Fabrication of nanocrystalline alloys Cu–Cr–Mo super satured solid solution by mechanical alloying

This work discusses the extension of solid solubility of Cr and Mo in Cu processed by mechanical alloying. Three alloys processed, Cu–5Cr–5Mo, Cu–10Cr–10Mo and Cu–15Cr–15Mo (weight%) using a SPEX mill. Gibbs free energy of mixing values 10, 15 and 20 kJ mol⁻¹ were calculated for these three alloys respectively by using the Miedema's model. The crystallite size decreases and dislocation density increases when the milling time increases, so Gibbs free energy storage in powders increases by the presence of crystalline defects. The energy produced by crystallite boundaries and strain dislocations were estimated and compared with Gibbs free energy of mixing values. The energy storage values by the presence of crystalline defects were higher than Gibbs free energy of mixing at 120 h for Cu–5Cr–5Mo, 130 h for Cu–10Cr–10Mo and 150 h for Cu–15Cr–15Mo. During milling, crystalline defects are produced that increases the Gibbs free energy storage and thus the Gibbs free energy curves are moved upwards and hence the solubility limit changes. Therefore, the three alloys form solid solutions after these milling time, which are supported with the XRD results.     

Role of nitrogen addition in stabilizing the γ phase of Biomedical Co–29Cr–6Mo alloy

Three-dimensional atom probe observations and thermodynamic calculations revealed that the mechanism of γ-phase stabilization by N addition in the Co–29Cr–6Mo alloy is different from that in stainless steel: N addition does not lower the free energy of the γ phase in Co–29Cr–6Mo but increases the energy barrier and thus lowers the kinetic rate of the γ → ? transition through the formation of Cr–N short range order.     

Corrosion in the Flue Gas Cleaning System of a Biomass-Fired Power Plant

After only a few years operation, corrosion damage was observed in the flue gas cleaning system of a biomass power plant. The corrosion was on the lower part of the gas/gas heat exchanger fabricated from A242 weathering steel, where UNS S31600 bolts were used to attach sealing strips to the rotor. Thick iron oxides (up to 5 mm) had formed on the weathering steel, and these oxides also contained chlorine and sulfur. In this area of the heat exchanger, weathering steel has not had the optimal wet/dry cycles required to achieve a protective oxide. Due to the thick growing oxide on the rotor, the UNS S31600 bolts were under stress and this together with the presence of accumulated chlorine between the sealing strips and bolts resulted in stress corrosion cracking and rupture. In addition, Zn-K-Cl deposits were agglomerated in the duct after the DeNOx unit. Zn was also a constituent of corrosion products in various places in the ducts resulting in hygroscopic compounds. The presence of Zn in these cases was not from the fuel and is assumed to have originated from Zn containing primer (used to protect the plant during construction) reacting with flue gas constituents containing chlorine (KCl and HCl).     

As-cast microstructures and behavior at high temperature of chromium-rich cobalt-based alloys containing hafnium carbides

Hafnium is often used to improve the high temperature oxidation resistance of superalloys but not to form carbides for strengthen them against creep. In this work hafnium was added in cobalt-based alloys for verifying that HfC can be obtained in cobalt-based alloys and for characterizing their behavior at a very temperature. Three Co–25Cr–0.25 and 0.50C alloys containing 3.7 and 7.4 Hf to promote HfC carbides, and four Co–25Cr– 0 to 1C alloys for comparison (all contents in wt.%), were cast and exposed at 1200 °C for 50 h in synthetic air. The HfC carbides formed instead chromium carbides during solidification, in eutectic with matrix and as dispersed compact particles. During the stage at 1200 °C the HfC carbides did not significantly evolve, even near the oxidation front despite oxidation early become very fast and generalized. At the same time the chromium carbides present in the Co–Cr–C alloys totally disappeared in the same conditions. Such HfC-alloys potentially bring efficient and sustainable mechanical strengthening at high temperature, but their hot oxidation resistance must be significantly improved.     

Super austenitic stainless steels — a promising replacement for the currently used type 316L stainless steel as the construction material for flue-gas desulphurization plant

Potentiodynamic anodic cyclic polarization experiments on type 316L stainless steel and 6Mo super austenitic stainless steels were carried out in simulated flue-gas desulphurization (FGD) environment in order to assess the localized corrosion resistance. The pitting corrosion resistance was higher in the case of the super austenitic stainless steel containing 6Mo and a higher amount of nitrogen. The pit-protection potential of these alloys was more noble than the corrosion potential, indicating the higher repassivation tendency of actively growing pits in these alloys. The accelerated leaching study conducted for the above alloys showed that the super austenitic stainless steels have a little tendency for leaching of metal ions such as iron, chromium and nickel at different impressed potentials. This may be due to surface segregation of nitrogen as CrN, which would, in turn, enrich a chromium and molybdenum mixed oxide film and thus impedes the release of metal ions. The present study indicates that the 6Mo super austenitics can be adopted as a promising replacement for the currently used type 316L stainless steel as the construction material for FGD plants.     

Effects of trace impurities on the adherence of oxide to ultra low carbon steel

The effects of trace impurities on the adherence of oxides to ultra low carbon steels were investigated. Three steels, of differing chromium, aluminium, silicon, nitrogen and oxygen content, were oxidized at 850 K in 10% CO₂-90% N₂ gas and then oxidized at 770 K in 10% CO₂-10% O₂-80% N₂ gas. Surface analysers, i.e. a Mössbauer spectrometer, Auger electron spectrometer and/or ion micro analyser, were utilized in the present study because of the very thin layer of oxides formed (1.0 to 2.0 µm). The following results were obtained. The oxide on the Al-containing steel spalled at the outer-inner oxide interface, where aluminium was enriched and many cavities appeared. The inner oxide layer, oxidized by the transport of CO₂ or O₂ gases along the outer layer grain boundaries or micropores, became thick. This generated a stress at the outer-inner layer interface with the resultant formation of cavities. On the other hand, the oxide on the Cr-containing steel showed good adherence to the metal. The inner layer, enriched with chromium, did not become thick and had no cavities at the interface. The addition of chromium to the Al-containing steel resulted in good oxide adherence because chromium acted as a barrier to the aluminium enrichment.     

Comparison of conventional and active screen plasma nitriding of hard chromium electroplated steel

This paper considers the nitriding behavior of hard chromium electroplated steel by conventional plasma nitriding (CPN) and active screen plasma nitriding (ASPN) methods. Indentation test along the cross-section of the treated samples reveals that duplex treatment performed by two methods exhibits almost the same hardnesses. Furthermore, an increase in the time of plasma nitriding from 5 h to 10 h restores 30% of the hardness decline. Morphological studies show that surface particles formed on active screen plasma nitrided samples have orderly formed geometrical shapes while in conventional plasma nitriding they are in cauliflower shape. The reason for reaction between chromium and nitrogen seems to be the difference between thermal expansion coefficient of chromium oxide, chromium and steel substrate which results in partial breakdown of the oxide film. Moreover, the reducing of chromium oxide by hydrogen promotes the process. It looks as if nitriding treatment changes the corrosion behavior of the chromium coating from severe localized to uniform corrosion. Also active screen plasma nitriding treatment lowers the anodic dissolution 50-100 orders of magnitude which would be the result of full closure of surface microcracks.     

Electrochemical and surface analytical study of the anodic oxidation of Fe–18% Cr steel in molten NaOH–Li2CO3 mixtures

The anodic oxidation behaviour of a Fe–18%Cr steel in molten NaOH–Li₂CO₃ (50:50 mol%) at 470 °C was characterised by in-situ and ex-situ methods. In-situ electrochemical impedance spectroscopic (EIS) measurements showed that the steady state ionic conductivity of the oxide decreased with potential in the passive range. This indicates an increasing layer thickness or a decrease in the amount of mobile current carriers in the oxide with increasing potential. The two high-frequency time constants in the impedance spectra are most probably related to the semiconductor properties of the corrosion film and interfacial charge transfer, whereas the low-frequency time constant corresponds to the ionic defect transport through the oxide. The surface composition of the corrosion layers was estimated by X-ray Photoelectron Spectroscopy (XPS). The results show that the films can be regarded as mixed iron oxides containing a certain amount of chromium, sodium and lithium.     

Corrosion of armco iron and model Fe−Cr-Al alloys in oxygen-containing lead melts

We studied the influence of oxygen-containing (6·10⁻³ wt.% O) lead on the corrosion of Armco iron and Fe−16Cr, Fe−16Cr−1Al alloys at a temperature of 650°C under stationary conditions. The front of corrosion propagates according to a linear law and this process is periodically repeated. In each period, an oxide film based on Fe₃O₄ magnetite is formed on the surface of the metal and lead penetrates into the suboxide zone. This leads to the exfoliation the external oxide film and then the process is repeated. Under the indicated testing conditions, alloying with chromium and aluminum intensifies the process of corrosion in iron. Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 33, No. 2, pp. 84–88, March–April, 1997.     

Stress corrosion crack initiation in Alloy 690 in high temperature water

Initiation of stress corrosion cracks in Alloy 690 in high temperature water is a rare occurrence and depends on the method by which the sample is loaded. Only in dynamic straining experiments is crack initiation consistently observed. Stress relaxation in constant deflection tests, and lack of a means of rupturing the oxide film in constant load tests are the principle reasons for the difficulty of initiating cracks in these tests. These observations, combined with those from the much more susceptible Alloy 600 form the basis for a mechanism stress corrosion crack (SCC) initiation of Alloy 690. SCC initiation is proposed to occur in three stages: an oxidation stage in which a protective film of Cr₂O₃ is formed on the surface over grain boundaries, an incubation stage in which successive cycles of oxide film rupture and repair depletes the grain boundary of chromium, and a nucleation stage in which the chromium depleted grain boundary is no longer able to support growth of a protective chromium oxide layer, resulting in formation and rupture of oxides down the grain boundary. The mechanism is supported by the available literature on oxidation and crack initiation of Alloy 690 in hydrogenated primary water conditions.     

Design of composite porous cermets synthesized by hydrothermal treatment of CrAl powder followed by calcination

The microstructure of the porous Cr–Al metal–oxide cermet was studied by means of XRD, SEM, EDX as well as IR and Raman spectroscopy. This cermet was synthesized by mechanical alloying of Cr–Al powders in an AGO-2 planetary ball mill followed by hydrothermal treatment in a special stainless steel die and calcination in air. As a result, a highly porous monolith comprised of metal-like particles randomly distributed in the oxide matrix (Cr₂O₃ and Al₂O₃) was formed. Two types of the composite cores were found in cermets. The first one consisted of chromium phase containing nanoparticles sized from 50 to 140 nm and Al-enriched phase at the interfaces. The second one consisted of new chromium oxide phases with hexagonal Cr₂N-like and fcc CrN-like structures probably with Cr₂O and CrO stoichiometry. These new phases were stabilized within aggregates of the nanocomposite particles containing inclusions of alumina. The relations between different preparation stages and the cermet microstructure are discussed.     

Characterization and corrosion behavior of electroless Ni–P/nano-SiC coating inside the CO2 containing media in the presence of acetic acid

In this research, Ni–P and Ni–P/nano-SiC coatings were applied on the X70 steel substrate successfully without any surfactant. Then, CO₂ corrosion in the presence of acetic acid (HAc) was investigated using electrochemical techniques. Scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) techniques were used for surface analyses of the coatings. The electrochemical behavior of corrosion was investigated using polarization test and electrochemical impedance spectroscopy (EIS). XRD pattern of Ni–P/nano-SiC coating was very similar to that of Ni–P coating. EDS results demonstrated the presence of SiC particles in the coating. SEM images confirmed the presence of SiC nano-particles with almost uniform distribution in the coating. The corrosion current density was less in the Ni–P and Ni–P/nano-SiC coated samples than uncoated X70 steel. Ni–P/nano-SiC coated sample had the most corrosion resistance because of less effective metallic area available for corrosive media. The overall protection mechanism of Ni–P and Ni–P/nano-SiC coatings was achieved by formation of a layer of adsorbed hypophosphite anions (H₂PO₂⁻).     

AES study of passive films formed on a type 316 austenitic stainless-steels in a stress field

The type AISI 316 stainless steel, in addition to the principal alloying elements chromium and nickel, contains 2.5–3.5% of molybdenum. This element is added to improve the mechanical properties and the pitting resistance of austenitic alloys. Concerning the Stress Corrosion Cracking (SCC) resistance of austenitic stainless steels, molybdenum additions to alloys have a variable effect: the effect is detrimental for small additions of Mo, and it is beneficial for the alloy containing more than 4% Mo. Thus the Mo concentration on passive film plays an important role on the SCC resistance of steels. On the other hand, in a previous investigation, it was shown that the composition of passive films formed on the stressed 302 alloy depended on the compressive or tensile nature of stresses. Consequently, the aim of the present work is to study the composition of passive films formed on 316 steel and the migration of molybdenum in a stress field. Thus, Auger electron spectroscopy spectra were recorded to determine the chemical composition of the passive films formed on both sides of the type AISI 316 stainless steel U-bend samples. The results obtained show that the behaviour of chromium and oxygen in passive films formed on 316 steel in the stress field was nearly similar to that formed on 302 steel. Concerning the molybdenum diffusion outwards the passive film formed on the 316 steel was reduced by either the tensile or compressive stress field.     

钼酸锂对碳钢在溴化锂溶液中腐蚀的影响

本文通过静态挂片法研究了钼酸锂对碳钢在溴化锂溶液中缓蚀的作用，并通过极化曲线探讨其缓蚀作用的机理，用扫描电镜观察形成的表面膜形态，用X-射线衍射法对形成的表面膜层结构进行分析。实验表明钼酸锂对碳钢在溴化锂溶液中有一定的缓蚀作用，其中添加0．020mol／L钼酸锂和0．20mol／L氢氧化锂显著抑制溴化锂溶液对碳钢的腐蚀，其可能是在碳钢表面形成一层三氧化二铁、钼酸锂和三氧化钼组成的钝化镆，阻滞阳极反应从而起到缓蚀作用。     

Investigation on powder metallurgy Cr–Si–Ta–Al alloy target for high-resistance thin film resistors with low temperature coefficient of resistance

The sputtering target for high-resistance thin film resistors plays a decisive role in temperature coefficient of resistance (TCR). Silicon-rich chromium (Cr)–silicon (Si) target was designed and smelted for high-resistance thin film resistors with low TCR. Valve metal tantalum (Ta) and aluminum (Al) were introduced to the Cr–Si target to improve the performance of the target prepared. The measures for grain refining in smelting Cr–Si–Ta–Al target were taken to improve the performance of the prepared target. The mechanism and role of grain refinement were discussed in the paper. The phase structure of the prepared target was detected by X-ray diffraction (XRD). Rate of temperature drop was studied to reduce the internal stress of alloy target and conquer the easy cracking disadvantage of silicon-rich target. The electrical properties of sputtered thin film resistors were tested to evaluate the performance of the prepared target indirectly.     

Surface characterization of multiple coated H11 hot work tool steel by plasma nitriding and hard chromium electroplating processes

Formation of multi-layer coating by plasma nitriding and hard chromium electroplating on the surface of H11 hot work tool steel was investigated. Specimens were coated via a triple process containing plasma nitriding, hard chromium electroplating and plasma nitriding. Surface composition has been studied by X-ray diffraction analysis. The surface morphology and elemental analysis was examined by using scanning electron microscopy. Wear tests were conducted by the use of pin-on-disk method, a cemented tungsten carbide pin and 1000 gF load. Polarization corrosion tests were carried out in distilled water solution containing 3% NaCl. The improvement in hardness distribution after third step is discussed in considering the forward and backward diffusion of nitrogen in the chromium interlayer. Also, the formed phases in the hybrid coating were determined to be CrN + Cr₂N + Cr + Fe_2-3N + Fe₄N. Wear results showed that although the multi-layer coated specimens have higher wear resistance in comparison with the reference specimen, their wear resistance is less than that of two and one layer coated specimens due to micro-ploughing and removal of hard surface nitrides from the surface. By increasing the third step time and temperature, the wear resistance of specimens increases due to higher diffusion of nitrogen in the chromium layer. But polarization results showed that triple coated specimens have the lowest corrosion rate.     

Characterization of deformation instability in modified 9Cr–1Mo steel during thermo-mechanical processing

In this study, various existing instability criteria were employed to delineate the unstable flow regions in modified 9Cr–1Mo steel during hot deformation. Experimental stress–strain data obtained from isothermal hot compression tests, in a wide range of temperatures (1123–1373 K) and strain rates (10⁻³–10 s⁻¹), were employed to develop instability maps. The domains of these instability maps were validated through detailed microstructural study. It has been observed that Hart’s stability criterion, Jonas’s criterion and Semiatin’s criterion under-predicts the instability regions in the studied temperatures and strain rates regime. Gegel’s and Alexander’s criteria as well as Murty’s metallurgical instability criterion, on the other hand, found to over-predict the instability domains. The instability map developed based on Dynamic Materials Model criterion has been found to precisely predict the instability domains. This instability map revealed four major unstable domains. Microscopic examination in these domains revealed that the instability is manifested in the specimens either as localized deformation band primarily along one of the diagonal or inhomogeneous distribution of martensite lath in the prior austenite grains.     

High-temperature aging time-induced composition and thickness evolution in the native oxides film on Sn solder substrate

The presence of a native oxides film on Pb-free Sn solder joint surface is inevitable and its structure could be influenced by high-temperature aging due to heat release of electronic device itself during application. In this work, the effect of high-temperature aging time at 90 °C on composition and thickness evolution of a native oxides film, formed on pure Sn solder substrate after 24 h exposure to 90% RH atmosphere at 25 °C, has been characterized by AR-XPS and EIS. Results indicate that the outer layer of the as-obtained native oxide film consists of more Sn(OH)₄ and less SnO₂, and the inner layer consists of more SnO and less Sn(OH)₂. The aging process initially accelerates both the dehydration/oxidation of hydroxides/stannous oxides in the film and the fresh Sn substrate to form SnO₂, contributing to an increasing thickness and improved corrosion resistance of the film. However, an extended aging time deteriorates the structure of the film with more cracks and lowered corrosion resistance. Moreover, comparison finds that, without any assumption on models of resistivity distribution through the oxides film, the film thickness determination by Cole–Cole representation of the EIS results fits well with that by AR-XPS analyses.

     

Deckschichtbildung und Korrosionsverhalten des hochsiliciumlegierten austenitischen Sonderstahls X 2 CrNiSi 18 15 in Salpetersäuren

The Formation of Surface Films and the Corrosion Resistance of the Silicon Containing Austenitic Steel X 2 CrNiSi 18 15 in Nitric Acids The oxide layers consist of two parts: the lower one is mainly Croxide, the upper one is SiO₂. ESCA and AAS measurements were made in order to study the formation of the surface layer on the austenitic steel X2 CrNiSi 18 15 in nitric acids. A film, which is chromium oxide, is formed first. On top of this film a second film consisting of SiO₂ grows. The elements nickel and manganese are not found in the formation of the oxide films. Immediately below the oxide layer the steel is enrichened with chromium and depleted of iron. In order to find the in some cases very small corrosion rates (5 · 10^?5 mm · a^?1) in a reproducible manner, the amounts of iron, chromium and nickel which had been dissolved were measured by means of the AAS method as a function of time. For stationary samples apparent activation energies of 65.2 kJ/Mol (azeotropic nitric acid) and 37.5 kJ/Mol (highly concentrated nitric acid), respectively, were found. These data confirm the assumption that the corrosion rate is determined by reactions at the phase boundaries. No appreciable influence of the flow velocity on the corrosion rate was detected.     

Evaluation techniques of metallic biomaterials in vitro

Metals and alloys are widely used as biomedical materials and are important in medicine and they cannot be replaced with ceramics or polymers at present mainly because of their high strength and toughness. Since safety is the most important property of biomaterials, corrosion-resistant materials such as stainless steel, Co–Cr–Mo alloy, commercially pure titanium, and titanium alloys are employed as biomaterials. Evaluation techniques for corrosion with culturing cells, the characterization of reconstruction of surface oxide film, fretting fatigue, cytotoxicity, and biocompatibility are reviewed in this paper. These techniques are original and characteristics in the field of biomaterials that should contribute to the proper evaluations of biomaterials in vitro.