Microstructural effects on high-cycle fatigue-crack initiation in A356.2 casting alloy

The effects of various microconstituents on crack initiation and propagation in high-cycle fatigue (HCF) were investigated in an aluminum casting alloy (A356.2). Fatigue cracking was induced in both axial and bending loading conditions at strain/stress ratios of −1, 0.1, and 0.2. The secondary dendrite arm spacing (SDAS) and porosity (maximum size and density distribution) were quantified in the directionally solidified casting alloy. Using scanning electron microscopy, we observed that cracks initiate at near-surface porosity, at oxides, and within the eutectic microconstituents, depending on the SDAS. When the SDAS is greater than ∼ 25 to 28 μm, the fatigue cracks initiate from surface and subsurface porosity. When the SDAS is less than ∼ 25 to 28 μm, the fatigue cracks initiate from the interdendritic eutectic constituents, where the silicon particles are segregated. Fatigue cracks initiated at oxide inclusions whenever they were near the surface, regardless of the SDAS. The fatigue life of a specimen whose crack initiated at a large eutectic constituent was about equal to that when the crack initiated at a pore or oxide of comparable size.     

球团矿CO还原过程中微观结构的变化

根据高喷煤比的高炉多相流模拟研究数据,对球团矿进行CO还原反应实验,运用扫描电镜等手段对不同还原度的球团矿做微观结构分析,以探究不同还原阶段引起球团矿强度下降原因.微观结构分析表明,还原过程中从高价铁氧化物到低价铁氧化物变化过程中晶格的增大,新生铁相基体孔隙的增加、裂纹的产生以及渣铁分离等是导致强度下降的主要原因.     

玄武岩三维细观孔隙模型重构与直接拉伸数值试验

由于岩石材料的不透明性和多孔隙特性, 通过传统的物理试验或数值模拟很难真实体现其内部三维细观结构. 本文基于CT扫描技术、边缘检测算法、滤波算法、三维点阵映射与重构算法, 构建了可以表征玄武岩试样内部孔隙结构的三维细观非均匀数值模型. 结合并行计算进行直接拉伸数值试验, 研究了内部孔隙结构特征对试样破坏机制及抗拉强度的影响. 研究结果表明: 加载初期在试样孔隙处产生初始裂纹, 随着荷载的增加初始裂纹逐渐沿横向扩展最终形成宏观拉伸破坏裂纹, 并且孔隙含量和分布位置对试样拉伸断裂的位置具有重要影响. 随着孔隙率增高, 试样破坏过程中的声发射数目和能量逐渐减小. 拉伸破坏模式呈现脆性破坏特征, 同时孔隙的存在削弱了试样的抗拉强度.      

Directional solidification of eutectics: New prospects for refractory oxide cermaics (review)

Conclusions The directional solidification of the eutectic in the refractory oxide systems results in the formation of a plate-shaped or fibrous oriented composite structure. The theoretical fundamentals of the method of directionally solidified eutectics, developed on the basis of the metallic systems, can also be applied extensively to refractory oxide DSEs so that their structure and composition can be controlled in the required direction.The orientation relations between the phases of the refractory oxide DSEs and the construction of the interphase boundaries result in the formation of semicoherent interphase boundaries. This is one of the conditions for the higher and thermal stability of the materials.The mechanical properties of the materials based on refractory oxide DSEs are controlled by the structure of the oriented eutectic. Disruption of flat frontal growth in solidification leads to the formation of a cellular structure. The cell boundaries are the weakest area of the material and control its behavior during failure.The materials based on the direction solidified refractory oxide eutectics have unique set of the applied properties and are highly promising for use as structural materials at elevated (up to 1500°C) temperatures.The materials made of the refractory oxide DSEs can be used in practice after developing high-productivity methods of their growth which would make it possible to produce components of the required shape.Translated from Poroshkovaya Metallurgiya, No. 8(320), pp. 58–69, August, 1989.     

Microstructure and Properties of Cu-Fe-Cr-Ag Alloy Prepared by Directional Solidification and Upward Continuous Casting

A Cu-Fe-Cr-Ag alloy was prepared by directional solidification (DS) and upward continuous casting (UCC) to study the effect of different casting methods on the structure and properties of Cu-Fe-Cr-Ag. The results showed that the directionally solidified Cu-Fe-Cr-Ag alloy had excellent mechanical properties and conductivity. After cold drawing and isothermal aging, the peak tensile strength (789 MPa) and peak conductivity (65.5 pct IACS) of directionally solidified Cu-Fe-Cr-Ag alloy were 21 MPa and 4.7 pct IACS higher, respectively, than those that of the upward continuously casted Cu-Fe-Cr-Ag alloy. Compared to upward continuously casted Cu-Fe-Cr-Ag alloy, the Fe dendrites in directionally solidified Cu-Fe-Cr-Ag alloy were much finer, more uniform, and arranged along the direction of the magnetic field. Cu and Ag formed a Cu-Ag eutectic structure at the edge of the directionally solidified Cu-Fe-Cr-Ag alloy rod. After multi-stage thermomechanical treatment, Ag was mainly distributed around the material and formed a structure similar to Ag-clad Cu. The directionally solidified Cu-Fe-Cr-Ag alloy had a smaller lattice constant and finer Fe fibers. The small lattice constant and Fe fibers and the special distribution of Ag lead to the excellent comprehensive performance of the directionally solidified Cu-Fe-Cr-Ag alloy.

     

Strengths and failure mechanisms of a Co-15Cr-13TaC directionally solidified eutectic alloy

The tensile and stress rupture properties of a Co(Cr)-TaC directionally solidified eutectic alloy have been investigated and compared to those of a single phase, directionally solidified Co(Cr) alloy corresponding in composition to that of the eutectic matrix. The temperature for 100 h stress rupture life at 20,000 psi (138 MN/m²) is about 200°F (111°C) better than that of any cast nickel-base superalloy now used in aircraft or land gas turbines. The degree of superiority becomes progressively less at higher stresses, and at 50,000 psi (345 MN/m²), the temperature for 100 h stress rupture life in the eutectic is about 150°F (83°C) less than for several high strength superalloys. This behavior is related to a bimodal stress rupture mechanism. A model predicts that at low stresses, failure is controlled by the stress rupture behavior of the matrix; and at high stresses failure occurs by a stress relaxation mechanism which causes early fiber failure. Fractographic observations are in agreement with the existence of two stress rupture mechanisms. It was also observed that both stress rupture mechanisms can occur at the same temperature, with specimens failing by the fiber-related mode having lives 1 to 2 orders of magnitude in time less than those which fail by the matrix-related mode at the same stress level. Both authors were associated with the General Electric Corporate Research and Development Center, Schenectady, N. Y. at the time this study was performed     

新型高强抗热腐蚀定向凝固高温合金DZ409

在高温合金设计理论的指导下,综合考虑重型燃机叶片用耐热腐蚀定向凝固高温合金对持久强度、热腐蚀抗力、长期组织稳定性等多方面的要求,设计出 7 种成分的合金.通过热力学平衡相和电子空位数计算,优选3 种成分合金开展力学性能、抗热腐蚀、组织稳定性等研究,进行试验筛选工作,研制出新型抗腐蚀定向凝固高温合金 DZ409 合金.DZ409 合金具有良好的抗腐蚀性能、组织稳定性以及高的力学性能,可用作重型燃机的叶片材料.     

Thermal and grain-structure simulation in a land-based turbine blade directionally solidified with the liquid metal cooling process

The thermal field and the grain structure of a cored superalloy turbine blade, which has been directionally solidified with the liquid metal cooling (LMC) process, has been simulated in three dimensions using a cellular automaton (CA) coupled with finite-element (CAFE) model. The cooling induced by the liquid aluminum bath has been replaced by a heat-transfer coefficient, whose temperature- and time-dependence has been adjusted on the basis of natural convection simulations and dimensionless analyses. The simulated grain structure and crystallographic texture have been compared with the microstructure, and the electron back-scattered diffraction (EBSD) results were obtained for a real blade. In both the experiment and the simulation, it has been found that the grains do not exhibit a well-defined <001> texture, even near the top of the blade, mainly as a result of a concave liquidus surface. In order to improve the texture and decrease the number of stray crystals, the LMC process was then optimized by changing several parameters. The baffle geometry, the liquid bath level, and the thermal conductivity of the ceramic mold were found to be the dominant parameters. Using the optimized design, the effect of the withdrawal rate on the resulting grain structure was investigated.     

Experimental-calculation study of ion-plasma thermal barrier coatings for turbine blades made of intermetallic nickel superalloys

The ion-plasma thermal barrier coatings deposited onto samples and blades made of intermetallic VKNA-1V and VKNA-25 alloys are tested in a laboratory. The external ceramic layer of the thermal barrier coatings (TBC) is formed by magnetron sputtering of zirconium alloy targets and has a columnar structure. The influence of NiCrAlY(Re, Ta, Hf) + AlNiY(Hf) + ZrYGdO TBC on the long-term strength at a test temperature of 1200°C and on the high-cycle fatigue at a temperature of 900°C is studied. Blades with TBC are subjected to thermal cycling tests in the temperature range 950 ? 400°C and 1050 ? 400°C during air cooling and in the range 950 ? 200°C during water cooling at 500 cycles. The temperature fields in the cross section of a blade airfoil during thermal cycling are calculated. The laws of formation of fracture zones and the development of thermal fatigue cracks under the conditions that are close to the operating conditions of nozzle TBC-containing blades are investigated.     

Cracking mechanisms in thermally cycled Ti-6AI-4V reinforced with SiC fibers

A titanium alloy (Ti-6A1-4V) reinforced with continuous SiC fibers (SCS-6) was thermally cycled between 200 ‡C and 700 ‡C in air and argon. The composite mechanical properties deteriorate with an increasing number of cycles in air because of matrix cracks emanating from the specimen surface. These cracks also give oxygen access to fibers, further resulting in fiber degradation. The following matrix cracking mechanisms are examined: (1) thermal fatigue by internal stresses resulting from the mismatch of thermal expansion between fibers and matrix, (2) matrix oxygen embrittlement, and (3) ratcheting from oxide accumulating within cracks. Matrix stresses are determined using an analytical model, considering stress relaxation by matrix creep and the temperature dependence of materials properties. Matrix fatigue from these cycli-cally varying stresses (mechanism (1)) cannot solely account for the observed crack depth; oxygen embrittlement of the crack tip (mechanism (2)) is concluded to be another necessary damage mechanism. Furthermore, an approximate solution for the stress intensity resulting from crack wedging by oxide formation (mechanism (3)) is given, which may be an operating mech-anism as well for long cracks. S.H. THOMIN, formerly Graduate Student, Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA     

Fabrication of Porous Iron with Directional Pores through Thermal Decomposition of Chromium Nitride

Lotus-type porous iron with long directional pores was fabricated by a continuous zone melting technique through thermal decomposition of chromium nitride Cr_1.18N. Nitrogen decomposed from the nitride powders dissolves in the molten iron. Insoluble nitrogen evolves the directional gas pores when the melt is solidified in a direction. The porosity increases with increasing transference velocity, while the pore diameter is almost constant. The porosity change with the transference velocity is attributed to the difference in decomposition rate of chromium nitride. The compound Cr_1.18N is composed of CrN and Cr₂N, the latter of which is considered to evolve the pores because of the coincidence of heating rate of the continuous zone melting with that for the decomposition of Cr₂N.     

Electrical properties of Al-In-Sn alloys directionally solidified in high and low gravitational fields

Al-ln-Sn alloys have been directionally solidified in the NASA KC-135 aircraft which flies a series of parabolas to generate high (high-g) and low gravity (low-g) forces parallel to the longitudinal growth axis. Thus, for a given sample successive sections can be identified which were solidified in high-g and in low-g. Measurements on the electronic properties of the samples reveal that (1) the resistivity of the low-g sections is larger (about a factor of 10) than that of the high-g sections; (2) the low-g sections behave conductively like a semi-metal, while the high-g sections are essentially metallic; and (3) both high-g and low-g sections are superconducting but the superconducting transition temperature of the low-g sections is 1 K higher than that of the high-g sections.     

TRT透平机叶片断裂分析

对某厂2号TRT透平机一级动叶片中断裂叶片的化学成分、宏观断口以及金相组织进行了分析,结果表明,该叶片属于腐蚀疲劳断裂,其疲劳裂纹起源于叶片进气边边缘部位的腐蚀麻点处,腐蚀麻点是由于叶片与湿积灰中的腐蚀性介质Cl-,SO4-等发生反应造成的。     

A thermodynamic prediction for microporosity formation in aluminum-rich Al-Cu alloys

A computer model is used to predict the formation and the amount of microporosity in directionally solidified Al-4.5 wt pct Cu alloy. The model considers the interplay between so-called “solidification shrinkage” and “gas porosity” that are often thought to be two contributing and different causes of interdendritic porosity. There is an accounting of the alloy element, Cu, and of dissolved hydrogen in the solid- and liquid-phase during solidification. Consistent with thermodynamics, therefore, a prediction of forming the gas-phase in the interdendritic liquid is made. The local pressure within the interdendritic liquid is calculated by macrosegregation theory that considers the convection of the interdendritic liquid, which is driven by density variations within the mushy zone. Process variables that have been investigated include the effects of thermal gradients and solidification rate, and the effect of the concentration of hydrogen on the formation and the amount of interdendritic porosity. These calculations show that for an initial hydrogen content less than approximately 0.03 ppm, no interdendritic porosity results. For initial hydrogen contents in the range of 0.03 to 1 ppm, there is interdendritic porosity. The amount is sensitive to the thermal gradient and solidification rate; an increase in either or both of these variables decreases the amount of interdendritic porosity.     

Influence of microstructure on high-cycle fatigue of Ti-6Al-4V: Bimodal vs. lamellar structures

The high-cycle fatigue (HCF) of titanium alloy turbine engine components remains a principal cause of failures in military aircraft engines. A recent initiative sponsored by the United States Air Force has focused on the major drivers for such failures in Ti-6Al-4V, a commonly used turbine blade alloy, specifically for fan and compressor blades. However, as most of this research has been directed toward a single processing/heat-treated condition, the bimodal (solution-treated and overaged (STOA)) microstructure, there have been few studies to examine the role of microstructure. Accordingly, the present work examines how the overall resistance to high-cycle fatigue in Ti-6Al-4V compares between the bimodal microstructure and a coarser lamellar (β-annealed) microstructure. Several aspects of the HCF problem are examined. These include the question of fatigue thresholds for through-thickness large and short cracks; microstructurally small, semi-elliptical surface cracks; and cracks subjected to pure tensile (mode I) and mixed-mode (mode I+II) loading over a range of load ratios (ratio of minimum to maximum load) from 0.1 to 0.98, together with the role of prior damage due to sub-ballistic impacts (foreign-object damage (FOD)). Although differences are not large, it appears that the coarse lamellar microstructure has improved smooth-bar stress-life (S-N) properties in the HCF regime and superior resistance to fatigue-crack propagation (in pure mode I loading) in the presence of cracks that are large compared to the scale of the microstructure; however, this increased resistance to crack growth compared to the bimodal structure is eliminated at extremely high load ratios. Similarly, under mixed-mode loading, the lamellar microstructure is generally superior. In contrast, in the presence of microstructurally small cracks, there is little difference in the HCF properties of the two microstructures. Similarly, resistance to HCF failure following FOD is comparable in the two microstructures, although a higher proportion of FOD-induced microcracks are formed in the lamellar structure following high-velocity impact damage. This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans, Louisiana, under the auspices of the following ASM committees: Materials Science Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee.     

Effect of Hot Isostatic Pressing on Fatigue Properties of Laser Melting Deposited AerMet100 Steel

The ultra-high strength steel AerMet100 was fabricated by laser melting deposition (LMD) process. The effect of hot isostatic pressing (HIP) on high-cycle fatigue properties of the LMD AerMet100 steel was investigated, and the influence of defects on fatigue behavior was discussed. Results showed that the LMD AerMet100 steel had fine directionally solidified cellular-dendrite structure and coarse columnar prior austenite grains. Metallurgical defects such as gas pore and lack-of-fusion porosity were produced during the laser deposition process. After HIP treatment, the number and size of metallurgical defects had remarkably decreased. Moreover, high-cycle fatigue properties of the alloys after HIP treatment were superior to the as-deposited alloys.     

Effect of micro-pores on cracks formation in metallurgical coke

The relationships of micro-pores and cracks in metallurgical coke have been investigated by optical microscope and field emission scanning electron microscope, using surface section samples. The pores have circular, elliptical and irregular shapes with smooth outlines, formed during the thermoplastic stage of the coking process. They often associate with connecting cracks between neighbouring pores. In case of elliptical pores, the connecting cracks are usually oriented along the longer axis of the pore. The connecting cracks can be developed between the pores, depending on their size and the distance between them. The coke with a large number of small pores rather than with a small number of larger pores will have lower strength due to the increased amount of connecting cracks. When compared with circular pores, elliptical and flattened pores have a lower ability to resist load pressure. Nano-sized pores have polygonal outlines, indicating an ‘explosion’-type formation in the solidified matrix.     

铈锆酸镧涂层高温时效行为和热震性能研究

采用等离子喷涂制备了铈锆酸镧涂层,采用扫描电子显微镜(SEM)、X射线衍射分析(XRD)、图像分析法等研究了喷涂功率对沉积态涂层表面和截面微观结构、孔隙率等的影响规律,研究了涂层在1200℃、1300℃高温100h时效下相稳定性、微观结构、孔隙率的变化,比较了不同喷涂功率涂层的抗热震性能。研究结果表明:随着等离子喷涂功率的增加,喷涂过程中半熔融颗粒比例减小,涂层的孔隙率减小。涂层经1200℃、1300℃高温保温100 h后仍然具有单一的烧绿石结构,随着热处理温度升高,涂层孔隙率减小。研究了不同功率喷涂的涂层从1250℃到冷水中的热震行为,失效机制分析表明:陶瓷层与粘结层热应力不匹配造成陶瓷层底部产生裂纹是导致涂层失效的主要方式。     

Modeling of Microporosity Size Distribution in Aluminum Alloy A356

Porosity is one of the most common defects to degrade the mechanical properties of aluminum alloys. Prediction of pore size, therefore, is critical to optimize the quality of castings. Moreover, to the design engineer, knowledge of the inherent pore population in a casting is essential to avoid potential fatigue failure of the component. In this work, the size distribution of the porosity was modeled based on the assumptions that the hydrogen pores are nucleated heterogeneously and that the nucleation site distribution is a Gaussian function of hydrogen supersaturation in the melt. The pore growth is simulated as a hydrogen-diffusion-controlled process, which is driven by the hydrogen concentration gradient at the pore liquid interface. Directionally solidified A356 (Al-7Si-0.3Mg) alloy castings were used to evaluate the predictive capability of the proposed model. The cast pore volume fraction and size distributions were measured using X-ray microtomography (XMT). Comparison of the experimental and simulation results showed that good agreement could be obtained in terms of both porosity fraction and size distribution. The model can effectively evaluate the effect of hydrogen content, heterogeneous pore nucleation population, cooling conditions, and degassing time on microporosity formation.