5.1. Nanoalloys of Group 11 (Cu, Ag, Au) 865 5.1.1. Cu−Ag 866 5.1.2. Cu−Au 867 5.1.3. Ag−Au 870 5.1.4. Cu−Ag−Au 872 5.2. Nanoalloys of Group 10 (Ni, Pd, Pt) 872 5.2.1. Ni−Pd 872 * To whom correspondence should be addressed. Phone: +39010 3536214. Fax:+39010 311066. E-mail: ferrando@fisica.unige.it. † Universita di Genova. ‡ Argonne National Laboratory. § University of Birmingham. | As of October 1, 2007, Chemical Sciences and Engineering Division. Volume 108, Number 3

Nanoalloys: From Theory to Applications of Alloy Clusters and Nanoparticles

Bulk metallic glasses (BMGs) have been classified according to the atomic size difference, heat of mixing (� H mix ) and period of the constituent elements in the periodic table. The BMGs discovered to date are classified into seven groups on the basis of a previous result by Inoue. The seven groups are as follows: (G-I) ETM/Ln-LTM/BM-Al/Ga, (G-II) ETM/Ln-LTM/BM-Metalloid, (G-III) Al/Ga-LTM/BMMetalloid, (G-IV) IIA-ETM/Ln-LTM/BM, (G-V) LTM/BM-Metalloid, (G-VI) ETM/Ln-LTM/BM and (G-VII) IIA-LTM/BM, where ETM, Ln, LTM, BM and IIA refer to early transition, lanthanide, late transition, group IIIB–IVB and group IIA-group metals, respectively. The main alloying element of ternary G-I, G-V and G-VII, ternary G-II and G-IV, and ternary G-VI BMGs is the largest, intermediate and smallest atomic radius compared to the other alloying elements, respectively. The main alloying element of ternary BMGs belonging to G-I, G-V, G-VI and G

Classification of Bulk Metallic Glasses by Atomic Size Difference, Heat of Mixing and Period of Constituent Elements and Its Application to Characterization of the Main Alloying Element

https://www.tcd.ie/Physics/people/Graham.Cross/SF%20Poster%202011/Schuh-ActaMater07-Mechanical%20behaviour%20of%20amorphous%20alloys.pdf

Mechanical behavior of amorphous alloys

Unlike the well-defined long-range order that characterizes crystalline metals, the atomic arrangements in amorphous alloys remain mysterious at present. Despite intense research activity on metallic glasses and relentless pursuit of their structural description, the details of how the atoms are packed in amorphous metals are generally far less understood than for the case of network-forming glasses. Here we use a combination of state-of-the-art experimental and computational techniques to resolve the atomic-level structure of amorphous alloys. By analysing a range of model binary systems that involve different chemistry and atomic size ratios, we elucidate the different types of short-range order as well as the nature of the medium-range order. Our findings provide a reality check for the atomic structural models proposed over the years, and have implications for understanding the nature, forming ability and properties of metallic glasses.

http://slapper.apam.columbia.edu/bib/papers/sheng_n06.pdf

Atomic packing and short-to-medium-range order in metallic glasses

Atomic packing and short-to-medium range order in metallic glasses

We present a new semi-empirical potential suitable for molecular dynamics simulations of liquid and amorphous Cu–Zr alloys. To provide input data for developing the potential, new experimental measurements of the structure factors for amorphous Cu64.5Zr35.5 alloy were performed. In this work, we propose a new method to include diffraction data in the potential development procedure, which also includes fitting to first-principles and liquid density and enthalpy of mixing data. To refine the new potential, we used first-principles and liquid enthalpy of mixing data published earlier combined with the densities of liquid Cu64.5Zr35.5 measured over a range of temperatures. We show that the potential predicts a liquid-to-glass transition temperature that agrees reasonably well with experimental data. Finally, we compare the new potential with two previously developed semi-empirical potentials for Cu–Zr alloys and examine their comparative and contrasting descriptions of structure and properties for Cu64.5Zr35...

Development of suitable interatomic potentials for simulation of liquid and amorphous Cu–Zr alloys

We propose a method of using atomistic computer simulations to obtain partial pair correlation functions from wide angle diffraction experiments with metallic liquids and their glasses. In this method, a model is first created using a semiempirical interatomic potential and then an additional atomic force is added to improve the agreement with experimental diffraction data. To illustrate this approach, the structure of an amorphous Cu64.5Zr35.5 alloy is highlighted, where we present the results for the semiempirical many-body potential and fitting to x-ray diffraction data. While only x-ray diffraction data were used in the present work, the method can be easily adapted to the case when there are also data from neutron diffraction or even in combination. Moreover, this method can be employed in the case of multicomponent systems when the data of several diffraction experiments can be combined.

Using atomistic computer simulations to analyze x-ray diffraction data from metallic glasses

Thermal barrier coating (TBC) systems, needed for higher thrust with increased efficiency in gas turbines, typically consist of an alumina-scale forming metallic bond coat and a ceramic topcoat. The durability and reliability of TBC systems are critically linked to the oxidation behavior of the bond coat. Ideally, the bond coat should oxidize to form a slow-growing, non-porous and adherent thermally grown oxide (TGO) scale layer of α-Al2O3. The ability to promote such ideal TGO formation depends critically on the composition and microstructure of the bond coat, together with the presence of minor elements (metal and non-metal) that with time diffuse into the coating from the substrate during service. An experimental program was undertaken to attain a more detailed fundamental understanding of the phase equilibria in the Ni-Al-Pt system and the influences of alloy composition on the formation, growth and spallation behavior of the resulting TGO scales formed during isothermal and thermal cycling tests at 1150°C. Additional studies were conducted to determine the influence of platinum on interdiffusion behavior in the Ni-Al system, and how this influence would impact coating/substrate interdiffusion. It will be shown that platinum has a profound effect on the oxidation and interdiffusion behaviors, to the extent that novel advanced coating systems can be developed. Introduction The demand for improved performance in high-temperature mechanical systems has led to increasingly severe operating environments, particularly for the components in advanced gas-turbine engines. Future improvements in gas-turbine performance will require even higher operating efficiencies, longer operating lifetimes, reduced emissions and, therefore, higher turbine operating temperatures. Advanced cooling schemes coupled with thermal barrier coatings (TBCs) can enable the current families of nickel-base superalloys to meet the materials needs for the engines of tomorrow. Thermal barrier coating systems currently provide average metal temperature reductions of about 80°C, while potential benefits are estimated to be greater than 170°C. However, lack of reliability, more than any other design factor, limits the general use of TBC systems for gas turbines. Commercial advanced TBC systems are typically two-layered, consisting of a ceramic topcoat and an underlying metallic bond coat. The properties of the ceramic topcoat are such that it has a low thermal conductivity, high oxygen permeability, and a relatively high coefficient of thermal expansion. The topcoat is also made “strain tolerant” by depositing a structure that contains numerous pores and/or pathways. The consequently high oxygen permeability of the topcoat imposes the constraint that the metallic bond coat must be resistant to oxidation attack. The bond coat should therefore be sufficiently rich in aluminum to form a protective, thermally grown oxide (TGO) scale of α-Al2O3. In addition to imparting oxidation resistance, the TGO serves to bond the ceramic topcoat to the substrate/bond coat system. Notwithstanding, it is generally found that spallation and/or cracking of the growing TGO scale is the ultimate failure mechanism of Materials Science Forum Online: 2004-08-15 ISSN: 1662-9752, Vols. 461-464, pp 213-222 doi:10.4028/www.scientific.net/MSF.461-464.213 © 2004 Trans Tech Publications Ltd, Switzerland All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications Ltd, www.scientific.net. (Semanticscholar.org-11/03/20,15:01:32) commercial TBCs, particularly EB-PVD TBCs [1-3]. Thus, improving the adhesion and integrity of the interfacial TGO scale is critical to the development of more reliable TBCs. Current bond coats are typically either an MCrAlY overlay (where M = Ni,Co, or both) or a platinum-modified diffusion aluminide (β-NiAl-Pt). The composition and phase constitutions of such bond coats vary. Both the MCrAlY and NiAl-Pt types of coating were originally developed to enhance long-term oxidation and corrosion protection of turbine components rather than specifically as bond coats. The oxidation resistance provided by these coatings allowed alloy developers to maximize the high-temperature mechanical properties of nickel-base superalloys. The original coatings required a high aluminum content in order to ensure re-healing of the Al2O3 scale after repeated cracking and spalling during service. In the case of TBC systems, however, Al2O3 healing after scale spallation is not an important requirement for ceramic topcoat adhesion. This is because the adhesion, and therefore the reliability, of a TBC system is dictated primarily by the first spallation event of the TGO scale. As a consequence, the currently used bond coats do not necessarily possess optimum compositions and/or structures for prime reliant TBC systems. In assessing a given bond coat system, it is also important to realize that its composition and structure change with time in service due to both TGO-scale growth and interdiffusion with the substrate. The occurrence of coating/substrate interdiffusion decreases the concentration of aluminum in the coating, thereby reducing the ability of the coating to sustain exclusive Al2O3-scale growth, particularly in the event of localized detachment and/or microcracking, and introduces unwanted elements (e.g., sulfur and titanium) which can promote oxide-scale spallation [4]. A further consequence of coating/substrate interdiffusion, particularly for the next generation of superalloys with up to 6 wt.% rhenium, is the formation of topologically close-packed (TCP) phases in the region of the original coating/substrate interface, which can be deleterious to the mechanical properties of the superalloy substrate. The results presented in this paper are from a larger US Office of Naval Research sponsored study that is concerned with gaining a more detailed fundamental understanding of the influences of alloy composition and microstructure on the adhesion/spallation resistance of TGO scales for the purpose of improving the reliability and durability of advanced TBC systems. An experimental approach was taken initially, from which the important chemical and microstructural factors governing TGO growth and adhesion were determined. The starting point for this study was a determination of the (partial) Ni-Al-Pt phase diagram, which to date has only been shown to be speculative [5] (see Fig.1). Oxidation and interdiffusion experiments were then conducted on the various Ni-Al-Pt bulk alloys processed to determine the potential effects of phase constitution and Pt content on overall coating performance. Experimental Procedures Ni-Al-Pt alloys were prepared by argon arc melting high-purity pieces of the constituents. To ensure homogenization and equilibration, all alloys were annealed at 1100 ̊C or 1150 ̊C for at least one week in a flowing argon atmosphere and then quenched in water to retain their high-temperature structure. The alloys were then cut into coupon samples and polished to a 600-grit finish for the further testing. The equilibrated alloy samples were first analyzed using X-ray diffraction (XRD) for phase identification and then prepared for metallographic analyses by cold mounting them in an epoxy resin followed by polishing to a 0.5 μm finish. Microstructure observations were initially carried out on etched samples using an optical microscope. Concentration profiles were obtained from unetched (i.e., re-polished) samples by either energy (EDS) or wavelength (WDS) dispersive spectrometry, with the former utilizing a scanning electron microscope (SEM) and the latter an 214 High Temperature Corrosion and Protection of Materials 6

Effects of Platinum on the Interdiffusion and Oxidation Behavior of Ni-Al-Based Alloys

The suspension plasma spray (SPS) process was used to produce coatings from yttria-stabilized zirconia (YSZ) powders with median diameters of 15 μm and 80 nm. The powder-ethanol suspensions made with 15-μm diameter YSZ particles formed coatings with microstructures typical of the air plasma spray (APS) process, while suspensions made with 80-nm diameter YSZ powder yielded a coarse columnar microstructure not observed in APS coatings. To explain the formation mechanisms of these different microstructures, a hypothesis is presented which relates the dependence of YSZ droplet flight paths on droplet diameter to variations in deposition behavior. The thermal conductivity (kth) of columnar SPS coatings was measured as a function of temperature in the as-sprayed condition and after a 50 h, 1200 °C heat treatment. Coatings produced from suspensions containing 80 nm YSZ particles at powder concentrations of 2, 8, and 11 wt.% exhibited significantly different kth values. These differences are connected to microstructural variations between the SPS coatings produced by the three suspension formulations. Heat treatment increased the kth of the coatings generated from suspensions containing 2 and 11 wt.% of 80 nm YSZ powder, but this kth increase was less than has been observed in APS coatings.

/pdf/column-formation-in-suspension-plasma-sprayed-coatings-and-4lr4spodpa.pdf

Column Formation in Suspension Plasma-Sprayed Coatings and Resultant Thermal Properties

Spherical, monosized yttria precursor particles were obtained by homogeneous precipitation in aqueous solutions by reaction with the thermal decomposition products of urea. Increasing [Y 3+ above 0.05 M resulted in a deviation from spherical morphology and caused agglomeration of particles. Over the concentration range studied, excess urea did not affect particle morphology, but increased the yield. Increasing aging time appeared to increase particle size as well as to improve yield, as long as the urea was not depleted. The approximate chemical composition of the precipitate was YOHCO 3 . The YOHCO 3 particles formed were amorphous to X-rays, and underwent a two-stage thermal decomposition, first forming Y 2 O 2 CO 3 near 180°C, and then cubic Y 2 O 3 above 610°C. Generation of CO 3 2− appeared to be crucial to the formation of the solid phase. Heating the aqueous yttrium solution with trichloroacetic acid (CCl 3 COOH) instead of urea as the precipitating agent produced a solid phase, while heating the same solution with formamide (HCONH 2 ) substituted for urea formed no precipitate.

Daniel J. Sordelet

Papers

Development of suitable interatomic potentials for simulation of liquid and amorphous Cu–Zr alloys

Using atomistic computer simulations to analyze x-ray diffraction data from metallic glasses

Effects of Platinum on the Interdiffusion and Oxidation Behavior of Ni-Al-Based Alloys

Column Formation in Suspension Plasma-Sprayed Coatings and Resultant Thermal Properties

Preparation of spherical, monosized Y2O3 precursor particles