Showing papers on "Alloy published in 2012"

Precipitation and Hardening in Magnesium Alloys

Abstract: Magnesium alloys have received an increasing interest in the past 12 years for potential applications in the automotive, aircraft, aerospace, and electronic industries. Many of these alloys are strong because of solid-state precipitates that are produced by an age-hardening process. Although some strength improvements of existing magnesium alloys have been made and some novel alloys with improved strength have been developed, the strength level that has been achieved so far is still substantially lower than that obtained in counterpart aluminum alloys. Further improvements in the alloy strength require a better understanding of the structure, morphology, orientation of precipitates, effects of precipitate morphology, and orientation on the strengthening and microstructural factors that are important in controlling the nucleation and growth of these precipitates. In this review, precipitation in most precipitation-hardenable magnesium alloys is reviewed, and its relationship with strengthening is examined. It is demonstrated that the precipitation phenomena in these alloys, especially in the very early stage of the precipitation process, are still far from being well understood, and many fundamental issues remain unsolved even after some extensive and concerted efforts made in the past 12 years. The challenges associated with precipitation hardening and age hardening are identified and discussed, and guidelines are outlined for the rational design and development of higher strength, and ultimately ultrahigh strength, magnesium alloys via precipitation hardening.

Alloy Design and Properties Optimization of High-Entropy Alloys

Abstract: This article reviews the recent work on the high-entropy alloys (HEAs) in our group and others. HEAs usually contain five or more elements, and thus, the phase diagram of HEAs is often not available to be used to design the alloys. We have proposed that the parameters of δ and Ω can be used to predict the phase formation of HEAs, namely Ω ≥ 1.1 and δ ≤ 6.6%, which are required to form solid-solution phases. To test this criterion, alloys of TiZrNbMoVx and CoCrFeNiAlNbx were prepared. Their microstructures mainly consist of simple body-centered cubic solid solutions at low Nb contents. TiZrNbMoVx alloys possess excellent mechanical properties. Bridgman solidification was also used to control the microstructure of the CoCrFeNiAl alloy, and its plasticity was improved to be about 30%. To our surprise, the CoCrFeNiAl HEAs exhibit no apparent ductile-to-brittle transition even when the temperatures are lowered from 298 K to 77 K.

Effect of Nb addition on the microstructure and properties of AlCoCrFeNi high-entropy alloy

Abstract: The microstructures and properties of the AlCoCrFeNbxNi high-entropy alloys (HEAs) were investigated. Two phases were found in the prepared AlCoCrFeNbxNi HEAs: one is body-centered-cubic (BCC) solid solution phase; the other is the Laves phase of (CoCr)Nb type. The microstructures of the alloy series vary from hypoeutectic to hypereutectic, and the compressive yield strength and Vickers hardness have an approximately linear increase with increasing Nb content. The residual magnetization (Mr) reaches a maximum for AlCoCrFeNb0.1Ni alloy, which is 6.106 emu/g. The factor of Ω, which is defined as entropy of mixing times 1000 over enthalpy of mixing, well predicts the phase formation for the multicomponents alloys.

Fabrication of Metal and Alloy Components by Additive Manufacturing: Examples of 3D Materials Science

Abstract: Objective This paper provides a brief review of relatively new additive manufacturing technologies for the fabrication of unusual and complex metal and alloy products by laser and electron beam melting. A number of process features and product microstructures are illustrated utilizing 3D optical and transmission electron microscope image compositions representing examples of 3D materials science. Methods Processing methods involving electron beam melting (EBM) and a process referred to as direct metal laser sintering (DMLS), often called selective laser melting (SLM) are described along with the use of light (optical) microscopy (OM), transmission electron microscopy (TEM), and X-ray diffraction (XRD) to elucidate microstructural phenomena. Results Examples of EBM and SLM studies are presented in 3D image compositions. These include EBM of Ti-6Al-4V, Cu, Co-base superalloy and Inconel 625; and SLM of 17-4 PH stainless steel, Inconel 718 and Inconel 625. Conclusions 3D image compositions constituting 3D materials science provide effective visualization for directional solidification-related phenomena associated with the EBM and SLM fabrication of a range of metals and alloys, especially microstructures and microstructural architectures.

Solvothermal Synthesis of Platinum Alloy Nanoparticles for Oxygen Reduction Electrocatalysis

Abstract: Platinum alloy nanoparticles show great promise as electrocatalysts for the oxygen reduction reaction (ORR) in fuel cell cathodes. We report here on the use of N,N-dimethylformamide (DMF) as both solvent and reductant in the solvothermal synthesis of Pt alloy nanoparticles (NPs), with a particular focus on Pt–Ni alloys. Well-faceted alloy nanocrystals were generated with this method, including predominantly cubic and cuboctahedral nanocrystals of Pt3Ni, and octahedral and truncated octahedral nanocrystals of PtNi. X-ray diffraction (XRD) and high angle annular dark field scanning transmission electron microscopy (HAADF-STEM), coupled with energy dispersive spectroscopy (EDS), were used to characterize crystallite morphology and composition. ORR activities of the alloy nanoparticles were measured with a rotating disk electrode (RDE) technique. While some Pt3Ni alloy nanoparticle catalysts showed specific activities greater than 1000 μA/cm2Pt, alloy catalysts prepared with a nominal composition of PtNi disp...

Core-shell compositional fine structures of dealloyed Pt(x)Ni(1-x) nanoparticles and their impact on oxygen reduction catalysis.

Abstract: Using aberration-corrected scanning transmission electron microscopy and electron energy loss spectroscopy line profiles with Angstrom resolution, we uncover novel core–shell fine structures in a series of catalytically active dealloyed PtxNi1–x core–shell nanoparticles, showing the formation of unusual near-surface Ni-enriched inner shells. The radial location and the composition of the Ni-enriched inner shells were sensitively dependent on the initial alloy compositions. We further discuss how these self-organized Ni-enriched inner shells play a key role in maintaining surface lattice strain and thus control the surface catalytic activity for oxygen reduction.

Photocatalytic H2O2 Production from Ethanol/O2 System Using TiO2 Loaded with Au–Ag Bimetallic Alloy Nanoparticles

Abstract: TiO2 loaded with Au–Ag bimetallic alloy particles efficiently produces H2O2 from an O2-saturated ethanol/water mixture under UV irradiation. This is achieved via the double effects created by the alloy particles. One is the efficient photocatalytic reduction of O2 on the Au atoms promoting enhanced H2O2 formation, due to the efficient separation of photoformed electron–hole pairs at the alloy/TiO2 heterojunction. Second is the suppressed photocatalytic decomposition of formed H2O2 due to the decreased adsorption of H2O2 onto the Au atoms.

Reversible Insertion of Sodium in Tin

Abstract: The electrochemistry and the structural changes that occur during sodium insertion and removal from tin are studied by in-situ X-ray diffraction at 30°C. The Sn vs. Na voltage curve has four distinct plateaus, corresponding to four two-phase regions during sodiation, and indicating that four Na-Sn binary alloys are formed. The alloy formed at full sodiation was found to be Na15Si4, as expected from the Na-Sn binary system at equilibrium. The three intermediate Na-Sn phases that form during sodiation have X-ray diffraction patterns that do not correspond to any known equilibrium phase of Na-Sn. More work is needed to characterize these new binary Na-Sn phases.

Tensile properties of an AlCrCuNiFeCo high-entropy alloy in as-cast and wrought conditions

Abstract: Extensive multistep forging at 950 °C was applied to the cast AlCuCrFeNiCo high-entropy alloy to transform the cast coarse dendritic structure into a fine equiaxed duplex structure consisting of the mixture of BCC and FCC phases, with the average grain/particle size of ∼1.5 ± 0.9 μm. Tensile properties of the alloy in the as-cast and forged conditions were determined in the temperature range of 20–1000 °C. The hot forged alloy was stronger and more ductile during testing at room temperature, than the as-cast alloy. The yield stress (YS), ultimate tensile strength (UTS), and tensile ductility ( δ ) of the forged condition were 1040 MPa, 1170 MPa, and 1%, respectively, against 790 MPa, 790 MPa and 0.2% for the as-cast condition. In both conditions, the alloy showed brittle to ductile transition (BDT), with a noticeable increase in the tensile ductility within a narrow temperature range. In the as-cast condition, this transition occurred between 700 and 800 °C, while in the forged condition, it was observed between 600 and 700 °C. With an increase in the testing temperature above the BDT, a continuous decrease in tensile flow stress and an increase in tensile ductility were observed. In the temperature range of 800–1000 °C, the forged alloy showed superplastic behavior. The tensile elongation was above 400% and reached 860% at 1000 °C.

Microstructures and mechanical properties of multiprincipal component CoCrFeNiTix alloys

Abstract: The purpose of this study is to investigate the effects of Ti addition on the microstructures and mechanical properties of multiprincipal component CoCrFeNiTi x ( x values in molar ratio, x =0, 0.3, and 0.5) alloys. The CoCrFeNi quaternary alloy displayed a crystalline structure constructed by a simple face-centered cubic solid solution, whereas a plate-like structure consisting of a mixture of (Ni, Ti)-rich R phase and (Cr, Fe)-rich σ phase was observed within the face-centered cubic matrix of a CoCrFeNiTi 0.3 alloy. In a CoCrFeNiTi 0.5 alloy, an face-centered cubic matrix, a (Ti, Co)-rich Laves phase, and R+σ mixed phases were discovered. The compressive strength of the alloys rose by approximately 75% after the addition of Ti. Alloys with high levels of Ti content had high yield stress values and low ductility values. The solid-solution strengthening of the face-centered cubic matrix and the secondary-phase hardening were the two main factors that strengthened the alloy. The CoCrFeNiTi 0.3 alloy exhibited a compressive strength of 1529 MPa and a fracture strain of 0.60; this indicates that this material shows potential for the development of a ductile, high-strength alloy.

Role of electronic perturbation in stability and activity of Pt-based alloy nanocatalysts for oxygen reduction.

Abstract: The design of electrocatalysts for polymer electrolyte membrane fuel cells must satsify two equally important fundamental principles: optimization of electrocatalytic activity and long-term stability in acid media (pH <1) at high potential (0.8 V). We report here a solution-based approach to the preparation of Pt-based alloy with early transition metals and realistic parameters for the stability and activity of Pt(3)M (M = Y, Zr, Ti, Ni, and Co) nanocatalysts for oxygen reduction reaction (ORR). The enhanced stability and activity of Pt-based alloy nanocatalysts in ORR and the relationship between electronic structure modification and stability were studied by experiment and DFT calculations. Stability correlates with the d-band fillings and the heat of alloy formation of Pt(3)M alloys, which in turn depends on the degree of the electronic perturbation due to alloying. This concept provides realistic parameters for rational catalyst design in Pt-based alloy systems.

Oxidation behavior of a refractory NbCrMo0.5Ta0.5TiZr alloy

Abstract: Isothermal oxidation behavior of a refractory high-entropy NbCrMo0.5Ta0.5TiZr alloy was studied during heating at 1273 K for 100 h in flowing air. Continuous weight gain occurred during oxidation, and the time dependence of the weight gain per unit surface area was described by a parabolic dependence with the time exponent n = 0.6. X-ray diffraction and scanning electron microscopy accompanied by energy-dispersive X-ray spectroscopy showed that the continuous oxide scale was made of complex oxides and only local (on the submicron levels) redistribution of the alloying elements occurred during oxidation. The alloy has a better combination of mechanical properties and oxidation resistance than commercial Nb alloys and earlier reported developmental Nb–Si–Al–Ti and Nb–Si–Mo alloys.

Reviews on the Influences of Alloying elements on the Microstructure and Mechanical Properties of Aluminum Alloys and Aluminum Alloy Composites

Abstract: In recent year's aluminum and aluminum alloys are widely used in automotive industries. These are light weight (density of about 2.7g/cc),having good malleability and formability, high corrosion resistance and high electrical and thermal conductivity. High machinability and workability of aluminum alloys are prone to porosity due to gases dissolved during melting processes. However, in the engineering application pure aluminum and its alloys still have some problems such as relatively low strength, unstable mechanical properties. The microstructure can be modified and mechanical properties can be improved by alloying, cold working and heat treatment in this regards, this paper reports the influences of some alloying elements on the microstructures and mechanical properties of Aluminum alloys and aluminum alloy composites.

Microstructure and Compressive Properties of NbTiVTaAl x High Entropy Alloys

Abstract: The novel refractory high entropy alloys with the compositions of NbTiVTaAl x were prepared under a high-purity argon atmosphere and their microstructure and compressive properties at room temperature were investigated. Despite containing many constituents, all alloys had a single solid solution phase with body-centered cubic (BCC) structure, and possessed high compressive yield strength and ductility, which should be attributed to solid solution strengthening. © 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of MRS-Taiwan Keywords: High entropy alloy; solid solution; yield strength; ductility; solid solution strengthening 1. Introduction Recently, high-entropy alloys (HEAs), defined as alloys that generally have at least 5 major metallic elements and each of which has an atomic percentage between 5 % and 35% [1], have attracted increasing attentions. According to the regular solution model, the alloys have very high entropy of mixing, which makes HEAs usually form FCC and/or BCC solid solutions rather than intermetallic compounds or other complex ordered phases, and the total number of phases is well below the maximum equilibrium number allowed by the Gibbs phase rule [1-4]. In the past decade, a number of these HEAs have been explored

Effect of the LPSO volume fraction on the microstructure and mechanical properties of Mg–Y2X–ZnX alloys

Abstract: The influence of the volume fraction of long-period stacking ordered structure (LPSO) on the microstructure and mechanical properties in three extruded Mg100-3x Y2x Zn x alloys (x = 0.5, 1 and 1.5 at.%) has been studied. Two structures of LPSO phase coexist in these extruded alloys, 18R and 14H. The 18R structure transforms to 14H structure gradually in the course of the extrusion process. For the three alloys, the grain size in the vicinity of LPSO phase particles is refined because of a particle-stimulated nucleation (PSN) mechanism. The reinforcing effect of the LPSO phase is active up to 523 K. Above this temperature, grain size effect becomes important. Accordingly, MgY1Zn0,5 extruded alloy shows the Highest mechanical strength for temperatures greater than 523 K.