Influence of Solution Temperature on Microstructure and Mechanical Properties of Directionally Solidified Superalloy DZ22

The microstructure changed markedly withincreasing solution temperature, i.e eutecticγ' phases and coarse primary γ' phase areredissolved continuously into matrix, whichwas finished at 1250-1260℃, and more and finerγ' phases are reprecipitated. The grain boundarymorphology changed gradually to "fine line"form. The dendritic segregation of elementsdecreased. With raising solution temperaturethe longitudinal stress-rupture life at highand intermediate temperature were enhanced,but the transversal stress-rupture life atintermediate temperature decreased obviously.The incipient melting temperature of DZ22 alloywas measured to be 1230-1240℃ and may beincreased by homogenization at 1150℃. In thispaper the principle of determining solutiontemperature of DS alloys was discussed. Itwas suggested that the solution temperatureof alloy DZ22 selected at 1200-1210℃ is appropri-ate.     

Effects of heat treatments on the microstructure and mechanical properties of Rene 80

Effects of heat treatments on room temperature mechanical properties and stress-rupture properties of Rene 80 have been investigated. The microstructures were analyzed by optical microscope and scanning electron microscope after each step of heat treatments. With the decrease of aging temperature, the average size of γ′ phase decreases, but the volume fraction of γ′ phase increases. The lower aging temperature suppresses the growing of the coarse γ′ particles, but facilitates the growth of the fine γ′ particles. After the optimum heat treatment, the ultimate tensile strength and yield strength are respectively higher than 1040 MPa and 950 MPa, the stress-rupture life at 871 °C/310 MPa is higher than 170 h with excellent ductility. The improved tensile strength and stress-rupture life are primarily due to the increased volume fraction of γ′ phase. The borides precipitate at grain boundaries at about 913 °C. The primary MC is found to decompose into M₆C at about 873 °C and M₂₃C₆ at 840–873 °C at grain boundaries. The precipitate of the carbides may partly contribute to the improved mechanical properties.     

Cavitation Damage of the single-crystal nickel-base superalloy SRR99 during creep at 850°C

Various heat treatments of the single-crystal nickel-base superalloy SRR99 have been carried out to produce a wide range of initial γ′ phase morphology. The cavitation damage has been studied after creep fracture at 850°C and stresses between 400MPa and 650 Mpa. The crep fracture surfaces and longitudinal sections of crept specimens have been examined by image analysing system in order to determine the size distribution and the area fraction of (001)-planes cracks on fracture surface characterise the creep crack damage level. The cracking morphology in fractured material as a function of stress and γ′-phase size has also been investigated. The results show, that crack propagation occurred anisotropically on (001) crystallographic planes perpendicular to the aplied stress along the γ/γ′ interfaces. It was found, that creep cracks dependent on the shape and size of γ′-phases developing at the temperature and stress level.     

Creep behaviour of single crystal nickel base superalloy

Abstract

The creep activation energy and structure constant at the different creep stages have been calculated, and the microstructures have been observed by SEM and TEM. The results showed that the internal stress σo decreased with an increase in temperature. Over the stress and temperature range, there are different activation energies, time exponents, and structure constants at different creep stages. The change in microstructure has an influence on creep resistance in this superalloy (Ni-6.0AI-7.0Ta-8.5Mo, wt-%). It is shown that the dislocation climb is the major deformation mechanism during tensile creep stages I and II, but during the tertiary stage, the creep resistance decreased as a result of dislocations shearing into the γ′ rafts. Creep fracture occurs mainly by the cavities and microcracks produced at the γ′/γ phase interface due to the interaction of multislips.     

Effect of Solution Annealing on Microstructure and Mechanical Properties of a Ni-Cr-W-Fe Alloy

The effect of solution annealing on the microstructure and mechanical properties of a Ni-Cr-W-Fe alloy developed for advanced 700?C ultra-supercritical power plants was investigated. Test samples in this study were subjected to different solution treatments and the same aging treatment(at 760?C for 1 h).When solution annealing temperature was elevated from 1020?C to 1150?C, the stress-rupture life at750?C/320 MPa was increased from 60 h to 300 h, the stress-rupture elongation was enhanced from12% to 17%, and the elongation of the tensile at 750?C was improved from 11% to 24%. All tensile and stress-rupture samples displayed an intergranular dimple mixed fracture. Intergranular micro-cracks had a great relationship with the morphology of grain boundary carbides. Most carbides retained the morphology of globular shape and continuous thin plate. After tensile and stress-rupture tests, a few carbides were converted into lamellar. The results showed that intergranular micro-cracks were easier to form at continuous thin plate carbides than at globular shape carbides. Lamellar carbides hardly caused the nucleation of micro-cracks. Besides, grain boundaries sliding and elements diffusion during stressrupture tests led to the formation of precipitate free zones, which accelerated the extension of microcracks and influenced the stress-rupture life.     

Precipitate coarsening and its effects on the hot deformation behavior of the recently developedγ’-strengthened superalloys

Recently,theγ’-strengthened superalloys are of great interests in high temperature applications due to their excellent high temperature strength which is derived from theγ’strengthening phase.For theseγ’-strengthened superalloys,the changes in morphology,size and distribution ofγ’precipitates due to coarsening during thermal exposure have a significant impact on the properties of alloys.This article briefly summarizes the recent advances on the coarsening behavior of gamma prime precipitates in the recently-developedγ’-strengthened superalloys and its effects on the hot deformation behavior of superalloys,drawing specific examples on Allvac^■718 Plus TM and Ni3 Al-based intermetallic superalloys.It is found that the particle size plays an important role in morphological evolution ofγ’precipitates.For instance,the morphology ofγ’precipitates evolves from cuboidal to strip-like or other complex structures in Ni3 Al-based intermetallic alloys,while theγ’precipitates in Allvac^■718 Plus alloy always present nearspherical morphology due to the relatively small initial particle size.The Lifshitz-Slyozof-Wagner(LSW)theory and its modifications,as well as Trans-Interface Diffusion Controlled(TIDC)theory have been applied to describing the coarsening kinetics ofγ’precipitates.Additionally,the hot deformation behavior ofγ’-strengthened superalloy is found to be greatly influenced by the coarsening ofγ’precipitates.     

Effects of Al on microstructural stability and related stress-rupture properties of a third-generation single crystal superalloy

To examine the influences of minor modification of Al content on the microstructural stabilities and stress rupture properties,two alloys with minor difference in Al content were exposed isothermally at 1100 ℃ for 100 h,500 h,and 1000 h,respectively.The microstructures were characterized before and after thermal exposure.It was found that when Al content was decreased by 0.4 wt %,the volume fractionγ' decreased by 4 %,the size of γ' increased by 40 nm,the matrix channel width increased by 5 nm,and the misfit degree of γ/γ' phases increased by 0.006 % after heat treatment(HT).During thermal exposure,the alloy with low Al content had a better resistance to coarsening of γ' phase and precipitation of μphase.The γ' particles of the alloy with high AI content tended to connect each other and coarsened along 100directions;however,the γ' particles of the alloy with low Al content remained cubic after 500 h.A serious coarsening behavior took place in the two alloys after aging for 1000 h.The structural stabilities were significantly improved.However,the change of 0.4 wt % Al content was found to have little effect on the high temperature stress-rupture properties.     

Influence of Co, W and Ti on the Stress-rupture Lives of a Single Crystal Nickel-base Superalloy

The influence of Co, W and Ti on stress-rupture lives of a Ni-Cr-AI-Mo-Ta-Co-W-Ti single crystal nickel-base superalloy has been investigated using a L9 （34） orthogonal array design （OAD） by statistical analysis. At a selected composition range, Ti content was the most important factor to the effect of the stress-rupture lives and then followed by Co content. W content had the minimum effect on stress-rupture lives. The optimal alloy should contain 10 wt pct Co, 8 wt pct W and zero Ti. The optimized alloy also had good microstructural stability during thermal exposure at 870℃ for 500 h.     

不同取向DD3单晶合金扩散连接接头组织及性能

采用D1F非晶态箔中间层合金对0°+30°,0°+60°取向组合的DD3单晶合金试样进行了TLP扩散焊,研究了被焊单晶合金试样相互之间的取向对接头组织和性能的影响.结果表明:当被焊二母材取向不一致时,由于在焊缝中央存在较大块状γ′相组成的界面,且此界面与外加应力方向垂直,是高温应力作用下的薄弱环节,从而使接头持久性能明...     

Soviet superalloy ZHS6-K,effect of B/C modification

A Soviet nickel base cast superalloy, ZHS6-K, has been examined for its response to B/C modification. Three alloy chemistry variants, with 0.17C/0.02B, 0.06C/0.1 B and 0.03C/0.2B (wt%) have been investment cast and investigated for microstructure, heat-treatment, tensile properties, stress-rupture properties and hot corrosion resistance. The B/C modification of the alloy results in some improvement in the yield strength. However, the tensile ductility and the stress-rupture properties are considerably degraded. The nature of the hot corrosion is of a pitting type for the ZHS6-K, whereas it is of a broad front type for the high boron alloys.     

Rohr-Innendruck-Zeitstanduntersuchungen bei überlagerter Heißdampf-Korrosion. Ausmaß und Erscheinungsform der spannungsbedingt verstärkten Oberflächen- und Gefüge-Oxidation von Rohrmaterial der Leigierung Incoloy 800

Stress-Rupture Testing under Superimposed Steam Corrosion . Extent and Morphology of Stress-Assisted Surface and Bulk Oxidation of Incoloy Alloy 800 Thin Wall Tubing. Continuing long-time stress-rupture testing under biaxial stresses and superimposed steam corrosion of Incoloy Alloy 800 thin wall tubing, the extent and morphology of stress-assisted additional surface and bulk oxidation was evaluated by quantitative metallography. This paper describes the stress-assisted additional corrosion as function of tangential stress or corresponding rupture-life, creep strain, and creep rate. The results are presented and explained on the basis of surface and bulk penetrating oxide morphology.     

Continuous γ′ precipitation in directionally solidified IN738 LC alloy

Abstract

A microstructural characterisation of continuously precipitated γ′ particles in IN738 LC alloy directionally solidified at various cooling rates has been carried out. With decreasing cooling rate there is a trend for the γ′ morphology to vary from rounded particles to irregular cuboids and, finally, a clusterlike formation. In the interdendritic regions, microsegregation leads to an increase in the γ′ solvus temperature, and γ′ precipitation begins at higher temperatures than in the dendrite cores, resulting in differences in particle size and morphology. Observations on the effects of solution treatment and aging on the development of γ′ dispersions are also reported.     

Structural Dependence of Creep/Fatigue Behaviour of Single Crystal Ni-base Superalloy

The effect of different initial microstructures deftned by γ' precipitate morphology has been investigated at the creep/fatigue conditions of 900℃ and 500 MPa. The wave form of stress as a function of time for cyclic load was of trapezoidal shape with a hold time of 10s at the upper stress level. The TEM was employed to examine the deformation process in strengthened γ' matrix in dependence of γ' precipitate morphology. The fracture lifetime and cycle number up to fracture were the criteria to evaluate the additional cyclic component efFect on the course of deformation     

Weakening Mechanisms of Platelike σ Phase on Ni-base Superalloys

On the basis of an investigation on σ phase inNi-base cast superalloy K24 and the results aboutσ phase in other Ni-base superalloys,an embrittl-ing mechanism and a softening mechanism,bywhich platelike σ phase weakens the Ni-basesuperalloys,have been proposed.It is consideredthat the platelike morphology and the habit precipi-tation along{111}of σ phase are necessary condi-tions for both mechanisms.The embrittling mecha-nism is dominant at room temperature and highstrain rate,and the softening mechanism is domi-nant at high temperature and low strain rate.Ac-cording to the idea of the softening mechanism andthe analyses of σ phase and alloy compositions,it isconsidered that Nb,Mo and W in the alloys may beresistant to the detrimental effect of σ phase on thestress-rupture properties of the alloys.     

Hydrothermal synthesis of uniform rock salt (α-) MnS transformation from wurtzite (γ-) MnS

Monodisperse α-MnS octahedrons from γ-MnS have been synthesized on a large scale by means of a complexing-agent-assisted solution route. The as-prepared α-MnS microcrystals have a uniform octahedral morphology with an edge of 170–200 nm. It was found that suitable additives could induce the polymorph transformation from metastable γ-MnS to stable α-MnS and modify the morphology of the obtained products. The crystalline structure and morphology of the resultant products were characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM).     

Directionally solidified Soviet superalloy,ZHS6-K

Directional solidification of the Soviet superalloy, ZHS6-K, has been carried out in an argon atmosphere. Tensile and stress-rupture properties have been measured for the investment cast and directionally solidified (DS) alloy. The DS alloy shows a several fold increase in rupture life and ductility compared with the investment cast alloy It also shows improved tensile properties. Stress-rupture and tensile fracture behaviour has been examined.     

Study of the low-cycle fatigue of steel ÉI415 after prior creep

Experimental studies have been carried out on the effect of prior creep on the resistance of steel ÉI415 to low-cycle fatigue. It has been shown that partial exhaustion of the stress-rupture strength life at high stresses does not reduce the low-cycle strength and deformation properties of the material in question.Moscow Energy Institute. Translated from Problemy Prochnosti, No. 10, pp. 23–26, October, 1989.     

Microstructure and Stress-Rupture Life of Polycrystal, Directionally Solidified, and Single Crystal Castings of Nickel-Based IN 939 Superalloy

A comparative investigation of microstructural and mechanical properties (stress-rupture life) in conventionally cast, directionally solidified, and single crystal IN 939 superalloy has been undertaken. Directional castings possess only a few columnar grains, all oriented in the <100> crystallographic direction, whereas only one grain is present in a single crystal. Single crystals are characterized by the highest values of stress-rupture life, much higher than those of directionally solidified and, especially, polycrystal castings, which is accounted for by the absence of grain boundaries.     

σ-Phase Embrittlement in Some ferritic-austenitic stainless steels

Four commercially established ferritic-austenitic stainless steel grades have been investigated with respect to σ-phase embrittlement by exposure in the temperature interval 700–900 °C. The results illustrate the fact that this type of steels is rather susceptible to such an embrittlement, and this has to be considered when selecting steel grades for different applications. It is also shown that an increased austenite content in fact promotes σ-phase formation. Molybdenum as alloying element is found to favour σ-phase much stronger than chromium (4–5 times counted per wt%), hence enlarging the critical temperature range considerably towards higher temperature. Further, a simple “σ-equivalent” has been developed by which semiquantitative comparisons of the susceptibility to σ-phase embrittlement might be made between different steel grades, based on their alloy composition.     

β→α Transformation of γ-Phase in Sintered WC-Co Cemented Carbides

Varying the morphology and the structure of γ-phase (Co-base Co-W-C solid solution) by means of altering the cooling rate and the preparing method of liquid sintered WC-Co cemented carbides samples, the mechanism of fcc→hcp transformation of γ-phase in WC-Co alloy has been explored. The results show that, the cooling rate is an important affecting factor on fcc→hcp transformation of γ-phase and the fcc→hcp transformation is mainly a diffusive type when cooling WC-Co samples above room temperature