Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： Single crystal (SC) superalloy is a kind of complex structure and multi phase materials. With the increase of the degree of alloying and the content of refractory elements, or the more complicated structure and larger size of the casting made of SC superalloy, it is essential important to suppress the formation of solidification defects to improve the quality and performance of the blades. The microstructure and solidification defects of single crystal alloy are not only related to the composition of the alloy, but also depend on its solidification characteristics and technological conditions. The paper first summarizes the research progress of the solidification characteristics for advanced SC superalloys, focusing on analysis of the effects of solidification characteristics and processing parameters on the formation and its mechanics for two typical directional solidification defects, crystallographic orientation deviation and stray grains. Then some methods and approaches to suppress such defect formation for complex single crystal blade have been reviewed.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： Homogeneous distribution of primary dendritic arm spacing (PDAS) is required to achieve uniform mechanical properties in final product of single-crystal superalloys. In this work, the dendrite characterization and orientation of Ni-based single-crystal DD6 superalloy have been deeply investigated using different methods, which include minimum spanning tree (MST), Voronoi polygon- based approach, fast Fourier transform (FFT), as well as EBSD and RO-XRD. The investigation results indicate that the mean PDAS of DD6 superalloy is about 325.7 mm and its variation ratio is 7.38%. The measured Voronoi polygon parameters suggest that the number of nearest-neighbor dendrite ranges from 5.87 to 5.93, approximating six nearest neighbors in the spatial distribution of dendrite microstructures. However, the change in ratio of six nearest number proportion has exceeded 30% for the twenty specimens. The MST method shows that the change in branch length measured from the twenty specimens achieves 26.95%. Also, the analysis results of FFT imply that the dendrite microstructures of DD6 superalloy evolve apparently. These results give the proof that the dendrite microstructures of DD6 superalloy vary with the solidified distance. Additionally, the deviation angles between preferential orientations of DD6 with the axial direction of specimen were measured by EBSD and RO-XRD, respectively. The deviation angle values of DD6 superalloy in this experiment are both within 10�. The reason for the deviation angle measured by RO-XRD being smaller is well explained due to the fact of selecting the diffraction intensity maximum angles. Furthermore, the EBSD results indicate that the orientations of DD6 superalloy prepared by grain selector can be well controlled along the Z-axial direction, but do not work in other two X and Y directions.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： TiAl-based alloys have recently received considerable attention as one of the promising candidates for application in aero engine blades by replacing the Ni-based superalloys because of their unique properties, such as high specific strength, high specific stiffness and good oxidation resistance. However, there are some shortcomings limiting the application of TiAl-based alloys, namely, their brittleness and poor processing properties. Nevertheless, aero engine blades usually suffer a variety of cyclic loadings during the period of services, which finally results in fatigue failure. According to statistics, fatigue failure, mainly high cycle fatigue (HCF), occupies almost 80% failure modes of gas turbine blades in aero engines. Consequently, more and more researches about fatigue behavior of blade materials have been done in the last tens of years. However, there are less relevant results about TiAl-based alloys, especially HCF properties. Recently, the advancement of directional solidification (DS) of TiAlbased alloys using cold crucible has revealed that the ductility can be enhanced at room and elevated temperature. For purpose to verify the influence of DS structures on the tensile and HCF properties, TiAl-based alloy in composition with Ti-47Al-2Cr-2Nb (atomic fraction, %) was prepared and evaluated in this work. Directionally solidified Ti-47Al-2Cr-2Nb alloy ingots with different withdrawal rates (1.0, 1.2 and 1.4 mm/min) were prepared by cold crucible method under alter electromagnetic field in a vacuum furnace. Based on these ingots, macro and microstructures have been characterized by methods of digital camera, OM, SEM and XRD. Furthermore, the tensile properties at room and high temperature (800 ℃) as well as HCF properties at room temperature have been measured respectively. So, the relationship between microstructures and mechanical properties of TiAl-based alloy, especially HCF properties, was demonstrated reasonably and mechanism in which HCF cracks propagated was discussed. The results show that the comprehensive mechanical properties of Ti-47Al-2Cr-2Nb alloy can be significantly improved after directionally solidified using cold crucible. The tensile strength reaches 652 MPa at room temperature with the maximum elongation of 1.5%. Meanwhile, the tensile strength at 800 ℃ attains 490 MPa with the elongation of 5.0%. Based on the data of HCF test at room temperature with the stress ratio of 0.1, the equations of stress amplitude-number of cycles to failure (S-N) curve at different withdrawal rates are calculated. The fatigue limits are 300 and 247 MPa with the withdrawal rates of 1.0 and 1.2 mm/min, respectively, namely, with the increase of withdrawal rate, the fatigue fracture resistance decreases. The mode of HCF fracture of directionally solidified Ti-47Al-2Cr-2Nb alloy behaves in brittle cleavage fracture. And micro-cracks which can propagate along and perpendicular to the lamellae at the same time are observed between a2/g lamellae and around B2 phases.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： TiAl-Nb alloys have been determined as the advanced direction for the development of the high temperature TiAl alloys, so being one of the advanced materials for turbines of aircraft engines and gas-burning power-generation plants. However, highly-Nb addition can lead to the complex solidification behavior of TiAl-Nb alloy and multi- phase microstructure, which is important for the mechanical properties during the alloy design. Bridgman type directional solidification experiments were conducted for Ti-46Al-(8, 9, 10)Nb alloy. The effect of the growth rate and Nb content on the microstructure, phase transition and microsegregation was investigated, and finally the selection diagram of the phase transition and the microstructure of the directional solidified TiAl-Nb alloy were obtained. The results show that the planar-cellular-dendritic evolution of solid-liquid interface can be observed with the increase of the growth rate. Meanwhile the fully b phase solidification changes to the peritectic solidification with the increase of the growth rate, and correspondingly the final microstructure is composed of the a2/g lamellar structure and a multiphase microstructure (B2 phase, a2/g lamellar structure) respectively. The increase of the b-stabilizer Nb content can promote the fully b phase solidification and the formation of the multiphase microstructure (B2 phase, a2/g lamellar structure). The contribution of the growth rate and the Nb content to the phase transition and the microstructure is connected with the solute segregation (S- segregation, b- segregation) closely. The increase of the S-segregation amplitude can easily promote the peritectic reaction, which always leads to the highly solute segregation and the concentrated distribution of plenty of B2 phase in the core of the dendrite. b-segregation is the mainly origin of the B2 phase formation, in which the Nb enrichment in the retained b phase directly determines the morphology and the dimension of the B2 phase. Finally according to the selection diagram of the solidification process and the microstructure of the Ti-46Al alloy with the growth rate and the Nb content, the high Nb content and the low growth rate during fully b solidification should be selected for the prefer microstructure with the homogeneous distribution and the low solute segregation.
Subjects: Materials Science >> Materials Science (General) submitted time 2017-03-31 Cooperative journals: 《金属学报》
Abstract：通过对3种不同W(6%-8%, 质量分数, 下同)含量的第三代镍基单晶高温合金铸态, 热处理态和热暴露后的组织观察和成分分析, 研究了W对元素偏析, 热处理组织及热暴露过程中组织演化的影响. 结果表明: W含量的提高对合金元素的铸态偏析, 完全热处理后的γ′相形貌, 尺寸和体积分数无明显影响. 在950 ℃热暴露过程中, W含量的提高抑制了γ′相的粗化, 但加速了γ′相的连接变形. 3种合金在热暴露过程中析出的TCP相主要为μ相和σ相, 且TCP相析出量随W含量的增加缓慢增大. 此外, 3种合金在1000 ℃热暴露时TCP相析出量最大, 在950 ℃热暴露时次之, 在1050 ℃热暴露时析出量最小.
Subjects: Materials Science >> Materials Science (General) submitted time 2017-03-31 Cooperative journals: 《金属学报》
Abstract：将直流电流作用于定向凝固过程中的Ti-48Al-2Cr-2Nb合金。并分析了合金的凝固组织、相组成和片层组织，测试了合金的显微硬度及800℃压缩力学性能。结果表明，电流在一定程度上促进了合金凝固组织的细化及成分的均匀性，减少或消除了片层间偏析。随着电流密度的增大，平均晶粒尺寸和片层厚度呈现先减小后增大的趋势，α2相相对含量先增大而后减小，合金的显微硬度、压缩断裂与屈服强度也呈现先增大后减小的趋势。平均晶粒尺寸最小约0.46 mm，片层间距最小为0.19 μm，分别比未加载电流时降低70%和29%，α2相相对含量从18.5%增至39.4%。片层间距或晶粒尺寸越小，合金的强度越高并且变形能力愈均匀，塑性也越好。合金的最大显微硬度值达到542 HV，合金的压缩屈服强度与断裂强度分别达到1200 和1365 MPa，与未施加电流时相比均有所提高。加载直流电流引起固-液界面相前沿过冷度减小，可认为是TiAl二元相图中的L→β+L→α+β的包晶反应成分向富Al侧微小偏移，此时初生β相增多，从而造成了TiAl合金室温相组织α2相的相对含量增加。
Subjects: Materials Science >> Materials Science (General) submitted time 2016-11-15 Cooperative journals: 《金属学报》
Abstract：采用熔体超温处理技术对新型含Re和Ru的镍基单晶高温合金熔体进行了不同温度的超温处理，利用电子探针系统研究了定向凝固过程中合金元素的溶质分布状态及溶质分配系数随熔体超温处理温度变化的规律。结果表明，在平界面定向凝固条件下，熔体超温处理对Al、Ta、Ru、Re、W元素的分布和分配系数有明显的影响，而对Mo、Cr、Co元素无明显影响。随熔体超温处理温度的升高，Al、Ta元素在固液界面前沿液相一侧的分布呈现先增高后降低的趋势，而Re、W、Ru元素则呈现相反的规律。当熔体从1500℃过热到1700℃时，Al、Ta、Re、W元素的溶质分配系数趋近于1；进一步提高熔体温度至1800℃，各元素溶质分配系数逐渐偏离1，然而熔体超温处理温度对于Ru、Mo、Cr、Co的溶质分配系数的变化影响不大。熔体超温处理使合金熔体结构发生变化，并进而影响元素分布，是导致溶质分配系数发生变化的主要原因。
Subjects: Materials Science >> Materials Science (General) submitted time 2016-11-10 Cooperative journals: 《金属学报》
Abstract：本文采用冷坩埚定向凝固技术成功地制备了定向柱状晶的Ti-47Al-2Cr-2Nb合金铸锭，在改变抽拉速度的情况下，所获得的铸锭具有不同片层间距的双相全片层组织，并在其中分布着少量B2相。通过拉伸性能实验，测得其室温极限抗拉强度最高达到652MPa，伸长率最大达到1.5%，而高温极限抗拉强度可达490MPa，伸长率为5.0%。通过高周疲劳性能实验，绘制了抽拉速度分别为1.0mm/min和1.2mm/min时定向凝固试样的疲劳寿命S-N曲线，以及应力比R为0.1时的疲劳极限值。对比断口形貌分析表明：室温时试样拉伸断裂的方式为脆性断裂，而试样经高周疲劳实验后断裂的方式为脆性解理断裂；高温时试样拉伸断裂的方式则为大部分脆性断裂与少部分延性断裂并存。对高周疲劳试样断口的分析表明疲劳裂纹在相界面和B2相附近萌生，据此基于塑性钝化理论分析了其裂纹扩展机制，并绘制了高周疲劳裂纹扩展模型图。
Subjects: Materials Science >> Materials Science (General) submitted time 2016-11-04 Cooperative journals: 《金属学报》
Abstract：通过Bridgman定向凝固技术对Al-12%Ni（质量分数）过共晶合金在1~100μm/s的生长速率范围内进行恒速定向凝固以及不同初始生长条件和变速比下的跃迁加速定向凝固实验，获得耦合共晶组织的条件。研究发现，生长速率为1μm/s时，经过一定距离生长后，组织中无单独生长的初生相，获得耦合共晶组织；而当生长速率为2～100μm/s时，初生Al3Ni相领先生长且呈现典型小平面生长特性。跃迁加速实验表明，跃迁加速前的初始组织对最终组织有决定性影响，只有当跃迁前的组织中无单独生长的初生相时，跃迁后才可在较高生长速率下获得耦合共晶组织。同时定向凝固可以有效提高Al-12%Ni合金的强度和塑性，跃迁加速获得的耦合共晶组织延伸率也得到进一步提高。
Hits
Hits
Downloads
Comment