Showing papers on "Alloy published in 2002"

Oxygen Reduction on Carbon-Supported Pt−Ni and Pt−Co Alloy Catalysts

Abstract: We describe a comparative study of the oxygen reduction reaction on two carbon-supported Pt-based alloy catalysts in aqueous acidic electrolyte at low temperature. Both alloys have the bulk compositions of 50 and 75 at. % Pt, with the alloying elements being Ni and Co. Comparison is made to a pure Pt catalyst on the same carbon support, Vulcan XC-72, having the same metal loading (20 wt %) and nominally the same particle size (4 ± 2 nm). High-resolution electron microscopy was used to determine the size and shape of the particles as well as the particle size distribution on all catalysts. Electrochemical measurements were performed using the thin-film rotating ring−disk electrode method in 0.1 M HClO4 at 20−60 °C. Hydrogen adsorption pseudocapacitance was used to determine the number of Pt surface atoms and to estimate the surface composition of the alloy catalysts. Kinetic analysis in comparison to pure Pt revealed a small activity enhancement (per Pt surface atom) of ca. 1.5 for the 25 at. % Ni and Co c...

Surface Composition Effects in Electrocatalysis: Kinetics of Oxygen Reduction on Well-Defined Pt3Ni and Pt3Co Alloy Surfaces

Abstract: The oxygen reduction reaction (ORR) has been studied on polycrystalline Pt3Ni and Pt3Co alloys in acid electrolytes using the rotating ring disk electrode (RRDE) method. Preparation and characterization of alloy surfaces were performed in ultrahigh vacuum (UHV). Clearly defined surface composition was determined via low-energy ion-scattering (LEIS) spectroscopy. Polycrystalline bulk alloys of Pt3Ni and Pt3Co were prepared in UHV having two different surface compositions: one with 75% Pt and the other with 100% Pt. The latter we call a “Pt-skin” structure and is produced by an exchange of Pt and Co in the first two layers. The base voltammetry in 0.1 M HClO4 solution of the 75% Pt alloy surface indicated a decrease of Hupd pseudocapacitance (ca. 30−40 μC/cm2) consistent with the surface composition determined in UHV. With the exception of the “Pt-skin” surface on Pt3Ni, all the alloy electrodes exhibited stable i−E curves with repeated cycling between 0.05 and 1.0 V at all temperatures. Activities of Pt-a...

Superelastic guiding member

Abstract: An improved guiding member for use within a body lumen having a unique combination of superelastic characteristics. The superelastic alloy material has a composition consisting of about 30% to about 52% (atomic) titanium, and about 38% to 52% nickel and may have one or more elements selected from the group consisting of iron, cobalt, platinum, palladium, vanadium, copper, zirconium, hafnium and niobium. The alloy material is subjected to thermomechanical processing which includes a final cold working of about 10 to about 75% and then a heat treatment at a temperature between about 450° and about 600° C. and preferably about 475° to about 550° C. Before the heat treatment the cold worked alloy material is preferably subjected to mechanical straightening. The alloy material is preferably subjected to stresses equal to about 5 to about 50% of the room temperature ultimate yield stress of the material during the thermal treatment. The guiding member using such improved material exhibits a stress-induced austenite-to-martensite phase transformation at an exceptionally high constant yield strength of over 90 ksi for solid members and over 70 ksi for tubular members with a broad recoverable strain of at least about 4% during the phase transformation. An essentially whip free product is obtained.

Decomposition of Austenite in Austenitic Stainless Steels

Abstract: Austenitic stainless steels are probably the most important class of corrosion resistant metallic materials. In order to attain their good corrosion properties they rely essentially on two factors: a high chromium content that is responsible for the protective oxide film layer and a high nickel content that is responsible for the steel to remain austenitic. Thus the base composition is normally a Fe-Cr-Ni alloy. In practice the situation is much more complex with several other elements being present, such as, Mo, Mn, C, N among others. In such a complex situation one almost never has a single austenite phase but other phases invariably form. Those phases are, with few exceptions, undesirable and they can be detrimental to the corrosion and mechanical properties. It is therefore of considerable importance to study the formation of such phases. In this work the decomposition of austenite in austenitic stainless steels is reviewed in detail. First the binary equilibrium diagrams relevant to the system Fe-Cr-Ni are briefly presented as well as other diagrams, such as the Schaeffler diagram, that traditionally have been used to predict the phases present in these steels as a function of composition. Secondly the precipitation of carbides and intermetallic phases is presented in detail including nucleation sites and orientation relationships and the influence of several factors such as composition, previous deformation and solution annealing temperature. Next, the occurrence of other constituents such as nitrides, sulfides and borides is discussed. TTT diagrams are also briefly presented. Finally the formation of martensite in these steels is discussed.

Metallic glass matrix composite with precipitated ductile reinforcement

Abstract: We report a composite material consisting of precipitated micron-scale Ta-rich solid solution particles distributed in a bulk metallic glass matrix. The reinforcing ductile particles are precipitated during melting of the master alloy of glass-forming (Zr70Ni10Cu20)82Ta8Al10, by using previously prepared metastable Zr–Ta solid solution binary ingots. Upon cooling from the melt, the matrix undergoes a glass transition to produce an amorphous phase while the particles of precipitated Ta solid solution are distributed in the glass matrix. The resulting material not only shows high strength (∼2.1 GPa), but also has dramatically enhanced plastic strain to failure in uniaxial compression relative to single-phase bulk metallic glasses. The composite also displays limited tensile ductility.

Cu-Ni Cermet Anodes for Direct Oxidation of Methane in Solid-Oxide Fuel Cells

Abstract: We have examined the use of Cu-Ni alloys as anodes for the direct oxidation of methane in solid-oxide fuel cells (SOFC) at 1073 K. Ceramic-metal (cermet) composites having alloy compositions of 0, 10, 20, 50 and 100% Ni were exposed to dry methane at 1073 K for 1.5 h to demonstrate that carbon formation is greatly suppressed on the Cu-Ni alloys compared to that of pure Ni. Increased reduction temperatures also reduced the carbon formation on the alloys. The performance of a fuel cell made with a Cu(80%)-Ni(20%) cermet was tested in dry methane for 500 h and showed a significant increase in power density with time. Impedance spectra of similar fuel cells suggest that small carbon deposits are formed with time and that the increase in performance is due to enhanced electronic conductivity in the anode. Finally, the implications of the use of metal alloys for SOFC applications are discussed.

Recent Progress in Bulk Glassy Alloys

Abstract: For thousands of years, metallic alloys have been among the most important materials used by mankind, and their importance as engineering materials remains as great now as ever. Without exception, all bulk metallic alloys used to date consist of crystalline materials with three-dimensional periodic atomic configurations. The instability of the liquid phase of metallic alloys below melting temperature had been thought to be a universal phenomenon, making the formation of a crystalline phase of the bulk metallic alloy unavoidable. In order to prevent transition from a liquid to a crystalline phase, extremely high cooling rates of the order 10 6 K/s are required and the alloys exhibiting critical cooling rates of 10 5 to 10 7 K/s are known as amorphous/glass forming alloys. 1, 2) As a result of the requirement for rapid cooling, amorphous alloys have usually been produced in a thin sheet form with thicknesses below 0.05 mm. The conventionally accepted concept that the supercooled liquid phase of metallic alloys is always unstable has been broken through by the recent successes in forming bulk glassy alloys in a number of transition metal-based alloy systems using the copper mold casting technique. 3‐5) In recent years, studies of the stabilization of metallic supercooled liquid and the resulting bulk glassy alloys have been significant not only for fundamental science but for engineering applications as well. In this paper we review our recent results on the formation and properties of bulk glassy alloys.

ZrNbCuNiAl bulk metallic glass matrix composites containing dendritic bcc phase precipitates

Abstract: We report on phase formation of a multicomponent Zr66.4Nb6.4Cu10.5Ni8.7Al8 glass-forming alloy upon copper mold casting. A bcc phase embedded in a glassy matrix forms for moldcast bulk samples yielding an in-situ bulk metallic glass matrix composite upon slow cooling from the melt. Upon annealing, the first exothermic transformation of the material is related to precipitation of an icosahedral phase from the glassy matrix. The formation of the bcc phase-containing metallic glass composite is strongly governed by the alloy composition and the actual cooling rate during solidification. Room-temperature compression tests reveal significant yielding and plastic deformation before failure.

Development of Low Rigidity β-type Titanium Alloy for Biomedical Applications

Abstract: The low rigidity type titanium alloy, Ti–29Nb–13Ta–4.6Zr was designed, and then the practical level ingot of the alloy was successfully fabricated by Levicast method. The mechanical and biological compatibilities of the alloys were investigated in this study. The following results were obtained. The mechanical performance of tensile properties and fatigue strength of the alloy are equal to or greater than those of conventional biomedical Ti–6Al–4V ELI. Young’s modulus of the alloy is much lower than that of Ti–6Al–4V ELI, and increases with the precipitation of α phase or ω phase in the β matrix phase. The compatibility of the alloy with bone of the alloy is excellent. Low rigidity of the alloy is effective to enhance the healing of bone fracture and remodeling of bone. The bioactive coating layer of hydroxyapatite can be formed on the alloy.

Formation of Immiscible Alloy Powders with Egg-Type Microstructure

Abstract: The egg-type core microstructure where one alloy encases another has previously been obtained during experiments in space. Working with copper-iron base alloys prepared by conventional gas atomization, we were able to obtain this microstructure under gravity conditions. The minor liquid phase always formed the core of the egg, and it sometimes also formed a shell layer. The origin of the formation of this core microstructure can be explained by Marangoni motion on the basis of the temperature dependence of the interfacial energy, which shows that this type of powder can be formed even if the cooling rate is very high.

Creep and microstructure of magnesium-aluminum-calcium based alloys

Abstract: This article describes the creep and microstructure of Mg-Al-Ca-based magnesium alloys (designated as ACX alloys, where A stands for aluminum; C for calcium; and X for strontium or silicon) developed for automotive powertrain applications. Important creep parameters, i.e., secondary creep rate and creep strength, for the new alloys are reported. Creep properties of the new alloys are significantly better than those of the AE42 (Mg-4 pct* Al-2 pct RE**) alloy, which is the benchmark creep-resistant magnesium die-casting alloy. Creep mechanisms for different temperature/stress regimes are proposed. A ternary intermetallic phase, (Mg,Al)2Ca, was identified in the microstructure of the ACX alloys and is proposed to be responsible for the improved creep resistance of the alloys.

Development of low rigidity β-type titanium alloy for biomedical applications : Biomaterials and bioengineering

Abstract: The low rigidity type titanium alloy, Ti-29Nb-13Ta-4.6Zr was designed, and then the practical level ingot of the alloy was successfully fabricated by Levicast method. The mechanical and biological compatibilities of the alloys were investigated in this study. The following results were obtained. The mechanical performance of tensile properties and fatigue strength of the alloy are equal to or greater than those of conventional biomedical Ti-6Al-4V ELI. Young's modulus of the alloy is much lower than that of Ti-6Al-4V ELI, and increases with the precipitation of a phase or ω phase in the β matrix phase. The compatibility of the alloy with bone of the alloy is excellent. Low rigidity of the alloy is effective to enhance the healing of bone fracture and remodeling of bone. The bioactive coating layer of hydroxyapatite can be formed on the alloy.

Processing and mechanical properties of autogenous titanium implant materials

Abstract: Pure titanium and some of its alloys are currently considered as the most attractive metallic materials for biomedical applications due to their excellent mechanical properties, corrosion resistance, and biocompatibility. It has been demonstrated that titanium and titanium alloys are well accepted by human tissues as compared to other metals such as SUS316L stainless steel and Co–Cr–Mo type alloy. In the present study, highly porous titanium foams with porosities ≤80% are produced by using a novel powder metallurgical process, which includes the adding of the selected spacers into the starting powders. The optimal process parameters are investigated. The porous titanium foams are characterized by using optical microscopy and scanning electron microscopy. The distribution of the pore size is measured by quantitative image analyses. The mechanical properties are investigated by compressive tests. This open-cellular titanium foams, with the pore size of 200–500 μm are expected to be a very promising biomaterial candidates for bone implants because its porous structure permits the ingrowths of new-bone tissues and the transport of body fluids.

Surface segregation effects in electrocatalysis: Kinetics of oxygen reduction reaction on polycrystalline Pt3Ni alloy surfaces

Abstract: Submitted to the Journal of Electroanalytical Chemistry, November 6, 2002 Surface Segregation Effects in Electrocatalysis: Kinetics of Oxygen Reduction Reaction on Polycrystalline Pt Ni Alloy Surfaces** V. Stamenkovic*, T.J. Schmidt - , P.N. Ross and N . M . Markovic Materials Sciences Division, Lawrence Berkeley National Laboratory University of California at Berkeley, CA 94720, USA Abstract Effects of surface segregation on the oxygen reduction reaction (ORR) have been studied on a polycrystalline Pt Ni alloy in acid electrolyte using ultra high vacuum (UHV) surface sensitive probes and the rotating ring disk electrode (RRDE) method. Preparation, modification and Depending on the characterization of alloy surfaces were done in ultra high vacuum (UHV). preparation method, two different surface compositions of the Pt Ni alloy are produced: a sputtered surface with 75 % Pt and an annealed surface (950 K ) with 100 % Pt. The latter surface is designated as the Pt-skin structure, and is a consequence of surface segregation, i.e., replacement of N i with Pt atoms in the first few atomic layers. Definitive surface compositions were established by low energy ion scattering spectroscopy (LEISS). The cyclic voltammetry of the Pt-skin surface as well as the pseudcapacitance in the hydrogen adsorption/desorption potential region is similar to a polycrystalline Pt electrode. Activities of ORR on Pt Ni alloy surfaces were compared to polycrystalline Pt in 0.1M H C l O electrolyte for the observed temperature range of 293 Pt Ni (75% Pt) > Pt with the maximum catalytic enhancement obtained for the Pt-skin being 4 times

Formation, Thermal Stability and Mechanical Properties of Cu-Zr-Al Bulk Glassy Alloys

Abstract: New Cu-based bulk glassy alloys with large supercooled liquid region and high mechanical strength were found to be formed in Cu-Zr-Al ternary system. The large supercooled liquid region exceeding 70 K was obtained in the composition range of 40 at%Zr at 2.5 at%Al, 37.5%Zr to 47.5%Zr at 5%Al and 42.5%Zr at 7.5%Al. The largest ΔT x was 74 K for Cu 55 Zr 40 Al 5 and Cu 50 Zr 42.5 Al 7.5 alloys and the highest T g /T l was 0.62 for the former alloy. The alloys with large ΔT x values above 70 K were formed into a bulk glassy rod form with diameters up to 3 mm by copper mold casting and the glassy alloy rods exhibit high compressive strength of 1885 to 2210 MPa and Young's modulus of 102 to 115 GPa combined with elastic elongation of 1.60 to 1.95% and plastic elongation of 0 to 0.4%. The addition of 2.5 to 7.5°%Al to Cu-Zr alloys was very effective for the increase in glass-forming ability as well as the stabilization of supercooled liquid. The effectiveness can be interpreted on the basis of the concept of the formation of the unique glassy structure in special multi-component alloys with the three empirical component rules.

Material flow and microstructure in the friction stir butt welds of the same and dissimilar aluminum alloys

Abstract: The material flow and microstructural evolution in the friction stir welds of a 6061-Al alloy to itself and of a 6061-Al alloy to 2024-Al alloy plates of 12.7 mm in thickness were studied under different welding conditions. The results showed that plastic deformation, flow, and mechanical mixing of the material exhibit distinct asymmetry characteristics at both sides of the same and dissimilar welds. The microstructure in dissimilar 6061-Al/2024-Al welds is significantly different from that in the welds of a 6061-Al alloy to itself. Vortex-like structures featured by the concentric flow lines for a weld of 6061-Al alloy to itself, and alternative lamellae with different alloy constituents for a weld of 6061-Al to 2024-Al alloy, are attributed to the stirring action of the threaded tool, in situ extrusion, and traverse motion along the welding direction. The mutual mixing in the dissimilar metal welds is intimate and far from complete. However, the bonding between the two Al-alloys is clearly complete. Three different regions in the nugget zone of dissimilar 6061-Al/2024-Al welds are classified by the mechanically mixed region (MMR) characterized by the relatively dispersed particles of different alloy constituents, the stirring-induced plastic flow region (SPFR) consisting of alternative vortex-like lamellae of the two Al-alloys, and the unmixed region (UMR) consisting of fine equiaxed grains of the 6061-Al alloy. Within all of these three regions, the material is able to withstand a very high degree of plastic deformation due to the presence of dynamic recovery or recrystallization of the microstructure. The degree of material mixing, the thickness of the deformed Al-alloy lamellae, and the material flow patterns depend on the related positions in the nugget zone and the processing parameters. Distinct fluctuations of hardness are found to correspond to the microstructural changes throughout the nugget zone of dissimilar welds.

New Bulk Glassy Ni-Based Alloys with High Strength of 3000 MPa

Abstract: New Ni-based bulk glassy alloys with high strength and good ductility were synthesized for the first time in Ni- Nb-Ti-Zr base system by the mold-clamp or copper mold casting method. The bulk glassy Ni 53 Nb 20 Ti 10 Zr 8 Co 6 Cu 3 alloy has a rod shape with diameters up to 3mm or a sheet shape with thickness up to 1 mm. The glass transition temperature (T g ) and the supercooled liquid region defined by the difference between T g and crystallization temperature (T g ). ΔT x (= T x - T g ) are 846 and 51 K, respectively, and no distinct change in T g . T x and ΔT x with sample diameter is seen. The Ni-based alloy is located in the vicinity of eutectic composition and has a high reduced glass transition temperature (T g /T m ) of 0.67. The Ni-based bulk glassy alloy also exhibits good mechanical properties. i.e., tensile fracture strength of 2700 MPa, tensile fracture elongation of 2. 1%, compressive fracture strength of 3010 MPa and compressive fracture elongation of 2.4%. It is noticed that the tensile fracture strength is the highest among all bulk glassy alloys developed up to date. The success of synthesizing the new Ni-based bulk glassy alloy with good mechanical properties is promising for future uses as a new type of high strength material.