bluuebunny/crossref_metadata_2025 huggingface dataset:
Dislocation Glide Resistance in a Model Quasicrystalline Lattice
Authors: Tamura R. , Takeuchi S. , Edagawa K.
Published: April 2011

The glide resistance of edge dislocations gliding along a two-dimensional quasiperiodic lattice (Burkov II model of the decagonal quasicrystal) has been calculated. The glide resistance consists of τphason and τPeierls components and the τPeierls component depends strongly on the orientation of the dislocation. For the orientation of large τPeierls component, the τphason component is about half of the τPeierls component for individual dislocation glide but becomes negligibly small for glide of a pair of dislocations. The largest τPeierls component is about 0.1G (G: the shear modulus).
Phase Transitions in Quasicrystals Induced by Friction and Wear
Authors: Dong Chuang , Wu Jinsong , Zhang Liming , Dubois Jean-Marie , Brunet Pierre , Zhou Qinggang , Wang Dehe , Zhang Huichen
Published: April 2011

Certain ductility may occur during friction tests on quasicrystalline materials that are intrinsically brittle. This is, at least in part, due to a solid-state phase transition from the icosahedral to a BCC phase. The present paper first summarizes phase transition features of quasicrystals and then examines the microstructural mechanism of scratch indentation on an icosahedral Al-Cu-Fe sample. The last part of this paper is devoted to a discussion of the correlation of this phase with respect to quasicrystals.
Influence of Grain Orientation on Friction and Wear Behavior in Quasicrystalline Alloys
Authors: Duckham Alan , Shechtman Dan , Grushko Benjamin
Published: April 2011

The influence of grain orientation on the friction and wear behavior of single-phase quasicrystalline alloys is investigated. Pin-on-disc wear tests and friction coefficient measurements have been performed on bulk samples of icosahedral phase AlPdMn and decagonal phase AlNiCo alloys. Wear behavior has been evaluated by optical microscopy, scanning electron microscopy (SEM) and electron back-scattered diffraction (EBSD). Coefficient of friction values of the decagonal AlNiCo alloy are found to be widely scattered (between 0.03 and 0.55) compared to the far more consistent values of the icosahedral alloys. This is explained by considering the significant variation in wear behavior that is observed between different – sometimes adjacent – oriented grains in the decagonal specimens.
Comparative Study of the Tribological and Oxidative Properties of AlPdMn Quasicrystals and Their Cubic Approximants
Authors: Mancinelli Chris , Ko Jeff S. , Jenks Cynthia J. , Thiel Patricia A. , Ross Amy R. , Lograsso Thomas A. , Gellman Andrew J.
Published: April 2011

An experimental comparison has been made between the properties of the surfaces of an Al70Pd21Mn9 quasicrystal and its Al48Pd42Mn10 approximant. The Al70Pd21Mn9 sample was a single grain icosahedral quasicrystal cut to expose its five-fold symmetric (000001) surface. The approximant was polycrystalline β-phase Al48Pd42Mn10, which has a CsCl-type cubic structure. Surfaces of both were prepared under ultra-high vacuum (UHV) conditions and then used for comparative measurements of their frictional properties and oxidation rates. Both materials are oxidized by reaction with O2 to form a thin film of aluminum oxide that ultimately passivates their surfaces. The interesting difference between the two is that the rate of oxidation of the approximant is significantly higher than that of the quasicrystal in spite of the fact that the bulk Al concentration of the approximant is lower than that of the quasicrystal. Friction measurements were made under UHV conditions between pairs of quasicrystals and pairs of approximants whose surfaces were either clean or oxidized to varying degrees. The friction between pairs of the approximant surfaces is significantly higher than that measured between the quasicrystal surfaces under all conditions of surface oxidation.
An Investigation of Sputtered Al-Cu-Fe-Cr Quasicrystalline Films Via Synchrotron Diffraction
Authors: Daniels M.J. , Maciejewski J. , Zabinski J.S. , Rek Z.U. , Yalisove S.M. , Bilello J.C.
Published: April 2011

Analysis of the phase relationships of Al-Cu-Fe-Cr based sputtered films, before and after various annealing treatments, was carried out via grazing incidence x-ray scattering (GIXS) at SSRL on beamline 7-2. The “as-deposited” films were composed primarily of nanoscale grains whose structure were mainly quasicrystalline (QC) and its approximants, plus a nano-crystalline phase was also observed in one sample. None of these precursor films exhibited any degree of amorphous character. GIXS studies, in the symmetric and asymmetric modes, revealed that grain growth due to the post-deposition heat treatments caused a decagonal QC phase to evolve and dominate the crystalline phases, which composed the film. These ≍10 µm thick films were created by sputter deposition with a DC magnetron source via an Al-Cu-Fe-Cr powder pressed target, giving a nominal sample composition of Al75Cu4Fe8Cr10O3. Two types of annealing treatments were performed using an Argon atmosphere. Anneal of a precursor film for 4 hours at 500°C showed a large fraction of decagonal and approximant phases, while anneal of a precursor film for 2 hours at 400°C and 2 hours at 550°C showed a large fraction of the crystalline phase.
Aluminum-Based Quasicrystals Studied by Slow Positron Beam Technique
Authors: Sato K. , Uchiyama H. , Kanazawa I. , Tamura R. , Kimura K. , Komori F. , Suzuki R. , Ohdaira T. , Takeuchi S.
Published: April 2011

Icosahedral quasicrystals Al71.5Pd20.3Mn8.2, Al70.7Pd21.34Re7.96, Al62.5Cu25.5Fe12.5, and α-Al68.31Mn21.21Si10.48 1/1- approximant were investigated by using a monoenergetic slow positron beam. The structural vacancy densities in the first three samples were determined to be 5.0×1020, 7.7×1020, and 4.7×1020 cm−3, respectively, by analyzing the measured S-parameter.
Experimental Studies and Molecular Dynamics Simulations of the Sliding Contact of Metallic Glass
Authors: Fu Xi-Yong , Falk Michael L. , Rigney David A.
Published: April 2011

Tribological properties of bulk metallic glass Zr41.2Ti13.8Cu12.5Ni10.0Be22.5 were studied experimentally using a pin/disk geometry without lubrication. Experimental observations were compared with 2D molecular dynamics simulations of amorphous material in sliding contact. The friction coefficient and the wear rate of bulk metallic glass (BMG) depend on normal load and test environment. The sliding of annealed BMG re-amorphizes devitrified material. A mechanically mixed layer is generated during sliding; this layer has enhanced oxygen content if the sliding is in air. The MD simulations show that atomic scale mixing occurs across the sliding interface. The growth kinetics of the mixing process scales with the square root of time. In the simulations, a low density region is generated near the sliding interface; it corresponds spatially to the softer layer detected in experiments. Subsurface displacement profiles produced by sliding and by simulation are very similar and are consistent with the flow patterns expected from a simple Navier-Stokes analysis when the stress state involves both compression and shear.
Abrasive Wear Resistance of Bulk Metallic Glasses
Authors: Greer A. Lindsay , Myung Wha-Nam
Published: April 2011

This paper reviews work on the wear of metallic glasses in general, as well as reporting recent results on the abrasive wear of bulk metallic glasses. The distinctive mechanical properties of metallic glasses make their wear resistance of fundamental interest. Metallic glasses, and the partially or fully crystalline materials derived from them, can have very good resistance to sliding and abrasive wear. Standard wear laws are followed, with behaviour similar to that of conventional hardened alloys. The microhardness and abrasive wear resistance are measured for four bulk metallic glasses (based on La, Mg, Pd or Zr). The hardness and wear resistance correlate well with the Young's modulus of the glass.
Plasticity at Crack Tips in Zr-Based Bulk Metallic Glasses
Authors: Li Jing , Gu Xiaofeng , Xing Li-Qian , Livi Ken , Hufnagel T. C.
Published: April 2011

We have examined the structure of plastic deformation zones ahead of the tips of microcracks in Zr-based bulk metallic glass Zr57Ti5Cu20Ni8Al10. We have used an axially aligned dark field transmission electron microscopy technique, with the objective aperture placed to form images using electrons from several different areas of the diffraction patterns. We also compared Fourier transforms of the high resolution transmission electron microscopy images of deformed and undeformed regions to extract additional structural information. The plastic zones produce enhanced low-angle scattering of electrons and an apparent broadening of the amorphous halo, suggesting increased disorder and the presence of voids in the deformed zones. These results are consistent with an increased degree of atomic-scale disorder and enhanced free volume in highly deformed regions, which may provide a partial explanation of the manner in which plastic deformation occurs in metallic glasses.
Shaping of Bulk Metallic Glasses by Simultaneous Application of Electrical Current and Low Stress
Authors: Yavari A.R. , Oliveira M. F. de , Botta W.J.
Published: April 2011

Using the intrinsic materials properties of bulk metallic glasses (BMG), namely electrical resistivities two orders of magnitude higher than good conductors and a Newtonian viscous-flow regime of deformability, a new electromechanical process has been developed for shaping, joining and engraving of BMGs. The wider the liquid supercooled region between the glass transition temperature Tg and the crystallisation temperature Tx of the bulk metallic glass, the easier the application of the new process. In this range, the undercooled liquid deforms in a quasi-Newtonian way, allowing thermomechanical shaping in the low viscosity range as for oxide glasses. The new electromechanical processing technology has been used for economical and rapid shaping at low applied stresses by eliminating the thermal mass of the furnace and the need to heat the deformation dies. The process parameters are adaptable for the full maintenance of the glassy state or when desired, for appropriate compositions, for nanocrystallisation during the joining or shaping operation.
Glass Formation and Glass Forming Ability of Alloys Near Eutectic Composition
Authors: Li Y.
Published: April 2011

Onset temperature, Tm and offset temperature (liquidus) Tl of melting of a series of bulk glass forming alloys based on La, Mg, and Pd have been measured by studying systematically the melting behaviour of these alloys using DTA or DSC. Bulk metallic glass formation has been found to be most effective at or near their eutectic points and less effective for off-eutectic alloys. Reduced glass transition temperature Trg given by Tg/Tl is found to show a stronger correlation with critical cooling rate or critical section thickness for glass formation than Trg given by Tg/Tm.
Calculation of Glass-Forming Ability in the Ni-Zr and Ni-Ti Systems from Interatomic Potentials
Authors: Lai W.S. , Liu B.X.
Published: April 2011

For the Ni-Zr and Ni-Ti systems, Molecular-dynamics (MD) simulations are conducted to compare the relative stability of the terminal solid solutions versus the corresponding amorphous states as a function of solute concentrations. It turns out that the terminal solid solutions transform into an amorphous state spontaneously when the solute concentrations are beyond the maximum allowable values, i.e. the critical solubilities, determined to be 14 at.% Zr in Ni and 25 at.% Ni in Zr for Ni-Zr system and 38 at.% Ti in Ni and 15 at.% Ni in Ti for the Ni-Ti system, respectively. The glass-forming ranges are therefore deduced to be within the respective critical solubilities, i.e. 14-75 at.% Zr and 38-85 at.% Ti for the Ni-Zr and Ni-Ti systems, respectively, which are compatible with those from experiments and/or from the generalized Lindemann criterion. Moreover, MD simulation also reveals that solid-state amorphization does take place and that the growth of the amorphous interlayer follows exactly a t½ law. Besides, a solubility criterion is proposed that the lower the maximum solid solubility the less stable is the lattice of the metal upon solid-state reaction and it can explain the fact that the growing speed of amorphous interlayer toward Ni (melting point = 1528 K) is greater than that directed to the Zr (2128 K) lattice, while it is smaller than that to Ti (1941 K) side.
Critical Cooling Rate and Thermal Stability in Zr-Ti-Cu-Ni-Be Bulk Metallic Glasses
Authors: Waniuk Theodore A. , Schroers Jan , Johnson William L.
Published: April 2011

The crystallization behavior of a series of alloys in the Zr-Ti-Cu-Ni-Be system is studied. Upon cooling from the molten state with different rates, alloys with compositions ranging along a tie line from (Zr75Ti25)55(Ni45Cu55)22.5Be22.5 (Vit1) to (Zr85Ti15)55(Ni57Cu43)17.5Be27.5 (Vit4) show a continuous increase in the critical cooling rate to suppress crystallization. In contrast, thermal analysis of the same alloys shows that the undercooled liquid region, the temperature difference between the glass transition temperature and the crystallization temperature, is largest for compositions midway between the two endpoints, revealing that glass forming ability does not correlate with thermal stability. The relationship between the change in glass forming ability and thermal stability is discussed with reference to a chemical decomposition process.
Synthesis and Stability of Amorphous Al Alloys
Authors: Perepezko J. H. , Wu R.I. , Hebert R. , Wilde G.
Published: April 2011

The recent innovations in metallic glasses have led to new alloy classes that may be vitrified and a re-examination of the commonly used criteria for glass formation and stability. The new alloy classes are usually at least ternary systems and often higher order that can be grouped into two general categories. In one case large, bulk volumes may be slowly cooled to the glassy state which signifies a nucleation controlled synthesis. The other important class is represented by Al and Fe based glasses that can be synthesized only by rapid solidification processes such as melt spinning. These glasses are often called marginal glass formers that are synthesized under growth controlled kinetic conditions. With marginal glass forming alloys the termination of the amorphous state upon heating is often not characterized by a clear glass transition signal, Tg, but instead by the rapid onset of a primary crystallization reaction that represents the partial crystallization into a high number density of nanocrystals of the major component (i.e. Al or Fe) dispersed within a residual amorphous matrix. However, a closer examination by modulated or dynamic differential scanning calorimetry (DDSC) has identified a true reversible Tg signal that confirms the amorphous state and has revealed additional low temperature annealing behavior that impacts the overall thermal stability and microstructure evolution. At the same time alternate synthesis strategies involving deformation alloying by intense cold rolling reveals that the primary crystallization reaction can be bypassed. Alternatively other approaches have demonstrated that by suitable alloying it is possible to innoculate the primary crystallization reaction and increase the density of primary nanocrystals by about an order of magnitude. These developments represent a major level of microstructure control that have an impact on the structural performance and stability.
Formation of Nano Icosahedral Quasicrystalline Phase in Zr-based Binary and Ternary Glassy Alloys
Authors: Saida Junji , Matsushita Mitsuhide , Inoue Akihisa
Published: April 2011

It is found that a nano icosahedral phase with diameters below 50 nm is formed as a primary phase in the Zr70Ni10M20, Zr70TM10Pd20, Zr70Au10Pd20 and Zr75Pt10Pd15 ternary and Zr70Pd30 binary glassy alloys. The nanoscale icosahedral phase in the diameter range below 10 nm was also found to be formed directly in the melt-spun Zr80Pt20 binary alloy. These icosahedral phases transform to the crystalline phase(s) at the higher annealing temperature. The nucleation kinetics for the precipitation of the icosahedral phase from supercooled liquid were examined in the Zr70Pd30 and Zr70Ni10Pd20 glassy alloys. It was clarified that the transformation of both alloys proceeds in the diffusion-controlled growth mode with increasing nucleation rate. The formation of the nanometer-scale icosahedral phase is due to the transformation mode. The activation energy of nucleation is evaluated to be 267 kJmol−1 for the binary alloy and 311 kJmol−1 for the ternary alloy. The difference between the two alloy systems seems to originate from the difference in the number of atoms for rearrgements in the nucleation mode. The short-range ordering is observed in the as-quenched Zr70Pd30 glassy alloy, which is indicative of the icosahedral structure. The formation of the nano-scale icosahedral phase in the Zr-based binary and ternary alloys is due to the existence of an icosahedral short-range order in the glassy or liquid state. It is suggested that the icosahedral short-range order is stabilized by the restraint of the long-range atomic rearrangements that lead to the transition to a periodic structure by the strong chemical affinities of Pd or Pt with Zr.
Local Magnetic Structure and Magnetic Properties of Nd50Fe40Si10-xAlx Bulk Glassy Hard Magnets
Authors: Chiriac H. , Lupu N. , Vandenberghe R.E. , Rao K.V.
Published: April 2011

The large values over 300 kA/m at room temperature were found for the coercive field of Nd50Fe40Si10−xAlx (x = 0; 10) amorphous melt-spun ribbons and cast rods. The dependence of the coercive field on the thickness and the preparation method, namely the cooling rate, is ascribed to the existence in the amorphous matrix of very small ferromagnetic clusters, which approach of a single magnetic domain. Coercivity increases substantially up to 600 kA/m by decreasing the temperature down to 200 K, although the measurements are carried out in low fields (up to 800 kA/m). The Mässbauer spectra can be fitted to two Fe sites with magnetic moments ranging from 1.25 to 2.20νB depending on the additional element.
R&D Status and Needs for Improved EB-PVD Thermal Barrier Coating Performance
Authors: Leyens C. , Schulz U. , Bartsch M. , Peters M.
Published: April 2011

The key issues for thermal barrier coating development are high temperature capability and durability under thermal cyclic conditions as experienced in the hot section of gas turbines. Due to the complexity of the system and the interaction of the constituents, performance improvements require a systems approach. However, there are issues closely related to the ceramic top coating and the bond coat, respectively. Reduced thermal conductivity, sintering, and stresses within the ceramic coating are addressed in the paper as well as factors affecting failure of the TBC by spallation. The latter is primarily governed by the formation and growth of the thermally grown oxide scale and therefore related to the bond coat. A strategy for lifetime assessment of TBCs is discussed.
Non-Destructive Characterisation of Thermal Barrier Coatings Using Impedance Spectroscopy
Authors: Mei Junfa , Wang Xin , Xiao Ping
Published: April 2011

Impedance spectroscopy (IS) has been used to characterise the degradation of thermal barrier coatings (TBCs) due to thermal treatments at 1100°C for a period up to 200 hrs. The growth of the oxide layer at the yttria stabilised zirconia (YSZ)/bond coat interface in TBCs can be examined by measuring the impedance diagrams. From the analysis of the impedance diagrams of TBCs, the electrical properties of YSZ were found to be nearly constant during the thermal treatments, indicating there was little change in the microstructure and composition of YSZ. However, there was a clear change in the electrical properties of the oxide layer in the TBCs after thermal treatments, suggesting both microstructural and composition changes occurred in the oxide layer. These studies indicate that the IS is a very useful method in non-destructive characterisation of the degradation of TBCs.
In-Plane Cracking Behavior Near and Away from Interface of Thermal Barrier Coatings and Thermally Grown Oxides
Authors: Zhang Z. , Kameda J. , Swanson A. H. , Sakurai S.
Published: April 2011

The initiation characteristics of in-plane cracks near and away from the interface of thermal barrier coatings (TBC) and thermally grown oxides (TGO) have been studied using a protruded four-point bend testing technique together with a finite element analysis. In-plane TBC cracks were initiated near and away from the TBC/TGO interface, respectively, in protruded specimens without and with grooved substrates. It was shown that the onset of in-plane TBC cracks near or away from the interface in the protruded TBC tests was controlled by the out-of-plane tensile stress but not by the principal tensile stress acting upon an inclined plane to the interface. The critical local tensile stress for the initiation of TBC cracks near the interface was found to be 20% lower than that away from the interface. The TBC cracking near and away from the TBC/TGO interface is discussed in light of the residual stress distribution through the TBC thickness.
Measuring the Properties of Diffusion Aluminide Bond Coat Layers with Microsample Tensile Testing
Authors: Pan Deng , Wright P.K. , Hemker K.J.
Published: April 2011

The bond coat layer plays an important role in the TBC system by providing oxidation resistance and a foundation for the ceramic top coat. The thermal cyclic durability of a TBC is thought to be strongly dependent on the physical and mechanical properties of the bond coat layer. Attempts to measure these properties for as-deposited and thermally cycled diffusion aluminide bond coats have been greatly inhibited by the limited thickness (∼60 μm) of these coatings. In the present study, novel high temperature microsample tensile testing technique is employed to measure the Young's modulus (E), coefficient of thermal expansion (CTE), yield strength and stress relaxation behavior of an as-deposited (Ni, Pt)Al bond coat in the temperature range of 25 oC to 1150 °C. Preliminary results for these material parameters are reported here. They are being used as material inputs for an independent finite element analysis (FEA) study of the development of stresses in the TBC layers during thermal cycling.
A Numerical Model of Ratcheting in Thermal Barrier Systems
Authors: Karlsson Anette M. , Evans Anthony G.
Published: April 2011

Morphological instabilities in thermally grown oxide, observed in a range of thermal barrier systems, have been simulated by developing and using a numerical code. The simulations are based on a range of phenomena and constituent properties, such as plasticity in the bond coat and growth strains in the TGO at high temperature. One of the key implications is that the incidence of reverse yielding upon reheating is a necessary condition for morphological instabilities. That is, whenever the condition for reverse yielding is satisfied during the initial cycles, the imperfection amplitude increases with thermal cycling (ratcheting). Otherwise, shakedown occurs, i.e., the imperfection amplitude stops growing.
Phase Identification in Heated Thermal Barrier Coatings using Microbeam X-ray Diffraction Combined with Quantitative X-ray Mapping
Authors: Verkouteren Jennifer R. , Marinenko Ryna B. , Bright David S.
Published: April 2011

Phase evolution in a yttria-partially-stabilized zirconia (Y-PSZ) thermal barrier coating (TBC) was studied by using quantitative electron probe x-ray mapping combined with microbeam x-ray diffraction. The as-sprayed coating contains a single t' phase with the composition of the feedstock material. After annealing at 1400 °C for 1 hour, the yttria content of the t' phase increases slightly to compensate for the formation of 0.06 mass fraction of a t phase containing 0.01 mole fraction YO1.5. These results are in contrast to those determined by bulk x-ray and neutron diffraction, and therefore highlight the need for multiple techniques of phase analysis in Y-PSZ.
Development of a Creep Resistant Tial-Base Alloy
Authors: Deevi S.C. , Zhang W.J. , Liu C.T. , Reddy B.V.
Published: April 2011

In this paper, the mechanical and physical properties of a Ti-47Al-4(Nb, W, B) alloy (CTI-8) developed by Chrysalis Technologies are presented. The properties of CTI-8 alloy are compared with other TiAl-base alloys and currently used materials for high temperature applications. The CTI-8 alloy exhibits excellent creep resistance, good high temperature strength and better physical properties than the currently used materials. The comparisons suggest that CTI-8 has a great potential in aerospace and automotive industries.
Creep Behavior and Microstructural Stability of Lamellar γ-T1AI (Cr, Mo, Si, B) with Extremely Fine Lamellar Spacing
Authors: Schillinger Wolfram , Zhang Dezhi , Dehm Gerhard , Bartels Arno , Clemens Helmut
Published: April 2011

γ-T1AI (Cr, Mo, Si, B) specimens with two different fine lamellar microstructures were produced by vacuum arc melting followed by a two-stage heat treatment. The average lamellar spacing was determined to be 200 nm and 25–50 nm, respectively. Creep tests at 700°C showed a very strong primary creep for both samples. After annealing for 24 hours at 1000 °C the primary creep for both materials is significantly decreased. The steady-state creep for the specimens with the wider lamellar spacing appears to be similar to the creep behavior prior to annealing while the creep rate of the material with the previously smaller lamellar spacing is significantly higher. Optical microscopy and TEM-studies show that the microstructure of the specimens with the wider lamellar specing is nearly unchanged, whereas the previously finer material was completely recrystallized to a globular microstructure with a low creep resistance. The dissolution of the fine lamellar microstructure was also observed during creep tests at 800 °C as manifested in an acceleration of the creep rate. It is concluded that extremely fine lamellar microstructures come along with a very high dislocation density and internal stresses which causes the observed high primary creep. The microstructure has a composition far away from the thermodynamical equilibrium which leads to a dissolution of the structure even at relatively low temperatures close to the intended operating temperature of γ-T1AI structural parts. As a consequence this limits the benefit of fine lamellar microstructures on the creep behavior.
Creep Mechanisms in Equiaxed and Lamellar Ti-48Al
Authors: Viswanathan G. B. , Karthikeyan S. , Vasudevan V. K. , Mills M. J.
Published: April 2011

Minimum creep rates as a function of stress have been obtained for Ti-48Al binary alloy with a near gamma and a fully lamellar microstructure. TEM investigation reveals that deformation structures in both microstructures are dominated by jogged screw 1/2[110] dislocations in γ phase. A modif ied jogged screw model is adopted to predict minimum creep rates where the rate controlling step is assumed to be the non-conservative motion of 1/2[110] unit dislocations. In the case of equiaxed microstructure where the deformation was mostly uniform, the creep rates predicted by this model were in agreement with experimental values. Conversely, deformation in lamellar microstructures were highly inhomogeneous where the density of jogged 1/2[110] unit dislocations were seen in varying proportions depending on the width of the γ laths. The creep rates and stress exponents in these microstructures is explained in terms of active volume fractions of γ laths participating in deformation for a given applied stress.
Anomalous Strengthening Mechanism in NbSi2-Based Silicide Single Crystals
Authors: Umakoshi Yukichi , Nakano Takayoshi , Azuma Masafumi
Published: April 2011

Anomalous strengthening behavior of NbSi2-based silicide single crystals with C40 structure was examined focusing on the effect of substitutional alloying elements and the formation of a solute dragging atmosphere. After pre-straining to 1% at temperatures between 600 and 1600°C, single crystals were deformed at 400°C to examine the effects of dislocation sources and segregation of solute atoms around dislocations. The stress amplitude of yield drop was derived by the segregation of solute atoms at the superlattice intrinsic stacking fault (SISF) between two superpartials. Introduction of dislocation sources by pre-straining remarkably improved fracture strain, but the segregation of solute atoms around dislocations during pre-straining at higher temperatures induced a rapid drop of fracture strain. Change in the SISF energy by static and dynamic aging was observed to obtain evidence of segregation of solute atoms at the SISF between two superpartials.
Microstructures and thermoelectric power of the higher manganese silicide alloys
Authors: Kakubo Kimiko , Kimura Yoshisato , Mishima Yoshinao
Published: April 2011

In the present work, we investigate microstructures and thermoelectric power of the binary higher manganese silicide (hereafter denoted as HMS) base alloys prepared by ingot metallurgy, i.e., arc-melting in an Ar gas atmosphere. Alloys with 63.5, 64.0 and 64.5 at% Si have a small amount of second phase, either the primary metallic MnSi phase or Si solid solution (hereafter denoted as (Si)). A slight revision is thereby made on the HMS single phase region of the Mn-Si binary phase diagram. The presence of MnSi seems to reduce the thermoelectric power of the HMS, while that of (Si) enhances it. An alloy having the HMS/(Si) eutectic microstructure, 67.9at%Si, shows the highest thermoelectric power in the Mn-Si binary alloys examined. Moreover, effect of doping the p-type elements, Fe and Cr, on the thermoelectric power and microstructural features is also investigated for a 63.5at%Si alloy which consists mostly of HMS phase with a small amount of (Si) phase. It is shown that both elements, especially 0.5 at% Cr addition, enhance the thermoelectric power of the alloys.
The Role of Carbon and Vacancies in Determining the Hardness of FeAl Intermetallic in the Quenched and the Aged States
Authors: García-Oca C. , Morris D.G. , Muñoz-Morris M.A. , Deevi S.C.
Published: April 2011

The role of quenched-in vacancies in FeAl intermetallics on producing considerable hardening is well known, as is the softening on annealing as vacancies are annihilated. The present study examines quench hardening and anneal softening by quenched-in vacancies and interstitial carbon solute in Fe-40Al-C. Interstitial carbon is seen to be a more potent hardening agent than the vacancy, while the co-annihilation of vacancies and carbon atoms from solution during annealing leads to dislocation loop debris, and equiaxed or plate-like carbide precipitation, according to the annealing conditions. The processes occurring have been followed by detailed TEM studies, and are discussed in terms of the relative solubilities and diffusion rates of vacancies and carbon. The relevance of such interstitial solute hardening to the behaviour of other FeAl intermetallics is also briefly considered.
High-Temperature Strength of Ir-Based Refractory Superalloys
Authors: Yamabe-Mitarai Yoko
Published: April 2011

Ir-based refractory superalloys with an fcc and L12two-phase structure similar to that in Ni-based superalloys but with considerably higher melting temperatures are proposed. First, the microstructure and strength are shown for several kinds of binary alloys. Then, the effect that the third element and the combination of Ni-Al with an fcc and L12two-phase structure in binary alloys had on the lattice misfit, microstructure, and strength was investigated. The precipitation-hardening effect in Ir-based alloys was investigated and discussed in terms of precipitate morphology and lattice misfit. The creep properties of Ir-based alloys were also investigated.
High-Temperature Materials Based on Quaternary Ir-Nb-Ni-Al Alloys
Authors: Yu X. , Yamabe-Mitarai Y. , Ro Y. , Nakaza S. , Harada H.
Published: April 2011

A novel method to develop new quaternary alloys with an fcc/L12 coherent structure is proposed. This paper reviews the development of quaternary Ir-Nb-Ni-Al alloys. The microstructure, lattice misfit, and compressive 0.2% flow stress of 15 kinds of alloys were investigated systematically. Two kinds of coherent structures, fcc/L12-Ir3Nb and fcc/ L12-Ni3Al, were observed in most alloys. Two two-phase structures, fcc+L12-Ir3Nb and fcc+L12-Ni3Al, were observed in Ir-rich and Ni-rich regions, respectively. The lattice misfits of quaternary Ir-Nb-Ni-Al alloys were higher than those of Ni- or Ir-base binary alloys. The compressive 0.2% flow stresses of quaternary alloys increased dramatically compared with those of Ni-base superalloys. The quaternary alloys located in the Ir-rich region were not only had higher strength but also better ductility than Ir-base binary alloys. The potential use of quaternary alloys is discussed.
Hydrogen Pulverization in Intermetallic-based Alloys
Authors: Semboshi Satoshi , Masahashi Naoya , Hanada Shuji
Published: April 2011

Pulverization behavior and microstructure evolution with hydrogenation in hydrogen absorbing Ta-Ni intermetallic-based alloys, such as Ta2Ni with Ta solid solution (Tass), TiMn2 with TiMn and Nb3Al with Nb solid solution (Nbss), are investigated to elucidate the mechanism of the hydrogen pulverization. Ta-10at.%Ni consisting of Ta solid solution (Tass) and Ta2Ni Laves phase is pulverized to coarse powder over 100 μm in hydrogenation. Crack propagation occurs preferentially in the brittle Ta2Ni phase rather than in the ductile Tass phase. When the volume fraction of brittle Ta2Ni increases with increasing Ni content, hydrogen pulverization is enhanced. The lattice parameter of Tass increases by hydrogenation, while it does not change in Ta2Ni. In addition, nano-sized regions with Moiré patterns are produced in Tass and Debye rings corresponding to tantalum hydride β-TaH appear in the diffraction pattern. These features are very similar to those of TiMn2 based alloy and Nb3 Al based alloys in the literature. Based on the present results along with those in the literature it is concluded that the hydrogen pulverization is attributable to (1) the absorption of a large amount of hydrogen in constituent phase(s), (2) the large strain introduced by lattice expansion and the hydride formation, and (3) the ease of crack nucleation and propagation in brittle constituent phase(s).
High Temperature Compressive Properties and Deformation Microstructure of Fe-25Al-10Ti Intermetallic Alloy
Authors: Zhu Su-Ming , Sakamoto Kazushi , Tamura Makoto , Iwasaki Kunihiko
Published: April 2011

The compressive properties of Fe-25Al-10Ti intermetallic alloy were studied as a function of temperature and strain rate. Optical microscopy and transmission electron microscopy (TEM) were used to examine the deformation microstructure. The alloy exhibited strong tendency of strain hardening at low temperatures. A positive temperature dependence of strength was observed in the 673 – 873 K range. The mechanical behavior was interpreted in terms of dislocation structures. Based on the analysis of the stress-strain response, strain rate sensitivity and deformation microstructure, possible hot working range was proposed.
Mechanical Properties of Fe3Al Intermetallic Matrix Composites
Authors: Park B.G. , Ko S.H. , Park Y.H.
Published: April 2011

Iron aluminides are of considerable interest due to their low cost, relatively high melting point, relatively low density, and excellent resistance to oxidation, sulfidation and molten salts. However, poor ductility and fracture toughness at room temperature hinder their use as a structural material. Refining of the microstructure is known to be one method to increase the room temperature ductility. Mechanical alloying (MA) is an easy way to obtain fine microstructures. In addition, pulse discharge sintering (PDS) is a new technology which suppresses grain growth during sintering because the sparks generated during sintering break the surface oxide layer of the powder particles and thus speed up the sintering process. Therefore, the combination of MA and PDS processes results in final products with very fine microstructures. Fe-28at.%Al alloy and its composite reinforced with 5vol.% of TiB2particles were fabricated by the MA-PDS process. The mechanical properties of these materials were improved significantly as compared to conventionally processed materials.
Tailoring the thermal expansion anisotropy of Mo5Si3
Authors: Schneibel Joachim H. , Rawn Claudia J. , Fu Chong Long
Published: April 2011

The silicide Mo5Si3, with the W5Si3 structure, exhibits a high anisotropy of its coefficients of thermal expansion (CTEs) in the a and c directions, namely, CTE(c)/CTE(a)=2.2. In order to determine whether the CTE anisotropy can be controlled, molybdenum was partially substituted with Nb. CTEs were determined by high temperature x-ray powder diffraction. Partial substitution with 44 at. % Nb reduced the value of CTE(c)/CTE(a) to a value near 1. For higher Nb concentrations, CTE(c)/CTE(a) increased again. When nearly all the Mo was replaced by Nb, the crystal structure changed to the Cr5B3 structure type and the CTE anisotropy decreased to a value near 1. Our thermal expansion results are interpreted in terms of the site occupation of Nb, the Nb-induced increase in the interatomic spacing of the Mo atom chains along the c-direction, and the reduction in the anisotropy of the lattice anharmonicity in the a and c directions.
High-Temperature Compression Strength of Directionally Solidified Nb-Mo-W-Ti-Si In-Situ Composites
Authors: Hirai Hisatoshi , Tabaru Tatsuo , Sha Jiangbo , Ueno Hidetoshi , Kitahara Akira , Hanada Shuji
Published: April 2011

Directionally solidified Nb-xMo-22Ti-18Si (x = 10, 20 and 30 mol%) and Nb-10Mo-yW-10Ti-18Si (y = 0, 5, 10 and 15 mol%) alloys were prepared. All of the alloys consisted of Nb solid solution and (Nb, Mo, (W,) Ti)5Si3 silicide. In compression tests performed on the Nb-xMo-22Ti-18Si alloys, a sample with x = 10 had the highest maximum stress at 1670 K, but it showed a sharp stress drop after reaching the maximum stress. Samples with x = 20 and 30 showed lower yield and lower maximum stress than that with x = 10, and they showed a small stress drop and an almost constant flow stress of 330 and 400 MPa, respectively. The sample with x = 20 showed a minimum creep rate (εm) of 9.6 × 10-7 s-1 at 1670 K at a stress of 200 MPa. The yield stress of the Nb-10Mo-yW-10Ti-18Si alloys increased with increasing W content, although the stress decreased gradually after reaching the maximum stress and showed no steady state deformation. The εm of the sample with y = 15 was 1.4 × 10-7 s-1 at 1670 K at a stress of 200 MPa, which was much slower than that of Nb-20Mo-22Ti-18Si.
Void Morphology In NiAl
Authors: Zakaria M. , Munroe P.R.
Published: April 2011

Void formation in stoichiometric NiAl was studied through controlled heat treatments and transmission electron microscopy. Voids formed at temperatures as low as 400°C, but dissolved during annealing at 900°C. Both cuboidal and rhombic dodecahedral voids were observed, often at the same annealing temperature. At higher annealing temperatures (>800°C) extensive dislocation climb was noted. The relative incidence of void formation and dislocation climb can be related to the mobility of vacancies at each annealing temperature. Further, differences in void shape can be described in terms of their relative surface energy and mode of nucleation.
Fracture Strength of Grain Boundaries in Ni3Al
Authors: Su Jian-Qing , Demura M. , Hirano T.
Published: April 2011

The fracture strength of grain boundaries in Ni3Al was quantitatively measured by performing tensile tests on miniature bicrystal specimens with various grain boundary types. A fairly good relationship between fracture mode, fracture strength and Σ value was established. The Σ1, Σ3 and Σ9 boundaries were found to be strongly crack-resistant but the adjacent bulk fractured. In contrast, Σ5, Σ7, Σ11, Σ13 and random boundaries were found to be less crack-resistant and fractured at lower stresses than the fracture strength of the bulk. In the latter case, the fracture strength of the Σ11 and Σ13 boundaries was higher than that of the Σ5 and Σ7 boundaries. Most random boundaries were as weak as the Σ5 and Σ7 boundaries, but some exhibited high fracture strength comparable to that of the Σ11 and Σ13 boundaries.
Atomistic Modeling Of Ternary And Quaternary Ordered Intermetallic Alloys
Authors: Bozzolo Guillermo , Khalil Joseph , Bartow Matthew , Noebe Ronald D.
Published: April 2011

The structure of ternary and quaternary NiAl-based ordered intermetallic alloys is studied using the BFS method for alloys. A simple calculational procedure, based on the determination of the energetics of local environments surrounding defect atoms is introduced and applied to the study of the defect structure and phase formation in NiAl-based systems. The procedure is illustrated with two different examples: 1) the phase structure of Ni-Al-Fe alloys, focusing on the concentration dependence of the site preference behavior of Fe in NiAl, and 2) the precipitation of a β′ phase in NiAl(Ti,Hf) alloys, focusing on the role of Hf in lowering the solubility limit of Ti in NiAl, thus enhancing the precipitation of a Heusler Ni2Al(Ti,Hf) phase.
Direct Molecular Dynamics Simulations of Diffusion Mechanisms in NiAl
Authors: Farkas D. , Soulé de Bas B.
Published: April 2011

Molecular dynamics simulations of the diffusion process in ordered B2 NiAl at high temperature were performed using an embedded atom interatomic potential. Diffusion occurs through a variety of cyclic mechanisms that accomplish the motion of the vacancy through nearest neighbor jumps restoring order to the alloy at the end of the cycle. The traditionally postulated 6-jump cycle is only one of the various cycles observed and some of these are quite complex. A detailed sequential analysis of the observed 6-jump cycles was performed and the results are analyzed in terms of the activation energies for individual jumps calculated using molecular statics simulations.
Long-Term Creep and Oxidation Behavior of a Laves Phase-Strengthened NiAl-Ta-Cr Alloy for Gas Turbine Applications
Authors: Palm Martin , Sauthoff Gerhard
Published: April 2011

A Laves phase strengthened NiAl-Ta-Cr alloy (IP 75) has been developed for structural applications in gas turbines as well as in heat exchangers. It has a lower density and a higher thermal conductivity as well as a higher melting point than conventional superalloys. Different methods for alloy production have been established including investment casting (IC) and a powder metallurgical method (PM). Creep properties have been determined in compression and tension. Tension tests up to 10000 hours were performed at 900 °C and 1000 °C for both PM and IC materials. The oxidation behavior of the PM and IC material was studied for up to 1000 hours by thermogravimetry in air at constant temperature between 600 °C and 1300 °C. The results are compared with results obtained from long-term isothermal and cyclic oxidation experiments in air at 1200 °C for 17900 hours. Microstructures and scales were examined by light optical and scanning microscopy, X-ray powder diffraction and electron probe microanalysis.
Static and Dynamic Strain Ageing in Two-Phase Gamma Titanium Aluminides
Authors: Christoph U. , Appel F.
Published: April 2011

The deformation behaviour of two-phase titanium aluminides was investigated in the intermediate temperature interval 450–750 K where the Portevin-LeChatelier effect occurs. The effect has been studied by static strain ageing experiments. A wide range of alloy compositions was investigated to identify the relevant defect species. Accordingly, dislocation pinning occurs with fast kinetics and is characterized by a relatively small activation energy of 0.7 eV, which is not consistent with a conventional diffusion process. Furthermore, the strain ageing phenomena are most pronounced in Ti-rich alloys. This gives rise to the speculation that antisite defects are involved in the pinning process. The implications of the ageing processes on the deformation behaviour of two-phase titanium aluminide alloys will be discussed.
Metal-Alloy Nanocluster Formation in Silica Glass by Sequential Ion Implantation
Authors: Battaglin G. , Cattaruzza E. , Gonella F. , D'Acapito F. , Fernandez C. de Julian , Mattei G. , Maurizio C. , Mazzoldi P. , Sada C.
Published: April 2011

Sequential ion implantation of two metal species in silica glass may give rise to the formation of alloy metal nanoclusters. Composite materials with peculiar optical properties can be therefore fabricated, with application in integrated-optical devices. In the presented experiments, different couples of transition elements were sequentially implanted in a silica glass. The resulting systems, in some cases after annealing in either reducing or oxidizing atmosphere, were studied by several analytical techniques, such as transmission electron microscopy and X-ray absorption spectroscopy. The formation of different alloy nanoclusters was evidenced, together with the cluster crystalline structure and the chemical environment of the dopant species.
The control of gold nanocluster sizes in dielectric thin films via ion beam assisted deposition
Authors: Schiestel S. , Carosella C.A. , Hubler G.K. , Qadri S.B. , Knies D. , Stroud R.M.
Published: April 2011

Gold nanoclusters in silica were deposited by co-evaporation of gold and silica in the presence of an argon ion beam. Gold clusters are formed in-situ during the deposition process. The damage induced by the ion beam controls the cluster size, and clusters with a diameter between 15 – 30 Å are obtained. The Au clusters show a highly compressive strain as measured by X-ray diffraction and lattice parameters up to 3.5 % smaller than in bulk gold are observed. Post- deposition annealing in air leads to cluster growth and release of the strain. Annealing near the melting point of gold causes the clusters to grow exponentially up to 180 Å. This is accompanied by a blue shift of the Mie absorption peak. Furthermore the formation of a superlattice was observed by TEM and confirmed by small angle XRD measurements.
Synthesis of B-C-N Thin Films by Ion-Beam-Assisted Deposition and Their Mechanical Properties
Authors: Matsumuro Akihito , Kato Yoshimasa , Ohta Hidenobu
Published: April 2011

B-C-N films have been synthesized by an ion-beam-assisted deposition technique, in which boron and carbon were evaporated by electron beam and a mixed nitrogen and argon ion beam was simulataneously irradiated onto silicon (100) substrates. The ratio of argon ions to nitrogen ions was varied by the flow rate ratio of Ar and N2 gases fed into the ion source. In this experiment, the influence of the ratio on the mechanical properties and the microstructure were investigated. Nano indentation studies show the maximum hardness up to 23 GPa at the gas ratio of 25.50 %. The films prepared under the appropriate conditions indicated low friction coefficients of 0.04-0.08 against a sapphire ball and excellent wear resistance. The existence of a cubic B-C-N like phase in the film, which has been predicted as one of new hard materials, was revealed by various micro-structural analyses. It was concluded that the excellent mechanical properties prepared at the optimum gas ratio could be attributed to the appearnce of the new cubic like phase.