Showing papers on "Alloy published in 2011"

Oxidation resistance of graphene-coated Cu and Cu/Ni alloy

Abstract: The ability to protect refined metals from reactive environments is vital to many industrial and academic applications. Current solutions, however, typically introduce several negative effects, including increased thickness and changes in the metal physical properties. In this paper, we demonstrate for the first time the ability of graphene films grown by chemical vapor deposition to protect the surface of the metallic growth substrates of Cu and Cu/Ni alloy from air oxidation. In particular, graphene prevents the formation of any oxide on the protected metal surfaces, thus allowing pure metal surfaces only one atom away from reactive environments. SEM, Raman spectroscopy, and XPS studies show that the metal surface is well protected from oxidation even after heating at 200 °C in air for up to 4 h. Our work further shows that graphene provides effective resistance against hydrogen peroxide. This protection method offers significant advantages and can be used on any metal that catalyzes graphene growth.

Shape and composition-controlled platinum alloy nanocrystals using carbon monoxide as reducing agent.

Abstract: The shape of metal alloy nanocrystals plays an important role in catalytic performances. Many methods developed so far in controlling the morphologies of nanocrystals are however limited by the synthesis that is often material and shape specific. Here we show using a gas reducing agent in liquid solution (GRAILS) method, different Pt alloy (Pt-M, M = Co, Fe, Ni, Pd) nanocrystals with cubic and octahedral morphologies can be prepared under the same kind of reducing reaction condition. A broad range of compositions can also be obtained for these Pt alloy nanocrystals. Thus, this GRAILS method is a general approach to the preparation of uniform shape and composition-controlled Pt alloy nanocrystals. The area-specific oxygen reduction reaction (ORR) activities of Pt(3)Ni catalysts at 0.9 V are 0.85 mA/cm(2)(Pt) for the nanocubes, and 1.26 mA/cm(2)(Pt) for the nanooctahedra. The ORR mass activity of the octahedral Pt(3)Ni catalyst reaches 0.44 A/mg(Pt).

Shape-selective synthesis and facet-dependent enhanced electrocatalytic activity and durability of monodisperse sub-10 nm Pt-Pd tetrahedrons and cubes

Abstract: Monodisperse single-crystalline sub-10 nm Pt-Pd nanotetrahedrons (NTs) and nanocubes (NCs) were synthesized with high shape selectivity via one-pot hydrothermal routes with small ions as efficient facet-selective agents. These alloy nanocrystals showed facet-dependent enhanced electrocatalytic activity and durability for methanol electrooxidations with commercial Pt/C catalyst as a reference. The (100)-facet-enclosed Pt-Pd NCs demonstrated a higher activity, whereas the (111)-facet-enclosed Pt-Pd NTs exhibited a better durability.

Microstructure and properties of a refractory NbCrMo0.5Ta0.5TiZr alloy

Abstract: A new refractory alloy, Nb 20 Cr 20 Mo 10 Ta 10 Ti 20 Zr 20 , was produced by vacuum arc melting. To close shrinkage porosity, it was hot isostatically pressed (HIPd) at T = 1723 K and P = 207 MPa for 3 h. In both as-solidified and HIPd conditions, the alloy contained three phases: two body centered cubic (BCC1 and BCC2) and one face centered cubic (FCC). The BCC1 phase was enriched with Nb, Mo and Ta and depleted with Zr and Cr, and its lattice parameter after HIP was a = 324.76 ± 0.16 pm. The BCC2 phase was enriched with Zr and Ti and considerably depleted with Mo, Cr and Ta, and its lattice parameter after HIP was estimated to be a = 341.0 ± 1.0 pm. The FCC phase was highly enriched with Cr and it was identified as a Laves C15 phase, (Zr,Ta)(Cr,Mo,Nb) 2 , with the lattice parameter a = 733.38 ± 0.18 pm. The volume fractions of the BCC1, BCC2 and FCC phases were 67%, 16% and 17%, respectively. The alloy density and Vickers microhardness were ρ = 8.23 ± 0.01 g/cm 3 and H v = 5288 ± 71 MPa. The alloy had compression yield strength of 1595 MPa at 296 K, 983 MPa at 1073 K, 546 MPa at 1273 K and 171 MPa at 1473 K. During deformation at 296 K and 1073 K, the alloy showed a mixture of ductile and brittle fracture after plastic compression strain of ∼5–6%. No macroscopic fracture was observed after 50% compression strain at 1273 K and 1473 K. Phase transformations and particle coarsening considerably accelerated by the plastic deformation occurred in the temperature range of 1073–1473 K.

Biodegradable Mg and Mg alloys: Corrosion and biocompatibility

Abstract: This review discusses the current state and challenges in understanding Mg alloy corrosion behavior, in view of use of these materials for biodegradable implants. After the description of some basic and specific aspects of Mg alloy corrosion, the influence of specific biological environments on the corrosion behavior of Mg alloys is summarized. Interactions between corroding Mg surfaces and cells are shortly discussed. Moreover, approaches to control the corrosion and biological performance of Mg alloys by surface modification are presented. The article tries to provide general, instead of alloy specific, information on the topics covered.

On the superior hot hardness and softening resistance of AlCoCrxFeMo0.5Ni high-entropy alloys

Abstract: Effects of Cr content on microstructures and hot hardness of AlCoCrxFeMo0.5Ni high-entropy alloys (x = 0–2.0) were investigated. The cast microstructure of AlCoCrxFeMo0.5Ni consists of B2 and σ phases, both being multi-element solid solutions. Increasing Cr content increases the volume fraction of σ phase and causes the matrix phase of the dendrite to change from B2 phase to σ phase. The alloy hardness increases from Hv 601 at x = 0 to Hv 867 at x = 2.0 as a result of increasing amount of hard σ phase. A phase diagram for the AlCoCrxFeMo0.5Ni alloy system is constructed based on SEM, HTXRD and DTA analyses, providing useful information for understanding and designing high-entropy alloys. All the AlCoCrxFeMo0.5Ni alloys possess higher hot-hardness level than that of Ni-based superalloys, In 718 and In 718 H, from room temperature to 1273 K. Cr-1.5 and Cr-2.0 alloys exhibit a transition temperature higher than that of Co-based alloy T-800 by about 200 K and have respective hardness values, Hv 374 and Hv 450 at 1273 K, being much higher than those, around Hv 127, of In 718 and In 718 H. The mechanism of larger strengthening and softening resistance is related with B2 and σ phases, both having multi-principal-element effect. The AlCoCrxFeMo0.5Ni alloy system has a potential in high-temperature applications.

Microstructural Architecture, Microstructures, and Mechanical Properties for a Nickel-Base Superalloy Fabricated by Electron Beam Melting

Abstract: Microstructures and a microstructural, columnar architecture as well as mechanical behavior of as-fabricated and processed INCONEL alloy 625 components produced by additive manufacturing using electron beam melting (EBM) of prealloyed precursor powder are examined in this study. As-fabricated and hot-isostatically pressed (“hipped”) [at 1393 K (1120 °C)] cylinders examined by optical metallography (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive (X-ray) spectrometry (EDS), and X-ray diffraction (XRD) exhibited an initial EBM-developed γ″ (bct) Ni3Nb precipitate platelet columnar architecture within columnar [200] textured γ (fcc) Ni-Cr grains aligned in the cylinder axis, parallel to the EBM build direction. Upon annealing at 1393 K (1120 °C) (hot-isostatic press (HIP)), these precipitate columns dissolve and the columnar, γ, grains recrystallized forming generally equiaxed grains (with coherent {111} annealing twins), containing NbCr2 laves precipitates. Microindentation hardnesses decreased from ~2.7 to ~2.2 GPa following hot-isostatic pressing (“hipping”), and the corresponding engineering (0.2 pct) offset yield stress decreased from 0.41 to 0.33 GPa, while the UTS increased from 0.75 to 0.77 GPa. However, the corresponding elongation increased from 44 to 69 pct for the hipped components.

Facile Synthesis of Pd-Pt Alloy Nanocages and Their Enhanced Performance for Preferential Oxidation of CO in Excess Hydrogen

Abstract: This article describes a new method for the facile synthesis of Pd–Pt alloy nanocages with hollow interiors and porous walls by using Pd nanocubes as sacrificial templates. Differing from our previous work (Zhang, H.; Jin, M. S.; Wang, J. G.; Li, W. Y.; Camargo, P. H. C.; Kim, M. J.; Yang, D. R.; Xie, Z. X.; Xia, Y. Synthesis of Pd-Pt Bimetallic Nanocrystals with a Concave Structure through a Bromide-Induced Galvanic Replacement Reaction. J. Am. Chem. Soc.2011, 133, 6078–6079), we complemented the galvanic replacement (between Pd nanocubes and PtCl42–) with a coreduction process (for PdCl42– from the galvanic reaction and PtCl42– from the feeding) to generate Pd–Pt alloy nanocages in one step. We found that the rate of galvanic replacement (as determined by the concentrations of Br– and PtCl42– and temperature) and the rates of coreduction (as determined by the type of reductant and temperature) played important roles in controlling the morphology of resultant Pd–Pt alloy nanocages. The Pd–Pt nanocages ex...