bluuebunny/crossref_metadata_2025 huggingface dataset:
Joining of Metal Films to Carbon-Carbon Composite Material by Metal Plasma Immersion Ion Implantation
Authors: Anders André , Anders Simone , Brown Ian G. , Chow Peter
Published: March 2011

Adhesion of metal films to carbon-carbon composite materials is a problem when using conventional techniques such as sputter deposition. Metal plasma immersion ion implantation is a novel technique which in combination with metal plasma deposition can produce metal-tocomposite bonding with very good adhesion characteristics. The substrate is immersed in a metal plasma which is produced by a pulsed vacuum arc. When the substrate is biased to high negative voltage the metal ions are accelerated toward and implanted into the substrate. A repetitively pulsed bias (Its pulses) is used to avoid arcing and other deleterious effects. Between high voltage pulses, metal plasma is deposited onto the surface with an energy typical of vacuum arcs, about 50–100 eV. The underlying idea of this mixed implantation-deposition technique is the formation of an extended substrate-film intermixed layer. We have demonstrated the technique for nickel films on carbon-carbon composite materials.
Bonding, Structure and Properties of Metal/Ceramic Interfaces
Authors: Howe James M.
Published: March 2011

This paper reviews the effects of chemical bonding, reaction, interfacial structure, fabrication, specimen geometry and testing conditions on the strength and fracture behavior of metal/ceramic interfaces. It is shown that a number of important properties of metal/ceramic interfaces such as the wetting behavior and work of adhesion can be qualitatively predicted from simple bonding models based on the elements in the metal and ceramic. In addition, the interfacial structure can often be predicted from principles of equilibrium thermodynamics and minimization of interfacial energy for relatively thick metal/ceramic layers. More quantitative description of interfacial structure employing atomistic calculations has been performed for simple interfaces and this area is progressing. The fracture behavior of metal/ceramic interfaces is a complicated process which depends on many factors such as the specimen geometry and loading conditions, strength of the interfacial bond, thermal, elastic and fracture properties of the metal and ceramic, thickness of the metal layer and testing environment. Advances in this area include the development of favorable specimen geometries for measuring interface properties and an understanding of the relationship among the phase angle of loading, crack trajectory and interface fracture energy for these geometries. Conversely, little is known about the stress corrosion and fatigue behavior of metal/ceramic interfaces although data on these time dependent failure modes are beginning to appear in the literature. Much progress has been made but considerably more work isneeded to understand the properties of metal/ceramic interfaces.
First principles study of interfacial adhesion: The Mo/MoSi2 Interface With and Without Impurities
Authors: Hong T. , Smith J. R. , Srolovitz D. J.
Published: March 2011

Adhesive properties of the Mo(001)//MoSi2 (001) heterophase interface with and without C, O, B, S, and Nb impurities are calculated using a first principles local density functional approach. The adhesive energy and interfacial strength of the impurity-free interface are 10% to 15% smaller than the respective values for cleavage along the (001) planes of Mo and MoSi2. All of the impurities were found to decrease the Mo//MoSi2 adhesive energy. The substitutional impurities S and Nb decrease the interfacial strength, while the interstitial impurities C, O, and B increase it. All of the impurities increase the interfacial spacing in proportion to their covalent radii. The impurity effects on adhesion may be described in terms of competing bonding and strain effects.
Towards an Understanding of the Reaction Sequences in Brazed Joints: Investigation of the Ti-Cu-O Ternary at 945°C.
Authors: Kelkar G. P. , Carim A. H. , Spear K. E.
Published: March 2011

The joining of ceramics using active metal braze alloys is an attractive technique and has been used for a variety of material systems. The braze alloys usually contain an active element such as Ti which can reduce the ceramic and form a strong bond across the interface. Microstructural characterizations of brazed interfaces have been widely reported, though reasons for the formation of reaction products and their sequence is not clear due to the lack of thermodynamic information on the phases formed.

The Ag-Cu-Ti/Al2O3 system contains an interface where Ti, the active element in the braze alloy, reduces Al2O3 to form layers of reaction products which include (Ti,Cu,Al)6O, an M6X type compound. In this study we have looked at the Ti-Cu-O system as a first step towards understanding the Ag-Cu-Ti/Al2O3 system. A section of the Ti-Cu-O ternary was investigated at 945°C. The system contains two M6X type compounds, Ti4Cu2O and Ti3Cu3O, which have independent single phase fields. A Ti activity diagram was generated from the available thermodynamic information and the knowledge of the ternary section. The two reaction sequences reported for such interfaces were analyzed based on the activity diagram.
Local Mechanical Stability of the Aluminum / Carbon (Amorphous) and Aluminum / SiO2 (Amorphous) Interface at Extrinsic Dislocations
Authors: McCarty E. D. , Hackney S. A.
Published: March 2011

Interfaces between Al and amorphous C or amorphous Sio2 were prepared by sputter deposition of the ceramic phases onto the sputter cleaned surfaces of large grain Al. In situ TEM was used to study the behavior of the extrinsic dislocations (slip traces) at the metal/ceramic interface formed by the motion of Al lattice dislocations which intersect the interphase boundary plane. The proximity of the extrinsic lattice dislocation to the metal/ceramic interface places this interphase boundary under a highly localized stress. Relaxation behavior of extrinsic defects at the interface region is a function of the defect standoff distance from the interface, image stress and the applied stress. An experimental technique is proposed to estimate the minimum shear stress the various interfacial regions are able to withstand. Research supported by the Department of Energy.
Sintering of MoSi2
Authors: Petrovic J.J. , Idasetima J.S.
Published: March 2011

Despite the fundamental nature of sintering and its importance as a low cost fabrication process, little information exists on the sintering behavior of the structural silicide Mosi2. The sintering of commercial Mosi2 powders in the range of 1-10 μm was investigated as a function of sintering temperature, sintering time, and sintering atmosphere. Initial densities for uniaxially cold pressed powders were in the range of 47-56% theoretical. A maximum sintered density of 90% of theoretical was achieved for 1 μm Mosi2 powders after sintering for 100 hours at 1600 °C in an argon atmosphere. Larger 10 μm Mosi2 powders achieved lower sintered densities under these conditions. Avenues to optimize the sintering behavior of Mosi2 are suggested.
Combustion Synthesis of MoSi2 and MoSi2-Composites
Authors: Monroe K. , Govindarajan S. , Moore J.J. , Mishra B. , Olson D.L. , Disam J.
Published: March 2011

Mosi2 and Mosi2-composites are potential candidate materials for both high-temperature coatings and structural components. The application of combustion synthesis and hot pressing in the production of dense Mosi2 and Mosi2- composites, i.e., Mosi2-SiC, Mosi2 -Mo, is currently being studied. Selection of compositions was based on oxidation resistance, thermal expansion, thermodynamic stability, and compatibility. Functionally-graded materials (FGM's) of these composite material systems are also currently being studied.
Synthesis of High-Temperature Silicides Via Rapid Solid-State Metathesis
Authors: Jacubinas Richard M. , Kaner Richard B.
Published: March 2011

Mosi2 has a very favorable combination of materials properties, including a high melting point (2020°C), high strength at elevated temperatures, and resistance to high temperature oxidation and corrosion. These properties make it a good candidate for a hightemperature structural material; however, it has very poor ductility. A great deal of research has focused on improving the ductility of this alloy through various preparative routes. We have synthesized Mosi2, as well as WSi2, TaSi2, and NbSi2, using rapid solid-state metathesis reactions between a high oxidation state metal halide and an alkaline earth silicide. These reactions take advantage of the large exothermic heat of formation of the alkaline earth halide and can reach temperatures as high as the melting point of the product silicides. In addition, this approach yields crystalline products in seconds. The synthetic technique will be discussed along with characterization results.
Synthesis and Properties of MoSi2/SiC Processed by Low Pressure Plasma Co-Injection and Deposition
Authors: Lawrynowicz D.E. , Wolfenstine J. , Nutt S. , Lavernia E.J. , Bailey D.E. , Sickinger A.
Published: March 2011

Low-pressure plasma deposition (LPPD) and co-injection has been used to fabricate a MoSi2 composite reinforced with 15 µm SiC particles. The microstructure and creep behavior of the LPPD processed composite are reported and discussed. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) showed the structure of the composite to be lamellar and energy dispersive X-ray analysis (EDAX) identified the phases present in the material as: MoSi2, Mo5Si3, SiO2, and SiC. Density characterization revealed a porosity of less than 1.0 vol. %, indicating a nearly fully dense material. A high concentration of SiO2 (∼8.0 vol. %) present in the MoSi2/SiC composite may be attributed to possible contamination of the starting powders before or during LPPD. Sublimation of SiC during co-injection led to a low volume fraction (< 2.0 vol. %) of reinforcement in the composite. The creep rate of the LPPD MoSi2/SiC was higher relative to that of MoSi2/SiC composites fabricated by powder metallurgy (PM) techniques. On the basis of the results of this study it has become evident that alternative processing methods such as LPPD insitu processing may be better suited for the fabrication of elevated volume fraction MoSi2/SiC composites.
A Review of Creep of Silicides and Composites
Authors: Sadananda K. , Feng C.R.
Published: March 2011

A review of creep behavior of molybdenum disilicides and their composites is presented. Creep rates of these silicides are compared with those of other high temperature materials such as superalloys, ceramic-ceramic composites, intermetallics including aluminides (nickel and titanium), berylides (vanadium and niobium), and refractory metals(molybdenum and tungsten). Creep rates of silicides are shown to be very sensitive to grain size even in the power-law creep regime with grain size exponent of the order of five and above. In addition, the results show that with increase in volume fraction of reinforcements there is a decrease in creep rates for volume percentages less than 25%. To achieve significant improvement in creep strength volume percentages of reinforcements greater than 25% are required. This weakening effect at low volume percentages is related to accompanying decrease in grain size with the addition of reinforcements. Addition of carbon to MoSi2 eliminated the silica present at grain boundaries and converted it to SiC. The SiC thus formed, inhibited grain growth during hot pressing. Thus although addition of carbon enhanced creep resistance, its effect is masked by the accompanying decrease in grain size. Thus, grain size plays a dominant role in the creep of molydisilicide composites. It is shown that among all the materials molybdenum disilicides possess very high creep resistance comparable to ceramic-ceramic composites.
Plasticity Enhancement Processes in MoSi2-Base Materials
Authors: Gibala R. , Chang H. , Czarnik C.M.
Published: March 2011

Low temperature plasticity enhancement processes observed in body-centered cubic metals and B2 ordered alloys can be observed in MoSi2 in appropriate stress-temperature-strain rate regimes. We illustrate effects of surface films (ZrO2) and dispersoids (TiC) in enhancing plasticity of MoSi2 in the ductile-to-brittle transition range of temperatures, 900-1400°C. We also show, through experiments involving high temperature (1300°C) prestrain, that effective operation of dislocation generation processes can extend the low temperature range to which MoSi2 can be plastically deformed. These results illustrate that approaches to enhance toughness of MoSi2 need not be limited to ductile phases characterized by weak interfaces.
Creep Behavior of MoSi2-SiC Composites
Authors: Butt Darryl P. , Maloy Stuart A. , Kung H. , Korzekwa David A. , Petrovic John J.
Published: March 2011

Using a cylindrical indenter, the indentation creep behavior of hot pressed and HIPed MoSi2-SiC composites containing 0-40% SiC by volume, was characterized at 1000-1200°C, 258-362 MPa. The addition of SiC affects the creep behavior of MoSi2 in a complex manner by pinning grain boundaries during pressing, thus leading to smaller MoSi2 grains; by obstructing or altering both dislocation motion and grain boundary sliding; and by increasing the overall yield stress of the material. Comparisons are made between indentation and compressive creep studies. It is shown that under certain conditions, compressive creep and indentation creep measurements yield comparable results after correcting for effective stresses and strain rates beneath the indenter.
Creep Rupture of MoSi2/SiCp Composites
Authors: French Jonathan D. , Wiederhorn Sheldon M. , Petrovic John J.
Published: March 2011

We studied the creep rupture of a series of MoSi2 materials reinforced with SiC particles. Particulate contents were in the range of 0 to 40 volume percent. Temperature and stress ranges were 1050°C to 1200°C and 10 MPa to 50 MPa, which gave failures ranging from 1 hour to 1500 hours. The creep curves show an extensive tertiary regime, accounting for 25-95% of the total lifetime. Tertiary creep increases with increasing stress and temperature. Cavitation occurs throughout the creep life, and tertiary creep is associated with the linkage of cavities into large cracks. The creep life improves with increasing SiC particle content, with a concurrent loss of creep ductility. Significant improvement occurs only when the particle content is greater than 30 volume percent. Our studies suggest that the creep and creep rupture behavior of MoSi2 can be further improved by increasing the content of SiC particles.
The Temperature and Strain Rate Dependence of the Flow Stress in MoSi2 Single Crystals
Authors: Maloy S.A. , Mitchell T.E. , Petrovic John J. , Heuer A.H. , Lewandowski J.J.
Published: March 2011

The temperature dependence of the flow stress and deformation mechanisms of single crystal MoSi2 have been determined for compression along three different orientations, [001], [021] and [771], at two different strain rates, 1 × 10−5/s and 1 × 10−4/s, and at temperatures between 900 and 1600°C. The flow stress along [021] is slightly higher than that along [771] while both orientations gave a much lower flow stress than that along [001]. Along [021], slip occurs on the {110} 1/2<111> slip system between 1000 and 1200°C, while at 1300-1400°C, slip occurs on the {013}<100> slip system. Along [771], deformation occurs by the [001]<100> slip system while cross-slip onto {013} and [011] planes is observed at 1000-1300°C except that slip occurs on the {013{1/2<331> slip system at 1000-1100°C for faster strain rates. Along [001], slip occurs on the {013}1/2<331> system at 900-1100°C while slip is observed on the {011} 1/2<111> system at 1300-1600°C. Strain rate jump tests from 1×10−5/s to 5x10−5/s at 1 100°C revealed a stress exponent of 7 along [771] and 20 along [021], while a rate jump test from lx10−5/s to 2x 10−5/s along [001] at 1400°C gave a stress exponent of 3.9.
The Effect of Grain Size and SiC Particulates on the Strength and Ductility of MoSi2
Authors: Ghosh Amit K. , Basu Ajoy
Published: March 2011

Monolithic MoSi2 and MoSi2 reinforced with SiC particulate reinforcements have been produced with widely varying grain sizes. Monotonic compression behavior of these materials has been examined in the temperature range of room temperature to 1200°C. Based on these results, the ductile-to-brittle transition temperature at low deformation rate of MoSi2 and its composite appears to be near 900°C and may decrease slightly with decreasing grain size. Tension and compression creep behavior of these materials were also studied between 1100 and 1400°C. Creep strength of the matrix decreases with decreasing grain size even in the nominally dislocation climb-glide regime. While 20 vol% SiC particulates provide a reasonable amount of creep strengthening of the fine grain MoSi2 matrix, strengthening due to coarser grain size can be significantly higher in these materials. These results parallel those on other intermetallics.
Brittle Composites Modeling: Compazisons with MoSi2/ZrO2
Authors: Chen S. P. , Lesar R. , Rollett A. D.
Published: March 2011

We have calculated the mechanical properties of brittle composites with spring-network (SN) model. The composites that we studied involve the transformation toughening effects and the accompanying micro-cracking. Our simulation results are consistent with experiments of MoSi2 toughened with ZrO2. By monitoring the stress changes due to the transformation and micro-cracking we are able to separate, for the first time, the contributions from these two competing effects. We also found that the fracture toughness of the composite increases as the modulus, interfacial cohesion of particle increases.
Design and Synthesize of New Ternary Zirconium Silicide Intermetallic Compounds with the 16H Crystal Structure
Authors: Ikarashi Yukinori , Ishizaki Kozo
Published: March 2011

The zirconium silicide intermetallic compounds with the 16H crystal structure are good materials for high temperature structural applications, because of their high melting point and low density. Their shortcoming is low ductility. To increase ductility, the coordination numbers are increased to increase isotropy of bondings, and to have more metallic nature of bonds.

Combination of elements, silicon, zirconium and other atoms for ternary silicides are selected by theoretical consideration of the coordination numbers. As the coordination numbers increase, the atomic bonds increase metallic properties, and their ductility increases. Higher coordination numbers are obtained by substituting yttrium atoms onto zirconium 6g point set sites.
Cyclic Oxidation Resistance of MoSi2/20v% Nb Composites
Authors: Alman D.E. , Stoloff N.S.
Published: March 2011

The effect of Nb morphology on the 1200°C cyclic oxidation resistance of MoSi2/2Ov%Nb composites was investigated. Niobium was incorporated into MoSi2 as particles, random short fibers and continuous aligned fibers. After oxidation, it was found all that the composites had lost weight and essentially disintegrated. This was due to spalling of both the Nb2O5 scale and the MoSi2 matrix. The spalling of the matrix was a result of cracks originating from the oxidized Nb and propagating through the MoSi2 matrix. These cracks arose from two sources: (1) the volume expansion associated with the transformation of Nb to Nb2O5 and (2) the difference in thermal expansion between Nb2O5 and MoSi2. Composites with smaller diameter Nb reinforcements did not disintegrate as rapidly as composites with larger sized Nb reinforcements. This was attributed to the effect of reinforcement size on CTE mismatch cracking.
Thermal Fatigue of MoSi2 and a MoSi2-10 vol% TiC Composite
Authors: Kush M. T. , Holmes J. W. , Gibala R.
Published: March 2011

Induction heating of disk shaped specimens was used to study thermal fatigue behavior of hot pressed MoSi2, hot pressed and hot isostatically pressed (HIPed) MoSi2, and a MoSi2-10 vol% TiC particulate composite. Specimens were subjected to 5 s, 30 s, and 300 s heating and cooling cycles between temperature limits of 700°C and 1200°C. Specimens of both the hot pressed and the hot pressed and HIPed polycrystalline MoSi2 material failed by transgranular cracking on the initial temperature ramp of the 5 s and 30 s cycles. Only when the severity of the thermal cycle was reduced (300 s heating/300 s cooling) was thermal fatigue cracking avoided in the HIPed MoSi2. In sharp contrast, the MoSi2-10% TiC composite remained uncracked when subjected to the severe 30 s heating/30 s cooling cycle. The fatigue results are discussed with reference to the initial microstructure of the specimens and the stress-strain history of the specimens which was obtained by a thermoelastic finite element analysis.
Fabrication and Characterization of a Functionally Gradient Mo-MoSi2 Composite
Authors: Agarwal Gaurav , Lin Wen-Yi , Speyer Robert. F.
Published: March 2011

For the application of immersed combustion-radiant tubes in glass melters, a Mo-MoSi2 functionally gradient composite was developed. A gradient profile mixture with varying amounts of powders of these two compounds was hot pressed at 1650°C and 17.4 MPa. Mo5Si3, and Mo3Si reaction products were detected by XRD in the hot pressed composite. Limited cracking was apparent in regions where pure Mo regions applied tensile stresses to regions rich in Mo3Si product. Cyclic heat treatments in a reducing atmosphere showed no signs of further CTE mismatch induced cracking, but did indicate a propensity to creep under load at ∼ 1400°C. A functionally gradient material with MoSi2 on all exterior surfaces showed excellent oxidation protection to the Mo center.
Development of High-Strength-Fabricable Tantalum-Base Alloys
Authors: Buckman R. W.
Published: March 2011

In the 1950s, Ta-7.5%W and the Ta-2.5%W were the only tantalum alloys of commercial significance. An intensive alloy development effort occurred between 1958 and 1968 in response to Air Force and Navy aerospace needs for high-temperature, oxidation-resistant alloys for rocket and air-breathing engines and airframe applications. Compatibility with oxidation-resistant coatings, high-temperature short-time strength, fabricability and weldability were of prime importance. These programs led to the development of Ta-10w%W, Ta-30w%Nb-7.5w%V, T-111(Ta-8w%W-2w%Hf), and T-222(Ta-10w%W-2.5w%Hf-O.Olw%C). T-111, with its demonstrated compatibility with liquid alkali metals, and combination of strength, fabricability and weldability, was selected by NASA as the baseline reference alloy for space nuclear power systems studies. Significant quantities of T- 111 and T-222 were produced in the 1960s. Today, however, production is limited to unalloyed tantalum and the tantalum-tungsten binaries because of the demand of the chemical industry for materials with outstanding acid corrosion resistance. To again produce T-11 and T-222 on a commercial basis will require relearning by the refractory metal alloy producers. The current lack of experience in the refractory metal industry with these high temperature alloys will necessitate recovery of the expertise needed for the United States to effectively compete in this technology arena.
Synthesis and Properties of In-Situ MoSi2/SiC Composites
Authors: Jayashankar S. , Riddle S.E. , Kaufman M. J.
Published: March 2011

Compositionally-tailored, silica-free, MoSi2/SiC composites with SiC content ranging from 0 to 40 percent were synthesized through a novel processing scheme involving mechanical alloying and in-situ reactions for the formation of the reinforcement. Room temperature indentation fracture toughness and hardness measurements were obtained from these silica-free composites and were compared with values obtained from silica-containing, conventionally-processed MoSi2/SiC composites.
Mechanical Working of Refractory Alloys Produced by Melting Processes
Authors: Hughes John R.
Published: March 2011

The elements generally considered to be the “major” refractory metals are niobium (Nb) and tantalum (Ta) from group VA of the periodic table, and molybdenum (Mo) and tungsten (W) from group VIA. By virtue of their high melting points, these metals and their alloys constitute an important class of materials. When they are produced by melting processes, the resulting ingots usually have large grain sizes, which need to be refined during mechanical working. While the unalloyed refractory metals have varied room-or-low-temperature applications, they require alloy additions, in many cases substantial, to meet the requirements of most of their hightemperature applications. This often results in high flow stresses at elevated working temperatures, and a concomitant decrease in low-temperature ductility. As BCC materials, the refractory metals have ductile-to-brittle transition temperatures, which vary widely from one metal to another and with other metallurgical variables. All of these factors must be taken into account in order to develop successful processing schedules for the refractory metals.
Effect of Cooling Rate on the Tensile Yield Strength and Ductility Of B2 Compound in Nb-40at.% Ti-15at.%Al Alloy
Authors: Hou D.-H. , Fraser H.L.
Published: March 2011

The effect of cooling rate on the tensile properties of specimens of the Nb-40Ti-15A1 alloy (in at.%) subjected to various heat treatments has been studied. This alloy has the B2 crystal structure and an order-disorder transition temperature between 1020°C and 1100°C. Two heat treatments have been carried out; the first one involves an 1100°C/1hr heat treatment followed by furnace cooling, air cooling or water quenching. The second type of heat treatment involves re-heating the furnace-cooled and water-quenched specimens at 400°C for 10 minutes or 900°C for 30 minutes, followed by either furnace cooling or water quenching. Tensile properties, SEM fractographs and microstructures of these specimens have been assessed. It is shown that specimens furnace-cooled from 1100°C have higher strength and less ductility than the water quenched ones. An observed microstructural feature associated with cooling rates is the difference in anti-phase domain (APD) size. Discussions are focused on possible cooling rate related phenomena that could affect the tensile properties. It is proposed that the degree of long range ordering, not the APD size, is the dominant factor for the observed cooling rate effect on the tensile properties.
Refractory and Silicide Laves Phases
Authors: Livingston James D.
Published: March 2011

Present knowledge and understanding of deformation mechanisms, mechanical properties, and dislocations in Laves phases are reviewed. Although the amount of study applied to alloys containing these compounds has been relatively limited, several systems with promising high-temperature properties have been identified, including alloys hardened by binary chromides, ternary aluminides, and ternary silicides. Studies of model alloy systems have suggested possible approaches to increase the room-temperature ductility and toughness of Laves phases. Fundamental studies of the effects of stoichiometry, alloying, atom sizes, electronic structures, stacking fault energy, and other variables on dislocation mobility are needed.
Effect of Intermetallic Compounds on the Properties of Tantalum
Authors: Kumar P. , Mosheim C.E. , Michaluk C.A.
Published: March 2011

While pure tantalum has excellent corrosion resistance and formability, its high temperature properties and thermal stability are marginal for some intended applications. Traditional approaches for improving these properties have been dispersion and solid solution strengthening. Modifications of properties via an intermetallic precipitation was not considered until recently.

Results of an on-going investigation on the processing and evaluation of silicidestrengthened tantalum are presented. Yttrium silicide-containing Tantalum samples were produced via the P/M method. Evaluation consisted of microstructural, mechanical, chemical and functional tests. Results were compared with those of commercially available tantalum. Intermetallics precipitates were found to be very potent in altering these properties.

While the preliminary results are encouraging, extensive functional testing is required to assure that there is no unexpected adverse effect.
Ternary MoSi2 Compounds for High Temperature Structural Applications
Authors: Chin S. , Anton D. L. , Giamei A. F.
Published: March 2011

The microstructure and phase composition of MoSi2 modified with Al, B, Ge, Hf, Nb, and Re have been investigated. B and Hf substitutions for Si and Mo, respectively, exhibited very low solubilities in MoSi2. Al and Nb substitutions for Si and Mo, respectively, changed the crystal structure from tetragonal C11b to hexagonal C40. Phase boundaries and solubility limits were determined for Al and Nb substitutions. Ge and Re substitutions for Si and Mo, respectively, exhibited complete solubility and maintained the tetragonal Cllb crystal structure. The mechanical properties evaluation as determined by four-point flexural testing indicate a ductile-to-brittle transition temperature (DBTT) of 1250-1350°C for all of the modifications evaluated. Isothermal oxidation testing at 1400°C indicates no significant debit in oxidation resistance of MoSi2 that can be attributed to alloying, however, alloys containing higher concentrations of the ternary elements may exhibit reduced oxidation resistances.
Identification of the 0.15 eV Donor Defect in Bulk GaAs
Authors: Fang Z-Q. , Hemsky J. W. , Look D. C.
Published: March 2011

The well-known 0.15-eV Hall-effect center appearing in bulk, n-type GaAs quenches under IR illumination and recovers via an Auger-like process at a rate similar to the Auger rate of EL2. On the other hand, the 0.15-eV VAs-related center produced by 1-MeV electron irradiation does not quench at all. Based on these data and a detailed theoretical analysis by Baraff and Schluter, we argue that the bulk 0.15-eV center is related to the AsGa-VAs defect or a related complex.
Effects of Processing and Prolonged High Temperature Exposure on the Microstructure of Nb-1Zr-C Sheet
Authors: Uz Mehmet , Titran R. H.
Published: March 2011

High temperature stability of the microstructure of Nb-lZr sheet containing 0.1 and 0.06 wt.%C was studied as affected by processing and prolonged 1350-K exposure with and without applied stress. Sheets were fabricated by cold rolling bars that were single-, double- or triple-extruded at 1900 K. Creep samples were double-annealed (1 h @ 1755 K + 2 h @ 1475 K) prior to testing at 1350 K for 10,000 - 34,500 h. The microstructures of the as-cast, extruded, rolled, DA and crept samples were characterized using various metallographic and analytical methods. The precipitates were rather coarse Nb2C initially, but transformed to finer (≤1 µm) carbides of (Zr,Nb)C with each subsequent high temperature process. The grain size, and the relative amount and morphology of (Zr,Nb)C were found to be affected by the number of extrusions and to some extent by C-content. However, the microstructures of all the crept samples were similar with (Zr,Nb)C distributed throughout the matrix indicating that prolonged exposure to 1350 K gave rise to complete transformation of Nb2 C to (Zr,Nb)C regardless of the processing history. These and other observations are presented with the emphasis on the correlation between processing, microstructure and creep properties.
Tensile and Stress-Rupture Behavior of Hafnium Carbide Dispersed Molybdenum and Tungsten Base Alloy Wires
Authors: Yun H.M. , Titran R.H.
Published: March 2011

The tensile strain rate sensitivity and the stress-rupture strength of Mo-base and W-base alloy wires, 380 µm in diameter, were determined over the temperature range from 1200 to 1600 K. Three molybdenum alloy wires; Mo + 1.1 wt% hafnium carbide (MoHfC), Mo + 25 wt% W + 1.1 wt% hafnium carbide (MoHfC+25W) and Mo + 45 wt% W + 1.1 wt% hafnium carbide (MoHfC+45W), and a W + 0.4 wt% hafnium carbide (WHfC) tungsten alloy wire were evaluated.

The tensile strength of all wires studied was found to have a positive strain rate sensitivity. The strain rate dependency increased with increasing temperature and is associated with grain broadening of the initial fibrous structures. The hafnium carbide dispersed W-base and Mo-base alloys have superior tensile and stress-rupture properties than those without HfC. On a density compensated basis the MoHfC wires exhibit superior tensile and stress-rupture strengths to the WHfC wires up to approximately 1400 K. Addition of tungsten in the Mo-alloy wires was found to increase the long-term stress-rupture strength at temperatures above 1400 K.
Microstructural Evolution and Densification Kinetics During Sintering of Oxide-Dispersed Tungsten Alloys
Authors: Chen Li-Chyong , Bewlay Bernard P.
Published: March 2011

The present paper discusses the role of ceria and hafnia dispersions in tungsten alloys on the microstructural evolution and densification kinetics during sintering. Densification kinetics were measured using dilatometry, and microstructural changes were examined using scanning electron microscopy and Auger electron spectroscopy. Activation energies for sintering were obtained by analyzing the shift of the iso-density points as a function of linear heating rate. Sintering of both tungsten and ceria-dispersed tungsten were found to be controlled by grain boundary diffusion, with apparent activation energies of 318±21 and 385±15 kJ/mole, respectively. However, densification of hafnia-dispersed tungsten is not controlled by a single mechanism. Under different conditions hafnia can enhance or retard densification; the mechanisms associated with this behavior are discussed. In particular, the relationships between sintering behavior and the tungsten-ceria and tungsten-hafnia interfaces are examined. Comparison with conventional oxide dispersoids, such as thoria, will also be made.
C11b-C40 Transformation-Induced 1/4<111> Faults in Mosi2
Authors: Kad B.K. , Vecchio K.S. , Bewlay B.P. , Asaro R.J.
Published: March 2011

Previous experimental evidence for the transformation of MoSi2 from the high temperature C40 structure to the low temperature C11b structure has been re-examined such that the fault character and the existence of the C40 structure is now questioned. New observations of faults in single crystals prepared over a range of growth rates from 1cm/hr to 30cm/hr are presented. Transformation-induced stacking faults, which were previously described as 1/4<111> lying on {110), have been re-identified as 1/6[001] condensation faults on (001). These faults were probably produced by silicon loss during crystal growth. The contribution that these faults may make to macroscopic strain under the action of diffusion-assisted mechanisms at elevated temperatures is discussed.
Microstructures and Mechanical Behavior of Nb-Ti Base Beta + Silicide Alloys
Authors: Subramanian P. R. , Mendiratta M. G. , Dimiduk D. M.
Published: March 2011

Studies on Nb/Nb5Si3 based in-situ composites have demonstrated an acceptable balance of low-temperature damage tolerance and high-temperature strength/creep resistance. However, catastrophic oxidation and embrittlement of these materials limit their usefulness in structural applications. Alloying studies were initiated at Wright Laboratory with the aim of obtaining incremental improvement in the overall oxidation response of the Nb/Nb5Si3 system, while seeking microstructurally similar systems. The results showed that reduced metal recession rates and oxygen embrittlement can be obtained by Ti and Al additions to Nb-Si base alloys. This paper focuses on the effect of Ti and Al alloying additions to Nb-Si base alloys on phase equilibria, microstructures, temperature dependence of strength, low-temperature toughness, and environmental resistance.
Vacuum Plasma Spray Forming of High Temperature Silicides
Authors: Sampath S. , Herman H.
Published: March 2011

Molybdenum disilicide and its composites have been fabricated by a number of researchers in recent years through vacuum plasma spray (VPS) forming. VPS is capable of producing dense, fine grained deposits of these high temperature intermetallics and is a promising technique for near-net-shape manufacturing. Reviewed here is VPS forming of high temperature silicide intermetallics, principally Mosi2 and its composites. A discussion will be given of the processing-structure-properties relationship of the spray formed silicide.
Recrystallization Kinetics During Fast Thermal Annealing of Pfn+ Implanted Silicon+
Authors: Adekova W. O. , Muller J. C. , Siffert P. , Pedulli L.
Published: March 2011

The damage recovery and electrical activation of PFn+ (1 ≤n ⩽,5) implanted silicon layers during fast thermal annealing has been investigated. The <100> oriented wafers were implanted by glow discharge with 30 keV PFn+ ions at a dose of 2.5–3.1015 ions/cm2 and subsequently annealed using incoherent light pulses in the temperature range 600–1100°C with irradiation times of 1–15 secs.

Our results show that the maximum electrical activity is obtained by about 750°C, 15 sec and the same activity is reached for shorter annealing
times and higher temperatures (typically 820°/ 5 sec; 950°C / 1 sec). These values and Rutherford Backscattering analysis reveal that the velocity of regrowth of the PFn+-implantation amorphized layer is lower than in the case of P+ implantation and that the former requires a higher activation energync≃3.4 eV.

TEM analysis reveals precipitates in 820°C/4 s annealed samples with the appearance of dislocation loops at 980°C/4 s annealing. Finally two characterizable defect levels (ETT = EV + 180 meV, ET2 = EV + 542 meV) are seen to remain in the PFn+ implanted samples examined by DLTS, even after annealing at 1100°C.
Regrowth Rates of Amorphous Layers in Silicon-on-Sapphire Films
Authors: Timans P. J. , McMahon R. A. , Ahmed H.
Published: March 2011

The rate and direction of regrowth of amorphous layers, created by self-implantation, in silicon-on-sapphire (SOS) have been studied using time
resolved reflectivity (TRR) experiments performed simultaneously at two wavelengths. Regrowth of an amorphous layer towards the surface was observed in specimens implanted with 3.1015Si+/cm2 at 50keV and regrowth of a buried amorphous layer, from a surface seed towards the sapphire, was observed in specimens implanted with 1.1015Si+/cm2 at 175keV. Rapid isothermal heating to regrow the layers was performed in an electron beam annealing system. The combination of 514.5nm and 632.8nm wavelengths was found to be particularly useful for TRR studies since the high absorption in amorphous silicon, at the shorter wavelength, means that the TRR trace is not complicated by reflection from the silicon-sapphire interface until regrowth is nearly complete. The dual wavelength method removes ambiguity about the position of the amorphous to crystalline interface and the direction of regrowth. The temperature dependence of the refractive index of silicon leads to large changes in the reflectivity of SOS films as they are heated. The combination of regrowth rate observations and reflectivity measurements during heating has been used to characterize the isothermal heating cycle, avoiding the difficulties of using pyrometers operating at the useful near infra-red wavelengths, where sapphire is transparent.
Metastable Activation in Rapid Thermal Annealed Arsenic Implanted Silicon
Authors: Kamgar Avid , Baiocchi F. A.
Published: March 2011

Low temperature (600–800&C) thermal treatment subsequent to high temperature short time anneal of As implanted Si has resulted in significant clustering of As atoms in Si. Si wafers implanted with As at a variety of doses and energies were subjected to single and multiple thermal anneals using incoherent tungsten radiation. Sheet resistance (R▩ ), and Rutherford backscattering (RBS) and channeling measurements were carried out on the wafers. The metastable behavior observed in the activation of As was attributed to electrical deactivation of As atoms caused by clustering. From the temperature dependence of the rate of increase in R▩ we extracted a value of 1.1 eV for the activation energy of electronic deactivation of As atoms in agreement with the value obtained earlier for the activation of As clustering. We also report, for the first time, observation of a structure in the Si RBS spectrum which represents a defective layer in Si caused by the clustering As atoms.
Rapic Thermal Annealing of W-TI Bilayers on Silicon
Authors: Wei C. S. , Van der Spiegel J. , Setton M. , Santiago J. , Tanielian M. , Blackstone S.
Published: March 2011

W-Ti bilayers on Si have been processed by fast thermal annealing in air, in N2/H2 (5%), and vacuum (1 E-3 and 1 E-6 Torr). The results indicate that the W layer on top of Ti acts as an effective protection barrier against oxidation of Ti in all atmospheres except air. Titanium-rich silicides are formed after 500 °C, while TiSi2 is completed at about 600 °C. At 700 °C a bilayer of WSi2/TiSi2 is found. Silicon is found to be the diffusing specie during the formation of WSi2. Finally, a ternary silicide, Ti0.6W0.4Si2, starts to form after 780 °C. The final ternary silicide phase has'an Mlectrical resistivity of about 60 μΩ-cm.
Material and Electrical Properties of Gate Dielectrics Grown by Rapid Thermal Processing
Authors: Nulman J. , Krusius J. P. , Renteln P.
Published: March 2011

The material and electrical characteristics of silicon dielectric films prepared via Rapid Thermal Processing (RTP) are described. A commercial RTP system with heat provided by tungsten-halogen lamps was used. Silicon dioxide films were grown in pure oxygen and in oxygen with 4% hydrogen chloride ambients. As grown films were either annealed in a nitrogen ambient or nitrided in an ammonia ambient. Film thickness ranges from 4 to 70 nm for RTP times from 0 to 300 s at 1150 C. Current-voltage and capacitance-voltage methods were used for electrical characteristics. Ellipsometry, Auger and TEM were used for material characterization.
Rapid Optical Annealing for Improved GaAs Fet Uniformity
Authors: Kazior Thomas E. , Tabatabaie-Alavi Kamal
Published: March 2011

An Eaton Nova Rapid Optical Annealer (ROA 400) has been used to activate n and n+ Si implants for use in power and low noise FET structures for GaAs MMIC's. PECVD SiN capped 3" SI GaAs wafers were annealed at temperatures ranging from 800 to 970 °C for times ranging from 0 (transient light pulse) to 20 sec. Doping profiles were determined using a Polaron concentration profiler; FATFET's were used for measuring drift mobility; and short gate (l.0μm gate length) FET's were fabricated to establish activation uniformity and to determine d.c. and r.f. performance. Results have indicated peak implant activation as high as 90% and electron mobilities of up to 4700cm2/V-sec for carrier concentrations of 1.3×1017/cm3 – results comparable to conventional furnace annealing. The most significant improvement of optical annealing comes in device uniformity. Saturated current uniformities of < ±3% have been achieved over 3" wafers with excellent reproducibility from wafer to wafer. Power FET structures with zero bias gm of 120mS/mm with uniformities of <±5mS/mm have been measured. R.f. measurements on these devices yielded output powers of >500mW/mm with power added efficiencies as high as 35%.
The Annealing of Phosphorus-Ion-Implanted Copper Indium Disulfide by A Pulsed Electron Beam
Authors: Lin J. L. , Lue J. T. , Leng H. Y. , Yang M. H. , Hwang H. L.
Published: March 2011

Pulsed Electron beam annealing of phosphorus implanted CuInS2 has been found to be an efficient method in p-type doping of CuInS2. A sheet resistance as low as 10.1 Ω/‮, a sheet carrier concentration as high as l.0 ×1016 cm−2, and a hole mobility as high as 499 cm2 /V.s have been obtained. The irradiation energy density for the best doping condition was determined to be in the ranges between 11–13 J/cm2. Using Van der Pauw/Hall technique in conjunction with a chemical etching technique, the effective carrier concentration profiles have been determined with a maximum carrier concentration of ∼9×1019cm−3. Excellent p-n CuInS2 homojunctions have been fabricated by electron-beam pulse annealing with anideality factor of 1.75.
METALLIC EPITAXY OF TRANSITION METALS ON SEMICONDUCTORS
Authors: Prinz G.A.
Published: March 2011

Although epitaxial growth of single crystal metal films was reported at least five decades ago, the availability of modern ultra high vacuum technology and sensitive characterization of thin film properties has opened many new directions in this area of materials research. Of considerable interest is the utilization of the interfacial forces associated with epitaxial growth to stabilize structures which may exist in a metastable minimumenergy configuration. The problem of metastable structural phases of elemental metals will be discussed with particular emphasis upon the 3-d transition metals. In these materials the magnetic properties both depend dramatically upon the structural phase and contribute toward determining the stable minimum energy configuration.
GROWTH AND CHARACTERIZATION OF EPITAXIAL Si/CoSi2 AND Si/CoSi2/Si HETEROSTRUCTURES
Authors: HUNT B.D. , LEWIS N. , HALL E.L. , JTURNER L.G. , SCHOWALTER L.J. , OLANOTO MASAKO , HASHIMOTO SHIN
Published: March 2011

Thin (<200Å), epitaxial CoSi2 films have been grown on (111) Siwafers in a UHV system using a variety of growth techniques including solid phase epitaxy (SPE), reactive deposition epitaxy (RDE), and molecular beam epitaxy (MBE). SEN and TEN studies reveal significant variations in the epitaxial silicide surface morphology as a function of the sillciqd formation method. Pinhole densities are generally greater than 107 cm-2, although some reduction can be achieved by utilizing proper growth techniques. Si epilayers were deposited over the CoSi2 films inthe temperature range from 550ºC to 800ºC, and the reesuulttinng structures have been characterized using SEM, cross—sectional TEN, and ion channeling measurements. These measurements show that the Si epitaxial quality increases with growth temperature, although the average Si surface roughness and the CoSi2 pinhole density also increase as the growth temperature is raised.
ELECTRONIC AND CRYSTALLOGRAPHIC STRUCTURE OF IRON FILMS EPITAXIALLY GROWN ON Cu SINGLE CRYSTALS
Authors: MONTANO P.A. , LEE Y.C. , MARCANO J. , MIN H.
Published: March 2011

The crystallographic and electronic structure of iron films epitaxially grown on Cu single crystals was investigated by LEED, AS, EELS and UPS. The Cu(l00), Cu(ll0) and Cu(lll)faces were used as substrates for the growth of the iron films. In our study emphasis was placed on the modification of surface properties due to the Fe/Cu interface. Significant variation with respect to α—Fe was observed in the electronic structure of fcc iron formed on the various crystal faces.
STRAIN IN MAGNETIC EuS/SrS SUPERLATTICES INVESTIGATED BY He ION CHANNELING
Authors: MANTL S. , BUCHAL CH. , STRITZKER B. , SAFTIC B.
Published: March 2011

He ion channeling experiments indicate that molecular beam grown epitaxial multilayers of EuS/SrS on (111) Si are under strain. Angular scans of the normalized He backscattering yields showed that inclined [100] directions vary by 0.28 ± 0.05º between the first two 30 nm thick layers due to tetragonal distortion of the EuS and SrS lattices. Comparison of theoretical estimated kink angles and experimental results indicated cormplete elastic accommodation of the 0.9 % lattice mismatch between EuS and SrS in the superlattice. In contrast, thin EuS layers on (111) Si are epitaxial but incommensurate because of the large 10 % lattice mismatch.
X-RAY AND TRANSPORT STUDIES OF RUTHENIUM-IRIDIUM BICRYSTAL SUPERLATTICES
Authors: CLARKE ROY , LAMELAS F. , UHER C. , CUNNINGHAM J.E. , FLYNN C.P.
Published: March 2011

The stacking structure and epitaxial ordering are studied in MBE-grown Ru-Ir superlattices of wavelengths λ = 41\ and 62\. These samples span the fcc-hcp crossover previously found to occur when the Ir component is thinner than about 10 monolayers. Evidence for a new stacking sequence of period 3λ is found in the λ=41\ sample, possibly resulting from stacking faults in the Ir layers. The superconducting transition temperature is significantly higher in the 41\ sample (Tc=0.19K) than in the 62\ sample (Tc=0.15K).