bluuebunny/crossref_metadata_2025 huggingface dataset:
Quasi-Static Brittle Fracture
Authors: Pfaendtner J. A. , Muthiah R. C. , McMahon C. J.
Published: April 2011

Recent work on the phenomenon of diffusion-controlled quasi-static brittle fracture, known as dynamic embrittlement, is reviewed here with reference to sulfur-induced embrittlement of steel, tin-induced embrittlement of Cu-Sn alloys, and oxygen-induced embrittlement of copper-based and nickel-based alloys. The mechanisms of this generic form of intergranular brittle fracture are discussed.
The Influence of Microstructure on the Fracture and Tensile Properties of Die Cast and Thixomolded Magnesium Alloys
Authors: Thoma Paul E. , Hays Colleen , Baik Aekyung
Published: April 2011

In this investigation, the fracture and tensile properties of die cast and thixomolded magnesium alloys are compared and shown to be dependent on the microstructure of the test specimens. The principal features of a die cast structure that inhibit crack propagation and contribute to high ductility are the presence of highly structured primary α-magnesium dendrites and a continuous network of the α-magnesium phase. The lower tensile properties of thixomolded magnesium alloys are attributed to the presence of nodular a-magnesium dendrites. Cracks propagate around the nodular dendrites through the divorced eutectic that surrounds the nodular α-magnesium dendrites. In the thixomolded alloys, there is also a less developed continuous network of the α-magnesium phase.
Effect of Bottom Electrode System on Ferroelectric Properties of Sputter Deposited PZT Capacitors
Authors: Hong Kwon , Yu Yong Sik , Lee Shang Kyoo
Published: April 2011

RF magnetron sputter deposited PZT films were prepared on various bottom electrode systems such as Pt/Ti/SiO2/Si, IrO2/SiO2/Si, Pt/IrO2/SiO2/Si and Ir/IrO2/SiO2/Si substrates using the ceramic PZT target with Pb1.1(Zr0.52Ti0.48)O3 composition. In order to obtain single perovskite phase, PZT films were sputter-deposited at room temperature under Ar only plasma condition followed by high temperature annealing under oxygen ambient. Regardless of the bottom electrode system, reasonable ferroelectric properties such as 2Pp, Vc could be obtained by 650°C post annealing. Their values were over 20μ C/cm2 and below ± 1.OV when drive voltage was ± 3V, respectively. Hybrid electrode system, namely, thick Pt or Ir and thin IrO2 shows good leakage current characteristics, of ∼ 10−8A/cm2. Fatigue properties of PZT capacitor depend on the test condition. However, retention after 3×104sec showed a degradation of 15% when writing voltage was −5V irrespective of bottom electrode system. Imprint characteristics also showed good results within ± 1 as a figure of merit (FOM).
Characterization of Ruo2 Films Prepared by Rf Reactive Magnetron Sputtering
Authors: Meng Li-jian , Santos M.P. dos
Published: April 2011

Ruthenium dioxide films have been prepared by rf reactive magnetron sputtering at different oxygen partial pressures and total sputtering pressures. The films have been characterized by scanning electron microscopy, X-ray diffraction and electrical conductivity. The films prepared at low oxygen partial pressure and total pressure show a strong preferred orientation along the [110] direction. As both pressures increased, the peak intensity decreases. All the films are subject to a compressive stress. As the total pressure is decreased and the oxygen partial pressure is increased, the stress increases. When the total pressure is lower than 6 × 10−3 mbar and the oxygen partial pressure is higher than 1 × 103 mbar, the films peeled off automatically from the substrate because of the high stress. The films prepared at high oxygen partial pressure and high total pressure have a rough surface and those prepared at low pressure show smooth surface. In this paper, these phenomena have been discussed. In addition, the electrical properties of the films are also discussed.
Fabrication of PZT Based Capacitor with SrRuO3 Electrode for Memory Device Applications
Authors: Kumar Ashok , Rahman H. , Shamsuzzoha M.
Published: April 2011

Strontium ruthenium oxide (SrRuO3) was deposited on Pt/(100)Si substrates at varying temperatures and 300 mTorr oxygen pressure using the pulsed laser deposition method and was found to be highly crystalline and textured when deposited over 450°C. After achieving highly crystalline SrRuO3 films, capacitors using the ferroelectric material - PZT (PbZr0.5Ti0.48O3) were successfully fabricated on Pt/(100)Si substrates. The ferroelectric properties of the films were determined by the RT66A Standardized Ferroelectric Test System. The structural properties of the films were analyzed by X-ray diffraction. Transmission electron microscopy was used to determine the crystallinity and quality of interfaces among different layers.
The Phase Transition of Bi-Pt Alloys at the Interface of Pt/SrBi2Ta2O9 and its Effect on Interface Roughness
Authors: Shin Dong Suk , Lee Ho Nyung , Kim Yong Tae , Park Young K. , Choi In-Hoon
Published: April 2011

Pt/SrBi2Ta2O9(SBT)/CeO2/Si (MFIS) structures were investigated for observing the change of electrical properties and morphology of interface of Pt/SBT after post-annealing of Pt top electrodes. The morphology of Pt/SBT interface became smooth and Bi oxide was formed at the bottom of Pt top electrode after post-annealing Pt top electrode. In order to describe the origin of these changes, Bi-oxide/Pt/SiO2/Si structure was investigated with annealing temperatures about the reaction between Bi oxide and Pt. We can describe that the smooth interface of Pt/SBT and the consumption of metallic Bi, which the reason why electrical properties were drastically improved, is induced by the melting of Pt-Bi alloys and formation of Bi-oxide after post-annealing Pt top electrode.
Sol-Gel Growth of High-Quality Pb(Zr,Ti)O3 Films on RUO2 Using Seed Layers
Authors: Norga G. J. , Fè Laura , Wouters D. J. , Bartic A. , Maes H.E.
Published: April 2011

New approaches for improving fatigue performance of sol-gel PZT based ferroelectric capacitors with RuO2 electrodes are discussed. The use of thin (∼5 nm) high-Ti PZT seed layer was found to be effective for improving fatigue performance. For an optimized seedlayer thickness, FECAPs with excellent fatigue characteristics (less than 10% decrease in Pr after 1011 cycles) were obtained.
The Stress State and Domain Structure of Epitaxial PbZr0.2Ti0.8O3 Films on (001) SrTiO3 with and without La0.5Sr0.5CoO3 Electrode Layer
Authors: Alpay S.P. , Nagarajan V. , Bendersky L.A. , Vaudin M.D. , Aggarwal S. , Ramesh R. , Roytburd A.L.
Published: April 2011

The domain structure of the 400 nm thick PbZr0.2Ti0.8O3 (PZT) films with different electrode layer configurations was investigated by x-ray diffraction and transmission electron microscopy. The c-domain fractions of the PZT films with no electrode layer, with a 50 nm electrode layer between the film and the substrate, and with 50 nm electrode layers on top and bottom of the PZT film were found to be equal. This means that depolarizing fields do not affect the polydomain structure of the film. Calculations of the in-plane strains based on the lattice parameters of the La0.5Sr0.5CoO3 (LSCO) layer in the above configurations led to the conclusion that the bottom electrode layer is coherently strained to match the substrate.
Ferroelectric Behavior of Li-Doped ZnO Thin Films on Si(100) by Pulsed Laser Deposition
Authors: Joseph M. , Tabata H. , Tkawai T.
Published: April 2011

Thin films of Li-doped ZnO of different compositions (Zn1−xLix)O, x=0.1, 0.17 and 0.3 have been prepared on Si(100) substrate for the first time by pulsed laser deposition. These films are characterized for their structural, surface morphology and ferroelectric nature. A memory window of 1.2V has been observed in capacitance-voltage measurement.
The Effect of (Ba,Sr) and (Mn,Fe,W) Dopants on the Microwave Properties of BaxSr1−xTiO3 Thin Films
Authors: Chang Wontae , Horwitz James S. , Kim Won-Jeong , Pond Jeffrey M. , Kirchoefer Steven W. , Chrisey Douglas B.
Published: April 2011

Single phase BaxSr1−xTiO3 (x=0.5 and 0.6) thin films (∼5000Å thick) have been deposited onto (100) MgO single crystal substrate with (Ba,Sr) compensated and/or (Mn,Fe,W) doped targets using pulsed laser deposition (PLD). The room temperature capacitance (C) and dielectric Q (1/tanδ) have been measured at microwave frequencies of I to 20 GHz as a function of electric field (0-80kV/cm). Microstructural defects associated with cation and anion vacancies have been observed in BaxSr1−xTiO3 films. Compensation of the ablation target with excess Ba and Sr tends to increase the dielectric constant and the dielectric Q. A film deposited with (Ba,Sr) compensated target has been obtained with 25% tuning, where % tuning is defined as {(C(0)-C(E))/C(O))×100, and dielectric Q of ∼ 100 at room temperature (1 — 10 GHz) for DC bias field (E=67 kV/cm). A further increase in the dielectric Q is observed by the addition of donor/acceptor dopants such as Mn, Fe, and W (Q≈100-240). The effects of (Ba, Sr) compensation and (Mn,Fe,W) doping on the film structure and dielectric properties are discussed.
Electrode Structures for Integration of Ferroelectric or High Dielectric Constant Films in Semiconductor Devices
Authors: Grill Alfred
Published: April 2011

The preparation of ferroelectric and high-dielectric perovskite materials, which is performed at high temperatures in oxidizing environments, imposes strong limitations on the choice of suitable electrode materials which can be used for integration of these materials with semiconductor devices. Because of the complex compositions of the perovskites and of some of the electrode materials the two can interact and result in the deterioration of the structures. The electrode materials have, therefore, to be used often in combination with suitable barriers which block diffusion of the elements of the perovskite and of the Si device and prevent interactions between the components. These requirements can result in complex, multilayered electrode/barrier structures that can affect the crystallization of the perovskite material and its electrical properties as well as the perovskite properties during subsequent processing steps (e.g. in forming gas anneals). The present paper will review the different electrode/barrier structures that have been proposed for integration of ferroelectric thin films with semiconductor devices and discuss their effects on the properties of ferroelectric and high-dielectric materials.
Dendritic Macromolecules: A Fourth Major Class of Polymer Architecture – New Properties Driven by Architecture
Authors: Tomalia D. A. , Uppuluri S. , Swanson D. R. , Brothers H. M. , Piehler L. T. , Li J. , Meier D. J. , Hagnauer G. L. , Balogh L.
Published: April 2011

This new architectural class of macromolecules has received substantial attention during the past decade. Three dendritic subclasses, which include (a) random hyperbranched (i.e., one-pot ABx, polymerizations), (b) dendritic grafted (i.e., Combburst® polymers) and (c) regular dendrons/dendrimers (e.g., Starburst® dendrimers) have been synthesized and characterized at a well-defined level in our laboratory. It is clear that their precisely controlled, nanoscale dimensions and architecture play critical roles in influencing physical properties and performance characteristics. Furthermore, these parameters have also distinguished dendrimers as fundamental modules for many nanotechnology applications, as well as for the construction of a new class of larger nanoscale entities which we have termed core-shell tecto(dendrimers). This account will overview these activities and focus on certain unique de Gennes dense packing (or congestion phenomena) and nanoscale container properties that have emerged from this novel architecture.
The Hot Carrier Degradation and Device Characteristics with Variation of Pre-Metal Dielectric Materials
Authors: Seo Y. J. , Choi W. S. , Kim S. Y. , Kim C. I. , Chang E. G. , Lee W. S.
Published: April 2011

It is very important to select superior interlayer Pre Metal Dielectric (PMD) materials which can act as a penetration barrier to various impurities created by the Chemical Mechanical Polishing (CMP) processes. In this paper, hot carrier degradation and device characteristics were investigated with material variation of PMD-1 layers, which were split by LP-TEOS, SR-oxide, PE-oxynitride, PE-nitride and PE-TEOS films. It was observed that the PE-oxynitride and PEnitride using plasma was greatly deteriorated in hot carrier effect in comparison with silicon oxide. Consequently, it is clearly shown that silicon oxide turned out to be a better PMD-1 material than both PE-oxynitride and PE-nitride. From the results, it is suggested that LP-TEOS film is the best PMD-1 material among the silicon oxide samples.
Ammonia RF-Plasma Treatment of Tubular ePTFE Vascular Prostheses : A Preliminary Study
Authors: Mantovanil D. , Castonguay M. , Pageaul J. F. , Fiset M. , Laroche G.
Published: April 2011

A cylindrically-configured plasma treatment system in Radio Frequency Glow Discharges fed with ammonia was used to modify the internal surface of ePTFE arterial prostheses. The effects of RF-power on the surface chemical composition were characterized by XPS. Results show that the treatment at 20 W, for 250 seconds and under an ammonia pressure of 300 mtorr yielded a good compromise between ablation and substitution phenomena on the surface. With this treatment, fluorine content was decreased, while up to 20 % of the surface atoms were substituted by nitrogen.
Study of Defect Numbers and Distributions in Pecvd S102 Transparent Barrier Coatings on Pet
Authors: Sobrinho A. S. Da Silva , Czeremuszkin G. , Latrèche M. , Wertheimer M. R.
Published: April 2011

Transparent ceramic barrier films are the object of increasing interest in packaging, pharmaceutical, optical, and electronics industries, since they can reduce the permeation rate of oxygen (OTR) and/or water vapor (WVTR) by several orders of magnitude. We prepare silicon dioxide (Si02) or nitride (“SiN”) barrier layers by plasma-enhanced chemical vapor deposition (PECVD). We have shown that OTR (and WVTR) decrease with increasing coating thickness, d, to a certain asymptotic minimum value, when d ≥ 50 nm; the residual permeation is attributed to the presence of microscopic defects in the coating. Since the coatings are transparent and very thin, the detection of defects and the analysis of their origin are very difficult.

We have recently developed techniques based on reactive ion etching (RIE) in oxygen plasma to render visible micrometer- or sub-micrometer-sized defects in transparent barrier coatings on transparent polymers. These techniques can be used to better understand the origins of defects in these coatings on a microscopic scale, as well as for mapping and counting defect density on a macroscopic scale (tens of cm2 or more).

In this article we present a correlation between measured 02 transmission rate (OTR) values and the number density and size distribution of defects in SiO2 barrier coatings. Excellent agreement between measured and calculated OTR values allows us to confirm that residual OTR is indeed controlled by pinhole defects; our results also compare well with published data for aluminized PET.
Monte Carlo Prediction of Alloy Structure
Authors: Good Brian S. , Bozzolo Guillermo , Noebe Ronald D.
Published: April 2011

As the performance demands on metal alloys of interest to the aerospace community continue to increase, the complexity of these alloys has tended to grow as well, with corresponding increases in the cost and time needed to develop future generations of materials. While the traditional empirical approach to designing these materials has proven effective, as the complexity of the materials grows, such methods will become less efficient, limiting prospects for further improvement. The incorporation of theoretically- and computationally-based input into the design process has the potential to both improve the quality of, and reduce the time to design, new materials.

Among the most fundamental issues in new materials design, the prediction of the crystallographic structure of solids has long been a goal of researchers in the field of computer simulation. To become an effective tool for assisting in the process of alloy design, a computational scheme must make use of an energy method fast enough to allow consideration of a large number of candidate structures, and an efficient energy minimization scheme.

In this work, we discuss our application of Monte Carlo methods, using the BFS semi-empirical energy method developed in our laboratory, to the problem of prediction of the crystallographic structure of multi-component metal alloys. Using a simple Metropolis Monte Carlo method in conjunction with an accurate and transferable method for computing energetics, we have investigated the crystallographic structure of a variety of fcc- and bcc-based intermetallic compounds, and have considered the influence of alloying additions in compounds having up to five constituents. We discuss the successes and limitations of the methodology, and the prospects for obtaining accurate prediction of physical properties from our results. We also emphasize the role of this type of methodology as part of a synergistic experimental-theoretical effort for alloy design.
The Abrasion of Aluminum, Platinum, and Nickel by Martian Dust as Determined by the Mars Pathfinder Wheel Abrasion Experiment
Authors: Ferguson Dale , Siebert Mark , Wilt David , Kolecki Joseph
Published: April 2011

The Mars Pathfinder Wheel Abrasion Experiment (WAE) spun a specially prepared wheel with strips of aluminum, platinum, and nickel, in the Martian soil. These materials were chosen because of their differing hardnesses, their ability to stick to anodized aluminum, and their comparative chemical inertness under Earth launch and Mars landing conditions. Abrasion of those samples was detected by the change in their specular reflectances of sunlight as measured by a photovoltaic sensor mounted above the wheel. The degree of abrasion occurring on the samples is discussed, along with comparisons to the abrasion seen in Earth-based laboratory experiments using Martian soil analogs. Conclusions are reached about the hardness, grain size, and angularity of the Martian soil particles, and the precautions which must be undertaken to avoid abrasion on moving parts exposed to the Martian dust.
Growth Competition During Double Recalescence in Fe-Cr-Ni Alloys
Authors: Matson D. M.
Published: April 2011

The rapid solidification of a Fe- 12wt%Cr- 16wt%Ni alloy was investigated under containerless processing conditions using both ground-based electromagnetic levitation equipment and aboard the shuttle Columbia using the TEMPUS facility. A high-speed digital video technique was used to image growth of the metastable ferritic phase and the stable austenitic phase into the undercooled melt. Above a critical undercooling, the metastable phase nucleates first. After a delay, a second thermal rise is observed during transformation to the stable phase. Double recalescence events were observed at temperatures consistent with the To, temperature of the bcc phase thus defining a value of the critical undercooling for metastable nucleation which is significantly lower than previously predicted. For a given liquid temperature the velocity of the stable fcc phase is greater than that of the metastable bcc phase. The velocity for growth of the stable phase into the semi-solid which forms during primary metastable recalescence was also measured and found to be independent of the initial undercooling. A model based on competitive growth of the two phases successfully predicts the limit where double recalescence events may be detected.
The Isothermal Dendritic Growth Experiment: Evolution of Teleoperational Control of Materials Research in Microgravity
Authors: Lacombe J.C. , Koss M.B. , Lupulescu A.O. , Frei J.E. , Glicksman M.E.
Published: April 2011

Exactly one year ago, the Isothermal Dendritic Growth Experiment (IDGE) completed its third and final orbital space flight aboard the United States Microgravity Payload (USMP) on STS-87. The IDGE conducted 180 experiments on dendritic growth in 5-9's succinonitrile (SCN), a BCC material used on USMP-2 and USMP-3, and over 100 experiments on 4-9's pivalic acid (PVA), an FCC material used on USMP-4. IDGE film and telemetry data provide benchmark tip velocity and radii versus supercooling for critically testing transport theory and the interfacial physics of diffusion-limited dendritic growth. Post-flight application of optical tomography is providing the first tip shape data allowing quantitative tests of three-dimensional phase field calculations. Several new discoveries were made during each flight concerning the behavior of dendrites at low driving forces, and the influences of time-dependent pattern features and noise. A summary of these scientific highlights will be provided.

The IDGE instrument was upgraded on each successive flight, improving its optics and electronics, especially the capability for teleoperational control. Near real-time, full gray-scale video was accommodated on USMP-4, allowing investigation of non-steady-state features and time-dependent growth dynamics. A short example of video from space will be shown. USMP-4 science was teleoperated by a student cadre for 16 days from a remote site established by NASA at RPI. This operational experience provides valuable insights, which will be drawn upon for future microgravity experiments to be conducted on the International Space Station.
Vapor Grown Carbon Fiber Reinforced Aluminum Matrix Composites for Enhanced Thermal Conductivity
Authors: Ting J.-M. , Tang C. , Lake P.
Published: April 2011

Aluminum matrix composites reinforced with high thermal conductivity vapor grown carbon fiber (VGCF) were developed for improved thermal efficiencies in electronic devices. The carbon fiber was heat treated to increase its thermal conductivity. Various aluminum matrix composites were fabricated by the densification of fiber preforms using a pressure casting technique. Uniformity of the density was examined using optical microscopy. A scanning electron microscope equipped with a microprobe was utilized to examine the mechanical integrity of the composite. Mechanical properties, including tension, compression and flexural properties, were measured. While the results of the mechanical property measurements indicate moderate values, the composite exhibited remarkable thermal conductivity that reached 642 W/m.K, three times that of aluminum, at a fiber volume fraction of 36.5%, following closely the rule of mixture.
Large Scale Teleoperation on the Lunar Surface
Authors: Konesky Gregory
Published: April 2011

The popular success of the Mars Pathfinder mission, especially in terms of individual access to “live”pictures over the internet, demonstrates substantial mass appeal of space exploration on a personal level. Teleoperation provides an inexpensive approach to remote presence, but is distance-limited due to finite signal propagation times. The proximity of the Lunar surface permits near real-time teleoperation, and through a Virtual Reality approach, presents a sense of “being there” without the difficulty of getting there (and back).

While limited single user teleoperated Lunar Rovers concepts have been proposed (Luna Corp., Inc.), an on-going and economically self supporting venture would require simultaneous teleoperation of many vehicles on a large scale. In addition, each vehicle would carry several simultaneous stereoscopic remote viewing television camera heads, whose position is controlled by the head movements of Earth bound viewers. The overall concept is similar to a small fleet of touring busses, although some smaller scout vehicles may be included, as well as a tow truck. An access fee structure for Earth bound teleoperators provides for return on investment.

Technical issues for an on-going operation in the Lunar environment include temperature extremes, high vacuum, solar radiation, micrometeorites, abrasive nature of Lunar dust and its capacity to electrostatically attach to surfaces are discussed, as are their impact on vehicle design and operation. Optical data links are considered to handle bandwidth requirements of 500 simultaneous users. The economics of scale, on multiple levels, demonstrates the feasibility of large scale teleoperation. Various base-line studies are presented.
Dislocation Dynamics in Ni3Al: Experiments and Computations
Authors: Bhate N. , Kumar S. , Phillips R. , Clifton R. J.
Published: April 2011

A modified electrolytic etchant has been successfully used to observe dislocation etch-pits on {100} surfaces of single crystal Ni3AI containing Hf and B additions. Four-point bending tests have been used to obtain the dependence of dislocation velocity on resolved shear stress at room temperature. A comparison with earlier studies reveal that the rate of change of dislocation velocity with resolved shear stress, to a first approximation, is independent of alloying. Atomic level simulations have been performed using the embedded atom method (EAM) to study dislocation core structures and frictional stress in Ni3Al.
Effect of Off-Stoichiometry on the Deformation Behavior of Ni3AI Binary Polycrystals
Authors: Matterstock B. , Conforto E. , Kruml T. , Bonneville J. , Martin J. L.
Published: April 2011

Polycrystals of binary Ni3Al with different compositions are deformed in compression between 300K and 1300K. They exhibit a strength anomaly with peak temperatures between 800K and 950K. Below the peak, the aluminium rich compounds are the strongest ones while the nickel rich compounds are the weakest ones. As the aluminium content increases, the complex stacking fault energy, measured through weak beam experiments, increases as well. This confirms the cross slip mechanism of mobile dislocation exhaustion through Kear-Wilsdorf lock formation. A ‘ductile-to-brittle transition’ is observed at 1000K. Fracture occurs along grain boundaries, the strength of which decreases as environmental effects are enhanced at high temperatures.
Atomistic Studies of Jogged Screw Dislocations in γ-TiAl Alloys
Authors: Chen K. Y. , Li M. , Zhou S. J.
Published: April 2011

The behavior of jogged screw dislocations in γ-TiAl alloys have been investigated with largescale molecular dynamics (MD) simulations. We find a new mechanism for formation of pinning points in jogged screw dislocations. We also find that the critical height for the jogs in ±[110] directions on the (001) plane to move nonconservatively is between 3r0 and 4r0, where r0 is the nearest neighbor distance of aluminum atoms. Interstitials and vacancies are created during the nonconservative motions of the jogs. In addition, the formations of dislocation dipole and loops around the jogs are also observed.
Thermal Defects in B2 Iron Aluminide
Authors: Collins Gary S. , Peng Luke S.-J , Wei Mingzhong
Published: April 2011

Thermal defects in B2 FeAl samples with compositions between 49.5 and 54.7 at.% Fe were investigated using perturbed angular correlation of gamma rays (PAC). Vacancies on the Fe-sublattice were detected through quadrupole interactions induced at adjacent 111In/Cd probe atoms on the Al-sublattice. Five high-frequency quadrupole-interaction signals were detected (greater than 50 Mrad/s) that are attributed to complexes involving 1, 2, 3, 4 and (with less certainty) 5 Fe-vacancies in the first neighbor shells of the probes. These attributions are based on (1) a comparison between measured quadrupole interaction parameters and point-charge calculations of electric-field gradients for possible vacancy-probe complexes; and (2) numerical simulation of the evolution of site fractions of probes in the complexes at lower temperatures. Measurements were made at temperatures up to 950 C. Assuming that the equilibrium high-temperature is the triple defect (2 Fe-vacancies and one Fe-antisite atom), measurements over the range 600–900 °C yield a formation enthalpy of 1.1(1) eV for the triple defect. Below about 600 °C, Fe-vacancies are quenched-in with a fractional concentration of the order of 1 at.% close to stoichiometry. However, quenched-in vacancies continue to jump over short distances and trap next to the impurity probes atoms in day-long measurements down to 200 °C. Simulations of site fractions below 700 °C were used to determine binding enthalpies of vacancies with probe complexes. Binding enthalpies obtained for the first four vacancies were 0.23, 0.23, 0.15 and 0.18 eV. Simulations in the range 200–700 °C suggest a negative value for the formation entropy. The negative value indicates either that triple defects stiffen the B2 lattice or that repulsive defectdefect interactions become important at the high defect concentrations in FeAl.
Stoichiometric and Off-Stoichiometric Alloying in Silicide Compound Ti5Si3 By Nb or Cr Additions
Authors: Zhang Lanting , Wu Jiansheng
Published: April 2011

Titanium silicide Ti5Si3 whose melting temperature is 2130°C bears the potential for very hightemperature application. This paper reports our results on the alloying behaviour of Nb or Cr addition to this compound. Total energy calculation shows that the substitution of Ti by Nb or Cr atoms in Ti5Si3 crystal stiffens the bonding between the atoms. In experiment, two means of alloying are considered: stoichiometric and off-stoichiometric alloying. Stoichiometric alloying in Ti5Si3 results in compounds consisting of single Ti5Si3 phase while off-stoichiometric alloying yields hypereutectic microstructure with Ti5Si3 being the primary phase. The Ti5Si3 phase in both cases dissolves certain amount of Nb or Cr alloying element and its composition agrees with the stoichiometric composition of (Ti,Nb) 5Si3 or (Ti,Cr) 5Si3 The moduli of the stoichiometric alloys increase with the increase of alloying element addition, indicating an enhancement in Ti5Si3crystal.
Fracture and Fatigue of Refractory Metal Intermetallic Composites
Authors: Zinsser William A. , Solv'yev Sergey , Lewandowski John J.
Published: April 2011

The fracture and fatigue crack growth characteristics of niobium silicide/Nb(ss) in-situ composites as well as a variety of Nb alloys were determined over a range of temperatures and loading rates. It is shown that the in-situ composites exhibited nominally rate-independent and temperatureindependent values for the toughness with an average toughness exceeding 24 MPa.m0.5. The fatigue behavior of both the Nb alloys and in-situ composites revealed that both the Paris Law exponents and fatigue threshold values were similar to that exhibited by metallic materials.
Processing, Microstructure, and Properties of Multiphase Mo Silicide Alloys
Authors: Liu C. T. , Schneibel J. H. , Heatherly L.
Published: April 2011

Multiphase Mo silicide alloys containing T2 (Mo5SiB2), Mo3Si and Mo phases were prepared by both melting & casting (M&C) and powder metallurgical (PM) processes. Glassy phases are observed in PM materials but not in M&C materials. Microstructural studies indicate that the primary phase is Mo-rich solid solution in alloys containing ≤(9.4Si+13.8B, at. %) and T2 in alloys with ≥(9.8Si+14.6B). An eutectic composition is estimated to be close to Mo–9.6Si–14.2B. The mechanical properties of multiphase silicide alloys were determined by hardness, tensile and bending tests at room temperature. The multiphase alloy MSB-18 (Mo–9.4Si–13.8B) possesses a flexure strength distinctly higher than that of MoSi2 and other Mo5Si3 silicide alloys containing no Mo particles. Also, MSB-18 is tougher than MoSi2 by a factor of 4.
Influence of Ni Addition on Mechanical Properties and Fracture Behaviors of Ir-15Nb Two-Phase Refractory Superalloys
Authors: Gu Yuefeng , Yamabe-Mitarai Y. , Ro Y. , Yokokawa T. , Harada H.
Published: April 2011

Ir-based alloys with fcc-L12 two-phase coherent structure, which are called “refractory superalloys”, have good potentiality as structure materials used at ultra-high temperatures up to 2000 °C. Preliminary results showed that the refractory superalloys failed predominately by brittle intergranular fracture at room temperature even though they showed higher strength at that temperature. This paper will present the influence of nickel (Ni) addition on mechanical properties and fracture behaviors of one of these alloys, Ir-15at%Nb. The results indicated that Ni addition was beneficial to the compression ductility and strength of two-phase Ir-15at%Nb alloy when Ni content was below the optimum content.
The Influence of Microstructure on Superplastic Behaviours of γ-TiAl Alloys
Authors: Sun J. , Wu J. S. , Hu G. X. , He Y. H. , Huang B. Y.
Published: April 2011

In this work, superplastic behaviours in Ti-33A1–3Cr-0.5Mo (wt%) γ-TiAl alloys with two different initial microstructures of near gamma (NG) and duplex (DM) structure were investigated with respect to the effect of testing temperatures and strain rates. At 1050°C and a strain rate of 8×10–5 S–1, a maximum elongation of 570% was observed for NG-TiAl and a maximum elongation of 467% for DM-TiAl. The relations of flow stress and strain rate sensitivity vs. strain rates at different temperatures were also determined by incremental strain rate tests. The results showed that the value of strain rate sensitivity is higher and the flow stress is lower for NG than those for DM at the same condition. The microstructural evolution during superplastic deformation was examined and correlated to the mechanical properties for these two alloys. The influence of microstructure on the superplastic behaviours of γ-TiAl alloys, and possible superplastic deformation mechanisms were finally discussed.
Fragmentation of α2 Plates in a Fully Lamellar TiAl During Creep
Authors: Wang J. G. , Hsiung L. M. , Nieh T. G.
Published: April 2011

The fragmentation and spheroidization of α2 laths in a fully-lamellar TiAl alloy during creep were examined. Three possible mechanisms, Rayleigh's perturbation model, subgrain boundary groove mechanism and intersection of deformation twins with α2 lamellae were presented and discussed. During creep deformation, the pile-up of interfacial dislocations leads to a change of planar interface, which, in turn, causes a difference in local chemical potential, and further results in the spheroidization of α2 lamellae. On the other hand, the deformation of the α2 phase is expected to be induced by the high local stress concentration introduced by the pile up of interfacial dislocations. The dynamic recovery process may lead to the formation of subgrain boundaries in the α2 lamellae, which results in the spheroidization and termination of α2 lamellae with the aid of diffusion during creep.
The Effects of Nanostructure on the Strengthening of NiAl
Authors: Chen T. , Thadhani N. N. , Hampikian J. M.
Published: April 2011

The relationship between microhardness and grain size was investigated for nanocrystalline and coarse-grained stoichiometric intermetallic NiAl. The nanocrystalline NiAl specimens were synthesized through mechanical alloying with a high-energy Spex 8000 shaker mill and consolidated by shock compaction at a peak pressure of 4–6 GPa, to 83% dense compacts. The nanocrystalline NiAl compacts were also sintered at 1073, 1173 and 1473 K for 2 h. The Vickers hardness of consolidated and sintered NiAl was determined by microhardness testing, and the grain size and microstructure were investigated with transmission electron microscopy. It was found that the hardness values increased with decreasing grain size of the NiAl alloy. The Vickers hardness values were approximately 650±16, 690±6, 800±43 HV, respectively for NiAl with grain-sizes corresponding to approximately 27±18, 11±6 and 9±6 nm. The possible strengthening mechanisms operating in NiAl are discussed.
Equilibrium Defects and Concentrations in Nickel Aluminide
Authors: Bai Bin , Collins Gary S.
Published: April 2011

Perturbed angular correlation of gamma rays was applied to determine properties of equilibrium defects in B2 NiAl near the stoichiometric composition. Point defects were detected through quadrupole interactions they induce at In probe atoms on the Al sublattice. Well-resolved signals were observed for probe atoms having zero, one or two Ni-vacancies (VNi) in the first neighbor shell. The fractions of probes in different sites are analyzed using a thermodynamic model to determine defect properties as follows. The equilibrium high-temperature defect is determined to be the triple defect combination (two VNi and one Ni-antisite atom) through the variation of the vacancy concentration with composition and not, for example, the Schottky vacancy pair. The binding enthalpy of VNi with a probe atom was determined to be in the range 0.18–0.24 eV. Site fractions were measured for three samples having 50.03, 50.14 and 50.91 at.% Ni at temperatures up to 1300 C. Vacancy concentrations were deduced from the site fractions and binding enthalpy. The equilibrium constant for formation of the triple defect was determined as a function of temperature from the vacancy concentrations and sample compositions. The formation enthalpy was found to be in the range 1.65–1.83 eV, depending on the binding enthalpy. The formation entropy was found to be -3.2(4) kB. The large, negative value of the formation entropy probably cannot be explained in terms of a binding entropy, and we speculate that triple defects harden the B2 lattice, perhaps by disrupting the well-known 1/3 <111> ‘soft mode’ lattice instability in B2 and bcc materials.
Effects of Deviation from Stoichiometry on Deformation Behavior of Hard-Oriented NiAl Single Crystals
Authors: Srinivasan R. , Savage M. F. , Noebe R. D. , Mills M. J.
Published: April 2011

Ni-44A1, Ni-50Al and NiAl-0.3 at.% Hf single crystals have been studied in compression to understand the effects that alloying additions and deviation from stoichiometry can have on the mechanical response of NiAl-based single crystals. While all three single crystals deform through a<111> slip at lower temperatures, the active slip systems differ at higher temperatures. Climb of a<010> dislocations contributes to deformation in Ni-50AI single crystals beyond the slip transition temperature, while Ni-44Al and NiAl-0.3Hf crystals deform through a<101> glide. But several microstructural differences have been observed in the mode of deformation between Ni-44Al and NiAl-0.3Hf crystals. In addition, significant strengthening is exhibited in the Hf-doped crystals at higher temperatures. The post-deformation microstructure is also observed to be sensitive to both strain and strain rate. A possible explanation is offered for some of the observed differences in deformation behavior between the three alloys.
Joining of NiAl to Nickel-Base Alloys by Transient Liquid Phase Bonding
Authors: Abdo Z. A. M. , Guan Y. , Gale W. F.
Published: April 2011

A transmission and scanning electron microscope investigation is undertaken to study microstructural development during transient liquid phase (TLP) bonding of NiA1 to Ni-base substrates. The bonds were produced through a conventional technique employing a Cu foil interlayer or a widegap technique using a composite preform containing powders of NiAl and Cu. The time required for completion of isothermal solidification was greatly reduced in wide-gap bonds as compared to conventional bonds. Microstructural features of conventional TLP bonds of polycrystalline-NiAl/Ni were controlled by the ratio of Al: Cu across the joint. The precipitation of the a phase encountered in polycrystalline- NiAl/Martin Marietta 247 superalloy (MM247) bonds was suppressed in wide-gap bonds of single crystal-NiAl(Hf) and MM247. In general, the extent of second phase precipitation, in the asbonded condition, was greatly reduced by the use of the wide-gap technique. However, extensive precipitation of HfC and W-rich phases was observed after post-bond heat treatments.
Crystal Structure Of Hexagonal Phases And Its Relation To Icosahedral Quasicrystalline Phase In Zn-Mg-RE (RE = Rare Earth) System
Authors: Takakura Hiroyuki , Sato T. J. , Tsai A. P. , Sato A. , Yamamoto A.
Published: April 2011

A relationship between hexagonal phases in the Zn-Mg-RE (RE=rare earth) system and icosahedral quasicrystalline phase is discussed. There are three hexagonal phases in the vicinity of Zn6Mg3RE1which are related with a lattice parameters by a ratio 3:5:7 but with approximately the samecparameters. A structural model for the phase with medium a lattice parameter is proposed based on the structures of the other two phases recently determined. We also show a preliminary result of X-ray diffraction study of the Zn-Mg-Ho icosahedral phase.
Structural Phase Transitions Of Decagonal Quasicrystals
Authors: Steurer W.
Published: April 2011

A two step model for the transformation of decagonal quasicrystals (DQC) to their crystalline approximants is presented. First, a partially disordered approximant domain structure results rapidly from small atomic displacements. Subsequently, atomic diffusion leads very slowly to an ordered approximant crystal (AC) structure. The model is discussed on atomic scale. Its basic idea is to describe the structure of a quasicrystal (QC) as incommensurately modulated structure (IMS). In that description, the average structure (AS) of a QC is at the same time the AS of all its irrational and rational approximants. In this approach, the positional quasicrystal- to-approximant-crystal (QC-AC) transformation can be performed by atomic displacements smaller than any interatomic distance. This mechanism, however, leads inherently to to a certain amount of chemical disorder. In most cases also some positional disorder will result. The tools developed for the description of phase transitions of IMS can analogously be used for QC. Examples for one- and two-dimensional quasiperiodic structures are discussed.
Room Temperature Oxidation Of AI-Cu-Fe and Al-Cu-Fe-Cr Quasicrystals
Authors: Pinhero Patrick J. , Sordelet Daniel J. , Anderegg James W. , Brunet Pierre , Dubois Jean-Marie , Thiel Patricia A.
Published: April 2011

We have investigated formation of oxides on quasicrystalline and crystalline alloy surfaces of similar composition, in different oxidizing environments. This includes a comparison between a quaternary orthorhombic approximate of Al-Cu-Fe-Cr quasicrystal and the ternary Al-Cu-Fe quasicrystalline and crystalline phases. We noted that each sample showed the following common trends: preferential oxidation of the Al, enrichment in the concentration of Al present at the surface upon oxidation, water concentration is directly related to oxide thickness, and the oxide thickness displays a strong correlation with the bulk concentration of Al in the sample.
A New Series Of Al-Pd-Based Ordered Icosahedral Quasicrystals And Their Electrical Resistivities
Authors: Tamura R , Asao T , Tamura M , Takeuchif S , Shibuya T
Published: April 2011

X-ray measurements on samples of Al-Pd-(Fe,Ru,Os) systems with various annealing temperatures and times have been performed in order to gain insights into formations of icosahedral (I) phases and related approximants and to produce single phases of them. As a result, well ordered I phases have been obtained in Al-Pd-(Ru,Os) systems when annealed at 1000 °C for several hours. Formations of the (1/0-1/0-1/0) and the (2/1-2/1-2/1) approximants have been observed in wide compositional ranges in Al-Pd-(Fe,Ru,Os) and Al-Pd-(Ru,Os) systems, respectively. Electrical resistivity measurements of I phases and related approximants are performed as a function of temperature. The resistivity of I phases at room temperature is as high as that of highly resistive I phases such as Al-Pd-Mn and AI-Cu-(Fe,Ru) systems and the resistivity ratio (P12K/P300K) reaches about 1.5 in Al-Pd-Ru I phases. A negative temperature coefficient of the resistivity is observed in (1/0-1/0-1/0) approximants of Al-Pd-(Fe,Ru) systems, which indicates the electronic transport is quite anomalous even in the lowest approximant of the I phase.
Influence Of Hydrogen On The Thermal Stability Of Zr-Based Quasicrystals
Authors: Zander D. , Köster U. , Eliaz N. , Eliezer D. , Plachke D.
Published: April 2011

The high number of tetrahedrally coordinated sites for interstitial hydrogen and the favorable hydrogen-metal chemistry make quasicrystalline Zr-Cu-Ni-Al alloys a candidate for hydrogen storage applications. The icosahedral phase in Zr69.5Cu12Ni11Al7.5 has been shown to absorb electrochemically hydrogen up to H/M = 1.6. Only partially desorption of hydrogen was observed by TDA at about 500°C. Since hydrogen desorption seems to be hindered by a surface barrier, the hydrogen remains in the quasicrystal thus influencing their thermal stability. Such effects of hydrogen were studied by DSC and microstructural investigations. With increasing hydrogen content the decomposition of icosahedral Zr69.5. Cu12Ni11A17.5 is shifted to lower temperatures. Quasicrystals decompose by a discontinous transformation into the tetragonal Zr2Cu phase, the tetragonal Zr2Ni and the hexagonal Zr6NiAl2. Hydrogenation does not influence the phases formed during decomposition, but leds to the formation of a finer microstructure. We assume that the defects formed during the hydrogenation accelerate the nucleation of the stable crystalline phase.
Experimental Observations And Theoretical Modelling Of Equilibrium Quasicrystalline Phase In A Commercial Maraging Steel
Authors: Nilsson J.-O. , Liu P. , Dzugutov M.
Published: April 2011

A recently developed maraging steel of type 12%Cr-9%Ni-4%Mo-2%Cu-l%Ti in which precipitation strengthening is caused by quasicrystalline precipitates is presented. The steel is used in its tempered condition as surgical instruments such as surgical needles and dental reamers. The high strength and the resistance to tempering induced softening is in part attributable to the formation of quasicrystalline precipitates of icosahedral symmetry. Profuse nucleation in combination with slow coarsening of precipitates are explicable in terms of a low surface energy associated with quasicrystallinity. There is experimental evidence supporting the conclusion that the quasicrystalline precipitates in the present material are formed under thermal equilibrium conditions. To rationalise this conclusion, we compare it with the thermodynamics and kinetic properties of an equilibrium quasicrystalline phase simulated by molecular dynamics. The simulation has demonstrated that the stability of this quasicrystal with respect to the crystalline ground state may be attributed to the large configurational entropy arising from the thermally activated phason dynamics. We argue that this mechanism, suggested by the random tiling model, may account for the observed equilibrium quasicrystalline precipitates in metallic alloys.
Self-Similar Porosity In Quasicrystals
Authors: Janot C. , Loreto L. , Farinato R. , Mancini L. , Baruchel J. , Gastaldi J.
Published: April 2011

Single grains of real quasicrystals have been investigated using X-ray imaging techniques at a third-generation synchrotron radiation source (ESRF). Facetted microholes have been observed. The experimental results are discussed with reference to a description of the quasicrystal structure in terms of overlapping atomic clusters (self-similar packing or random covering modifications). Relations between holes and quasicrystal formation are also discussed.