Chromium Content Of Alloy Research Articles

Influence of thermal conductivity on evolution of grain morphology during laser-based directed energy deposition of CoCrxFeNi high entropy alloys

A model CoCrxFeNi high entropy alloy system was chosen to investigate the effect of thermal conductivity on the evolution of spatial orientation of columnar grain morphology during laser directed energy deposition. The thermal conductivity of the alloy was varied by tuning the chromium content in CoCrxFeNi alloys. CoCrxFeNi alloys with chromium content ranging between 0 and 24 at% were produced under identical processing conditions of laser direct energy deposition method. Electron back scattered diffraction images of the cross-sections revealed that the spatial orientation of grain morphology transitioned from zigzag columnar grains from layer-to-layer to near-continuous columnar grains across multiple layers with increased content of chromium in CoCrxFeNi. A multi-scale multi-physics finite element based thermo-kinetic model with consideration to multi-track multi-layer nature of the laser direct energy deposition technique was adopted to correlate the process thermo-kinetics with the evolution of microstructure. The computation calculations revealed that the decrease in thermal conductivity from 36 W/m.K (CoFeNi) to 26 W/m.K (CoCr24FeNi) at the melting point reduced the cooling rate from 3.3 ×104 °C/s to 1.3 ×104 °C/s in CoFeNi and CoCr24FeNi alloys, respectively. This subsequently evolved into increased degree of remelting of previously deposited layers and caused the direction of resultant thermal gradient to reorient towards the build direction. Such reorientation in resultant thermal gradient contributed to transition of spatial orientation of grain morphology from columnar zigzag microstructures from layer-to-layer to near continuous columnar grains across multiple layers. The present observation was verified compared with other reported metallic systems exhibiting zigzag and continuous columnar grains, and a unified mechanism underlining the role of thermal conductivity on microstructure evolution was proposed.

The Importance of Phase Composition for Corrosion Resistance of Borided Layers Produced on Nickel Alloys.

The plasma paste boriding process was used for production of the borided layers on pure nickel and nickel-chromium alloys. The produced layers consisted of nickel borides only (in the case of nickel) or a nickel and chromium borides mixture (in the case Ni–Cr alloys). The objective of this investigation was to indicate the importance of the presence of chromium for corrosion resistance of non-borided alloys, as well as to indicate the influence of phase composition of borided layers on their corrosion resistance. Pure nickel was characterized by higher corrosion resistance, in comparison to the nickel-based alloys. Increased chromium content in nickel alloys resulted in their high susceptibility for pitting corrosion. All borided samples were characterized by higher corrosion resistance than the non-borided samples. However, the phase composition of borided layers influenced their corrosion resistance. Due to the microstructure which consisted of one type of borides (nickel borides), borided nickel had the highest resistance to corrosion, whereas the presence of chromium borides in layers produced on nickel-chromium alloys caused a decrease in corrosion resistance.

Tunable magnetoresistance and thermopower in interconnected NiCr and CoCr nanowire networks

Magnetoresistance and thermopower of crossed NiCr and CoCr nanowire networks have been measured as a function of temperature and chromium content in dilute alloys. At low temperatures, it is found that the impurity effect leads to negative anisotropic magnetoresistance, an observation that even persists until room temperature in diluted CoCr alloy nanowires. The addition of a small amount of Cr in nickel nanowires also abruptly reverses the sign of the thermopower from −20 μV/K for pure Ni up to +18 μV/K for the dilute alloys, implying the switching from n- to p-type conduction. These results are consistent with pronounced changes in the density of states for the majority spin electrons. The high room-temperature power factors of these magnetic nanowire networks (in the range of 1–10 mW/K2 m) provide interesting perspectives for designing n- and p-type legs for flexible spin thermoelectric devices.

11B NMR STUDY OF AMORPHOUS ALLOYS Fe85-xCrxB15 (x = 0-20)

Amorphous alloys Fe85-xCrxB15 (x = 0-20), prepared by rapid quenching of the melt, have been studied by pulsed nuclear magnetic resonance (NMR) on nuclei 11B at a temperature of 4,2 K. The amorphous alloys consist of nanoclusters with short-range order of the tetragonal t-(Fe, Cr)3B and α-(Fe, Cr) phases which contain chromium atoms in the nearest surroundings of boron atoms. The average magnetic moments of magnetic atoms in the nearest sphere of boron atoms in nanoclusters and their dependence on the chromium content in alloys are estimated.

The Effect of Nickel Alloy Chromium Content in Indirect-Fired CO2 Power Cycle Environments

Power cycles in the fossil energy, nuclear, and concentrated solar industries, using pure supercritical CO2 (sCO2) as the working fluid, have been proposed that offer significantly improved conversion efficiency and lower levelized cost of electricity over current power systems. High creep strength nickel-base alloys are proposed for use at the higher temperature portions of the power cycles. In this study a series of Ni-xCr model alloys (where x = 5, 12, 14, 16, 18, 20, 22, 24) were exposed to both sCO2 at 700°C/200 bar for 1500 h and atmospheric pressure CO2 at 700°C for over 5000 h. In both tests, all Ni-Cr alloys, even the low-chrome Ni5Cr alloy, maintained low oxidation rates and chromia scales. For CO2+H2O+O2 mixtures at 750°C/1 bar, all alloys except Ni5Cr exhibited low oxidation rates and chromia scales. However, in other environments (air at 700°C/1 bar and steam at 700°C/200 bar) more Cr was needed to maintain low oxidation rates and chromia scales. This result shows that Ni-Cr alloys appear to be quite robust with respect to oxidation resistance in the pure and predominantly CO2 environments found in sCO2 power cycles.

Correlation between microstructure, hardness, wear and electrochemical behaviour in 8.0%, 16.0% and 20.0% (by wt) chromium white irons

Microstructures, hardness, wear and corrosion behaviour of high chromium white irons with 8, 16, and 20 wt% chromium have been compared. In all the alloys, the matrix microstructure contains pearlite, martensite and retained austenite in varying proportions, while the carbide content increases from 26 vol% in alloy1 to 42 vol% in alloy2 and then to 57 vol% in alloy3. Presence of all these phases has been confirmed by XRD and SEM studies. EDS analysis confirms the formation of duplex carbide made of M3C shell around M7C3. The hardness (macro and micro) increases with the increase of chromium in the alloys. The wear of the alloys at 19.6N and 49.0N load produces closely spaced wear tracks with evidences of oxidation, edge-chipping, cracking and delamination of oxide layers. The higher chromium content of alloy 2 and 3 causes more oxidation of the worn surface under frictional heat developed during wear tests. At low wear load alloy 2 and 3 show better wear resistance, while at higher load alloy3 behaves at par with Ni-hard IV. Alloy3 appears to be most corrosion resistance exhibiting a wider passive range and with lower corrosion current density (icorr) than that of alloys 1 and 2.

THE DEGRADATION OF THE PROTECTIVE SCALE ON BINARY FeCr ALLOYS (Fe-2.25Cr, Fe-10Cr, Fe-18Cr AND Fe-25Cr) IN CO2 AND IN CO2 + H2O ENVIRONMENT AT 600oC

The oxidation behaviour of the binary alloys Fe-2.25, Fe-10Cr, Fe-18Cr and Fe-25Cr in dry and wet O2 at 600oC is investigated by isothermal exposures of carefully polished samples for up to 168 hours. The oxidized samples are investigated gravimetrically and the oxides formed are studied by X-ray diffraction. X-ray photoelectron spectroscopy is used for depth pro􀂿 ling of the thin oxides. The scale surface is imaged by SEM. Cross sections through the scale are analyzed by SEM/EDX for imaging and for measuring the chemical composition. The oxidation behavior of the four FeCr alloys is intermediate between those of iron and chromium. Fe-2.25Cr oxidizes in a way similar to iron in both environments, forming a poorly protective scale consisting of FeCr spinel at the bottom, magnetite in the middle and a hematite cap layer. In dry O2, Fe-10Cr, Fe-18Cr and Fe-25Cr form a thin and protective (Fe,Cr)2O3 oxide similar to the chromia 􀂿 lm formed on pure chromium. In wet O2, Fe-10Cr, Fe-18Cr and Fe-25Cr initially form the same kind of protective oxide 􀂿 lm as in dry conditions. After an incubation time that depends on alloy chromium content, all three alloys go into breakaway oxidation and form thick, poorly protective scales similar to those formed on Fe-2.25Cr. Breakaway oxidation in wet O2 is triggered by the evaporation of CrO2(OH)2 from the protective (Fe,Cr)2O3 oxide.

An application of learning machine methods in prediction of wear rate of wear resistant casting parts

In this paper, a method of floating ball wear rate identification, using two machine-learning techniques Support Vector Machine (SVM) and Improved Support Vector Machine (ISVM) are proposed. Both models are used to relate the wear rate and technological parameters of the wear resistant drip moulding using different kernel functions. The models for determining the wear rate of white iron casting with low chromium content (flotation balls), was trained and tested by using the existing exploitation data from the Bor Flotation Plant, Serbia. In order to select the best model parameters the statistical indicators for both models are presented. Results show that the ERBF (SVM) and ERBF+POLY (ISVM) achieved the best classification accuracy compare to other kernels used: the absolute mean error of ERB (SVM) is 5.85%, while the error of ERBF+POLY (ISVM) is 6.67%. The tuned ISVM model with mixture of kernels is able to accurately predict the wear rate and can be used to define the optimum chromium content in liquid metal alloys for the casting of flotation balls.

Oxidation of Binary FeCr Alloys (Fe–2.25Cr, Fe–10Cr, Fe–18Cr and Fe–25Cr) in O2 and in O2 + H2O Environment at 600 °C

The oxidation behaviour of the binary alloys Fe–2.25Cr, Fe–10Cr, Fe–18Cr and Fe–25Cr (wt%) in dry and wet O2 at 600 °C is investigated by isothermal exposures of carefully polished samples for up to 168 h. The oxidized samples are investigated gravimetrically and the oxides formed are studied by X-ray diffraction. X-ray photoelectron spectroscopy is used for depth profiling of the thin oxides. The scale surface is imaged by SEM. Cross-sections through the scale are analyzed by SEM/EDX for imaging and for measuring the chemical composition. The oxidation behavior of the four FeCr alloys is intermediate between those of iron and chromium. Fe–2.25Cr oxidizes in a way similar to iron in both environments, forming a poorly protective scale consisting of FeCr spinel at the bottom, magnetite in the middle and a hematite cap layer. In dry O2, Fe–10Cr, Fe–18Cr and Fe–25Cr form a thin and protective (Fe,Cr)2O3 oxide similar to the chromia film formed on pure chromium. In wet O2, Fe–10Cr, Fe–18Cr and Fe–25Cr initially form the same kind of protective oxide film as in dry conditions. After an incubation time that depends on alloy chromium content, all three alloys go into breakaway oxidation and form thick, poorly protective scales similar to those formed on Fe–2.25Cr. Breakaway oxidation in wet O2 is triggered by the evaporation of CrO2(OH)2 from the protective (Fe,Cr)2O3 oxide.

Structure and properties of Cu-Cr alloys subjected to shear under pressure and subsequent heating

The effects of the chromium content (0.75, 9.85, 27%) in Cu-Cr alloys, their initial state, and the heating temperature on their thermal stability after severe plastic deformation are studied by microhardness and electrical resistivity measurements and metallography. The microhardness and electrical resistivity of Cu-Cr alloys are determined as functions of the heating temperature in the range 50–600°C, and microhardness distributions over the diameter of samples differing in the chromium content are plotted. The difference in the strengthening and softening of low- and high-alloy copper-chromium alloys subjected to high-pressure torsion is shown to be related to the initial structure of the material.

The Effect of Water Vapour on the Oxidation of High Speed Steel, Kinetics and Scale Adhesion

An investigation has been carried out into the effects of steam on the adhesion of oxide scales grown on four different chemical compositions of high speed steels. The oxidation tests were carried out in a thermobalance at 650 °C for up to 14.4 ks. The mass gain of the specimens increased with increasing contents of water vapour, and in no case were parabolic kinetics obeyed. Increasing the alloy chromium content reduced the oxidation rate in dry conditions but increases in the V and W content had no effect. In moist conditions, the alloys containing increased Cr and W had the lowest oxidation rates followed by the alloy with increased V levels, however these effects were small. The “adhesion” of the oxide scale was determined using indentation with a Rockwell C diamond and the pull adhesion test to determine the interfacial toughness and the tensile strength of the oxide metal bond, respectively. Generally, the results of the investigation indicated that oxide adhesion was lowest for the specimens exposed to dry conditions, and that with increasing water vapour content, the scale was more adherent. It is noteworthy that whereas the scale formed in dry conditions spalled at the metal oxide interface, that formed in moist gas failed cohesively within the scale, mostly at the Cr spinel/magnetite interface. Examination of the fractured scale after indentation shows that the outer oxide layer became more porous as the water vapour content of the gas increased. Indentation test results showed that an increase in the V concentration was deleterious to oxide adhesion in both the dry and wet environments, whereas an increase in the Cr-content had little effect on adhesion. The results with the pull test were less clear.

ChemInform Abstract: The Corrosion Behavior of Sputter-Deposited Amorphous Chromium- Zirconium Alloys in 6 M HCl Solution.

Abstract Amorphous binary CrZr alloys containing 11–66 at% Zr were successfully prepared by a magnetron sputtering method. Amorphous CrZr alloys thus prepared were spontaneously passive in 6 M HCl solution. Their corrosion rates were four orders of magnitude lower than chromium metal and one-third of zirconium metal and decreased with alloy chromium content. XPS analysis revealed that airformed films and passive films on the amorphous CrZr alloys were composed of double oxyhydroxides of chromium and zirconium. This appears to be responsible for the high corrosion resistance of the amorphous CrZr alloys in comparison with their constituent elements; the formation of chromium-containing passive film is more protective than the film on zirconium metal, despite the fact that chromium metal dissolves actively in 6 M HCl solution.

Corrosion Behavior of Stainless Steels in Simulated PWR Primary Water—Effect of Chromium Content in Alloys and Dissolved Hydrogen—

The structure and composition of surface oxide films on austenitic stainless steels in hydrogenated high-temperature water were examined by changing the chromium content in alloys and the concentration of dissolved hydrogen in high-temperature water. Auger electron spectroscopy, X-ray diffraction and analytical transmission electron microscopy revealed that the oxide films had a double-layer structure: ironbased spinels as the outer layer and chromium-rich spinel oxide as the inner layer. Increasing the chromium content suppressed the corrosion rate and produced fine oxide particles with a higher chromium concentration in the inner layer. Increasing the concentration of dissolved hydrogen enhanced the corrosion rate without a notable change in oxide structure. These influences are considered to originate from changes in cation diffusion through the inner layer, such as a decrease in the lattice diffusion of iron in the inner layer due to a higher concentration of chromium in the oxide as a diffusion barrier for a high chromium content in the alloys and due to a lower oxygen partial pressure for a higher concentration of dissolved hydrogen.

Corrosion Behavior of Stainless Steels in Simulated PWR Primary Water —Effect of Chromium Content in Alloys and Dissolved Hydrogen—

The structure and composition of surface oxide films on austenitic stainless steels in hydrogenated high-temperature water were examined by changing the chromium content in alloys and the concentration of dissolved hydrogen in high-temperature water. Auger electron spectroscopy, X-ray diffraction and analytical transmission electron microscopy revealed that the oxide films had a double-layer structure: ironbased spinels as the outer layer and chromium-rich spinel oxide as the inner layer. Increasing the chromium content suppressed the corrosion rate and produced fine oxide particles with a higher chromium concentration in the inner layer. Increasing the concentration of dissolved hydrogen enhanced the corrosion rate without a notable change in oxide structure. These influences are considered to originate from changes in cation diffusion through the inner layer, such as a decrease in the lattice diffusion of iron in the inner layer due to a higher concentration of chromium in the oxide as a diffusion barrier for a high chromium content in the alloys and due to a lower oxygen partial pressure for a higher concentration of dissolved hydrogen.

Tunnel scanning microscopy and spectroscopy in studying Fe-Cr stainless steels

Scanning tunnel microscopy (STM) and scanning tunnel spectroscopy (STS) are used in studying highly pure iron and chromium and Fe-x%Cr (2.4 ≤ x ≤ 60 wt %) alloys at the air boundary, as well as some of the alloys in 0.01 N H2SO4. The obtained probability coefficients (α) of the electron tunnel transfer from a specimen to the needle and the slopes (β) of the logarithmic U t /log (I t ) dependences as functions of the chromium content in Fe-Cr alloys confirm the critical compositions of the alloys containing ∼6.5 and 10–13% chromium, which is in accord with the results of the steady-state and transient electrochemical measurements. A correlation between these critical compositions of the alloys and sharp changes in the histograms of α and β values is observed. Pronounced extreme properties of the Fe-25.2% Cr alloy surface, which corresponds to the inclusion of Cr atoms in tetrahedral voids of the alloy crystal lattice, are noticed.

Cyclic Oxidation of Heat Resisting Steels

Standard cast, heat resisting steels containing 25–29 w/o (weight percent) chromium and 30–36 w/o nickel together with cast alloys containing 45 or 60 w/o nickel plus low levels of aluminium were subjected to cyclic oxidation in air at 1000 and 1150°C. The standard materials suffered rapid weight loss which was somewhat mitigated by the presence of cerium. The 45 w/o nickel alloys were much more resistant and the 60 w/o nickel alloys showed superior resistance to cyclic oxidation. This improvement was due to alumina formation at or near the alloy surface. In the absence of aluminium, alloys underwent subsurface chromium carbide oxidation at a rate independent of alloy chromium content. This effect is shown to be a consequence of rapid oxygen diffusion along internal phase boundaries.

The effect of alloying elements on the corrosion behaviour of sputter-deposited amorphous Mn–Ta–Cr alloys in 1 M H2SO4

The effect alloying tantalum and chromium on the corrosion resistance of amorphous manganese-base alloys was studied by weight-loss, electrochemical and XPS techniques. Amorphous alloys were formed in a wide composition range by the addition of tantalum to manganese-chromium alloys. The corrosion rates of Mn–Ta–Cr alloys in 1 M H 2SO 4 open to air at 30°C were about four and six orders of magnitude lower than those of Mn–Ta and Mn–Cr alloys containing the same amount of manganese, respectively, and decreased with increasing alloy chromium content. Thus, additions of chromium and tantalum to manganese were synergistically effective in enhancing the corrosion resistance of Mn–Ta–Cr alloys. The beneficial effect of chromium in the presence of tantalum is explained in terms of enhancement passivity and cathodic reactions of the alloys.

Metal dusting of 9 - 20% Cr steels in increased pressure environments at 560°C

The susceptibility of steels to catastrophic failure by metal dusting is known to depend upon the activity of the carburising environment, alloy chromium content, grain size, surface condition and temperature. This paper presents results of first tests investigating the influence of gas pressure on metal dusting of 9-20% Cr containing steels, by increasing the test gas pressure to approximately five bar over atmospheric pressure. Cross-sectional micrographs show attack morphologies such as intergranular and internal carbide formation, as well as the classic uniform and pitting types of attack.

Hydrogen embrittlement of Ni-Cr-Fe alloys

The purpose of this work was to investigate the role of chromium on hydrogen embrittlement of Ni-Cr-Fe alloys and thus to develop a better understanding of the low-temperature stress corrosion cracking (SCC) phenomenon. The effect of chromium on hydrogen embrittlement was examined using tensile tests followed by material evaluation via scanning electron microscopy (SEM) and light optical microscopy. Four alloys were prepared with chromium contents ranging from 6 to 35 wt pct. In the uncharged condition, ductility, as measured by the percent elongation or reduction in area, increased as the alloy chromium content increased. Hydrogen appeared to have only minor effects on the mechanical properties of the low-chromium alloys. The addition of hydrogen had a marked effect on the ductility of the higher-chromium alloys. In the 26 pct chromium alloy, the elongation to failure was reduced from 53 to 14 pct, with a change in fracture mode from mixed ductile dimple and ductile intergranular failure to a brittle appearing intergranular failure. A maximum in embrittlement was observed in the 26 pct Cr alloy. The maximum in embrittlement coincided with the minimum in stacking-fault energy. It is proposed that the increased hydrogen embrittlement in the high-chromium alloys is due to increased slip planarity caused by the lower stacking-fault energy. Slip planarity did not appear to affect the fracture of the uncharged specimens.

Corrosion-resistant NiCrMo alloys in hot concentrated sulphuric acid with active carbon

Abstract Dewpoint corrosion by sulphuric acid has often been observed in the heat exchangers of coal-burning plants. If ashes contain active carbon, corrosion becomes particularly serious. Various nickel-base alloys with high corrosion resistance in such environments have been designed. The corrosion behaviour of these alloys was examined in hot concentrated sulphuric acid with and without active carbon. The corrosion resistance of ternary NiCiMo alloys in the acids without active carbon increases with increasing alloy molybdenum content. When active carbon is added to the acids, the corrosion rate increases significantly. This is due to acceleration of transpassive dissolution of molybdenum as a result of enhancement of cathodic activity with the addition of active carbon. The corrosion resistance in the acids with active carbon is enhanced by an increase in alloy chromium content, which improves the passivity in the acids.