bluuebunny/crossref_metadata_2025 huggingface dataset:
Novel Carbon Fibers by Laser Assisted Chemical Vapor Deposition (Lcvd)
Authors: Wallenberger Frederick T. , Diefendorf R. Judd , Frischknecht Kyle D. , Nordine Paul C.
Published: April 2011

Chemically pure carbon fibers with small fiber diameters and high growth rates were obtained by laser assisted chemical vapor deposition (LCVD) using high reactor pressures and a unique rate control mechanism. Depending upon growth conditions and gases, these fibers were either flexible (elastic), brittle (thickened) or graphitic (strong). The elastic carbon fibers were uniform and appear to represent a novel form of carbon. The reactants were acetylene, ethylene or methane, and the reaction pressures ranged from 1.9 to 7.5 bar. The highest fiber growth rate was 0.33 mrn/s, and the lowest fiber diameter was 10 μm.
Thermal Stability Studies of Diamond-Like Carbon Films
Authors: Parmeter John E. , Tallant David R. , Siegal Michael P.
Published: April 2011

Thin films of amorphous carbon/hydrogen, also known as diamond-like carbon or DLC, are of interest as an economical alternative to diamond in a variety of coatings applications. We have investigated the thermal stability of DLC films deposited onto tungsten and aluminum substrates via plasma CVD of methane. These films contain approximately 40 atom % hydrogen, and based on Auger spectra the carbon in the films is estimated to be approximately 60 % sp3 hybridized and 40 % sp2 hybridized. Thermal desorption, Auger, and Raman measurements all indicate that the DLC films are stable to 250–300° C. Between 300 and 500° C, thermal evolution of hydrogen from the films is accompanied by the conversion of carbon from sp3 to sp2 hybridization, and Raman spectra indicate the conversion of the overall film structure from DLC to micro-crystalline graphite or so-called “glassy” carbon. These results suggest that DLC of this type is potentially useful for applications in which the temperature does not exceed 250° C.
Hardness Measurements on Anode and Cathode Mounted, Plasma Deposited Amorphous Hydrogenated Carbon
Authors: Lin Shu-Han , Feldman Bernard J. , Li Dong , Chung Yip-Wah , Wong Mingshow , Sproul William D.
Published: April 2011

We have measured the microhardness of various amorphous hydrogenated carbon thin films that were grown on both the anode and the cathode of our plasma deposition system. We observe no difference in microhardness between films with the same optical bandgap, whether grown on the cathode or on the anode. We conclude that the ion bombardment during cathode growth does not contribute to the hardness of the films. In contrast, we observe a large variation in microhardness as a function of optical bandgap. We conclude that it is increasing hydrogen concentration, found primarily in the clusters and not in the crosslinks, that decreases the hardness of our films. Finally, we observe that the addition of nitrogen to our films does not change the microhardness; again, the hardness is primarily determined by the hydrogen concentration.
Influence of Silicon Incorporation on the Properties of Hard a-C:H Films
Authors: Neto A.L. Baia , Santos R.A. , Freire F.L. , Beyer W. , Camargo S.S.
Published: April 2011

- Results on the properties of silicon incorporated hard a-C:H films deposited on the cathode of a glow discharge decomposition system using methane and silane gaseous mixtures are reported. Obtained samples showed an increased deposition rate, high hardness comparable to that of a-C:H, and a large decrease of the internal stress upon silicon incorporation. The structure of the samples seems to be similar to diamond-like a-C:H, as revealed by the total hydrogen content, infrared absorption and hydrogen effusion experiments. Our results indicate that the observed reduction of residual internal stress may be attributed to a less compact material and/or to a smaller density of voids containing hydrogen in comparison to pure a-C:H.
Pressure-Enhanced Interdiffusion in Amorphous Si/Ge Multilayers: Implications for Defect-Mediated Diffusion
Authors: Theiss Steven D. , Spaepen F. , Aziz M. J.
Published: April 2011

We have studied the pressure-dependence of the interdiffusion rate in amorphous Si/Ge multilayers. Samples were annealed in an externally-heated diamond anvil cell at 693 K at pressures ranging from 0 to 3.1 GPa. Interdiffusion rates were determined by ex situ x-ray diffraction measurements of the decay of the artificial Bragg peaks associated with the multilayer periodicity. Scaling experiments were performed to factor out the effects of concentration and time dependence in the diffusivity. All samples showed a consistent increase in diffusivity with applied pressure, characterized by a negative activation volume ranging from −44±3 to −37±2 percent of the atomic volume of Si for films ranging in average Si composition from 25 to 71 percent, respectively. These results are consistent with a model for diffusion in amorphous Si and Ge based on dangling bond migration.
Amorphous SIC-N Coatings: Its Properties and Applications
Authors: Yee Andrew L. , Ong Hockchun , Xiong Fulin , Chang R. P. H.
Published: April 2011

Laser ablation has been used to deposit a:SiC-N films from a CVD SiC target. Depositions were carried out either in vacuum or in a nitrogen ambient ranging from 10 to 100 mT. The mechanical properties of the films versus nitrogen background pressure were assessed using nanoindentation and surface acoustic wave measurements. Deflected optical beam testing, pull testing and atomic force microscopy were used to determine film stress, film-substrate adhesion and film roughness, respectively. The optical properties were also characterized as a function of nitrogen back pressure using spectroscopic ellipsometry which determined refractive indices and predicted film compositions. Fourier transform infrared spectroscopy determined the bonding structures in the films, and finally Rutherford backscattering spectroscopy measurements were also performed. Based on the best mechanical and optical properties, the optimum film deposition conditions were obtained. It was observed that with an increase in nitrogen background pressure, oxygen content in the film, in the form of SiOx, increased significantly affecting the overall properties of the a:SiC-N films.
Modelling and Measurements of Stress-Controlled Interdiffusion in Multilayered Amorphous Alloys
Authors: Yang F. L. , Shih W. C. , Greer A. L.
Published: April 2011

The decay of X-ray satellite intensities is used to measure interdiffusion in amorphous Ni55Zr45 multilayers as a function of repeat distance, time and temperature. The data are compared to analytical expressions and a numerical simulation based on the analysis of Stephenson for stress effects. It is concluded that stress effects are very strong and that the analysis fits them quantitatively in a system such as a-Ni-Zr with marked diffusional asymmetry.
Temperature Dependence of the Intrinsic Stress and Biaxial Modulus of Plasma Deposited Silicon Nitride and Silicon Oxynitride Films
Authors: Harding David R. , Ogbuji Linus T.
Published: April 2011

The intrinsic stress and biaxial modulus of thin films of amorphous silicon nitride (0.5μm) and silicon oxynitride (1.5 to 2.5 μm) were measured following deposition (Ts=260 to 610°C), and then at discrete intervals throughout an annealing cycle. The biaxial modulus and intrinsic stress of the silicon nitride films increased from 110 GPa and 130 MPa, respectively, to 180 GPa and 680 MPa as the deposition temperature increased from Ts=340°C to Ts=610°C. The elemental composition was unaffected by the deposition temperature. Annealing at 1100°C increased the intrinsic stress to ≃ 1.8 GPa, as nitrogen and hydrogen were lost. Films with “near-stoichiometric” compositions (SiN1.1; H = 12 at.%) did not crack.

Adding oxygen to form silicon oxynitride lowered both the biaxial modulus (20–30 GPa) and the intrinsic stress (−50 to 100 MPa). All the silicon oxynitride compositions (SiO0.3N1.0 to SiO1.7N0.5) were unstable when annealed above the deposition temperature (260°C). Films grown using mostly nitrous oxide (R = N2O/(N20+NH3) < 0.5) oxidized at 350°C to form silica. Simultaneously, the biaxial modulus and intrinsic stress increased to 100 GPa and 170 MPa, respectively. Films grown from mostly ammonia (R <0.5) lost nitrogen and hydrogen and cracked when the temperature exceeded the deposition temperature by 40 to 90°C. The stress induced by the elemental loss was ≃ 600 MPa.
Mechanical Stresses During Solid State Amorphization of Zr/Co Multilayers
Authors: Moske M. , Samwer K.
Published: April 2011

Thin film structures of crystalline Zr and Co, deposited in UHV, are investigated by bending beam technique and by X-ray diffraction. During isothermal annealing and interdiffusion reaction of the thin film double layer and multilayer packages large compressive stresses are generated while an amorphous ZrCo-phase is formed. This can, at first hand, be understood in terms of the Kirkendall effect where Co atoms, as the main moving species, lead to a volume increase of the film beyond the Co interface. The observed change in Zr lattice spacing in accordance with the evolution of mechanical stress indicates that the compressive stress is built up particularly within the Zr layer due to the solution of Co in Zr grains during the initial amorphization reaction. Film structures, having Co already present in the crystalline Zr layer after film deposition, show a decrease in reaction kinetics combined with a lower stress level, indicating that the interdiffusion reaction is depending on the stress state in the Zr grains. At late stages of annealing in high vacuum a sudden increase of additional compressive stress is observed, which could be attributed to the oxidation of Zr, very likely due to the formation of diffusion paths for oxygen through the Co (cap-) layer (Kirkendall voids). Such oxidation behavior was not observed with samples measured in situ in UHV directly after film deposition.
Emission of a Half-Rectangular Dislocation Loop from the Surface of a Compositionally Graded Epilayer
Authors: Zhang Tong-Yi
Published: April 2011

When a half-rectangular perfect or partial dislocation loop is emitted from the surface of a strained epilayer, the optimum dislocation shape occurs at a saddle point of the total energy profile as a function of the dislocation width and length. The activation energy and the optimum shape are calculated for either a compositionally graded or uniform epilayer. The results indicate that 1) dislocation emission is more difficult in a compositionally graded layer than in a compositionally uniform layer; and 2) the ratio of threading dislocation density to mismatch dislocation density is lower in a compositionally graded layer than in a compositionally uniform layer. Theoretical results agree with experimental observations [1,2].
In Situ Measurement of Young’s Modulus and Residual Stress of Thin Electroless Nickel Films for Mems Applications
Authors: Roy S. , Furukawa S. , Miyajima H. , Mehregany M.
Published: April 2011

This paper reports in situ measurement of Young’s modulus and residual stress of electroless nickel films through the use of microfabricated nickel test structures, including electrostatic microactuators and passive devices. The test structures are fabricated in a new surface micromachining process, termed “nickel surface micromachining”, using electroless plated nickel as the structural layer and polysilicon as the sacrificial layer. Subsequent to fabrication, lateral resonant-type electrostatic microactuators of different geometries are resonated by electrical excitation. Using the measured resonant frequencies and knowledge of the device geometry, the Young’s modulus of the film is determined. The passive electroless nickel microstructures deform upon completion of the fabrication process due to residual stress in the film. Measurement of this deformation in conjunction with an appropriate mechanical model is used to determine the residual stress in the films.
Nanohardness Measurements on Thin Films
Authors: Upadhyaya K. R. , Saimoto S. , Timsit R. S.
Published: April 2011

Nanohardness measurements have been carried out on barrier-type anodic aluminum oxide films grown on super-purity aluminum and on a 600 nm thick Al-1wt%Cu blanket film deposited on a silicon wafer. This work was performed using a custom-built displacement-controlled apparatus. The anodic oxide films range in thickness from 20 to 100 nm. The material systems investigated correspond respectively to hard-film/soft-substrate and soft-film/hard-substrate combinations. The experimental evidence suggests that indentation of anodic films begins when the penetration-depth is approximately 1/5th the film thickness. There is also evidence that the measured hardness for Al-1wt%Cu film stems largely from the thin film material when the probing depth does not exceed approximately 1/5th the film thickness. Beyond this depth the substrate influences the film deformation behaviour and the measured hardness gradually reaches the hardness of the substrate material. The oxide films on aluminum show unexpected deformation properties.
Nano-Hardness, Nano-Friction and Nano-Wear of Ultra-Thin Overcoats
Authors: Bogy D. B. , Jiang Zhaoguo
Published: April 2011

Thin film magnetic disks require protective overcoats, usually some form of carbon, to guard against corrosion and wear from interaction with the read/write transducer. In current products these films are less than 25 nm in thickness. This paper summarizes developments using scanning probe microscopes with sharp diamond tips (15 – 100 nm radius) to obtain indentation hardness tests with 5 nm deep indentations. We discuss an accelerated wear test that can measure wear at depths on the order of 1 nm. Finally material characterizations related to friction over sub-micron scans are discussed.

A novel observation has been made when studying the dependence of friction coefficient on normal load: below a critical load, which is material and tip dependent, no observable wear occurs, and the coefficient of friction is about 0.05. Above the critical load the coefficient is load dependent and increases to a value more usually associated with the materials being tested. A study of fatigue wear was made in the “no-wear” regime with three different results. For some materials, fatigue wear occurred with multiple passes, when none was apparent for a single pass. Other materials showed no fatigue wear, and one material, silicon, showed a build-up or “negative-wear” under multiple passes. Interpretations and implications of these results are discussed.
Studies on Wear Mechanism of Ultrathin Protective Carbon Overcoat by Micro-Wear Scan Technique
Authors: Wu T. W.
Published: April 2011

By combining the micro-wear and microscratch techniques in a microindenter, a new methodology to investigate the mechanical properties of ultrathin coatings has been developed. With a 0.18 μm/sec scratch speed along a Y-direction while oscillating the indenter tip along the X-direction at a 6.55 Hz frequency, uniform wear tracks of approximately 20 μm × 90 μm in dimension can be created under either a constant or a gradually increased applied normal load. The wear-scan methodology has rendered several immediate advantages over the conventional micro-scratch technique, offering features such as a much larger testing area thus facilitating the failure and morphology analyses afterwards, the possibility of studying the wear debris generation and the flexibility in testing characteristics. Two thin film disks with different hydrogenated carbon overcoats have been tested by using the micro-wear scan technique. The results in the mechanical measurements, the wear processes and the morphology evolution are illustrated and discussed.
Tribology and Surface Mechanical Properties of Excimer Laser Nitrided Titanium
Authors: Jervis T. R. , Hirvonen J. P. , Zocco T. G. , Tesmer J. R.
Published: April 2011

We have nitrided Ti-6A1-4V alloys using excimer laser pulses at 1.2 J-cm−2 in high purity N2 at approximately one atmosphere. Substantial nitrogen and oxygen incorporation resulted from multiple pulse processing. The surface microstructure that results is of fine grained precipitates in the Ti alloy matrix. Formation of a TiN surface film was not observed. We have examined the tribological and mechanical properties of these surfaces using pin-on-disk and nanoindenter techniques respectively. Nitrogen alloying results in a period of reduced friction and torque noise in the pin-on-disk measurements. At higher N concentrations, abrasive wear of the pin is observed, but with little deterioration of the disk surface. This is consistent with the formation of a transfer film at the sliding interface. Nanoindenter measurements of the surfaces show increasing hardness proportional to nitrogen incorporation. The tribological properties can therefore be understood to result from a combination of tribochemical effects and increased surface hardness.
Hardness and Tribological Effects of Ion Implantation on Electroplated Chromium
Authors: Noll Kathryn , SteinbrÜchel Christoph
Published: April 2011

The effects of ion implantation on two types of electroplated chromium were studied. Both hard chromium and low contraction chromium were plated onto samples of AISI 4340 grade steel and subsequently implanted with N2+ or Ar+ at ion energies of 150 keV and 75 keV, respectively. The dose was varied from 9.4 x 1015 to 3.1 x 1018 atoms/cm2 and the implantation was conducted at both room temperature and 500 °C. The properties of the resulting surface layers were characterized using secondary ion mass spectrometry and Knoop microhardness. The dry sliding wear and friction behavior was also compared between implanted and unimplanted layers. An increase in hardness was observed for both types of electroplated chromium. The wear rate and friction coefficient were reduced in the nitrogen implanted samples. At the intermediate doses tested, the elevated temperature condition provided improved wear and friction properties compared to the room temperature condition.
Thermo-Mechanical Properties of Terfenol-D Thin Films
Authors: Su Quanmin , Zheng Y. , Wuttig Manfred
Published: April 2011

The thermo-mechanical properties of Terfenol-D thin films deposited on Si substrates werestudied by static and dynamic measurements of film/substrate composite cantilevers. The Curie transition, δE effect and mechanical damping of the film were measuredsimultaneously. The stress in the film was controlled by annealing below the recrystallization temperature and determined to vary from -500 MPa, compression, in as deposited films to +480 MPa, tension, in annealed films. The Curie temperature shifts from 80ºC to 140ºC as the tension increases while the structure of the film remains amorphous. The stress change induced by annealing also drastically effects the film's damping characteristics. The δE effect of the amorphous material, about 20%, wasused to estimate the magnetostriction, λs≈4.10-3.
Smart Thin Film TiNi/Pzt Heterostructures
Authors: Mercado Peter G. , Jardine A.Peter
Published: April 2011

Thin film layers of shape memory alloys and ferroelectric ceramics can produce a family of ‘smart’ heterostructures capable of performing both sensing and actuating functions. Important issues in the synthesis of these active structures are the ability to generate the appropriate crystalline phases of each material, while producing defect-free homogeneous high quality films. The compatibility of sol-gel processed Pb(Zr,Ti)O3 (PZT) thin films with thin film shape memory effect TiNi substrates were investigated. Thin film TiNi was deposited on quartz substrates by physical sputter deposition utilizing a TiNi target in a ultra-high vacuum chamber, and followed by in-situ vacuum annealing. PZT was deposited on TiNi by sol-gel and spin coating processes. The ferroelectric tetragonal phase of PZT was obtained by a 600 °C anneal for 5m in air. The heterostructures were nominally defect-free, unlike those obtained through deposition onto bulk TiNi substrates.x
Evolution of the R-Phase Transformation Temperature of Ti50Ni50-xFex Shape Memory Alloys with Iron Concentration
Authors: GÓmez R , Marquina V. , Ridaura R. , Marquina M.L. , JimÉnez M. , Aburto S. , Escudero R. , Morales F. , Rios-Jara D. , Canales A.
Published: April 2011

The evolution of the R-phase transformation temperature with iron concentration is studied by Mössbauer spectroscopy (MS), magnetic susceptibility and calorimetric measurements in a series of three Ti50Ni50-xFex, samples, with x = 1, 2 and 3. The changes in the Mössbauer spectra, from a single line at room temperature to a quadrupole doublet when the temperature is lowered below 280 K, are related to structural transformations occurring in the system. The temperature changes in the magnetic susceptibility are interpreted in terms of a gap opening characteristic of charge density waves (CDW) formation when the R-phase transformation takes place. The calorimetric measurements show specific heat fluctuations near room temperature in two of the samples (x = 1 and 3) whose peaks shift during the cooling and heating cycles.
Deformation and Failure Modes Under Compressive Loading in Lamellar Tial
Authors: Dao Ming , Kad Bimal K. , Asaro Robert J.
Published: April 2011

A physically based micromechanical model is applied to study finite compressive deformation and failure in lamellar TiAl microstructures. Orientation dependent yielding of lamellar TiAl single crystals is specifically modeled. Finite element computations show that deformation is inherently non-uniform in the lamellar microstructure. Intergranular fracture (at low strains) and internal buckling (at high strains) are found to be two of the most important failure modes.
Grain Size Effect on the Creep Behavior of Monolithic MoSi2
Authors: Feng C.R. , Sadananda K.
Published: April 2011

The grain size effect on the creep behavior of hot-pressed monolithic molybdenum disilicide was investigated at 1200°C in a 19–255 MPa stress range. The creep-stress exponent, n, increased from 1 at low stresses to 4 at high stresses. The grain size exponent, p, varied from 0, to 3.5 and to 8 depending on the grain size, the creep-stress exponent, and creep history.
Microstructure of Zr-25at. %Al Melt Spun Ribbons.
Authors: Lutterotti L. , Pradhan S. K. , Gialanella S. , Yavari A. R.
Published: April 2011

Following a previous study in which we presented some microstructural aspects of meltspun ribbons having a composition close to Zr-25 at.% Al, we discuss now the crystallography of the phases observed in similar samples. We performed X-Ray diffraction analyses of ribbons and refined the observed crystallographic structures. We could identify a number of stable and metastable structures, according to the actual composition of the ribbons. We also estimated the percentage of each one of these phases. We did the same for some ribbons annealed at 750°C for several times. In this way we could follow the kinetics leading from the initial as-spun condition to the final one, featuring the L12 ordered Zr3Al, as the major phase, and other intermetallic phases of the Zr-Al phase diagram.
Metastable Phase Transformations in Ti-40Al-10V Alloy
Authors: Shao G. , Tsakiropoulos P. , Miodownik A. P.
Published: April 2011

The microstructures in arc melted ingots and melt spun ribbons have been investigated by electron microscopy and thermodynamic modelling has been used to study the phase transformations. In the ingot, solidification starts with the bcc β phase and at room temperature the structure consists of B2, ωordered, γ and α2 phases. The calculated equilibrium phase transformation sequence during cooling is L → L+ β→β→β + α→β2+α → α+β2+γ → α2+γ + B2. The phase transformation sequence is dramatically changed by rapid quenching from the melt. Athermal ordered w phase is formed in metastable B2 and the α→α2 ordering process is completely suppressed in the melt spun ribbons. The volume fraction of the α precipitates is also dependent on cooling rates.
Dynamic Restoration Mechanism of a Fe3Al Based Alloy During Elevated Temperature Deformation
Authors: Chen Mingwei , Shan Aidang , Lin Dongliang
Published: April 2011

By TEM and metallographic examination, we found that dynamic recovery and following continuous recrystallization, which connects with the elevated temperature ductility, takes place in a Fe3Al based alloy during elevated temperature deformation. Dynamic restoration consists of the following process: (1) by climbing or cross-slipping, glide dislocations change into dislocation arrays, (2) the dislocation array attracts lattice dislocations, (3) with increasing dislocation density, a non-equilibrium subboundary forms, which is easy to migrate, slide or rotate under external force, and (4) when the misorientation angle of the subboundary increases to a critical value, the subboundary changes into a grain boundary with continuous misfit. By relaxation, the boundary then changes into a grain boundary consisting of periodical structure units.
Mechanical Properties of Polycrystalline NiAl from Miniaturized Disk-Bend Tests
Authors: Eck S. J. , Ardell A. J.
Published: April 2011

Disk-shaped specimens of polycrystalline NiAl (grain size ∽25 μm), 3 mm in diameter and ranging in thickness from 165 to 370 μm, were indented in their centers to various indentation loads, F, using a Vickers indenter. The yield stress, σy, and apparent fracture stress, σfa, were measured as a function of F, with the indented side in tension, using a miniaturized disk-bend test (MDBT). Fracture does not originate at the indentation for F ≤ 39.2 N. Within this regime of behavior both σy and σfa are slightly larger than in unindented specimens, although σfa passes through a sharp minimum for F < 10 N. For F ≥ 39.2 N fracture originates at the indentations. In this regime σy is essentially constant, but significantly larger than σy at smaller values of F, while σfa decreases steadily with increasing F. We attribute the larger values of σy to strain hardening within the relatively large plastic zone surrounding the indentation. The reduction of σfa with increasing F in this regime occurs because the indentation serves as the point of failure. Fractography reveals NiAl fractures primarily in an intergranular manner. A preliminary estimate of the fracture toughness is 8.54 MPa·a.m1/2, which overestimates the true fracture toughness because σfa exceeds the true fracture stresses.
Intermediate Temperature Creep of Ni3Al
Authors: Rong T. S. , Jones I. P. , Smallman R. E.
Published: April 2011

The creep behaviour and dislocation structure of polycrystalline Ni3Al have been investigated at intermediate temperatures, where macroscopic inverse creep may or may not be present depending upon both the temperature and the stress applied. Microstructural observations indicate that superdislocations slip on the cube cross-slip plane more easily at lower temperatures, and is one of the reasons causing the creep strength to vary anomalously with temperature. Although inverse creep is produced by the operation of cube cross-slip, it is a necessary but not sufficient condition, i.e. the operation of cube cross-slip may not necessarily result in macroscopic inverse creep.
Trends in Dislocation Core Structures and Mechanical Behavior in B2 Aluminide
Authors: Vailhe C. , Farkas D.
Published: April 2011

In an effort to understand the deformation mechanism in high temperature B2 intermetallics, atomistic simulations were carried out for dislocation cores in a series of compounds exhibiting the B2 structure (FeAl, NiAl, CoAl). A comparison was made on the basis of core structures, dislocation splittings and Peierls stress values. The (110) and (112) γ surfaces were computed for these three compounds. The importance of the APB values and the maximum shear faults for explaining the dislocation behavior is discussed.
Influence of Dislocation-Solute Interactions on The Mechanical Properties of Zirconium-Doped NiAl
Authors: Jayaram R. , Miller M.K.
Published: April 2011

Atom probe field ion microscopy (APFIM) has been used to characterize NiAl microalloyed with molybdenum and zirconium. Field ion images and atom probe analyses revealed segregation of zirconium to dislocation strain fields and ribbon-like morphological features that are probably related to dislocations. These results provide direct experimental evidence in support of the suggestion that the tremendous increase in the ductile-to-brittle transition temperature (DBTT) in zirconium-doped NiAl is due to pinning of dislocations by zirconium atoms. Atom probe analyses also revealed segregation of zirconium to grain boundaries. This result is consistent with the change from an intergranular fracture mode in undoped NiAl to a mixture of intergranular and transgranular fracture mode in zirconium-doped NiAl. The NiAl matrix was severely depleted of the solutes molybdenum and zirconium. Small Mo-rich precipitates, detected in the matrix and grain boundaries, are likely to contribute to the significant increase in the room-temperature yield stress of microalloyed NiAl through a precipitation hardening mechanism.
Dislocation Motion in λ Tial Studied by in situ Straining Experiments in the Hvem
Authors: Messerschmidt Ulrich , Bartsch Martin , Häussler Dietrich , Aindow Mark , Hattenhauer Rainer , Jones Ian P.
Published: April 2011

Micro-tensile specimens of coarse-grained Ti52at%Al crystals have been deformed in situ in a high voltage electron microscope at room temperature. In addition to some twinning, “simple” 1/2〈110] dislocations as well as superdislocations were moving, with the simple dislocations prevailing even if their orientation factor is lower than that of the superdislocations. Both types of dislocations are pinned, probably by small precipitates having a distance along the dislocations of about 100 nm. The precipitates consist most probably of Al2O3. Under stress, the dislocations bow out between the obstacles. The bowing is stronger for 1/2〈110] dislocations. An effective stress of about 41 MPa is estimated from their curvature. The kinematic behaviour of the dislocations is in accord with precipitation hardening. The dislocations are generated by the double-cross slip mechanism. Their density within the slip bands corresponds to a long-range internal stress of about 40 MPa. These data are consistent with the flow stress of PST crystals in the easy orientation, taken from the literature.
Influence of the Grain Size of Fully Lamellar Structure on the Mechanical Properties of a TiAl Based Alloy
Authors: Pu Z. J. , Ma J. L. , Wu K. H.
Published: April 2011

A preliminary investigation was conducted for developing a thermomechanical procedure to refine the lamellar colony size of a fully lamellar structure and improve the balance of the mechanical properties of TiAl based alloys. The main experimental results are as follows: 1) The lamellar colony size in a fully lamellar structure can be reduced to 50 μm using a proper thermomechanical procedure. 2) The influence of colony size on the compressive yield stress of the fully lamellar structure is complex. In the large colony size region, the yield stress can be described by the Hall-Petch relation. In the small colony size region, the yield stress decreases as the colony size decreases. 3) The TiAl alloy, with a fine fully lamellar structure, has the best balance of tensile properties and fracture toughness.
Deformation Behavior and Dislocation Mechanisms in Tial Alloys
Authors: Sriram S. , Vasudevan Vijay K. , Dimiduk Dennis M.
Published: April 2011

Polycrystalline Ti-50 and Ti-52A1 (in at.%) alloys, with large grain sizes (50Al-300μm, 52Al-500μm) and containing low (∼250 wt.ppm) levels of oxygen were deformed under compression over a wide temperature range (77-1173°K). The 0.2% proof stress-temperature profiles comprise of three distinct regimes: Regime I (between 77-∼600°K), regime II (between ∼600-1073°K), and regime III (above 1073°K). Deformation temperature influences the types of dislocations present, the nature of superdislocation dissociations, and the morphological characteristics of both ordinary and superdislocations. Collectively, experiment and theory suggest that the flow properties at low temperatures (regime I) are controlled by lattice friction, whereas at higher temperatures (regime II) the properties are dislocation obstacle controlled.
On the Correlation of the Hardness of B2 Compounds with Point Defect Concentrations
Authors: Pike L. M. , Chang Y. A. , Liu C. T.
Published: April 2011

Point defects such as vacancies and anti-site atoms are known to strongly affect the mechanical properties of B2 compounds. The variations in the hardness of these compounds with composition and quenching temperature can often be correlated with the concentrations of these point defects. Simple themodynamic models using a quasi-chemical approach can be used to estimate the concentrations of point defects with composition and temperature. These estimates can often be supported by experimental methods. Knowledge of the concentrations of defects can then be compared to the variations in hardness using solid solution strengthening models. A consistent relationship is found for several B2 compounds. The hardening rates of vacancies are found to be greater than those of anti-site atoms. B2 compounds having the anti-structure defect structure (AuZn, FeCo) are investigated as well as those with the triple-defect structure (FeAl, NiAl, CoAl).
Quantitative Observations in Dislocation Mechanisms in Gamma TiAl
Authors: Viguier B. , Cieslar M. , Hemker K. J. , Martin J. L.
Published: April 2011

Quantitative TEM observations have been made on a series of gamma Ti47Al51Mn2 polycrystals that were deformed at different temperatures. Special attention has been given to determining the statistical variation of defect densities that occur at the different temperatures. The results, which are in good agreement with mechanical testing [J], indicate that three different mechanisms control deformation in this alloy: lattice friction and the formation of faulted dipoles at low temperatures, the pinning of ordinary dislocations at intermediate temperatures, and the bowing and looping of dislocations at high temperatures. The anomalous flow strength of this alloy has been found to be related to the intrinsic pinning of ordinary dislocations. Details of this pinning and subsequent unzipping process, which are the basis for the new local-pinning-unzipping (LPU) models[2-4], are outlined within.
The Effect of Room Temperature Prestrain on the Yield Stress of Single Crystals of γ’ in the Anomalous Regime
Authors: Ezz S.S. , Hirsch P.B.
Published: April 2011

The yield stress of a specimen prestrained at 294 K increases only slowly or is constant up to a temperature above which the yield stress of a virgin specimen is greater. Above that temperature, the yield stress tends to increase as for a virgin crystal. For sufficiently high prestresses, the yield stress anomaly disappears completely. The results are interpreted in terms of a model in which the operation of Frank-Read sources is rate controlling.
The Microchemistry Studies of Ductile Ll2 Ni-Based Intermetallics
Authors: Masahashi Naoya
Published: April 2011

Auger electron microscopy (AES) analysis of boron doped Ni3Al have shown boron segregation at grain boundary (GB). Boron segregation was enhanced with increasing bulk boron content independent of stoichiometry, suggesting that hypo-stoichiometric alloy is intrinsically ductile even without boron. A slight Ni cosegregation is confirmed using atom probe field ion microscope (APFIM). In Ni3(Si,Ti), no distinct composition fluctuation was identified between matrix and GB vicinity. These results suggest that atomic bonding atmosphere modification from covalent to metallic in the vicinity of GB is one of factors to modify ductility for Ll2-type intermetallics.
Oxide Dispersion in Direct-Cast Gamma TiAl-Based Alloy
Authors: Hanamura Toshihiro , Hashimoto Keizo
Published: April 2011

The objective of this study is to evaluate the high temperature behavior of γ TiAl-based alloy sheets containing AI2O3 particles, produced by a combination of vacuum induction melting, use of a CaO crucible, and direct sheet casting, over a wide temperature range. Alumina particles, having a tendency to coagulate during solidification of a TiAl ingot, are finely dispersed due to the disturbance of high frequency induction, and frozen without having enough time to grow in size by direct sheet casting. The TiAl sheet thus produced shows remarkable high temperature tensile strength which exceeds that of conventional ingots having the same composition and various different structures. This is determined to be attributable to the dispersion strengthening of finely dispersed AI2O3 particles whose diameter is from 100 to 500nm. Moreover, because of the small size of these alumina particles, the TiAl sheet does not show any significant retardation in high temperature ductility, which is often the case in conventional ceramic-reinforced intermetallic compound composites.
Hot Oxidation Resistance of an Intermetallic Nb-Ti-Al Alloy
Authors: Bannykh Oleg A. , Sheftel Elena N. , Usmanova Galina S. , Sharapov Anatoly A.
Published: April 2011

Structure, phase composition and air oxidation behavior in the temperature range 800 to 1400°C of a Nb-Ti-Al-based intermetallic alloy with the chemical composition (wt %): Nb-47.0; Ti-23.9; Al-21.0, V-4.4, and Cr-4.1 have been studied. The alloy structure is two-phase -σ (Nb2Al type) and γ (TiAl type). Preliminary air oxidation at 1400°C decreases the oxidation rate at 1150°C by a factor 2 to 3. It is connected with the formation of a protective scale (rutile and corundum with chromium and vanadium additions), refining of the alloy structure, and the formation of an internally oxidized underscale.
Directionally Solidified Mullite Fibers
Authors: Sayir Ali , Farmer Serene C.
Published: April 2011

Directionally solidified fibers with nominal mullite compositions of 3Al2O3·2SiO2 were grown by the laser heated float zone (LHFZ) method at NASA Lewis. High resolution digital images from an optical microscope evidence the formation of a liquid-liquid miscibility gap during crystal growth. Experimental evidence shows that the formation of mullite in aluminosilicate melts is in fact preceded by liquid immiscibility. The average fiber tensile strength is 1.15 GPa at room temperature. The mullite fibers retained 80 % of their room temperature tensile strength at 1450 °C. SEM analysis revealed that the fibers were strongly faceted and that the facets act as critical flaws. Examined in TEM, these mullite single crystals are free of dislocations, low angle boundaries and voids. Single crystal mullite showed a high degree of oxygen vacancy ordering. Regardless of the starting composition, the degree of order observed in polycrystalline fibers was lower than that observed in the mullite single crystals.
A Numerical Analysis of Crack Growth in Microcracking Ceramic and Ceramic Composites
Authors: Biner S. B.
Published: April 2011

A set of numerical analyses of crack growth was performed to elucidate the influence of microcracking on the fracture behavior of microcracking ceramic and ceramic composites. The random nucleation, orientation and size effects of discrete nucleating microcracks and resulting interactions are fully accounted for in a hybrid finite element model. The results obtained from the finite element analysis are compared with the continuum description of the microcracking. Although continuum description can provide a reasonable estimation of shielding, it fails to resolve the details of the micromechanism of toughening resulting from microcracking, since not every shielding event during the course of crack extension corresponds to an increase in the Rcurve. Moreover, as seen in the composite cases, the local events leading to toughening behavior may not be associated with the microcracking, even in the presence of large population of microcracks.
Stress Transfer from Axially Loaded Fiber to Matrix in a Microcomposite
Authors: Hsueh Chun-Hway
Published: April 2011

The shear lag model has been used extensively to analyze the stress transfer in a singe fiberreinforced composite (i.e., a microcomposite). To achieve analytical solutions, various simplifications have been adopted in the stress analysis. Questions regarding the adequacy of those simplifications are discussed in the present study for the following two cases: bonded interfaces and frictional interfaces. Specifically, simplifications regarding (1) Poisson's effect, and (2) the radial dependences of axial stresses in the fiber and the matrix are addressed. For bonded interfaces, the former can be ignored, and the latter can generally be ignored. However, when the volume fraction of the fiber is high, the radial dependence of the axial stress in the fiber should be considered. For frictional interfaces, the latter can be ignored, but the former should be considered; however, it can be considered in an average sense to simplify the analysis. Comparisons among results obtained from analyses with various simplifications are made.
Single-Fiber Push-In vs. Single-Fiber Push-Out: A Comparison Between Two Test Methods to Determine the Interfacial Properties of Brittle Matrix Composites
Authors: Ferber M. K. , Lara-Curzio E. , Russ S. E. , Chawla K. K.
Published: April 2011

Interfacial properties of Nicalon™-reinforced brittle matrix composites were determined from single-fiber load-controlled push-in tests using a Mechanical Properties Microprobe (Nanoindenter) and single-fiber displacement-controlled push-out tests using the Interfacial Test System (ITS). A comparison between the results from these two tests is presented along with a discussion of data analysis techniques.
Microstructures of BN/SiC Coatings on Nicalon Fibers
Authors: Dickerson R.M. , Singh M.
Published: April 2011

The microstructures of Nicalon silicon carbide (SiC) fibers and layered coatings of boron nitride (BN) followed by chemical vapor infiltrated silicon carbide (CVI-SiC) were characterized using optical and electron microscopy. Two different precursors and reactions were used to produce the BN layers while the deposition of CVI silicon carbide were nearly identical. Coated tows were examined in cross-section to characterize the chemistry and structures of the constituents and the interfaces. One BN precursor yielded three sublayers while the other gave a relatively homogeneous nanocrystalline layer.
Thermal Shock Behavior of Fiber-Reinforced Ceramic Composites
Authors: Singh Raj N. , Wang Hongyu
Published: April 2011

The influence of fiber type and method of composite fabrication on the thermal shock behavior of 2-D fiber-reinforced ceramic composites is studied. Thermal shock tests are performed using a water quench technique, and thermal shock damage is characterized by both destructive and nondestructive techniques. It is shown that the composites possessed superior resistance to thermal shock damage than the monolithic ceramics. Catastrophic failure due to severe thermal stresses is prevented in composites and a significant portion of their original strength is retained at a quench temperature difference up to 1000°C. These results along with an analysis of the thermal shock damage mechanism based on the destructive and nondestructive tests is described.
High-Temperature Flow of SiC Continuous Fiber-Glass Ceramic Matrix Composites: The Effect of Interface/Interphase Ductility
Authors: Nair B.G. , Cooper R.F. , Almquist J.N. , Plesha M.E.
Published: April 2011

The elevated temperature rheology of continuous SiC (Nicalon®) calcium aluminosilicate glass-ceramic matrix composites is evaluated in uniaxial compression creep experiments (-σ1 = 20-to-40 MPa; T = 1300–1320°C). The steady state strain rate is demonstrated.to be highly sensitive to the orientation of the reinforcement relative to the maximum compressional stress, with highest bulk specimen strain rates noted for conditions in vWhich the sliding between the fiber and the matrix is optimized as a kinetic flow response (i.e., a fiber orientation of approximately 40-50° from σ1). One further discovers that the temperature sensitivity (i.e., activation energy) of flow increases as the amount of interface flow/sliding increases. The experimental results suggest that the high-temperature, low-stress interface response in this composite system is related to the ductile flow of the “Planar” SiO2 reaction-layer interphase that exists (in addition to the well-recognized planar carbon interphase) in these materials. The results of these simple experiments are used to calibrate a microscale-to-macroscale rheologic model in which the fibermatrix interface is described by a viscous constitutive relationship.
Effects of Helium Pre-Implantation on the Microstructure and Mechanical Properties Of Irradiated 316 Stainless Steel
Authors: Toloczko Mychailo B. , Tedeski Gregg R. , Lucas Glenn E. , Odette G. Robert , Stoller Roger E. , Hamilton Margaret L.
Published: April 2011

Transmission electron microscopy (TEM) specimens of a First Core heat of 316 stainless steel, in both the solution annealed and 20% cold worked condition, were irradiated to 46 dpa at 420°C, to 49 dpa at 520°C, and to 34 dpa at 600°C in FFTF/;MOTA. Prior to irradiation, about half of the specimens were pre-implanted with approximately 100 appm of helium, and of these, several of the solution annealed and pre-implanted specimens were aged at 800°C for 2h. Post-irradiation density measurements showed small differences in density between the unimplanted alloys and their helium implanted counterparts. Microstructural observations on specimens irradiated at 420°C and 520°C showed relatively minor differences in defect distributions between the unimplanted and the helium implanted materials; in all cases the defect sizes and number densities were consistent with data in the literature. Where possible, irradiation hardening of the alloys was experimentally evaluated by microhardness and shear punch; experimentally obtained values were compared to values calculated using a computer model based on barrier hardening and the microstructural data. All methods indicated relatively small effects of helium implantation, and both measured and calculated values were in agreement with the range of values reported in the literature.
Amorphization and the Effect of Implanted Ions in Sic
Authors: Snead L. L. , Zinkle S. J.
Published: April 2011

The effects of implanted ion chemistry and displacement damage on the amorphization threshold dose of SiC were studied using cross-section transmission electron microscopy. Room temperature as well as 200 and 400°C irradiations were carried out with 3.6 MeV Fe, 1.8 MeV Cl, 1 MeV He or 0.56 MeV Si ions. The room temperature amorphization threshold dose in irradiated regions well separated from the implanted ions was found to range from 0.3 to 0.5 dpa for the four different ion species. The threshold dose for amorphization in the He, Si and Fe ion-implanted regions was also σ0.3 to 0.5 dpa. On the other hand, the amorphization threshold in the Climplanted region was only about 0.1 dpa. The volume change associated with amorphization was σ17&. No evidence for amorphization was obtained in specimens irradiated at 200 or 400°C.