Wrought cobalt- base superalloys

Wrought cobalt-base superalloys are used extensively in gas turbine engines because of their excellent high-temperature creep and fatigue strengths and resistance to hot corrosion attack. In addition, the unique character of the oxide scales that form on some of the alloys provides outstanding resistance to high-temperature sliding wear. This article provides a review of the evolutionary development of wrought cobalt-base alloys in terms of alloy design and physical metallurgy. The topics include solid-so-lution strengthening, carbide precipitation characteristics, and attempts to introduce age hardening. The use of PHACOMP to enhance thermal stability characteristics and the incorporation of rare-earth ele-ments to improve oxidation resistance is also reviewed and discussed. The further development of cobalt-base superalloys has been severely hampered by past political events, which have accentuated the strategic vulnerability of cobalt as a base or as an alloying element. Consequently, alternative alloys have been developed that use little or no cobalt. One such alternative, Haynes® 230^TMalloy, is discussed briefly.     

Steam Oxidation Resistance of Advanced Steels and Ni-Based Alloys at 700 °C for 1000 h

The aim of the work was to investigate corrosion resistance of highly alloyed steels and Ni-based alloys in a steam atmosphere for 1000 h at 700 °C. In these steam oxidation experiments, two solid solution strengthened alloys; Haynes^® 230^®, 617 alloy, two gamma-prime (γ′) strengthened alloys; 263 and Haynes^® 282^® and three Cr+Ni- rich stainless steels: 309S, 310S and HR3C austenitic steels were exposed. The study showed that the materials exposed commonly developed thin oxide scales; in Ni-based alloys, these consisted of mainly MnCr₂O₄ spinels and Cr₂O₃, with the exception of 617 alloy where NiCr₂O₄ spinels and Cr₂O₃ were found. In Fe-based alloys, Cr₂O₃, MnCr₂O₄ spinels, Fe,Mn(SiO)₄, and finally Fe₃O₄ developed. No evaporation of chromia has been found within 1000 h test period. Furthermore, the development of TiO₂ was not observed into a large extent in Haynes^® 282^® and 263 alloy, in contrast to the study performed at 800 °C under the same steam environment conditions.     

Long-Term Performance of High-Temperature Foil Alloys in Water Vapor Containing Environment. Part II: Chromia Vaporization Behavior

The long-term oxidation resistance of various Ni- andFe-base foil alloys was studied in air plus 10 vol.% water vapor exposed for 360 days at 760 and 871 °C, and has been reported in the Part I. The long-term Cr-consumption rate for all alloys was measured to develop a better understanding of oxidation behavior in water vapor containing environment. It was found that Cr-consumption rates varied from alloy-to-alloy, but results were generally in accordance with oxidation performance of the alloys. Subsequently, the measured Cr-consumption rate for HAYNES^®230^® alloy was compared to a practical approach of Cr₂O₃ vaporization rate calculations proposed by Young and Pint based on classical gas transport theory.     

Steam Oxidation Behavior of Advanced Steels and Ni-Based Alloys at 800 °C

This publication studies the steam oxidation behavior of advanced steels (309S, 310S and HR3C) and Ni-based alloys (Haynes^® 230^®, alloy 263, alloy 617 and Haynes^® 282^®) exposed at 800 °C for 2000 h under 1 bar pressure, in a pure water steam system. The results revealed that all exposed materials showed relatively low weight gain, with no spallation of the oxide scale within the 2000 h of exposure. XRD analysis showed that Ni-based alloys developed an oxide scale consisting of four main phases: Cr₂O₃ (alloy 617, Haynes^® 282^®, alloy 263 and Haynes^® 230^®), MnCr₂O₄ (alloy 617, Haynes^® 282^® and Haynes^® 230^®), NiCr₂O₄ (alloy 617) and TiO₂ (alloy 263, Haynes^® 282^®). In contrast, advanced steels showed the development of Cr₂O₃, MnCr₂O₄, Mn₇SiO₁₂, FeMn(SiO₄) and SiO₂ phases. The steel with the highest Cr content showed the formation of Fe₃O₄ and the thickest oxide scale.     

Metallic interconnects for solid oxide fuel cell: Performance of reactive element oxide coating during long time exposure

One of challenges in improving the performance and cost‐effectiveness of SOFCs (solid oxide fuel cells) is the development of suitable interconnects materials. Chromia‐forming alloys and especially ferritic stainless steels, like Crofer22APU, are considered to be among the most promising candidate materials as interconnects in SOFC stacks. However, the performance of chromia‐forming materials can be limited by the low electronic conductivity of the oxide scale (high ASR – area specific resistance – value). Such degradation are unacceptable regarding the long‐term operation (>40 000 h). A previous study 1 demonstrated that in air, the addition of a nanometric reactive element oxide (La₂O₃) layer applied by metal organic chemical vapor deposition (MOCVD) drastically improved both corrosion rate and electrical properties of Crofer22APU and Haynes230 alloys for 100 h at 800 °C. In this present study coating performances were checked after 10 months (7500 h) and 20 months (15 000 h) at 800 °C in air. The corrosion products were carefully analyzed by SEM, EDX, and XRD. ASR measurements are realized after long time exposure. This study demonstrates that the Crofer22APU alloy has a good oxidation resistance after 15 000 h in air but this alloy has an ASR value equal to 0.370 Ω cm². The coatings composed of a thin reactive element oxide such as La₂O₃ resulted in an important improvement in the high temperature oxidation resistance; the ASR values are equal to 0.154 Ω cm². Haynes230 alloy has a better oxidation resistance but the formation of an insulating Al₂O₃/SiO₂ layer could be detrimental.     

Oxidation of a chromia-forming nickel base alloy at high temperature in mixed diluted CO/H2O atmospheres

Corrosion of a chromia-forming nickel base alloy, Haynes 230^®, has been investigated under impure helium containing a few Pa of CO and H₂O at 900 °C. It has been found that this alloy reacts simultaneously with CO and H₂O. Oxidation by CO has been revealed to occur mainly in the first hours. CO diffuses through the scale via short-circuit pathways and oxidizes Al, Cr and Si at the oxide/metal interface. Kinetics of CO oxidation has been investigated and several rate limiting steps are proposed. In the long term, H₂O is the major oxidant of chromia-forming nickel base alloys in impure helium.     

Substrate Effect on the High-Temperature Oxidation Behavior of a Pt-Modified Aluminide Coating. Part I: Influence of the Initial Chemical Composition of the Coating Surface

The effect of substrate composition on the oxidation behavior of the industrial NiPtAl coating RT22 ^TM was investigated by studying the isothermal and cyclic-oxidation behavior of this coating deposited on three different Ni-base superalloys (CMSX-4 ^TM , SCB ^TM and IN792 ^TM ). Isothermal tests were performed at 900, 1050 and 1150°C for 100 h. Cyclic oxidation was studied at 900°C with a holding time of 300 h for up to 52 cycles (i.e, 15,600 h at 900°C). Thermogravimetric analysis (TGA), X-ray diffraction (XRD), microstructural and analytical investigations using scanning-electron microscopy (SEM) and transmission-electron microscopy (TEM), both equipped with energy-dispersive X-ray spectroscopy (EDS) were performed to characterize the oxidation behavior of the systems studied. An effect of the superalloy substrate was observed and related to the initial chemical composition of the coating surface which depends on the superalloy and the associated heat treatments. The effect of the substrate’s alloying elements is discussed. Particularly the influence of Ti and Ta that formed rutile-type oxides inducing oxide-scale cracking and spallation. The excellent resistance to cyclic oxidation of the coating systems studied at 900°C was also demonstrated from very long duration tests of 15,600 h.     

The one-step synthesis of dense titanium-carbide tiles

Self-propagating, high-temperature synthesis (SHS), which is an attractive process for forming alloys, cermets, ceramics, and composites, has been combined with a rapid quasi-isostatic consolidation technology called the Ceracon^TM process. This one-step synthesis and densification route has been applied to the rapid fabrication of large 15 cm × 15 cm × 2.5 cm titanium carbide tiles. A cost analysis of this process based on prototype quantities shows that the cost of the process is 30–50% of that of current manufacturing processes.     

Role of phase diagram calculations in the Design of Fe-TM-B alloys

Previous work on rapidly solidified Fe-TM-B alloys (whereTM are selected transition metals) has shown thatin situ metal matrix composites can be produced with a high elastic modulus and reasonable ductility. To understand the effect of various microstructural features, it is necessary to prepare model alloys that contain well-defined proportions of selected pairs of phases with specified compositions. To this effect, phase diagram assessments on the relevant ternary and quaternary systems were performed. This paper shows how the calculations have been used to design a matrix of test compositions to allow an evaluation of the contribution made by (1) a ferritic, (2) a martensitic, or (3) an austenitic matrix, combined with the different boride present in the alloys. Examination of the alloys selected from the Fe-Cr-B and Fe-Cr-Mo-B systems has confirmed the predicted phase equilibria. The paper helps demonstrate the role played by phase diagram calculations in the design of the test alloys which has been done in a highly cost-effective manner. This paper was presented at the International Phase Diagram Prediction Symposium sponsored by the ASM/MSD Thermodynamics and Phase Equilibria Committee at Materials Week, October21–23,1991, in Cincinnati, Ohio. The symposium was organized by John Morral, University of Connecticut, and Philip Nash, Illinois Institute of Technology     

Sn对Mg-5Li合金铸态和挤压态组织的影响

在氩气保护气氛下熔炼,得到Mg-5Li-xSn（x=0.15,0.25和0.65,质量分数）系列合金。通过光学显微镜、扫描电镜、X射线衍射仪和能谱仪分析合金的显微组织。结果表明,Mg-5Li合金中添加的Sn元素可以起到明显的晶粒细化作用,当Sn含量从0.15%增加到0.65%时,铸态合金的平均晶粒尺寸从556μm细化到345μm,相应的挤压态合金的晶粒从33μm减小到23μm。近似网状的第二相Mg2Sn分布在铸态合金的晶界上,挤压之后,颗粒状的Mg2Sn主要分布在晶粒内部。这些金属间化合物在挤压动态再结晶中可以作为有效的形核质点,从而起到细化晶粒的作用。     

Hold-time effect on the elevated-temperature crack growth behavior of solid-solution-strengthened superalloys

The fatigue crack growth behavior of two solid-solution-strengthened superalloys, Ni-based HAYNES^® 230 and HASTELLOY^® X, was studied at 816 and 927 °C in laboratory air. The fatigue crack growth tests were conducted following a baseline triangular waveform of 0.33 Hz. Various hold times were introduced at the maximum load to study the hold-time effect. Fracture mechanics parameters, K, C¹, C_t, and (C_t)_avg, were applied to correlate the crack growth rates at different temperatures for both HAYNES 230 and HASTELLOY X alloys. For both alloys, the fatigue cracking path was mainly transgranular at 816 and 927 °C. The cracking path became dominantly intergranular if the hold time increased to 2 min, indicating that the time-dependent creep damage mechanisms were in control. When the time-dependent damage dominated (temperature ⩾816 °C and hold time ⩾2 min), the crack growth rates can be correlated with C_t or (C_t)_avg parameters. The C_t and (C_t)_avg parameters were capable of consolidating data from different temperatures and different alloys.     

Structure formation and properties of rapidly quenched aluminum alloys

Aluminum alloys in a rapidly quenched state are studied by four technological methods, namely, fabricating foils and wire with unique properties, fabricating granulated alloys, surface strengthening and alloying articles by a laser or an electron beam treatment, and realizing superplasticity. In rapid quenching of alloys, crystallization occurs with cooling the melt at a rate of at least 10³–10⁴ K/sec. Such cooling can be realized in practice by several methods, for example, by drop crystallization on a rotating heat-conducting substrate, spraying of the melt in a cooling medium, quenching a thin foil on cooled rolls in liquid rolling, or melting a thin surface zone of a quite massive part. The present paper describes a study of flakes of aluminum alloys obtained by the method of drop crystallization. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 5, pp. 31–34, May, 1997.     

Phase transformations in heat treatment of Al−Li alloys and optimization of operational properties of semifinished products from these alloys

In recent years Al−Li alloys have become very popular in the aircraft industry. These alloys are characterized by a high specific strength and rigidity which increases substantially the load ratio and fuel efficiency of the parts. As a rule, Al−Li alloys are used in an underaged state, which provides a high, ductility over the height of the semifinished products. However, the threshold stress (σ_cr) in corrosion cracking in this state does not usually exceed 25–50 N/mm², and the yield strength is not always satisfactory. The results of a study of the mechanical properties and corrosion resistance of some Al−Li alloys are described and the ways for optimizing these properties are described on the basis of the phase transformations in the alloys. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 1, pp. 24–30, January, 1998.     

Mechanical properties of the cast intermetallic alloy Ti-43Al-7(Nb,Mo)-0.2B (at %) after heat treatment

A new approach to obtaining fine-grained structure in intermetallic-compound alloys such as γ-TiAl + α₂-Ti₃Al has been suggested. This approach is based on the use of alloys that solidify as the β phase, which contain β-stabilizing additives such as Nb and Mo and are characterized by the small size of crystallites already in the cast state; in these alloys, a simple heat treatment makes it possible to substantially decrease the fraction of the lamellar component and to increase the content of the β(B2) phase. It is shown on the example of the Ti-43Al-7(Nb,Mo)-0.2B (at %) alloy that this heat treatment ensures superplastic properties in the material in the temperature range of T = 1050–1130°C at a deformation rate $ \dot \varepsilon A new approach to obtaining fine-grained structure in intermetallic-compound alloys such as γ-TiAl + α₂-Ti₃Al has been suggested. This approach is based on the use of alloys that solidify as the β phase, which contain β-stabilizing additives such as Nb and Mo and are characterized by the small size of crystallites already in the cast state; in these alloys, a simple heat treatment makes it possible to substantially decrease the fraction of the lamellar component and to increase the content of the β(B2) phase. It is shown on the example of the Ti-43Al-7(Nb,Mo)-0.2B (at %) alloy that this heat treatment ensures superplastic properties in the material in the temperature range of T = 1050–1130°C at a deformation rate = 1.7 × 10⁻⁴ K⁻¹. Under these temperature-strain-rate conditions, relative elongations such as δ = 160–230% and low flow stresses such as σ = 36–100 MPa characteristic of superplastic flow have been obtained. It has been shown for the first time for the intermetallic γ-TiAl + ga₂-Ti₃Al alloy that a sheet semifinished product cut out from an ingot subjected only to heat treatment can have plasticity acceptable for press forming. Original Russian Text ? V.M. Imayev, R.M. Imayev, T.G. Khismatullin, 2008, published in Fizika Metallov i Metallovedenie, 2008, Vol. 105, No. 5, pp. 516–522. The author is also known by the name Imayev. The name used here is a transliteration under the BSI/ANSI scheme adopted by this journal.—Ed.     

On the Stress corrosion cracking behaviour of aluminium alloy sheet in an aqueous solution of 3% NaCl + 0.3% H2O2

The stress corrosion cracking (SCC) behaviour of aluminium alloy sheet was investigated in the long transverse direction using the slow strain rate testing technique. The synthetic environment used was an aqueous solution of 3% NaCl + 0.3% H₂O₂. No indications of SCC sensitivity are observed for the alloys 2024-T351, 8090-T81, and 2091 CPHK-T8X. The alloys 2091 T8X and 6061-T4 are found to be susceptible to intergranular stress corrosion cracking. At strain rates below 4 · 10^?7 s^?1, the slow strain rate testing technique indicates a slight SCC sensitivity with alloy 6013-T6. Fractography reveals transgranular stress corrosion cracking. Transgranular stress corrosion cracking is also observed with 6061-T4 specimens which are dynamically strained at strain rates below 5 · 10^?7 s^?1. Aqueous 3% NaCl solution with hydrogen peroxide addition promotes pitting and intergranular corrosion. The loss of ductility caused by these corrosion processes interferes with the evaluation of the results of the slow strain rate testing technique.     

Metal dusting behaviour of several nickel‐ and cobalt‐base alloys in CO?H2?H2O atmosphere

The metal dusting behaviour of total 11 nickel‐ and cobalt‐base alloys at 680 °C in a gas of 68%CO? 31%H₂? 1%H₂O (a_C = 19.0, = 5.4 × 10^?25 atm) was investigated. All samples were electropolished and reacted in a thermal cycling apparatus. On the basis of their reaction kinetics, these alloys can be classified into three groups: the first, with rapid carbon uptake and significant metal wastage, consists of alloys of relatively high iron content (AC 66, 800H and NS‐163); the second, with intermediate rates, consists of some Co‐base alloys (HAYNES 188, HAYNES 25 and ULTIMET) and the third, with very low reaction rates, consists of nickel‐base alloys with high chromium levels (601, HAYNES HR 160, 230, G‐35 and EN 105). An external chromia scale protected group 3 alloys from carburization and dusting. However, this protective scale was damaged and not rehealed for group 1 and group 2 alloys, allowing carbon attack. In all cases, coke deposited on the surface with two typical morphologies: filaments and graphite particle clusters. Subsurface spinel formation in high iron‐content alloys led to rapid dusting due to the significant volume expansion. Alloy carbon permeability was calculated from a simple law of mixtures, and shown to correlate reasonably well with initial dusting rate except for one cobalt‐base alloy in which iron spinel formation was significant.     

The kinetics of phosphate film formation on the type MA 21 magnesium-lithium alloys

Kinetics of phosphating MA 21 magnesium-lithium alloys in a zinc-nitrate-phosphate solution (0.25 M Zn²⁺+0.34M PO ₄ ³⁻ +0.17M NO ₃ ⁻ +0.02M F⁻, pH = 2.7) at 25°C is studied with various surface preparation procedures. The effect of the alloy surface pretreatment in titanium-containing activating solutions on the electrochemical process rate and the initial stage of the phosphate film crystallization is revealed. Peculiarities of the phosphate film growth during different stages of the magnesium-lithium alloy phosphating are elucidated. Combining the activating pretreatment with zinc-phosphate bath allows accelerated applying finegrain thin phosphate conversion layers with enhanced protective effect on magnesium-lithium alloys. Original Russian Text ? L.A. Isaicheva, N.M. Trepak, L.K. Il’ina, A.L. L’vov, 2006, published in Zashchita Metallov, 2006, Vol. 42, No. 4, pp. 425–429.     

Korrosionsermüdung von Aluminium- und Magnesium-Gußlegierungen

Corrosion fatigue of cast aluminium- and cast magnesium alloys The fatigue properties of the aluminium cast alloys AlSi7Mg and AlSi10Mg, magnesium high pressure die-cast AZ91 hp and AM60 hp, as well as low-pressure permanent mould casting AZ91 hp were determined at numbers of cycles to failure between 10⁵ and 10⁹ in ambient air and in saltwater spray. The investigations were performed at constant amplitudes and in addition at varying load amplitudes for aluminium cast alloy AlSi10Mg and low-pressure permanent mould casting AZ91 hp with a high-frequency testing facility (20 kHz ultrasound). The SN-curves of the aluminium cast alloys and of the magnesium low-pressure permanent mould casting are steeper in corrosive environment than in laboratory air. No endurance limit could be detected up to 10⁹ cycles in both environments. The magnesium high pressure die-cast alloys do not have an endurance limit in saltwater spray, whereas an endurance limit could be detected in laboratory air. The life time curves for in-service loading conditions are about parallel to the SN-curves for ambient air, but steeper for corrosive environment. The reason for the reduced fatigue properties in corrosive environment are accelerated crack initiation and higher crack propagation rates. Material defects may lead to a large scatter of the fatigue data for testing in air as well as in saltwater spray. Corrosion pits are formed on the surface of both light-weight alloys at higher number of cycles. They become crack initiation sites.     

Deformable alloys based on the Al - Mg - Se system

Aluminum alloys with magnesium that are deformable and not strengthened by heat treatment are widely used as a structural meterial due to their good weldability, high corrosion resistance, and high ductility. However, even the strongest alloys of this system, containing 5–6% Mg, have low strength characteristics. For example, annealed sheets of alloy AMg6 have _b = 340 N/mm² and _0.2 = 180 N/mm². The present work concerns the possibility of strengthening AI - Mg alloys by an additional alloying with scandium.Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 6, pp. 33 – 36, June, 1996.     

The high-temperature corrosion of Fe-Nb alloys in a H2/H2O/H2S gas mixture

The corrosion of Fe-Nb alloys containing up to 40 wt.% Nb has been studied over the temperature range 600–980°C in a mixed gas of constant composition having sulfur and oxygen pressures ranging from 10^–8 to 10^–4 atm. and from 10^–27 to 10^–18 atm., respectively. All alloys were two-phase, consisting of an Fe-rich solid solution and an intermetallic compound, Fe₂Nb. The scales formed on the Fe-Nb alloys were duplex, consisting of an outer layer of iron sulfide (FeS) and an inner complex layer of Fe_xNbS₂(FeNb₂S₄ or FeNb₃S₆), FeS and unreacted Fe₂Nb. No oxides were detected at any temperature. The addition of Nb reduced the corrosion rate. The corrosion kinetics of Fe-Nb alloys followed the parabolic rate law, regardless of alloy composition and temperature. Platinum markers, attached to the original alloy surfaces, were always located at the interface between the inner and outer scales.