High Temperature Oxidation of FGH96 P/M Superalloy

High temperature oxidation behaviors of FGH96 P/M superalloy have been studied in air at temperatures ranging from 600 to 1000℃. By means of isothermal oxidation testing,X-ray diffraction,SEM(scanning electron microscopy),and EDS(energy dispersive X-ray spectroscopy) analyses,the oxidation kinetics as well as the composition and morphology of scales were investigated. Thermodynamic calculations were used to explain the oxidation mechanism. The results showed that as the oxidation temperature increased,the oxidation rate,the scale thickness,and scale spallation increased. FGH96 P/M superalloy exhibits good oxidation resistance at temperature below 800℃. The oxidation kinetics follows an approximately parabolic rate law,and the oxide layer was mainly composed of Cr2O3,TiO2,and a little amount of NiCr2O4. The oxidation is controlled by the transmission of chromium,titanium,and oxygen through the oxide scale.     

基于数值模拟的镍基高温合金电渣重熔工艺优化

利用MeltFlow软件对镍基高温合金电渣重熔过程进行数值模拟计算,探究了电渣重熔过程中温度场、流场、熔池形貌及微观组织的分布特点,通过工业试验验证模拟的准确性,定量分析熔速对熔炼过程的影响规律,提出了一种改善铸锭凝固质量的工艺优化方法。结果表明,渣池内的温度相对较高且分布均匀,熔池形貌近似“V”型。铸锭一次枝晶间距从中心到边缘沿径向逐渐减小,而二次枝晶间距没有明显差别。对比试验数据,数值模拟结果误差较小,可以准确预测镍基高温合金电渣重熔过程中的熔池形貌和枝晶间距。随着熔化速率的减小,金属熔池深度降低,ESR铸锭二次枝晶间距逐渐减小,且宏观偏析程度得到改善,黑斑产生概率逐渐降低。控制熔化速率由0.47 kg/min降低至0.45 kg/min,有利于获得凝固质量较好的镍基高温合金电渣重熔铸锭。     

Precipitate coarsening and its effects on the hot deformation behavior of the recently developedγ’-strengthened superalloys

Recently,theγ’-strengthened superalloys are of great interests in high temperature applications due to their excellent high temperature strength which is derived from theγ’strengthening phase.For theseγ’-strengthened superalloys,the changes in morphology,size and distribution ofγ’precipitates due to coarsening during thermal exposure have a significant impact on the properties of alloys.This article briefly summarizes the recent advances on the coarsening behavior of gamma prime precipitates in the recently-developedγ’-strengthened superalloys and its effects on the hot deformation behavior of superalloys,drawing specific examples on Allvac^■718 Plus TM and Ni3 Al-based intermetallic superalloys.It is found that the particle size plays an important role in morphological evolution ofγ’precipitates.For instance,the morphology ofγ’precipitates evolves from cuboidal to strip-like or other complex structures in Ni3 Al-based intermetallic alloys,while theγ’precipitates in Allvac^■718 Plus alloy always present nearspherical morphology due to the relatively small initial particle size.The Lifshitz-Slyozof-Wagner(LSW)theory and its modifications,as well as Trans-Interface Diffusion Controlled(TIDC)theory have been applied to describing the coarsening kinetics ofγ’precipitates.Additionally,the hot deformation behavior ofγ’-strengthened superalloy is found to be greatly influenced by the coarsening ofγ’precipitates.     

Effect of strain rate on microstructure evolution of a nickel-based superalloy during hot deformation

The hot deformation behavior of a nickel-based superalloy was investigated by means of isothermal compression tests in the strain rate range of 0.001–10 s⁻¹ at 1110 °C. Transmission electron microscope (TEM) and electron backscatter diffraction (EBSD) technique were used to study the effect of strain rate on the microstructure evolution of the alloy during hot deformation. The results revealed that the dynamic recrystallization (DRX) process was stimulated at high strain rates ($\dot{\epsilon }⩾5{\text{s}}^{-1}$) due to the high dislocation density and adiabatic temperature rise. Meanwhile, high nucleation of DRX and low grain growth led to the fine DRX grains. In the strain rate rage of 0.001–1 s⁻¹, the volume fraction of DRX grains increased with the decreasing strain rate, and the grain growth gradually governed the DRX process. Moreover, the strain rate has an important effect on DDRX and CDRX during hot deformation. On the other hand, particular attention was also paid to the evolution of twin boundaries during hot deformation. It was found that there was a lower fraction of Σ3 boundaries at the intermediate strain rate of 1 s⁻¹, while the fractions of Σ3 boundaries were much higher at both the lower strain rates ($\dot{\epsilon }⩽0.1{\text{s}}^{-1}$) and higher strain rates ($\dot{\epsilon }⩾5{\text{s}}^{-1}$).     

A quantitative metallographic assessment of the evolution of porosity during processing and creep in single crystal Ni‐base super alloys

The present work reviews previous research on the evolution of porosity. It presents new results from a detailed study on the evolution of porosity during casting, heat treatment and creep of a single crystal Ni‐base superalloy subjected to uniaxial tensile creep at 1050 °C and 160 MPa in [001] and [110] directions. A quantitative metallographic study was performed on carefully polished metallographic cross sections, monitoring sampling fields of 4500 × 1000 µm² using the back scatter contrast of an analytical scanning electron microscope; evolutions of pore sizes and pore form factors were analyzed and all important details which were previously revealed in a synchrotron study could be reproduced. In addition, it was observed that micro cracks form at larger cast pores. They interlink and thus initiate final rupture. The [110] tensile creep tests showed lower rupture strains than the [001] experiments. In agreement with earlier work, this can be rationalized on the basis of aligned porosity along primary dendrites.     

单晶高温合金DD6再结晶晶界析出相特征及其形成机制

对单晶高温合金DD6进行表面吹砂处理,然后进行固溶与时效真空热处理,采用SEM,TEM,EPMA和Thermo-Calc的方法研究了单晶合金DD6再结晶晶界析出相的特征及其形成机制.结果表明,经过吹砂处理的DD6合金在固溶与时效热处理过程中发生再结晶.再结晶晶界出现析出相,分析表明析出相为M_6C碳化物,该碳化物呈粒状析出,尺寸约为0.5μm,数量极少,富含W,Re和Mo,且Cr,Nb和Co的含量与合金名义成分差别不大,而Al,Ta和Ni含量较低.由于再结晶晶界上C元素的聚集效应,C原子在晶界上达到一定浓度后即与一定数量的W,Mo等μ相形成元素发生相变反应,抑制了合金析出μ相的倾向.又因为DD6合金W含量较高,而Cr含量较低,抑制了M_(23)C_6碳化物,有利于析出M_6C碳化物.     

Hot Corrosion Behaviors of Ni-14%Cr Alloys in Molten Sulfate

The corrosion behaviors and mechanism of Ni-14%Cr alloys in molten salt with Na2SO4 at 1000℃ was investigated by means of XRD analysis and SEM observation. Results show that the alloys were subjected to the accelerated corrosion in molten salt and the corrosion kinetic of the alloy obeys the parabolic law. On the other hand, the corrosion-resistance of the alloy could be improved significantly by increasing of chromium content. Corrosion products may be divided into three layers, i. e. NiCr2O4, CoCr2O4, CoN...     

DD15单晶高温合金持久性能各向异性

研究了[001]、[011]和[111]3种不同取向的DD15单晶高温合金的热处理组织、980 ℃/300 MPa和1 150℃/120MPa的持久性能、持久断口和断裂组织.结果表明在与定向凝固方向垂直的截面上,3种取向合金具有明显不同的热处理组织形貌,γ'相的形状分别为规则的正方形、矩形和多边形.合金在980℃/30...     

Effect of metallurgical evolution during post-weld aging treatment on localised corrosion of resistance spot welding joints of A286 superalloy

The combined effect of resistance spot welding and precipitation hardening on the localised corrosion of A286 superalloy is studied. The specimens tested by double loop electrochemical potentiokinetic reactivation were welded in the solution treated condition, and then subjected to different precipitation hardening treatments. For both base metal and weld nugget, the maximum localised corrosion is reached when η phase is clearly observable. The fact that the localised corrosion resistance of weld nugget is different from that shown by base metal may be explained by the segregation of Ni and Ti towards the interdendritic region of weld nugget (studied by using scanning electron microscope/energy dispersive X-ray analysis).     

Simulating the effect of aluminizing on a CoNiCrAlY-coated Ni-base superalloy

MCrAlY (M = Ni, Co) coatings are commonly used on gas-turbine components as oxidation resistant overlay coatings and bondcoats for thermal barrier systems. The present work focuses on the effect of the aluminizing process on the CoNiCrAlY coating microstructure. In the as-received condition the outer part of the coating consisted mostly of β-(Ni,Co)Al with interspersed precipitates of Cr-rich carbide and Cr-rich boride precipitates. Formation of σ-CoCr was observed at the interface between the β-layer and the inner initial CoNiCrAlY microstructure. Scanning electron microscopy (SEM) combined with energy and wavelength-dispersive X-ray spectroscopy (EDX/WDX) was employed to characterize the aluminized CoNiCrAlY coating. Phases were then identified by electron backscatter diffraction (EBSD). Detailed microstructural studies of the coating were corroborated with the help of coupled thermodynamic-kinetic calculations to model the aluminizing process. The calculations were performed with the in-house developed code employing the commercially available thermodynamic and kinetic databases (ThermoCalc). The mechanisms of the observed microstructural changes were elucidated with the help of the modelling results.