Nondestructive evaluation of cracks in a paramagnetic specimen with low conductivity by penetration of magnetic fluid

This paper proposes a nondestructive testing method for detecting a crack in an Inconel^® specimen. Because of the low conductivity and permeability of Inconel, cracks are difficult to detect using the electromagnetic method, until now. In our method, magnetic fluid penetrates a surface crack in the Inconel specimen, and the distribution of the magnetic field is measured using a scan-type magnetic camera. The magnetic field leakage and induced current methods were examined. With our method, cracks can be evaluated. This research can be used to detect and evaluate cracks on non-ferrous and non-metallic materials such as ceramic, composite, and synthetic resins.     

Grain boundary energy vs. misorientation in Inconel® 600 alloy as measured by thermal groove and OIM analysis correlation

Samples of Inconel^® 600 foil were annealed to form a bamboo structure with grain boundary grooves. Grain boundary misorientation was measured using OIM and groove root angle was measured by AFM. Analysis of the data showed a misorientation vs. grain boundary plot that agrees with literature trends for FCC materials.     

Effect of Ca addition on the as-cast microstructure and creep properties of Mg-5Zn-5Sn magnesium alloy

The effect of Ca addition on the as-cast microstructure and creep properties of Mg-5Zn-5Sn magnesium alloy was investigated. The results indicate that adding 1.0 wt.% Ca to Mg-5Zn-5Sn alloy can effectively refine the as-cast microstructure of the alloy, and the CaMgSn phase with high thermal stability is formed in the alloy. In addition, adding 1.0 wt.% Ca to Mg-5Zn-5Sn alloy can also improve the creep properties of the alloy. After adding 1.0 wt.% Ca to Mg-5Zn-5Sn alloy, the second creep rate of the alloy at 150°C and 50 MPa for 100 h decreases from 4.67 × 10⁻⁸ to 1.43 × 10⁻⁸ s⁻¹. The strengthening mechanism is mainly attributed to the microstructural refinement and the formation of CaMgSn phase.     

Einfluß der Aufkohlung auf das Kriechverhalten einer FeNiCr-Hochtemperaturlegierung

Effects of Carburization of the Creep Behaviour of a FeNiCr-High Temperature Alloy Incoloy 800 and Incoloy 800 doped with 1% Nb were carburized at 1000 °C in CH₄–H₂ to 0.83% C (mass content). The undoped alloy shows relatively coarse large M₂₃C₆ carbides at the grain boundaries, the alloy with 1% Nb has mainly fine carbides in the grains. Creep experiments were performed with the carburized and uncarburized specimens at 1000 °C, in which creep rates were attained in the range 10^?9… 10^?7 s^?1 of secondary creep. The stress dependence of the creep rate indicates two creep mechanisms: diffusion creep at low stresses and dislocation creep at high stresses. The diffusion creep is faster for both alloys after carburization. The dislocation creep is retarded by carburization for the undoped alloy. At about equal creep rate ε = 10^?7 s^?1 the carburized specimens have a longer lifetime. The fracture is brittle for Incoloy 800 in the uncarburized and carburized state, characterized by void and crack formation and poor reduction in area. The fracture of the carburized Incoloy 800 with 1% Nb is rather ductile with less void formation. The results indicate that carburization does not deteriorate the creep behaviour of the FeNiCr alloy if the reached carbon content is not too high. An addition of Nb is very favorable for the creep properties after carburization.     

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.     

Oxidation and Creep Limited Lifetime of Kanthal APMT®, a Dispersion Strengthened FeCrAlMo Alloy Designed for Strength and Oxidation Resistance at High Temperatures

Kanthal APMT^® is an FeCrAlMo alloy optimized for continuous service up to 1,250 °C (~2,300 °F). Rapid solidification powder metallurgy applied on this FeCrAlMo composition provided an oxide dispersion strengthened microstructure. The alloy exhibits an attractive combination of resistance to oxidation and corrosion and excellent form stability. In this study, oxidation and corrosion properties were investigated, as well as mechanical properties at elevated temperature. It was shown that an adherent alumina layer on the alloy surface formed during service that provided excellent resistance to corrosion attacks in most industrial atmospheres and gave great advantages compared to chromia forming high temperature Ni-base alloys in terms of maximum operating temperature and life. Focus was set on oxidation and creep properties but also other important aspects are discussed.     

Effect of exposure in steam or argon on the creep properties of Ni‐based alloys

Although expensive, Ni‐based superalloys are of interest for the ultrasupercritical steam program because of their good creep and oxidation resistance at temperatures above 700 °C. As the effect of steam oxidation on the alloy mechanical properties is unknown, creep specimens of alloy CCA617, 740, and 230 were pre‐oxidized for 2000 and 4000 h in steam at 800 °C before testing in air at the same temperature. Compared with as fabricated material, exposure in steam decreased the creep properties of alloy CCA617, had less of an effect on alloy 740, and did not affect alloy 230. Testing of a specimen repolished after steam exposure as well as microstructural observations indicate that the oxidation affected zone at the specimen surface is not responsible for the properties degradation. Surprisingly, a similar time anneal in an inert environment resulted in a drastic decrease of creep rupture life and an increase in the creep rate and elongation at rupture. Transmission electron microscopy analysis revealed that the mechanical properties decrease for alloy CCA617 is related to the absence of γ′ precipitates within the grains.     

Deformation of a multiphase Mo–9.4Si–13.8B alloy at elevated temperatures

A 3-phase silicide alloy, Mo–9.4Si–13.8B (at.%), was prepared via powder metallurgy techniques. The tensile properties of the alloy at elevated temperatures were evaluated in vacuum at temperatures ranging from 1350 to 1550°C and strain rates ranging from 5.0×10⁻⁴ to 1×10⁻³ s⁻¹. The alloy was found to exhibit a stress exponent of about 2.8 and relatively a high activation energy 740 kJ/mol. Also, it displayed unusually large tensile ductility (>100%) at T>1400°C. The deformation mechanism as well as large ductility are discussed in the light of the microstructural observations. The alloy has a very good mechanical strength at elevated temperatures, comparable to some of the most advanced tungsten-based alloys.     

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.     

Steady-state creep of a particulate SiC/6061 Al composite

The creep behavior of a 10 vol.% silicon carbide particulate reinforced 6061 Al composite produced by powder metallurgy (PM) has been examined by creep tests in both tension and compression at 400°C. The tensile creep data covering minimum creep rates of the orders 10⁻⁹ to 10⁻⁴/s show an apparent stress exponent n_app≈13, but a comparison with compressive creep data reveals that some high strain-rate data in tension are due to the transition to the tertiary stage. Analysis of the data is made only for the steady-state creep rate, together with that for an unreinforced PM 6061 Al alloy, by incorporating a threshold stress. This gives a stress exponent n=3 for the matrix alloy, whereas the composite data show such a trend that the n value gradually changes from 3 to 1 as the effective stress increases. A new method of steady-state creep data analysis is formulated by taking account of the interface-confined diffusional flow and thereby the finding above is reasonably assessed.     

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.     

Tension–compression creep asymmetry in a turbine disc superalloy: roles of internal stress and thermal ageing

The tension and compression creep behaviour of an as-received and pre-aged IN100 disc alloy have been characterised in order to validate a previous hypothesis that the unusual response of low and even negative initial creep rates in tension was caused by the presence of an internal stress field within the alloy. Absolute values of initial creep rates in compression were found to be much greater than in tension and this asymmetric creep response is conclusive proof of the presence of an internal compressive stress field in the alloy matrix. The asymmetry was virtually removed by pre-ageing prior to creep and this is attributed to the decay of the internal stress. These features have been simulated using a microstructure-based creep model incorporating an evolving internal stress field. The model also simulates the additional (and complicating) reduction in general creep strength that is thought to be due to coarsening and dissolution of the smallest particles of the tri-modal γ^′ distribution in the alloy. The net consequence of these two competing thermal processes is that the short-term creep response is dominated by the initial magnitude of the internal stress field whereas coarsening and dissolution of the smallest γ^′ particles determines the long-term behaviour.     

Hot deformation behavior of novel imitation-gold copper alloy

The hot deformation behavior of a novel imitation-gold copper alloy was investigated with Gleeble–1500 thermo-mechanical simulator in the temperature range of 650–770 °C and strain rate range of 0.001–1.0 s^?1. The hot deformation constitutive equation was established and the thermal activation energy was obtained to be 249.60 kJ/mol. The processing map at a strain of 1.2 was developed. And there are two optimal regions in processing map, namely 650–680 °C, 0.001–0.01 s^?1 and 740–770 °C, 0.01–0.1 s^?1. Optical microscopy was employed to investigate the microstructure evolution of the alloy in the process of deformation. Recrystallized grains and twin crystals were found in microstructures of the hot deformed alloy.     

The controlling creep processes in TiAl alloys at low and high stresses

The creep response of a nearly-lamellar Ti–47Al–4(W, Nb, B) alloy is studied at 760 °C in a wide stress range 100–500 MPa. The alloy exhibits excellent creep resistance with a minimum creep rate of 1.2×10⁻¹⁰/s at 100 MPa and the time to 0.5% creep strain of 1132 h at 140 MPa. The controlling creep process is probed by analysis of the post-creep dislocation structure and by observation of incubation period during stress reduction test. The results indicate that creep is controlled by dislocation climb at low stresses (Class II type) and by jog-dragged dislocation glide at high stresses (Class I type). The transition from Class II to Class I type creep occurs at about 180 MPa. The excellent creep resistance of the studied alloy compared to other W containing TiAl alloys is attributed to its highly stable lamellar microstructure consisting eventually of coarse gamma laths.     

Creep mechanism of as-cast Mg-6Al-6Nd alloy

The creep mechanism of as-cast Mg-6Al-6Nd alloy was studied. The stress exponent for creep is 5.8 under the applied stresses of 50–70 MPa at 175°C. The activation energy for creep is 189 kJ·mol⁻¹ under the applied stress of 70 MPa in the range of 150–200°C. The true stress exponent and threshold stress for creep are calculated as 4.96 and 10.2 MPa, respectively. The true stress exponent indicates that its creep mechanism belongs to the dislocation climb-controlled creep, which is in agreement with the microstructure changes before and after creep. The high value for stress exponent is attributed to the interaction of Al₁₁Nd₃ phase with dislocations. The activation energy is more than the self-diffusion activation energy of Mg, which is attributed to the load transfer taking place from the matrix to Al₁₁Nd₃ phase during creep.     

Infrared brazing of Ti50Ni50 shape memory alloy using two Ag–Cu–Ti active braze alloys

Microstructural evolution, shape memory effect and shear strength of infrared brazed Ti₅₀Ni₅₀ shape memory alloy using Cusil-ABA^® and Ticusil^® active braze alloys are investigated. The Ag–Cu eutectic braze alloy can readily wet Ti₅₀Ni₅₀ substrate by minor titanium additions. The brazed Ti₅₀Ni₅₀/Cusil-ABA^®/Ti₅₀Ni₅₀ joint is mainly comprised of Cu-rich, Ag-rich and CuNiTi phases. On the other hand, the brazed Ti₅₀Ni₅₀/Ticusil^®/Ti₅₀Ni₅₀ joint consists of Ag-rich, Cu-rich and TiCu₂ phases. Because the chemical composition of Ticusil braze alloy is located inside the huge miscibility gap, the molten braze tends to be separated into two liquids during brazing. One is rich in Ag, and the other is rich in both Cu and Ti. The Ag-rich liquid does not react with Ti₅₀Ni₅₀ substrate. In contrast, the copper content is depleted from the matrix of brazed joint due to the formation of interfacial TiCu₂ phase. The TiCu₂ phase is less detrimental to the shape memory effect than CuNiTi phase during the shape recovery bending test. Shear strength of brazed joints exceeds 200 MPa for both braze alloys if the brazing time exceeds 180 s. However, thick interfacial CuNiTi and TiCu₂ layers can deteriorate the shear strength.     

High temperature deformation behaviors of a high Nb containing TiAl alloy

In the present paper, high temperature tensile and creep behaviors of Ti–45Al–9(Nb,W,B,Y) alloy with duplex (DP) microstructure were investigated. In addition to tensile tests at 815 °C and a strain rate range of 1 × 10⁻⁴ s⁻¹−1 × 10⁻³ s⁻¹ and tensile, creep tests at 760 °C and 815 °C under the stress of 180 MPa, the microstructure evolutions during tensile and creep tests were studied. The results show that high temperature high Nb containing TiAl alloy with DP microstructure has a good balance between ductility and strength and intermediate creep resistance. The tensile properties have the strain rate dependence, and ultimate tensile strength (UTS) and yield strength (YS) vs. strain rate obey a single-logarithm linear relationship. Minimum creep rate is affected by the test temperature and stress. Using loading change experiment a stress exponent of 4.3 is determined. DP microstructure is unstable after long-term exposure at high temperatures, and the spheroidization of lamella and recrystallization along grain boundaries occur during the high temperature deformation. It is assumed that the diffusion-assisted climb of dislocations might be the controlling mechanism at the minimum creep rate stage.     

The Influence of Al Content on the High Temperature Oxidation Properties of State-of-the-Art Cast Ni-base Alloys

The aim of the study was to investigate the impact of the Al and Cr content on the oxidation behaviour of the commercial spun cast Ni-base alloy Centralloy^® 60 HT R, an Al₂O₃ former. State-of-the-art Ni-base alloys intended for service temperatures above 1,000 °C rely on secondary carbide or γ′ formation in order to obtain sufficient creep strength and formation of either Cr₂O₃ or Al₂O₃ for corrosion protection. Decreased Cr and increased Al contents promote γ′-formation. The oxidation resistance suffers with decreased Cr and the ductility and thermal shock resistance becomes catastrophic with increased Al. 60 HT R has Cr and Al contents that result in a good compromise between oxidation and creep resistance. Samples from a cast of 60 HT R with modified Al content have been exposed in artificial air for up to 500 h at 1,200 °C. The Cr₂O₃ former G4879 has been co-exposed as a reference. The exposed samples were evaluated using gravimetry, optical microscopy and SEM/EDX. The results showed that too much Al, 10 % (even though the oxidation properties were passable but worse than the for 60 HT R), destabilizes the austenitic matrix, resulting in a non-controllable multiphase structure.     

Effect of subgrain structure on the creep strength of alloy 1207

The effect of severe plastic deformation (SPD) on the creep resistance of the Al-6%Cu-0.48Mn-0.52Mg-0.3Sc-0.1Zr alloy has been examined in a temperature range of 125–180°C. It has been shown that SPD performed by the method of equal-channel angular pressing at 300°C to a true strain of ～1 leads to the formation of a well-defined subgrain structure, which is retained upon solution treatment before quenching because of the presence in the alloy of ultradisperse Al₃(Sc, Zr) particles with coherent boundaries. It was established that the creep strength at 125–150°C of the as-cast alloy and of the deformed material is approximately the same. At 180°C, the creep rate of the deformed aluminum alloy is almost an order of magnitude lower than that of the as-cast alloy. The reasons for the influence of the subgrain structure on the creep strength of the Al-Cu-Sc-Zr alloy are discussed.     

Nitridation of HAYNES<Superscript>®</Superscript> NS-163<Superscript>®</Superscript> Alloy: Thermodynamics and Kinetics

HAYNES^® NS-163^® alloy is a new cobalt-base alloy that derives its exceptional high-temperature strength from a dispersion of fine-scale stable (Ti,Nb)N nitrides in the microstructure. In the present study, these nitrides were formed by means of a gas nitriding process. Resulting microstructures in the partially and fully nitrided conditions were examined. The nature and sequence of formation of the observed nitride compounds are rationalized by comparing their respective Gibbs free energies of formation. Nitride nucleation densities and morphologies are discussed in terms of the locally varying speed of the reaction front.