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Dynamical x-ray diffraction from an icosahedral Al-Pd-Mn quasicrystal

Description: Primary extinction effects in diffraction from single grains of Al-Pd- Mn, and presumably many other FCI alloys, may be significant and should be corrected for prior to use of diffraction data in structural determinations. Probes based on dynamical diffraction effects, such as x-ray standing wave fluorescence, multiple beam interference, and x-ray transmission topographs, may now be used to study the bulk and surface structure of some quasicrystals. The observation of dynamical diffraction from icosahedral Al-Pd-Mn is a striking confirmation of the fact that quasicrystals can present a degree of structural perfection comparable to that found in the best periodic intermetallic crystals.
Date: April 23, 1996
Creator: Kycia, S.
Item Type: Refine your search to only Report
Partner: UNT Libraries Government Documents Department

Spin-density-wave magnetism in dilute copper-manganese alloys

Description: Elastic neutron-scattering measurements on two samples of Cu alloyed with 1.3% Mn and 0.55% Mn show that the spin-density-wave (SDW) features found in more concentrated alloys persist in the limit of very dilute alloys. These features consist of temperature-dependent incommensurate peaks in magnetic neutron scattering, with positions and strengths which are fully consistent with those in the concentrated alloys. The implications of these measurements are twofold. First, it is clear from this data that SDW magnetic ordering occurs across the entire range of CuMn alloys which have typically been interpreted as spin glasses. Second, the more fundamental significance of this work is the suggestion via extrapolation that a peak in the magnetic susceptibility x(q) occurs in pure copper, at a value of q given by the Fermi-surface diameter 2k{sub F}.
Date: February 1, 1995
Creator: Lamelas, F.J.; Werner, S.A.; Shapiro, S.M. & Mydosh, J.A.
Item Type: Refine your search to only Article
Partner: UNT Libraries Government Documents Department

OPERATION OF A TRITIUM GLOVEBOX CLEAN-UP SYSTEM USING ZIRCONIUM MANGANESE IRON AND ZIRCONIUM TWO IRON METAL GETTERS

Description: A metal hydride-based tritium clean-up system has been successfully operated for more than four years on an 11 m{sup 3} helium/nitrogen glovebox which was used for handling metal tritide powders. The clean-up system consists of two beds: (1) a Zr-Mn-Fe (in a 10% by weight Al binder, SAES ST909) bed operating at 675 C followed by (2) a Zr{sub 2}Fe (SAES ST198) bed operating at 250 C. The Zr-Mn-Fe bed serves to condition the gas stream by cracking hydrogenous impurities (such as H{sub 2}O and hydrocarbons) and absorbing oxygen and carbon. The Zr{sub 2}Fe bed absorbs the hydrogen isotopes from the flowing stream by forming a solid hydride compound. These beds contain 3 kilograms of Zr{sub 2}Fe and have been loaded routinely with 230-250 STP liters of hydrogen isotopes in earlier trials. The Zr-Mn-Fe alloy exhibits an anomaly during activation, namely an exotherm upon initial exposure to nitrogen. The purpose of this work is to better understand this reaction. Nitrogen absorption studies were done in order to quantify the nitrogen taken up by the getter and to characterize the reaction kinetics. In addition, ST909 phases before and after the reaction were studied with x-ray diffraction.
Date: August 1, 2000
Creator: LARSON, E. & COOK, K.
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Partner: UNT Libraries Government Documents Department

PdMn and PdFe: New Materials for Temperature Measurement Near 2K

Description: Interest in the critical dynamics of superfluid <SUP>4</SUP> He in microgravity conditions has motivated the development of new high resolution thermometry technol- ogy for use in space experiments near 2K. The current material commonly used as the temperature sensing element for high resolution thermometers (HRTs) is copper ammonium bromide [Cu(NH<SUB>4</SUB>)<SUB>2</SUB>Br<SUB>4</SUB>2H<SUB>2</SUB>0) or "CAB", which undergoes a ferromagnetic phase transition at 1.8K1. HRTs made from CAB have demonstrated low drift (< 10fK/s) and a temperature resolu- tion of O.lnK. Unfortunately, paramagnetic salts such as CAB are difficult to prepare and handle, corrosive to most metals, and become dehydrated if kept, under vacuum conditions at room temperature. We have developed a magnetic thermometer using dilute magnetic alloys of Mn or Fe dissolved in a pure Pd matrix. These metallic thermometers are easy to fabricate, chemically inert, and mechanically robust. Unlike salts, they may be directly soldered to the stage to be measured. Also, the Curie temperature can be varied by changing the concentration of Fe or Mn, making them available for use in a wide temperature range. Susceptibility measurements, as well as preliminary noise and drifl measurements, show them, to have sub-nK resolution, with a drift of less than 10<SUP>-13</SUP> K/s.
Date: May 17, 1999
Creator: Adriaans, M. J.; Aselage, T. L.; Day, P. K.; Duncan, R. V.; Klemme, B. J. & Sergatskov, D. A.
Item Type: Refine your search to only Article
Partner: UNT Libraries Government Documents Department

First-principles exchange interactions between ferro and antiferromagnetic films: Co on NiMn, a case study

Description: Heisenberg exchange parameters at the interface of antiferromagnetic NiMn with ferromagnetic Co are calculated from first-principles. The authors use a layer version of the Korringa-Kohn-Rostocker multiple scattering approach and an expression, which is based on the expansion of the band energy, to calculate the exchange parameters from the underlying electronic structure. For bulk systems, the parameter sets yield Curie temperatures that are in good agreement with experimental values. In the interface region, the inter-layer interactions in NiMn change significantly compared to the bulk while the intra-layer interactions are almost unchanged.
Date: December 31, 1997
Creator: Schulthess, T.C. & Butler, W.H.
Item Type: Refine your search to only Article
Partner: UNT Libraries Government Documents Department

THE EFFECT OF MANGANESE AND SILICON CONTENT ON THE HOT WORKABILITY OF TYPE 304 STAINLESS STEEL CONTAINING 2 w/o BORON

Description: Increased hot workability of Type 304 stainless steel containing approximately 2 wt% boron was obtained by maintaining the manganese and silicon contents of the alloy above minimum levels of 1.40 and 0.60 wt%, respectively. Close control of the range of temperatures over which hot working was carried out was also demonstrated to be of importance. However, over the range investigated variation in the temperature of hot working was of secondary significance as compared with alloy composition. (auth)
Date: October 1, 1961
Creator: hymes, L.C. & Balai, N.
Item Type: Refine your search to only Report
Partner: UNT Libraries Government Documents Department

Structure of the modulated magnetic phase of Mn{sub 3}Sn

Description: The triangular antiferromagnetic order that occurs in hexagonal Mn{sub 3}Sn below 420 K becomes modulated below 270 K in what is generally described as a helical modulation propagating along the c axis with a period of about 10 or 12 c{sub 0}. However, such a modulation is inconsistent with a recently observed energy gap in the magnetic excitation spectra for this phase. We have reexamined the magnetic structure of this modulated phase by neutron diffraction and find two distinct modulations, both propagating along the c axis and both with periods that are commensurate with the lattice below about 120 K. The satellite reflections associated with these two modulations have quite different intensities and temperature dependences suggesting coexistent modulated phases. For the dominant 11 c{sub 0} phase, we observe a 3rd harmonic with an intensity that is 4% of the 1st harmonic but find no 5th or 7th harmonic to a sensitivity of about 10{sup {minus}4}. The presence of this 3rd harmonic shows a significant distortion of the helical modulation wave. The observed harmonic intensity distribution was compared with magnetic structure factor calculations for configurational models with moment directions bunched around the easy axes in the basal plane. The results of this comparison are given in this paper.
Date: November 1, 1993
Creator: Cable, J. W.; Wakabayashi, N. & Radhakrishna, P.
Item Type: Refine your search to only Article
Partner: UNT Libraries Government Documents Department

DEVELOPMENT OF CORROSION-RESISTANT ALLOYS FOR USE AS CONTAINER MATERIALS FOR DECLADDING SOLUTIONS OR AS WELDING ALLOYS

Description: Twenty-four experimental alloys were developed and evaluated as container materials or welding alloys for use with Sulfex and Niflex decladding solutions. Niflex solutions which were more corrosive than Sulfex solutions to most of the experimental alloys, produced severe localized attack on weldments made on vacuum-melted Hastelloy F with the experimental alloys. However, several of the alloys, when self-welded, were not selectively attacked. Some of these showed a substantial improvement in resistance to the decladding solutions. The most promising alloys were based on either 45 wt.% nickel--22 wt.% chromium or 50 wt.% nickel--25 wt.% chromium, with at least 6 wt.% molybdenum, and 1 wt.% titanium, 0.6 wt.% manganese, 0.4 wt. % silicon, 0.02 wt.% carbon, and the balance, iron. The alloy most resistant to both solutions contained 6 wt.% molybdenum and 1 wt.% copper in the 50 wt.% nickel--25 wt.% chromium base. Its corrosion rate of 22 mils per month in Niflex, with no selective attack, was significantly lower than the 105 mils per month recorded for Hastelloy F. Even lower rates would be expected under the less stringent conditions of actual process operation. Indications are that more resistance might be obtained by increasing the chromium and nickel contents. (auth)
Date: August 1, 1960
Creator: Peterson, C.L.; Drennen, D.C.; Langston, M.E.; Hall, A.M. & Boyd, W.K.
Item Type: Refine your search to only Report
Partner: UNT Libraries Government Documents Department

Aqueous Corrosion of Magnesium Alloys

Description: BS>The aqueous corrosion of Mg alloys was investigated at 53 to 150 deg C. Corrosion rates rose rapidly with temperature, reaching about 3 mils per day at 150 deg C for AZ-31 STAMg-2.5 to 3.5 wt.% Al-0.7 to 1.3 wt.% Zn-0.2 wt.% Mn!. Additions of small amounts of Cu and/or Ni to the basic AZ-31 composition reduced the corrosion rate at 150 deg by a factor of about two. Sn may be advantageously substituted for Zn in AZ-31. Control of the pH in the range between 6 and 7 and maintenance of a fluoride concentration in the range between 1 and 10 ppm reduced the corrosion rate of AZ-31 to about 0.1 mil per day at 150 deg C. (auth)
Date: May 1, 1960
Creator: Greenberg, S. & Ruther, W. E.
Item Type: Refine your search to only Report
Partner: UNT Libraries Government Documents Department

Large Components Test Loop System Temperature Limit of Error

Description: Tests were conducted to determine the limit of error of the temperature measuring system for high thermal-stress tests on moderator cans and to determine a means for the calibration of chromel-alumel thermocouples after installation. Results and recommendations are included. (J.R.D.)
Date: March 27, 1959
Creator: Gerber, M. D.
Item Type: Refine your search to only Report
Partner: UNT Libraries Government Documents Department

Surface structures of Al-Pd-Mn and Al-Cu-Fe icosahedral quasicrystals

Description: In this dissertation, the author reports on the surface structure of i-Al-Pd-Mn twofold, threefold, fivefold and i-Al-Cu-Fe fivefold surfaces. The LEED studies indicate the existence of two distinct stages in the regrowth of all four surfaces after Ar{sup +} sputtering. In the first stage, upon annealing at relatively low temperature: 500K--800K (depending on different surfaces), a cubic phase appears. The cubic LEED patterns transform irreversibly to unreconstructed quasicrystalline patterns upon annealing to higher temperatures, indicating that the cubic overlayers are metastable. Based upon the data for three chemically-identical, but symmetrically-inequivalent surfaces, a model is developed for the relation between the cubic overlayers and the quasicrystalline substrate. The model is based upon the related symmetries of cubic close-packed and icosahedral-packed materials. These results may be general among Al-rich, icosahedral materials. STM study of Al-Pd-Mn fivefold surface shows that terrace-step-kink structures start to form on the surface after annealing above 700K. Large, atomic ally-flat terraces were formed after annealing at 900K. Fine structures with fivefold icosahedral symmetry were found on those terraces. Data analysis and comparison of the STM images and structure model of icosahedral Al-Pd-Mn suggest that the fine structures in the STM images may be the pseudo Mackay (PMI) clusters which are the structure units of the structure model. Based upon his results, he can conclude that quasicrystalline structures are the stable structures of quasicrystal surfaces. In other words, quasicrystalline structures extend from the bulk to the surface. As a result of the effort reported in this dissertation, he believes that he has increased his understanding of the surface structure of icosahedral quasicrystals to a new level.
Date: February 12, 1999
Creator: Shen, Z.
Item Type: Refine your search to only Thesis or Dissertation
Partner: UNT Libraries Government Documents Department

PdMn and PdFe: New Materials for Temperature Measurement Near 2K

Description: Interest in the critical dynamics of superfluid He in microgravity conditions has motivated the development of new high resolution thermometry technology for use in space experiments near 2K. The current material commonly used as the temperature sensing element for high resolution thermometers (HRTs) is copper ammonium bromide (Cu(NH{sub 4}){sub 2}Br{sub 4}2H{sub 2}O) or CAB, which undergoes a ferromagnetic phase transition at 1.8K. HRTs made from CAB have demonstrated low drift (&lt;10fK/s) and a temperature resolution of 0.1nK. Unfortunately, paramagnetic salts such as CAB are difficult to prepare and handle, corrosive to most metals, and become dehydrated if kept under vacuum conditions at room temperature. We have developed a magnetic thermometer using dilute magnetic alloys of Mn or Fe dissolved in a pure Pd matrix. These metallic thermometers are easy to fabricate, chemically inert, and mechanically robust. Unlike salts, they may be directly soldered to the stage to be measured. Also, the Curie temperature can be varied by changing the concentration of Fe or Mn, making them available for use in a wide temperature range. Susceptibility measurements, as well as preliminary noise and drift measurements, show them to have sub-nK resolution with a drift of less than 10{sup {minus}13} K/s.
Date: February 1, 1999
Creator: Aselage, T.L.; Duncan, R.V.; Klemme, B.J.; Sergatskov, D.A.; Adriaans, M.J. & Day, P.K.
Item Type: Refine your search to only Report
Partner: UNT Libraries Government Documents Department

Creep fracture during solute-drag creep and superplastic deformation

Description: Creep fracture behavior has been studied in Al-Mg and Al-Mg-Mn alloys undergoing solute-drag creep and in microduplex stainless steel undergoing both solute-drag creep and superplastic deformation. Failure in these materials is found to be controlled by two mechanisms, neck formation and cavitation. The mechanism of creep fracture during solute-drag creep in Al-Mg is found to change from necking-controlled fracture to cavitation-controlled fracture as Mn content is increased. Binary Al-Mg material fails by neck formation during solute-drag creep, and cavities are formed primarily in the neck region due to high hydrostatic stresses. Ternary alloys of Al-Mg- Mn containing 0.25 and 0.50 wt % Mn exhibit more uniform cavitation, with the 0.50 Mn alloy clearly failing by cavity interlinkage. Failure in the microduplex stainless steel is dominated by neck formation during solute-drag creep deformation but is controlled by cavity growth and interlinkage during superplastic deformation. Cavitation was measured at several strains, and found to increase as an exponential function of strain. An important aspect of cavity growth in the stainless steel is the long latency time before significant cavitation occurs. For a short latency period, cavitation acts to significantly reduce ductility below that allowed by neck growth alone. This effect is most pronounced in materials with a high strain-rate sensitivity, for which neck growth occurs very slowly.
Date: October 1, 1996
Creator: Taleff, E.M.; Lesuer, D.R.; Syn, C.K. & Henshall, G.A.
Item Type: Refine your search to only Article
Partner: UNT Libraries Government Documents Department

Charge-density wave and magnetic phase diagram of chromium alloys

Description: The magnetic phase diagrams of all dilute Cr alloys can be explained by a simple theoretical model with coupled spin- and charge-density waves and a finite electron reservoir. If the charge-density wave and electron reservoir are sufficiently large, the paramagnetic to commensurate spin-density wave transition becomes strongly first order, as found in Cr{sub 1- x}Fe{sub x} and Cr{sub 1-x}Si{sub x} alloys. The observed discontinuity of the slope dT{sub N}/dx at the triple point and the bending of the CI phase boundary are also natural consequences of this model.
Date: August 8, 1996
Creator: Fishman, R.S. & Jiang, X.W.
Item Type: Refine your search to only Article
Partner: UNT Libraries Government Documents Department

Microstructural characterization of a Zr-Ti-Ni-Mn-V-Cr based AB{sub 2}-type battery alloy

Description: Transmission Electron Microscopy (TEM), combined with X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) was employed to investigate a proprietary and multicomponent AB{sub 2} type Nickel-Metal Hydride (Ni-MH) battery alloy. This material was prepared by High Pressure Gas Atomization (HPGA) and examined in both the as-atomized and heat treated condition. TEM examination showed a heavily faulted dendritic growth structure in as-atomized powder. Selected Area Diffraction (SAD) showed that this region consisted of both a cubic C15 structure with lattice constant a=7.03 and a hexagonal C14 structure with lattice parameter a=4.97 {angstrom}, c=8.11 {angstrom}. The Orientation Relationship (OR) between the C14 and C15 structures was determined to be (111)[1{bar 1}0]{sub C15}//(0001)[11{bar 2}0]{sub C14}. An interdendritic phase possessing the C14 structure was also seen. There was also a very fine grain region consisting of the C14 structure. Upon heat treatment, the faulted structure became more defined and appeared as intercalation layers within the grains. Spherical particles rich in Zr and Ni appeared scattered at the grain boundaries instead of the C14 interdendritic phase. The polycrystalline region also changed to a mixture of C14 and C15 structures. These results as well as phase stability of the C15 and C14 structures based on a consideration of atomic size factor and the average electron concentration are discussed.
Date: January 1, 1999
Creator: Shi, Zhan
Item Type: Refine your search to only Thesis or Dissertation
Partner: UNT Libraries Government Documents Department

Electron irradiation-induced mechanical property changes in reactor pressure vessel alloys

Description: High-energy electrons were used to study tensile property changes in simple Fe-Cu and Fe-Cu-Mn alloys irradiated at 288C. A comparison was made with neutron irradiation data on the same alloys. An apparent effect of alloy chemistry was observed in which the presence of Mn affected embrittlement differently for electron and neutron irradiation. Comparison of previous experimental studies with the present experimental results indicates that electrons may be more efficient than fast neutrons at producing embrittlement.
Date: November 1, 1995
Creator: Alexander, D.E.; Rehn, L.E.; Odette, G.R. & Lucas, G.E.
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Partner: UNT Libraries Government Documents Department

Neutron diffraction study of the magnetic structures of CeMn{sub 2}Ge{sub 2} and CeMn{sub 2}Si{sub 2}

Description: The magnetic properties of the layered compounds of the form RMn{sub 2}X{sub 2} (R = Rare Earth, X = Si, Ge) have been thought to be sensitive to the intralayer Mn-Mn distance. Thus it has been reported that the Mn moments in CeMn{sub 2}Si{sub 2} are aligned antiferromagnetically (AF) below T{sub N} = 380K, while the Mn moments in CeMn{sub 2}Ge{sub 2} are ferromagnetic (FM) below T{sub C} = 316K. Recently, however, there has been some debate about the actual magnetic structures of this family of compounds, and for this reason the authors have performed high-resolution neutron powder diffraction measurements on these compounds for temperatures between 12K and 550K. The measurements indicate that at high temperatures both compounds are paramagnetic. Below T{sub N} = 380K CeMn{sub 2}Si{sub 2} becomes a collinear AF, with a structure similar to that reported by Siek et al. in which the magnetic propagation vector is {tau} = (0 0 1). CeMn{sub 2}Ge{sub 2} on the other hand, exhibits two different magnetic transitions. At T{sub N} {approx} 415K there is a transition to a collinear AF phase characterized by the commensurate propagation wavevector {tau} = (1 0 1). At T{sub C} = 318K there is a transition to a conical structure with a ferromagnetic component along the c-axis and a helical component in the ab plane. The helical component is characterized by the incommensurate propagation vector {tau} = (1 0 1-q{sub z}), where q{sub z} is temperature dependent. These findings are consistent with the recent results of Welter et al.
Date: December 31, 1995
Creator: Fernandez-Baca, J.A.; Chakoumakos, B.C.; Hill, P. & Ali, N.
Item Type: Refine your search to only Report
Partner: UNT Libraries Government Documents Department