Evolution of micro-pores in a single crystal nickel-based superalloy during 980°C creep

The evolution of micro-pores in a single crystal nickel-based superalloy during creep at 980 °C/220 MPa was investigated by X-ray computed tomography. Time-dependent ex-situ 3D information including the number, volume fraction, distribution and morphology of micro-pores was analyzed. The results reveal that the signifi cant formation and growth of micro-pores occur at the end of secondary/beginning of tertiary creep stage. The irregular large pores as well as high density pores located at strain...     

取向偏离度对镍基单晶高温合金DD10低周疲劳行为的影响

本文研究了特定试验条件下,镍基单晶DD10取向偏离[001]的程度对其低周疲劳行为的影响。结果表明：取向偏离度对DD10合金的低周疲劳行为影响明显。疲劳寿命随取向偏离度的增加而明显降低,在同等偏离度下,晶体取向靠近[001]-[011]的合金低周寿命优于取向靠近[001]-[11]的合金;分析发现：不同取向偏离度合金DD10低周疲劳寿命的差异源于其每循环周次的可累计塑性变形的差异。另一方面,取向偏离度对疲劳总应力幅和塑性应变幅的影响却截然相反,但对疲劳裂纹萌生和扩展方式影响不大。     

Rejuvenation Heat Treatment of the Second-Generation Single-Crystal Superalloy DD6

The second-generation single-crystal superalloy DD6 with [001] orientation was prepared by screw selecting method in the directionally solidified furnace. The long-term aging of the alloy after full heat treatment was performed at1100 °C for 400 h. Then the rejuvenation heat treatment 1300 °C/4 h/AC ? 1120 °C/4 h/AC ? 870 °C/24 h/AC was carried out. The stress rupture properties were investigated at 760 °C/800 MPa, 850 °C/550 MPa, 980 °C/250 MPa and1100 °C/140 MPa after different heat treatments. The microstructures of the alloy at different conditions were studied by SEM. The results show that c0 phase of the alloy became very irregular and larger after long-term aging at 1100 °C for 400 h. A very small amount of needle-shaped TCP phase precipitated in the dendrite core. The coarsened c0 phase and TCP phase dissolved entirely after rejuvenation heat treatment. The microstructure was restored and almost same with the original microstructure. The stress rupture life of the alloy decreased in different degrees at various test conditions after long-term aging. The stress rupture life of the alloy after rejuvenation heat treatment all restores to the original specimen more than 80%at different conditions. The microstructure degradation of the alloy during long-term aging includes coarsening of the c0 phase,P-type raft and precipitation of TCP phase, which results in the degeneration of stress rupture property. The rejuvenation heat treatment succeeds in restoring the original microstructure and stress rupture properties of the alloy.     

Technological Aspect of Processing Maps for the AA2099 Alloy

Results of an experimental and modelling study of forming processes in the AA2099 Al–Cu–Li alloy, for a wide range of temperatures, strains and strain rates, are presented. The analyses are based on tensile testing at 20 °C at a strain rate of 0.02 s-1and uniaxial compression testing in the temperature range 400–550 °C at strain rates ranging from0.001 to 100 s-1, for constant values of true strain of 0.5 and 0.9. The stability of plastic deformation and its relationship with a sensitivity of stress to strain rate are considered. The power dissipation efficiency coefficient, g(%), and the flow instability parameter, n B 0, were determined. The complex processing maps for hot working were determined and quantified, including process frames for basic forging processes: conventional forging and for near-superplastic and isothermal conditions. A significant aspect is the convergence of power dissipation when passing through the 500 °C peak.Deformation, temperature and strain-rate-dependent microstructures at 500 °C for strain rates of 0.1, 1, 10 and 100 s-1are described and analysed for the conventional die forging process frame, corresponding to 465–523 °C and strain rates of50–100 s-1.     

Deformation Behaviors of an Additive-Manufactured Ni32Co30Cr10Fe10Al18 Eutectic High Entropy Alloy at Ambient and Elevated Temperatures

Eutectic high-entropy alloys (EHEAs) that have superior formability are attractive for direct laser deposition technology. In this study, a regular-shaped bulk Ni₃₂Co₃₀Cr₁₀Fe₁₀Al₁₈ EHEA without apparent pores and micro-cracks was successfully fabricated by direct laser deposition. The as-deposited alloy showed a high tensile strength of 1.3 GPa with a ductility of 35% at ambient temperature and a tensile strength of 320 MPa at 760 °C. The deformation mechanisms of the as-deposited alloy at ambient and elevated temperatures were investigated by coupling the in-situ tensile test with a scanning electron microscope. It is revealed that the excellent combination of strength and ductility originated from the synergic effects of the FCC and B2 phases in eutectic lamellae. And the generation of cracks along phase boundaries restricted its high-temperature strength above 760 °C.     

Temperature Effects on the Microstructures of Mg-Gd-Y Alloy Processed by Multi-direction Impact Forging

A high strain rate multi-directional impact forging(MDIF) was applied to a solutionized Mg-Gd-Y-Zr alloy in the temperature range of 350-500℃.Results demonstrate that the dominant deformation mode is twinning at a temperature below 400℃,whereas at a medium temperature of 450℃ considerable continuous dynamic recrystallization was promoted by{10-12} extension twins.At a higher temperature of 500℃,twinning activation was suppressed.New DRX grains were observed but their sizes were much bigger than those resulting from the MDIFed 50 passes at 450℃,which are ascribed to the larger grain boundary mobility and atomic diffusion at 500℃.Moreover,a non-basal weak texture was gained afterward MDIF at each temperature,which is credited to the MDIF process and the minor strain applied in each pass.     

Improved Spallation Resistance of the Oxide Scale by Hf/Y Co-doping in Ni-Based Superalloy at High Temperature

Ni-based superalloys added with comparably higher concentrations of single-doped Hf and co-doped Hf/Y are prepared by vacuum induction melting (VIM). The oxidation properties up to 300 h at 900 °C, 1000 °C, and 1100 °C are investigated. The undoped alloy exhibited a minimum oxidation rate at 900 °C and 1000 °C. The co-doped alloy showed a higher oxidation rate; however, it possesses better scale adhesion, and no spallation occurs. Hf-doped alloy showed a lower oxidation rate and better scale adhesion at 900 °C and 1000 °C, but exhibited a shorter lifetime at 1100 °C. The spallation of the Hf-doped alloy is attributed to the precipitation of the HfO₂ in and beneath the oxide scale. The spallation in the undoped alloy is accredited to the thermal expansion mismatch between the growing oxide scale and superalloy substrate. Incorporating two reactive elements (REs) in alloy minimized the precipitation of RE oxide in the oxide scale, diminished internal oxidation in the alloy, and decreased oxide scale spallation.     

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.     

Characterization of the Hot Deformation Behavior of Cu–Cr–Zr Alloy by Processing Maps

Hot deformation behavior of the Cu–Cr–Zr alloy was investigated using hot compressive tests in the temperature range of 650–850 °C and strain rate range of 0.001–10 s-1. The constitutive equation of the alloy based on the hyperbolic-sine equation was established to characterize the flow stress as a function of strain rate and deformation temperature. The critical conditions for the occurrence of dynamic recrystallization were determined based on the alloy strain hardening rate curves. Based on the dynamic material model, the processing maps at the strains of 0.3, 0.4 and 0.5were obtained. When the true strain was 0.5, greater power dissipation efficiency was observed at 800–850 °C and under0.001–0.1 s-1, with the peak efficiency of 47%. The evolution of DRX microstructure strongly depends on the deformation temperature and the strain rate. Based on the processing maps and microstructure evolution, the optimal hot working conditions for the Cu–Cr–Zr alloy are in the temperature range of 800–850 °C and the strain rate range of 0.001–0.1 s-1.     

In Situ DIC Study on LCF Behavior of Retired Weld Joint Subjected to Prolonged Service at Elevated Temperature

By using digital image correlation(DIC), low-cycle fatigue(LCF) behavior of CrMoV weld joint taken from a retired gas turbine rotor after 15-year service was investigated at 500 °C and 540 °C. The most remarkable plastic strain was observed in the weld metal(WM), which was up to 6 times of the global strain at mid-life cycle. Due to the cyclic accumulation of local deformation, the stress–strain hysteresis loops relative to tensile or compressive strain concentration area in WM displayed the rat...     

Effect of Ni and Mn on the Mechanical Properties of 22Cr Microduplex Stainless Steel

The tensile properties of 22Cr–2Ni–4Mn–0.2N micro-duplex stainless steels with different Ni and Mn contents were investigated. Duplex stainless steels were vacuum induction melted and hot rolled, then annealed at 1,000–1,100 °C, at which temperature both the austenite and ferrite phases were stable. The volume fraction of the ferrite phase was markedly affected by the alloying elements of Mn and Ni; 1 wt% of Mn was equivalent to 0.4 wt% of Ni. All of the steels tested at room temperature showed the common strain-hardening behavior, while the steels tested at lower temperatures(-30 or-50 °C)showed a distinct inflection point in their stress–strain curves, which resulted from the transformation of the austenite to straininduced martensite. The onset strain(e0) of the inflection point in the stress–strain curve depended on the Md30 value of the steel. Testing at lower temperatures resulted in smaller e0 and consequently higher strengths and fracture strains(ef). The tensile behavior was examined from the perspective of austenite stability of the micro-duplex stainless steels with the different Ni and Mn contents.     

吹砂对DD6单晶高温合金表面完整性和高周疲劳强度的影响EI北大核心CSCD

将标准热处理的试样分别采用粒径为150、124和100μm白刚玉砂在0.5 MPa压力下吹砂,研究吹砂对第二代单晶高温合金DD6表面完整性的影响;对未吹砂和粒径150μm吹砂试样分别进行760和980℃旋转弯曲高周疲劳性能测试,研究吹砂对DD6合金疲劳性能的影响。结果表明:吹砂会破坏单晶高温合金的表面完整性,使表面出现砂粒切削造成的不规则凹坑,改变表面形貌;砂粒粒径增加,表面粗糙度和显微硬度均增大;吹砂使大量位错在γ相通道中滑移,靠近表面区域位错密度较大;并且,大量位错剪切γ’相,形成反相畴界和层错;吹砂造成形变强化、引入残余应力;150μm、0.5 MPa吹砂对DD6合金760℃旋转弯曲疲劳性能基本无影响,但会降低合金980℃疲劳性能,对低应力幅区疲劳寿命影响较大,使疲劳强度下降约7.3%。缺口效应、氧化损伤、形变强化和残余压应力的耦合作用导致吹砂与不吹砂试样疲劳寿命产生差异。     

Thermochemistry of GaBO3 and phase equilibria in the Ga2O3–B2O3 system

Employing a Tian-Calvet-type calorimeter operating in the scanning mode at temperatures from 1120 to 1220 K, the enthalpy change, Δ_dH, associated with the decomposition of GaBO₃ (=1/2β-Ga₂O₃+1/2B₂O₃(liq.)) and the corresponding decomposition temperature, T_d, were determined: Δ_dH=30.34±0.6 kJ/mol, T_d=1190±5 K. Using the transposed-temperature-drop method the thermal enthalpy, H(T)−H(295 K), of GaBO₃ was measured as a function of temperature, T, in the region from 760 to 1610 K; the results obtained are

[H(T)−H(295 K)]/(J/mol)=104.8·(T/K)−31 300 (760 K<T<1190 K),

[H(T)−H(295 K)]/(J/mol)=138.8·(T/K)−41 480 (1190 K<T<1590 K).

On the basis of the experimental results, the enthalpy and entropy of formation, Δ_fH and Δ_fS, respectively, of GaBO₃ from the component oxides were derived:

Δ_fH=−30.34 kJ/mol,Δ_fS=−25.50 J/(K·mol) at 1190 K,

Δ_fH=−10.55 kJ/mol,Δ_fS=−5.48 J/(K·mol) at 298 K.

The enthalpy versus temperature curve shows, apart from a step associated with the decomposition of GaBO₃, a further step at 1593 K which is attributed to a monotectic equilibrium.     

Effect of C/SiC Volume Ratios on Mechanical and Oxidation Behaviors of Cf/C-SiC Composites Fabricated by Chemical Vapor Infiltration Technique

C/SiC volume ratios in carbon fiber-reinforced carbon-silicon carbide (C_f/C-SiC) composites may influence greatly mechanical and oxidation properties of the composites, but have not been well investigated yet. Herein, C_f/C-SiC composites with different C/SiC volume ratios were fabricated by chemical vapor infiltration (CVI) technique through alternating the thickness of a pyrocarbon (PyC) interlayer. The composites with C/SiC volume ratios of 0.37 and 0.84 exhibited the better comprehensive mechanical properties. The CS0.37 showed the highest flexural strength of 340.6 MPa, and CS0.84 had the maximum tensile strength of 139.1 MPa. The excellent mechanical properties were closely related to the relatively low C/SiC volume ratios and porosities, optimum interfacial bonding and reduced matrix micro-cracks. The composite with a low C/SiC volume ratio of 0.10 showed the best anti-oxidation performance due to its high SiC content. The oxidation mechanisms at 1100 °C and 1400 °C were discussed by considering the effect of the C/SiC volume ratios, pores and matrix micro-cracks, oxidation of carbon phase and SiC.     

Influence of aluminide diffusion coating on the tensile properties of the Ni-base superalloy René 80

Ni-base superalloy René 80 is widely used in manufacturing aircraft turbine blades. The service temperature of this alloy is in the range of 760-982 °C. Although this alloy possesses suitable mechanical, oxidation and hot corrosion properties, it is coated in order to increase its wear, oxidation, erosion and hot corrosion properties against harmful environmental service conditions.In this paper the influence of applying diffusion coating (CODEP-B) on the tensile properties of René 80 has been studied in the temperature range of 22-982 °C. Experimental results show that the tensile properties of the coated specimens are relatively lower than that of uncoated ones in the same conditions. But in the service conditions, coating could have some useful and positive effects practically.In aircraft turbines in service conditions, the maximum strain the blades experience is always bellow 1% and in this range of strain no crack initiates and generates on the applied coating surface. Thus in this situation, the coating would certainly have protective properties preventing from oxidation, hot corrosion and erosion of the base metal.     

Temperature Effect on Early Oxidation Behavior of NiCoCrAlY Coatings: Microstructure and Phase Transformation

Initial oxidation behavior of NiCoCrAlY coating prepared by arc-ion plating has been studied in air at 900, 1000 and 1100 °C. The results showed that phase transformation from transient θ-Al₂O₃ to α-Al₂O₃ was highly related to the temperature and oxidation time. The oxide scale in the initial stage was mainly composed of θ-Al₂O₃ at 900 °C. Instead, more amount of α-Al₂O₃ emerged out with increasing oxidation temperature. The elemental distribution after oxidation confirmed that faster chromium diffusion to the oxide scale played an important role in the speedy transformation from θ-Al₂O₃ to α-Al₂O₃. Y segregation at scale/coating interface resulted in less cavity formation and hence improved the oxide scale adherence.     

A Local Damage Approach to Predict Crack Initiation in Type AISI 316L(N) Stainless Steel

A local damage approach based on plastic strain equivalent to uniform strain and grain diameter of the material is proposed for prediction of crack initiation. Plane strain, plane stress, and 3D FEM simulations are carried out for compact tension (CT) geometry with blunt notch of different a/W ratios under mode-I loading. Elastic-plastic fracture parameters have been estimated based on certain assumptions on blunting at notch tip and micromechanisms of events leading to onset of crack. The various crack initiation parameters evaluated based on proposed local damage approach and initial assumptions have been verified by conducting experiments on CT specimens and subsequent scanning electron microscopy study on fracture surface. The laboratory scale experimental results of AISI 316L(N) stainless steel material are in good agreement with FEM-predicted fracture parameters for notch type of stress raisers. The local damage approach and FEM procedure established in the present study would be easily extendable to the analysis of stress raisers in components for the prediction of crack initiation under elastic-plastic condition.     

Influence of Sintering Temperature on Microstructure and Mechanical Properties of Al_2O_3 Ceramic via 3D Stereolithography

In this work,aluminum oxide(Al_2O_3) ceramic samples were fabricated by 3D stereolithography printing.Printing process was followed by debinding and sintering.In addition,the effect of sintering temperature on micro structure and properties was investigated.Flexure strength was observed to increase with increasing sintering temperature due to fewer pores,fewer defects and stronger grain boundary bonding of samples at higher sintering temperatures.Maximum flexure strength of 138.5 MPa was obtained when sintering temperature was 150℃.Furthermore,the shrinkage along length direction decreased with the decreasing sintering temperature until reaching minimum value of 1.02% after sintering at 1200℃.After sintering at 1280℃,flexure strength was 24.0 MPa and the shrinkage along length direction was 2.1%,which meets demands of ceramic core.     

Hot Deformation and Dynamic Recrystallization Behavior of Austenite-Based Low-Density Fe–Mn–Al–C Steel

The hot deformation and dynamic recrystallization(DRX) behavior of austenite-based Fe–27Mn–11.5Al–0.95 C steel with a density of 6.55 g cm-3were investigated by compressive deformation at the temperature range of900–1150 °C and strain rate of 0.01–10 s-1. Typical DRX behavior was observed under chosen deformation conditions and yield-point-elongation-like effect caused by DRX of d-ferrite. The flow stress characteristics were determined by DRX of the d-ferrite at early stage and the austenite at later stage, respectively. On the basis of hyperbolic sine function and linear fitting, the calculated thermal activation energy for the experimental steel was 294.204 k J mol-1. The occurrence of DRX for both the austenite and the d-ferrite was estimated and plotted by related Zener–Hollomon equations. A DRX kinetic model of the steel was established by flow stress and peak strain without considering dynamic recovery and d-ferrite DRX. The effects of deformation temperature and strain rate on DRX volume fraction were discussed in detail. Increasing deformation temperature or strain rate contributes to DRX of both the austenite and the d-ferrite, whereas a lower strain rate leads to the austenite grains growth and the d-ferrite evolution, from banded to island-like structure.     

Effect of Heat Treatment on the Microstructure,Mechanical Properties and Corrosion Resistance of Selective Laser Melted 304L Stainless Steel

The influence of heat treatment holding temperatures from 600 to 1300 °C on the microstructure, mechanical properties and corrosion resistance in selective laser melted(SLMed) 304L stainless steel is investigated in this work. The results reveal that there is no remarkable microstructure change after holding at 600 °C for 2 h, while recrystallization leads to a slight decrease in grain size in the temperature range of 700–900 °C. The heat treatment at temperatures from 1000 to 1300 °C for 2 h ob...