Cr Al Alloys Research Articles

The third-element effect in the oxidation of Ni– xCr–7Al ( x = 0, 5, 10, 15 at.%) alloys in 1 atm O 2 at 900–1000 °C

The oxidation in 1 atm of pure oxygen of Ni–Cr–Al alloys with a constant aluminum content of 7 at.% and containing 5, 10 and 15 at.% Cr was studied at 900 and 1000 °C and compared to the behavior of the corresponding binary Ni–Al alloy (Ni–7Al). A dense external scale of NiO overlying a zone of internal oxide precipitates formed on Ni–7Al and Ni–5Cr–7Al at both temperatures. Conversely, an external Al 2O 3 layer formed on Ni–10Cr–7Al at both temperatures and on Ni–15Cr–7Al at 900 °C, while the scales grown initially on Ni–15Cr–7Al at 1000 °C were more complex, but eventually developed an innermost protective alumina layer. Thus, the addition of sufficient chromium levels to Ni–7Al produced a classical third-element effect, inducing the transition between internal and external oxidation of aluminum. This effect is interpreted on the basis of an extension to ternary alloys of a criterion first proposed by Wagner for the transition between internal and external oxidation of the most reactive component in binary alloys.

Temporal evolution of the nanostructure and phase compositions in a model Ni–Al–Cr alloy

In a Ni-5.2 Al-14.2 Cr at.% alloy with moderate solute supersaturations and a very small γ/ γ′ lattice parameter misfit, the nanostructural and compositional pathways during γ′(L1 2) precipitation at 873 K are investigated using atom-probe tomography, conventional transmission electron microscopy, and hardness measurements. Nucleation of high number densities ( N v > 10 23 m −3) of solute-rich precipitates (mean radius = 〈 R〉 = 0.75 nm), with a critical nucleus composition of Ni-18.3 ± 0.9 Al-9.3 ± 0.7 Cr at.%, initiates between 0.0833 and 0.167 h. With increasing aging time (a) the solute concentrations decay in spheroidal precipitates (〈 R〉 < 10 nm); (b) the observed early-stage coalescence peaks at maximum N v in coincidence with the smallest interprecipitate spacing; and (c) the reaction enters a quasi-stationary regime where growth and coarsening operate concomitantly. During this quasi-stationary regime, the γ (face-centered cubic)-matrix solute supersaturations decay with a power-law dependence of about −1/3, while the dependencies of 〈 R〉 and N v are 0.29 ± 0.05 and −0.64 ± 0.06 at a coarsening rate slower than model predications. Coarsening models allow both equilibrium phase compositions to be determined from the compositional measurements. The observed early-stage coalescence is discussed in further detail.

Laser Developed Al-Cr Surface Alloys: Microstructure, Mechanical and Wear Behaviour

Surface alloys with composition ranging from 10 to 20% Cr were produced by laser surface alloying. Their microstructure consists of faceted plate-like Al4Cr intermetallic compound particles dispersed in a matrix of α-Al solid solution. During remelting, heterogeneous nucleation of eutectic Al7Cr/α-Al occurred in the undercooled liquid ahead of the columnar solid-liquid interface, followed by equiaxial solidification, resulting in a microstructure formed of equiaxed cells. Al-Cr alloys present Young’s modulus and hardness values that increase with increasing volume fraction of intermetallic compounds. Wear resistance, measured in dry sliding conditions, increases with increasing load due to the protective effect of a stable mechanically mixed layer that forms at the surface of the samples and the steel counterbody. Alloys formed of equiaxed eutectic cells provide better wear resistance than those formed of large plate-like particles since a thinner, more stable and harder mechanically mixed layer is formed, which offers best protection against wear.

Phase Separation and Stability of &lt;I&gt;L&lt;/I&gt;2&lt;SUB&gt;1&lt;/SUB&gt;-Type Phase in Co&lt;SUB&gt;2&lt;/SUB&gt;(Cr&lt;SUB&gt;1&amp;minus;&lt;I&gt;x&lt;/I&gt;&lt;/SUB&gt;Fe&lt;I&gt;&lt;SUB&gt;x&lt;/SUB&gt;&lt;/I&gt;)(Ga&lt;SUB&gt;1&amp;minus;&lt;I&gt;y&lt;/I&gt;&lt;/SUB&gt;Al&lt;I&gt;&lt;SUB&gt;y&lt;/SUB&gt;&lt;/I&gt;) Alloys

The B2/L2 1 order-disorder transition and ferromagnetic/paramagnetic transition temperatures in Co 2 (Cr 1-x Fe x )(Ga 1-y Al y ) alloys were determined by differential scanning calorimetry and transmission electron microscopy. It was found that spinodal decomposition cannot be suppressed by quenching in the Al-rich portion of Co 2 Cr(Ga 1-y Al y ) alloys, leading to lowering of Curie temperature T c . The transition temperature T B2/L2 1 t from the B2 to L2 1 -type phase decrease with increasing y for both the Co 2 CrrGa 1-y Al y ) and Co 2 Fe(Ga 1-y Al y ) alloy systems. The T c in the Ga-rich portion of the L2 1 -type Co 2 Cr(Ga 1-y Al y ) alloy system is almost constant at about 480K, whereas that in the B2-type Co 2 Fe(Ga 1-y Al y ) alloy system slightly increases from about 1100K to about 1170K. By extrapolation from the Ga-rich side of Co 2 Cr(Ga 1-y Al y ) alloys, T B2/L2 1 t and Tc of the metastable Co 2 CrAl alloy were determined as 800 and 470K, respectively. An optimal concentration area for obtaining excellent half-metallic properties and high stability of the L2 1 -type phase in the Co 2 (Cr 1-x Fe x )(Ga 1-y Al y ) alloys is proposed.

Comments on “Growth Rates of Alumina Scales on Fe–Cr–Al Alloys”

This comment illustrates the importance of data analysis in the determination of scale growth kinetics. It proposes a general method permitting to distinguish parabolic kinetics and power-law kinetics and to determine what scale growth kinetics is better suited to the analysis of a given data set.

Gaseous corrosion resistance of Fe–Al-based alloys containing Cr additions: Part I: Kinetic results

Iron–aluminum-based weld overlay claddings have recently been considered as corrosion resistant coatings. In this particular study, 10 alloys were exposed at 500 °C to three different corrosive environments ranging from a highly sulfidizing to an oxidizing atmosphere. Corrosion kinetics were determined for exposed alloys and cross-sectional microscopy was used to relate the corrosion products to the corrosion rates. It was found that critical aluminum and chromium concentrations were required to prevent rapid corrosion kinetics during 100 h of exposure. Titanium was found to help the corrosion resistance of a Fe–Al–Cr alloy when exposed to a highly sulfidizing environment, but had an insignificant effect on the corrosion kinetics of alloys exposed to a mixed oxidizing/sulfidizing and an oxidizing atmosphere. The total aluminum and chromium concentration was taken as the alloy content factor ( ϕ = at% Al + at% Cr) in order to numerically represent alloys based on the alloying elements. Critical alloying content values were found for each environment based on the corrosion kinetics. It was determined that 19 at% Al and 1 at% Cr was required to prevent rapid corrosion kinetics in all three corrosive environments.

The effect of aluminum content on phase constitution and heat treatment behavior of Ti–Cr–Al alloys for healthcare application

As life expectancy steadily increases, developing reliable functional materials for healthcare applications gains importance. Titanium and its alloys, while attractive for such applications, are expensive. The present investigation suggests that it may be possible to reduce costs by using new, low-cost beta Ti alloys. To assess their reliability, the heat treatment behavior of beta Ti alloys, Ti–7 mass% Cr with varying Al content (0%, 1.5%, 3.0% and 4.5%), was investigated through electrical resistivity and Vickers hardness measurements. In the Ti–7Cr–0Al alloy quenched from 1173 K, only the beta phase was identified by X-ray diffraction (XRD). In Ti–7Cr–1.5 to 4.5 Al alloys, XRD detected both beta and orthorhombic martensite. On isochronal heat treatment behavior of Ti–7Cr–3.0, 4.5 Al alloys, resistivity at liquid nitrogen temperature and resistivity ratio increased between 423 and 523 K.These increases are due to reverse transformation of orthorhombic martensite to the metastable beta phase.

Criteria for the formation of protective Al 2O 3 scales on Fe–Al and Fe–Cr–Al alloys

The conditions for the formation of external alumina scales on binary Fe–Al alloys and the nature of the third-element effect due to chromium additions have been investigated by studying the oxidation at 1000 °C in 1 atm O 2 of a binary Fe–10 at.% Al alloy (Fe–10Al) and of two ternary Fe–Cr–10 at.% Al alloys containing 5 and 10 at.% chromium (Fe–5Cr–10Al and Fe–10Cr–10Al, respectively). An Al-rich scale developed initially on Fe–10Al was subsequently replaced by a multi-layered scale containing mixtures of Fe and Al oxides plus a large number of Fe-rich oxide nodules: internal aluminum oxidation was essentially absent from this alloy. Addition of 5 at.% chromium to Fe–10Al did not suppress the formation of nodules, but they were eventually healed by the growth of an alumina layer at their base, resulting in a significant reduction of the oxidation rate. Finally, the alloy with 10 at.% Cr formed continuous external alumina scales without any Fe-rich nodule. Thus, the addition of sufficient amounts of chromium to Fe–10Al produces a third-element effect as expected. However, the process found in this alloy system does not involve a prevention of the internal oxidation of Al. Instead, it shows a transition from the growth of mixed Fe- and Al-rich external scales directly to an external Al 2O 3 scale formation. An interpretation of this kind of mechanism involving a third-element is presented along with a prediction of the critical Al contents required to produce the various possible scaling modes on binary Fe–Al alloys.

CORROSION BEHAVIOUR OF AGE HARDENABLE ALUMINUM–CHROMIUM ALLOYS

The corrosion behaviour of binary Al-Cr alloys and the ternary alloys with minor Zr additions were investigated. Immersion tests and current potential measurements were conducted with solution heat treated and aged specimens under exposure of varying levels of salinity and pH. Electrochemical corrosion parameters were evaluated from the potentiodynamic polarization plots of the binary and ternary alloys and were indicative of the relative extent of corrosion resistance in the alloys under different conditions. Results of immersion studies indicate that for binary Al-Cr alloys, the corrosion behaviour was not influenced by prior ageing temperature whereas for ternary alloys, the ageing temperature appeared to influence the corrosion behaviour. These results are further substantiated in microstructural observations from transmission electron microscopic (TEM) analysis and optical metallography was used as a supporting study to reveal the morphology of the corroded surface.On a investigué le comportement de corrosion d’alliages binaires Al–Cr et d’alliages ternaires avec additions mineures de Zr. On a effectué des épreuves d’immersion et des mesures de courant–potentiel avec des échantillons traités par mise en solution et trempe puis vieillis, par exposition à différents degrés de salinité et de pH. On a évalué les paramètres électrochimiques de corrosion à partir des courbes de polarisation potentiodynamique des alliages binaires et ternaires. Ces paramètres étaient indicatifs du degré relatif de résistance à la corrosion des alliages sous différentes conditions. Les résultats des études d’immersion indiquaient que pour les alliages binaires Al–Cr, le comportement de corrosion n’était pas influencé par la température antérieure de vieillissement alors que pour les alliages ternaires, la température de vieillissement semblait influencer le comportement de corrosion. Ces résultats sont davantage établis par les observations de la microstructure lors de l’analyse de microscopie électronique à transmission (TEM). On a utilisé la métallographie optique comme étude de support pour révéler la morphologie de la surface corrodée.

Residual stresses in alumina scales grown on different types of Fe–Cr–Al alloys: effect of specimen geometry and cooling rate

Residual stress measurements in alumina scales grown on Fe–Cr–Al alloys were conducted using the ruby fluorescence technique and the results compared with common theoretical considerations. The oxidation experiments were carried out at 1200 °C using different Fe–Cr–Al-based materials, different substrate thickness values and varying cooling rates. In case of a conventional wrought alloy the cooling rate significantly influences the residual stress state, whereas in case of a stronger, dispersion strengthened (ODS) alloy hardly any effect is visible. The results indicate at the very beginning of the oxidation process a high oxide growth stress level (up to −1.6 GPa), which does not substantially depend on the alloy type or specimen geometry. The growth stresses relax with time and this relaxation at oxidation temperature is faster in case of the wrought alloy than for the ODS alloy. Additionally, for all studied systems except the 2 mm thick PM 2000 specimens a transition from compressive to apparent tensile growth stresses after longer exposure times occurs. For very thin Aluchrom YHf specimens (hM = 50 μm) this transition might be explained by nearly total Al-depletion of the substrate material and the resulting decrease of the thermal expansion coefficient. However, the validity of the theoretical calculations based on an ideal flat surface cannot be assured at least for such thin substrates. In fact, geometrical irregularities, a non-even relaxation of the stress or micro-cracking in the scale are likely to affect the calculated residual and/or growth stresses even for thicker specimens.

Residual stresses in alumina scales grown on different types of Fe–Cr–Al alloys: effect of specimen geometry and cooling rate

Abstract Residual stress measurements in alumina scales grown on Fe–Cr–Al alloys were conducted using the ruby fluorescence technique and the results compared with common theoretical considerations. The oxidation experiments were carried out at 1200 °C using different Fe–Cr–Al-based materials, different substrate thickness values and varying cooling rates. In case of a conventional wrought alloy the cooling rate significantly influences the residual stress state, whereas in case of a stronger, dispersion strengthened (ODS) alloy hardly any effect is visible. The results indicate at the very beginning of the oxidation process a high oxide growth stress level (up to −1.6 GPa), which does not substantially depend on the alloy type or specimen geometry. The growth stresses relax with time and this relaxation at oxidation temperature is faster in case of the wrought alloy than for the ODS alloy. Additionally, for all studied systems except the 2 mm thick PM 2000 specimens a transition from compressive to apparent tensile growth stresses after longer exposure times occurs. For very thin Aluchrom YHf specimens ( h M = 50 μm) this transition might be explained by nearly total Al-depletion of the substrate material and the resulting decrease of the thermal expansion coefficient. However, the validity of the theoretical calculations based on an ideal flat surface cannot be assured at least for such thin substrates. In fact, geometrical irregularities, a non-even relaxation of the stress or micro-cracking in the scale are likely to affect the calculated residual and/or growth stresses even for thicker specimens.

The Effect of Water Vapor on Passive-Layer Stability and Corrosion Behavior of Fe–Al–Cr Base Alloys

Although it has been reported that additions of water vapor to high-temperature corrosion environments accelerate corrosion, the role of water vapor on increasing the corrosion rate of Fe–Al–Cr alloys and coatings is not very well understood. In order to better understand these effects, multiple Fe–Al–Cr base alloys were tested at 500°C for 100 hr in three different multi-component corrosive gases with and without water vapor. The three gases were a sulfidizing gas, a mixed oxidizing/sulfidizing gas, and an oxidizing gas. Thermogravimetric testing showed that the corrosion kinetics increased when water vapor up to 6% was added to the atmospheres. The surfaces of the exposed samples were considered carefully to determine if the addition of water vapor changed the morphology of the corrosion products and, more importantly, affected the passive layer. It has previously been shown that the formation of nodules can be caused by the inability of the passive layer to re-heal itself after the presence of a defect allowing fast growing non-protective corrosion products to externally grow from the surface. In this study, the amount of external nodules that formed on the surface of the alloys was shown to increase with the addition of water vapor. An increase in the amount of external nodules present due to additions of water vapor gives an indication that water vapor accelerates the passive-layer breakdown. It was observed that the scale morphology correlated well with the corrosion kinetics for the exposed alloys, which showed that water vapor increased the corrosion rates of the alloys.

Growth Rates of Alumina Scales on Fe–Cr–Al Alloys

The growth rates of alumina scales formed on high-temperature alloys often show a strong deviation from classical parabolic kinetics, and frequently scale growth rates near to a cubic time dependence are observed. This is e.g. the case for most RE doped Fe–Cr–Al-alloys oxidized at temperatures in the range of 1000 to 1300°C. For a number of components made of Fe–Cr–Al alloys, which e.g. prevail in car catalyst carriers, gas burners or hot-gas filters, the detailed knowledge of the rate law governing scaling kinetics is an absolutely necessary requirement for a reliable prediction of the component lifetime and/or the materials application limits. It seems that for ideally gas-tight α-Al2O3 scales a near-cubic time dependence is more a rule than an exception, although the frequently used methods for evaluating measured oxidation data might lead to the erroneous conclusion that the scale growth is governed by a transient-oxidation stage followed by parabolic oxidation. Deviations from an ideal, near-cubic time dependence of the oxidation kinetics of the Fe–Cr–Al alloys are caused by scale cracking during thermal cycling and/or local inhomogenities in the alloy and/or the scale.

Le diagramme Al-Cr-Zn à 460○C

Today the baths of galvanisation contain different chemical elements: zinc, but also aluminium, iron and chromium. To understand the formation of solid phases and the reactions that take place in the molten zinc bath, the zinc rich corner of the quaternary diagram Fe-Zn-Al-Cr has to be investigated. The base of this quaternary diagram is composed by three ternary diagrams: Fe-Al-Zn, Fe-Cr-Zn and Al-Cr-Zn, the two first were already known, but not the last one. The main purpose of this study is the exploration of the ternary diagram Al-Cr-Zn with different experiences: immersion of chromium in aluminium added zinc bath, PVD of zinc on a Al-Cr alloy, galvanizing procedure, mechanical alloying, evaporation of zinc. Each new ternary phase is characterized by EDS and XRD. This experimental results allows the optimization of this ternary diagram by the modelisation software Thermo-Calc.

Co-Deposition of Al-Cr and Al-Ni Alloys using Potential Pulse Technique in Molten Salt

Co-Deposition of Al-Cr and Al-Ni Alloys using Potential Pulse Technique in Molten Salt

Ｔｉ‐Ａｌ‐Ｃｒ β固溶体中における相互拡散

The interdiffusion experiments of Ti-rich β Ti–Al–Cr alloys have been performed in the temperature range from 1273 to 1473 K. The concentration profiles indicate that the diffusion distance of Cr is somewhat longer than that of Al in the solid solutions. The diffusion paths show S-shaped curves. The formation of S-shaped diffusion paths is attributed to the difference between the diffusion distance of Cr and Al. The direct and indirect interdiffusion coefficients are positive in the ternary alloys, and the four interdiffusion coefficients are not sensitive to the solute concentrations. The ratio of indirect coefficient to direct one suggests that repulsive interactions exist between Al and Cr atoms in the Ti–Al–Cr alloys. In addition, the ratio values of converted interdiffusion coefficients in the ternary alloys suggest that the interactions between Ti (solvent) and Cr atoms are attractive in the present alloy.

Effect of alloying elements on the electrochemical polarization behavior and passive film of Fe–Mn base alloys in various aqueous solutions

The corrosion properties of austenitic Fe–Mn, Fe–Mn–Al, Fe–Mn–Cr and Fe–Mn–Al–Cr alloys with compositions of 23–30 wt% Mn, 2.8–8.2 wt% Al and 4.9–6.9 wt% Cr in various aqueous solutions of pH −0.8 to 15.3 and the passivating mechanism induced by the presence of Al, Cr, or Al and Cr have been studied using electrochemical measurements and Auger electron spectroscopic/X-ray photoelectron spectroscopic analysis. Binary Fe–Mn alloys can be passivated only in 10–50% NaOH solutions, and alloying of binary Fe–Mn alloy with Al or Cr or combination of Al and Cr seems not so obviously beneficial to corrosion resistance in HNO 3 or Na 2SO 4 solutions. All of the experimental Fe–Mn based alloys and steels for comparison cannot passivate in either 10% HCl or 3.5% NaCl solution. The Fe–Mn based alloys containing Al or Cr or Al and Cr can passivate in 10–50% HNO 3 or 1 mol l −1 Na 2SO 4 solutions and rainwater. In general, Fe–Mn based alloys can passivate in oxidizing acid, neutral and basic solution, but cannot passivate in reducing acid or solution containing active Cl − ions. In the passive film formed on the surface of Fe–Mn base alloys in various aqueous solutions, bound water and hydroxides are present at the surface of the film, while mixed oxides of Al, Cr, Mn and Fe are located in the inner part. The resistance to corrosion is imparted by a barrier film of bound water, hydroxides and oxides of Al, Cr or Fe, while the Mn oxides in passive film reduce the corrosion resistance.

The local internal stress in ferromagnetic alloys. Dislocation–solute interaction model for local internal stress source

In order to elucidate the Raleigh and Left Shift phenomena encountered in the investigations into damping capacity (internal friction) of ferromagnetic alloys (Acta Metall Sin 34 (1998) 1039; Trans Jpn Inst Met 25 (1984) 891; Trans Jpn Inst Met 20 (1979) 409; Mater Sci Forum 119–121 (1993) 415; Metall Mater Trans A 25 (1994) 111), the solute atom model (Mater Design (in press)) for the local internal stress source is modified and replaced by the dislocation–solute interaction model proposed by the authors. The coercive force as well as crystal lattice parameter of FeCrAl and FeCrAlSi alloys was measured and the local internal stress of these alloys was discussed. It is pointed out that only when the alloy is fully re-crystallized, the potential of damping capacity could be fully developed, and at the pre-requisite of which do not lower the energy density of domain walls (Mater Design 21 (2000) 541), to choose those alloying element that distort crystal lattice slightly is benefit to improve the damping capacity of ferromagnetic alloys.

High-temperature relaxation in a Fe–Cr–Al Alloy

Two relaxational internal friction peaks were found in a (wt%)Fe–25Cr–5Al alloy. The low-temperature peak is related to Zener relaxation and the high-temperature one to grain-boundary relaxation. Their activation energy values are 2.55 (±0.14) eV for the Zener peak and 4.07(±0.15) eV for the grain-boundary relaxation peak, respectively. Grain-boundary relaxation strength remarkably increases with decreasing grain size, while the Zener peak is independent of the grain size. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Microstructural evidence for counter-diffusion of aluminum and oxygen during the growth of alumina scales

A novel, two-stage oxidation experiment is described that enables the outward diffusion of cations in alumina scales during high-temperature oxidation to be analyzed on the basis of microstructural changes in the surface morphology of the scale. Using this technique, observations of aluminum out-diffusion along α-Al2O3 grain boundaries during oxidation of Fe–Cr–Al alloys, nickel aluminides and platinum-modified NiAl bond-coats are made. Although microstructural evidence for the inward grain boundary diffusion of oxygen is more difficult to obtain, it still can be demonstrated by the growth of the oxide above interface cavities on nickel aluminides and inside internal cracks in the alumina scales during cyclic oxidation of zirconia top-coated material. SEM examination of the crack surfaces after scale spallation provides a vivid illustration of two simultaneous processes, aluminum outward and oxygen inward diffusion along grain boundaries in the scale.