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.     

Effects of Crystal Orientations on the Low-Cycle Fatigue of a Single-Crystal Nickel-Based Superalloy at 980 °C

The influence of crystal orientations on the low-cycle fatigue(LCF) behavior of a 3Re-bearing Ni-based single-crystal superalloy at 980 °C has been investigated. It is found that the orientation dependence of the fatigue life not only depends on the elastic modulus, but also the number of active slip planes and the plasticity of materials determine the LCF life,especially for the [011] and [111] specimens. The [011] and [111] specimens with better plasticity withstand relatively concentrated inelastic deformation caused by fewer active slip planes, compared to the [001] specimens resisting widespread deformation caused by a higher number of active slip planes. Additionally, fatigue fracture is also influenced by cyclic plastic deformation mechanisms of the alloy with crystal orientations, and the [001] specimens are plastically deformed by wave slip mechanism and fracture along the non-crystallographic plane, while the [011] and [111] specimens are plastically deformed by planar slip mechanism and fracture along the crystallographic planes. Moreover, casting pores,eutectics, inclusions and surface oxide layers not only initiate the crack, but also reduce the stress concentration around crack tips. Our results throw light upon the effect of inelastic strain on the LCF life and analyze the cyclic plastic deformation for the alloy with different orientations.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

Phase equilibria of low-Ni side in Nb-Ni-Ti system at 1100℃

The phase equilibria related to liquid phase in low-Ni side of Nb-Ni-Ti system at 1100℃ were investigated through scanning electron microscope(SEM),electron probe micro-analyzer(EPMA),X-ray diffraction(XRD) and differential scanning calorimetry(DSC).The results show that there exists liquid phase in equilibrium with continuous solid solution(Nb,βTi) and compound TiNi in low-Ni side of Nb-Ni-Ti system at 1100℃.The liquid phase region originates from low-Ni side of binary TiNi system and extends t...     

Influence of Hot Corrosion on High Temperature Creep Rupture for Single Crystal Superalloy DD3

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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.     

The Improvement of Oxidation Resistance of a Re-Based Diffusion Barrier/Ni�CAl Coating on the Single-Crystal Ni-Based TMS-82+ Superalloy

Oxidation behavior of a Re-based diffusion barrier/Ni?CAl coated single-crystal (SC) Ni-based TMS-82+ superalloy was studied to compare with those of the base and Ni?CAl coated superalloys under cyclic air at 1150 °C for 200 h. The base superalloy showed a negative mass gain due to extensive oxide spallation, and the Ni?CAl coated superalloy without the diffusion barrier started to spall slightly after about 90 h. The oxidation resistance of the Ni?CAl coated superalloy with the Re-based diffusion barrier was greatly improved due to the formation of a dense ??-Al₂O₃ layer in the scale. The Re-based alloy was an effective diffusion barrier layer against inward diffusion of Al and outward diffusion of alloying elements in the alloy substrate due to the reduced thickness of interdiffusion zone with small amount of detrimental precipitates and higher content of Al in the Ni?CAl coating that supplied enough Al for formation of the ??-Al₂O₃ layer.     

Microstructure Evolution of Extruded Mg-6Gd Alloy Under 175 ℃ and 150 MPa

The tensile creep behavior of extruded Mg-6 Gd alloy,having the tensile yield strength of～ 110 MPa at 175 ℃,has been investigated under 175 ℃ and 150 MPa. In this study, the extruded Mg-6 Gd sample exhibits the total tensile strain of ～10.5% after the creep time of 1100 h,and the fast plastic strain of ～4.6% at the beginning of the creep test. The microstructure result suggests that the dislocation deformation is the main deformation mode during creep, and the grains with orientation close to(0001) II ED disappear after creep. The creep process containing a low creep strain has no effective promotion for the precipitation compared with the aging process without strain. The origination of creep crack is related to the formation of precipitate-free zone during creep. The work offers an important implication to research the microstructure evolution under an applied stress in a weak aging response Mg alloy.     

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 M_(23)C_6 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_(23)C_6 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_(23)C_6 carbide, which strongly supports the presence of Suzuki segregation.     

Effects of Si Addition and Long-Term Thermal Exposure on the Tensile Properties of a Ni–Mo–Cr Superalloy

The effect of long-term thermal exposure on the grain boundary carbides and the tensile behavior of two kinds of Ni–Mo–Cr superalloys with different silicon contents(0 and 0.46 wt%) was investigated. Experimental results showed granular M2C carbides formed at the grain boundaries after exposure for 100 h for the non-silicon alloy. Furthermore, these fine granular M2C carbides will transform into plate-like M6C carbides as exposure time increases. For the Si-containing alloys,only the granular M6C carbides formed at the grain boundaries during the whole exposure time. The coarsening of the grain boundary carbides occurred in both alloys with increasing exposure time. In addition, the coarsening kinetics of the grain boundary carbides for the non-silicon alloy is faster than that of the standard alloy. The tensile properties of both alloys are improved after exposure for 100 h due to the formation of nano-sized grain boundary carbides. The grain boundary carbides are coarsened more seriously for non-silicon alloys than that of Si-containing alloys, resulting in a more significant decrease in the tensile strength and elongation for the former case. Silicon additions can effectively inhibit the severe coarsening of the grain boundary carbides and thus avoid the obvious deterioration of the tensile properties after a long-term thermal exposure.     

Al2O3/Ti2AlN复合材料在900℃,1000℃和1100℃空气中的氧化行为

利用综合热分析仪、背散射扫描电镜(BSE)和能谱分析(EDS)对Al₂O₃/Ti₂AlN复合材料在900 ℃,1 000 ℃和1 100 ℃/20 h空气中连续氧化20h后的氧化增重及氧化层截面进行了研究。结果表明：Al₂O₃/Ti₂AlN复合材料在空气中的氧化行为符合抛物线规律,在900 ℃,1 000 ℃和1 100 ℃/20 h氧化增重分别为2.78×10_-2 kg/m₂、10.4 ×10_-2 kg/m₂、21.9 ×10_-2 kg/m₂,抛物线速率常数相应为1.08×10_-8 kg₂/m₄s、1.44×10_-7 kg₂/m₄s、6.56×10_-7 kg₂/m₄s,氧化激活能为274 kJ/mol。氧化层主要由TiO₂和Al₂O₃组成的,连续的Al₂O₃次外层可以提高其抗氧化性能。氧化层结构的改变是由于氧化温度对Ti₄₊、Al₃₊由基体表面向外扩散和O_2-向内扩散的影响,以及TiO₂和Al₂O₃在不同温度下的形核生长速率导致的。对Al₂O₃/Ti₂AlN而言,控制材料与氧化气氛的界面是提高该材料抗氧化性能的关键。     

Improvement of the Oxidation Resistance of the Single-Crystal Ni-Based TMS-82+ Superalloy by Ni–Al Coatings with/without the Diffusion Barrier

Oxidation behavior of the uncoated base, Ni–Al coated and Re–Cr-Ni plus Ni–Al coated single-crystal (SC) Ni-based TMS-82+ superalloy is studied under cyclic air at 900 °C for 200 h to assess the oxidation resistance. Regardless of the coating processing, Ni–Al coating is effective in improving the oxidation resistance due to the formation of a continuous α-Al₂O₃ layer in the scale. For the uncoated base superalloy, the mass-gain curves are fitted by a subparabolic relationship, and complex oxide products including predominately NiO, some CrTaO₄, α-Al₂O₃, Cr₂O₃, a minor of spinels of (Ni, Co)Al₂O₄, AlTaO₄ and θ-Al₂O₃ are detected. Time-dependence of the oxide growth rate for both coated superalloy with/without the diffusion barrier is explained by the parabolic relationship. The oxide scales consist predominately of α-Al₂O₃ and a minor of θ-Al₂O₃. The diffusion barrier of σ-phase plays a negligible effect on the oxidation resistance during the cyclic exposure environment. The amount of detrimental γ′-phase and topologically close-packed (TCP) phases in the interdiffusion zone in the coated superalloy with the diffusion barrier is greatly reduced compared with that without the diffusion barrier due to the distinct barrier effect limiting diffusion of elements between the bond-coat and the substrate.     

The Oxidation of a Directionally Solidified Ni-Al-Cr3C2 Alloy at 1100 and 1200°C in Oxygen

The oxidation behavior of a directionallysolidified Ni-Al-Cr₃C₂ alloy ofover all composition Ni-12.3Cr-6.9Al-1.8C (wt.%) hasbeen investigated at 1100 and 1200°C under 1 atmoxygen. Experiments have also been carried out on specimens having the samecomposition but with a nonaligned structure. At1100°C, in both cases and unlike conventionalnickel-base superalloys with the same chromium andaluminum contents, aluminium was found to oxidize internallybeneath an external Cr₂O₃ scale.Although the volume fraction of the internalprecipitates was significant, they showed no tendency tocoalesce into a compact subsurface layer, but formed randomly distributed clustersin the alloy matrix. The kinetics of oxidation andmorphologies of the oxide scales were not substantiallyaffected either by thermal cycling or by the alloy microstructure. At the higher temperature,continuous Al₂O₃ scales formedbeneath thick layers of transient nickel andnickel-chromium-rich oxides; no internal precipitationof aluminum-rich oxides was observed. However, internal degradation of thedirectionally solidified specimens at 1200°C wasquite significant, due to in situ oxidation of primarycarbides. The multilayered scales formed at 1200°C spalled extensively on cooling as a consequenceof loss of contact, starting preferentially at theintersections of the Cr₂C₃ fiberswith the alloy-scale interface. The observed behaviorcan be attributed to a reduction in the availability of chromiumbecause of the multiphase structure of the alloy; this,in turn, resulted in an increase in the flux of oxygeninward, leading to internal oxidation of aluminum at 1100°C. The almost exclusive externaloxidation of aluminum becomes possible at 1200°C,probably because of an increase in the diffusivity ofaluminum in the alloy matrix.     

The Sulfidation/Oxidation Resistance of Two Ni-Cr-Al-Y Alloys at 700℃

The high temperature corrosion resistance of Ni-25.9Cr-13.5Al-1.2Y-0.6Si and Ni-10.2Co-12.4Cr-16.0Al-0.5Y-0.2Hf alloys was assessed in sulfidation/oxidation environments.In the environment with a sulfur partial pressure of 1Pa. and an oxygen partial pressure of 10^-19Pα,both these alloys exhibited three distinct stages in the weight gain-time curve when tested at 700℃.In the initial stage, selective sulfidation of Cr suppressed the formation of the other metal sulfides,resulting in lower weight gains.In the transient stage, breakdown and cracking of Cr sulfides and insufficient concentration of Cr at the outer zone led to the rapid formation of Ni sulfides and a rapid increase in weight.In the steady-state stage, corrosion was controlled by the diffusion of anions and/or cations, which led to a parabolic rate law.