General laws governing variation in properties of cast alloys of the Al-Zn-Mg system

Among existing high-strength corrosion-resistant aluminum alloys, those of the Al-Mn-Zn system are most promising. They have a different phase composition, depending on the content of magnesium and Zinc: + (Al₃Mg₂), + + T(Al₂Mg₃Zn₃), + T, + T + (MgZn2) and + The majority of industrial Al-Mg-Zn alloys correspond to the phase regions + T and + T + with respect to composition. A high level of strength and satisfactory overall corrosion resistance are characteristic for these alloys. Al-Mg-Zn alloys may, however, tend to the most dangerous form of corrosion - stress-induced corrosion cracking. Using methods of experiment planning in the study, we investigated Al-Zn-Mg alloys of various compositions for the purpose of selecting alloy compositions with a high level of mechanical properties and stress-induced corrosion cracking.All-Union Scientific-Research Institute of Aviation Materials. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 12, pp. 24–28, December, 1994.     

Analytical study of surface and bulk oxidation of Astroloy

Nickel-base alloys, such as Astroloy, used for aeronautical turbine disks, are sensitive to time-dependent cracking in environments containing oxygen. The mosaic structure of the alloy consisting islands (200 nm average size) surrounded by the -phase (100 nm thick) induces complex oxidation phenomena. Various analytical approaches allow the delineation of all the steps from segregation to oxidation occurring on the surface of such a duplex structure. The protection of Astroloy by its outer oxide layer against oxygen penetration was studied also, using alternative ¹⁶O₂ then ¹⁸O₂ oxidation. In association with STEM studies, it is shown that the outer oxide scale is not a real barrier against oxygen penetration and that inner precipitation of chronium (+ aluminium and titanium)-enriched oxides, takes place especially in the structure.     

Sulfidation properties of an Fe-26.6 at.% Cr alloy at temperatures of 973–1173 K in H2S-H2 atmospheres at sulfur Pressures 104–10−6 Pa

An investigation is reported on the sulfidation properties of an Fe-26.6 at. % Cr alloy at 973, 1073, and 1173 K in H₂S-H₂ atmospheres at sulfur pressures 10⁴ 10^–6 Pa. The sulfidation kinetics when plotted according to a parabolic relationship usually exhibited an early slow transient period before onset of parabolic kinetics. Scales contained up to three layers. A triplex (CrFe)S_x/(CrFe)₃S₄/-(FeCr)S_x scale was formed at high sulfur pressures (range I), a single-phase (FeCr)S_x or a duplex (CrFe)S_x/(FeCr)S scale at intermediate sulfur pressures (range II), and a single-phase (CrFe)S_x scale at low sulfur pressures (range III). These pressure ranges at 973 K were: range I = 10^–2Pa, 10^–2 > (range II) 10^–5 Pa, and range III .     

Kinetics and mechanisms of the oxidation of cobalt at 600–800°C

Two-phase layered scales comprising CoO and Co ₃O₄ formed on cobalt during oxidation at 600°, 700°, and 800°C and at oxygen partial pressures in the range 0.001–1 atm. The kinetics, which were obtained by thermogravimetric analysis, obeyed a parabolic rate law after an initial, non-parabolic stage of oxidation. The monoxide consisted of relatively large grains (10 ) and the spinel comprised small grains (3 ) for all conditions of oxidation. Grain boundary diffusion of cations played a significant role in the growth of the spinel layer. Thermogravimetric data and the steady-state ratio of the oxide layer thicknesses were employed to calculate the rates of thickening of the individual oxide layers and the rate of oxidation of CoO to Co₃O₄.     

Oxygen-18 tracer study of the passive thermal oxidation of silicon

This work focuses on the thermal oxidation of silicon near 1273 K using the double-tracer oxidation method. The results confirm that oxidation occurs by the transport of electrically neutral non-network oxygen through the interstitial space of the vitreous silica (-SiO₂) scale. Simultaneously, self- (or isotopic-) diffusion occurs in the network, resulting in characteristic isotopic fraction distributions near the gas-scale interface. The self-diffusion coefficients calculated from these profiles agree with those reported for tracer diffusion in -SiO₂, and the diffusion coefficient calculated from the scale growth is consistent with reported O₂ permeation data. An important parameter that describes the double-oxidation behavior is the ratio of the value of /(D _nt),where is the scale thickness grown during the second oxidation, D_n is the network self-diffusion coefficient for oxygen, and t is the time of the second oxidation.     

Oxidation of Ni–Al-Base Electrodeposited Composite Coatings. I: Oxidation Kinetics and Morphology at 800°C

The oxidation of nickel-matrix/aluminum-particle composite coatings was studied using thermogravimetric (TG) analysis in air at 800°C for up to 100 hr. Long-term oxidation behavior was investigated with furnace exposures up to 2000 hr. The coatings were applied to nickel substrates by the composite electrodeposition technique and vacuum heat treated for 3-hr at 825°C prior to oxidation testing. The heat-treated coatings contained a two-phase (Ni)+(Ni₃Al) microstructure and the overall coating composition was approximately 7 wt.% Al. Also examined were uncoated nickel substrates and bulk Ni–Al alloys containing 6.2, 9.0, and 14 wt.% Al. For all samples, mass-gain kinetics were obtained from thermogravimetric (TG) experiments and furnace exposures and the composition and morphology of the oxidation products were examined using optical microscopy, scanning-electron microscopy (SEM), electron-probe microanalysis (EPMA), and X-ray diffraction (XRD). An outer NiO layer and an inner -Al₂O₃ layer formed on the composite-coating surface. The addition of a small amount of Si (about 1–2 at.%) was found to have little effect on Ni–Al composite-coating oxidation behavior. The Ni–Al coatings behave similarly to bulk + (Ni₃Al) or single-phase (Ni₃Al). In addition, at lower temperatures, such as 800°C, the coatings benefit from a small grain size that enhances Al diffusion to the surface to form the protective alumina layer. Based on oxidation kinetics and morphology, a critical Al content of about 6 wt.% was found, below which internal oxidation and higher oxidation mass gains were observed.     

Oxidation Behavior of Nanocrystalline Al Alloys Containing 5 and 10 at.% Ti

The oxidation behavior of mechanically alloyed(MA) Al-Ti alloys containing 5 and 10 at.% Ti wereinvestigated at 500-600°C under 1 atm of oxygen. Ateach temperature, alloys oxidized linearly during the initial stage and later followed the parabolicrate law. During the initial stage, the oxidation ratesof nanocrystalline (50 nm) Al-Ti alloys were fasterthan those of conventional (200 nm) alloys. It is suggested that more grain boundaries innanocrystalline alloys provide more nucleation sites foroxides, so that the oxide scales grew faster as denseprotective layers. During the parabolic stage, the nanocrystalline alloys had greater oxidationresistance than conventional alloys because of the denseprotective layer. Oxide scales on both alloys consistedof a mixture of -Al₂O₃ andTiO₂ in the outer layer and-Al₂O₃ near the alloy as aprotective layer.     

The formation of solid solution oxides during internal oxidation

The diffusion processes occurring when binary alloys react with oxygen to form an oxide that contains both alloy components in solid solution, either exclusively as internal oxide or in combination with a surface scale, have been analyzed and compared with experimental results for Fe-Mn and Ni-Co alloys. The experimental results available for the Fe-Mn system were obtained under conditions of exclusive internal oxidation, and good agreement was obtained between calculated and experimental results. In the Ni-Co system, a surface scale and a zone of internal oxidation develop. Agreement between calculated and experimental depths of internal penetration is acceptable if the diffusivity of oxygen in the alloy is 3.8×10^–6 cm²/sec at 1100°C. Agreement between calculated and experimental concentration profiles is not very good.List of Principal Symbols B alloy component with higher affinity for oxygen - BO more stable scale component - a _O activity of oxygen - D _O diffusivity of oxygen in the alloy - D _O ^eff effective diffusion coefficient of oxygen in alloy - f volume or mass fraction of internal oxide - f _max maximum volume or mass fraction of internal oxide - G _AO , G _BO free energies of formation of oxides AO and BO, respectively - N _B mole fraction of component B in the alloy - N _B ^O bulk mole fraction of component B in the alloy - N _BO mole fraction of oxide BO in oxide phase - N _O atomic fraction of oxygen dissolved in alloy - N _O ^I ,N _O ^II atomic fraction of oxygen dissolved in alloy at the internal oxide-surface scale and alloy-internal oxide interfaces, respectively - R gas constant - r ratio of number of moles of precipitated oxide to total number of moles of metallic constituents in the alloy - T temperature - t time - X ₁,X ₂ positions of internal oxide-surface scale and internal oxide-alloy interfaces, respectively - x position coordinate - defined as [–1/RTG _BO ] - ₁, ₂ dimensionless rate constants describing rate of displacement of the internal oxide-surface scale and internal oxide/alloy interfaces, respectively - _O Henry's law activity coefficient for oxygen dissolved in alloy - defined as [–1/RT(G _BO –G _AO )]     

Composition Dependence of the Kinetics and Mechanisms of Thermal Oxidation of Titanium-Tantalum Alloys

The oxidation behavior of titanium-tantalumalloys was investigated with respective concentrationsof each element ranging from 0 to 100 wt.%. Alloys wereexposed to argon-20% oxygen at 800 to 1400°C. Theslowest oxidation rates were observed in alloys with5-20% Ta. The oxidation kinetics of alloys containingless than approximately 40% Ta were approximatelyparabolic. Pure Ta exhibited nearly linear kinetics. Alloys containing 50% or more Taexhibited paralinear kinetics. The activation energiesfor oxidation ranged between 232 kJ/mole for pure Ti and119 kJ/mole for pure Ta, with the activation energies of the alloys falling between these values andgenerally decreasing with increasing Ta content. Theactivation energies for oxidation of the end members, Tiand Ta, agree well with published values for the activation energies for diffusion of oxygenin -Ti and Ta. Scale formation in the alloys wasfound to be complex exhibiting various layers of Ti-,Ta-, and TiTa-oxides. The outermost layer of the oxidized alloys was predominately rutile(TiO₂). Beneath the TiO₂ grew avariety of other oxides with the Ta content generallyincreasing with proximity to the metal-oxide interface.It was found that the most oxidation-resistant alloys hadcompositions falling between Ti₅Ta andTi-15Ta. Although Ta stabilizes the -phase of Ti,the kinetics of oxidation appeared to be rate limited byoxygen transport through the oxygen-stabilized -phase.However, the kinetics are complicated by the formationof a complex oxide, which cracks periodically. Tantalumappears to increase the compositional range ofoxygen-stabilized -phase and reduces both the solubilityof oxygen and diffusivity of Ti in the - and-phases.     

Electrochemical Behavior of Nickel Hydride in Sodium Hydroxide Solutions

Electrochemical behavior of nickel hydride Ni₂H (-phase) is studied in 0.01–1 N NaOH by using common (VA) and cyclic (CVA) voltammetry, chronocoulometry, amperometry, and potentiometry. The limiting anodic and cathodic currents in VA and CVA curves are caused by the hydride decomposition via the following scheme Ni₂H -phase Ni + H_abs, where the intermediate -phase contains 0.003 at. % H, that is, one tenth that in the saturated -phase (0.03 at. %). At open circuit, the hydride maintains the equilibrium hydrogen potential. In the first 30 min, the hydrogen ionization from hydride is limited by solid-state diffusion and, later, the hydride decomposition. The anodic process involves ionization of sorbed hydrogen, while the cathodic process represents its electrochemical desorption: H₂O + H_ads + e H₂ + OH^–. The hysteresis observed in the cathodic CVA and open-circuit chronograms of the hydride potential in the beginning of anodic dissolution reflect the changes in the surface coverage of hydride with adsorbed hydrogen. The rate constant of hydride decomposition k, the rate Vitself, and the equilibrium constant K are as follows: k = k = 8 × 10^–5 s^–3, V = 3 × 10^–5 C/cm², and K = 10. The kinetic parameters of hydrogen electrochemical ionization from the hydride are b _a = 0.12 V and = 0.5.     

18O/SIMS characterization of the growth mechanism of doped and undoped α-Al2O3

Sequential oxidation experiments at 1200°C and 1500°C using¹⁶O and >95%¹⁸O-enriched environments were conducted on undoped and Y- and Zr-doped -NiAl and FeCrAl alloys. After oxidation, samples were analyzed by SIMS sputter depth profiling. At 1200°C, a clear pattern was established where the undoped -Al₂O₃ was found to grow by the simultaneous transport of both Al and O. Zr-doped -Al₂O₃ was found to grow mainly by the inward transport of oxygen. The profiles in all cases indicate O diffusion primarily by shortcircuit pathways. Results at 1500°C (only on -NiAl) indicated a similar behavior but were less conclusive. Y and Zr were found to segregate to the oxide grain boundaries at 1200°C and 1500°C. The segregation of these dopants is believed to impede the cation transport in the -Al₂O₃ scale and thereby change the oxidation mechanism.     

Two-phase ferritic-martensitic steels

Conclusions The results of laboratory and production experiments showed that the main condition for providing the necessary combination of properties of two-phase ferritic-martensitic steels (high tensile strength, low yield strength, high plasticity and work hardenability) is obtaining the specified quantity of the hardening phase (martensite) in the structure (20–28% M to obtain _t 550 MPa or 10–18% M for _t 450 MPa). The specified ratio of the structural constituents under conditions of mass production of two-phase ferritic-martensitic steels may be guaranteed only with the use of steels containing carbon and alloy elements within the necessary limits and also with strict observance of the heat-treatment cycle. Without the use of metallographic measurements as the criteria for obtaining the optimum structural condition in addition to the required values of strength and plasticity it is necessary to use the _0.2/_t ratio, which must not be greater than 0.5–0.6 with the absence of yield points on the tensile curve (without special temper rolling).As the result of the combination of work done in the Central Scientific-Research Institute of Ferrous Metallurgy together with plants of the Ministries of Ferrous Metallurgy and of the Automobile Industry at present the production is being introduced and experimental production lots of heat-treated two-phase steels of the following types with guaranteed mechanical properties are being supplied:At the Novolipetsk Metallurgical Combine cold-rolled 0.7–2.0-mm sheet of 06KhG(S)Yu and 06G2SYu steels with _0.2 = 260–320 MPa, _t 550 MPa, ₄ 30%, and _0.2/_t 0.5–0.6.At the Cherepovets Metallurgical Combine hot-rolled 2–6-mm sheet of 09G2(S) steel with _0.2 = 260–320 MPa, _t 550 MPa, ₄ 25%, and _0.2/_t 0.5–0.6.At the Beloretsk Metallurgical Combine (billets melted and rolled at the Cherepovets Metallurgical Combine) heat-treated cold-drawn wire up to 10.5 mm in diameter of 06KhGR steel with _0.2 250 MPa, _t 530 MPa, and ₁₀ 30%.For successful development of the production and use of two-phase ferritic-martensitic steels the combined work of the designers of machine building plants on determination of the most effective types of parts, of engineers on correction of the method taking into consideration the specifics of the properties of two-phase ferritic-martensitic steels, and of users on determining the service properties of parts of these steels is necessary. The fulfillment of such a combination of work will make possible th timely formulation of the further work of the metallurgical industry and determination of the required volume and product range of rolled products of two-phase ferritic-martensitic steels.I. P. Bardin Central Scientific-Research Institute of Ferrous Metallurgy. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 11, pp. 25–29, November, 1984.     

Oxidation ofγ′-Ni3Al andγ′-Ni3Al(Si) intermetallic compounds at low-oxygen pressures

The oxidation behavior of -Ni₃Al and -Ni₃Al(Si) (Ni₇₅Al₂₀Si₅) intermetallic compounds was studied at 1073 K and oxygen partial pressures of 59×10^–6 atm, 1.2×10^–14 atm, and 1.2×10^–19 atm by means of a manometric apparatus, and Rhines packs of NiO/Ni and FeO/Fe, respectively. Oxidation kinetics were determined either by recording weight gains or by measuring the internal-oxidation-zone depths. The structures and morphologies of oxides were also studied. Relatively low oxidation rates occurred for both compounds when oxidized in the manometric apparatus, while fast internal oxidation was observed for both compounds in the NiO/Ni pack. The fastest oxidation occurred in the -Ni₃Al compound in the FeO/Fe pack. A healing Al₂O₃ layer was formed on the surface of -Ni₃Al(Si) compound in the FeO/Fe pack, indicating a synergistic effect between the solute elements in the compound at the oxygen pressures corresponding to the dissociation of FeO. The oxidation rate was found to depend on the volume expansion associated with solute-atom oxidation.     

Improvement of the application of an electrochemical method for the determination of transport properties of anα-alumina scale. Part II: Influence of yttrium and palladium on alumina scales developed on aβ-NiAl alloy

An electrochemical method based on analysis of potential-current curves at various appliedP _{O 2} for an alumina scale developed at 1100°C on either undoped or yttrium-palladium doped -NiAl alloys was used to determine transport parameters in the scale. In the case of scales formed on undoped or Y-doped NiAl alloys, thet _i variations, at least in theP _{o 2} range experimentally studied, are very close: the scale consists of an outer domain characterized byt _i0.1, thent _i increases with decreasingP _{o 2}. The ionic conductivity in both cases shows a V-shape and decreases in the presence of Y. The presence of palladium notably modifies the shape of thet _i variation and induces an inversion of the shape of ₁ variation withP _{o 2}. The analysis of these results suggests that at lowP _{O 2} yttrium, localized in the inner part of the scale, decreasest _i, creates defect complexes and intergranular yttrogarnet precipitates, and prevents the outer part of the scale from contamination by nickel, which is thought also to give defect complexes when present. The variation of the oxygen chemical potential inside the scale shows a progressive variation of _o. The calculated oxidation constants are close to the experimental ones, which indicates that the alumina scale growth is ensured by grain-boundary diffusion of ionic species.     

Oxidation of Ni–Al-Base Electrodeposited Composite Coatings. II: Oxidation Kinetics and Morphology at 1000°C

The oxidation behavior of nickel-matrix/aluminum-particle composite coat–ings was studied using thermogravimetric (TG) analysis and long-term furnace exposure in air at 1000°C. The coatings were applied by the composite-electrodeposition technique and vacuum heat treated for 3 hr at 825°C prior to oxidation testing. The heat-treated coatings consisted of a two-phase mixture of (Ni)+ (Ni₃Al). During short-term exposure at 1000°C, a thin -Al₂O₃ layer developed below a matrix of spinel NiAl₂O₄, with -Al₂O₃ needles at the outer oxide surface. After 100 hr of oxidation, remnants of -Al₂O₃ are present with spinel at the surface and an inner layer of -Al₂O₃. After 1000–2000 hr, a relatively thick layer of -Al₂O₃ is found below a thin, outer spinel layer. Oxidation kinetics are controlled by the slow growth of the inner Al₂O₃ layer at short-term and intermediate exposures. At long times, an increase in mass gain is found due to oxidation at the coating–substrate interface and enhanced scale formation possibly in areas of reduced Al content. Ternary Si additions to Ni–Al composite coatings were found to have little effect on oxidation performance. Comparison of coatings with bulk Ni–Al alloys showed that low Al -alloys exhibit a healing Al₂O₃ layer after transient Ni-rich oxide growth. Higher Al alloys display Al₂O₃-controlled kinetics with low mass gain during TG analysis.     

The effect of thermodynamic parameters of the adsorption surfactants by lead on the lead’s corrosion rate

For several surfactants, free energies of adsorption (G_a⁰) are calculated relative to a symmetrically chosen standard state. It is shown that they characterize the strength of organic molecule-metal bond. The effect of these surfactants on active corrosion of lead in acetic acid solution, as well as correlation between the inhibition coefficient and the free energy of adsorption (G_a⁰), is studied by measuring the polarization resistance. The corrosion rate of lead in acetic acid solution with and without adding surfactants was studied gravimetrically. In this case, the corrosion rate was virtually independent of surfactant additives and their chemical nature; this is associated with the oxidation of lead surface.Translated from Zashchita Metallov, Vol. 41, No. 1, 2005, pp. 56–60.Original Russian Text Copyright © 2005 by Afanasev, Akulova, Yakovleva.     

Effect of alloying element content on ion-nitriding behavior of Fe-Ti alloys

The ion-nitriding behavior of iron alloys with a titanium content of between 1.07 and 2.58 wt.% was investigated in the -phase region. The behavior was found to be analogous to the internal oxidation behavior of iron alloys: An internal-nitriding layer, where small TiN precipitates are dispersed, as well as a very thin surface layer of -Fe₄N were formed. A parabolic rate law holds for growth of the internal-nitriding layer. The kinetics of growth of the internalnitriding layer is discussed according to the rate equation of internal-oxidation, giving the diffusion coefficient of nitrogen, D _N ^app , in the layer. The measured D _N ^app decreases as the volume fraction of TiN, f, increases, indicating that the diffusion of nitrogen is apparently inhibited by the existence of TiN precipitates. Furthermore, the diffusion coefficient of nitrogen in -iron was evaluated by extrapolating D _N ^app to f=0, being in good agreement with that reported previously. The f-dependence of D _N ^app is discussed in terms of the effective area for diffusion of nitrogen in -iron.     

Improvement of the application of an electrochemical method for the determination of transport properties of an alumina scale. Part I: Alumina scale on aβ-NiAl alloy

An electrochemical method based on analyzing the potential-current curves, plotted at various applied P_{O 2} for an alumina scale developed at 1100°C on a -NiAl alloy, was used to determine transport parameters in the scale. The variation of V_o vs P_{O 2} indicates that the scale consists of an inside zone characterized by an ionic-transport number t_i0.4 and an outside part with t_i0.1. This leads to a value of the oxygen pressure at the NiAl/Al₂O₃ interface 2×10^–27 atm, not far from the equilibrium value. The variation of the oxygen chemical potential inside the scale was determined, showing a steep variation of µ_O in the middle of the scale. The calculated oxidation constant is close to the experimental one, which would indicate that mainly charged species are responsible for the alumina scale growth.     

The kinetics and mechanism of catastrophic oxidation of metals

A set of independent methods has been used to study the catastrophic oxidation of copper in the system Cu–Me_xO_y (where Me is Bi, W, Mo, or V). Two stages of the catastrophic oxidation have been revealed: a rapid stage (K10^–4 kg² m^–4 sec^–1) and a super rapid stage when the metal is oxidized within1–5 sec. The weight ratios of metal to oxidizer and the partial oxygen pressure for the superrapid copper oxidation have been established. The mechanism of the catastrophic oxidation of metals is considered.     

TEM Study of Scale Microstructures Formed on Ni–10Cr and Ni–10Cr–5Al Alloys with and Without Y Addition

A study was conducted to investigate the effect of Y addition on the isothermal-oxidation behaviors of Ni–10Cr, Ni–10Cr–0.5Y, Ni–10Cr–5Al, and Ni–10Cr–5Al–0.5Y alloys. The alloys were oxidized in air for 50 hr at 1000°C. The oxides formed on the alloys were characterized using primarily cross-sectional transmission-electron microscopy techniques along with light microscopy, scanning-electron microscopy, and X-ray diffraction. Although the Al-containing alloys showed comparatively better oxidation behavior, all alloys exhibited nonprotective scaling, as suggested by the thick oxides formed. The major component of the outer oxide was NiO. However, modified Y-containing alloys formed protective layers (i.e., -Cr₂O₃ for NiCrY and -Al₂O₃ for NiCrAlY) at the scale–alloy interface following the nonprotective scaling. The spalling resistance of the modified Y-containing alloys was better than their counterpart unmodified Y-free alloys, while their overall oxidation mechanism remained unchanged after Y addition.