Oxidation of a Co-base Superalloy

The oxidation behavior of a cast polycrystalline Co-base superalloy was studied at temperatures from 900 to 1050℃ and analyzed by thermogravimetric analysis （TGA）, X-ray diffraction （XRD） and scanning electron microscopy （SEM）. The results indicate that a cast Co-base superalloy follows the subparabolic oxidation kinetics at 900 and 1000℃, which are controlled by the growth of the inner Cr-rich layer, and that after oxidation at 1050℃ for 200 h, it almost exhibits the linear oxidation kinetics possible due to the volatility of Cr-rich oxide. A mixed scale forms on the alloy after prolonged oxidation. The oxide scale formed at 900 and 1000℃ is composed of an outer layer of spinel and an inner continuous Cr-rich layer and at I050℃ is composed of a very discontinuous Cr-rich layer.

Effect of Rare-earth Element La on Co-base Superalloy

The effect of La element on the mechanical properties and constitution nearby grain boundary in a Co-base superalloy Co-22Ni-22Cr-14k during 760℃ ageing has been investigated by AES and material testing. Results show that element La can be in a state of solid solution and segregates mainly at the vicinity of grain boundaries. The prior segregation of La atoms can affect segregation behaviour of other elements, so that exerts influence on microstructure of the alloy, Particularly increasing of La content can decrease P concentration at the vicinity of grain boundaries but not decrease S concentration. S together with La produce cosegregation effect. The content of La has no obvious effect on high temperature strength. When the content of La comes up to 0.02wt-％, it can obviously improve the high temperature ductility.

MICROSTRUCTURE AND MECHANICAL PROPERTIES OF Fe-Ni-Co-BASE SUPERALLOY AFTER LONGTERM AGING AT 650℃

The microstructure and mechanical properties of a Ni-Fe-Co base superalloy with low thermal expansion coefficient after aging at 650℃ for 200,500,1000 and 2000 h have been investigated.It was found that they are stable at 650℃.This alloy is believed to.fill the re- quirements for long time service to aircraft engine.

Effects of secondary precipitation on recrystallization in Co-base superalloy DZ40M

A series of experimental studies were conducted on the recrystallization of directionally solidified cobalt-base superalloy DZ40M. It is found that the secondary M23C6 precipitation influences the size and shape of the recrystal grains. When the annealing temperature is below 1 473 K, a large amount of the fine secondary M23C6 precipitations are distributed around the primary carbides, and such carbides impede the movement of grain boundary because the effect, the size and shape of recrystal grains become irregularly. When the temperature exceeds 1 473 K, the recrystal grains grow rapidly due to the dissolved secondary M23C6 precipitation.

Revealing the Void Formation Mechanism during Superplastic Deformation of a Fine-Grained Ni-Co-Base Superalloy

The mechanism behind void formation during superplasticity remains a subject of uncertainty.This study presented a novel insight into the void formation in a fine-grained Ni-Co-based superalloy during superplasticity.It was observed that the dissolution ofγ′-particles resulted in the creation of vacancies due to differences in atomic size between the matrix and the particles.These vacancies acted as inclusions,leading to the formation of micro-voids.Notably,excessive void formation correlated with higher particle dissolution was experimentally observed,highlighting a direct relationship between void formation and particle dissolution.

PHASE TRANSFORMATION OF Al-Ti COATINGS ON Co-BASE SUPERALLOY DZ40M DURING HIGH TEMPERATURE OXIDATION

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Influence of welding on low cycle fatigue properties of Co-based superalloy FSX-414

Co-based superalloys such as FSX-414 have been recently used in gas turbine first stage nozzles. During service, nozzles are exposed to low cycle fatigue, which can lead to cracking of these components. The cracks on these nozzles are usually welded with ttmgsten arc welding （TIG） using Co-based filler metals. In this paper, the effect of TIG on the tensile and low cycle fatigue properties of Co-based superalloy FSX-414 was studied at 950℃. The experimental results show that the yield and ultimate tensile stresses of welded and unwelded specimens are comparable to each other. But toughness of welded specimens is lower than that of unwelded ones. The low cycle fatigue properties of FSX-414 were studied at a strain rate of 3.3×10^-4 s^-1, strain ratio R=-1 （R=emin/emax） and Aet （total strain change） from 0.8% to 2%. In welded specimens, at high strain cycling, the nucleation and growth of cracks occur in the welded zone. But at Aet=0.8%, fracture occurs in the same zones of unwelded specimens. The results show that the total fatigue lives of the welded specimens are shorter than those of unwelded ones. In all of the low cycle fatigue tests, softening phenomena are observed.

MICROSTRUCTURES AND PROPERTIES OF DZ40M Co-BASED SUPERALLOY AFTER LONG-TERM AGEING AT 850℃

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Composition effect on elastic properties of model NiCo-based superalloys

NiCo-based superalloys exhibit higher strength and creep resistance over conventional superalloys.Compositional effects on elastic properties of the γ and γ' phases in newly-developed NiCo-based superalloys were investigated by first-principles calculation combined with special quasi-random structures.The lattice constant,bulk modulus,and elastic constants vary linearly with the Co concentration in the NiCo solution.In the selected(Ni,Co)3(Al,W)and(Ni,Co)3(Al,Ti)model γ' phase,the lattice constant,and bulk modulus show a linear trend with alloying element concentrations.The addition of Co,Ti,and W can regulate lattice mismatch and increase the bulk modulus,simultaneously.W-addition shows excellent performance in strengthening the elastic properties in the γ' phase.Systems become unstable with higher W and Ni contents,e.g.,(Ni0.75Co0.25)3(Al0.25 W0.75),and become brittle with higher W and Co addition,e.g.,Co3(Al0.25 W0.75).Furthermore,Co,Ti,and W can increase the elastic constants on the whole,and such high elastic constants always correspond to a high elastic modulus.The anisotropy index always corresponds to the nature of Young's modulus in a specific direction.

Dynamic Strain Aging in a Newly Developed Ni-Co-Base Superalloy with Low Stacking Fault Energy

Characteristics of dynamic strain aging （DSA） in a Ni-Co-base superalloy were studied by tensile tests at temperatures ranging from 250 ℃ to 550 ℃ and strain rates ranging from 3 x 10-5 to 8 x 10-4 s-1. Serrated flow in the tensile stress-strain curves was observed in the temperature range from 300 ℃ to 500 ℃. Normal DSA behavior was found at temperatures ranging from 300 ℃ to 350 ℃, while inverse DSA behavior was observed at temperatures ranging from 400 ℃ to 500 ℃. The yield strength, ultimate tensile strength, elongation, work hardening index, and fracture features were not affected by temperature and strain rates in DSA regime. Negative strain-rate sensitivity of flow stress was observed in DSA regime. The analysis suggests that the ordering of the substitutional solutes around some defects like mobile dislocations and stacking faults due to the thermal activated process may cause the serrations on the tensile curves.

Laser engineered net shaping of Co-based superalloys

Laser engineered net shaping(LENS) process was investigated using Co-based superalloy powder with a high power continuous wave CO2 laser. Thin wall part with smooth surface was obtained by LENS of layer-by-layer deposition of the powder materials. This thin wall sample was tested for metallographic examinations, micro-hardness, X-ray diffraction and mechanical property test. Microstructural results show that the layers possess rapid solidification microstructural feature, fine dendritic crystal and M7C3-type carbides (essentially chromium-rich carbide) dispersed in theγ(Co,Cr) phase matrix. Dendrite spacing as well as the solidification mode can be controlled through control process parameters. In addition, this microstructural feature of the as-formed Co-base sample leads to an evident hardening and a superior tensile strength and toughness.

EFFECT OF TRACE ELEMENT B ON MICROSTRUCTURE OF DIRECTIONALLY SOLIDIFIED Co-BASE SUPER ALLOY DZX40M

The effect of trace element B on microstructure of directionally solidified new type Co-base superalloy is studied systematically in this paper. It is found that the addition of B makes the incipient melting temperature of the alloy decline remarkably. Compared with the alloy having a boron content of 0.05 %, the incipient melting temperature of the alloy with 0.7 % B decreases by 197℃. The primary and secondary dendrite arm spacings increase and the volume fraction of interdendritic carbides with fine microstructure becomes bigger with the enhancement of boron content. When the alloy is with 0.7wt %B, M3B2 precipitates in the interdendrites.

Effect of Yttrium on Microstructure and Hot Corrosion Preformance of Laser Clad Co-Based Alloy

The structure of the yttrium modified Co-base alloy layers formed by laser cladding on 2Cr13 and 1Cr18Ni9Ti steel surfaces and its hot corrosion performance have been investigated systematically.The re- sults show that the Y-containing cobalt base clad alloy has a finer microstructure and higher corrosion re- sistance to the salt mixture of 75% Na_2SO_4+25%NaCl at high temperature.The unique properties are ob- tained with addition of 0.875% Y for the formation of a continuous and compact oxide scale.The compact scale may act as a barrier for the inward diffusion of oxygen and sulphur and also for the outward diffusion of alloying elements.

Microstructure and wear resistance of Fe-based and Co-based coating of AISI H13

Fe-based and Co-based cladding layers were prepared on the surface of AISI H13 hot die steel by laser cladding technology.The microstructure,hardness and abrasion resistance of the two cladding layers were studied by means of optical microscope,scanning electron microscope,rockwell hardness tester,and high temperature friction and wear tester.Also,the red hardness of the cladding layers was measured,after holding the layers at 600℃ for 1 hour by muffle furnace and repeated 4 times.The rockwell hardness values of the substrate,the Fe-based and the Co-based alloy coating measured were HRC 47,HRC 52 and HRC 48,respectively.The red hardness values of the substrate and the Fe-based cladding layer were decreased,while that of the Co-based cladding layer was increased.The Co-based cladding layer has the minimal wear loss weight and friction coefficient among them.The wear mechanisms of the substrate,the Fe-based layer and the Cobased layer attribute mainly to abrasive wear,adhesion wear,and both of them,respectively.

Thermal Fatigue Behaviour of Co-Based Alloy Coating Obtained by Laser Surface Melt-Casting on High Temperature Alloy

A thermal fatigue behaviour of Co-based alloy coating obtained by laser surface meltcasting on the high temperature alloy GH33 was studied.The results show that after each time of thermal cycling,the final residual stress was formed in the melt-casting layer which is attributed to the thermal stress and structural stress.Through the first 50 times of thermal cycling,the morphology of coating still inherits the laser casting one,but the dendrites get bigger;After the second 50 times of thermal cycling,corrosion pits emerge from coating,and mostly in the places where coating and substrate meet.The fatigue damage type of coating belongs to stress corrosion.

Microstructures and properties of Co-based alloy coatings prepared on surface of H13 steel

The Co-based alloy coatings had been prepared by laser cladding and vacuum fusion sintering. Microstructures of the coatings were investigated and the performance of thermal cycling was also tested using scanning electron microscopy （ SEM） and X-ray diffraction （ XRD ）. The results show that the coatings and substrates combine well. The main phase compositions of laser cladding coating are T-Co, Cr23 C6 and Ni2 9 Cro. 7 Feo. 36, while vacuum fusion sintering coating consists of Co, Cr7 C3, and Ni2.9 Cro. 7 Feo. 36. After thermal cycling, the minimum hot cracking width of laser cladding coating is 14 μm; moreover, laser cladding coating maintains high hardness and hot-cracking susceptibility. Those are beneficial to high temperature wear resistance of hot work dies.

A review of linear friction welding of Ni -based superalloys

Ni-based superalloys are one of the most important materials employed in high-temperature applications within the aerospace and nuclear energy industries and in gas turbines due to their excellent corrosion,radiation,fatigue resistance,and high-temperature strength.Linear friction welding(LFW)is a new joining technology with near-net-forming characteristics that can be used for the manu-facture and repair of a wide range of aerospace components.This paper reviews published works on LFW of Ni-based superalloys with the aim of understanding the characteristics of frictional heat generation and extrusion deformation,microstructures,mechanical proper-ties,flash morphology,residual stresses,creep,and fatigue of Ni-based superalloy weldments produced with LFW to enable future optim-um utilization of the LFW process.

Competitive oxidation behavior of Ni-based superalloy GH4738 at extreme temperature

A high thrust-to-weight ratio poses challenges to the high-temperature performance of Ni-based superalloys. The oxidation behavior of GH4738 at extreme temperatures has been investigated by isothermal and non-isothermal experiments. As a result of the competitive diffusion of alloying elements, the oxide scale included an outermost porous oxide layer (OOL), an inner relatively dense oxide layer (IOL), and an internal oxide zone (IOZ), depending on the temperature and time. A high temperature led to the formation of large voids at the IOL/IOZ interface. At 1200℃, the continuity of the Cr-rich oxide layer in the IOL was destroyed, and thus, spallation occurred. Extension of oxidation time contributed to the size of Al-rich oxide particles with the increase in the IOZ. Based on this finding,the oxidation kinetics of GH4738 was discussed, and the corresponding oxidation behavior at 900-1100℃ was predicted.

High-Temperature Electric Properties and X-ray Photoemission Spectra of Layered Co-Based Oxides Bi_(2-x)La_xSr_2Co_2O_(8-δ)

Layered Co-based ceramics with a nominal composition Bi2-xLaxSr2Co2O8-δ (x=0.0, 0.4, 0.8, short by BLC-222) were prepared using conventional solid state reaction method. X-ray photoemission spectroscopy (XPS) was used to investigate their electronic structures. The cobalt ions are highly mixed valences of Co3+ and Co4+. The fraction of Co4+ almost keeps unchanged with the increase of x. The O-1s photoemission spectra show that there are lattice oxygen and chemical absorbed oxygen in all the samples. The substitution of Bi3+ by La3+ results in a change from metallic-like behavior to semiconductor behavior. This abnormal phenomenon means that La3+ plays a key role in effecting the electrical transport property of BLC-222. The O-Co covalence bond is strengthened by the increase of La3+, which results in the decrease of conductivity.

Preparation, Structures and Magnetic Properties of Two Co-based Clusters with Triangular Core and Cuboidal Core Geometries

Two novel Co-based clusters with the 2-(hydroxylmethyl)pyridine(hmpH)ligand,formulated as[Co3(hmp)6(hmpH)]×2NO3×3H2O(ZTU-3)and[Co4(hmp)4(CH3CO2)2(H2O)4]×2NO3(ZTU-4),have been successfully synthesized and structurally characterized.ZTU-3 features a triangular core geometry,while ZTU-4 exhibits a cuboidal core geometry.In addition,the magnetic properties of ZTU-3 and ZTU-4 are also all investigated.