Microstructural investigation and castability anticipation in modern Ti/Al/Nb-containing nickel-based superalloys

A quantitative relation between the γ/γ′ and γ/Laves intermetallics was investigated with the change of chemical composition, i.e., Ti, Al and Nb in the third generation of nickel-based superalloys. The results demonstrated that the maximum amount of intermetallic eutectics （i.e., 41.5%, mass fraction） has been formed in 9.8% （Ti＋Al）. It is predicted that high level of intermetallics formed in the 3GSA-HNM-1 （γ-9.8%（Ti＋Al）） deteriorates its castability. The type and morphology of eutectic intermetallics change and the amount considerably diminishes by decreasing Ti＋Al in 3GSA-HNM-2 （γ-7.6%（Ti＋Al）, 1.5% Nb）. Thus, it is predicted that the castability for the 3GSA-HNM-2 improves. The amount of Laves intermetallics shows an ascending behavior again, however, with less intensity by increasing the Nb content in the 3GSA-HNM-3 （γ-5.7%（Ti＋Al）, 2.9% Nb）. It can be concluded that for 3GSA-HNM-3 with composition of γ-5.7%（Ti＋Al） and 2.9% Nb, the optimized castability can be anticipated, because the minimum amount of eutectic intermetallics （i.e., 4.7%） is formed.

Review on synergistic damage effect of irradiation and corrosion on reactor structural alloys

The synergistic damage effect of irradiation and corrosion of reactor structural materials has been a prominent research focus.This paper provides a comprehensive review of the synergistic effects on the third-and fourth-generation fission nuclear energy structural materials used in pressurized water reactors and molten salt reactors.The competitive mechanisms of multiple influencing factors,such as the irradiation dose,corrosion type,and environmental temperature,are summarized in this paper.Conceptual approaches are proposed to alleviate the synergistic damage caused by irradiation and corrosion,thereby promoting in-depth research in the future and solving this key challenge for the structural materials used in reactors.

Recent progress in self-repairing coatings for corrosion protection on magnesium alloys and perspective of porous solids as novel carrier and barrier

Featuring low density and high specific strength, magnesium(Mg) alloys have attracted wide interests in the fields of portable devices and automotive industry. However, the active chemical and electrochemical properties make them susceptible to corrosion in humid, seawater, soil,and chemical medium. Various strategies have revealed certain merits of protecting Mg alloys. Therein, engineering self-repairing coatings is considered as an effective strategy, because they can enable the timely repair for damaged areas, which brings about long-term protection for Mg alloys. In this review, self-repairing coatings on Mg alloys are summarized from two aspects, namely shape restoring coatings and function restoring coatings. Shape restoring coatings benefit for swelling, shrinking, or reassociating reversible chemical bonds to return to the original state and morphology when coatings broken;function self-repairing coatings depend on the release of inhibitors to generate new passive layers on the damaged areas. With the advancement of coating research and to fulfill the demanding requirements of applications, it is an inevitable trend to develop coatings that can integrate multiple functions(such as stimulus response, self-repairing, corrosion warning,and so on). As a novel carrier and barrier, porous solids, especially covalent organic frameworks(COFs), have been respected as the future development of self-repairing coatings on Mg alloys, due to their unique, diverse structures and adjustable functions.

Oxidation behavior and mechanism of porous nickel-based alloy between 850 and 1000 °C

The oxidation behavior and mechanism of a porous Ni?Cr?Al?Fe alloy in the temperature range from850to1000°Cwere investigated by optical microscopy,scanning electron microscopy(SEM)and energy dispersive spectroscopy(EDS),X-raydiffraction(XRD)analyses and X-ray photoelectron spectroscopy(XPS).The results show that the oxidation kinetics at950and1000°C of this porous alloy is pseudo-parabolic type.Complex layers composed of external Cr2O3/NiCr2O4and internalα-Al2O3areformed on the surface of the oxidized porous alloys.γ?phases favor the formation of NiO/Cr2O3/NiCr2O4during the initial oxidation.Many fast diffusion paths contribute to the development of the oxide layers.The decrease of the open porosity and the permeabilitywith exposure time extending and temperature increasing can be controlled within a certain range.

In-situ building of multiscale porous NiFeZn/NiZn-Ni heterojunction for superior overall water splitting

The development of efficient nonprecious bifunctional electrocatalysts for water electrolysis is crucial to enhance the sluggish kinetics of the oxygen evolution reaction(OER)and hydrogen evolution reaction(HER).A self-supporting,multiscale porous NiFeZn/NiZn-Ni catalyst with a triple interface heterojunction on nickel foam(NF)(NiFeZn/NiZn-Ni/NF)was in-situ fabricated using an electroplating-annealing-etching strategy.The unique multiinterface engineering and three-dimensional porous scaffold significantly modify the mass transport and electron interaction,resulting in superior bifunctional electrocatalytic performance for water splitting.The NiFeZn/NiZn-Ni/NF catalyst demonstrates low overpotentials of 187 m V for HER and 320 mV for OER at a current density of 600 mA/cm^(2),along with high durability over 150 h in alkaline solution.Furthermore,an electrolytic cell assembled with NiFeZn/NiZn-Ni/NF as both the cathode and anode achieves the current densities of 600 and 1000 m A/cm^(2) at cell voltages of 1.796 and 1.901 V,respectively,maintaining the high stability at 50 mA/cm^(2) for over 100 h.These findings highlight the potential of NiFeZn/NiZn-Ni/NF as a cost-effective and highly efficient bifunctional electrocatalyst for overall water splitting.

Electronic Properties of Passive Films Formed on G3 and G30 Nickel-based Alloys in Bicarbonate/Carbonate Buffer Solution

The electronic properties of passive films formed on G3 and G30 alloys in bicarbonate/carbonate buffer solution were comparatively studied by electrochemical impedance spectra（EIS） and Mott-Schottky analysis, the chemical composition of the passive film formed on G3 alloy was detected by X-ray photoelectron spectroscopy （XPS）. The results show that passive film on G3 alloy had better protection than that on G30 alloy. The transfer resistance, film resistance and diffusion resistance of the passive films on both alloys increased with increasing formation potential, prolonging formation time, increasing pH value, decreasing formation temperature, and decreasing chloride and sulphide ions concentration. Mott-Schottky plot reveals that the passive films on the two alloys show a p-n semi-conductive character. XPS analysis indicates that the passive film on G3 alloy was composed of an inner Cr oxide and an outer Fe, Mo/Ni oxides.

ADVANCED NICKEL-BASED AND NICKEL-IRON-BASED SUPERALLOYS FOR CIVIL ENGINEERING APPLICATIONS

The use of high-temperature materials is especially important in power station construction, heating systems engineering, furnace industry, chemical and petrochemical industry, waste incineration plants, coal gasification plants and for flying gas turbines in civil and military aircrafts and helicopters. Particularly in recent years, the development of new processes and the drive to improve the economics of existing processes have increased the requirements significantly so that it is necessary to change from well-proven materials to new alloys. Hitherto, heat resistant ferritic steels sufficed in conventional power station constructions for temperatures up to 550℃ newly developed ferritic/martensitic steels provide sufficient strength up to about 600 - 620℃. In new processes, e.g. fluidized-bed combustion of coal, process temperatures up to 900℃ occur. However, this is not the upper limit, since in combustion engines, e.g. gas turbines. Material temperatures up to 1100℃ are reached locally. Similar development trends can also be identified in the petrochemical industry and in the heat treatment and furnace engineering. The advance to ever higher material temperatures now not only has the consequence of having to use materials with enhanced high-strength properties, considerable attention now also has to be given to their chemical stability in corrosive media. Therefore not only examples of the use of high-temperature alloys for practical applications will be given but also be contributed to some general rules for material selection with regard to their high-temperature strength and corrosion resistance.

THE ROLE OF NIOBIUM IN NICKEL-BASED SUPERALLOYS AND CHARACTERIZATION OF PM ALLOY EP741NP

The role of niobium in nickel-based superalloys is reviewed. The importance of niobium as a strengthener is discussed. New developments in nickel-based superalloys are also briefly mentioned, including some results that show improved resistance to sulfidation by niobium. Research results from a current program on the role of niobium in the Russian powder metallurgy alloy EP741NP are presented. Future research plans on the role of niobium in superalloys are also discussed.

Corrosion performance and applicability evaluation of nickel-based alloys for oil and gas production tubing applications

The sulfide stress corrosion cracking（ SSC） performance of G3 and 028 nickel-based alloys w as studied using slow strain rate test（ SSRT） and the four-point bend（ FPB） test under simulated dow nhole conditions. The effect of high temperature,high H2 S / CO2 partial pressure,and the presence of sulfur on SSC susceptibility w as investigated. The G3 alloy w as found to have a higher SSC resistance than the 028 alloy. Presence of sulfur and temperature bear a strong influence on the SSC performance of the metals,particularly on the 028 alloy. The applicability of 028 and G3 alloys may be expanded and both could safely be used beyond the limits set by the ISO15156-3 standard.

Phase transformation and damping behavior of lightweight porous TiNiCu alloys fabricated by powder metallurgy process

Porous TiNiCu ternary shape memory alloys （SMAs） were successfully fabricated by powder metallurgy method. The microstructure, martensitic transformation behavior, damping performance and mechanical properties of the fabricated alloys were intensively studied. It is found that the apparent density of alloys decreases with increasing the Cu content, the porous Ti50Ni40Cu10 alloy exhibits wide endothermic and exothermic peaks arisen from the hysteresis of martensitic transformations, while the porous Ti50Ni30Cu20 alloy shows much stronger and narrower endothermic and exothermic peaks owing to the B2-B19 transformation taking place easily. Moreover, the porous Ti50Ni40Cu10 alloy shows a lower shape recovery rate than the porous Ti50Ni50 alloy, while the porous Ti50Ni30Cu20 alloy behaves reversely. In addition, the damping capacity （or internal friction, IF） of the porous TiNiCu alloys increases with increasing the Cu content. The porous Ti50Ni30Cu20 alloy has very high equivalent internal friction, with the maximum equivalent internal friction value five times higher than that of the porous Ti50Ni50 alloy.

Research on CMT welding of nickel-based alloy with stainless steel

Cold Metal Transfer （CMT） welding technique is a new welding technique introduced by Fronins company. CMT welding of nickel-based alloy with stainless steel was carried out using CuSi3 filler wire in this paper. Effects of welding parameters, including welding current, welding speed, etc, on weld surface appearance were tested. Microstructure and mechanical properties of CMT weld were studied. The results shaw that the thickness of interface reaction layer of the nickel- based alloy is 14. 3 μm, which is only 4. 33% of base material. The weld is made up of two phases, α-copper and iron-based solid solution. Rupture occurs initially at the welded seam near the edge of stainless steel in shear test. The maximum shear strength of the CuSi3 welded joint is 184. 9 MPa.

Preparation and characterization of porous NiTi alloys synthesized by microwave sintering using Mg space holder

To obtain the lightweight,high strength,and high damping capacity porous NiTi alloys,the microwave sintering coupled with the Mg space holder technique was employed to prepare the porous NiTi alloys.The microstructure,mechanical properties,phase transformation behavior,superelasticity,and damping capacity of the porous NiTi alloys were investigated.The results show that the porous NiTi alloys are mainly composed of the B2 NiTi phase with a few B19'NiTi phase as the sintering temperature is lower than or equal to 900℃.With increasing the sintering temperature,the porosities of the porous NiTi alloys gradually decrease and the compressive strength increases first,reaching the maximum value at 900℃,and then decreases.With increasing the Mg content from 1 wt.%to 7 wt.%,the porosities of the porous NiTi alloys increase from 37.8%to 47.1%,while the compressive strength decreases from 2058 to 1146 MPa.Compared with the NH4HCO3 space holder,the phase transformation behavior of the porous NiTi alloys prepared with Mg space holder changes,and all of the compressive strength,superelasticity,shape memory effect and damping capacity are greatly improved.

Fabrication of lotus-type porous Mg−Mn alloys by metal/gas eutectic unidirectional solidification

Lotus-type porous Mg–xMn(x=0,1,2 and 3 wt.%)alloys were fabricated by metal/gas eutectic unidirectional solidification(the Gasar process).The effects of Mn addition and the fabrication process on the porosity,pore diameter and microstructure of the porous Mg-Mn alloy were investigated.Mn addition improved the Mn precipitates and increased the porosity and pore diameter.With increasing hydrogen pressure from 0.1 to 0.6 MPa,the overall porosity of the Mg-2wt.%Mn ingot decreased from 55.3%to 38.4%,and the average pore diameter also decreased from 2465 to 312μm.Based on a theoretical model of the change in the porosity with the hydrogen pressure,the calculated results were in good agreement with the experimental results.It is shown that this technique is a promising method to fabricate Gasar Mg–Mn alloys with uniform and controllable pore structure.

Role of Ag addition on microstructure,mechanical properties,corrosion behavior and biocompatibility of porous Ti-30 at%Ta shape memory alloys

In the present study,the thermal,mechanical,and biological properties of xAg/Ti-30Ta(x=0,0.41,0.82 and 2.48 at%)shape memory alloys(SMAs)were investigated.The study was conducted using optical and scanning electron microscopy(SEM),X-ray diffractometry(XRD),compression test,and shape memory testing.The xAg/Ti-Ta was made using a powder metallurgy technique and microwave-sintering process.The results revealed that the addition of Ag has a significant effect on the pore size and shape,whereas the smallest pore size of 11μm was found with the addition of 0.41 at%along with a relative density of 72%.The fracture stress and strain increased with the addition of Ag,reaching the minimum values around 0.41 at%Ag.Therefore,this composition showed the maximum stress and strain at fracture region.Moreover,0.82 Ag/Ti-Ta shows more excellent corrosion resistance and biocompatibility than other percentages,obtaining almost the same behaviour of the pure Ti and Ti-6Al-4V alloys,which can be recommended for their promising and potential response for biomaterial applications.

Pore structure and mechanical properties of directionally solidi ed porous aluminum alloys

Porous aluminum alloys produced by the metal-gas eutectic method or GASAR process need to be performed under a certain pressure of hydrogen, and to carry over melt to a tailor-made apparatus that ensures directional solidif ication. Hydrogen is driven out of the melt, and then the quasi-cylindrical pores normal to the solidif ication front are usually formed. In the research, the effects of processing parameters(saturation pressure, solidif ication pressure, temperature, and holding time) on the pore structure and porosity of porous aluminum alloys were analyzed. The mechanical properties of Al-Mg alloys were studied by the compressive tests, and the advantages of the porous structure were indicated. By using the GASAR method, pure aluminum, Al-3wt.%Mg, Al-6wt.%Mg and Al-35wt.%Mg alloys with oriented pores have been successfully produced under processing conditions of varying gas pressure, and the relationship between the f inal pore structure and the solidif ication pressure, as well as the inf luences of Mg quantity on the pore size, porosity and mechanical properties of AlMg alloy were investigated. The results show that a higher pressure of solidif ication tends to yield smaller pores in aluminum and its alloys. In the case of Al-Mg alloys, it was proved that with the increasing of Mg amount, the mechanical properties of the alloys sharply deteriorate. However, since Al-3%Mg and Al-6wt.%Mg alloys are ductile metals, their porous samples have greater compressive strength than that of the dense samples due to the existence of pores. It gives the opportunity to use them in industry at the same conditions as dense alloys with savings in weight and material consumption.

Nickel-base superalloys for USC power plants

The advanced ultra-supercritical power plants of the future will utilize steam pressures and temperatures that are too high for traditional ferritic steels,thus requiring austenitic materials.Older nickel-base superalloys such as 263 and 617 were initially evaluated under the European THERMIE project beginning in the 1990s.An entirely new age-hardened alloy 740 which possesses exceptional fireside corrosion resistance and creep strength was also developed for boiler tubing capable of serving at 700C.Subsequently,interest in the USA considered other product forms such as steam header piping and steam turbine forgings for service as high as 760C.A more stable and weldable alloy version now called 740H was developed to meet these more demanding conditions.This paper summarizes the current status of work on alloys 740 and 740H.

Simulation and experiments of ultrasonic propagation in nickel-based alloy weldments

In order to obtain good understanding of complicated beam propagation behaviors in nickel-based alloy weldments , ray tracing simulation is established to predict the ultrasonic beam path in a special welded structure of dissimilar steels. Also experimental examinations are carried out to measure the ultrasonic beam paths in the weldment. Then comparisons of the modeling predictions with experimental results are presented to reveal the complicated beam propagation behaviors.

NICKEL-BASE ALLOY SHEET ALLOYS USED IN AEROSPACE APPLICATIONS

Many gas turbine components are made from nickel alloy sheet. Most are used for directing or containing gases at high temperatures and pressures where metal temperatures can be as high as 1090℃ (2000°F). These applications included combustor systems, casings and liners, transition and exhaust ducting, afterburners, and thrust reversere. Light weight components and sub-assemblies call for alloy sheet with high levels of stength and oxidation resistance. Complex component design calls for excellent ductility and ease of fabrication.The wide range of nickel alloy sheet alloys presently used in aircraft and land-based gas turbines is briefly described and typical properties presented. New sheet alloy developments, involving INCONEL￣* alloys 625LCF, 718SPF and MA754, are presented including the process routes involved and material properties.

Evolution of helium bubbles in nickel-based alloy by post-implantation annealing

Nickel-based alloys have been considered as candidate structural materials used in generation IV nuclear reactors serving at high temperatures.In the present study,alloy 617 was irradiated with 180-keV helium ions to a fluence of 3.6×10^(17) ions/cm^(2) at room temperature.Throughout the cross-section transmission electron microscopy(TEM)image,numerous over-pressurized helium bubbles in spherical shape are observed with the actual concentration profile a little deeper than the SRIM predicted result.Post-implantation annealing was conducted at 700℃for 2 h to investigate the bubble evolution.The long-range migration of helium bubbles occurred during the annealing process,which makes the bubbles of the peak region transform into a faceted shape as well.Then the coarsening mechanism of helium bubbles at different depths is discussed and related to the migration and coalescence(MC)mechanism.With the diffusion of nickel atoms slowed down by the alloy elements,the migration and coalescence of bubbles are suppressed in alloy 617,leading to a better helium irradiation resistance.

Effects of Mg content on pore structure and electrochemical corrosion behaviors of porous Al-Mg alloys

Porous Al-Mg alloys with different nominal compositions were successfully fabricated via elemental powder reactive synthesis, and the phase composition, pore structure, and corrosion resistance were characterized with X-ray diffractometer, scanning electron microscope and electrochemical analyzer. The volume expansion ratio, open porosity and corrosion resistance in 3.5%(mass fraction) Na Cl aqueous solution of the alloys increase at first and then decrease with the increase of Mg content. The maxima of volume expansion ratio and open porosity are 18.3% and 28.1% for the porous Al-56%Mg(mass fraction) alloy, while there is the best corrosion resistance for the porous Al-37.5% Mg(mass fraction) alloy. The pore formation mechanism can be explained by Kirkendall effect, and the corrosion resistance can be mainly affected by the phase composition for the porous Al-Mg alloys. They would be of the potential application for filtration in the chloride environment.