Low-Cycle Fatigue and Creep-Fatigue Behaviors of a Second-Generation Nickel-Based Single-Crystal Superalloy at 760 ℃

Low-cycle fatigue (LCF) behaviors of a second-generation nickel-based single-crystal superalloys with [001] orientation at 760 °C have been investigated. Different strain amplitudes were introduced to investigate the creep-fatigue effects. The LCF life of none tensile holding (NTH) was higher than that of the 60-s tensile hold (TH) at any strain amplitude. As the strain amplitude was 0.7%, the stacking and cross-slip dislocations appeared together at the γ/γ’ coherent microstructure in both TH and NTH specimens. At the strain amplitude of 0.9%, plenty of the cross-slip dislocations appeared in γ channel and other dislocations were stacking at γ/γ’ interfaces. However, the SFs still appeared in γ’ phase with 60-s TH which caused cyclic softening. As the strain amplitude increased up to 1.2%, the dislocations are piling up at the γ/γ’ interfaces and cutting through the γ’ phase in both TH and NTH tests, which caused cyclic hardening. The influences of strain amplitude and holding time were complicated. Different stress response behaviors occurred in different loading conditions. The surface characteristic and fracture mechanism were observed by scanning electron microscopy. This result is helpful for building the relationship of various blade fatigue failure modes, cyclic stress response and microstructure deformation under different strain amplitudes.     

Microstructural Evolution of Oxidation Film on a Single Crystal Nickel-Based Superalloy at 980 °C

The isothermal oxidation behavior of a single crystal nickel-based superalloy was investigated at 980 °C through XRD, SEM/EDX and EPMA. The mass gain process exhibited two periods: an initial stage followed by a steady-state stage. Based on the experimental results, the rapid formation of alumina and NiO was responsible for the initial stage of mass gain, and the formation of complex spinels phases may dramatically effect on the steady stage. The microstructure of oxidation film, from the top surface down to the base material, was clarified as Ni-rich oxides, Ni–Cr oxides, Cr–Ta–Co oxides, Ni–Al oxides and finally a continuous Al₂O₃. In addition, AlN formed in the γ′-free zone. The effect of oxidation film evolution on the oxidation kinetics and mechanism were discussed.     

Oxidation Behavior of the Nickel-Based Superalloy DZ125 at 980℃

The isothermal oxidation behavior of the nickel-based superalloy DZ125 was investigated at 980 °C through SEM/EDX and XRD. The weight loss process exhibited three periods—initial, transient and steady stages, which correspond to the formation of three layers on the surface. The outer layer was composed of Ni O, whereas the intermediate layer mainly consisted of spinels and was accompanied with Ta-rich oxide. Alumina was evident as the inner continuous layer close to the substrate. The first formation of alumina was responsible for the initial stage of weight loss, and the formation of Ni O and intermediate layer may dramatically affect on the transient and steady stage, respectively. The oxide scales effect on growth mechanism was discussed.     

Anisotropic Stress Rupture Properties of a 3rd-Generation Nickel-Based Single-Crystal Superalloy at 1100℃/150 MPa

The influence of orientation on the stress rupture behaviors of a 3 rd-generation nickel-based single-crystal superalloy was investigated at 1100℃/150 MPa.It is found that the stress rupture anisotropy is shown at 1100℃,but not so obvious compared with that at intermediate temperatures.The [001] specimens display the longest rupture life,[111] specimens show the shortest rupture life,and [011] specimens exhibit the intermediate life.Detailed observations show that the final fracture is caused by crack initiation and propagation,and the anisotropy of three oriented specimens is related to the fracture modes,γ/γ' microstructures,interfacial dislocation networks and cutting mechanisms in y' phase.For [001] specimens,N-type rafted structures are formed which can well hinder the slip and climb of dislocations.Besides,the regular interfacial dislocation networks can prevent dislocations from cutting into y' phase,leading to the improvement of the creep resistance.For [011] specimens,±45°rafted structures and irregular networks result in less strain hardening.For [111] specimens,a large number of crack propagation paths and inhomogeneous deformations caused by irregular rafted structures deteriorate the property and result in the shortest life.Furthermore,a[100] superdislocations with low mobility are widely formed in[001] and [011] specimens which suggests the low creep strain rate during steady creep stage,whereas superdislocations in[111] specimens possess high mobility,which indicates the high strain rate and corresponding poor stress rupture property.     

Oxidation Behavior of a Single-Crystal Ni-Base Superalloy in Air. I: At 800 and 900°C

The oxidation behavior of a Single-crystal Ni-base superalloy was studied using discontinuous thermogravimetric analysis (TGA) and prolonged exposure in air at 800 and 900°C. The mass gain of specimens at 900°C was found to be lower than that of specimens at 800°C because of the formation of a protective inner -Al₂O₃ layer at 900°C. A subparabolic time dependence (n=0.16 at 800°C and n=0.10 at 900°C) of the oxide growth rate was determined at both temperatures. At 800°C, the superalloy exhibited nonuniform oxidation—in some areas a thin scale with an outer NiO layer and an inner layer of an Al-rich oxide was found and, in other areas, complex oxides [CrTaO₄, NiCr₂O₄, (Ni,Co)Al₂O₄, etc.] below the NiO outer layer formed by growing into the superalloy. The scale formed at 900°C is more uniform than that formed at 800°C, consisting of several layers: an NiO outer layer, spinel-rich sublayer, a CrTaO₄-rich layer, and an -Al₂O₃ inner layer. The -Al₂O₃ inner layer provides good oxidation protection and the specimen mass gain is low for test up to 1925 hr.     

Effect of Orientation on Stress-Rupture Property and Related Deformation Microstructure of a Ni-Base Re-containing Single-Crystal Superalloy at 900 ℃

Stress-rupture properties of a Ni-base Re-containing single-crystal superalloy with three orientations have been tested under 900 ℃/445 MPa. An obvious anisotropy of stress-rupture property is attributed to orientation reliant deformation microstructure. The good strength in [001] orientation is attributed to the rapid multiplication of dislocations active in horizontal channels and later γ' cutting via dislocations pair coupled with anti-phase boundary. The microtwin formation largely limits the strength and plasticity as a result of the continuous shearing across γ/γ' microstructure by {111} 112 slip activated in [011] orientation. The property in [111] orientation results mainly from the lateral cross-slip movements of the screw dislocations within connected matrix channels as well as the precipitate shearing by coplanar dislocations. Microcracks all initially originate from the interdendritic micropores in three orientations. The critical temperature of stress-rupture anisotropy could be increased by a high level of refractory solutes especially Re.     

High Temperature Oxidation of the Haynes 282© Nickel-Based Superalloy

Nickel-based superalloys are used in applications where corrosion and oxidation resistance at high temperatures are required together with microstructural stability. Superalloys with different metallurgical characteristics are therefore currently being developed; the high temperature behaviour of these systems must be evaluated. In this investigation, the isothermal oxidation resistance of a Haynes 282^© nickel-based superalloy was studied by gravimetric means in the temperature range 800–1,000 °C for relatively short exposure times up to 150 h. The results from the tests suggest that the parabolic rate law describes the oxidation kinetics of the alloy. The chemical composition of the oxides present in the scale comprised an outer TiO₂ layer and an inner Cr₂O₃ layer, with the latter located at the metal/oxide interface_. In addition, the formation of an internal oxidation zone of Al₂O₃ and TiO₂ was also observed at all temperatures. The role of oxide formation on microstructural changes experienced by the alloy is discussed.     

Sulfidation Behavior of Inconel 738 Superalloy at 500–900°C

The high-temperature sulfidation behavior of the cast nickel-base superalloy Inconel 738 (IN-738) was studied over the temperature range 500–900°C in pure sulfur vapor over the range 10²–10⁴ Pa. The sulfidation kinetics followed the parabolic rate law in all cases. The sulfidation rates increased with increasing temperature and sulfur pressure. The scales formed were bilayered and temperature-dependent. At T700°C, the outer scale consisted of mostly NiS (with dissolved Co) and minor (CoS₂ and NiCo₂S₄, while the inner layer was a heterophasic mixture of NiS, NiCo₂S₄, and minor amounts of Al₂S₃ and chromium sulfide (Cr₂S₃/Cr₃S₄). At T750°C, the outer scale consisted of mostly Ni₃S₂ (with dissolved Co) and minor amounts of Co₃S₄ and Cr₂S₃/Cr₃S₄, while the inner layer was a complex, heterophasic mixture of Ni₃S₂, Cr₂S₃/Cr₃S₄, CoCr₂S₄, and minor Al₂S₃. Platinum markers were found to be located at the interface between the inner and outer scales, suggesting that the outer scale grew by the outward transport of cations and the inner scale grew by the inward transport of sulfur. The formation of Al₂S₃ and Cr₂S₃/Cr₃S₄ partly blocked the transport of cations through the inner scale and consequently reduced the sulfidation rates as compared to pure nickel.     

Oxidation Kinetics and Oxide Scale Characterization of Nickel-Based Superalloy IN738LC at 900 °C

The high-temperature isothermal oxidation behavior of the polycrystalline nickel-based superalloy IN738LC was investigated at 900 °C in air for up to 1000 h. The results from the tests suggest that the alloy showed single-stage parabolic oxidation behavior during isothermal oxidation. The oxidized samples were characterized using SEM and SEM/EDS, and the results show that the alloy is comprised of an outer dense chromia scale with titania proving Type II oxidation behavior. In addition, the formation of a spinel composition adjacent to the external layer and a discontinuous needle-shaped alumina scale in the alloy subsurface zone were also observed. The depletion of gamma prime (γ′) phase leads to a precipitate-free zone formation in the subscale zone. A JMatPro thermodynamic analysis showed that an increase in titanium content from 1 to 3.44 wt.% increased the chromium activity by 50%. Therefore, the results suggest that the presence of high amounts of titanium (~3.44 wt.%) in IN738LC increased the oxidation kinetics by increasing the chromium scale growth rate and resulting in an oxidation rate constant of 2.79 × 10^?6 mg² cm^?4 s^?1.     

Effect of Pressures on the Corrosion Behaviours of Materials at 625°C

The corrosion behaviors of austenitic stainless steels (SS) 310, 304 and Ni- and Fe-based A-286 exposed to 0.1 MPa, 8 MPa and 29 MPa at 625°C for 1000 h were investigated. These represent exposure to superheated steam, subcritical and supercritical water (SCW) at 625°C, respectively. As SS 310 showed the smallest weight change, the oxide cross-sections made from 310 samples were examined by transmission electron microscopy. The results revealed a single-layer oxide at 0.1 MPa and dual-layer oxides at 8 MPa and 29 MPa, followed by a Cr-depleted region into the austenite substrate. The compositions of the inner oxides at 8 MPa and 29 MPa are Cr-rich and largely similar to those of the single-layer oxides at 0.1 MPa exposure. These results suggest that corrosion testing in superheated steam may be a suitable surrogate for scoping tests of materials under SCW conditions at >650°C.     

Oxidation Behavior of a Single-Crystal Ni-Base Superalloy in Air—II: At 1000, 1100, and 1150°C

The oxidation behavior of a single-crystal (SC) Ni-base superalloy was studied over the temperature range from 1000–1150°C and analysed by TGA, XRD, EDAX, and SEM. The results indicated that the SC Ni-base superalloy exhibited parabolic oxidation kinetics, which were controlled by the growth of the inner -Al₂O₃ layer. A mixed scale formed on the SC Ni-base superalloy after prolonged oxidation. The scale consisted of an outer layer of spinel, a sublayer of mainly -Al₂O₃ with small amount of spinel adjoined by a very thin and even discontinuous layer of CrTaO₄-rich oxide, and an inner -Al₂O₃ layer. The inner -Al₂O₃ layer provided good protection. No internal oxides or nitrides were observed below the inner -Al₂O₃ layer after 1000 hr at 1000°C, and after 200 hr at 1100 and 1150°C.     

Formation Mechanism of Lamellar M 23C6 Carbide in a Cobalt-Base Superalloy During Thermal Exposure at 1000 °C

The precipitation of the lamellar-shaped M ₂₃C₆ carbide within the dendritic matrix of a cobalt-base superalloy during thermal exposure at 1000 °C has been investigated. Such a precipitation is not commonly observed in cobalt-base superalloys. It is found that M ₂₃C₆ 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 M ₂₃C₆ carbide, which strongly supports the presence of Suzuki segregation.     

Effect of Surface Roughness on the Oxidation Behavior of a Directionally Solidified Ni-Based Superalloy at 1,100℃

The effect of surface roughness on the oxidation behavior of a directionally solidified Ni-based superalloy was investigated by surface mapping microscope,scanning electron microscope and X-ray diffraction.It was found that specimens with surface roughness of 0.05 urn exhibit the best oxidation resistance,while specimens with surface roughness of 0.14 μm behave worse than specimens with surface roughness of 0.83 μm.The specimens with surface roughness of 0.05 μm have the best oxidation resistance,which is mainly due to the smallest surface area exposed in air and thinnest work-hardening layer.The Al_2O_3 layer alleviates the oxidation process of the specimens with surface roughness of 0.83 μm,and this is the possible reason for the better oxidation resistance of samples with surface roughness of 0.83 μm than samples with surface roughness of 0.14 μm.     

Effects of heat shock at 800 °C on mechanical properties of γ-TiAl alloys

Effect of heat shock on the mechanical properties and the microstructure of TiAl alloys (Ti–47Al–2Cr–2Nb and Ti–45.3Al–2Cr–2Nb–0.1W–0.15B) with near fully lamellar structure were investigated. After heat shock process from room temperature to 800 °C for 500 cycles, the microstructure demonstrated that lamellar microstructure has been destructed by the presentation of some γ and α₂ block phases in lamellar structure, resulting in the ductility of as-polished Ti–47Al–2Cr–2Nb alloy decreased by about 70% and the ultimate tensile strength (UTS) reduced by about 25%, and the ductility of as-unpolished Ti–47Al–2Cr–2Nb alloy decreased by more than 70% and the ultimate tensile strength (UTS) was reduced by about 35%. The ductility of Ti–45.3Al–2Cr–2Nb–0.1W–0.15B alloy decreased by about 60% and the ultimate tensile strength (UTS) reduced by about 18% after heat shock, which was resulted from the appearance of small α₂ block phase at interfaces of lamellar colonies and microcracks at the interfaces of ribbon boride and lamellar structure.     

Effects of rare earth elements on microstructure and tensile properties of Al-Si-Cu alloy at 250 °C

The effects of rare earth elements (La, Sm) on the high-temperature (250 °C) microstructure and mechanical properties of Al-Si-Cu alloys were analyzed. The experimental results show that with the addition of La and Sm, the α-Al was significantly refined, and the eutectic Si changed from acicular to rod-like and granular. XRD and SEM analysis shows that the rare earth phases in the alloy were mainly AlSiRe and AlRe. Fracture morphology observations show the fracture mode of the alloy changes from brittle and ductile fracture to ductile fracture. With the increase of La or Sm contents, the mechanical properties of the alloys at 250 °C increase at first, and then decrease. When the contents of La and Sm are 0.4wt.% and 0.2wt.%, the tensile strength of the alloy reaches maximum of 143.91 MPa and 201.48 MPa, respectively.

     

Modelling and analysis of the oxidation influence on creep behaviour of thin-walled structures of the single-crystal nickel-base superalloy René N5 at 980 °C

Creep test results of thin-walled specimens of the single-crystal nickel-base superalloy René N5 at 980 °C under vacuum as well as under air show different creep properties depending on material thickness and atmosphere. The differences in creep strength and strain were analysed based on a creep-oxidation model. The model specifies the primary and secondary creep stages of thin-walled specimens by a sequence of layers. The model takes different zones affected by oxidation into account. Four layers were experimentally observed and considered in the model: oxide layer, γ′-free layer, γ′-reduced layer and the two-phase substrate in the sample as centre. Material parameters for growth laws of each layer were identified both by experimental analyses and by thermodynamic simulations. The final creep-oxidation model characterizes the creep behaviour of samples with small thicknesses and low initial stress with high accuracy.     

Effects of Homogenization Treatment on the Microsegregation of a Ni–Co Based Superalloy Produced by Directional Solidification

To reduce microsegregation,a series of homogenization treatments were carried out on a Ni-Co based superalloy prepared through directional solidification(DS).The element segregation characteristics and microstructural evolution were investigated by optical microscopy(OM),scanning electron microscopy(SEM),and electron probe microanalysis(EPMA).The results show that the elements are non-uniformly distributed in the solidified superalloy,in which W and Ti have the greatest tendency of microsegregation.Furthermore,severe microsegregation leads to complicated precipitations,including η-Ni_3Ti and eutectic(γ+γ').EPMA results show that Al and Mo are uniformly distributed between the eutectic(γ+γ')and γ matrix,whereas Ti is segregated in the eutectic(γ+γ') and η phases.The positive segregation element Ti,which is continuously rejected into the remaining liquid during γ matrix solidification,promotes the formation of eutectic(γ+γ') and the transformation of the η phase.According to the homogenization effect,the optimal single-stage homogenization process of this alloy is 1180 ℃ for 2 h because of the sufficient diffusion segregation of the elements.In the present study,a kinetic diffusion model was built to reflect the degree of element segregation during homogenization,and the diffusion coefficients of W and Ti were estimated.     

Effects of constant load on corrosion cracking of 304 ASS at 600°C in molten salts

The effects of constant load (CL) on corrosion cracking of 304 austenitic stainless steel (ASS) in molten salts (binary nitrate salts, 60 wt% NaNO₃, and 40 wt% KNO₃) were investigated. A pair of novel clamping fixture was designed to fix the tensile sample, making the gauge section immersed in molten salts. The CL tensile tests were applied with the initial stress of 212 MPa and the temperature of 600°C. Optical microscopy, scanning electron microscopy, and energy dispersive spectroscopy were employed to characterize the deformation behavior and corrosion features. The results show that corrosion of 304 ASS in molten salts was aggravated by the continuous deformation due to the cracking of protective oxide scale. Grain boundaries were oxidized and weakened by the molten salt, leading to a crack propagating. Compared with the creep test result, a reduced rupture life of 304 ASS under constant loading in molten salts was observed.     

Effect of Interactions Among Elements on Diffusion Process Associated with γ′ Coarsening in a Ni-Based Single-Crystal Superalloy

Coarsening of cuboidal γ' precipitates and relevant diffusion process in Ni-based single-crystal superalloy CMSX-4 were investigated at 1000,1020 and 1040℃ for specific times.The y' coarsening kinetics followed a cubic rate law with time and was presumably controlled by bulk diffusion of elements in y matrix.The associated diffusion activation energy was experimentally determined to be about 300 kJ/mol when it is considered the temperature-dependent thermo-physical parameters in modified Lifshitz-Slyozov-Wagner theory.The influence of temperature on γ/γ' microstructure is briefly discussed based on pseudo-binary [Ni]-[Al] phase diagram.Interactions among elements can effectively raise the local vacancy formation and vacancy-atom exchange barriers close to γ-and γ'-partitioning elements,respectively.Thus,it can significantly reduce the inter-coupling migrations of atoms during the macroscopic cross-diffusion process associated with γ' coarsening of Ni-based superalloys.     

Experimental Investigation on the LCF Behavior Affected by Manufacturing Defects and Creep Damage of One Selective Laser Melting Nickel-Based Superalloy at 815℃

Uniaxial tensile tests and stress-controlled low-cycle fatigue(LCF) and creep-fatigue interaction(CFI) tests of Inconel 625 alloy manufactured by selective laser melting(SLM) were performed at 815℃ in air environments.The microstructure was characterized by optical microscopy and scanning electron microscopy after testing.The results confirmed that significant embrittlement and large scatter in LCF life are resulted from manufacturing defects.The CFI life is decreased sharply to approximately dozens of cycles with the accumulated creep strain;however,the selected dwell time(i.e.,60 s and 300 s)exhibits low sensitivity to the fracture time and elongation to failure.The embrittlement of SLM Inconel 625 was proposed to be due to the low grain uniformity and precipitation of carbides at the grain boundaries.Due to the quality of the SLM process,the accelerated initiation and propagation of fatigue crack are caused by the present unmelted powder particles,which result in the large dispersion of LCF life.Meanwhile,due to the accumulation of creep damage,cracks in the CFI test are initiated along the grain boundaries and then linked together,contributing to a significant decline in fatigue life.