41 Matching Results

Search Results

Advanced search parameters have been applied.

Glass forming ranges of amorphous alloys

Description: Two main methods of synthesis of amorphous metallic alloys are based on the rapid solidification of molten alloys and on isothermal solid state reactions between pure metals. We discuss recent calculations for the glass forming range for both techniques and we compare the predicted homogeneity ranges of the amorphous phase with experiments. 10 refs., 2 figs.
Date: January 1, 1988
Creator: Schwarz, R.B.
Partner: UNT Libraries Government Documents Department

Formation of amorphous alloys by solid state reactions. [Ni-Ti]

Description: Amorphous metallic alloys have been traditionally prepared by the rapid solidification of molten alloys. New methods of synthesis have been recently developed which are based on isothermal solid state reactions between pure metals. These are carried out at temperatures that kinetically favor the interdiffusion of the species but precludes the nucleation of intermetallics. Other novel methods of synthesis rely on the mechanical attrition of crystalline alloy powders. The paper reviews the basic concepts behind the novel methods of synthesis and compares the amorphous alloys prepared by these techniques with those obtained by the rapid solidification of melts.
Date: January 1, 1987
Creator: Schwarz, R.B.
Partner: UNT Libraries Government Documents Department

Metastable alloy phases prepared by solid state reactions and ball milling

Description: The main methods of synthesis of amorphous metallic alloys are based on (1) the rapid solidification of alloys from the liquid or vapor phase, (2) solid state interdiffusion reactions between pure metals having negative heats of mixing, and (3) mechanical attrition of crystalline intermetallics. We describe methods (2) and (3), developed around 1983, and we discuss recent advances in the understanding of the glass forming ranges for the three methods of synthesis. 43 refs., 10 figs.
Date: January 1, 1988
Creator: Schwarz, R.B.
Partner: UNT Libraries Government Documents Department

Amorphous metal alloys produced by mechanical alloying

Description: Mechanical alloying is a powder metallurgy method used in commercial production of high temperature superalloys. Under specific conditions, mechanical alloying allows the synthesis of amorphous metal alloys from mixtures of pure metal powders or from the powders of intermetallic compounds. Because the amorphizing transformation during mechanical alloying is a solid state reaction, most of the difficulties related to the amorphization by rapid solidification of melts can be avoided. Mechanical alloying allows the synthesis of amorphous alloys from metals with high melting temperatures and the resulting alloys have interesting properties such as high hardness and high crystallization temperatures. We used mechanical alloying for the synthesis of amorphous alloys in the binary alloy systems Nl-Sn, Nb-Si and Al-Hf. The amorphous alloy powders were characterized by x-ray diffraction, differential scanning calorimetry, scanning and transmission electron microscopy and hardness measurements. Produced alloys were compacted by hot pressing and the obtained compacts were characterized by x-ray diffraction, density and hardness measurements and by optical and scanning electron microscopy. The results of the amorphization and compaction studies are presented and discussed together with the characteristics of the mechanical alloying process. 15 refs., 5 figs.
Date: January 1, 1989
Creator: Tiainen, T.J. & Schwarz, R.B.
Partner: UNT Libraries Government Documents Department

Chemical short-range order in dense random packed models. [Ni/sub 35/Ti/sub 65/; Ni/sub 35/Zr/sub 65/]

Description: A dense random packed model of an amorphous alloy was used to calculate the total and partial reduced radial distribution functions, the Bhathia-Thorton number-concentration fluctuations, and the number-concentration interference functions. The model was applied to amorphous Ni/sub 35/Ti/sub 65/ using atomic radii of 1.10 and 1.58 A/sup -1/ for nickel and zirconium, respectively. Chemical short-range order was included in the model by permuting nickel-zirconium nearest-neighbors atoms pairs in response to a decrease in the alloy's enthalpy. The permutations were found to decrease in the Warren-Cowley order parameter from zero to -0.38. The increase in chemical short range order is accompanied by the appearance of a peak in the partial interference function I/sub Ni-Ni/(K) at K = 1.9 A/sup -1/. The increase in chemical short range order and the prepeak in I/sub Ni-Ni/(K) are tentatively attributed to the formation of double tetrahedra with three zirconium atoms at the base and two nickel atoms at the apexes. 18 refs., 5 figs.
Date: January 1, 1987
Creator: Saw, C.K. & Schwarz, R.B.
Partner: UNT Libraries Government Documents Department

Remarks on solid state amorphizing transformations

Description: Amorphous solids can now be produced through a variety of laboratory synthesis techniques as well as through many naturally occurring processes. In general, we can classify the methods of synthesis of amorphous solids as follows: (1) rapid solidification of melts or vapors; (2) atomic disordering of crystalline lattices; (3) solid state reactions between pure elements; (4) solid-state transformations from metastable crystalline states; and (5) deposition from electrolytes. We give a short summary of the historical development of methods (1)-(4) (method (5) is clearly outside the focus of this conference) and we discuss the basic physical principles behind the methods. 21 refs., 1 fig.
Date: January 1, 1987
Creator: Schwarz, R.B. & Johnson, W.L.
Partner: UNT Libraries Government Documents Department

Calorimetry study of the synthesis of amorphous Ni-Ti alloys by mechanical alloying. [Ni33 Ti67]

Description: We synthesized amorphous Ni/sub 33/Ti/sub 67/ alloy powder by ball milling (a) a mixture of elemental nickel and titanium powders and (b) powders of the crystalline intermetallic NiTi/sub 2/. We characterized the reaction products as a function of ball-milling time by differential scanning calorimetry and x-ray diffraction. The measurements suggest that in process (a) the amorphous alloy forms by a solid-state interdiffusion reaction at the clean Ni/Ti interfaces generated by the mechanical attrition. In process (b), the crystalline alloy powder stores energy in the form of chemical disorder and lattice and point defects. The crystal-to-amorphous transformation occurs when the stored energy reaches a critical value. The achievement of the critical stored energy competes with the dynamic recovery of the lattice. 23 refs., 7 figs.
Date: January 1, 1988
Creator: Schwarz, R.B. & Petrich, R.R.
Partner: UNT Libraries Government Documents Department

Structure and properties of metal hydrides prepared by mechanical alloying

Description: Our research examines the structure and reversible hydrogen storage capacity of alloys based on the LaNi{sub 5} intermetallic. The alloys are prepared by mechanical alloying (MA), a technique particularly useful when alloying LaNi{sub 5} with low melting point elements such as tin and calcium. In LaNi{sub 5-y}Sn{sub y}, x-ray diffraction and Rietveld analysis show that tin preferentially occupies the Ni(3g) sites in the LaNi{sub 5} structure, and the unit cell volume increases linearly with tin content to a maximum tin solubility of 7.33 atomic percent (LaNi{sub 4.56}Sn{sub 0.44}). The addition of tin to LaNi{sub 5} causes (a) a logarithmic decrease in the plateau pressures for hydrogen absorption and desorption, which is consistent with the corresponding increase in the volume of the LaNi{sub 5} unit cell; (b) a decrease in the hysteresis between the pressures for hydride formation and decomposition, which is in agreement with a recent theoretical model for the effect; and (c) a linear decrease in the hydrogen storage capacity. Effect (c) is explained by a rigid-band model whereby electrons donated by the tin atoms occupy holes in the 3d band of LaNi{sub 5}, which could otherwise be occupied by electrons donated by the hydrogen atoms. Thermodynamic van`t Hoff analysis for these alloys show an increase in hydride formation enthalpy and no change in entropy with increasing tin concentration. LaNi{sub 5} with calcium additions shows enhanced kinetics of hydrogen absorption/desorption. The powder particles prepared by MA have a larger surface area than particles of the same overall size prepared by arc casting. All LaNi{sub 5}-based alloys prepared by MA in an inert environment require no activation for hydrogen absorption and suffer less comminution upon hydriding/dehydriding.
Date: September 1, 1995
Creator: Wasz, M.L. & Schwarz, R.B.
Partner: UNT Libraries Government Documents Department

Storage of hydrogen in powders with nanosized crystalline domains

Description: The term nanostructured materials is being used in the literature to denote different materials prepared by different techniques: (a) by the gas condensation of vapors in a partial vacuum and (b) by mechanical alloying, a high energy ball milling technique. The authors discuss the origin of this confusion and propose a differentiating nomenclature. They then discuss the use of particles with nanosized structures for the storage of hydrogen.
Date: September 1997
Creator: Schwarz, R. B.
Partner: UNT Libraries Government Documents Department

Metastable phase equilibria in co-deposited Ni sub 1-x Zr sub x thin films

Description: We determine the glass forming range (GFR) of co-deposited Ni{sub 1-x}Zr{sub x} (0 < x < 1) thin films by measuring their electrical resistance during in situ constant-heating-rate anneals. The measured GFR is continuous for 0.10 < x < 0.87. We calculate the GFR of Ni-Zr melts as a function of composition and cooling rate using homogeneous nucleation theory and a published CALPHAD-type thermodynamic modeling of the equilibrium phase diagram. Assuming that the main competition to the retention of the amorphous structure during the cooling of the liquid comes from the partitionless crystallization of the terminal solid solutions, we calculate that for dT/dt = 10{sup 12} K s{sup {minus}1}, the GFR extends to x = 0.05 and x = 0.96. Better agreement with the measured values is obtained assuming a lower effective'' cooling rate during the condensation of the films. 18 refs., 6 figs.
Date: January 1, 1991
Creator: Rubin, J.B. & Schwarz, R.B.
Partner: UNT Libraries Government Documents Department

Temperature kinetics during shock-wave consolidation of metallic powders

Description: Powders (60 ..mu..m diam) of constantan and pure copper were compressed statically into cylindrical greens (20.3 mm diam, 5.3 mm long) with a flat interface separating the two powders. A 20-mm propellant gun was used to accelerate a flyer of Lexan, copper, or aluminum, and generate in the green a shock wave with front parallel to the Cu/constantan interface. The voltages between opposite ends of the greens were measured as a function of time and for shock pressures between 1.3 and 9.4 GPa. When the shock wave arrives at the Cu/constantan interface, the voltage signal shows an abrupt increase, which lasts between 45 and 81 ns and leads to a peak temperature T/sub p/. After this, the hotter and cooler parts of the compact equilibrate and the temperature decreases to a value T/sub h/. With increasing shock pressure, T/sub h/ increases from 425 to 1215 K. The measurements of T/sub h/ are in excellent agreement with the temperatures calculated from the measured flyer velocity, the Hugoniot for copper powder, and thermodynamic data for the flyer and powders.
Date: January 1, 1985
Creator: Schwarz, R.B.; Kasiraj, P. & Vreeland, T. Jr.
Partner: UNT Libraries Government Documents Department

Kinetics of solid-state reactions in Ni-Zr thin films

Description: We have studied the kinetic of the solid-state amorphizing reaction in thin film multilayers of Ni and Zr. Crystalline Ni and Zr films were deposited in ultra-high vacuum onto platinum resistance thermometer embedded in alumina. An electronic feedback circuit controls the temperature of the substrata by adjusting the power dissipated by the platinum resistors. We find that structural relaxation in the as-deposited Ni and Zr films affects the initial stages of the reaction. For long reaction times there is a discontinuous change in the reaction rate. The time to reach this transition increases with film thickness and depends exponentially on 1/T, with an apparent activation energy of 3 eV atom{sup {minus}1}. 25 refs., 9 figs.
Date: January 1, 1991
Creator: Schwarz, R.B. & Rubin, J.B.
Partner: UNT Libraries Government Documents Department

Amorphous powders of Al-Hf prepared by mechanical alloying

Description: We synthesized amorphous Al/sub 50/Hf/sub 50/ alloy powder by mechanically alloying an equimolar mixture of crystalline powders of Al and Hf using hexane as a dispersant. We characterized the powder as a function of mechanical-alloying time by scanning electron microscopy, x-ray diffraction, and differential scanning calorimetry. Amorphous Al/sub 50/Hf/sub 50/ powder heated at 10 K s/sup /minus/1/ crystallizes polymorphously at 1003 K into orthorhombic AlHf (CrB-type structure). During mechanical alloying, some hexane decomposes and hydrogen and carbon are incorporated into the amorphous alloy powder. The hydrogen can be removed by annealing the powder by hot pressing at a temperature approximately 30 K below the crystallization temperature. The amorphous compacts have a diamond pyramidal hardness of 1025 DPH. 24 refs., 7 figs., 1 tab.
Date: January 1, 1988
Creator: Schwarz, R.B.; Hannigan, J.W.; Sheinberg, H. & Tiainen, T.
Partner: UNT Libraries Government Documents Department

Metastable phases in mechanically alloyed aluminum germanium powders

Description: Aluminum and germanium form a simple eutectic system with no stable intermetallic phase, and limited mutual solubility. We report the formation of a metastable rhombohedral,{gamma}{sub 1} phase by mechanically alloying aluminum and germanium powders. This phase, which appears for compositions between 20 and 50 at. % germanium, has also been observed in rapidly quenched alloys, but there is disagreement as to its composition. By measuring the heat of crystallization as a function of composition, we determined the composition of the {gamma}{sub 1} phase to be Al{sub 70}Ge{sub 30}. We also produced Al{sub 70}Ge{sub 30} by arc melting the pure elements, followed by splat-quenching at a cooling rate in the range of 10{sup 8} K s{sup {minus}1}. This method produced two metastable phases, one of which was found to be the {gamma}{sub 1} phase obtained by mechanical alloying. The other was a monoclinic phase reported earlier in the literature as {gamma}{sub 2}.
Date: March 1, 1993
Creator: Yvon, P. J. & Schwarz, R. B.
Partner: UNT Libraries Government Documents Department

Elastic properties of amorphous thin films studied by Rayleigh waves

Description: Physical vapor deposition in ultra-high vacuum was used to co-deposit nickel and zirconium onto quartz single crystals and grow amorphous Ni{sub 1-x}Zr{sub x} (0.1 < x < 0.87) thin film. A high-resolution surface acoustic wave technique was developed for in situ measurement of film shear moduli. The modulus has narrow maxima at x = 0. 17, 0.22, 0.43, 0.5, 0.63, and 0.72, reflecting short-range ordering and formation of aggregates in amorphous phase. It is proposed that the aggregates correspond to polytetrahedral atom arrangements limited in size by geometrical frustration.
Date: August 1, 1993
Creator: Schwarz, R. B. & Rubin, J. B.
Partner: UNT Libraries Government Documents Department

Bulk metallic glass formation in the Pd-Ni-P and Pd-Cu-P alloy systems

Description: Bulk metallic glasses were prepared in the Pd-Ni-P and Pd-Cu-P systems using a fluxing technique. The formation of bulk amorphous Pd-Cu-P alloys was reported here for the first time. For both alloy systems, bulk glass formation requires maintaining the phosphorus content near 20 at.%. In the Pd-Ni-P system, 10-mm diameter amorphous Pd{sub x}Ni{sub 80{minus}x}P{sub 20} rods can be formed for 25 {le} x {le} 60. In the Pd-Cu-P system, 7-mm diameter amorphous Pd{sub x}Cu{sub 80{minus}x}P{sub 20} rods can be produced for 40 {le} x {le} 60. From all the ternary alloys studied, Pd{sub 40}Ni{sub 40}P{sub 20} has the highest glass formability, and 25-mm diameter amorphous cylinders, 50 mm in length, can be easily fabricated. The glass stability of the Pd-Ni-P system is wider than that of the Pd-Cu-P system. For most bulk Pd-Ni-P glasses, {Delta}T > 90 K. The {Delta}T values of bulk amorphous Pd-Cu-P alloys are considerably smaller, ranging from 27 to 73 K. The elastic constants of bulk amorphous Pd-Ni-P and Pd-Cu-P alloys were determined using a resonant ultrasound spectroscopy technique. The Pd-Ni-P glasses are slightly stiffer than the Pd-Cu-P glasses. Within each alloy system, the Young`s modulus and the bulk modulus show little change with alloy composition. Of all the bulk glass forming systems so far investigated, the ternary Pd-Ni-P system has the best glass formability. This alloy was one of the first bulk glasses discovered, yet it still remains the best in terms of glass formability. Upon replacing part of Ni by Cu, the critical cooling rates are expected to be further reduced.
Date: December 11, 1996
Creator: Schwarz, R.B. & He, Y.
Partner: UNT Libraries Government Documents Department

Bulk amorphous metallic alloys: Synthesis by fluxing techniques and properties

Description: Bulk amorphous alloys having dimensions of at least 1 cm diameter have been prepared in the Pd-Ni-P, Pd-Cu-P, Pd-Cu-Ni-P, and Pd-Ni-Fe-P systems using a fluxing and water quenching technique. The compositions for bulk glass formation have been determined in these systems. For these bulk metallic glasses, the difference between the crystallization temperature T{sub x}, and the glass transition temperature T{sub g}, {Delta}T = T{sub x} - T{sub g}, ranges from 60 to 1 10 K. These large values of {Delta}T open the possibility for the fabrication of amorphous near net-shape components using techniques such as injection molding. The thermal, elastic, and magnetic properties of these alloys have been studied, and we have found that bulk amorphous Pd{sub 40}Ni{sub 22.5}Fe{sub 17.5}P{sub 20} has spin glass behavior for temperatures below 30 K. 65 refs., 14 figs., 3 tabs.
Date: May 1, 1997
Creator: He, Yi; Shen, Tongde & Schwarz, R.B.
Partner: UNT Libraries Government Documents Department

Synthesis and properties of bulk metallic glasses in Pd-Ni-P and Pd-Cu-P alloys

Description: Bulk amorphous Pd-Ni-P and Pd-Cu-P alloy rods with diameters 7-25 mm were synthesized over a wide composition range using a fluxing technique. For most bulk amorphous Pd-Ni-P alloys, the difference {Delta}T=T{sub x}-T{sub g} between the crystallization temperature T{sub x} and the glass transition temperature T{sub g} is larger than 90 K, while for bulk amorphous Pd-Cu-P alloys, {Delta}T varies from 27 to 73 K. Pd{sub 40}Ni{sub 40}P{sub 20} has the highest glass formability, and 300-g bulk amorphous cylinders, 25mm dia and 50mm long, can be easily produced. This size is not an upper limit. The paper presents the glass formation ranges for both ternary alloy systems and data on the thermal stability of the amorphous alloys, as well as their specific heat, density, and elastic properties.
Date: November 1, 1996
Creator: He, Y. & Schwarz, R.B.
Partner: UNT Libraries Government Documents Department

Vanadium-spinel composites for structural applications in hostile environments

Description: Vanadium-spinel composites are promising materials for structural applications in radiation environments. Powders of two Vanadium-spinel composites, 20/80 vol. %, were prepared by (a) ball milling mixtures of vanadium and spinel powders (alloy VSLP) and (b) through a self-sustained reaction synthesis of vanadium, MgO, and Al powders (alloy VSHP). These powders were consolidated by hot isostatic pressing. Most of the V and spinel domains in the the compacts are sub-micron in size. The compacts have K{sub c} toughness values of 3.9, about three times the toughness obtained by hipping mixtures of commercial powders.
Date: May 1, 1997
Creator: Schwarz, R.B.; Wetteland, C.J. & Shen, T.D.
Partner: UNT Libraries Government Documents Department

Sn-substituted LaNi{sub 5} alloys for metal hydride electrodes

Description: This research examines the efficacy of tin additions to LaNi{sub 5} in improving the hydrogen storage capacity of the material during charging/discharging. Alloys were prepared using high energy ball milling (mechanical alloying), a technique superior to arc casting for alloying elements with a wide disparity in melting points. Characterization by X-ray diffraction and Rietveld analysis shows that tin preferentially occupies the Ni(3g) sites in the LaNi{sub 5} structure, and the unit cell volume increases linearly with tin content to the maximum tin solubility of 7.33 atomic percent (LaNi{sub 4.56}Sn{sub 0.44}). The authors found that powders prepared by mechanical alloying and not exposed to air require no activation to induce hydrogen absorption. The hydrogen storage capacity in the gas and electrochemical phase was measured as a function of tin content. They found that with increasing tin, the plateau pressure decreases logarithmically, whereas the hydrogen storage capacity decreases linearly.
Date: May 1, 1995
Creator: Wasz, M.L.; Schwarz, R.B.; Srinivasan, S. & Sridhar Kumar, M.P.
Partner: UNT Libraries Government Documents Department

Local Probe into the Atomic Structure of Metallic Glasses using EELS

Description: Electron energy loss spectroscopy (EELS) is used to extract information on the topological arrangement of atoms around Pd in the bulk-glass-forming Pd{sub 60}Ni{sub 20}P{sub 20}. It is found that the environment around Pd in the glass is only a slight modification of the Pd crystalline structure. However, the modification is enough to allow this alloy to form a glass in bulk. In examining the differences between the structure of crystalline Pd and glassy Pd{sub 60}Ni{sub 20}P{sub 20} it is concluded that incorporation of Ni and P into the structure frustrates the structure enough that glass formation becomes easy.
Date: November 30, 1999
Creator: Alamgir, F.M. & Ito, Y. Schwarz, R.B.
Partner: UNT Libraries Government Documents Department