bluuebunny/crossref_metadata_2025 huggingface dataset:
The Importance of Secondary Phases in Glass Corrosion
Authors: Ebert William L. , Bates John K.
Published: March 2011

The analytical expression used to model glass reaction in computer simulations such as EQ6 is compared to the results of experiments used to support the simulations. The expression correctly predicts the acceleration observed in experiments performed at high glass surface area/1eachant volume ratios (SA/V) upon the formation of secondary phases. High resolution microscopic analysis of reacted glass samples suggests that the accelerated nature of the reaction after secondary phase formation is due to changes in the reaction affinity (i.e., is a solution effect) and not a change in the glass reaction mechanism. The composition of solutions in contact with reacted samples reflect the effects of the secondary phases predicted in the model. Experiments which lead to the generation of secondary phases within short reaction times can be used to identify important secondary phases which must be Included in the data base of computer simulations to correctly project long-term glass reaction behavior.
The Formation of Nial Phase and Layered Carbon Precipitates During Ni3Al/SiC Solid State Reactions
Authors: Chou T. C. , Nieh T. G.
Published: March 2011

Solid state reactions of Ni3A1 with SiC have been studied at 1000°C. Three layered reaction products consisting of NiAl Ni5,4Al1Si2, and Ni(5.4−x)A11Si2+C were formed in the reaction zone. The Ni5.4AllSi2layer showed carbon precipitation free, while modulated carbon bands were formed in the Ni(5.4−x)A11Si2+C layer. Carbon precipitates were found to exist in either a disordered or partially ordered (graphitic) state, depending upon their locations from the SiC reaction interface. The NiAl layer showed dramatic contrast difference compared to the Ni3Al and Ni5.4Al1Si2layers, and was bounded by the Ni3Al/NiAl and NiAl/Ni5.4A1lSi2phase boundaries. The kinetics of the NiAl formation was limited by diffusion, and the growth rate constant was measured to be 2 × 10−10cm2/s. The thickness of the reaction zone on the Ni3Al side was always greater than that on the SiC side, suggesting that the decomposition of the SiC may be a rate limiting step for the SiC/Ni3Al reactions. An Al-rejection model, based on a lower solid solubility of Al in the Ni-Al-Si ternary phase than in the Ni3A1 intermetallic compound, is proposed to explain the formation of NiAl phase. The rate limiting step for the Ni3Al/SiC reactions is discussed in light of discontinuous decomposition of SiC, which leads to the formation of alternating C and Ni(5 4−x)Al1Si2layers in the reaction zone. Preliminary results on C precipitation behavior in NiAl/SiC system are reported.
Microstructure and Mechanical Properties of Ni3Al-Based Alloys Reinforced with Particulates
Authors: Mckamey C. G. , Carmichael C. A.
Published: March 2011

Hot-extrusion was used to produce Ni3Al-based alloys to which 10 vol% TiN, NbC, HfO2, or HfN was added for reinforcement. The TiN, NbC, and HfO2 particulates produced Ni3Al-matrix composites in which no reaction was noted at the particle-matrix interface. However, the addition of HfN resulted in extensive reaction in which the hafnium appeared to diffuse into the matrix. Microstructures of this alloy showed a complex array of phases and voids where the HfN particles are presumed to have been originally. Hot hardness, compression, and compression creep tests were performed on specimens cut from the extruded bar of each alloy. No significant strengthening was observed for the alloys containing TiN, NbC, or HfO2. However the HfN-containing alloy did show significant strengthening in simple compression and compression creep. This presentation will include microstructures and the results of the mechanical properties tests.
Phase Diagram Relationships in the Ternary System Ti-Ai-V
Authors: Paruchuri Mohan , Massalski T.B.
Published: March 2011

Liquid-solid and solid state phase equilibria in the ternary system Ti-Al-V have been studied using a combination of several experimental techniques. A likely surface of primary seperation (i.e., the liquidus surface) is proposed in the form of the usual projection on the triangular base and the directions of the monovariant lines are defined. Four ternary invariant reactions have been identified in this system. Solid state equilibria have been determined at 900°C and are presented in the form of an isothermal section through the phase diagram. These are very similar to the relationships reported at 800°C by Hashimoto et al. [1].
Phase Transformations and Microstructural Development in Tial-Based Alloys
Authors: Beaven P. A. , Pfullmann TH. , Rogalla J. , Wagner R.
Published: March 2011

The main features of the phase transformations occurring in alloys based on Ti-48 at% Al are described in order to assess their roles in microstructural development during thermomechanical processing and subsequent heat treatment. It is shown that the presence of Al-rich interdendritic γ formed during solidification plays a critical role during these processes.
Phase Stability and Solidification Pathways in MoSi2 Based Alloys
Authors: Frankwicz P. S. , Perepezko J. H.
Published: March 2011

The phase equilibria developed upon substitution of Ti in MoSi2 along the pseudobinary MoSi2-TiSi2 section has been investigated using x-ray diffraction, and electron microscopy. The pseudobinary exhibits orthorhombic (C54), hexagonal (C40), and tetragonal (Cllb) crystalline structures. A hexagonal (C40) ternary silicide (Ti1-xMox)Si2 was observed to exhibit a relatively wide range of solubility with x in the range of 0.50 to 0.75, in contrast to MoSi2 which exhibits limited solubility for Ti. An analysis of the microstructural evolution following solidification reveals that the hexagonal β-(Ti,Mo)Si2 phase forms via a peritectic reaction between liquid and MoSi2 (Cllb). The primary dendritic structure during solidification in (Til-xMo2)Si2, where x z 0.80, has been determined to be tetragonal (Cllb) MoSi2. The results of this study confirm that substitutional alloying on the transition metal sublattice (Mo) is an effective approach to control of the phase stability and crystal structure in intersilicide reactions.
Metastable Phase Formation in Be-Nb Intermetallic Compounds
Authors: Brimhall J. L. , Charlot L. A. , Bruemmer S. M.
Published: March 2011

Amorphous structures or metastable crystalline phases are produced in sputter deposited Be-Nb alloys (5–15 at.% Nb) depending on the substrate temperature. The metastable phases transform to the stable Be12Nb, Be17Nb2 or Be3Nb phases on annealing at temperatures >800°C. No Be5Nb phase was found and the Be17Nb2 phase is stable to low temperature. The Be12Nb phase appeared to have a stoichiometric range of about 5.5 to 7.7 at.% Nb. The formation of the metastable phases is consistent with current models and theories.
Ab

Initio Calculations of Structural Energetics of Transition-Metal Aluminides and Silicides
Authors: Carlsson A. E. , Meschter P. J.
Published: March 2011

Total energies of binary and ternary -metral trialminides in the L12DO22 Do23Structures and binary Transition- metal disilicides in the C1lb, C40, C54, and C49 structures have been obtained by ab initio band-structure calculations. In aluminides the tetragonal Do22 and Do23 structures are stabilized relative to cubic L12P and in silicides the hexagonal C40 structure is stabilized relative to orthorhombic C54 and tetragonal C11b relative to C40, as the transition-metal d-electron count increases. The observed easier stabilization of L12 in Ti(AI,Fe) 3 relative to Nb(AI,Fe)is justified by the calculations. Location of the Fermi level in a quasigap in the density of states distribution rationalizes the observed structural stabilities in aluminides but not in silicides.
From Dislocation Cores to high Temperature Strain rate Effects in L12 Compounds
Authors: Vitek V. , Sodani Y. , Cserti J.
Published: March 2011

It is now generally accepted that in many L12 compounds the yielding behavior is controlled by the special features of the cores of screw dislocations. In this paper we first summarize results of the atomistic studies of the core structures of the <110> screw dislocations in these compounds. At this point we show that, depending on the atomic bonding, two distinct classes of L12 alloys exist. In the first, represented by alloys like Ni3Al, a glissile configuration of the core exists on {111} planes although a sessile configuration is energetically more favored. In the second class, represented by alloys like Pt3A1 and A13Ti modified by alloying into L12 structure, the cores of screw dislocations are always sessile. Using the results of the atomistic studies we present physical models explaining the temperature dependences of the yield stress in both classes of L12 alloys. At this point we also present a further development of the model for the anomalous temperature dependence of the yield stress in alloys like Ni3A1, originally put forward by Paidar et al. [15]. In this development strain rate effects are included and it is shown that the model explains not only the orientation dependences of the yield stress in the anomalous regime but also the very low strain rate sensitivity observed in this regime.
Dislocation Structures and Anomalous Yielding in Ordered Alloys
Authors: Hazzledine P. M. , Sun Y. Q.
Published: March 2011

The strain, strain rate and temperature dependencies of the yield stresses of the model L12materials Ni3Al, Ni3Ga and Co3Ti are described, as well as two associated properties, the initial work-hardening rate and the inverted creep. These mechanical properties, the electron microscope observations of <110> {111} APB coupled slip and the violations of Schmid's laws point to glissile-sessile-glissile transitions by cross slip of screw dislocations as the explanation for the yield stress anomaly. Similar explanations are probable for h.c.p. Be, B2CoTi and tetragonal TiAl and MoSi2. A different kind of model, based on the behavior of edge dislocations is required for h.c.p. Mg, B2 CuZn and DO19Ti3Al.
Room Temperature Deformation in “Soft” Orientation Nial Single Crystals
Authors: Field R.D. , Lahrman D.F. , Darolia R.
Published: March 2011

A detailed study of deformation of NiAl single crystals in two soft orientations, <110> and <111>, has been conducted. The Schmid factor favors {100} slip in the former and {110} slip in the latter. Detailed dislocation analysis, critical resolved shear stress measurements, and slip trace analysis have been performed to determine the nature of dislocation motion and interactions in this material. Particular attention is given to prismatic loops formed during deformation, since the shapes of these loops reveal the active slip planes. Similar loop morphologies observed in elevated temperature [001] oriented tensile specimens are also discussed.
Slip Transition in [001] Oriented Nial at High Temperatures
Authors: Kim J.T. , Gibala R.
Published: March 2011

The present work was undertaken to understand the transition in slip direction In [001] oriented NiAl as a function of temperature. Single crystals of [001] NiAl were deformed at room temperature to produce <111> dislocations and were subsequently annealed at various temperatures in order to see the effect of temperature on the stability of the <111> dislocation. These results were compared with corresponding dislocation substructures of specimens deformed directly at high temperatures. During annealing <111> dislocations are dissociated into <001>+<110> dislocations at 773–923 K. It is also observed that <111> dislocations can be dissociated into <001>+<110> dislocations during deformation in the vicinity of the transition temperature of around 773 K. The quantitative details of these changes depend on deformation or annealing temperatures.
Burgers Vector Transition in Fe-Ai-Ni Alloys
Authors: Patrick D. Keith , Chang Keh-Minn , Miracle Daniel B. , Lipsitt Harry A.
Published: March 2011

Polycrystalline NiAl exhibits <001> slip at temperatures below about 450°C. In Fe-Al alloys there is a transition in the Burgers vector from a/2<111> at low temperatures to a<001> at intermediate temperatures. The transition temperature is dependent upon composition.

A transition in the Burgers vector from a/2<111> to a<001> has been found to exist as a function of composition and temperature in the B2 pseudo-binary (Fe,Ni)-40Al (at%) system. At room temperature, this transition occurs at a composition between Fe-40Al-10Ni, which exhibits <111> slip, and Fe-40Al-3ONi, which exhibits <001> slip. At 400°C this transition shifts further toward the iron-rich compositions. Fe-40Al-10Ni exhibits both <111> and <001> slip, whereas Fe-40Al-20Ni exhibits <001> slip.
Deformation of the Orthorhombic Phase in Ti-Al-Nb Alloys
Authors: Banerjee D. , Rowe R.G. , Hall E.L.
Published: March 2011

The deformation structure of the alpha-2 and orthorhombic phase in two alloys of the Ti-Al-Nb system has been characterised by transmission electron microscopy after plastic deformation of about 2% in compression at room temperauture. The orthorhombic phase is shown to deform by all slip modes present in the alpha-2 phase. In addition, extensive “c” component slip is observed in the orthorhombic phase on slip planes that are not equivalent to those observed for “c”component slip in alpha-2. Dislocation dissociation on both basal and prismatic planes of dislocations without a c component is observed in both phases.
Dislocation Behaviour in the Octahedral Plane in Ni3Ga Single Crystals
Authors: Couret A. , Sun Y. , Hazzledine P.M.
Published: March 2011

Weak beam electron microscopy has been used to study the substructure in octahedral planes in Ni3Ga deformed in the range of the yield stress anomaly. The substructure is formed by screw dislocations in the Kear Wilsdorf configuration, which contain numerous superkinks. A statistical analysis of the superkink height has been performed. The effect of a first high temperature deformation on a second low temperature one has been investigated. The behaviour of the dislocations has been deduced from these experimental results and from those of the literature.
High-Temperature Deformation of Nb-18 Al
Authors: Marieb T. N. , Kaiser A. D. , Nutt S. R. , Anton D. L. , Shah D. M.
Published: March 2011

Specimens of Nb-18 at. % Al were bend-tested at 20° C, 1200° C, and 1600° C, and the deformed microstructures were characterized using TEM to determine phase distributions and slip systems. Material deformed at 20° C showed brittle fracture and was characterized by large grains of a Nb-Al phase showing B2 ordering. Lamellar colonies of heavily dislocated massively transformed Nb3Al (A15) were present at grain boundaries. The slip system for dislocations in Nb3Al, was determined to be {100}/<001>, with defects typically extending in <011> directions. Fracture at 1200° C was ductile, and TEM observations revealed a bcc Nb-Al(ss) matrix with second phase growth of Nb3Al and α -Al2O3. The Nb3Al phase contained total dislocations and slip-induced planar defects on {100} planes terminating in partial dislocations. In tilting experiments, the planar defects exhibited stacking fault contrast consistent with a displacement vector of R=a/4<100>. Fracture at 1600° C was also ductile, and the deformed material consisted of a niobium matrix with equiaxed inclusions of alumina resulting from internal oxidation during deformation. The creep resistance reported previously was attributed to a combination of dispersion hardening of the refractory metal matrix and inherent creep resistance of Nb3Al.
Electron Microscopic Observation on the Structure of Ni(Cu)AI Martensite
Authors: Kang Seung-Hyuk , Jeon Seung-Jun , Lee Hu-Chul
Published: March 2011

In an effort to seek a way to toughen NiAL intermetallic compounds, deformation characteristics of copper containing NiAL martensite were studied by electron microscopy. Martensite plates with fine {111} <112> internal twins were well developed across the grains in as extruded alloys. Initial linear work hardening up to 7 % strain was observed and the migration of twin interface and martensite plate interfaces were suggested as major deformation modes at this stage. Increase in strain hardening rate after 7 % strain was associated with the development of dislocation structure and growth of new strain induced twins within the internal twin free martensite plates.
Effect of Ta on the Structure and Dynamics of γ/α2 Interfaces in Tial
Authors: Singh S. R. , Howe J. M.
Published: March 2011

The structure of γ/α interfaces in binary and Ta-containing TiAl alloys were analyzed by HRTEM and image simulations. Growth of α2 was found to be due to a ledge mechanism, consisting of Shockley partial dislocations on alternate (111)γ planes. The interface is atomically flat between the ledges and addition of Ta was found to transform arrays of growth ledges in the binary alloy into islands on the plate faces in the Ta-containing alloy. These islands of α2 on the γ/α2 interfaces were 4–7nm wide and increased in size with decreasing ageing temperature. The height of the ledges and islands were always a multiple of the c-parameter (0.46nm) of the α2 phase. The islands were bounded by 90°(edge) and 30° screw) Shockley partial dislocations. The 30° partial dislocation cores were localized whereas the 90° partial dislocation cores appeared to be highly delocalized due to presence of a high density of kinks, which in one case was found to be about 0.65nm−1.These results are interpreted in terms of the growth mechanisms and morphology of the α2 phase.
Deformation and Defects in C36 Laves Phases
Authors: Livingston J. D. , Hall E. L.
Published: March 2011

MgNi2and MgCu0.4Zn1.6, two C36 Laves phases, were plastically deformed at elevated temperatures, and, the deformation-induced defects were studied by TEM. Extensive faulting was observed, consistent with the low stacking-fault energy of the C36 structure. HREM revealed multi-layer faults with a variety of stacking sequences.
Micromechanisms of Deformation and Fracture in a Ti3Al-Nb Alloy
Authors: Akkurt A.S. , Liu G. , Bond G.M.
Published: March 2011

The object of this study has been to gain a greater understanding of deformation and fracture processes in a Ti-24Al-11Nb (at %) alloy. The in-situ TEM deformation technique has been used to observe these processes as they occur. Material characterization studies revealed the existence of three different crystal structures in the material (α2,B2 and orthorhombic (0)). Slip is first initiated in the B2 and 0 phases. Although dislocations are observed in the α2 phase in the deformed material, they are seen only in high-stress regions and only in some laths. While numerous dislocations may be injected into the corresponding grain and phase boundaries, slip does not easily propagate into the α2 phase. Cracks are seen to originate most frequently in the regions transformed fully to α2 laths, and sometimes in the regions that contain α2 laths at prior β grain boundaries. Failure of grain or phase boundaries in the B2 or 0 phases where no α2 is present is not observed. Crack branching in the α2 phase is common, particularly in the vicinity of phase boundaries. Crack propagation in the B2 and 0 phases occurs by plastic thinning, and cracks formed in the α2 lath regions are either stopped in phase boundaries or blunted in the neighboring 0 or B2 grains.
Tough, Ductile High-Temperature Intermetallic Compounds: Results of a Four-Year Survey.
Authors: Fleischer R.L. , Briant C.L. , Field R.D.
Published: March 2011

A four-year survey of high-temperature intermetallic compounds has been aimed at identifying potentially useful structural materials for aerospace and aircraft engine applications. Since the good properties of high strength and stiffness at high temperatures are typically negated by brittleness at ambient temperature, new materials must have roomtemperature toughness or ductility. Screening has been done of 90 binary compounds with 20 different crystal structures, and 130 ternary or higher-order alloys. Testing typically included hardness vs. temperature, elastic modulus determination, and toughness evaluation via a room-temperature chisel test. Four alloy systems, including only two types that are of the simplest structures, showed substantial room-temperature toughness: Al-Ru, Ru-Sc, Ir-Nb, and Ru-Ta. Of these the last and the first are the most promising. Special features of the Ru- Ta (L1o) alloys are their room-temperature impact resistance and high-temperature strength. AIRu (B2) alloys can be tougher than the L1o structures and most are also ductile in compression at room temperature. Alloying experiments with B, Cr, and Sc show beneficial effects on ductility, oxidation resistance, and high-temperature strength.
Room Temperature Strength and Fracture of FeAl And NiAl.
Authors: Nagpal P. , Baker I. , Liu F. , Munroe P.R.
Published: March 2011

The yield strengths (σy) of FeA1 and NiAl of various aluminum concentrations were measured as a function of grain size, d, and the results fitted to σy=σo+ kd−1/2. It was found that for NiAl the lattice resistance (σo) and Hall-Petch slope (k) have minima at the stoichiometric composition and increase with decreasing aluminum concentration, whereas for FeAl these parameters have maxima at the stoichiometric composition and decrease off this composition. In tension tests, iron-rich FeAl was shown to be ductile whereas the stoichiometric alloy is brittle. In contrast, stoichiometric NiAl shows ductility but off-stoichiometric compositions are brittle. The fracture modes of the FeAl and NiAl are predominantly intergranular at the stoichiometric compositions and become increasingly transgranular with decreasing aluminum concentration.
On the High Temperature Behavior of Niobium Aluminide intermetallics
Authors: Rodriguez A. B. , Barth E. P. , Tien J. K.
Published: March 2011

A basic characterization of the mechanical and physical properties of a Niobium Aluminide intermetallic, NbAl3 , was undertaken. Four ingots of material were vacuum arc-melted, yielding a two phase microstructure in each case. Physical testing including coefficient of thermal expansion measurement was performed on the most promising ingot. Elastic modulus of this compound was measured in compression. Elevated temperature microindentation measurements have been performed to gain insight into the deformation behavior as a function of temperature. Microindentation measurements have also been used to study the time-dependent deformation of NbAl3.
Ductility and Strength in Mo Modified TiAl
Authors: Maeda T. , Okada M. , Shida Y.
Published: March 2011

An investigation has been made on the effect of ternary additions of Mo and other elements on the room temperature tensile ductility and high temperature strength, including creep rupture strength in TiAl based alloys. Mo modified Ti-rich TiAl in an annealed condition exhibited higher tensile ductility at room temperature than other well-known Cr or Mn modified alloys, this resulting from the refinment of mean grain size in the duplex structure of lamellar (γ+α2) and single phase (γ) rather than the crystalline tetragonality of the γ phase. Moreover, creep strength in as cast Ti-rich TiAl is improved with the addition of Mo, and lowered with the addition of Mn. These results suggest that Mo modified Ti-rich TiAl can enhance, the potential of TiAl as a high temperature material more than other ternary modified TiAl.
The Fatigue Behavior of γ/α2 Titanium Aluminides
Authors: Dowling W.E. , Donlon W.T. , Allison J.E.
Published: March 2011

Axial load controlled high cycle fatigue experiments were conducted on the γ/α2 alloy, Ti-48A1-1V-0.2C (at%), at 23 and 815°C. Four different microstructures, produced through thermomechanical processing, were evaluated to examine the influence of grain size and α2 content on fatigue behavior. The load controlled fatigue life was significantly reduced by increasing grain size and unaffected by α2 content at both 23 and 815°C. Although, α2 content did not greatly influence high cycle fatigue life, the room temperature crack initiation and fast fracture was changed from transgranular to partially intergranular as the volume fraction of α2 was reduced in the fine grain size material. The fatigue strength at 107 cycles (FS) to ultimate tensile strength (UTS) ratio was 0.8 to 0.9 at 23°C and 0.5 to 0.6 at 815°C for all microstructures examined. Low tensile ductility, high work hardening rate and the difficulty in forming strain local-izations all aided the high FS/UTS ratio. The dislocation microstructures produced by fatigue at room temperature were examined in the fine grained high α2 (ductile) microstructure. They consisted of loop patches of all <110] regular dislocations without any <101] or <011] super dislocations because of the large difference in CRSS for these dislocation. The inability to nucleate and move superdislocations inhibited the formation of persistent slip bands as is often found in high and intermediate stacking fault FCC metals.
Cyclic Deformation Of B2 Aluminides
Authors: Hartfield-Wünsch S.E. , Gibala R.
Published: March 2011

A study has been undertaken to understand the cyclic deformation behavior of singlephase B2 aluminides. The alloys chosen for this study were rapidly solidified powder extrusions of Fe60 Al40 (at.%) and Ni50 A130 Fe20 (at.%). These alloys were chosen to compare the cyclic deformation behavior of a material that deforms by <111> slip and thus meets the von Mises criterion for slip system compatibility (Fe60Al40) with one that deforms by <100> slip and does not meet the von Mises criterion (Ni50A130Fe20). Fully reversed tension/compression tests were run in plastic strain control at room temperature. Rapid cyclic hardening was observed for the Fe60Al40 alloy. However, intergranular failure occurred before saturation was reached at all plastic strain amplitudes. The Ni50A130Fe20 alloy displays no tensile ductility , with failure occurring before completion of the first cycle. For the Ni50A130Fe20 alloy, a lower extrusion temperature during materials processing, and compressive prestrain both increase the cyclic accumulated plastic strain to failure, as well as the monotonic tensile ductility. This made cyclic deformation of the Ni50Al30Fe20 alloy possible. In this alloy, limited cyclic hardening is followed by saturation. For both the Fe60Al40 and Ni50 Al30Fe20 alloys, significant ductility enhancement was achieved by the application of thin nickel films. All cyclic deformation results are compared to similar monotonic tension results and are discussed in terms of the dislocation substructures observed.
Flow and Fracture Behavior of NiAl in Relation to the Brittle-Toductile Transition Temperature
Authors: Noebe R.D. , Bowman R.R. , Cullers C.L. , Raj S.V.
Published: March 2011

NiAl has only three independent slip systems operating at low and intermediate temperatures whereas five independent deformation mechanisms are required to satisfy the von Mises criterion for general plasticity in polycrystalline materials. Yet, it is generally recognized that polycrystalline NiAl can be deformed extensively in compression at room temperature and that limited tensile ductility can be obtained in extruded materials. In order to determine whether these results are in conflict with the von Mises criterion, tension and compression tests were conducted on powder-extruded, binary NiAl between 300 and 1300 K. The results indicate that below the brittle-to-ductile transition temperature (BDTT) the failure mechanism in NiAl involves the initiation and propagation of cracks at the grain boundaries which is consistent with the von Mises analysis. Furthermore, evaluation of the flow behavior of NiAl indicates that the transition from brittle to ductile behavior with increasing temperature coincides with the onset of recovery mechanisms such as dislocation climb. The increase in ductility above the BDTT is therefore attributed to the climb of <001> type dislocations which in combination with dislocation glide enable grain boundary compatibility to be maintained at the higher temperatures.
The Compression Creep Behavior of Ni3Al-X Single Crystals
Authors: Miura Seiji , Hayashi Tohru , Takekawa Mitsuhiro , Mishima Yoshinao , Suzuki Tomoo
Published: March 2011

Compressive creep behavior is investigated in ternary Ni3Al single crystals containing Ti, Si, Hf and Cr with stress axes parallel to the crystallographic orientation near [001]. Then a comparison is made with the results of high temperature compression tests under a constant strain rate for the same orientation where plastic behavior is characterized by a distinct yield drop followed by steadystate deformation. It is found that the deformation mechanism for the two cases is identical, namely octahedral viscous flow being expressed by the state equation of the power-law type with a stress exponent of about 3 to 4. The effect of offstoichiometry on the creep resistance is then examined in Ni3(Al,5 at%Ti) alloys with different Ni concentrations. The results support the observation in the polycrystalline compound where the creep resistance increases with Ni concentration on both sides of stoichiometry exhibiting a discontinuity at stoichiometry. Finally, the apparent activation energy in the power-law type state equation for the steady state creep deformation is estimated for all the ternary alloys examined. They are in general in good agreement with that for diffusion of ternary elements in Ni3Al. However, the relative magnitude of the value can not simply be compared since the activation energy depends on deviations from stoichiometry.
The Strain Rate Sensitivity of Ni3(Al,Ta) Single Crystals.
Authors: Bonneville J. , Martin J.L.
Published: March 2011

In order to obtain more complete information about the strain rate sensitivity of the flow stress of L12alloys, Ni3Al,Ta) single crystals have been deformed in compression, over a range of temperatures (293–1273K), at two different strain rates and in stress relaxation experiments.

During the stress relaxation tests, at almost all the temperatures (except T~470K), a logarithmic decrease of the stress as a function of time has been observed. This is in fair agreement with the classical frame work of the thermal activation theory of dislocation glide. Thus, the corresponding apparent activation volumes have been determined and the nonmonotonic variation of this activation parameter with the temperature indicates that it is necessary to consider more than one plastic deformation process. These new results are discussed in terms of the previously published models which account for the plastic behaviours of the L12compounds.
Fractographic Observation of Fatigue Crack Growth in Cast Ni3Al-Based Alloys
Authors: Wanquan Yin , Wei Gao , Yaoxiao Zhu
Published: March 2011

Fatigue behaviour of cast Ni3Al-based alloys with the addition of alloying elements, such as Hf, Zr and Ti etc., has been investigated. It was found that: (1) The alloying elements change remarkably mechanical property and fatigue property of cast Ni3Al-based alloys, in which, the fatigue strenghts of No.1 and No.2 alloys are 215.6MPa and 205.8MPa respectively, while that of No.3 alloy is only 78MPa, for the same cyclic life of 107; (2) Fatigue cracks initiate at the (γ'+γ) /γ interface and “matrix channel” in No.1 and No.2 alloys are at the grain-boundary or micropore, of No.3 alloy often fatigue cracks initiated from the edge of specimen under the cyclic stress; (3) Fbitigue crack propagation of cast Ni3Al-based alloys follows “zigzag” propagation path, and it proceeds through the big blocky (γ'+γ) eutectic and “matrix channel” and along the (γ';+γ)/γ interface in No.1 and No.2 alloys, while the fatigue crack propagation of No.3 alloy sometimes follows the “zigzag” propagation path. Finally, the effect of alloying elements on the fatigue behaviour of cast Ni3Al-based alloys has been discussed.
High-Temperature Ductility of Intermetallic Nickel Aluminide (IC-218LZr)
Authors: Mirshams A.R. , Baldwin R.H. , Sikka V.K.
Published: March 2011

This paper reports the results of uniaxial and compression tests on IC-218LZr extruded bars at temperatures from 700 to 1000°C in air. Tensile results indicate reduction in ductility at temperatures between 750 to 900°C. However, the results show improvement in ductility above 950°C. Compression test results do not show a decrease in the ductility in the temperature range.
First-Principles Investigation of Mechanical Behavior of B2 Type Aluminides: Feal and Nial
Authors: Fu C. L. , Yoo M. H.
Published: March 2011

First-principles calculations of the elastic constants, shear fault energies, and cleavage strength of NiAl and FeAl are presented. For NiAl, we find that the dissociation of <111> superdislocations into partial dislocations is unlikely, because of a high antiphase boundary energy and a weak repulsive elastic force between partial dislocations. FeAl has a high ideal cleavage strength as a result of the directional d-bond formation at the Fe sites. The strong ordering behavior of NiAl is explained in terms of the Al-to-Ni charge transfer and the repulsive interaction between Al atoms. The spontaneous glide decomposition of the <111> superdislocations in NiAl is also discussed.
High Temperature Properties of Refractory Intermetallics
Authors: Anton D.L. , Hartford CT E. , Shah D.M. , Whitney Pratt , Hartford CT E.
Published: March 2011

On the basis of creep strength, ultimate tensile strength and oxidation resistance, seven intermetallic compounds with melting temperatures above 1600°C have been selected as possible candidate materials for high temperature structural applications in advanced aero-turbines. These compounds, Nb3Al, Cr3Si, Co2Nb, Cr2Nb, MoSi2, Mo5Si3 and Nb2Al, have been evaluated and their properties reported herein. All seven of the compounds displayed excellent creep resistance at 1200°C with Mo5Si3 and Nb2Al being the strongest. Nb3Al, with the precipitation of the niobium solid solution displayed the greatest low temperature toughness. The greatest ultimate tensile strengths were observed for Co2Nb and MoSi2, while MoSi2 was by far the most oxidation resistant.
High Temperature Deformation of Single Crystalline Cr3Si (A15) Intermetallic Compound
Authors: Chang C. Steve , Pope D. P.
Published: March 2011

High temperature compression tests were performed on Cr 3Si single crystalline and poly crystalline samples. Slip systems were determined to be of the {001}<010> type based on an analysis of slip traces and Laue spots. Single crystals show significant compressive ductility at temperatures above 0.7Tm. The implication of cube slip on the ductility of A15-type intermetallic compounds is discussed.
Miniaturized Disk-Bend Testing of Ni3 Al: Effect of Stoichiometry and Boron Content
Authors: Li H. , Ardell A. J.
Published: March 2011

The results of miniaturized disk-bend tests on samples of Ni 3Al of different stoichiometry and boron content are presented. The yield strengths and ductilities of alloys containing 24, 25 or 26 %Al. either boron-free or doped with 0.3 or 0.35 %B, were measured. Specimens 3 mm in diameter and approximately 200 μm thick were tested, some of these having been cut from the grip sections of previously tested tensile bars. The yield strengths were in excellent agreement with the results obtained from the uniaxial tensile tests. The load-displacement curves for the brittle alloys (all but the boron-doped 24 %Al alloy) exhibited a maximum load corresponding to crack initiation. The shapes of the deformed specimens confirmed the assumption that they deform as if they were clamped even though they are not. The fracture surfaces of the brittle alloys are consistent with intergranular failure. Nevertheless, the ductility of the alloys increases with decreasing Al content and decreasing grain size, even for the boron-free alloys which are all brittle. The fracture stress of the boron-doped 26 %Al alloy is about 30% greater than that of the boron-free alloy. It is argued that this is most likely a consequence of the depletion of aluminum at grain boundaries, coupled with boron segregation. Independent evidence suggests that this should increase the cohesive strength of grain boundaries in the boron-doped 26 %Al alloy.
Miniaturized Disk-Bend Testing and Microstructure of 3.8 Mev Zr3+ Irradiated Zr3Al
Authors: Chen F. C. , Ardell A. J. , Pedraza D. F.
Published: March 2011

The mechanical behavior of Zr3Al irradiated with 3.8 MeV Zr3+, ions has been investigated using a miniaturized disk bend test (MDBT). The samples tested were 3 mm in diameter and either nominally 100 or 250 μ m thick. They were irradiated at 250 °C to peak doses varying from 0.0041 to 2 displacements per atom (dpa). The yield stresses, σy, of the thin specimens were independent of dose, within experimental error, and comparable to that of unirradiated Zr3 Al. However, for the thick specimens σy was significantly larger for the irradiated material, possibly increasing to a maximum at a peak dose between 0.02 and 0.06 dpa. The relative insensitivity of σy of the thin specimens to irradiation, compared to the thick ones, is a surprising result for which we have no satisfactory explanation. Examination by transmission electron microscopy showed that Zr3Al begins to undergo chemical disorder at a dose of only 0.02 dpa and Is nearly completely disordered at 0.41 dpa. Electron diffraction indicates the presence of satellite reflections along 〈110〉 in {001} diffraction patterns. The nature of the defects responsible for these has not yet been determined, although rather similar effects have been observed in electron and Kr+-ion irradiated Zr3Al and attributed to elastic softening of the lattice. The possible roles of these defects, as well as disordering, are discussed.
Ternary Alloying of Gamma Titanium-Aluminides for Hot-Workability
Authors: Masahashi Naoya , Mizuhara Y. , Matsuo M. , Hashimoto K. , Kimura M. , Hanamura T. , Fujii H.
Published: March 2011

A Microstructurally controlled γ(TiAl)-based alloy containing β(bcc) has shown remarkable tensile properties at high temperatures. It exhibits over 450% tensile elongation at 1473K. The strain rate sensitivity factors are calculated to be over 0.3 above 1273K. A specific texture is not observed from transmission X-ray photograph. TEM observation reveals that the precipitated β phase located at grain boundaries elongates uniformly along γ grain boundaries and causes large ductility. We propose β/γ dual phase alloys as high temperature deformable materials.
Spray Forming of MoSi2 and MoSi2-Based Composites
Authors: Tiwari R. , Herman H. , Sampath S.
Published: March 2011

Vacuum plasma spraying has been used to fabricate dense MoSi2 and TiB2-reinforced MoSi2-based composites. The spray-formed deposits have high hardness and fracture toughness as compared to that achieved by other processing techniques. Annealing of the spray formed MoSi2 leads to a decrease in hardness and an increase in fracture toughness. Reinforcement with TiB2 leads to toughening and strengthening of the spray-formed MoSi2.
Solidification Microporosity in Equiaxed Multi-Component Nickel Aluminide
Authors: Cheng C.J. , Sekhar J.A.
Published: March 2011

Porosity formation during solidification in equiaxed microstructure of multicomponent nickel aluminide IC396M is examined. Two types of porosities are identified and related to the other scales of microstructure that evolve during solidification. A correlation is established between the relevent processing parameters and the microstructure variables.
Formation of Metastable Disordered Ni3Al by Pulsed Laser Induced Rapid Solidification
Authors: West Jeffrey A. , Manos James T. , Aziz Michael J.
Published: March 2011

Thin films of Ni3Al formed by co-evaporation onto insulating substrates form a single phase fcc disordered lattice upon rapid solidification following excimer laserinduced melting with an interface velocity of ~4 m/s. Transmission Electron Microscopy (TEM) and x-ray diffraction (XRD) analyses exhibit no superlattice diffraction at room temperature. Resistivity measurements, indicating that the disordered phase has a higher resistivity and much smaller temperature coefficient at room temperature than the stable ordered (L12) phase, permit us to monitor phase changes and ordering on a fast time-scale. Subsequent annealing recovers long-range order, with resistivity measurements indicating that reordering begins just below 300°C.
Cold-Rolling and Subsequent Annealing of Ti-Rich Tial Polysynthetically Twinned (PST) Crystals
Authors: Oh M.H. , Inui H. , Nishitani S.R. , Yamaguchi M.
Published: March 2011

Polysynthetically twinned (PST) crystals of Ti-rich TiAl have been grown and specimens cut from these crystals have been rolled at room temperature and subsequently annealed at 900°C and 1000°C. When the shear deformation parallel to the lamellar boundaries occurs during rolling, PST crystals of Ti-rich TiAl can be rolled to about 50% reduction in thickness at room temperature.

The recovery in microhardness occurs in two stages; the first stage associated with the decrease in dislocation density and the second stage connected with the annealing-out of deformation induced twins. The recrystallization mode depends on the amount of reduction. Up to 20% reduction, the lamellar structure is preserved even after the full recovery in hardness. When the amount of reduction exceeds 40%, a structure composed of equiaxed grains of TiAl is obtained after recrystallization. A mechanism of recrystallization of cold-rolled PST crystals, which may explain the dependence of recrystallization mode on the amount of reduction, is proposed.
Powder Processing of Fe3Al-Based Iron-Aluminide Alloys
Authors: Sikka V.K. , Baldwin R.H. , Reinshagen J.H. , Knibloe J.R. , Wright R.N.
Published: March 2011

The production, characterization, and consolidation of Fe3Al-based iron-aluminide powders are described. The consolidated powders were fabricated into sheet. The sheet was tensile tested from room temperature to 800°C and creep tested in the temperature range of 450 to 704°C. Mechanical properties were compared for nitrogen and argon atomized powders. Mechanical properties were also compared for argon atomized powders of Fe3Al alloys containing 2 and 5% Cr.
Production of Fe3Al-Based Intermetallic Alloys
Authors: Sikka Vinod K.
Published: March 2011

The melting of Fe3Al-based alloys at the Oak Ridge National Laboratory (ORNL) and commercial vendors is described. The melting processes evaluated include arc melting, air-induction melting (AIM), vacuum-induction melting (VIM), vacuum-arc remelting (VAR), and electroslag remelting (ESR). The quality of the ingots studied are based on internal soundness and the surface finish obtained. The ingots were analyzed for impurity levels observed in the alloys by various melting processes. Recommendations are made for viable processes for commercial melting of these alloys.
Fabrication of Dense Nickel Aluminide Monolithic and Composite Bodies By Combined Self Propagating High Temperature Synthesis and in Situ Containerless Hot Isostatic Pressing
Authors: Concannon Michael , Hodge Edwin S. , Nyce Andrew C. , Turmel Christopher P.
Published: March 2011

Monolithic Ni 3Al and composite Ni3Al/ceramic whisker bodies were fabricated by combined Self Propagating High Temperature Synthesis and in situ containerless Hot Isostatic Pressing (SHS/HIP), starting from cold pressed blends of nickel, aluminum, and boron powders, with and without additions of alumina whiskers. The resultant bodies were dense and had good ductility and mechanical strength at room temperature.
Phase Formation and Phase Stability in the Al-Ti Thin Film System
Authors: Parks L.R. , Lilienfeld D.A. , BØRgesen P. , Raj R.
Published: March 2011

This study focuses on the sequential formation of aluminide phases during annealing of titanium and aluminum thin film bilayers. The formation of titanium-rich intermetallic phases at higher annealing temperatures is emphasized. Using Rutherford Backscattering Spectrometry (RBS) analysis, and x-ray diffraction, phases formed as a function of temperature have been identified. The phases Al3Ti through Ti3Al were observed over the temperature range 450–750°C, where reaction with the SiO2 substrate occurred. All phases were present as discreet layers within the samples with several layered phases coexisting at the higher temperatures.
Chloride-Induced Localized Corrosion of Fe3Al-Type Iron Aluminides: Beneficial Effects of Cr And Mo Additions
Authors: Buchanan R. A. , Kim J. G.
Published: March 2011

In this study, an acid-chloride electrolyte at pH = 4 (H2SO4) and containing 200 ppm Cl- was used to define the effects of Cr concentration (0–6 at.%) and Mo additions (0–2 at.%) on the aqueous corrosion behavior of iron aluminides containing 28 at.% Al. For the Fe-28Al composition, cyclic-anodic-polarization testing indicated passivation, but with a relatively low breakdown potential for pitting corrosion, and a protection potential lower than the open-circuit corrosion potential. Cr additions alone proved beneficial by continuously increasing the pitting potential. However, even at the highest Cr level, 6%, the protection potential was still lower than the corrosion potential, indicating that pitting could initiate after an incubation period. Mo additions were found to raise the protection potential, such that at 1 and 2% Mo levels (4 % Cr), it was higher than the corrosion potential, indicating significantly improved resistance to the initiation of localized corrosion. Immersion testing showed that the latter compositions remained passivated with no localized corrosion for a period of four months, at which point the tests were terminated. The overall results indicated that for satisfactory resistance to chlorideinduced localized corrosion, both higher Cr levels (4–6 at.%) and Mo additions (1–2 at.%) are desirable.
Oxidation Behavior of Advanced Intermetallic Compounds
Authors: Mckee Douglas W. , Fleischer Robert L.
Published: March 2011

The oxidation behavior of a number of ordered high melting point intermetallic materials has been studied in flowing air in the temperature range 1000 to 1500°C. The alloys were selected for acceptable toughness and ductility at temperatures above 1000°C. Included were ruthenium aluminides, chromium silicide and compounds of iridium and ruthenium with elements such as titanium, tantalum, nickel, iron, cobalt, silicon, vanadium and rhenium. In general, alloys containing the metals iron, cobalt and nickel, and also titanium and tantalum, oxidized rapidly at 1000°C and above. Oxidation-resistant materials were those capable of forming scales of chromia/silica or alumina. Intermetallic compounds of chromium and ruthenium formed volatile oxides above 1200°C, but binary compounds of these elements with silicon and aluminum were highly oxidation resistant at lower temperatures.
Effects of Crystallinity on Electrical Properties and Structure of Semicrystalline Polyimides
Authors: Friler Jerome , Cebe Peggy
Published: March 2011

The semicrystalline polyimide studied here has been developed by NASA Langley Research Center and film processed by Foster Miller Inc. Aromatic polyimides posess a unique property combination. The outstanding thermal stability (Tg above 230 C and Tm above 355 C) in addition to such other properties such as excellent radiation and solvent resistance is supported by excellent mechanical and electrical properties. For instance the tensile modulus of LARC-CPI is 513 ksi at 25 C and 206 ksi at 232 C, G1c=37.8 in lb/cm2 [1].
Third Order Optical Nonlinearity of Soluble Polythiophenes
Authors: Okawa Haruki , Wada Tatsuo , Yamada Akira , Sasabe Hiroyuki
Published: March 2011

Soluble conjugated polymers, poly (3-alkylthiophene)s were prepared by chemically oxidative polymerization. The polymers had no absorption at wavelengths longer than 600 nm, indicating that the polymers were suitable for waveguide structures. Dispersion of refractive index (real and imaginary parts) was calculated from the reflectance data using the Kramers-Kronig relation. The dispersion agreed with the result obtained by ellipsometry. Third order optical nonlinearity, χ(3)(−3ω; ω, ω, ω), of the polymers was determined by optical third harmonic generation (THG). Enhancement of the χ(3) by three photon absorption was observed when a beam of 1.54 μm was used as a fundamental. Also, light propagation properties were investigated by using slab waveguides of the polymers.