bluuebunny/crossref_metadata_2025_split huggingface dataset:
An Investigation of Phase Equilibria in the Ternary Al-Ti-Ta System
Authors: Weaver M.L. , Guy S.L. , Stone R.K. , Kaufman M.J. 
Published: March 2011

ABSTRACTAn investigation of the phase equilibria and phase transformation behavior in the Al-Ti-Ta system was conducted with an emphasis on developing alloys with stable microstructures and properties desirable for advanced aerospace applications. A series of ternary alloys containing 5 to 50 atomic percent Ta and varying amounts of Ti and Al were prepared by arc melting and characterized in the as-cast and heat treated (1450°C) conditions. Three stable two-phase regions (β-σ, β-α2. and Al2 Ta-η) were identified and characterized at 1450°C. A stable ternary B2 phase was identified near the composition 25Al-5OTi-25Ta1 and several binary phases were found to exhibit extensive solid solubilities. The results were used to construct a partial isothermal section at 1450°C for this system and to assess the complex phase transformations that occur upon cooling.
Comparison of TEM Observations with Dislocation Core Structure Calcuiations in B2 Ordered Compounds
Authors: Farkas D. , Pasianot R. , Savino E. J. , Miracle D.B. 
Published: March 2011

ABSTRACTDislocation core structures have been calculated using atomistic computer simulations in NiAl and other B2 compounds. In the present work the calculated dislocation core structure are correlated with the known deforniatiorn behavior of B2 alloys. It is found that for the high ordering energy compounds <111> dislocations do not split in the simulations, in agreement with the experimental observations. It is also found that core structures for certain <111> and 1/2 <111> dislocations are spread in { 112} planes, which is consistent with the slip plane often reported for these dislocations. For the < 100> dislocations several orientations of the dislocation line produce sessile core configurations, whereas other orientations produce relatively more glissile cores. However, a structural transition of each of these dislocation cores may be required before < 100> dislocations become mobile, and this is consistent with the limited tensile ductility observed in NiAl “soft” single crystals below 200°C. Core structure simulations for < 110> dislocations are also reported and are discussed with respect to the importance of these dislocations in the deformation of NiAl.
Stress Induced Martensitic(SIM) Transformations In B2 NiAl Observed in Crack Propagation Computer Simulations
Authors: Kim Donghyun , Clapp P. C. , Rifkin J. A. 
Published: March 2011

ABSTRACTIn molecular dynamics studies of 10,000 atom arrays of stoichiometric B2 NiAl containing a crack under external stress in Mode I loading, it has been observed that a martensitic transformation generally occurs (starting in the vicinity of the crack tip) prior to the generation of dislocations and/or the propagation of the crack. The martensitic phase appears to be 2H, in agreement with experimental observations of SIM at higher Ni compositions (62 at % Ni). The interatomic interactions used in the simulations were the Embedded Atom Method (EAM) potentials developed by Voter and Chen [1] for the Ni-Al alloy system.
The Crystallography of Cleavage Fracture in Al3 Sc
Authors: Schneibel J.H. , Hazzledine P.M. 
Published: March 2011

ABSTRACTThe intermetallic compound Al3Sc is a trialuminide with the LI2 structure, which deforms easily in compression at room temperature, with yield stresses around 100 MPa. In tension Al3Sc fractures transgranularly in a brittle manner. The predominant cleavage plane is {011}. Regions, which are flat within experimental resolution are only occasionally observed. Numerous cleavage steps, which are aligned in 3 major crystallographic directions, are found. Some of these steps consist of {111} or {001} planes, but others are not distinctly crystallographic. Plastic deformation involving dislocation motion or twinning may have occurred at some of these steps. Reactions between different types of steps are also observed. One type of cleavage pattern found is strikingly similar to the typical appearance of fracture surfaces of fcc brass and Cu3Au after stress corrosion cracking. However, this particular pattern is only rarely observed in Al3Sc. Our observations indicate that an interpretation of cleavage fracture in Al3Sc in terms of surface energies alone is unlikely to be successful. A full understanding of the fracture morphology of Al3Sc will require detailed atomistic simulations taking dislocation motion and twinning into account.
Grain Boundary Structure as a Function of Aluminum Level in Ni3Al
Authors: Horton J.A. , Liu C.T. , Pennycook S.J. 
Published: March 2011

ABSTRACTUnderstanding the boron effect in Ni3Al currently centers on determining thestructure of the region near grain boundaries, especially the presence of a disordered γ phase. In this study, a series of alloys was examined by transmission electron microscopy (TEM) as a function of aluminum level, boron level, and cooling rate for thepresence of any grain boundary phases. The base alloy series all contained 0.5 at. % Hf. At 21.5 at. % Al,γ formed in the matrix as expected. However, in slowly cooled specimens with 22 and 22.5% Al, a second phase with thicknesses from 25 to 50 nm formed on some of the grain boundaries. At higher aluminum levels no evidence of any second phase was observed. The minimum width of grain boundary images formed by normal diffraction contrast imaging was generally 0.5 to 1 nm for grain boundaries that presumably had no second phase and were parallel to the electron beam. Therefore, a second phase with a thickness of 1 nm or less would not be discernable by TEM. High resolution Z-contrast imaging by scanning TEM of a very low angle boundary in directionally solidified (DS) material also showed no evidence of disordering, but did show a surprising amount of non-planarity of the boundary.Whereas slower cooling rates might be expected to aid the formation of any grain boundary phase and thereby increase the ductility, the ductility was slightly lower for the more slowly cooled alloys than for the furnace cooled alloys. From 21.5 to 24% Al, the tensile elongations remain fairly constant with values ranging from 45 to 52%. Above 24%, the ductility drops off rapidly but is still much higher for the boron-doped material than for the undoped material. At 25.2% Al, the ductility is still 4% as compared to essentially zero for undoped material. Therefore the ductility improvements in boron-doped Ni3Al do not require the presence of any grain boundary disordered phases discernable by TEM.
Interaction Between Lattice Dislocations and Grain Boundaries In Ordered Compounds
Authors: Pestman B.J. , De Hosson J. Th. M. , Vitek V. , Schapink F.W. 
Published: March 2011

ABSTRACTThe interaction of 1/2<1 1 0> screw dislocations with symmetric [1 1 0] tilt boundaries was investigated by atomistic simulations using many-body potentials representing ordered compounds. The calculations were performed with and without an applied shear stress. The observations were: absorption into the grain boundary, attraction of a lattice Shockley partial dislocation towards the grain boundary and transmission through the grain boundary under the influence of a shear stress. It was found that the interaction in ordered compounds shows similarities to the interaction in fcc.
CVM Simulation of the Resistivity in L10 and L12 Co-Pt Intermetallic Compounds
Authors: Sanchez J.M. , Cadeville M.C. , Pierron-Bohnes V. 
Published: March 2011

ABSTRACTThe contributions of the atomic and spin disorder to the resistivity of two intermetallic Co1−x Ptx (x=0.5 and 0.7) compounds have been previously determined from experimental studies and separated from the phonon contribution. Now, the atomic and magnetic contributions and their variation with temperature through the Curie and order-disorder transitions are analyzed using a statistical model. The components of the resistivity are described as sums of resistivities of individual tetrahedral clusters. The cluster probabilities are calculated in a magnetic (up and down spin model) CVM approximation, the parameters of which are those that reproduce the experimental Co-Pt phase diagram. The contribution of chemically (magnetically) ordered clusters to the resistivity due to the chemical (magnetic) disorder are taken equal to zero. It is the temperature dependence of all other cluster probabilities which determine the resistivity behavior. Although there are severe approximations in both the experimental analysis and the statistical model, the agreement between theory and experiment is very satisfactory, underscoring the strong interplay between chemical and magnetic order in such ferromagnetic compounds.
Effect of Aluminum Concentration And Boron Dopant on Environmental Embriitlement In Feal Aluminides
Authors: Liu C. T. , George E. P. 
Published: March 2011

ABSTRACTThe room-temperature tensile properties of FeAl aluminides were determined as functionsof aluminum concentration (35 to 43 at. % Al), test environment, and surface (oil) coating. The two lower aluminum alloys containing 35 and 36.5% Al are prone to severe environmental embrittlement, while the two higher aluminum alloys with 40 and 43% Al are much less sensitive to change in test environment and surface coating. The reason for the different behavior is that the grain boundaries are intrinsically weak in the higher aluminum alloys, and these weak boundaries dominate the low ductility and brittle fracture behavior of the 40 and 43% Al alloys. When boron is added to the 40% Al alloy as a grain-boundary strengthener, the environmental effect becomes prominent. In this case, the tensile ductility of the boron-doped alloy, just like that of the lower aluminum alloys, can be dramatically improved by control of test environment (e.g. dry oxygen vs air). Strong segregation of boron to the grain boundaries, with a segregation factor of 43, was revealed by Auger analyses.
Deformation of Polysynthetically Twinned (PST) Crystals of TiAl in Tension and Compression at Room Temperature
Authors: Inui H. , Nakamura A. , Yamaguchi M. 
Published: March 2011

ABSTRACTPolysynthetically twinned (PST) crystals of TiAl have been grown and deformed in tension and compression at room temperature. A room temperature tensile elongation of 12.8% which is far larger than any other reported values of room temperature ductility was obtained for a specimen whose tensile axis is inclined at 51° to the lamellar boundaries. Deformation mechanisms of PST crystals of TiAl are discussed on the basis of the results of electron microscope observations of deformation structures.
Mechanism of Ductility Improvement In L12 Compounds by Interstitial Elements
Authors: Chaki T. K. 
Published: March 2011

ABSTRACTInterstitial boron atoms are known to increase room temperature ductility in Ni-rich Ni3Al and Ni3Si. Inerstitial carbon atoms increase ductility in Ni-rich Ni3Si, but not in Ni3Al. It is argued that grain boundary segregation of B or C is not the cause of the increase in ductility. Instead, strong Ni-B bonding and antisite Ni defects reduce the strength of directional Ni-Al bonding in ordered Ni3Al so that Ni and Al atoms can relax more easily to fill up the microcavities at the grain boundaries, thereby strengthening the boundaries. The reduced Ni-Al bonding will also enhance the ductility in the interior of the grains so that dislocations can be easily generated to shield the tips of the cracks at the grain boundary. In Ni3Si, strong Si-C bonds distort the directionality of Ni-Si bonding. Evidence for this model is presented. Estimates of the concentration of interstitial atoms to achieve maximum ductility are made and the estimate agrees well with the experimental value.
A Comparative Study on the Influence of Nb and Ti Additions to Different Processed Atomized Nial Powders.
Authors: Laag Rainhard , Kaysser Wolfgang A. , Petzow GÜnter 
Published: March 2011

ABSTRACTPrealloyed powders of NiAl, NiAl-5Ti, NiAl-5Nb and NiAl-5Ti-5Nb (at.%) were gas atomized with an average particle size of 90 μm and cooling rates of > 104 K/s. The powders were attritor milled under Ar atmosphere reducing the average particle size to 1.5 μm. Sintering CIP compacts developed microstructure and density distributions, which allowed subsequent containerless HIPing to near full density (>98.5%) and a final grain size < 8 μm. Alloying of NiAl with 5at.% Nb or Ti increased its hardness, Young's modulus, toughness and creep resistance. The room temperature fracture path changed from primarily intergranular (NiAl) to primarily transgranular (Nb, Ti alloyed), increasing the Klc, values from 3.8 to 14.4 and 15.3 MPa√m, respectively. For comparison, HIPed materials from the as-atomized powders were also tested.
Improvements in Strength and Ductility of Feal-ZrB2 By Rapid Solidification
Authors: Morris David G. , Morris Maria A. 
Published: March 2011

ABSTRACTFe-35Al alloys containing various amounts of ZrB2 have been prepared by melt spinning and the microstructure and its stability examined. The mechanical properties are evaluated both on as cast materials as well as after high temperature heat treatments. The ZrB2 additions lead to a dramatic increase in hardness and strength. In addition, small amounts of ZrB2 lead to significant increases in ductility. Because the microstructure is fairly stable, these improvements in properties are maintained even after high temperature exposures.Alloys with up to about 1%ZrB2 have the dispersoid particles arranged in an imperfect cellular microstructure after rapid solidification. The cell walls contain many fine particles and are efficient barriers against dislocation propagation. Strain occurs as dislocations escape through gaps in the imperfect cell walls at a stress level that is controlled by the size of these gaps. Ductility is improved since the deformation that results from such controlled strain progression through the material is much more uniform and large stress and strain concentrations are avoided.
Cracking in the Weld Heat-Affected Zone of COntinuously Cast Sheet and Ingot of Ni3Al
Authors: Li Huaxin , Chaki T.K. 
Published: March 2011

ABSTRACTWeldability is a key issue for successful applications of ductile Ni3Al in structural components. Here we report the results of an investigation of gas tungsten arc welding behaviors of continuously cast thin sheet and cast ingot of a Ni3Al alloy, known as IC-50, containing boron and zirconium. Cracking in heat-affected zone (HAZ) was prevalent in both types of specimens. Both in cast sheet and ingot, liquation cracking was observed to occur along the grain boundaries in HAZ. Microstructural studies showed that continuously cast thin sheet consisted of columnar grains along the thickness. Easy cracking along the boundaries of columnar grains made cast sheets more susceptible to hot cracking than ingots. The possibility of segregation of boron and zirconium, causing liquation cracking, will be discussed.
The Structure and Properties of a Ti-25Al-10Nb-3V-1mo Alloy
Authors: Wang Wego , Scanlon Marietta R. , Wells Martin G.H. 
Published: March 2011

ABSTRACTSix super α2 titanium aluminide pancakes were produced by rapid omnidirectional compaction of prealloyed Ti-25Al-l0Nb-3V-lMo (at%) [Ti- 14.1Al-19.5Nb-3.2V-2Mo (wt%)] powder. Various solution and aging heat treatments were investigated to yield the best combination of tensile and stress rupture properties. The β transus temperature was found to be about 1,090 °C (1,990 °F). The alloy was heat treated with four different schedules to develop various structure combinations of α2 and β, and followed by a series of microstructural analyses and mechanical property determinations. The highest ultimate tensile strength and yield strength attained at room temperature were 1,174.9 and 977.7 MPa (170.4 and 141.8 ksi), respectively, for a specimen heat treated at 1,140 °C (2,084 °F) for 1 hr and aged at 816 °C (1,500 °F) for 4 hr + air cooling. However, the elongation was less than 2%. Under the same heat treatment condition, the ultimate tensile strength and yield strength measured at 427 °C (800 °F) were 1,139.1 and 711.6 MPa (165.2 and 103.2 ksi), respectively, with an elongation of 6.7%. The alloy also showed good stress rupture resistance. Compared with a similar hot isostatically pressed material, the current alloy shows superior mechanical properties due to a finer microstructure.
Development of Surface Treatment Techniques to Improve Oxidation Resistance of Titanium Aluminide
Authors: Yoshihara Michiko , Tanaka Ryohei , Suzuki Tetuya , Shimizu Masayuki 
Published: March 2011

ABSTRACTThe oxidation resistance of intermetallic compound TiAl was greatly improved in cyclic heating at 900°C by means of a new type of surface treatment, which is the heat treatment under a low partial pressure oxygen atmosphere. The treatment becomes more effective with increasing aluminum content in the specimen. It was presumed that the excellent oxidation resistance resulting from this method is due to preferential formation of a thin and tighit Al2 O3 surface layer, which is formed during the surface treatment at a temperature vicinity of 1000°C and air pressure of about 10−3 Pa. The oxidation resistance of Al2O3 film formed, however, was found to be insufficient at 950°C. A combined treatment consisting of the heat treatment under a low partial pressure oxygen atmosphere followed by a diffusion coating with aluminum showed an excellent resistance to cyclic oxidation even at 950°C.
Processing and Properties of Intermetallic Matrix Composites
Authors: Alman D.E. , Stoloff N.S. 
Published: March 2011

ABSTRACTThis paper is concerned with the processing and mechanical properties of intermetallic-matrix composites. The effects of processing variables on fabrication of compounds including Ni3Al, NiAl, TiTaAl2, MoSi2 and their composites is described. A key concern is with processing effects on microstructure, selection of compatible ceramic reinforcing phases, and whisker alignment through injection molding.
In-Situ Derivative Cyclic Voltabsorptometric Studies On Poly-3-Methylthiophene
Authors: Hoier Sally N. , Ginley David S. , Park Su-Moom 
Published: March 2011

ABSTRACTSpectroscopic behavior of poly-3-methylthiophene (P3MT) has been studied employing derivative cyclic volt-absorptometric (DCVA) techniques. In the DCVA technique, the derivative absorption signal (dA/dt) is recorded as a function of the applied potential. The dA/dt signals, the spectroscopic analog of electrochemical currents in cyclic voltammetry, are capable of monitoring the potential dependency for the absorption band effectively discriminating against nonfaradaic signals. The DCVA studies on the P3MT system show that the neutral form of P3MT, absorbing at 490 nm (at less than 0.3 V vs. Ag), changes to the radical cation form, which absorbs at 760 nm. Initially, the formation of the radical cation goes through an isosbestic point, indicating that the conversion of the neutral to radical (polaron) form is chemically reversible. However, upon increasing the electrode potential, the rate of the radical formation at 760 nm starts to decrease, with the formation of another band at about 1250 nm, attributable to a quinoid (bipolaron) form. This trend begins above about 0.6 V, shifting to a more positive voltage as the thickness of the film grows. This observation indicates that the electrochemical conversion of the neutral to radical form, followed by the quinoid form, is a slow process controlled by the diffusion of counter ions through the film. In-situ conductivity measurements as a function of applied potentials support the observed spectroscopic behavior.
Fracture Energy and Critical Strength of High Molecular Weight Glassy Polymers
Authors: Mikos Antonios G. , Peppas Nikolaos A. 
Published: March 2011

ABSTRACTThe fracture energy and the critical strength of glassy polymers with molecular weight larger than the critical value for the onset of chain entanglements are proportional to the number of chain segments entangled about a unit plane. A new molecular model is presented to calculate the crossing density of these chain segments when the segment length is a stochastic variable. The crossing density depends on the mesh size of the entanglement network and the number of entanglement network strands per unit volume. Theoretical predictions of the variation of the fracture energy and strength with the molecular weight are compared with experimental results for various glassy polymers.
Molecular Design of Stable Second Order Nonlinear Optical Polymers
Authors: Mandal Braja K. , Kumar Jayant , Tripathy Sukant 
Published: March 2011

ABSTRACTThe temporal and thermal stability of second order nonlinear optical properties of polymers is an important issue in the development of poly meric electro-optic devices. This has been addressed by introducing interchain crosslinking. A number of crosslinked systems developed by photochemical reactions have been presented.
Optical and Electro-Optical Properties of a Photocross-Linkable Polymer
Authors: Rahman A. K. M. , Mandal B. K. , Zhu X. F. , Kumar J. , Tripathy S. K. 
Published: March 2011

ABSTRACTPlanar waveguides from a photocross-linkable polymer have been fabricated on glass and Si0 2 on silicon using the spin coating process. The polymer has been cross-linked by exposure to UV radiation (λ = 254nm). Prism coupling technique has been used to couple a laser beam into the waveguide structure. The waveguiding parameters such as number of modes, loss, thickness, and index of refraction of the polymer have been determined before and after crosslinking. The refractive index of the polymer before cross-linking differs significantly from that after cross-linking. No anisotropy has been observed in the refractive indices for the uncross-linked or cross-linked samples. The refractive index and thickness of the polymer film before and after cross-linking have also been measured using an ellipsometer and found to be in very good agreement with those obtained by the prism coupling technique. Dye molecules with large second order hyperpolarizability were utilized as guests into the photocross-linkable polymer matrix for second order nonlinear optical applications. Electro-optic properties of this polymeric system are reported.
Carbon-13 Two Dimensional Chemical Shift Anisotropy Exchange Patterns in Blends of Polyphenylene Oxide and Polystyrene Near the Glass Transition
Authors: Inglefield P. T. , Jones A. A. , Wang P. , Zhang C. 
Published: March 2011

AbstractTwo dimensional solid state NMR exchange spectroscopy can be used to monitor slow motions on time scales comparable to dynamical mechanical spectroscopy. Polyphenylene oxide, carbon-13 labeled in a methyl position, was blended with polystyrene. The resulting compatible blends have single thermal glass transitions (Tg) at temperatures between those of the homopolymers. The chemical shift anisotropy line shape of the labeled methyl shows reorientational exchange at temperatures near the thermal glass transition. The 2D exchange pattern contains information on the rate and amplitude of the motion. The data indicate Brownian rotational angular diffusion of the phenylene oxide units with rates in the millisecond to second range. The motional description associated with the glass transition is contrasted with those appropriate to single component polymeric glasses. It is concluded that in the blend system the onset of motion occurs at a lower temperature relative to the DSC Tg than in the homopolymer and there is evidence for considerably more dynamic heterogeneity.