基于不同原始组织预设变形第四代单晶高温合金的再结晶行为

第四代单晶高温合金标准热处理试样和铸态试样压痕后分别在1100,1150,1200,1250,1300℃和1340℃退火处理,采用光学显微镜、扫描电镜、电子背散射仪研究不同条件的再结晶组织。结果表明:1100,1150,1200℃退火处理后,标准热处理试样和铸态试样都出现胞状再结晶。1250℃退火处理后,标准热处理试样和铸态试样都为混合再结晶。1300℃退火处理后,标准热处理试样再结晶组织全部为等轴再结晶,而铸态试样仍为混合再结晶。1340℃退火处理后,标准热处理试样和铸态试样都形成了等轴再结晶。随着退火温度升高,标准热处理试样和铸态试样的再结晶层深度明显增加,标准热处理试样再结晶深度明显大于铸态试样,相同条件下标准热处理试样的再结晶晶粒更容易长大。再结晶与基体的界面为小角度晶界、大角度晶界,而再结晶晶粒之间为小角度晶界、大角度晶界和孪晶界。孪晶在单晶高温合金再结晶的过程中发挥了重要作用。     

喷丸对DD6单晶高温合金拉伸性能的影响

采用扫描电子显微镜(SEM)、X射线(X-ray)和透射电子显微镜(TEM)等手段研究铸钢丸喷丸对第二代单晶高温合金DD6在500,600,650℃拉伸性能的影响。结果表明:500,600,650℃时,喷丸对DD6合金抗拉强度影响不大,略微提高屈服强度,显著降低伸长率和断面收缩率。喷丸DD6合金在流变应力上升到最高点后断裂,试样拉伸断裂后的横截面为圆形;未喷丸DD6合金拉伸曲线呈现双重阶段特征,试样拉伸断裂后的横截面为椭圆形。     

带有扭转小角度晶界DD6单晶高温合金的横向持久性能EI北大核心

在980℃/250MPa条件下,研究了DD6单晶高温合金扭转小角度晶界的横向持久性能。结果表明,0~4.0°晶界试样与[001]取向试样的持久寿命相近,表明DD6单晶高温合金的横向持久性能优异;4.0°晶界试样持久断裂不为沿晶断裂;随着晶界角度的进一步增大,合金的横向持久性能明显下降,断裂方式为沿晶断裂。     

一种单晶高温合金铸态下的再结晶行为（英文）

一种单晶高温合金试样铸态下压痕后分别在1100,1150,1200,1250,1300和1340℃温度下的真空保温处理4h,用包括电子背散射衍射(EBSD)在内的诸多方法研究了再结晶组织。结果表明,合金经1100,1150和1200℃保温处理后形成胞状再结晶,1340℃保温处理后形成了等轴再结晶,1250和1300℃保温处理后形成了混合再结晶。不连续的胞状再结晶首先在枝晶间的共晶上形成,而等轴再结晶首先在枝晶干形成和长大。1100和1150℃保温处理形成的胞状再结晶晶界大多数为小角度晶界。随着处理温度升高,晶界角度增加。除1100℃其它不同温度处理形成的再结晶晶粒都存在大量孪晶,随着处理温度升高,孪晶数量增多。     

单晶高温合金DD6拉伸性能各向异性

研究了[001],[011],[111]取向第二代单晶高温合金DD6的拉伸性能与断口组织。结果表明:DD6单晶高温合金存在拉伸各向异性,850℃以上[001]取向DD6单晶高温合金的抗拉强度与屈服强度分别高于[011],[111]取向合金的强度,[001],[011],[111]取向DD6单晶高温合金的拉伸断口具有类解理断裂与韧窝断裂的特征。     

Hf含量对第二代单晶高温合金拉伸性能的影响

在室温、760℃和980℃下,研究了Hf含量对第二代单晶高温合金DD6拉伸性能的影响,采用扫描电镜和透射电镜研究了合金拉伸断口和拉伸断裂显徽组织.结果表明,在本试验研究范围内,Hf含量对DD6合金的拉伸性能影响较小;不同Hf含量DD6合金室温下和760℃的拉伸断裂为类解理断裂,980℃的拉伸断裂为韧窝断裂.     

长期时效对DD6单晶高温合金组织和力学性能的影响

采用螺旋选晶法制备DD6合金单晶试棒,标准热处理后在980℃长期时效2000 h,研究980℃长期时效对DD6单晶高温合金的组织演化及力学性能的影响.结果表明:随着长期时效时间的延长,合金中γ'相的尺寸增大,2000 h后γ'相尺寸约为1μm,没有TCP相析出,合金具有较好的组织稳定性.2000 h长期时效试样在980℃/243 MPa下持久寿命为180.16 h,为热处理态的56.3％;在1070℃/130 MPa下持久寿命为144.42 h,为热处理态的35.31％,断裂模式均为微孔聚集型断裂;相比热处理态的合金,2000 h长期时效态试样760℃的抗拉强度和屈服强度分别降低5.55％和5.88％;980℃的抗拉强度和屈服强度分别下降11％和10.59％.     

K4169高温合金铸件铸造缺陷修复及疲劳性能研究

采用TlG焊对K4169高温合金薄片中铸造组织缺陷进行修复,并对修复前后的合金薄片进行疲劳性能测试和显微组织分析.结果表明:铸造缺陷对合金薄片疲劳性能的影响很大,含铸造缺陷的合金薄片疲劳寿命不到3万周次,而不含铸造缺陷的合金薄片疲劳寿命达7.1万周次.含铸造缺陷的样品经过TlG焊接修复后,疲劳寿命仍达6～9万周次,接近甚至超过了不含缺陷的试样疲劳寿命,这表明合适的焊接修复方法不会降低高温合金薄片的疲劳性能.铸件焊接修复后疲劳寿命高的原因是,焊缝中析出的较粗大相数量多、分布均匀,且疲劳裂纹扩展过程中产生较多的二次裂纹引发了裂纹偏移和扩展路径增长.     

DZ125定向凝固高温合金在不同状态下的超高周疲劳行为研究

研究了常规热处理和长期时效状态的DZ125定向凝固高温合金在室温和700℃下的超高周疲劳特征及其差异。利用超声疲劳试验机测试了合金的超高周疲劳性能,采用扫描电镜研究了合金的疲劳断口特征,借助电子探针和纳米压痕测量仪分别测试了合金枝晶间和枝晶干的成分和弹性模量。结果表明:两种热处理制度下DZ125合金的枝晶间和枝晶干的弹性模量以及合金元素分配和偏析比差异不大;DZ125合金在700℃下的超高周疲劳性能低于室温,合金在室温下存在明显的频率效应,但在700℃下不存在频率效应;室温和700℃下,合金经过长期时效后的疲劳性能均有所降低,特别在700℃下合金性能降低得更多。室温下,DZ125合金的两种热处理制度的超高周疲劳裂纹均起源于试样的表面;700℃下,两种热处理制度的超高周疲劳裂纹均起源于试样的亚表面。     

[011]取向DD6单晶高温合金薄壁持久性能

采用薄壁试样研究了760,980,1070℃条件下[011]取向第二代单晶高温合金DD6的持久性能.结果表明:与标准试样(直径5mm)相比,DD6单晶高温合金薄壁试样的持久寿命与之相当.对合金760℃/590MPa,980℃/270MPa及1070℃/270MPa条件下组织分析发现:760℃合金γ’相局部发生变形,98...     

Influence of residual fatigue damage on the fracture toughness parameters of an AA6082-T6 aluminium alloy

The present investigation has been carried out in order to study the influence of the previous accumulated fatigue damage induced during high cycle fatigue (HCF), on the fracture toughness parameters of an AA6082-T6 aluminium alloy. The results show that previous fatigue damage accumulated in HCF does not affect the tensile static mechanical properties of the material, but gives rise to a significant debit of the toughness properties on this aluminium alloy. The fracture toughness results have shown that the crack opening displacement at a crack extension of 0.2 mm (COD_0.2) decreases in the range of ∼18 to 36% whereas the value of the non-linear fracture mechanics parameter  J _0.2, decreases in the range of ∼11 to 25% at applied maximum stresses of 200 and 275 MPa, respectively. Optical microscopy observations conducted on the surface of the specimens subjected to HCF damage indicate the existence of microcracks ∼15 to 25 μm long nucleated along the grain boundaries of the material. Also, the scanning electron microscopy (SEM) observations of the fracture surfaces after the tearing tests show the predominance of a ductile fracture mechanism for the material prior to residual fatigue damage, whereas a mixed ductile–brittle fracture mechanism and the presence of flat facets were observed on the fracture surfaces of the specimens with a fatigue damage of 0.70.     

High‐cycle and very‐high‐cycle fatigue behaviour of a titanium alloy with equiaxed microstructure under different mean stresses

The fatigue behaviour of a titanium alloy Ti‐6Al‐4V with equiaxed microstructure (EM) under different values of tensile mean stress or stress ratio (R) was investigated from high‐cycle fatigue (HCF) to very‐high‐cycle fatigue (VHCF) regimes via ultrasonic axial cycling. The effect of mean stress or R on the fatigue strength of HCF and VHCF was addressed by Goodman, Gerber, and Authors' formula. Three types of crack initiation, namely, surface‐with‐RA (rough area), surface‐without‐RA, and interior‐with‐RA, were classified. The maximum value of stress intensity factor (SIF) at RA boundary for R < 0 keeps constant regardless of R in HCF and VHCF regimes. The SIF range at RA boundary for R > 0 also keeps constant regardless of R in VHCF regime, but this value decreases linearly with the increase of R for surface RA cases. The microstructure observation at RA regions gives a new result of nanograin formation only in the cases of negative stress ratios for the titanium alloy with EM, which is explained by the mechanism of numerous cyclic pressing.     

Cyclic crack growth and fracture resistance of Ti-6Al-6V-2Sn as influenced by recrystallization anneal and interstitial oxygen content

This study was undertaken to determine methods for metallurgical improvement of cyclic crack growth and fracture properties in Ti-6A1-6V-2Sn. Two heats of the alloy were studied: an + β processed material containing 0.165% interstitial oxygen and β-processed material containing 0.077% interstitial oxygen. All test specimens were given a mill anneal heat treatment, in addition some specimens from each heat were given a recrystallization anneal treatment Fatigue-crack growth rates were determined using compact tension specimens and fracture toughness was determined using 1 in. dynamic tear specimens for each material/heat treatment combination. The recrystallization anneal treatment resulted in significant improvements in the fracture resistance of both materials, despite metallographic evidence that die recrystallization-annealed β-processed material did not develop a fully recrystallized structure. Improvements in cyclic crack growth resistance resulting from the recrystallization anneal treatment, per se, were modest. However, the combined effects of recrystallizalion anneal plus a reduction B interstitial oxygen content significantly improved the cyclic crack growth properties of Ti-6Al-6V-2Sn.     

Fatigue behaviour of cast magnesium alloy AZ91 microstructurally modified by friction stir processing

Friction stir processing (FSP) was applied to cast magnesium alloy AZ91-F to modify the as-cast microstructure, and the effect of FSP on fatigue behaviour was discussed based on microstructural consideration, crack initiation, crack growth behaviour, and fracture surface analysis. Fully reversed axial fatigue tests have been performed using as-cast, T5-aged and their FSPed specimens (as-cast/FSP and T5/FSP). It was found that both FSPed specimens exhibited significantly higher fatigue strength than the as-cast and T5-aged specimens. FSP resulted in the break-up of coarse as-cast microstructure, grain refinement of the matrix, finely dispersed precipitates and increase of hardness, thereby both the crack initiation resistance and the crack growth resistance were considerably enhanced compared with the as-cast and T5-aged specimens, resulting in the improved fatigue strengths of the FSPed specimens.     

Fatigue crack propagation behaviour of Al–Zn–Ce alloys

This paper presents the investigation on fatigue crack growth behaviour of Al–Zn and Al–Zn–Ce alloys. Fatigue tests were carried out on as‐cast and heat‐treated CT specimens according to ASTM E647 testing standard. The test results showed that the addition of rare earth element (cerium) and heat treatments (T6 and T5) had very strong influence on fatigue strength. This enhancement was due to metallurgical changes in the alloy system. Cerium eliminates the porosities and refines microstructures of the alloy, showing the improved fatigue crack growth behaviour. In addition, the fatigue fractured specimens were examined using a scanning electron microscope to clarify the fracture initiation points.     

Microstructure-based fatigue modeling of cast A356-T6 alloy

High cycle fatigue (HCF) life in cast Al-Mg-Si alloys is particularly sensitive to the combination of microstructural inclusions and stress concentrations. Inclusions can range from large-scale shrinkage porosity with a tortuous surface profile to entrapped oxides introduced during the pour. When shrinkage porosity is controlled, the relevant microstructural initiation sites are often the larger Si particles within eutectic regions. In this paper, a HCF model is introduced which recognizes multiple inclusion severity scales for crack formation. The model addresses the role of constrained microplasticity around debonded particles or shrinkage pores in forming and growing microstructurally small fatigue cracks and is based on the cyclic crack tip displacement rather than linear elastic fracture mechanics stress intensity factor. Conditions for transitioning to long crack fatigue crack growth behavior are introduced. The model is applied to a cast A356-T6 Al alloy over a range of inclusion severities.     

Mechanical properties and microstructure of Al/Al laminated structure produced via ultrasonic consolidation process

Mechanical properties of a 2024 aluminium alloy laminated structure produced by the ultrasonic consolidation were investigated. In comparison with the monolithic aluminium alloy, the existence of laminated structure gave different fatigue and fracture mechanisms that associated with the layer interfaces. The Al/Al laminated specimens had the lower tensile strength but much higher fracture toughness than the monolithic Al specimens due to the exit of interface delaminations around the crack tip. The fatigue life of the laminated specimens was comparable to that of the monolithic Al specimens, though the initiation and propagation of the crack in the laminated specimens depended strongly on the microstructure of each material. The interface between layers could arrest the fatigue crack and impede the further propagation.     

两种组织TC11钛合金薄板弯曲疲劳性能的研究

本研究选取了具有（α＋β）等轴组织和α／β粗片层组织的TCll钛合金作为研究对象,采用研究了TCll合金薄板样品在恒总应变幅控制下的弯曲疲劳性能及其损伤行为,通过对疲劳开裂路径和断裂的观察与表征,探讨了疲劳损伤与组织结构间的关系。研究发现,恒总应变幅控制下的等轴组织TCll钛合金薄板的弯曲疲劳性能明显的高于粗片层组织合金。等轴组织合金的疲劳裂纹沿α相中的滑移带萌生并扩展,片层组织样品疲劳裂纹沿着α相或与片层垂直的方向扩展。     

Rotary bending high-cycle fatigue behavior of DD32 single crystal superalloy containing rhenium

Smooth and notched specimens of single crystal superalloy DD32 were subjected to rotary bending high-cycle fatigue (HCF) loading at different temperatures. The experimental results demonstrate that fatigue strengths of the smooth and notched specimens reach the maxima and the minimum notch sensitivity displays at 760 °C. DD32 alloy exhibits excellent HCF properties compared to SRR99 alloy under the same test condition. As for the smooth specimens, slip bands moving through γ and γ′ phases as well as dislocation bowing are the main deformation modes. As for the notched specimens, the deformations are carried out by dislocation loop bowing and shearing of PSBs mode at intermediate temperatures; at 900 °C, the minimum fatigue strength results from dislocation climbing deformation and the degradation of γ′ precipitates. The fine secondary γ′ precipitates advantage the recovery of dislocations and further deformation of the fatigue specimens.     

Surface characterization and influence of anodizing process on fatigue life of Al 7050 alloy

The present study investigates the influence of anodizing process on fatigue life of aluminium alloy 7050-T7451 by performing axial fatigue tests at stress ratio ‘R’ of 0.1. Effects of pre-treatments like degreasing and pickling employed prior to anodizing on fatigue life were studied. The post-exposure surface observations were made by scanning electron microscope (SEM) to characterize the effect of each treatment before fatigue testing. The surface observations have revealed that degreasing did not change the surface topography while pickling solution resulted in the formation of pits at the surface. Energy dispersive spectroscopy (EDS) was used to identify those constituent particles which were responsible for the pits formation. These pits are of primary concern with respect to accelerated fatigue crack initiation and subsequent anodic coating formation. The fatigue test results have shown that pickling process was detrimental in reducing the fatigue life significantly while less decrease has been observed for anodized specimens. Analyses of fracture surfaces of pickled specimens have revealed that the process completely changed the crack initiation mechanisms as compared to non-treated specimens and the crack initiation started at the pits. For most of the anodized specimens, fatigue cracks still initiated at the pits with very few cracks initiated from anodic coating. The decrease in fatigue life for pickled and anodized specimens as compared to bare condition has been attributed to decrease in initiation period and multi-site crack initiations. Multi-site crack initiation has resulted in rougher fractured surfaces for the pickled and anodized specimens as compare to bare specimens tested at same stress levels.