Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： The directionally solidificated (DS) Co-based superalloys are widely used in aircraft turbine vanes due to the good stress-rupture parameters and excellent hot corrosion resistance. The cyclic change of temperatures and complex stress state thermal fatigue (TF) cracks happen frequently in vanes during service. However, most of the work are conducted in Ni- based superalloys and there is rare report concerning the TF behavior of DS Cobased superalloys. Furthermore, due to the residual strain accumulated during processing, shot peening, grinding and recrystallization (RX) frequently occur when the DS components are exposed to high temperatures. It is believed that RX may change the microstructure, especially adding more grain boundaries to DS alloys, and result in the reduction of the mechanical properties of DS superalloys. Therefore, in this work, V-notch plate specimens with notch direction perpendicular and parallel to the DS orientation are machined from the DS plate. Local RX grains are prepared (local indented and then heat treated) in the notch areas of some samples. TF test is conducted between 1000 ℃ to room temperature. The effect of DS orientation and RX on TF properties of a DS Co-based superalloy is investigated. The results indicate that the cracks propagate along the interdendritic regions in the samples with notches parallel to the DS direction, which exhibites lower TF properties than samples with notches vertical to the DS direction. TF cracks initiate and propagate along RX boundaries in samples containing RX grains. Precipitation of M23C6 carbides is found along the RX boundaries during TF tests. Due to the oxidation at the tip of crack, M23C6 desquamates and leads to the formation of micro voids, which accelerates the crack propagation and decreases TF properties of tested alloy. In samples with notches parallel to the DS direction, cracks preferentially propagate along the RX grain boundaries.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： Advanced aero and power generation industry needs high-performance gas turbine. As key parts of gas turbine directionally solidified (DS) columnar grain and single crystal (SX) blades operate in heavy stress and high temperature conditions. The continuous demand for increasing turbine inlet temperature and aggressive environment has pushed alloy designers to develop DS and SX Ni-based blade alloys that contain high amount of alloying elements. DS process of blades using such alloys has become a challenging task. The small DS and SX blades are usually produced by high rate solidification (HRS) process. However, the growth of large DS and SX blades requires directional solidification with a sustained thermal gradient along the DS direction. By increasing the thermal gradient, the dendrites are refined, which results in a mechanically-superior DS and SX with reduced defects. One method to achieve consistent and higher thermal gradients is the utilization of the liquid metal cooling (LMC) process. In this method, heat extraction from the outer surface of the mold during DS relies on heat conduction rather than radiation in the conventional HRS process. The optimization of the LMC process is difficult and costly by experimental methods, especially for the complexly shaped industry gas turbine (IGT) blades because of the complicated process parameters associated with the technique. Numerical simulation is an efficient method to solve this problem. In this work, directionally solidified industry gas turbine hollow blades were prepared by high gradient LMC process. Liquid Sn was used as cooling medium. The temperature fields, macrostructures, primary dendrite arm spacing (PDAS) at various withdrawal rates during LMC process have been calculated with ProCAST software. The impact of withdrawal rate on formation of stray grains and freckles was predicted. The calculated results and the experimental observations agreed well. The solidification rates and cooling rates were found to increase with the increase of withdrawal rate. The axial thermal gradient was high and stable during the LMC process. It was found that stray grains would not block the growth of original grains at optimized withdrawal rate. No freckles were observed in the industry gas turbine hollow blades prepared by LMC technique due to the high cooling rate. Though the mean diameters of columnar grains in LMC blades were almost identical to that observed in HRS blades, the PDAS were more than 50% refined in LMC blades than those in HRS blades.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： Ni- based single crystal superalloys have been widely used for blades and vanes in gas turbine. However, recrystallization (RX) induced by residual strain has been a serious problem for the application of single crystal superalloys. In previous work, effect of microstructure, such as g', g/g' eutectics and carbides, as well as heat treatment parameters, on the RX behavior have been studied. However, the effect of alloy elements on the RX behavior has rarely been reported. Therefore, in this work, the effect of the important solution strengthening elements, W and Re, on the deformation and RX of solution heat treated Ni-based single crystal superalloys was investigated. At first, two single crystal superalloys were prepared, and W and Re were added into one alloy among them. After solution heat treatment, these two single crystal superalloys were deformed by shot-peening or Brinell indentation. Then these deformed samples were heat treated to observe the microstructure of RX. It indicated that RX depth decreased with the addition of W and Re irrespective of deformation mode and heat treatment temperature. Short time heat treatment experiment of indented and shot-peened samples both indicated that incubation period of RX was prolonged and nucleation of RX was slowed with the addition of W and Re, which verified that RX was suppressed by W and Re. After shot-peening, micro-hardness of the alloy with W and Re increased, but the depth of deformation zone was obviously reduced. Higher density of dislocation was found in the single crystal superalloy with W and Re, and also lots of dislocation tangles were observed. So, in this alloy, dislocation annihilated slowly, that is, recovery was slowed down, which prolonged the incubation period of RX. During the process of RX grain growth, the maximum RX grain boundary migration velocity was reduced with the addition of W and Re. Moreover, the change of mean RX grain boundary migration velocity showed the same trend with the micro-hardness along the direction of RX depth.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： As one of the most promising next generation reactors, the molten salt breeder reactor (MSBR) with excellent inherence security has attracted more and more attentions in recent years due to energy shortage and the security problem of traditional nuclear reactor. The most significant service characteristic of the structural material used in MSBR is the existence of FLiNaK molten salt compared with other nuclear reactors. FLiNaK molten salt is very corrosive to the structural material in the reactor, and affects the safety operation of nuclear power plants. A polycrystalline Ni-Mo-Cr-Fe superalloy was developed and used as an important structural material in MSBR at Oak Ridge National Laboratory (ORNL), but the corrosion mechanism of the alloy in FLiNaK molten salt has not been determined since the study terminated in 1970' s as some politic reasons. Alloy served in harsh environments, often using protective coating to improve the corrosion properties. While few works about the coating corrosion resistance in FLiNaK molten salt were reported at present. Al2O3 and Cr2O3 coatings usually have excellent corrosion resistance in molten salt, such as sulphate, nitrate and halide molten salt. But, whether the oxide film has corrosion resistance in FLiNaK molten salt has not been determined. In this work, the corrosion mechanism of alloy in FLiNaK molten salt was studied by using immersion corrosion experiment through the method of SEM, EDS and XRD. The influence of Al2O3 coating on corrosion resistance in FLiNaK molten salt was also investigated. The results show that the Al2O3 coating does not affect the exsolution corrosion characteristics of Cr and Mo elements in FLiNaK molten salt at 700 ℃ for 400 h. The different is that naked alloy exhibits intergranular corrosion characteristic, and the alloy with Al2O3 coating exhibits spot corrosion characteristic. The Al2O3 coating cannot improve the corrosion resistance of the alloy in FLiNaK molten salt. The Al2O3 film dissolved in molten salt and resulted in the exposure of the alloy surface. The corrosion rate was increased since the formation of corrosion cell between oxide film and the exposed alloy surface.
Subjects: Materials Science >> Materials Science (General) submitted time 2016-11-15 Cooperative journals: 《金属学报》
Abstract：对不同Re、W含量的铸态镍基单晶高温合金通过Brinell硬度计压痕变形, 分别在1230 ℃至1330 ℃温度下保温1 h, 研究了难熔元素Re和W对铸态单晶高温合金再结晶行为的影响. 结果表明: 再结晶晶粒在压痕表面形成, 并沿枝晶干向内扩展, 晶界迁移受到枝晶间粗大g'相和(g+g')共晶阻碍. 添加Re、W提高了铸态单晶高温合金的g'相溶解温度和(g+g')共晶含量, 导致单晶高温合金的再结晶温度升高. 热处理温度升高, 各单晶高温合金的再结晶面积随着枝晶间g'相和共晶含量的减少而增大. 相同热处理温度下, 由于不同成分单晶高温合金枝晶间粗大g'相和(g+g')共晶含量不同, 不含难熔元素Re、W的单晶高温合金再结晶面积最大, 含Re单晶高温合金的再结晶面积大于含W单晶高温合金, 同时添加Re和W的单晶高温合金再结晶面积最小.
Subjects: Materials Science >> Materials Science (General) submitted time 2016-11-04 Cooperative journals: 《金属学报》
Abstract：本文采用"V"型缺口分别垂直和平行于凝固方向的板状热疲劳试样, 并在缺口位置预制再结晶组织, 研究了在最高温度为1000℃, 最低温度为室温的冷热循环下, 缺口取向和再结晶对定向凝固钴基高温合金热疲劳性能的影响. 结果表明: 缺口取向垂直于凝固方向时, 基体在应力作用下循环氧化开裂; 缺口平行于凝固方向时, 热疲劳性能下降, 裂纹沿枝晶间扩展. 再结晶降低定向凝固钴基高温合金的热疲劳性能, 再结晶晶界氧化开裂, 实验中晶界析出的M23C6型碳化物氧化脱落后形成的孔洞加速了裂纹扩展; 连接枝晶间碳化物的再结晶晶界成为缺口平行于凝固方向时热疲劳裂纹的优先扩展通道。
Subjects: Materials Science >> Materials Science (General) submitted time 2016-11-04 Cooperative journals: 《金属学报》
Abstract：利用高温度梯度定向凝固-液态金属冷却(LMC)技术制备了重型燃机定向结晶空心高压涡轮叶片，采用ProCAST有限元模拟软件计算了LMC定向凝固工艺下，不同抽拉速率时空心定向结晶叶片凝固过程的温度场、晶粒组织以及一次枝晶间距(PDAS)，预测了抽拉速率对杂晶、雀斑等缺陷的影响。结果表明，模拟结果与实验结果吻合良好。随着抽拉速率增加，叶片的凝固速率、冷却速率均增加，远高于高速凝固法(HRS)的凝固速率、冷却速率；叶片不同部位达到最大纵向温度梯度时的抽拉速率不同，纵向温度梯度是评价定向工艺的有效方法；LMC工艺制备的燃机叶片消除了雀斑缺陷，PDAS远小于HRS工艺。
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