can be plastically deformed only above 1300°C, plastic flow is possible from temperatures as low as room temperature for single crystals with orientations other than [001]. Five slip systems, {110)<111], {011)<100], {010)<100], {023)<100] and {013)<331], are identified to be operative, depending on crystal orientation. An anomalous increase in CRSS is observed in the intermediate temperature range for the former three slip systems. Schmid’s law is generally valid for the soft modes, {110)<111], {011)<100] and {023)<100]. In contrast, the CRSS for the hard mode, {013)<331], strongly depends on crystal orientation with the higher values for orientations closer to [001].
Pip Processing, Microstructure and Properties of Si34N Fiber and Al2O3 Fiber Reinforced Silicon Nitride
Authors: Schwab Stuart T. , Page Richard A. , Davidson David L. , Graef Renee C. 
Published: April 2011

ABSTRACTPolymer infiltration/pyrolysis (PIP) processing has the potential to become an affordable means of manufacturing continuous fiber-reinforced ceramic-matrix components. The PIP method is very similar to the well-known polymer-matrix and carbon-carbon composite manufacturing techniques, the major difference being the use of a preceramic polymer in place of the organic polymer or carbon precursor. To date, the majority of research in the field of preceramic polymers has centered on precursors to silicon carbide (SiC). The Southwest Research Institute (SwRI) has focused on the development of polymeric precursors to silicon nitride (Si3N4) because its high-temperature strength, resistance to oxidation, and other properties make it an attractive candidate for many advanced high-temperature structural applications. PIP Si3N4 composites with NICALON SiC fiber reinforcement have exhibited good fracture toughness (KIC ∼ 16MPa·m1/ 2). We report here processing, microstructure and preliminary mechanical properties of two new PIP Si3N4 composites. One is reinforced with Tonen Si3N4 fiber (plain weave) while the other is reinforced with ALMAX Al2O3 fiber (8 Harness satin weave).
Processing and Properties of Some Alumina-Boride Composites
Authors: Edirisinghe Mohan J. 
Published: April 2011

ABSTRACTAlumina (Al2O3) test bars containing a small (5-10%) volume of titanium diboride (TiB2) or zirconium diboride (ZrB2) particles have been pressed and sintered (pressureless) in an argon atmosphere. The microstructure of the sintered bodies was characterized by X-ray diffraction and a range of microscopical techniques and shows that 3ppm (by volume) of oxygen present in the argon caused the boride particles in the surface regions of the test bars to oxidize during sintering, to a greater extent in the Al2O3-TiB2 composites. Mechanisms of oxidation are discussed. The boride particles retarded the densification of the composites, to a greater extent in the Al2O3-ZrB2 bodies. However, densification in the Al2O3-ZrB2 system was enhanced by sintering in an Ar-4% H2 atmosphere. The decrease in flexural strength due to the retardation of sintering has been overcome in both types of composite.
High Temperature Mechanical Properties of Al2O3/ZrO2(Y2O3)Fibers
Authors: Sayir A. , Farmer S. C. , Dickerson P. O. , Yun H. M. 
Published: April 2011

ABSTRACTIn-situ composite fibers produced by directional solidification of two phase oxide eutectics are one means of producing fibers with good strength and higher creep resistance than single crystal fibers. In this work, directionally solidified alumina-yttria stabilized zirconia eutectic fibers have been grown by the laser heated float zone (LHFZ) method at NASA Lewis. The average tensile strength of the alumina-zirconia (60.8 m/o Al2O3; 39.2 m/o ZrO2 (9.5 m/o Y2O3)) eutectic fibers was 1.2 GPa at room temperature. The high temperature tensile strength and creep resistance of the eutectic fiber were determined and compared to single crystal Al2O3.
Matrix Cracking In Brittle-Matrix Composites with Tailored Interfaces
Authors: Danchaivijit Sawai , Chao L-Y. , Shetfty D. K. 
Published: April 2011

ABSTRACTMatrix cracking from controlled through cracks with bridging filaments was studied in a model unidirectional composite of SiC filaments in an epoxy-bonded alumina matrix. An unbonded, frictional interface was produced by moderating the curing shrinkage of the epoxy with the alumina filler and coating the filaments with a releasing agent. Uniaxial tension test specimens (2.5 × 25 × 125 mm) with filament-bridged through cracks were fabricated by a novel two-step casting technique involving casting, precracking and joining of cracked and uncracked sections. Distinct matrix-cracking stresses, corresponding to the extension of the filamentbridged cracks, were measured in uniaxial tension tests using a high-sensitivity extensometer. The crack-length dependence of the matrix-cracking stress was found to be in good agreement with the prediction of a fracture-mechanics analysis that employed a new crack-closure force - crack-opening displacement relation in the calculation of the stress intensity for fiber-bridged cracks. The prediction was based on independent experimental measurements of the matrix fracture toughness (Kcm), the interfacial sliding friction stress (τ) and the residual stress in the matrix (σmI). The matrix-cracking stress for crack lengths (2a) greater than 3 mm was independent of the crack length and agreed with the prediction of the steady-state theory of Budiansky, Hutchinson and Evans[2]. Tests on specimens without the deliberately introduced cracks indicated a matrix-cracking stress significantly higher than the steady-state stress.
Strong Interface in CMCs, a Condition for Efficient Multilayered Interphases
Authors: Droillard Christine , Lamon Jacques , Bourrat Xavier 
Published: April 2011

ABSTRACTA fiber treatment was used to change the bonding strength of the Nicalon NLM 202 SiC fiber from weak to strong, in a series of 2D-SiC/SiC composites with multilayered interphases. The materials with the pre-treated fibers were compared to the same materials but reinforced with as received fibers. The stress-strain behavior and the fracture toughness were examined as a function of crack patterns identified by TEM. All the materials could be grouped into two distinct families: (i) materials reinforced with untreated fibers have a weak fiber bonding and are characterized by a low strength and a low toughness and (2) materials with the pre-treated fibers have a strong fiber bonding and are characterized by a high strength and a high toughness. This latter behavior is identified by TEM. It corresponds to a new interfacial behavior with a cohesive mode of interfacial cracking, involving branching and deflection by the successive interfaces. In the former family, the adhesive interfacial failure mode corresponds to the classical debond/sliding mechanism.
Interface Degradation in CAS/Nicalon During Elevated Temperature Aging
Authors: Plucknett Kevin P. , Cain Rebecca L. , Lewis M.H. 
Published: April 2011

ABSTRACTA CaO-Al203-SiO2 (CAS)/Nicalon glass-ceramic matrix composite has been subjected to elevated temperature oxidation heat-treatments between 375 and 1200°C, for up to 100 hours. Micro- and macro-mechanical properties have been determined by fiber push-down, using a mechanical properties microprobe, and flexure testing, respectively. Aging between 450 and 800°C results in significant property degradation, with reduced bending modulus and flexure strength, increased fiber sliding stress, and a transition to a purely brittle failure mode. Aging degradation is due to oxidative removal of the carbon interlayer, with the subsequent formation of a silica bond between fiber and matrix. At higher temperatures, carbon is retained due to the formation of a protective silica plug at exposed fiber ends, with the subsequent retention of composite properties. Short duration pre-treatment schedules, at 1000 or 1100°C, were developed to prevent intermediate temperature property degradation.
Use of CVD Ain Films as Protective Coatings in MoSi2 Composites
Authors: Cook Mark J. , Hudson John B. , Hillig William B. 
Published: April 2011

ABSTRACTIn many fiber reinforced composite systems, protective coatings on the fibers are required in order to protect the fiber from reaction with the matrix, to modify the mechanical properties of the fiber-matrix interface, or to protect the fiber during processing steps such as chemical vapor infiltration. We have developed techniques for coating single crystal sapphire fibers with thin AIN layers, using CVD from an organometallic precursor in a cold wall reactor. Side by side tests of coated and uncoated fibers introduced into a MoSi2 matrix, either by hot isostatic pressing or by reactive vapor infiltration (RVI), indicate that the coating inhibits both fibermatrix interdiffusion and volatilization of the fibers by HC1 produced in the RVI process.
Deformation Mechanisms in a Proton-Irradiated Austenitic Stainless Steel
Authors: Carter R.D. , Atzmon M. , Was G.S. , Bruemmer S.M. 
Published: April 2011

AbstractSamples of austenitic 304L stainless steel have been irradiated with 3.4 MeV protons to atotal dose of 1 dpa. The microstructure of the irradiated stainless steel has been quantified by transmission electron microscopy and shown to be similar to that found in neutron-irradiated core components. Constant extension rate tensile tests have been performed at strain rates of 3x10−7 s−1 and 3x10−8 s−1 to strains of up to 27% at 23°C and 288°C. The resulting microstructures were characterized using electron and optical microscopy. Deformation of the unirradiated material is similar to that reported by others in previous work on austenitic steels, consisting of dislocation source activation and formation of dense dislocation networks with increasing strain. In the irradiated samples tested at 288°C, deformation consists of dislocation source activation at grain boundaries punching dislocations through the grain interior to the opposing grain boundaries. The dislocations create channels that are free of radiation-produced defects and on which dislocation motion is concentrated. Dislocations in the channels pile-up at the boundaries, creating regions of highly localized stress concentration at the grain boundary. The mechanism by which this stress is relieved is still unknown. Deformation at 23°C consists of the nucleation and propagation of microtwins across the width of the grains.
Crystalline-To-Amorphous Transformation of Intermetallic Compounds in the Zr-Fe-M System Induced by Irradiation
Authors: Motta Arthur T. , Howe Lawrence M. , Okamoto Paul R. 
Published: April 2011

AbstractThe binary and ternary intermetallic compounds Zr3Fe, Zr2 Fe, (Zr0.5,Nb0.5)3Fe, Zr3(Fe0.9,Ni0.1) and Zr3(Fe0.5,Ni0.5) were subjected to 900 keV electron irradiation until amorphous to study the change in the dose-to-amorphization with temperature. The critical temperatures were observed to vary with dose rate, and with the type of compound. Hexagonal (Zr0.5,Nb0.5)3Fe had an appreciably lower critical temperature and higher dose to amorphization at low temperature than orthorombic Zr3Fe, whereas other orthorombic Zr3(Fex,NiI-x) compounds were essentially identical in behavior to Zr3Fe. The electron energy dependence of the dose-to-amorphization was studied in Zr3Fe between 250 and 900 keV. The analysis of the results gives displacement energies of EZrd = 26 eV, EFed = 18 eV in the Zr3Fe compound.
Ion Implantation of Iron into an Immiscible Copper Matrix
Authors: Murzin Ivan H. , Potter Donald I. 
Published: April 2011

AbstractFluences from 1×1017 to 9.6×1017 cm−2 of single and doubly charged iron ions accelerated through 170 kV were implanted into polycrystalline copper substrates at room temperature. An order of magnitude decrease in reflectivity was observed during implantation as a result of sputter-induced changes in surface topography and subsurface accumulation of iron. Both scanning electron microscopy and optical microscopy showed that the bombarded surfaces consisted of (110) and (100) grains having an intragranular, ridge-like structure. Significant height differences formed between grains as a result of their different sputtering yields. Transmission electron microscopy reveals that the implanted iron forms a face-centered-cubic metastable solid solution with the copper. Auger electron spectroscopy confirmed iron accumulation beneath the surface with maximum concentrations approaching in excess of 20 atomic percent.
Defect Production Mechanisms During keV Ion Irradiation: Results of Computer Simulations
Authors: Averback R.S. , Ghaly Mai , Zhu Huilong 
Published: April 2011

AbstractMD simulations have been employed to investigate damage processes during keV bombardment of metal targets. For self-ion irradiations of Au, Cu, and Pt in the range of 5-20 keV, we have found that both the amount and the character of the damage created in the surface depends sensitively on the details of the energy deposition along individual ion trajectories. In all of these cases, significantly more damage is produced and more atomic mixing takes place relative to corresponding recoil events in the crystal interior. In some cases, enormous craters are formed in an explosive event, while in others a convective flow of atoms to the surface leaves dislocations behind. The results of these simulations will be summarized and their significance for damage studies of ion irradiated materials, discussed.
Temperature and Dose Dependence of Metal Colloid Production in Alpha-Irradiated CaF2 Single Crystals
Authors: Weber W. J. , Exarhos G. J. , Wang L. M. 
Published: April 2011

AbstractSingle crystals of CaF2 were irradiated with 5.8 MeV alpha particles from a 244Cm source, and the absorbance spectra were recorded as a function of dose and temperature. Absorbance from the metal colloid band increases with dose at all temperatures. The colloid band exhibits a sharp increase in rate of growth over a narrow temperature range with a peak at 150°C. After a dose of 1.4 x 1016 α/cm2 at 150°C, the intensity of the colloid band is a factor of 7 to 8 higher than the intensity produced at 100 and 200°C. The optimum temperature (150°C) for colloid formation in CaF2 under the alpha-irradiation conditions (σ5 x 108 rad/h) of this study is significantly higher than the peak temperature (60°C) reported for colloid production in electron irradiated CaF2 at slightly higher dose rates (σ109 rad/h). Cryogenic transmission electron microscopy of the irradiated crystals reveals dislocation loops and small dark clusters as the dominant microstructural features.
Characterization of Collision Cascade Damage in Ca2La8(SiO4)6O2 by Hrtem
Authors: Wang L.M. , Webert W.J. 
Published: April 2011

AbstractCa2La8(SiO4)6O2 thin crystals become amorphous under ion beam irradiation. The ion dose required for complete amorphization of the thin crystal (critical amorphization dose, Dc) increased with the increasing irradiation temperature and decreased with ion mass at elevated temperatures. Samples irradiated with 1-1.5 MeV Ar+, Kr+ and Xe+ ions to doses much lower than Dc, in the temperature range from 20 to 498 K were used for a detailed HRTEM investigation to study the amorphization process. The residual collision cascade damage after irradiation appeared as nanometer scale amorphous domains. The images of these domains are extremely sensitive to the sample thickness. Small domains of cascade size were found only at the very thin edge of the sample. In thicker regions, amorphous domains appear after higher doses as the result of cascade overlap in projection. At higher temperatures, the observed amorphous domains are smaller indicating thermal recovery at the amorphous/crystalline interface. The amorphous domains are also larger in size after irradiation with ions of higher mass at a fixed ion dose. These results are consistent with the Dc-temperature curves determined by in situ TEM with the HVEM-Tandem Facility at Argonne National Laboratory. The width of the amorphous rim along the edge of the specimen grew with increasing ion dose suggesting that amorphization also proceeds from the sample surface. Images of the collision cascade damage were compared to the cascade sizes calculated with the TRIM code. Some digitally acquired HRTEM images of the cascade damage were processed to reveal more detailed information.
Cation Disorder Determined by MAS 27Al NMR in High Dose Neutron Irradiated Spinel
Authors: Cooper Elizabeth A. , Hughes Craig D. , Earl William L. , Sickafus Kurt E. , Hollenberg Glenn W. , Garner Frank A. , Bradt Richard C. 
Published: April 2011

AbstractSpinel (MgAl2O4) single crystals which had been neutron irradiated at (nominally) 400 and 750°C to high doses (53-250 dpa) were examined using 27Al magic angle spinning (MAS) nuclear magnetic resonance (NMR). The sensitivity of this procedure to a specific cation (Al) residing in different crystallographic environments allowed us to determine the distribution of the Al between the tetrahedral and octahedral cation sites in the spinel structure. Our results indicate that the Al was distributed nearly equally over both cation sites in the spinel, resulting in a nearly inverse structure.
Amorphization Threshold in Si-Implanted Strained Sige Alloy Layers
Authors: Simpson T.W. , Love D. , Endisch D. , Goldberg R.D. , Mitchell I.V. , Haynes T.E. , Baribeau J.-M. 
Published: April 2011

AbstractWe have examined the damage produced by Si-ion implantation into strained Si1-xGex epilayers. Damage accumulation in the implanted layers was monitored in situ by time-resolved reflectivity and measured by ion channelling techniques to determine the amorphization threshold in strained Si1-xGex, (x = 0.16 and 0.29) over the temperature range 30-110°C. The results are compared with previously reported measurements on unstrained Si1-xGex, and with the simple model used to describe those results. We report here data which lend support to this model and which indicate that pre-existing strain does not enhance damage accumulation in the alloy layer.
Point Defects in Gallium Arsenide Characterized by Positron Annihilation Spectroscopy and Deep Level Transient Spectroscopy
Authors: Mih R. , Gronsky R. 
Published: April 2011

AbstractPositron annihilation lifetime spectroscopy (PALS) is a unique technique for detection of vacancy related defects in both as-grown and irradiated materials. We present a systematic study of vacancy defects in stoichiometrically controlled p-type Gallium Arsenide grown by the Hot- Wall Czochralski method. Microstructural information based on PALS, was correlated to crystallographic data and electrical measurements. Vacancies were detected and compared to electrical levels detected by deep level transient spectroscopy and stoichiometry based on crystallographic data.
Monte Carlo Simulations for Bcc Alloys Under Irradiation: Phase Stability and Microstructural Evolutions
Authors: Soisson F. , Bellon P. , Barbu A. , Martin G. 
Published: April 2011

AbstractWhen a compound is maintained in far for equilibrium configurations by nuclear collisionsunder irradiation, the steady-state properties of the system can no longer be predicted from equilibrium thermodynamics. Here we propose Monte Carlo simulations for addressing the question of phase stability under irradiation. They are based on a kinetic model with two dynamics acting in parallel: thermally activated jumps of vacancies and ballistic events induced by nuclear collisions. Two transformations are studied: the A2-B2 order-disorder transition and the precipitation of copper in iron. In the former case a shift from second to first order of the A2-B2 transition, predicted by the model, has been experimentally checked by I MeV electron irradiations of a FeA1 alloy. In the latter case, the precipitation kinetics are determined by Monte Carlo simulations and are found to be in very good agreement with available experimental data.