Dual-System Activation of Ni-Base Superalloy under High Strain and High Temperature

Due to their superior combination of heat resistance, high temperature corrosion resistance, toughness and strength, nickel-based superalloys have become of extensive use in the aerospace industry. This research aims to explain why the fatigue life of Inconel-718 in preconditioned samples had larger fatigue lives than pristine samples. The hypothesis is that preconditioning at 700°C and 1.0% strain could lead to thermal activation of the {100} cubic slip plane alongside the {111} octahedral slip plane, potentially improving fatigue life. Using SEM and EBSD imaging, the microstructure of Inconel-718 samples were characterized before and after preconditioning. The directions of the slip bands that formed following the preconditioning were determined. The result was that the existence of both the cubic and octahedral slip systems was confirmed, leading to the thermal activation hypothesized. The existence of both slip planes was considered to be the reason behind the improved fatigue life due to better strain accommodation within the microstructure. It is suggested that focuses for future research includes conducting in-situ observation of slip activation and the application of preconditioning as a manufacturing method.

Optimization of the heat treatment of additively manufactured Ni-base superalloy IN718

Additive manufacturing(AM) of Ni-base superalloy components can lead to a significant reduction of weight in aerospace applications. AM of IN718 by selective laser melting results in a very fine dendritic microstructure with a high dislocation density due to the fast solidification process. The complex phase composition of this alloy, with three different types of precipitates and high residual stresses, necessitates adjustment of the conventional heat treatment for AM parts. To find an optimized heat treatment, the microstructures and mechanical properties of differently solution heat-treated samples were investigated by transmission and scanning electron microscopy, including electron backscatter diffraction, and compression tests. After a solution heat treatment(SHT), the Nb-rich Laves phase dissolves and the dislocation density is reduced, which eliminates the dendritic substructure. SHT at 930 or 954°C leads to the precipitation of the δ-phase, which reduces the volume fraction of the strengthening γ′-and γ′′-phases formed during the subsequent two stage aging treatment. With a higher SHT temperature of 1000°C, where no δ-phase is precipitated, higher γ′ and γ′′ volume fractions are achieved, which results in the optimum strength of all of the solution heat treated conditions.

Effect of solidification rate on MC-type carbide morphology in single crystal Ni-base superalloy AM3

In order to study the effect of the withdrawing rate on carbide morphology,MC-type carbide in single crystal superalloy AM3 was systematically investigated with sample growth rates from 3.5 μm/s to 500 μm/s.The carbide morphologies were investigated by scanning electron microscopy(SEM),and the electron probe microanalysis(EPMA) was used to characterize the carbide composition.The results indicate that the solidification rate is the important factor governing MC carbide growth morphology,size and distribution,composition and growth mechanism.With the increase of withdrawing rate,nodular,rod-like,Chinese script types of carbide morphology are observed.For the low withdrawing rate,with the increase of withdrawing rate,the carbide size becomes larger.For the case of dendritic interface,the carbide size becomes smaller with refinement of dendrites as withdrawing rate increases.The volume fraction of carbides increases with the withdrawing rate increasing.

Desulphurization during VIM Refining Ni-base Superalloy using CaO Crucible

The variation of S content during VIM refining Ni-base superalloy using CaO crucible was studied. It was foundthat the desulphurization process could not be carried out by only using CaO crucible. The role of Al additionto desulphurization was also studied. Combining with the results of XRD and composition analysis of the CaOcrucible, the mechanism of desulphurization was proposed. Thermodynamical calculation about the reaction betweenthe interface of CaO crucible and liquid metal has been discussed. This work indicated that under proper refiningtechnology the S content in the liquid Ni-base alloy could be reduced from 3×10-5 to 2×10-6~4×10-6.

γ'Precipitation in an Oxide Dispersion-Strengthened Ni-Base Superalloy

?precipitate morphology and its dissolution/precipitation kinetics in a special oxiddispersion-strengthened Ni-base superalloy have been investigated. Using an advanced computer programmebased on regression analysis, the volume fraction of γ'and its composition as a function of temperaturewere calculated. The lattice constants of both γ'precipitates nd γ matrix have been compared and theresultant lauice misfit computed. Finally, the concentration of AI remaining in γ matrix was determined.The results show generally good agreements with available data for ordinary superalloys. Cornparison ismade against experimental study by using differential scanning calorimetry.

Influence of long-term isothermal aging on microstructure and creep rupture properties of Ni-base superalloy M4706

After a standard heat treatment,the microstructural evolution with time during isothermal aging at 850°C and its effect on the creep rupture properties of the Ni-base superalloy M4706 at 870°C and 370 MPa are investigated.It is found that as the aging time increases from 0 to 5000 h,the average diameter of coarseγ′increases from 241 to 484 nm,and the distribution of the carbides at grain boundaries changes from discontinuous to continuous.Moreover,experimental observations on the microstructures of all the crept specimens reveal that dislocation bypassing controls the creep deformation.Thus,it is concluded that the transitions in the microstructures result in the degeneration of the creep rupture properties of the experimental alloy with aging time.

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

DYNAMICS OF PRECIPITATION AND TRANSFORMATION OF CARBIDES IN A Ni-BASE SUPERALLOY

The quantitative analytical method combining the electrolytic extraction with X-ray diffraction analysis to determine the carbides and TCP in a Ni-base superalloy was developed.The variation of decomposition of MC and precipitation of M_(23)C_6 and M_6C with hot exposure time at 850—1000℃ is shown to conform to hyperbolic law.The elemental con- stitution of M_(23)C_6 changes continutously during the hot exposure process.At the primary stage the constitution of the precipitates is(Cr_(0.67)Mo_(0.13)Ni_(0.13)Co_(0.07))_(23)C_6.Then with in- creasing Cr and decreasing Mo,Ni and Co it finally becomes(Cr_(0.88)Mo_(0.07)Ni_(0.03)Co_(0.02))_(23)C_6. The higher the hot exposure temperature,the faster the final constitution of M_(23)C_6 may be achieved.

EFFECT OF SULPHURIZING-OXIDIZING ENVIRONMENT ON CREEP CRACK GROWTH OF Ni-BASE SUPERALLOY GH30

An investigation was made on the creep crack growth behaviour under static load at 1023 K for Ni-base superalloy GH30 exposed to air and air+10% SO_2.The results showed that in the region of low stress intensity factor,the creep crack growth rate is higher in air+10% SO_2 than in air only,while in the high region,it is reverse.The fractograph of specimens has been analyzed,and the mechanism of creep crack growth together with the influences of sul- phur and oxygen has been discussed as well.

PROPERTIES OF YTTRALUMINIZING COATING ON Ni-BASE SUPERALLOYS

The yttraluminizing coating on the Ni-base superalloys has been developed by dipping the specimens in a specially made yttrium-containing slurry as an underlayer and over spraying a Cr-Al slurry as a protective layer,then heating to 1060℃ and holding for 1 h under argon atmosphere.Analysis by EPMA showed that the yttrium was concentrated in the intermediate zone of the coating as metallic compound Ni-5Y.After exposure to air up to 1000℃,the oxidation test revealed that the anti-oxidation resistance of the yttraluminizing coating with post-aluminizing is much superior to that with single aluminizing only.

EFFECT OF DUCTILE-BRITYLE TRANSITION TEMPERATURE OF Al-Si COATING ON FATIGUE PROPERTIES OF Ni-BASE SUPERALLOYS

The effect of Al content on the ductile-brittle transition temperature of Al-Si coating and the effect of fracture behaviour.of the Al-Si coating on the fatigue properties of Ni-base superallovs have been investigated to propose the theoretical basis of search for an optimum of mechanical properties of the high temperature coating on Ni-base alloys.

MC CARBIDE FLOATATION IN A HOT CORROSION RESISTANT SINGLE CRYSTAL Ni-BASE SUPERALLOY DURING DIRECTIONAL SOLIDIFICATION

The floating phenomenon of MC carbide(TiC)in a hot corrosion resistant single crystal Ni-base superalloy was observed during planar and cellular interface directional solidification.The explanation about the phenomenon is presented.

FATIGUE CRACK PROPAGATION OF Ni-BASE SUPERALLOYS

Time-dependent Fatigue Crack Propagation (FCP) behaviors of five Ni-base superalloys were investigated at various temperatures under fatigue with various holding times and sustained loading conditions. The new concept of damage zone is defined and employed to evaluate the alloys' resistance to hold-time FCP. A special testing procedure is designed to get the maximum damage zone of the alloys. Udimet 720 and Waspaloy show shorter damage zones than alloys 706 and 718. The fractographical analyses show that the fracture surfaces of the specimens under hold-time fatigue conditions are mixtures with intergranular and transgranular modes. As the extension of holding time per cycle, the portion of intergranular fracture increases, The effects of loading stress intensity, temperature, holding time, alloy chemistry, and alloy mi-crostructure on damage zone and the crack growth behaviors are studied. Hold-time usually increases the alloy's FCP rate, but there are few exemptions. For instance, the steady state hold-time FCP rate of Waspaloy at 760℃ is lower than that without hold-time. The beneficial effect of hold-time was attributed to the creep caused stress relaxation during the hold-time.

Effect of Solidification Interface Morphologies on Microstructures of a Ni-base Superalloy

A cast Ni-base superalloy K5 was directionally solidified and various solidification in- terfaces including plane front,cellular, cellular-dendritic and dendritic were obtained in a wider range of G/R ratio by using improved high withdrawal device and liquid metal cooling experi- mental sets.The precipitation pattern of some prin- cipal phases of the alloy and correlation of the vari- ous interfaces with microstructure were studied sys- tematically.It was indicated that the morphology of solidification interface of superalloy K5 varied with G/R ratio and that the solidification interface morphologies have a considerable effect on the fea- tures of phases both precipitated during solidification and post-solidification.Plane front and cellular directional solidification of superalloy K5 lead to a substantial decrease of MC carbide and elimination of γ-γ'eutectic,but make needle-shape M_6C carbide precipitate easily during ageing treatment.The finer dendritic structures solidified under the condition of higher cooling rate have less dendritic segregation and ideal microstructure.

Weakening Mechanisms of Platelike σ Phase on Ni-base Superalloys

On the basis of an investigation on σ phase in Ni-base cast superalloy K24 and the results about σ phase in other Ni-base superalloys,an embrittl- ing mechanism and a softening mechanism,by which platelike σ phase weakens the Ni-base superalloys,have been proposed.It is considered that 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 high strain rate,and the softening mechanism is domi- nant at high temperature and low strain rate.Ac- cording to the idea of the softening mechanism and the analyses of σ phase and alloy compositions,it is considered that Nb,Mo and W in the alloys may be resistant to the detrimental effect of σ phase on the stress-rupture properties of the alloys.

The Behaviour of Mg in γ-phase in a Ni-base Superalloy

1.IntroductionSince the biginning of 1960s,the effectof Mg upon superalloy has been studiedand some satisfactory developments havebeen achieved.The role of Mg on the alloycan be summarized as follows:(1)Themechanical plasticity of Ni-base superalloycan be improved by adding Mg asdesulphurizer,because of forming insolubleMgS instead of other low melting eutectic

Effects of electric-field treatment on a Ni-base superalloy

The effects of electric-field treatment on the microstructure and deformation behavior of a nickel-base superalloy were summarized.The results show that the electric-field treatment increases the ductility of the superalloy but has no evident influence on its static strength at both room and elevated temperatures,while,the strength increases but elongation changes weekly with the increasing tensile strain rate.It is found that the direction of microcrack propagation can be changed by the presence of the annealing twins during the tensile deformation,and it causes the increasing of the plastic deformation energy and delay of the fracture,which is considered as the reason for the increasing the ductility.

EFFECTS OF MICROSTRVCTURE ON FATIGUE CRACK GROWTH BEHMIOR IN Ni-BASE SUPERALLOY GH586

Threshold stress values, muging from ～8 to 16MPa·m1/2 can be obtained in a Ni-base alloy GH586 by varying the microstructure through heat treatments. The threshold and low crack growth rate behaviors at room temperature, with varying groin size and γ'-distribution, have been investigated. The results indicate that grain size is an important microstructurol parameter that affects fatigue crack growth threshold and propagation behaviors, as the values of △Kth increase with increasing grain size, but the γ' -distribution also has important effect. Analyses show that the effects of groin size on threshold and low crack-growth rate behavior result from heterogeneous deformation and roughness-induced crack closure due to crystallographic slipping and cracking in coarse microstructure. The higher △Kth and lower fatigue crack growth rate with increasing amounts of fine γ' phases are closely related to less damage accumulation level in deformation zone of crack-tip resulted from heterogeneous deformation due to dislocations' shearing γ' precipitates.

SOLIDIFICATION MICROSTRUCTURE OF Ni-BASE SUPERALLOY POWDER

The solidification microstructure and phase composition of Ni-base superalloy FGH95 pow- der with different mesh size have been investigated.The structure transition was found from dendrite in major into cellular structure as the powder size reduces and the cooling rate in- creases.The predominant phase was identified as MC-type carbide,with different morphologies,which may be related to their composition and the condition of solidification.Minor phases,such as boride,Laves and primary γ' are also present as ac- companiments of the carbide.

Effect of Melt Superheating on the Morphology of MC Carbide in a Cast Ni-base Superalloy M963

The effect of the melt superheating temperature on the as cast microstructure of a cast nickel base superalloy M963 has been investigated. The results show that the as cast microstructure of the alloy consists of γ solid solution matrix,γ′ precipitate in cubic shape, (γ+γ′) eutectic and MC carbide, and the morphology of MC carbide in the microstructure can be varied from coarse scriptlike, fine scriptlike to fine cubelike or discontinuous particles by increasing the melt superheating temperature. The mechanism of melt superheating is discussed by means of differential thermal analysis (DTA) technique.