Synthesis of a new tungsten-free gamma-gamma ` cobalt-based superalloy by tuning alloying additions

The paper presents the synthesis of a new class of gamma-gamma' cobalt-based superalloy that is free of tungsten as an alloying addition. It has much lower density and higher specific strength than the existing cobalt-based superalloys. The current superalloys have a base composition of Co-10Al and are further tuned by the addition of a binary combination of molybdenum and niobium, with the optimum composition of Co-10Al-5Mo-2Nb. The solvus temperature of the alloy (866 degrees C) can be further enhanced above 950 C by the addition of Ni to give the form Co-xNi-10Al-5Mo-2Nb, where x can be from 0 to 30 at.%. After heat treatment, these alloys exhibit a duplex microstructure with coherent cuboidal L1(2)-ordered precipitates (gamma') throughout the face-centred cubic matrix (gamma), yielding a microstructure that is very similar to nickel-based superalloys as well as recently developed Co-Al-W-based alloys. We show that the stability of the gamma' phase improves significantly with the nickel addition, which can be attributed to the increase in solvus temperature. A very high specific 0.2% proof stress of 94.3 MPa g(-1) cm(-3) at room temperature and 63.8 MPa g(-1) cm(-3) at 870 degrees C were obtained for alloy Co-30Ni-10Al-5Mo-2Nb. The remarkably high specific strength of these alloys makes this class of alloy a promising material for use at high temperature, including gas turbine applications. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

A new tungsten-free gamma-gamma ` Co-Al-Mo-Nb-based superalloy

We present the first report of a tungsten-free cobalt-based superalloy having a composition Co-10Al-5Mo-2Nb. The alloy is strengthened by cuboidal precipitates of metastable Co-3(Al,Mo,Nb) distributed throughout the microstructure. The precipitates are coherent with the face-centred cubic gamma-Co matrix and possess ordered Ll(2) structure. The microstructure is identical to the popular gamma-gamma' type nickel-based superalloys and that of recently reported Co-Al-W-based alloys. Being tungsten free, the reported alloy has higher specific proof stress compared to existing cobalt-based superalloys. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Studying the influence of substitutional elements on mechanical behavior of Alloy 718

In nickel-based superalloys, substitutional solute species have a strong impact on in service mechanical properties as well as on oxidation and corrosion resistances. In alloy 718, recent studies carried out by tensile tests highlighted the fact that refractory solute species are able to interact strongly with mobile dislocations during plastic deformation, generating dynamic strain ageing, and, in wide ranges of tests temperatures and strain rates, Portevin-Le Chatelier effect. The precise nature of the substitutional element responsible for such a dynamic interaction is still subject to debate. We addressed this question by means of mechanical spectroscopy studies of alloy 718 and various related alloys corresponding to monitored changes in the chemical composition. Only a single internal friction relaxation peak has been observed for all the studied alloys. By analyzing the damping behavior of these alloys at different imposed solicitation frequencies by sweeping a large temperatures range, the activation energies of the relaxation process and the type of mechanism involved have been determined. The process is a "Zener relaxation" in the alloys, i.e. a substitutional atoms dipole reorientation under applied stress. The results tend to prove that Niobium is not involved in the relaxation process whereas Molybdenum content seems to play an important role in the relaxation intensity.

Effect of mechanical coating with Ni and Ni-5% Al on the structure and electrochemical properties of the Mg-50% Ni alloy

The Mg-Ni metastable alloys (with amorphous or nanocrystalline structures) are promising candidates for anode application in nickel-metal hydride rechargeable batteries due to its large hydrogen absorbing capacity, low weight, availability, and relative low price. In spite of these interesting features, improvement on the cycle life performance must be achieved to allow its application in commercial products. In the present paper, the effect of mechanical coating of a Mg-50 at.% Ni alloy with Ni and Ni-5 at.% Al on the structure, powder morphology, and electrochemical properties is investigated. The coating additives, Mg-Ni alloy and resulting nanocomposites (i.e., Mg-Ni alloy + additive) were investigated by means of X-ray diffraction and scanning electron microscopy. The Mg-Ni alloy and nanocomposites were submitted to galvanostatic cycles of charge and discharge to evaluate their electrode performances. The mechanical coating with Ni and Ni-5% Al increased the maximum discharge capacity of the Mg-Ni alloy from of 221 to 257 and 273 mA h g(-1), respectively. Improvement on the cycle life performance was also achieved by mechanical coating.

Ultrasonic sensor signals and optimum path forest classifier for the microstructural characterization of thermally-aged inconel 625 alloy

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Debenzylation by Ni-A1 alloy

Nickel-aluminum alloy in aqueous sodium hydroxide-ethanol medium brings about facile hydro-enolrsis of benzyl ether function in substrates carrying a vicinal methoxy group. Simplicity of the reaction conditions and cheapness of the reagent are some of the advanrages of the procedure. Additionally, carbonyl functions when present are untouched and alkaline medium of the reaction makes it compatible with the presence of acid-sensitite groups.

Microstructure and texture evolution during accumulative roll bonding of aluminium alloy AA5086

In the present investigation, a strongly bonded strip of an aluminium-magnesium based alloy AA5086 is successfully produced through accumulative roll bonding (ARB). A maximum of up to eight passes has been used for the purpose. Microstructural characterization using electron backscatter diffraction (EBSD) technique indicates the formation of submicron sized (similar to 200-300 nm) subgrains inside the layered microstructure. The material is strongly textured where individual layers possess typical FCC rolling texture components. More than three times enhancement in 0.2% proof stress (PS) has been obtained after 8 passes due to grain refinement and strain hardening. (C) 2011 Elsevier B.V. All rights reserved.

Creep behaviour of short fibre reinforced QE22 magnesium alloy using impression creep test

Creep properties of QE22 magnesium based alloy and composites reinforced with 20 volume percent of short-fibers - Maftech (R), Saffil (R) or Supertech (R), were evaluated using the impression creep test. In the impression creep test, a load is applied with the help of a cylindrical tungsten carbide indenter of 1 mm diameter. This has advantages over conventional creep testing in terms of small specimen size requirement and simple machining. Depth of impression is recorded with time and steady state strain rate is obtained from the slope of the secondary strain (depth of impression divided by indenter diameter) vs. time plot. The results are compared with the creep obtained from conventional creep performed in tension on the same materials earlier. Microstructural examination of the plastically deformed regions is carried out to explain creep behaviour of these composites.

WEAR AND SLIDING-FRICTION CHARACTERISTICS OF STAINLESS STEEL SS316LN WITHOUT A COATING AND WITH A CrN COATING AT ELEVATED TEMPERATURES AND IN A VACUUM

Stainless steel of type AISI 316LN - one of the structural materials of fast neutron reactors - must have a long service life under conditions that subject it to different types of wear (galling, adhesion, fretting, and abrasion). Cobalt-based hard facings are generally avoided due to induced radioactivity. Nickel-based hard facings are strongly preferred instead. One alternative to both types of coatings is a hard-alloy coating of CrN. This article examines wear and friction characteristics during the sliding of uncoated steel SS316LN and the same steel with a CrN coating. In addition, a specially designed pin-on-disk tribometer is used to perform tests in a vacuum at temperatures of up to 1000 degrees C in order to study the effect of oxygen on the wear of these materials. The morphology of the wear surface and the structure of the subsurface were studied by scanning electron microscopy. The formation of an adhesion layer and the self-welding of mating parts are seen to take place in the microstructure at temperatures above 500 degrees C. It is also found that steel SS316LN undergoes shear strain during sliding wear. The friction coefficient depends on the oxygen content, load, and temperature, while the wear rate depends on the strain-hardening of the surface of the material being tested.

SEM and EDS Characterisation of Layering TiOx Growth onto the Cutting Tool Surface in Hard Drilling Processes of Ti-Al-V Alloys

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Crystallographic and electrochemical characteristics of melt-spun Ti-Zr-Ni-La alloys

Ti-based icosahedral quasicrystalline phase (I-phase) exhibited excellent hydrogen storage property for special structure. Unfortunately, the application as the negative electrode material of the nickel-metal hydride batteries was limited due to the poor electrochemical kinetics. Meanwhile, rare-earth element was beneficial to the electrochemical properties of Ti, Zr-based alloy.

Microstructure and Strengthening Mechanism of Die-Cast Mg-Gd Based Alloys

Mg-8Gd-2Y-Nd-0.3Zn (wt%) alloy was prepared by the high pressure die-cast technique. The microstructure, mechanical properties in the temperature range from room temperature to 573 K, and strengthening mechanism were investigated. It was confirmed that the Mg-Gd-based alloy with high Gd content exhibited outstanding die-cast character. The die-cast alloy was mainly composed of small cellular equiaxed dendrites and the matrix. The long lamellar-shaped stacking compound of Mg3X (X: Gd, Y, Nd, and Zn) and polygon-shaped.

不同基板预热温度对激光金属沉积成形过程温度场的影响

为降低成形过程的热应力,抑制成形过程裂缝的产生,减小成形过程试样和基板的翘曲变形,激光金属沉积成形往往需要进行基板预热,因此研究不同基板预热温度对激光金属沉积成形过程温度场的影响具有非常重要的意义.根据有限元分析中的"单元生死"技术,利用APDL编程建立了基板预热对激光金属沉积成形过程温度场影响的三维多道多层数值模拟模型,详细分析了基板未预热和分别预热到200,300,400,500,600℃时对沉积成形过程温度场和温度梯度的影响.通过中国科学院沈阳自动化研究所自行研制的激光金属沉积成形系统和基板预热系统,在与模拟过程相同的参数下,利用镍基合金粉末在基板未预热和分别预热到300,400,500,560℃时进行了成形试验,试验结果跟数值模拟结果吻合较好.

不同基板预热温度对激光金属沉积成形过程热应力影响的研究

研究不同基板预热温度对激光金属沉积成形过程热应力的影响,对于降低成形过程的热应力,抑制成形过程裂缝的产生,减小成形过程试样和基板的翘曲变形具有非常重要的意义。根据有限元分析中的"单元生死"技术,编程建立了基板预热对激光金属沉积成形过程热应力影响的三维多道多层数值模拟模型,详细分析了基板未预热和分别预热到200℃、300℃、400℃、500℃、600℃时对沉积成形过程VonMise’s热应力、X方向、Y方向以及Z方向热应力的影响。在与模拟过程相同的参数下,利用镍基合金粉末分别在基板未预热和分别预热到300℃、400℃、500℃、600℃时进行了成形试验,试验的结果跟数值模拟结果吻合较好。

Activity and coke formation of nickel and nickel carbide in dry reforming: A deactivation scheme from density functional theory

Dry reforming is a promising reaction to utilise the greenhouse gases CO2 and CH4. Nickel-based catalysts are the most popular catalysts for the reaction, and the coke formation on the catalysts is the main obstacle to the commercialisation of dry reforming. In this study, the whole reaction network of dry reformation on both flat and stepped nickel catalysts (Ni(111) and Ni(211)) as well as nickel carbide (flat: Ni3C(001); stepped: Ni3C(111)) is investigated using density functional theory calculations. The overall reaction energy profiles in the free energy landscape are obtained, and kinetic analyses are utilised to evaluate the activity of the four surfaces. By careful examination of our results, we find the following regarding the activity: (i) flat surfaces are more active than stepped surfaces for the dry reforming and (ii) metallic nickel catalysts are more active than those of nickel carbide, and therefore, the phase transformation from nickel to nickel carbide will reduce the activity. With respect to the coke formation, the following is found: (i) the coke formation probability can be measured by the rate ratio of CH oxidation pathway to C oxidation pathway (r(CH)/r(C)) and the barrier of CO dissociation, (ii) on Ni(111), the coke is unlikely to form, and (iii) the coke formations on the stepped surfaces of both nickel and nickel carbide can readily occur. A deactivation scheme, using which experimental results can be rationalised, is proposed.