Effect of long-term thermal exposure on microstructure and creep properties of DD5 single crystal superalloy

The effect of thermal exposure on the microstructure and creep properties of the Ni-based single crystal superalloy in different test conditions was studied.Long-term exposure was performed at 1,000 ℃ and 1,100 ℃ for 500 h prior to the creep tests.The creep lifetime is found to be improved after the long-term exposure at 1,000 ℃ for 500 h as a result of the formation of secondary M_(23)C_(6) in the interdendritic region.The coarsening of γ’ precipitates accompanied by the formation of TCP phase lead to the degradation of alloy,which is responsible for the reduction of the creep lifetime of Ni-base single crystal superalloy after long-term exposure at 1,100 ℃ for 500 h.The creep lifetime of 1,000 oC thermally exposed sample under the conditions of 1,093 ℃/137 MPa is lower than that of heat-treated state.Thermal exposure at 1,100 ℃ for 500 h causes the creep lifetime to drop drastically.

Influence of thermal exposure on microstructure and stress rupture properties of a new Re-containing single crystal Ni-based superalloy

In this study, the long-term thermal microstructural stability and related stress rupture lives of a new Re-containing Ni-based single-crystal superalloy, DD11, were investigated after high-temperature exposure for different lengths of time. The results show that the γ' precipitates retained a cuboidal morphology and the γ' size increased after short thermal exposure for 50 h at 1,070℃. As the thermal exposure time was prolonged to 500 h, the cuboidal γ' gradually changed into irregular raft-like morphology due to particles coalescence, and the morphology of the microstructure was almost unchanged after further thermal exposure up to 3,000 h. The stress rupture experiments at 1,070℃ and a tensile stress of 140 MPa showed that the rupture lives increased significantly after thermal exposure for 50 h and dropped dramatically with increasing exposure time up to 500 h but decreased slowly after exposure for more than 500 h. These results imply that stress rupture properties did not decrease when the γ' remained cuboidal but degraded to different extents during the γ' coarsening process. The coarsening of the γ' precipitates and change in morphology were regarded as the main factors leading to the degradation of the stress rupture lives. This study provides fundamental information on the high-temperature longterm microstructural stability and mechanical performance, which will be of great help for DD11 alloy optimization and engineering aeroengine applications.

Influence of Thermal Exposure on Mechanical Properties of C/Al Composite

In this paper were studied the microstructure and properties of the non-barrier coating compo- sites fabricated by a new technology.The bonding between C and A1 in the composites is quite well, and the composites have excellent properties.The ion probe and X-ray analyses indicate that there is Al_4C_3 phase in the composites and its amount in the composites increases when the composites are exposed.In the case of exposure the strength of the composites is reduced because of increasing amount of Al_4C_3.

Effect of thermal exposure on the microstructure and creep properties of a fourth-generation Ni-based single crystal superalloy

The effect of thermal exposure on microstructure and creep properties of a fourth-generation nickelbased single crystal superalloy was investigated.The thermal exposure of samples after the full heat treatment was carried out at 1000℃,1100℃ and 1140℃ for 100 h and 200 h.The γ’ coarsening,γ’ rafting and γ channel widening were observed in samples after thermal exposure.When the thermal exposure time was constant,the morphology of γ’ phase in the alloy evolved significantly with increasing aging temperature.The interracial dislocation networks in aged samples after creep ruptured gradually became irregular and sparse with the increase of exposure temperature.When the higher exposure temperature was used,enla rgement of the defect pores was observed in samples,the microcracks were more likely to initiate and propagate at the corner of these pores.After aging at 1000℃ for 100 h,the creep life at 1140℃/137 MPa was slightly longer than that of heat-treated sample,which could be attributed to the slightly coarsened γ’ phase,homogenization of refractor elements.In contrast,the creep life of sample exposed at 1140℃ for 100 h was greatly decreased.The decrease of creep life was dominated by the rafting of γ’phase,the irregular interfacial dislocation networks as well as the enlargement of homogenization pores.

Effect of Long-Term Thermal Exposures on Tensile Behaviors of K416B Nickel-Based Superalloy

The effect of long-term thermal exposure on the tensile behavior of a high W content nickel-based superalloy K416B was investigated.The microstructure and the deformation characteristics were observed by scanning electron microscopy and transmission electron microscopy,and the phase transformation of the alloy during long-term thermal exposure was analyzed by X-ray diffraction patterns and differential thermal analysis.Results showed that after thermal exposure at 1000℃,the MC carbides in the K416 B alloy decomposed into M_(6)C.During tensile deformation,dislocations slipping inγmatrix crossed over the M_(6)C by Orowan bowing mechanism.With the increase of thermal exposure time,the secondary M_(6)C reduced greatly the yield strength of the alloy at room temperature.Meanwhile,the continuous distribution of the secondary M_(6)C with great brittleness in the grain boundary could become the main source of crack,which might change the fracture characteristic of the alloy from trans-granular to intergranular.

Stability of NiCrAlY coating/titanium alloy system under pure thermal exposure

In this paper,NiCrAlY coating was deposited on TC4 titanium alloy using arc ion plating,and the stability of NiCrAlY coating/TC4 substrate system under pure thermal exposure was analyzed in a wide temperature range.During the thermal exposure,β→γ′→γphase transformation takes place in the NiCrAlY coating.The NiCrAlY coating phases totally decompose above the allotropic transformation temperature of TC4.The allotropic transformation ofα-Ti to β-Ti of the substrate significantly influences the stability of NiCrAlY coating/TC4 substrate system.NiCrAlY coating elements are mainly consumed by interfacial reactions below the allotropic transformation temperature of TC4.Above the allotropic transformation temperature of TC4,NiCrAlY coating elements are mainly consumed by element dissolution in β-Ti.The NiCrAlY coating totally degrades for the dissolution of coating elements in the substrate to form thickβ-Ti stabilized layer.

Effect of Long-term Thermal Exposure on Microstructure and Stress Rupture Properties of GH3535 Superalloy

The evolution of microstructure and the stress rupture properties of long term thermally exposed GH3535 alloy have been investigated. It was found that M6 C carbides presented in the solid solution heat treated samples. During long term thermal exposure at 700 C, fine M12 C carbides precipitated preferentially at grain boundaries. These carbides coexisted with the pre-exiting M6 C. The stress rupture life of700 C/1000 h exposed sample under creep testing at 650 C/324 MPa is 93 h. It is much longer than that of the solid solution samples. No noticeable changes could be detected in both the microstructure and stress rupture lives when the samples were exposed for time longer than 1000 h M12 C carbides were found to be beneficial to the creep properties. The cracks initiated at the interface of M6 C carbides and matrix, which led to a lower creep rupture life.

Microstructural evolution and its influence on the impact toughness of GH984G alloy during long-term thermal exposure

The microstructure evolution and its effect on the impact toughness of a new Ni-Fe based alloy GH984 G,used in 700℃ ultra-super critical coal-fired power plant,were investigated during thermal exposure at 650℃-750℃ for up to 10,000 h.The results show that the impact toughness at room tempe rature drops rapidly at the early stage during thermal exposure at 700℃ and then has no significant change even if after exposure for 10,000 h.The significant decline of the impact toughness is attributed to the coarsening of M_(23)C_(6) carbides at grain boundaries,which weakens the grain boundary strength and leads to the aging-induced grain boundary embrittlement.The M_(23)C_(6) carbides have almost no change with further thermal exposure and the impact toughness also remains stable.Additionally,the impact toughness rises with the increase of thermal exposure temperature.The size of γ' after thermal exposure at 750℃ for10,000 h is much bigger than that at 650℃ and 700℃ for 10,000 h.There fore,the intragranular strength decreases significantly due to the transformation of the interaction between γ' and dislocation from stro ngly coupled dislocation shearing to Orowan bowing.More plastic deformation occurs within grains after thermal exposure at 750℃ for 10,000 h,which increases the impact toughness.

Microstructural evolution and tensile property of 1Cr15Ni36W3Ti superalloy during thermal exposure

1Cr15Ni36W3Ti was thermally exposed at 580℃and 680℃,respectively,up to 3000 h.Theγ’phase and intergranular TiC carbides continuously coarsened during exposure.None ofη,laves orσphase was discovered in the exposed samples,indicating good microstructure stability under the present exposure conditions.The ripening process of theγ’phase could be well modelled utilizing the LSW theory.The evolutions of the yield and tensile strengths were monotonous during exposure at 580℃.However,a transition point in strengths was detected in the tensile samples exposed at 680℃for 300 h.Accordingly,the criticalγ’diameter was measured to be 13-14 nm.Theγ’/dislocations interaction mechanism transformed from shearing to looping with theγ’diameter exceeding the critical point.The combination of the weakly coupled dislocations model and the Orowan looping model yielded a critical diameter of 13.1 nm which coincided well with the measured one,indicating the applicability of these two strengthening models for 1 Cr15Ni36W3Ti.The present exposure conditions did not exert a profound effect on the fracture mode.All the tensile samples underwent a typically ductile fracture with a dimple pattern dominating the fracture surface.The dispersed deformation induced by the prevalence of dislocation looping in the over-aged tensile samples retarded the propagation of intergranular cracks.The declined precipitation hardening increment and the enhanced deformation homogeneity partially recovered the tensile ductility in the over-aged samples exposed at 680℃.

Precipitates evolution and tensile behavior of wrought Ni-based ATI 718Plus superalloy during long-term thermal exposure

In this paper,the microstructure evolution and tensile behavior of wrought ATI 718 Plus superalloy during long-term thermal exposure(LTTE) were investigated.The γ’ phase evolves from spherical morphology to cuboidal morphology at 800℃,which is related to increasing γ’/γ misfit because of the increase of elastic energy.The amounts of η phase obviously increased and γ’ precipitate free zones(PFZs) were found in 800-LTTE samples due to the increase of η phase amount by consuming Al,Ti,and Nb.According to the formula of “MC+γ→M_(23)C_(6)+η”,η phase is also related to the decomposition of MC carbide,which involves the diffusion and segregation of Nb and Ti.In addition,some short rod-like and irregular Cr,Mo-rich σ phases were prone to precipitate near η phases after LTTE at 800℃ for 500 and 1000 h.σ phase has a crystallographic relationship with ηphase as(002)_(σ)//(10■0)_(η),[0■0]_(σ)//[01■0]_(η).The deteriorated tensile strength after LTTE is mainly attributed to that:(1) the evident coarsening of γ’ phase results in that the deformation mechanism transforms from stacking faults(SFs) shearing to Orowan looping,which is also convinced by calculated critical resolved shear stress(CRSS);(2) the noticeable increasing number of η phase contributes to stress concentration,resulting in micro-cracks formation;and (3) the apparently increasing number of η phase promotes the PFZs formation.

Mechanical Behavior and Failure Mechanisms of Carbon Fiber Composite Pyramidal Core Sandwich Panel after Thermal Exposure

An attempt has been made here to evaluate the effect of thermal exposure on the mechanical behavior and failure mechanisms of carbon fiber composite sandwich panel with pyramidal truss core under axial compression. Analytical formulae for the collapse strength of composite sandwich panel after thermal exposure were derived. Axial compression tests of composite laminates and sandwich panels after thermal exposure were conducted at room temperature to assess the degradation caused by the thermal exposure. Experimental results showed that the failure of sandwich panel are not only temperature dependent, but are time dependent as well. The decrease in residual compressive strength is mainly attributed to the degradation of the matrix and the degradation of fiber-matrix interface, as well as the formation of cracks and pores when specimens are exposed to high temperature. The measured failure loads obtained in the experiments showed reasonable agreement with the analytical predictions.

Precipitation of Ti 2 Al phases at lamellar interfaces in a high-Nb-containing TiAl alloy during thermal exposure

The thermal stability of microstructures is crucial for determining the performance of alloys in extreme environments.In the present work,the microstructural evolution and precipitation behavior in a high Nb-containing Ti45Al8Nb alloy during thermal exposure at 950°C were investigated.It was found that excessα2 phases in the as-cast microstructure were unstable and tended to decompose during thermal expo-sure.Hexagonal Ti 2 Al phases precipitated at lamellar interfaces and had a[1¯10]_(γ)[11¯20]_(α2)[11¯20]_(Ti2Al),(002)_(γ)(1¯100)_(α2)(1¯100)_(Ti2Al)crystallographic orientation relationship(OR)with the matrix.Stacking faults(SFs)generated inα2 phases during theα2→γphase transformation provided favorable nucleation sites for Ti 2 Al phases.

Formation Mechanism of Lamellar M_(23)C_6 Carbide in a Cobalt-Base Superalloy During Thermal Exposure at 1000℃

The precipitation of the lamellar-shaped M23C6 carbide within the dendritic matrix of a cobalt-base superalloy during thermal exposure at 1000 ℃ has been investigated. Such a precipitation is not commonly observed in cobalt-base superalloys. It is found that M23C6 particles nucleate preferentially at stacking faults （SFs） in the dendritic matrix and grow along the SFs to develop a lamellar character. Additionally, a Cr depletion zone is observed in the vicinity of the lamellar M23C6 carbide, which strongly supports the presence of Suzuki segregation.

MICROSTRUCTURAL STABILITY OF LAMELLAR TiAl BASED ALLOYS AT HIGH TEMPERATURES

The microstructural stability of lamellar TiAl base alloys at high temperatures was studied by conventional and high resolution transmission electron microscopy. The influence of substructures on the thermal stability of lamellar structure was emphasized. These substructures produced by thermal mechanical treatments include the interfacial dislocations and ledges, the subgrain boundaries, the impinged T(Q) twins and misorientated lamellar interfaces. The microstructural change of three kinds of lamellar TiAl base alloys containing differents type and densities of substructures were compared during exposure at 800~1 000 ℃. It was found that the existence of such substructures could accelerate the degeneration of lamellar structure, leading to the rapid necking and break up of α 2 plates, the coarsening of γ plates, and the formation of new γ grains. As a result, the lamellar structure with substructures started to degenerate after thermal exposure at 800℃ for 4.5 h. While only slight coarsening was observed at the colony boundaries in the lamellar structure without substructures even after exposure at 900 ℃ for 7 d.

Development of Ti-Al-Ta-Nb-(Re)near-αhigh temperature titanium alloy:Microstructure,thermal stability and mechanical properties

Mechanical properties and microstructural stability under the service temperature are important to the high temperature titanium alloy.In order to evaluate the potential in increase the service temperature of Ti alloy,two near-αTi alloys with high content of Al asα-stabilizer and Ta,Nb and/or Re asβ-stabilizers were designed and prepared by ingot metallurgy and thermomechanical processing,and the microstructure and mechanical properties before and after thermal exposure at 650℃ for 100 h were characterized.The results indicated that due to the weakβ-stabilizing ability of Ta and Nb elements,only a small amount ofβphase was formed in Ti-10Al-4Ta-2Nb alloy.With a trace Re addition,theβphase was obviously increased in Ti-10Al-4Ta-2Nb-0.25Re,indicating that the Re was a strongβ-stabilizer.Under the same thermomechanical conditions,the Re addition decreased the volume fraction of primaryα(α_(p))phase and refined the secondaryα(α_(s))phase evidently.The primaryαphase presented an obvious core-shell structure in the Ti-10Al-4Ta-2Nb alloy,with higher Al concentration in the shell.While the core-shell structure was not obvious in the Re-containing alloy due to the Re decreases the diffusion of Al,Ta and Nb elements.A large number of orderedα_(2)precipitates can be observed in theα_(p)andα_(s) phases of two alloys.Theα_(2)precipitates continuously grew up during thermal exposure,however,their growth rate in theα_(s)phase of Re-containing alloy were lower than that of Ti-10Al-4Ta-2Nb alloy.Although plenty of orderedα_(2)precipitates formed in the Ti-10Al-4Ta-2Nb alloy,the alloy had a certain plasticity at room temperature.The trace Re addition evidently increased the tensile strength but caused the decrease of the plasticity.After thermal exposure,the strength was further increased,while the plasticity was decreased for both of alloys.

Improved properties and thermal stability of a titanium-stainless steel solid-state weld with a niobium interlayer

Stainless steel(SS)and titanium alloys can be welded in the solid-state by high speed oblique impact.Here,the effect of a niobium(Nb)interlayer on weld strength and thermal stability is evaluated.Both Ti/SS and Ti/Nb/SS welds were subjected to thermal exposure ranging from 300℃ to 950℃ for 1.5 h.Thermal exposure monotonically decreases the strength and toughness of the Ti/SS pair with a dramatic falloff in strength and change in failure mode from partial pullout failure to full interfacial fracture at 600℃.With the interlayer,toughness was increased versus baseline up to 700℃ thermal exposure and then intermetallic formation again caused falloff in properties.Guidelines for the production,properties and applications of these classes of welds are provided.

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 ofγ'increased by 40 nm,the matrix channel width increased by 5 nm,and the misfit degree ofγ/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 ofγ'phase and precipitation ofμphase.Theγ'particles of the alloy with high AI content tended to connect each other and coarsened along<100>directions;however,theγ'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.

A new type-γ'/γ''coprecipitation behavior and its evolution mechanism in wrought Ni-based ATI 718Plus superalloy

In the precipitation-hardened Ni-based superalloy,typified by ATI 718 Plus,the nano-scaleγandγphase in duplet or triple coprecipitate morphology can provide superior high-temperature strength.Thus,it is of great sense to study the evolution ofγ’/γ’’coprecipitate during long term service at elevated temperature.In this study,the new-typeγ’/γ’’coprecipitates with a sandwich or compact configuration were found firstly in wrought ATI 718 Plus superalloy during long term thermal exposure at 705℃.These co-structure of theγ’/γ’’precipitates evidently inhibit the coarsening ofγ’phase.The increase of thermal exposure time evidently leads to the increase of the volume fraction ofγ’/γ’’coprecipitate and transformation of sandwich-typeγ’/γ’’coprecipitate to compact-typeγ’/γ’’coprecipitate,which is characterized asγphase precipitate at several faces of theγphase.The main evolution mechanism ofγ’/γ’’coprecipitates is element segregation,especially the composition variations of Al+Ti and Nb and their ratio of Al+Ti/Nb.In addition,the interfacial energy betweenγ’’phase andγmatrix also plays a key role on theγ’/γ’’coprecipitates evolution.The calculated results show that the longer thermal exposure time leads to the higher interfacial energy,which is beneficial for nucleation and precipitation ofγ’’phase on the faces ofγ’phase.