bluuebunny/crossref_metadata_2025 huggingface dataset:
The Influence of Strain Rate on Dislocation Structures in γ/α2 Titanium Aluminides Deformed at 815°C
Authors: Donlon W.T. , Dowling W.E. , Allison J.E.
Published: March 2011

Elevated temperature (815°C) compression tests were performed at strain rates between 10−2 & 10−5 sec−1 on a γ/α2 alloy, Ti-48Al-1V-0.2C (at%), for two different α2 volume fractions. A large change in the flow stress was observed to occur for strain rates between 1.10−5 & 1.10−3 sec−1. The dislocations observed in the primary γ grains deformed at the high strain rates were “regular” 1/2<110]; screw dislocations. The dislocations observed at slow strain rates were also regular 1/2<110] dislocations, however their orientation was changed so that they were aligned close to [001]. Dislocation structures produced during high cycle fatigue are identical to those observed during high strain rate compression tests, while dislocation structures produced during tensile (ė- 8.10−4 sec−1 ) tests are identical to those observed in the slow strain rate compression tests. Analysis of the orientation of deformed primary γ-TiAl grains shows that the CRSS for {111}<101] slip must be at least 1.4 times the CRSS of {111}1/2<110] slip.
Deformation and Ductility in Alloys Based on Al3Ti
Authors: Morris David G. , Lerf Reto
Published: March 2011

Titanium trialuminide alloys suffer problems of intense segregation during solidification and extreme brittleness when subjected to tensile stresses. In this study, spray forming techniques have been used to obtain homogeneous materials of fine microstructure, containing various second phase particles, and the deformation behaviour of these materials examined over a range of temperatures.

The microstructure and deformation behaviour of an iron-modified titanium trialuminide with the L12structure is reported here.Deformation mechanisms are interpreted by analysis of dislocation structures and further confirmation is obtained by activation volume measurements: the importance of Peierls effects at low temperatures and cross slip at high temperatures is shown. The material remains brittle in tension both at room temperature and at high temperatures: the second phase particles in the material are yet not present in sufficient volume fraction to greatly affect plastic behaviour.
Energetics and Structural Effects of Boron Additive to Intermetallic Compound: γ-Tial
Authors: Khowash P.K. , Price D.L. , Cooper B.R.
Published: March 2011

Improving the low temperature ductility of the intermetallic compound γ - TiAl by alloying with small concentrations of an additive is of great practical importance. The difference in site selection energy of the additive plays an important role in the stability and behavior of the alloy. For boron in L10 TiAl, we have calculated the site selection energy using linearized combination of muffin-tin orbitals (LMTO) total energy calculations. For pure γ - TiAl, we found the equilibrium lattice structure by minimizing the total energy, and obtained good agreement with the experimental values. With the introduction of boron, a relaxation of the lattice around the boron additive is expected. For boron additives, we have calculated the forces on each atom leading to the minimization of the total energy as a function of the ionic positions in order to obtain the “true” stable structure of the alloy.
Computer Simulation of ao<110> Screw Dislocations in Ni3Al
Authors: Parthasarathy T.A. , Dimiduk D.M. , Woodward C. , Diller D.
Published: March 2011

Dissociation of the ao<110> screw dislocation in Ni3Al was studied using the embedded atom method of computer simulation. The dissociation occurred predominantly along the {111} plane, however, a {001}-plane step occurred in the APB at the center of the configuration. When a pair of ao/2<110> superpartials initially separated in the {111} plane was relaxed, the step formed once again but with a reduced height. When the pair was relaxed from larger distances the step was not formed. The results indicate that the elastic interaction “torque” due to elastic anisotropy is responsible for the formation of the {001} APB step. When a stress was applied to these dislocation configurations by simulation, results confirmed that the step in the APB and the octahedral cross-slipped-core dissociations can be significant barriers to glide of the screw dislocation.
Deformation Mechanisms in TiAl-Based Alloys Containing Low Oxygen
Authors: Sriram S. , Vasudevan Vijay K. , Dimiduk Dennis M.
Published: March 2011

The effects of oxygen on the deformation behavior of Ti-(48-52)Al alloys is reported. Two types of studies were conducted. In the first, high purity alloy buttons containing low oxygen (~250 ppm) were prepared, whereas in the second, alloys with additions of 1 at.% Er to scavenge the oxygen from the matrix were prepared. The alloys were heat treated to produce large grains and the microstructures characterized by analytical electron microscopy. Samples prepared from the heat treated alloys were electropolished and deformed in compression to a plastic strain of 1.0-1.5% at temperatures between 25 and 800°C and the yield stress measured. The morphology of deformation, that is, slip lines and the presence of twinning, was studied by optical microscopy and the dislocation structures were characterized by weak-beam imaging in the transmission electron microscope. The results of these various studies are presented and discussed in terms of recent developments regarding the factors that appear to control the dislocation structure and the mobility of dislocations.
The Effect of Grain Boundary Geometry on Intergranular Cracking in Ni3A1 at Room Temperature
Authors: Lin Hui , Pope David P.
Published: March 2011

The relation between intergranular fracture and grain boundary character was studied in Ni3Al using the electron backscattering pattern (EBSP) technique. A new and improved set of experiments have been performed in which the problems of forced crack propagation and statistical validity of numerical results encountered in the earlier study were eliminated. It was found that low angle and σ3 boundaries do have a substantially lower propensity for cracking as compared to other boundaries, but there is no correlation between grain boundary σ value and the propensity for cracking for low σ boundaries. These results are in general agreement with recent theoretical predictions of grain boundary structure.
The Microstructure of Titanium Beryllide, Tibe12
Authors: Banerjee D. , Jacobson L. , Zindell J. , Mitchell T.E.
Published: March 2011

Titanium beryllide (TiBe12) has been processed by various techniques, including inert gas arc melting, sputter deposition and as second phase particles in atomized powder of alloys ranging in composition from 1 to 35 weight percent Ti. The TiBe12 that formed in the various alloys was characterized by transmission electron microscopy. Defect structures such as microtwinning, antiphase domain boundaries and dislocations have been observed within the beryllide phase. Descriptions of these defect structures will be presented in this paper, together with a discussion of their origin.
Temperature Effects on the Strength and Toughness of Be12Nb
Authors: Bruemmer Stephen M. , Arey Bruce W. , Jacobson Rodney E. , Henager Charles H.
Published: March 2011

Strength and toughness of single-phase Be12Nb have been assessed at temperatures from ambient to 1200°C using four-point-bend and hardness tests. Vacuum-hot-pressed beryllide exhibits low bend strengths (∼110 MPa) and fracture toughnesses (∼2.5 MPa√m) at temperatures up to 500C. Measured bend strength then increases to a maximum at 1000°C, while KlC slowly increases through 1100 C. Significant plastic deformation is only observed at 1200 C for the test strain rate of 2×10−6/s. Hardness values were found to remain relatively constant at temperatures up to 800 C and then sharply decrease at higher temperatures. Results indicate that low toughness limits bend strength at temperatures below ∼1000 C and prompts the observed increase in strength at high temperatures.
Compression, Bend, and Tension Studies on Forged Al67Ti25Cr8 and Al66Ti25Mn9 L12 Compounds
Authors: Kumar K.S. , Brown S. A. , Whittenberger J.D.
Published: March 2011

Cast, homogenized, and isothermally forged aluminum-rich L12 compounds Al87Ti25Cr8 and Al66Ti25Mn9 were tested in compression as a function of temperature and as a function of strain rate at elevated temperatures (1000K and 1100K). Three-point bend specimens were tested as a function of temperature in the range 300K to 873K. Strain gages glued on the tensile side of the ambient and 473K specimens enabled direct strain measurements. A number of “buttonhead” tensile specimens were electro-discharge machined, fine polished, and tested between ambient and 1073K for yield strength and ductility as a function of temperature. Scanning electron microscope (SEM) examination of fracture surfaces from both the bend and tensile specimens revealed a gradual transition from transgranular cleavage to intergranular failure with increasing temperature.
Mechanical Behavior of Single Crystalline A166.8Ti27.4Fe5.8
Authors: Wu Z.L. , Pope D.P. , Vitek V.
Published: March 2011

The operating slip systems and flow behavior of single crystalline A166.8Ti27.4Fe5.8 having the L12 structure were investigated at elevated temperatures using specimens with compressive axes near [001], [113], [112], [013] and [133]. Plastic deformation was carried exclusively by octahedral slip at low temperatures, and by both octahedral and cube slip at high temperatures. The yield stress is roughly temperature-independent at intermediate temperatures, but changes rapidly with temperature at high and low temperatures. At 770°C fracture occurs by cleavage on {011}, {001}, {013} and {111} planes. Al2Ti and AlTi3 were found in the single crystal matrix. The effects of these second phases on the mechanical behavior are not the same: Al2Ti does not embrittle the material but AlTi3 does.
The Role of Twinning in Brittle Fracture Of Ti-Aluminides
Authors: Yoo M. H. , Fu C. L. , Lee J. K.
Published: March 2011

The important roles of (111)[112] twinning in cleavage fracture of single-phase γ-TiAl and TiAl3 are assessed on the basis of theoretically calculated elastic constants, shear fault energies, and cleavage energies. The importance of elastic anisotropy in both homogeneous twin nucleation theory and dislocation models for twin nucleation is emphasized. The twin-slip conjugate relationship makes an important contribution to the strain compatibility for localized plasticity at a crack tip of Mode-I type. The intrinsic brittleness of these aluminides is attributed to a combined effect of the low mobility of slip and twin dislocations and the relatively low cleavage strength. The effect of the resolved normal stress on the twin plane is to enhance twinning activity at a (110) crack tip, leading to transformation toughening of shear type.
Does A Threshold Stress For Creep Exist in Hfc-Dispersed Nial?
Authors: Daniel Whittenberger J. , Ray Ranjan , Jha Sunil C.
Published: March 2011

Recently it was proposed [S.C. Jha, et al., Mater. Sci. & Eng. A119 (1989) 103 & J.D. Whittenberger, et al., J. Mat. Sci. 25, (1990) 2771] on the basis of constant velocity testing at 1300 K that dispersion strengthened NiAl composites containing about 4 wt pct HfC possess threshold stresses for creep. Further 1300 K compression testing has been conducted on NiAl+4HfC,and diametrically opposite behavior has been found: for constant load creep tests normal power law behavior was observed (strain rate proportional to the stress raised to the power n); however additional constant velocity testing still indicates that the flow stress is essentially independent of strain rate below 10−6 s−1. Examination of NiAl+4.3HfC specimens after being deformed under constant velocity conditions revealed that the original hot extruded small grain structure could be converted to large, elongated grains during testing. Such a transformation appears to be responsible for the apparent threshold stress behavior in HfC dispersed NiAl.
Effect of Off-Stoichiometry on the Creep Behavior of Binary and Ternary Ni3Al
Authors: Hayashi Tohru , Shinoda Tetsumori , Mishima Yoshinao , Suzuki Tomoo
Published: March 2011

Compressive creep behavior is investigated In polycrystalline Ni3Al intermetallic compounds in which the focus is put on the effect of offstoichiometry and ternary additions of Ti, Ta, V and Co on the creep resistance of the compounds. In all cases, the steady-state creep rate has the power-law type dependence on the applied stress. The stress exponent thereby obtained leads us to regard the creep behavior of the compound to be of the Class I type. It is shown that the creep resistance Increases with increasing Ni concentration on both sides of the stoichiometric Ni3Al composition and a discontinuity exists in the variation at stoichiometry. The gap at the discontinuity becomes clearer by the addition of ternary element in most cases as compared to the binary Ni3Al. Although the exact reason for the phenomenon is not clearly understood at present, it is shown that it could be interpreted by the concentration dependence of activation energy in the power-law type creep equation.
Ternary Alloying of Refractory Intermetallics
Authors: Shah D. M. , Anton D. L.
Published: March 2011

One of the most attractive attributes of intermetallics is their potential for alloying to achieve the desired balance of engineering properties via the creative use of in-situ composite processing. Following our initial investigation of 20 binary refractory intermetallics, selected primarily on the basis of structure type; seven systems were down selected for ternary alloying, based on a balanced consideration of room temperature toughness, high temperature creep strength, oxidation resistance and ultimate strength. These systems were Nb3Al and Cr3Si with A15 structure, CO2Nb and Cr2Nb Laves phases with C14/C15 structure, MoSi2 with C11b structure, and Nb2Al and Mo5Si3 sigma phases.

The alloying potential of each of these binary systems is evaluated with respect to solubility limits, phase relationships, melting point suppression with alloying, and potential for enhancement of creep resistance. The results show that, with alloyed intermetallics, realistic creep strength is attainable and several opportunities exist for in-situ processing of intermetallic based composite systems to improve room temperature toughness.
Improvement of Room Temperature Ductility of Ni3Al by Unidirectional Solidification
Authors: Hirano Toshiyuki , Chung Sea-Sung , Mishima Yoshinao , Suzuki Tomoo
Published: March 2011

We present the new promising method to improve the ductility of intermetallic compound, Ni3Al. Stoichiometric and Alrich Ni3A1 were unidirectionally grown at the growth rate of 24 mm/h by a floating zone method (FZ-UDS). The stoichiometric Ni3Al had a columnar-grained and single phase structure, whereas the Al-rich Ni3Al had a Ni3Al matrix with martensite-like precipitates. These alloys exhibited a large tensile elongation along the growth direction at room temperature. Fracture mode is a complete transgranular fracture in the stoichiometric Ni 3Al alloy, and a mixture of transgranular fracture of the matrix and cleavage of the martensite-like precipitates with cracking at the phase boundary in the Al-rich Ni3Al alloys.
Microstructure and Mechanical Properties of Mechanically Alloyed NiAl
Authors: Hwang S. J. , Nash P. , Dollar M. , Dymek S.
Published: March 2011

Mechanical alloying (MA) has been used to produce NiAl powders from either elemental or prealloyed constituents. The powders were consolidated by hot extrusion resulting in material which was fully dense, with a grain size around 1 μm and a homogeneous distribution of oxide particles with sizes in the range 10 to 100 nm. TEM observation indicates the presence of a significant dislocation density after consolidation. Mechanical properties have been studied by compression testing from room temperature to 1300 K in air. Yield strengths ranged from 1453 MPa to 32 MPa depending on material and test temperature. Work hardening was observed at all test temperatures for both materials. Substantial ductility was observed even at room temperature where it exceeds 7.5 %. The effects of microstructure on the mechanical properties are discussed.
The Mechanical Properties of A Ni-30Al-2OFe-0.05Zr Intermetallic Alloy in the Temperature Range 300–1200 K.
Authors: Raj S. V. , Noebe R. D. , Locci I. E.
Published: March 2011

Tensile tests were conducted on an extruded Ni-30(at.%)Al-20Fe-0.05Zr intermetallic alloy in the temperature range 300–1200 K and the microstructures were characterized by optical, scanning and transmission electron microscopy. The alloy did not exhibit any room temperature ductility and failed by transgranular cleavage at a stress of about 710 MPa. This mode of fracture was observed at and below 873 K with the total ductility being less than 2.5%. However, the failure mode changed to dimpled fracture, triple point cracking and rupture above 873 K. Scanning electron microscopy of the fracture surfaces revealed that transgranular cleavage failure had always originated at pre-existing defects. In these cases, the fracture stress exhibited a good correlation with the defect size in accordance with linear elastic fracture mechanics, where the critical stress intensity factor was estimated to be about 4 MPa m1/2.
Structure/Property Relationships in a Rapidly Solidified and Annealed Ternary Iron Aluminide
Authors: Décamps B. , Gibsont M.A. , Morton A.J. , Wolfenden A.
Published: March 2011

Rapidly solidified strips of Fe66.5-A128.5-Cr5 alloy with the addition of 0.5wt.% TiB2, were produced by planar flow strip casting. Correlations of the microstructure and room temperature mechanical properties were made for strips in the as-cast condition and after annealing at 1273K for periods up to 4hrs. The results showed that grain size, tensile strength, ductility, hardness and Young's modulus were very stable for the times investigated. Studies of tensile fracture surfaces revealed essentially 100% transgranular cleavage in the ascast strip with a greater tendency for intergranular failure after prolonged exposure to high temperatures. TEM studies of this alloy revealed diffraction patterns characteristic of DO3 ordering but the dislocations observed in both ascast and deformed specimens were those typical of the B2 structure without any extended APBs. This is attributed to the very fine DO3, domain size . At room temperature <111> slip is predominant.
High Temperature Measurements of Neutron Diffuse Scattering in a FeAll9.Sat% Single Crystal. Influence of Magnetism on Chemical Order.
Authors: Pierron-Bohnes V. , Cadeville M.C. , Schaerpf O. , Finel A.
Published: March 2011

The influence of magnetism on atomic short range order is shown in a single crystalline FeAl19.sat% alloy in the disordered phase. The short range order has been measured in the paramagnetic state between 973 and 1573 K on D7 at ILL through neutron diffuse scattering in the [110] plane. The effective pair interaction potentials V1 , V2, V3 and V5 are calculated through an inverse cluster variation method. The results are compared to those obtained through X-ray diffuse scattering on the same alloy frozen in the ferromagnetic order state at 772 K. The effect of magnetic coupling on the atomic order is the most important on the 1st effective potential, which is smaller by about a factor 2 in the ferromagnetic phase than in the paramagnetic one.
Electronic Structure of Planar Faults in Tial
Authors: Woodward C. , Maclaren J. M. , Rao S.
Published: March 2011

The mechanical behavior of intermetallic alloys is related to crystal bonding and the influence of bonding on the core structure of dislocations formed in these compounds. These combined effects influence the deformation behavior in an, as yet, undefined manner. However, in a way that gives rise to unusual behavior, such as the anomalous temperature dependence of flow stress observed in TiAl. Recent studies have suggested a particular relationship between the directional bonding in TiAl and dislocation mobility. To better understand the flow behavior of intermetallics, and as a beginning toward bridging the gap between electronic structure and flow behavior, we have calculated the electronic structure of various planar faults in TiAl. The self consistent electronic structure has been determined using the layered Korringa Kohn Rostoker (LKKR) method which embeds the region containing the defect between two semi-infinite perfect crystals. Calculated defect energies agree reasonably well with other theoretical estimates, though overestimating experimental values. The changes in bonding taking place in the vicinity of the planar defect will be discussed and illustrated through the density of states and charge density plots.
Structure Dependence of Tensile Creep Behavior in Thermo-Mechanically-Treated Ti-50Mol%Al Intermetallics
Authors: Takahashi Tohru , Oikawa Hiroshi
Published: March 2011

Tensile creep properties have been studied in Ti-50mol%Al intermetallics prepared by ingot metallurgy. Four types of structure were obtained in as-cast material (C), homogenized material (CA), isothermally forged material (F) and forged-and-recrystallized material (FA). Structural effects was investigated by comparing creep properties such as minimum creep rates, fracture strains and creep life among C, CA, F, and FA materials. Creep characteristics were strongly dependent upon the structure.
High-Temperature Deformation Behavior of the γ Alloy Ti-48Ai-2Cr-2Nb
Authors: Shih Donald S. , Scarr Gary K.
Published: March 2011

The hot-workability of a two-phase (γ+α2) alloy, Ti-48A1-2Cr-2Nb, has been studied by conducting isothermal compression tests to 0.8 true strain over the temperature range of 975–1200°C at strain rates between 1×l0−1 and 3×10−3s−1. A deformation map showing temperature, strain rate, soundness of deformation, and isostress contours was constructed. Good workability is found from the low temperature/low strain rate regime to combinations of high temperature and either high or low strain rate. The upper-limit flow stress for good workability is between 450 and 500 MPa. Deformation induced softening occurs at all conditions. SEM and TEM examinations of the deformed specimens reveal that non-uniform deformation takes place at all strain rates, but cracking occurs mostly at high strain rates (e.g. 1×10−1s−1), especially combined with low temperatures. The cracking appears to progress primarily along γ/α2interfaces. It is thought that non-uniform deformation develops channels of shear bands, which in turn promote localized recrystallization, thus accommodating higher strains.
Preliminary Indications of a Slight Improvement in the Ductility of Ni3Ge With Boron
Authors: Fang J. , Schulson E. M.
Published: March 2011

Preliminary work was conducted to investigate whether 0.1 at% boron affects the ductility of Ni-rich Ni3Ge (Ni-23.5 at%Ge). Cracks were observed in the unmodified alloy around hardness indentations and on fresh surfaces upon abrasive cutting. No cracks appeared in the modified alloy. Also, SEM fractographs showed that with the boron addition the morphology changed from pure intergranular fracture to a mixture of intergranular and transgranular fracture, albeit predominantly intergranular. Tensile tests indicated that the boron addition slightly improves the ductility of Ni-rich Ni3 Ge.
Cavitation-Erosion Resistance of Thick-Film Thermally Sprayed Niti
Authors: Peter Jardine A. , Horan Y. , Herman H.
Published: March 2011

Thermal spray technologies provide a method for depositing high temperature intermetallics. Deposits of Shape Memory Effect NiTi were produced by Electric-Arc Spraying (EAS) of NiTi wire, Vacuum Plasma Spraying (VPS) of NiTi powder, and codeposition of Ni and Ti using VPS. NiTi deposits displayed shape memory effect properties, which are highly sensitive to processing conditions. They were found to also exhibit superior cavitation-erosion resistance. Under identical cavitation erosion conditions, weight losses of 0.41, 1.65 amd 7.71 mg were found VPS, arc-sprayed and blended Ni-Ti deposits respectively. Minimal amounts of NiTi was formed from co-spraying both Ni and Ti powders onto steel substrates.
Structures and Properties of Gamma-Tial Alloys Containing Interstitial Elements
Authors: Huang S.C. , Hall E.L.
Published: March 2011

This paper reports on the effects of carbon, nitrogen, boron, and oxygen on the microstructure and properties of gamma-TiAl alloys. It has been found that nitrogen and boron are capable of altering the solidification structure, whereas the others are not. The additions of >1 at.% B or 0.5 at.% N changed the cast structure from columnar to equiaxed with fine grain sizes. As a result, the strength has been significantly increased. Additional results on phase and structural stabilities will be presented.
Effect of APB Microsegregation on the Strength of Ni3Al with Ternary Additions
Authors: WU Y.P. , Sanchez J.M. , Tien J.K.
Published: March 2011

Solid solution strengthening for stoichiometric and non-stoichiometric alloys is studied using a microscopic model that takes into account the micro-segregation of the alloying additions and of the host atoms to antiphase boundaries. Experimental results for four ternary intermetallic compounds, Ni-Al-Si, Ni-Al-Ti, Ni-Al-Hf and Ni-Al-V, are analyzed in the light of the theoretical model. It is found that the athermal increment of yield strength for both stoichiometric and non-stoichiometric alloys can be explained using solid solution strengthening theory provided that the local concentration at the {111} antiphase boundaries between superpartial dislocations is used.
Homogenization of Ingot Metallurgy Near-Gamma Titanium Aluminides
Authors: Semiatin S.L. , Mcquay P. , Stucke M. , Kerr W.R. , Kim Y-W. , El-Soudani S.
Published: March 2011

The elimination of microsegregation and subsequent breakdown of the lamellar structure in near-gamma titanium aluminides was investigated through a series of high temperature homogenization heat treatments and isothermal hot compression tests. The homogenization heat treatment studies revealed that microsegregation could be eliminated after approximately five hours or one and one-half hours at temperatures either 15 C or 40 C, respectively, above the transus in the single phase alpha field. Subsequent attempts to breakdown the coarse prior alpha grain, lamellar structure were successful at low strain rates (∼ 10−3 s−1) and temperatures low in the alpha + gamma phase field, but led to wedge cracking and gross fracture at high strain rates (∼ 1 s−1) high in the alpha + gamma phase field. Subsequent annealing treatments to globularize the hot-worked structure in order to obtain a uniform, fine, and stable grain structure were also identified.
Tensile Fracture Behavior of a Cast Gamma-Titanium Aluminide
Authors: Seetharaman V. , Goetz R. L. , Semiatin S. L.
Published: March 2011

The fracture behavior of a cast gamma-titanium-aluminide alloy (Ti-49.5A1–2.5Nb-l. 1Mn) during deformation processing was investigated using hot-tension testing and isothermal-forging experiments. In uniaxial tension, fracture ductility increased with an increase in the test temperature and a decrease in the imposed strain rate. During open-die forging of this alloy under isothermal conditions, tensile stresses generated due to frictional effects and the associated inhomogeneous plastic deformation led to free-surface fracture via intergranular cracking. The failure observations under these two deformation modes were correlated successfully using a tensile-damage criterion.
High Temperature Corrosion Behaviour of Intermetallic Phases based on Ni3Al
Authors: Brill U. , Klöwer J.
Published: March 2011

Nickel aluminides based on Ni3Al were continuously cast into 12.4 mm diameter wire in an industrial production machine. Cyclic corrosion tests were carried out on this material in oxidizing, carburizing, sulfidizing and chloridizing atmospheres at temperatures between 750°C and 1100 °C in order to study the time and temperature dependence of high temperature corrosion. The results show, that Ni3Al has a very good oxidation resistance up to 1000 °C. The sulfidation resistance is poor, while resistance against carburization and chlorination is significantly better than that of the commercial alloy 800H andalloy 617.
Crystallization and Matrix Effects on the Electronic Properties of Polymers Doped with CT Salts of TCNQ
Authors: Fann Yang-Cheng , Jansen Susan A. , Hutchins Marygail
Published: March 2011

Molecularly doped polymer films have shown potential for applications in electrophotography and molecular electronics. The electrooptical and magnetic properties as well as conductivities are enhanced significantly by doping chargetransfer(CT) complexes or dye into a polymeric matrix, by comparison with nondoped polymer films.

Polycarbonate films doped with CT complexes based on 7,7′,8,8′- tetracyanoquinodimethane( TCNQ) were studied from 100K to 450K. Crystallization and formation of ‘nearly’ spaced aggregates within the polymer matrix were observed. These phenomena were affected by several factors such as the film thickness, evaporation process, temperature and concentration within the polymer matrix. Discontinuities in magnetic properties at 200K, possibly due to solvent encapsulation in the matrix, and at about 385K due to glass transition of the polymer were observed.

Characterization, conductivity measurements in comparison with pristine complexes, and photoirradiation studies on these doped films are presented.
Ab-Initio Calculations on PPS Oligomers
Authors: Musso G.F. , Piaggio P. , Cuniberti C. , Dellepiane G. , Borghesi A.
Published: March 2011

Ab-initio STO-3G* geometry optimizations have been carried out on the first two oligomers of poly (p-phenylene sulfide) (PPS), namely diphenyl sulfide (I) (neutral species, radical cation and dication) and 1, 4-bis(phenylthio)benzene (II) (neutral species and radical cation). Aim of the work is to obtain a first theoretical insight into the geometry relaxation effects occurring in the doping of PPS. The results show that i) geometry changes in the radical cation of I are rather small, being much more pronounced in the dication, and ii) in the radical cation of II a strong geometry modification occurs, the central phenyl ring now lying in the plane of the C-S bonds with a clear quinoid-type structure. These findings are in agreement with our experimental results on the same two oligomers.
Helical Sense and Twisting Power in Thermotropic Side–Chain Copolymers
Authors: Krishnamurthy Sushil , Chen Shaw H.
Published: March 2011

Thermotropic chiral nematic copolymers containing (S)–(–)–1–phenylethyl alcohol and (R)–(–)–methyl mandelate with methacrylate backbone and those containing (S)–(–)–1–phenylethylamine and cholesterol with acrylate/methacrylate mixed backbone structures have been synthesized and characterized. It is found that the helical sense is dictated by chiral/nematic molecular interactions of a steric nature, and that the helical twisting power (HTP) is favored by the structural similarity between the chiral and nematic components. Furthermore, it is shown that HTP can be optimized with a suitable nematogenic comonomer by regulating the backbone flexibility within the acrylate/methacrylate series, which facilitates supramolecular arrangement without affecting the HTP.
A Novel Unsymmetrical Poly(1-N-Carbazolyl) Diacetylene for Non Linear Optics
Authors: Garbarino G. , Dell'erba C. , Cuniberti C. , Piaggio P. , Musso G.F. , Comorefito D. , Lanzani G. , Dellepiane G.
Published: March 2011

Photoinduced absorption spectra of poly(1-N-carbazolyl-penta-1,3.diyne-5-acetoxy) show absorption peaks which depend on the sample preparation and on the temperature. The low temperature spectra show two well resolved absorptions centered at ∼0.5 and ∼1.2 eV respectively. In view of other experimental results on these systems, we have interpreted our data as evidence of charged bipolarons stabilized by interchain interactions.
Birefringence and Order of Liquid Crystal Polymer Solutions
Authors: Mattoussi Hedi , Srinivasarao Moran , Kaatz Philip G. , Berry Guy C.
Published: March 2011

Birefringence measurements of nematic solutions of the poly(p-phenylene benzobisthiazole), PBT, using interference between the ordinary and extraordinary waves created when a plane polarized wave enters the crystal, are described. The measured birefringence is high, e.g., ne – no varies from 0.05 to 0.09 at 633 nm for polymer concentrations 4×10−2 < c < 10×10−2(g/cm3). These data are discussed with the implications they have on the order parameter in these solutions.
Steady State and Transient Photoconductivity of Thin Film Polydiacetylene Single Crystals
Authors: Yang Y. , Lee J. Y. , Miller P. , Li L. , Kumar J. , Tripathy S. K. , Matsuda H. , Okada S. , Nakanishi H.
Published: March 2011

Charge carrier generation and transport mechanisms in polydiacetylene thin film single crystals., poly-PTS (2,4-hexadiyne-1,6-diol bis(p-toluenesulfonate)) and poly- BTFP (bis-(4-n-butyl-2,3,5,6-tetra-fluorophenyl) butadiyne), are studied by using steady state and transient photoconductivity techniques. The electric field dependence of the steady state photocurrent is superlinear for both samples. Dependence of photocurrent on incident light polarization has been investigated. The polarization dependence of photocurrent has completely different behavior for the polydiacetylene PTS and BTFP. Single-gap transmission line experiment has been designed to directly measure the drift velocity of PTS single crystals. A drift velocity of the order of 106cm/s was measured.
Nonlinear Optical Properties of Polyphosphazenes
Authors: Ferris K.F. , Risser S.M.
Published: March 2011

Theoretical second hyperpolarizabilities for a series of iso-electronic conjugated inorganic systems, including polyphosphazenes, have been evaluated using electronic structure methods. The nonlinear optical properties of these molecules are controlled by the electronegativity difference along the backbone, which may be modulated by the electron donating/accepting properties of the ligands.
Optimized Processing Condition for a Photocrosslinkable Stable Nonlinear Optical Polymer
Authors: Lee Jun Y. , Kim Woo H. , Mandal Braja K. , Kumar Jayant , Tripathy Sukant K.
Published: March 2011

The optimum processing condition for a photocrosslinking Nonlinear Optical(NLO) polymeric system was established. Parameters such as change in heat capacity at glass transition temperature, rate of decrease of absorbance of residual uncrosslinked chromophores, and measurement of mass loss upon heating were used to set up the optimum condition. Poly(vinyl cinnamate) was used as the photocrosslinkable polymer and 3- cinnamoyloxy-4-[4-(N, N-diethylamino)-2-cinnamoyloxy phenyl azo] nitrobenzene was used as the NLO molecule. Poling at 70°C for 5 minutes followed by photocrosslinking with a dose of 2.5 mW/cm 2 for 10 minutes results in a stable second order nonlinear optical material.
Theoretical Models of Cooperative Dynamics Near the Glass Transition
Authors: Jäckle Josef
Published: March 2011

It is shown that diffusion in the hard-square and hard-octahedron lattice gases at high particle concentration has cooperative properties resembling molecular relaxation in undercooled liquids near the glass transition. For these models a characteristic length of cooperativity is introduced by an underlying percolation problem, which determines whether permanently blocked particles exist in lattices of finite size. The percolation problem belongs to a general class of bootstrap percolation models. Salient Monte Carlo results for the concentration and size dependence of self diffusion in the hard-square lattice gas are presented. Similarities with the n-spin facilitated kinetic Ising models are also pointed out.
Physical Aging and Isothermal Relaxation in Glassy Polycarbonate Measured by Positron Annihilation Lifetime Spectroscopy
Authors: Hill A. J. , Jones P. L.
Published: March 2011

The free volume behaviour of polycarbonates annealed above and below the glass transition temperature, Tg, is studied. The free volume behaviour as a function of temperature and as a function of time during isothermal relaxation is compared for the different heat treatments. The relaxations which occur during contraction experiments performed after cooling from the heat treatment temperature result in a decrease in free volume for both sets of samples; however, the general decrease is much greater in the polycarbonate annealed below Tg. For the polycarbonate annealed below Tg, the free volume is partially recoverable if the samples are reheated to the original annealing temperature. Differences in the free volume relaxation behaviour during contraction experiments are discussed.
Free Volume Distribution in Polymer Liquid and Glass Evaluated by Molecular Dynamics Simulation
Authors: Roe R. J. , Rigby D.
Published: March 2011

Molecular dynamics simulation has been performed with dense systems of alkane-like chain molecules. Distribution of occupied and unoccupied space in the system was then evaluated by the technique of Voronoi tessellation of space and by enumeration of the cavities formed when hard spheres of diameter D were assumed placed on atomic centers. The distribution of cavity volumes is extremely broad and often exhibits a bimodal or trimodal character. These cavities change shape and size with time even at temperatures much below Tg.
Structural Relaxation, “Memory Behaviour”, Specific Heat Anomaly and Localized Motions in Amorphous Solids
Authors: Johari G. P.
Published: March 2011

Dielectric and mechanical relaxation spectroscopy of thermoplastics and thermosets show that, in the former, ageing is related only to the spontaneous structural relaxation towards a lower energy, but still disordered, structure and decreases the strength of the β- relaxation process. Ageing of a thermoset initially increases the strength of its (dielectric) β-relaxation as a result of diffusionallowed chemical reactions, which increase the number of dipolar groups, and later decreases it towards a limiting value. The decrease in the strength of β-relaxation on ageing of polymers and other amorphous solids suggests that structural relaxation causes a collapse of low-density, high-entropy, regions or “soft-sites” randomly distributed in the structure of a glassy material. The increase in the strength of β-relaxation observed in thermosets is higher than calculated from the extent of chemical reactions that occur during their structural relaxation at T<Tg. It is suggested that these effects should be included in the phenomenological theories for both the physical ageing and “memory behaviour” - the latter can be conveniently studied by both dielectric and mechanical spectroscopy and by differential scanning calorimetry. Structural relaxation also decreases the magnitude of the specific heat anomaly in amorphous solids and can be used to experimentally test the postulate that tunneling centres involved in the low-temperature phonon properties are related to the presence of “islands of mobility” or “soft-sites” in an otherwise rigid glassy matrix.
Study of Glass Transition of a Poly(Imide Siloxane) Copolymer Using Thermal Analysis
Authors: Shi L. T.
Published: March 2011

Films of a poly(imidc siloxane) thermoplastic copolymer with a thickness of 0.5 mm have been characterized using thermal analytic techniques including DSC, TMA, and DMA. It was found that the glass transition temperature depends not only on heating rates but also on loading conditions. The mechanism of the glass transition in this copolymer is shown to involve two interconnected steps, namely volume change and phase ordering.
Predictions of a Thermoviscoelastic Constitutive Equation for Specific Volume Relaxation in the Glass Transition Region
Authors: Song Dong-Ik , Caruthers James M.
Published: March 2011

Relaxation in the glass transition region is very important to the understanding of the mechanical properties of polymers. Both (i) traditional viscoelastic behavior resulting from application of a mechanical stress or strain and (ii) volume relaxation due to the temperature or pressure change. Structural relaxation near the glass transition region has mostly been probed via isothermal volume recovery after a step change in temperature. Key features of the isothermal volume recovery are well known and include (i) nonlinearity of the kinetics in terms of the magnitude of the initial departure from equilibrium, (ii) asymmetry of the relaxational behavior with respect to the sign of the temperature perturbation and (iii) memory effects related to the thermal history and physical aging.
Structure and Dynamics of Syndiotactic Poly(n-Butyl Methacrylate) Near the Glass Transition
Authors: Paiterson G. D. , Jue P. K. , Stevens J. R.
Published: March 2011

An optically homogeneous sample of highly syndiotactic poly(n-butyl methacrylate) (PBMA) has been prepared. The glass transition temperature was observed to be Tg=55C. Measurements of the scattered intensity of the sample followed the expected behavior for a pure liquid above Tg. The intensity rose as the sample was cooled further towards the glass transition temperature for atactic PBMA. These results illuminate the importance of regions of different stereoisomers in methacrylate polymers. Measurements of the Rayleigh- Brillouin spectrum were carried out from -15 to 130 C. The ratio of the the central peak intensity to the Brillouin intensities at temperatures above Tg was consistent with a viscoelastic liquid and had a magnitude near 3. The Brillouin linewidth remained large near Tg. and decreased continuosly with no apparent change in slope in the glass transition region. The presence and importance of rapid motions in polymers near the glass transistion is demonstrated by these results. Slowly relaxing density fluctuations near Tg. were measured by photon correlation spectroscopy. Relaxation functions were obtained from 10−6 to 10 s. Average relaxation times <τ> were obtained from the integral of the relaxation function and were found to follow the relation <τ>=Aexp(B/(T−T0)), where for our sample B=2940K and T0=273K. The observed relaxation function decayed over a wider time range as the sample was cooled. Quantitative analysis of this effect using the Williams-Watts empirical function yielded a decrease in β from 0.35 at 90 C to 0.18 at 65 C. The relaxation functions were also analyzed to give a distribution of relaxation rates. The observed distributions were bimodal at 70 and 80 C. The two features behaved in a manner consistent with the primary and secondary relaxations observed for methacrylate polymers by other techniques.
Theory of Spinodal Decomposition in Very Viscous Binary Liquid Mixtures
Authors: Jäckle Josef , Pieroth M.
Published: March 2011

We have developed a theory for the evolution of concentration fluctuations after a temperature quench which takes the effect of slow structural relaxation into account. In its linear and non-linear forms, it generalizes the theories of Cahn, Hilliard and Cook, and of Langer, Bar-on and Miller.
Bending and Torsional Fatigue of Nylon 66 Monofilaments
Authors: Toney Mary M. , Schwartz Peter
Published: March 2011

Using newly developed test equipment, the fatigue behavior of nylon 66 monofilaments was studied under two loading conditions, pure bending or simple torsion. In each case, the deformation was applied in balanced cycles of positive and negative strains. For each mode, results are expressed in terms of the measured decay in stiffness with numbers of cycles over a range of maximum applied strain levels. Fatigue lifetimes are presented in S-N format where the log number of cycles of fatigue for a 40% decay in stiffness (N) is plotted as a function of applied strain (S). The failure mechanism for these fibers in each fatigue mode reflects the morphology of semicrystalline oriented synthetic fibers. In torsion, many longitudinal cracks formed around the perimeter of the fiber as the result of cleavage of the relatively weak interfibrillar bonds in the nylon 66 fibers. In bending, cracks grew across the fiber at an oblique angle to the fiber axis along kink band boundaries.
Cooperative Domain Model for Yield Phenomenon
Authors: Matsuoka S.
Published: March 2011

A molecular model for cooperative segmental relaxation has been proposed, which leads to the Adam-Gibbs type dependence of the characteristic relaxation time on temperature and the configurational entropy. When the size distribution for the cooperative domains is introduced, the resulting relaxation spectrum is similar to the Kohlrausch-Williams-Watts (KWW) equation in the frequency range near the loss maximum, but the fit is actually better in the high frequency extremes where the KWW equation always underpredicts the intensity. The model also predicts the broad spread of the spectrum in the non-equilibrium state from the distribution of the apparent activation energy arising from the different sizes of domains.

We now extend this model to the yield phenomenon in glassy polymers. Under the stress, a domain has two alternatives to dissipate the strain energy. One way is to relieve the stress at the rate dictated by the linear viscoelastic relaxation time. This process is possible only when the strain rate is low. At above a certain strain rate, the stress reaches the strength of the domain, forcing to irreversibly break up the domain structure in the manner that can be described by the classical plasticity. It can be shown that the yield stress of a real polymer is a combination of the above two types of stresses. Namely, at a given strain rate, the small domains with short relaxation time will attain the linear viscoelastic steady stress while the larger domains reach the (limiting) plastic yield stress. Thus, the strain rate dependence of yield stress can be predicted from the linear viscoelastic relaxation spectra, even though the yield phenomenon is clearly in the realm of nonlinear viscoelasticity.
Torsional Relaxation and Volume Response During Physical Aging in Epoxy Glasses Subjected to Large Torsional Deformations
Authors: Santore Maria M. , McKenna Gregory B.
Published: March 2011

We present torsional dilatometry experiments to simultaneously measure torque relaxation and volume recovery in an epoxy glass quenched from above its glass transition temperature to below it. Two strain histories are employed: one with strains of equal sizes and a second where small strains follow a large one. The baseline for the thermally induced volume recovery is insensitive to intermittent torsional strains whose magnitude can be well into the non-linear regime. The torque relaxations from equal intermittent mechanical stimuli can be superposed by a time-aging time shift in a way that indicates fast changes cease to occur at ∼104 seconds after the quench, a “mechanical equilibration” time. Stimuli of different sizes can confound superposition via nonlinear effects, but do not affect the ultimate volume recovery of the glass or its mechanical equilibration time. Our results show the signature of rejuvenation; however, this may result from the nonlinear response of the material in a nonisochoric strain history. Our data firmly show that mechanical stimuli do not erase aging or rejuvenate this epoxy.
Binary Optics Microlens Arrays in CdTe.
Authors: Stem M.B. , Delaney W.F. , Holz M. , Kunz K.P. , Maschhoff K.R. , Welsch J.
Published: March 2011

Arrays of miniature focusing optics located at the focal plane can improve the performance of focal plane systems. By more completely collecting the light at the focal plane and concentrating it into a smaller spot size on the detector plane, the photodetector area can be substantially reduced. Increased gamma radiation hardening and noise reduction result from the decrease in photodetector surface area. Binary optics technology, a process for fabricating large arrays of diffractive optical elements, is especially attractive for infrared materials. In this paper, diffractive Fresnel microlens arrays containing over six thousand F/0.9 lenslets are patterned in the surface of CdTe substrates by successive photolithographic and Ar+ ion-beam-etching steps. Results on smaller arrays of monolithically integrated binary-optics lenslets with II-VI detectors, demonstrating enhanced photodetector responsivities, are presented for the first time.