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THE EFFECT OF MAGNETICALLY ALIGNED POWDER ON THE MAGNETOSTRICTION OF SINTERED RARE EARTH-IRON LAVES PHASE COMPOUNDS

Description: A powder metallurgical approach is utilized to prepare grain oriented Laves phase compounds of Tb{sub x}Dy{sub 1-x}Fe{sub 2}. The magnetostrains observed in the oriented compounds, though containing {approx}20% porosity as presently prepared, are far superior to those of arc cast and highly dense liquid phase sintered materials. Also, it is shown that the alignment achieved is strongly dependent on the Tb/Dy ratio.
Date: October 1, 1977
Creator: Malekzadeh, M. & Pickus, M.R.
Partner: UNT Libraries Government Documents Department

Point Defects in Binary Laves-Phase Alloys

Description: Point defects in the binary C15 NbCrQ and NbCoz, and C 14 NbFe2 systems on both sides of stoichiometry were studied by both bulk density and X-ray Iattiee parameter measurements. It was found that the vacancy concentrations in these systems after quenching from 1000"C are essentially zero. The constitutional defects on both sides of stoichiometry for these systems were found to be of the anti-site type in comparison with the model predictions. Thermal vacancies exhibiting a maximum at the stoichiometric composition were obtained in NbCr2 Laves phase alloys after quenching from 1400"C. However, there are essentially no thermal vacancies in NbFe2 alloys after quenching from 1300oC. Anti-site hardening was found on both sides of stoichiometry for all the tie Laves phase systems studied, while the thermal vacancies in NbCr2 alloys quenched from 1400'C were found to soften the Laves phase. The anti-site hardening of the Laves phases is similar to that of the B2 compounds and the thermal vacancy softening is unique to the Laves phase. Neither the anti-site defects nor the thermal vacancies affect the fracture toughness of the Laves phases significantly.
Date: January 11, 1999
Creator: Liaw, P.K.; Liu, C.T.; Pike, L.M. & Zhu, J.H.
Partner: UNT Libraries Government Documents Department

TEM investigation on the low temperature phase of HfV{sub 2}

Description: HfV2, a particularly attractive C15 Laves phase structural material, undergoes a structural transformation at 115K. A tentative reciprocal lattice for low-temperature HfV2 based on a combination of selected area diffraction patterns is illustrated. The results reveal structural characteristics inconsistent with earlier claims by other investigators. Further and detailed studies are under way, using a combination of CBED and synchrotron diffraction to determine the low-temperature structure`s crystal symmetry, lattice parameters, and atomic positions.
Date: April 1, 1995
Creator: Chu, Fuming & Mitchell, T.E.
Partner: UNT Libraries Government Documents Department

THE EFFECT OF MAGNETICALLY ALIGNED POWDER ON THE MAGNETOSTRICTION OF SINTERED RARE EARTH - IRON LAVES PHASE COMPOUNDS

Description: A powder metallurgical approach is utilized to prepare grain oriented Laves phase compounds of Tb{sub x}Dy{sub 1-x}Fe{sub 2}. The magnetostrains observed in the oriented compounds, though containing {approx}20% porosity as presently prepared, are far superior to those of arc cast and highly dense liquid phase sintered materials. Also, it is shown that the alignment achieved is strongly dependent on the Tb/Dy ratio.
Date: October 1, 1977
Creator: Malekzadeh, M. & Pickus, M.R.
Partner: UNT Libraries Government Documents Department

ALCHEMI of niobium dichrome/vanadium C15 Laves phase

Description: 33Nb42Cr25V was prepared by arc melting and annealing at 1400 C for 120 hr. Energy-dispersive x-ray spectra were collected over a range of {l_brace}400{l_brace} excitations between symmetry and beyond {l_brace}12 0 0{l_brace}. Results show that at least qualitatively V substitutes for Cr. Therefore, electronic effects must be more important than size effects in this case.
Date: May 1, 1996
Creator: Kotula, P.G.; Chu, Fuming; Mitchell, T.E.; Anderson, I.M. & Bentley, J.
Partner: UNT Libraries Government Documents Department

Point Defects in Binary Laves-Phase Alloys

Description: Point defect mechanisms in the binary C15 NbCr{sub 2} and NbCo{sub 2}, and C14 NbFe{sub 2} systems on both sides of stoichiometry was studied and clarified by both bulk density and X-ray lattice parameter measurements. It was found that the vacancy concentrations in these systems after quenching from 1000 C are essentially zero. The constitutional defects on both sides of stoichiometry for these systems were found to be of the anti-site type in comparison with the model predictions. However, thermal vacancies exhibiting a maximum at the stoichiometric composition were obtained in NbCr{sub 2} laves phase alloys after quenching from 1400 C. These could be completely eliminated by annealing at 1000 C. Anti-site hardening was found on both sides of stoichiometry for all three Laves phase systems studied. Furthermore, the thermal vacancies in NbCr{sub 2} alloys after quenching from 1400 C were found to soften the Laves phase. The anti-site hardening of the Laves phases is similar to that of the B2 compounds, while the thermal vacancy softening is unique to the Laves phase. Both the anti-site defects and thermal vacancies do not significantly affect the fracture toughness of the Laves phases.
Date: November 30, 1998
Creator: Liaw, P.K.; Liu, C.T.; Pike, L.M. & Zhu, J.H.
Partner: UNT Libraries Government Documents Department

The effect of stoichiometry in C15 HfCo[sub 2]

Description: In binary Laves phases (AB{sub 2}) that exhibit a homogeneity range, the ability to accommodate nonstoichiometry is related to the atomic size requirements for the topologically close-packed structure. Based upon a hard sphere model, Laves phases with a metallic atom size ratio R{sub A}/R{sub B} close to the ideal value of {approx}1.225 exhibit homogeneity ranges. The C15 phase, HfCo{sub 2}, has a radius ratio of R{sub H}/R{sub Co} = 1.26 and a reported range of solubility of 9 at.%. The solubility limits and constitutional defects have been investigated, and with this information, the elastic and mechanical properties as a function composition were experimentally investigated. The results indicate that anti-site substitution is the governing defect mechanism on both sides of stoichiometry, The elastic and mechanical properties show a maximum at stoichiometry (unlike most intermetallics), with the brittle behavior decreasing with increasing Co content. The properties are described in terms of geometric models of space filling.
Date: January 1, 2001
Creator: Thoma, D. J. (Dan J.); Chen, K. C. (Katherine C.); Baskes, M. I. (Michael I.) & Peterson, E. J. (Eric J.)
Partner: UNT Libraries Government Documents Department

High temperature oxidation of beryllium modified intermetallic compounds of the niobium-chromium system

Description: Intermetallic compounds based on the Laves phases NbCr2 and TiCr2 are of interest for high temperature applications. At temperatures where the high temperature strength and creep resistance of these compounds are excellent, their oxidation resistance is relatively poor. Beryllide compounds in the 3-Be, Ti-Be and Nb-Be systems (CrBe2, TiBel2, TiBez, NbBel2) on the other hand, exhibit excellent oxidation resistance even at temperaturets in excess of 1200' C, due to the formation of protective Be0 scales. The pure beryllides have relatively poor mechanical properties however. Therefore we have attempted to improve the oxidation resishnce of the Nb-Cr-Ti based Laves phases using alloying additions of beryllium. Alloys investigated include CrBe2, NbBe2 and NbCr2 with 10,25, and 33 atomic percent beryllium substituted for Cr. We have determined that there is a significant solubility of Be in NbCr2, Beryllium modified NbCr2 exhibits protective oxidation, with the growth of scales containing principally Cr2O3, and BeO.
Date: January 1, 2001
Creator: Hanrahan, Robert J.; Peters, Maria I.; Cooley, Jason C.; Hults, William L.; Thoma, Dan J. & Chen, Katherine C.
Partner: UNT Libraries Government Documents Department

Elastic constants of a Laves phase compound: C15 NbCr{sub 2}

Description: The single-crystal elastic constants of C15 NbCr{sub 2} have been computed by using a first-principles, self-consistent, full-potential total energy method. From these single-crystal elastic constants the isotropic elastic moduli are calculated using the Voigt and Reuss averages. The calculated values are in fair agreement with the experimental values. The implications of the results are discussed with regards to Poisson`s ratio and the direction dependence of Young`s modulus.
Date: April 1, 1997
Creator: Ormeci, A.; Chu, F.; Wills, J.M.; Chen, S.P.; Albers, R.C.; Thoma, D.J. et al.
Partner: UNT Libraries Government Documents Department

A Parametric Analysis of Solidification in Y(Fe,Ni,Cr)-Nb-C Alloys

Description: A parametric analysis is presented which summarizes the amount of total ({gamma}/NbC + {gamma}/Laves) and individual {gamma}/NbC and {gamma}/Laves constituents which form during solidification of {gamma}{sub (Fe,Ni,Cr)} alloys with variations in nominal Nb and C contents. Calculated results are presented for Fe base alloys and Ni base alloys. The results provide a quantitative rationale for understanding the relation between alloy composition and solidification microstructures and should provide useful insight into commercial alloys of similar composition.
Date: February 22, 1999
Creator: DuPont, J.N. & Robino, C.V.
Partner: UNT Libraries Government Documents Department

Slip, twinning, and transformation in Laves phases. Final technical report, 1 August 1990--January 31, 1997

Description: Currently, intermetallic compounds are being studied as potential high-temperature structural materials. Most of these studies are on phases such as Ni{sub 3}Al, TiAl, NiAl, and Ti{sub 3}Al, which have structures that are crystallographic derivatives of fcc, bcc, or hcp structures. However, most intermetallics have more complex crystal structures, and little is known about their deformation mechanisms and mechanical properties. By far the largest class of such compounds is that of the Laves phases, AB{sub 2} compounds having three different crystal structures: C15 (cubic), C14 (hexagonal), and C36 (dihexagonal). This research program explored room-temperature compressive deformation of two-phase alloys containing substantial amounts of Laves phase, and, by transmission electron microscopy, characterized the deformation-induced defects in these alloys. The results include: transmission electron microscopy analysis of defect structures to determine deformation mechanisms, use of indentation experiments to gain deeper understanding of deformation mechanisms, systematic study of effects of ternary alloying additions on mechanical properties, and assessment of effects of departure from stoichiometry on phase stability and mechanical properties of Laves phases.
Date: June 1, 1998
Creator: Allen, S.M.
Partner: UNT Libraries Government Documents Department

Theoretical and Experimental Investigation on the Low Temperature Properties of the NbCr{sub 2} Laves Phase

Description: This is the final report of a three-year, Laboratory Directed Research and Development (LDRD) project at Los Alamos National Laboratory (LANL). The goal of the project was to develop methodologies in which to define and improve the properties of NbCr{sub 2} so that the high temperature structural applications of alloys based upon this would not be limited by the low-temperature brittle behavior of the intermetallic. We accomplished this task by (1) understanding the defect structure and deformation mechanisms in Laves phases, (2) electronic and geometric contributions to phase stability and alloying behavior, and (3) novel processing of dual phase (Laves/bcc) structures. As a result alloys with properties that in many cases surpass superalloys were developed. For example, we have tailored alloy design strategies and processing routes in a metal alloy to achieve ambient temperature ultimate strengths of 2.35 GPa as well as ultimate strengths of 1.5 GPa at 1000 C. This results i n one of the strongest metal alloys that currently exist, while still having deformability at room temperature.
Date: June 3, 1999
Creator: Thoma, D.J.; Chu, F.; Chen, K.C.; Kotula, P.G.; Mitchell, T.E.; Wills, J.M. et al.
Partner: UNT Libraries Government Documents Department

Processing and Microstructure of Cr-Ta and Cr-Ta-Mo Composites Reinforced by the Cr2Ta Laves Phase

Description: The Cr-Ta alloy with an eutectic structure has a good combination of high strength and oxidation resistance at elevated temperatures up to 1200 C. It is an ideal candidate for ultrahigh-temperature applications. However, the material shows low ductility and fracture toughness at room temperature. A possible way to improve the ductility and fracture toughness is to obtain an aligned microstructure of eutectic Cr-based alloys, using a directional-solidification (DS) process, in which the feed materials with eutectic compositions are preferred. In the present work, a quantitative technique was developed to assist in monitoring and controlling the composition of the Cr-based alloys at each stage of the processing at elevated temperatures. The eutectic composition of the binary Cr-Ta alloy was determined to be Cr-9.7 at.% (atomic percent) Ta, and a drop-cast ternary Cr-9.7 at.% Ta-1.0 at.% Mo alloy was found to possess a fully eutectic structure.
Date: July 29, 2002
Creator: Wang, DF
Partner: UNT Libraries Government Documents Department

First principles total energy study of NbCr{sub 2} + V Laves phase ternary system

Description: The C15 NbCr{sub 2} + V Laves phase ternary system is studied by using a first-principles, self-consistent, full-potential total energy method. Equilibrium lattice parameters, cohesive energies, density of states and formation energies of substitutional defects are calculated. Results of all these calculations show that in the C15 NbCr{sub 2} + V compounds, V atoms substitute Cr atoms only.
Date: April 1, 1999
Creator: Ormeci, A.; Chen, S.P.; Wills, J.M. & Albers, R.C.
Partner: UNT Libraries Government Documents Department

Phase stability and elasticity of C15 transition-metal intermetallic compounds

Description: First-principle quantum mechanical calculations based on the local-density-functional theory have been performed to study the electronic, physical and metallurgical properties of C15 intermetallics MV{sub 2} (M = Zr, Hf, or Ta). The elastic constants of C15 HfV{sub 2} + Nb were measured by the resonant ultrasound spectroscopy technique. The phase stability of C15 HfV{sub 2} + Nb was studied by specific heat measurements and by transmission electron microscopy in a low temperature specimen holder. The total energies and their lattice volume dependence were used to obtain the equilibrium lattice constants and bulk modulus. The band structures at the X-point near the Fermi level were employed to understand the anomalous temperature dependence of shear modulus of the C15 intermetallics. It was found that the double degeneracy with a linear dispersion relation of electronic levels at the X-point near the Fermi surface is mainly responsible for the C15 anomalous elasticity. The density of states at the Fermi level, N(E{sub F}), and the Fermi surface geometry were obtained to understand the low temperature phase instability of C15 HfV{sub 2} and ZrV{sub 2} and the stability of C15 TaV{sub 2}. It was proposed that the large N(E{sub F}) and Fermi surface nesting are the physical reasons for the structural instability of the C15 HfV{sub 2} and ZrV{sub 2} at low temperatures. The relation between anomalous elasticity and structural instability of C15 HfV{sub 2} and ZrV{sub 2} is also discussed.
Date: March 1, 1995
Creator: Chu, F.; Mitchell, T.E.; Chen, S.P.; Sob, M.; Siegl, R. & Pope, D.P.
Partner: UNT Libraries Government Documents Department

The effect of microstructure and temperature on the oxidation behavior of two-phase Cr-Cr{sub 2}X (X = Nb, Ta) alloys

Description: The oxidation behavior of Cr(X) solid solution (Cr{sub ss}) and Cr{sub 2}X Laves phases (X = Nb, Ta) was studied individually and in combination at 950--1,100 C in air. The Cr{sub ss} phase was significantly more oxidation resistant than the Cr{sub 2}X Laves phase. At 950 C, two-phase alloys of Cr-Cr{sub 2}Nb and Cr-Cr{sub 2}Ta exhibited in-situ internal oxidation, in which remnants of the Cr{sub 2}X Laves phase were incorporated into a growing chromia scale. At 1,100 C, the Cr-Cr{sub 2}Nb alloys continued to exhibit in-situ internal oxidation, which resulted in extensive O/N penetration into the alloy ahead of the alloy-scale interface and catastrophic failure during cyclic oxidation. IN contrast, the Cr-Cr{sub 2}Ta alloys exhibited a transition to selective Cr oxidation and the formation of a continuous chromia scale. The oxidation mechanism is interpreted in terms of multiphase oxidation theory.
Date: November 1, 1998
Creator: Brady, M.P. & Tortorelli, P.F.
Partner: UNT Libraries Government Documents Department

Site occupancies in ternary C15 ordered Laves phases

Description: Site occupancies in three C15-structured AB{sub 2}(X) Laves phases have been determined by Atom Location by CHanneling Enhanced MIcroanalysis (ALCHEMI). In NbCr{sub 2}(V), the results were consistent with exclusive site occupancies of Nb for the A sublattice and Cr and V for the B sublattice. The B-site occupancy of V is not expected from atom size effects alone. In NbCr{sub 2}(Ti), the results were consistent with Ti partitioning mostly to the A sites with some anti-site defects likely. In HfV{sub 2}(Nb), the results were consistent with Nb partitioning between the A and B sites. The results of the ALCHEMI analyses of these ternary C15 Laves phase materials will be discussed with respect to previously determined phase diagrams and first-principles total energy and electronic structure calculations.
Date: December 31, 1996
Creator: Kotula, P.G.; Chu, F.; Thoma, D.J.; Mitchell, T.E.; Anderson, I.M. & Bentley, J.
Partner: UNT Libraries Government Documents Department

Laves intermetallics in stainless steel-zirconium alloys

Description: Laves intermetallics have a significant effect on properties of metal waste forms being developed at Argonne National Laboratory. These waste forms are stainless steel-zirconium alloys that will contain radioactive metal isotopes isolated from spent nuclear fuel by electrometallurgical treatment. The baseline waste form composition for stainless steel-clad fuels is stainless steel-15 wt.% zirconium (SS-15Zr). This article presents results of neutron diffraction measurements, heat-treatment studies and mechanical testing on SS-15Zr alloys. The Laves intermetallics in these alloys, labeled Zr(Fe,Cr,Ni){sub 2+x}, have both C36 and C15 crystal structures. A fraction of these intermetallics transform into (Fe,Cr,Ni){sub 23}Zr{sub 6} during high-temperature annealing; the authors have proposed a mechanism for this transformation. The SS-15Zr alloys show virtually no elongation in uniaxial tension, but exhibit good strength and ductility in compression tests. This article also presents neutron diffraction and microstructural data for a stainless steel-42 wt.% zirconium (SS-42Zr) alloy.
Date: May 1, 1997
Creator: Abraham, D.P.; McDeavitt, S.M. & Richardson, J.W. Jr.
Partner: UNT Libraries Government Documents Department

Alloy design and phase stability of the ternary alloy titanium-aluminum-niobium

Description: This is the final report of a three-year, Laboratory Directed Research and Development (LDRD) project at the Los Alamos National Laboratory (LANL). The authors have used high-precision electronic-structure and cluster-variational method (CVM) techniques to study aspects of alloy design involving the phase stability and ordering tendencies of the ternary system Ti-Al-Nb, and the correlation with these to changes in the electronic structure of these materials. This system is of great interest for aerospace applications due to its desirable mechanical properties and low densities. Total energies were computed for 18 binary and ternary bcc superstructures in order to derive parameters for CVM calculations, which showed important strong ordering tendencies in the alloy phase diagram as a function of temperature and alloy concentration. Structural optimization calculations were used to analyze structural instabilities for bcc, fcc, hcp, O-phase, and {omega}-phase structures. The authors discovered the mechanism for the role of Nb in the structural stability of the O-phase of the ternary intermetallic Ti{sub 2}AlNb. Calculations were also done to investigate the electronic properties associated with the structural stability of a related class of Laves Phase high-temperature structural materials NbCr{sub 2} and HfV{sub 2}.
Date: December 31, 1998
Creator: Albers, R.C.; Chen, Shao-Ping & Wills, J.M.
Partner: UNT Libraries Government Documents Department

Electron concentration and phase stability in NbCr2-based Laves phase alloys

Description: Phase stability in NbCr{sub 2}-based transition-metal Laves phases was studied, based on the data reported for binary X-Cr, Nb-X, and ternary Nb-Cr-X phase diagrams. It was shown that when the atomic size ratios are kept identical, the average electron concentration factor, e/a, is the dominating factor in controlling the phase stability of NbCr{sub 2}-based transition-metal Laves phases. The e/a ratios for different Laves polytypes were determined as followed: with e/a < 5.76, the C15 structure is stabilized; at an e/a range of 5.88--7.53, the C14 structure is stabilized; with e/a > 7.65, the C15 structure is stabilized again. A further increase in the electron concentration factor (e/a > 8) leads to the disordering of the alloy. The electron concentration effect on the phase stability of Mg-based Laves phases and transition-metal A{sub 3}B intermetallic compounds is also reviewed and compared with the present observations in transition-metal Laves phases. In order to verify the e/a/phase stability relationship experimentally, additions of Cu (with e/a = 11) were selected to replace Cr in the NbCr{sub 2} Laves phase. Experimental results for the ternary Nb-Cr-Cu system are reported and discussed in terms of the correlation between the e/a ratio and phase stability in NbCr{sub 2}-based Laves phases. A new phase was found, which has an average composition of Nb-47Cr-3Cu. Within the solubility limit, the electron concentration and phase stability relationship is obeyed in the Nb-Cr-Cu system.
Date: May 12, 1997
Creator: Zhu, J. H.; Liaw, P. K. & Liu, C. T.
Partner: UNT Libraries Government Documents Department

Formation of a metastable bcc solid solution and decomposition to a C15 Laves phase in melt-spun CrNb{sub 10}Ti{sub 10}

Description: A metastable, disordered bcc phase has been formed from the melt in a Cr-rich alloy of the Nb-Cr-Ti system, where large volume fractions of the Laves phase would develop under equilibrium solidification conditions. X-ray diffraction (XRD) studies and lattice constant determinations confirm that the melt-spun ribbons contain a bcc phase beyond its terminal solid solution limits. Solidification pathways are proposed based upon metastable and equilibrium phase diagrams. Microstructures have been studied using optical microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Cross-sectional TEM reveals tiny, dispersed Laves phase precipitates within the metastable bcc grains near the chill (wheel) side. Away from the wheel, the microstructure changes to a dendritic structure of the bcc and C15 Laves phase. Annealing of the melt-spun ribbons produces a fine, two phase microstructure of the equilibrium phases. Microstructures from the melt-spun and arc melted processing conditions are compared. The micro structural control afforded through this metastable processing route enables a methodology to tailor phase distributions for optimized toughness in Laves phase alloys.
Date: March 1, 1998
Creator: Chen, K.C.; Kotula, P.G.; Chu, F. & Thoma, D.J.
Partner: UNT Libraries Government Documents Department

Defects and site occupancies in Nb-Cr-Ti C15 Laves phase alloys

Description: A multiply-faulted dipole in the C15 Laves phase of composition Nb{sub 15}Cr{sub 68}Ti{sub 17} has been characterized. The dislocations all lie along [011] and show no residual contrast when imaged edge-on at B = [0{bar 1}{bar 1}] indicating that they are edge in character. Given this, the Burgers vectors for the Shockley partials are {+-} 1/6[2{bar 1}1] and those of the stair-rods are {+-} 1/6[0{bar 1}1]. This is consistent with a g {center_dot} b analysis which was performed. The stacking fault energy was determined to be 35 mJ/m{sup 2} which is higher than the experimental value of 25 mJ/m{sup 2} from NbCr{sub 2}. Simulations are currently underway to understand, given the site occupancies and observed trend in stacking fault energy, whether Ti would be expected to raise the stacking fault energy of the C15 Laves phase.
Date: September 1, 1998
Creator: Kotula, P.G.; Carter, C.B.; Chen, K.C.; Thoma, D.J.; Chu, F. & Mitchell, T.E.
Partner: UNT Libraries Government Documents Department

Fracture behavior of Cr{sub 2}Nb-based intermetallics

Description: Microstructural evaluations and mechanical testing of Laves-phase alloys based on Cr{sub 2}Nb were examined in order to determine phase relationships with heat treating temperatures up to 1,600 C. At ambient temperatures, single-phase Cr{sub 2}Nb alloys are very hard and brittle due to the complicated crystal structure (C-15). The following results were revealed through examination of the Cr-Cr{sub 2}Nb two-phase region: (a) with increasing amounts of the soft chromium-rich phase, the compression strength and hardness decrease; (b) the annealing treatments studied provided the best break-up of the brittle Laves-containing eutectic phase in the 94 at.% Cr-6 at.% Nb (CN-7) alloy; (c) annealing for 1 hour at 1,600 C + 4 hours at 1,200 C drastically improved the room temperature strength and compressive ductility over previous annealing treatments. Previous studies have shown that the introduction of a soft chromium phase has promising effects in improving the mechanical properties of brittle Cr{sub 2}Nb Laves-phase alloys.
Date: December 31, 1994
Creator: Cook, J.A.; Liaw, P.K. & Liu, C.T.
Partner: UNT Libraries Government Documents Department