时效温度对非平衡凝固K424合金析出相及硬度的影响

采用铜模喷铸与等温时效相结合的方法,研究了快冷K424高温合金的非平衡凝固组织及其沉淀相析出行为。结果表明,铜模喷铸合金组织发生了显著细化,二次枝晶间距相比铸态大幅降低,溶质偏析的减弱有利于抑制粗大碳化物及共晶组织形成。随铜模内径下降,枝晶形貌由无方向性等轴晶逐渐转变为择优生长的定向枝晶。经800~1000℃等温时效30min后,快冷合金的过饱和固溶体组织中开始析出γ′相。随时效温度提高,沉淀相尺寸及体积分数同时增加。900℃×30min时效后可获得大量细小均匀的球形γ′相,快冷合金的显微硬度(HV)高达478,相比原始非平衡态合金提高了13%。     

The influence of the processing route on the fragmentation of (Nb,Ti)C stringers and its role on mechanical properties and hydrogen embrittlement of nickel based alloy 718

The nickel-base superalloy 718 is a precipitation hardened alloy widely used in the nuclear fuel assembly of pressurized water reactors (PWR). However, the alloy can experience failure due to hydrogen embrittlement (HE). The processing route can influence the microstructure of the material and, therefore, the HE degree. In particular, the size and distribution of the (Nb,Ti)C particles can be affected by the processing. In this regard, the objective of this work was to analyze the influence of cold and hot deformation processing routes on the development of the microstructure, and the consequences on mechanical properties and hydrogen embrittlement. Tensile samples were hydrogenated through gaseous charging and compared to non-hydrogenated samples. Characterization was performed via scanning and transmission electron microscopies, as well as electron backscattered diffraction. The processing was effective to promote significant variations in average grain size and length fraction of special Σ3ⁿ boundaries, as well as reduction of average (Nb,Ti)C particle size, being these changes more intense for the cold-rolled route. For the mechanical properties, on one side, the cold-rolled route presented the highest increase in ductility for non-hydrogenated samples, while, on the other side, had the highest degree of embrittlement under hydrogen. This dual behavior was attributed to the interaction of hydrogen with the (Nb,Ti)C particles and stringers and its ensuing influence on the fracture processes.     

Element Segregation and Solidification Behavior of a Nb,Ti, Al Co-Strengthened Superalloy ЭК151

The as-cast microstructure, element segregation and solidification behavior of a multi-alloyed superalloy ЭК151 have been investigated. The results show that the severe element segregation leads to the complicated precipitations at the inter-dendritic region, including η-Ni_3(Ti, Nb), eutectic(γ + γ') and Laves, which shows the characteristics of both Ti, Al-strengthened and Nb-strengthened alloys. Differential thermal analysis, heating and quenching tests reveal the solidification sequence as follows: Liquids →γ matrix →(Nb, Ti)C →η-Ni 3(Ti, Nb) →eutectic( γ+γ') → Laves. The melting points are between 1250 and 1260 °C for(Nb, Ti)C, between 1200 and 1210 °C for η phase, between 1180 and 1190 °C for eutectic(γ+γ') and Laves. γ' initially precipitates from matrix at 1150 °C and achieves the maximum precipitation at 1130 °C. According to the microstructure evolution captured during solidification and composition analysis by an energy dispersive spectrometer and electron probe microanalyzer,(Nb, Ti)/Al ratio is put forward to explain the formation of η-Ni_3(Ti, Nb) and eutectic( γ+γ'). The solidification of γ matrix increased the Nb, Ti concentration in the residual liquids, so the high(Nb, Ti)/Al ratio would result in the formation of η-Ni_3(Ti, Nb); the precipitation of the phase consumed Nb and Ti and decreased the(Nb, Ti)/Al ratio in the liquid, which led to the precipitation of eutectic(γ + γ'). Laves formed by the sides of η-Ni_3(Ti, Nb) and in front of the eutectic( γ + γ') after Al, Ti were further depleted by the two phases and Cr, Co, Mo were rejected to liquids.     

Fabrication of Ni-Based Superalloys Containing Nb and Their High Temperature Oxidation Behaviors

Ni-based superalloys with different contents of Nb addition were fabricated by vacuum melting and casting. High temperature oxidation behaviors of the designed alloys were investigated by thermogravimetry. Microstructure and phase constituent of the oxidation scales were studied using scanning electron microscope and X-ray diffraction (XRD), respectively. Results showed that irrespective of the composition, all specimens obeyed the parabolic law during different stages. The scales on specimens B and C with 2.0 wt.% and 2.5 wt.% Nb addition, respectively, were mainly comprised of Cr₂O₃ and NiCr₂O₄. However, specimen A, with no Nb addition showed only NiO in the outer scale. The oxidation scales all exhibited a layered structure. With 2.0 wt.%Nb addition, the superalloy's high temperature oxidation resistance was enhanced by about 25%.     

Mean-stress and oxidation effects on fatigue behaviour of CM 247 DS LC alloy

During present investigations, an attempt was made to understand the effect of mean stress and oxidation on low cycle fatigue (LCF) behaviour of CM 247 DS LC alloy at T?=?850°C. A significant reduction in fatigue life was observed during LCF tests conducted at strain ratio (R) of 0 as compared to R?=??1. Reduction in fatigue life is attributed to the synergistic effect of mean stress, oxidation and the expanding precipitates within the grain boundaries which imparts high stresses at the grain boundary leading to the intergranular cracking. Additionally, to account the effect of mean stress on fatigue life of the alloy CM 247 DS LC empirical relations developed by Smith–Watson–Topper (SWT) and Morrow were used.     

Wear mechanisms and effects of monolithic Sialon ceramic tools in side milling of superalloy FGH96

FGH96 is one kind of nickel-based superalloy, being widely used in aero engine hot end components due to its superior mechanical properties, while its machinability has been a challenge because of rapid tool wear and low machining efficiency. Recently, ceramic tools have gained widespread attention for their superior performance maintained at high temperatures. This work aims to study the wear mechanisms and the wear effects of monolithic Sialon ceramic tool in the side milling of superalloy FGH96. The specific influences of average flank wear on the side milling forces, machining temperatures, surface quality and micro-hardness of subsurface and the wear modes in the early and serious stage are analyzed. The results indicate that the milling temperatures, machined surface roughness and the depth of hardening layer will increase as the wear increases. While parallel to the cutting direction, the machined surface roughness is about 1.8 μm without significant changes until entering the serious wear stage. The milling force tends to undergo a decrease when the VB is about 0.22 mm, since the temperature is close to the solute temperature of γ’ phase in this stage. The main wear modes of monolithic ceramic tools are adhesive wear and flanking in the flank surface.     

Effect of YSZ-incorporated glass-based composite coating on oxidation behavior of K438G superalloy at 1000 °C

A glass-based composite coating incorporating YSZ particles was prepared by sintering on K438G superalloy substrates. The YSZ additions increased the cyclic oxidation resistance at 1000 °C, while the formation of zircon resulting from interfacial reactions between YSZ and the glass matrix worked reversely. Besides, the YSZ inclusions changed the crystallization behavior of the glass matrix, and only anorthite precipitated during cyclic oxidation. Due to the synergy of sand-blasting and sealing effect of the glass-based coating, the oxidation behavior of K438G was changed and a layer of alumina instead of chromia formed at the substrate/coating interface. Furthermore, a gahnite layer formed at the alumina/gahnite interface because of interfacial reactions between alumina and the glass matrix, leading to the formation of a bi-layered thermally grown oxide. Thus, the alumina layer was protected from the attack of the active glass matrix. Accordingly, the coated K438G superalloy exhibited satisfactory oxidation resistance at 1000 °C.     

A comparison of the microstructure and creep behavior of cold rolled HAYNES 230 alloy™ and HAYNES 282 alloy™

HAYNES 282 and HAYNES 230 nickel-based superalloys were subjected to cold rolling deformation and heat treatments in order to investigate processing-microstructure-property relationships to understand the effects of thermomechanical processing on their microstructure and tensile-creep behavior. The sheet materials underwent four cycles of 20% reduction in thickness followed by a solution treatment. The resultant microstructures were characterized using electron backscattered diffraction and electron microscopy, and the high-temperature (973-1088 K (700-815 °C)) creep and fatigue behavior was evaluated and compared to the commercially available sheet alloys. The thermomechanical processing treatments did not significantly affect the grain boundary character distribution and almost half of all the boundaries in the microstructures were twin boundaries. The creep resistance was shown to degrade with the additional thermomechanical treatments, which resulted in finer equiaxed grain sizes. The HAYNES 282 alloy was shown to be significantly more creep resistant than the HAYNES 230 alloy. The fatigue behavior indicated that creep ratcheting occurred more prominently in the HAYNES 230 alloy than in the HAYNES 282 alloy and this was explained to be a result of the superior creep resistance exhibited by the HAYNES 282 alloy. Overall, this study suggests that additional energy-intensive processing treatments, beyond those involved in the commercially available sheet products, may not be beneficial for additional creep resistance.     

高温合金IN690管材挤压成形数值模拟

采用有限元软件对高温合金Incone1690 (IN690)高温高速热挤压工艺进行了数值分析,获得了不同工艺条件下变形材料内部的温度场、应变场、应力场、挤压力的变化规律.结果表明,随着挤压速度增加,变形的不均匀性增大,坯料的温度升高明显;随着变形温度升高,最大等效应力明显减小,其分布也趋于均匀化,使塑性提高.挤压力随着挤压速度的增大呈先减小后增大的趋势,随着挤压温度的增大而减小.数值模拟结果与试验结果的相对误差小于7.3％.IN690合金合理的挤压工艺参数是挤压速度为110～120mm/s,挤压温度为1 200℃.