高温合金的高温疲劳裂纹扩展门槛值试验

疲劳裂纹扩展门槛值反映材料抗裂纹扩展的能力,是重要的材料性能指标。室温疲劳试验已经成为标准试验方法,但是该方法能否拓展到高温下,并应用于高温合金,尚无试验数据。根据目前的测试技术,试验温度在600℃以下可以使用目测法进行裂纹扩展试验,而在600℃以上,由于试样表面氧化,目测无法观察,所以使用自动测量方法——直流电位法。但由于裂纹扩展门槛值试验时间很长(上百小时),试验难度非常大。该文介绍了粉末高温合金、变形高温合金以及定向凝固高温合金3种高温合金最高到850℃的疲劳裂纹扩展门槛值试验,使用目测法和直流电位法两种方法进行裂纹长度测量。结果表明:直流电位法可以用于高温疲劳裂纹扩展门槛值试验,但试验数据还存在一定的分散性,尚需进一步研究提高裂纹长度测量精度。     

TC4棒材室温疲劳寿命测定及S-N曲线的建立

本文利用升降法确定TC4棒材的疲劳极限,采用成组法确定高应力下疲劳寿命,数据处理后建立了TC4合金棒材疲劳试验S-N曲线。     

TNM-Ti Al合金室温高周疲劳性能研究

采用升降法与成组法对TNM-TiAl合金试样进行了应力比R=-1的室温拉压疲劳和R=0.1的室温拉伸疲劳试验，得到TNM-TiAl合金的P-S-N曲线，并对断口进行了分析。结果表明:TNM-TiAl合金对应力十分敏感，R=-1和R=0.1时的曲线整体呈较为平直的斜线，R=-1时的疲劳极限为414.7 MPa,R=0.1时的疲劳极限为285.6 MPa。R=0.1的S-N曲线远低于R=-1的S-N曲线;R=-1时，应力幅与疲劳寿命的关系满足Basquin方程。疲劳试件宏观断口较为粗糙，静态拉伸宏观断口平整，两者差异较大。拉伸断口整体分为裂纹萌生区与扩展区，其中起裂源均位于试样表面或板状试件的边角棱线处，起裂源区域包括γ相的解理断裂面、片层团的沿层解理面以及β₀相平整的穿晶断裂平面等特征。疲劳断口整体分为裂纹萌生区、扩展区与瞬断区，其中裂纹萌生区分为表面沿层起裂和γ相起裂。TNM-TiAl合金的疲劳断裂为脆性断裂，主要体现在扩展区上大量的片层团穿层断裂、扭折撕裂、γ相解理断裂和β₀相穿晶断裂。同寿命量级下，R=-1的断口与R=0.1的断口断裂类型...     

腐蚀环境下2E12航空铝合金疲劳裂纹扩展行为研究

采用SEM,TEM及疲劳性能测试等分析测试手段,系统地研究了2E12铝合金在室温空气、潮湿气体及盐雾环境下的疲劳裂纹扩展速率。结果发现2E12合金具有良好的疲劳性能,疲劳裂纹扩展速率优于国外的2524铝合金。利用修正的Paris公式分析腐蚀介质对合金的疲劳裂纹扩展性能影响轻重程度依次为;室温空气〈潮湿气体〈盐雾环境。不同环境条件下合金的疲劳裂纹形貌均表现微观裂纹扩展的晶界小平面、宏观裂纹扩展的疲劳条纹以及剪切撕裂的微观结构。在腐蚀环境下,宏观裂纹扩展表现出更多的脆性裂纹扩展特征,氢脆导致塑性区脆化及腐蚀诱导的阳极溶解是导致裂纹扩展性能降低的原因。     

返回料添加比例对K44合金热疲劳性能的影响

研究了返回料添加比例对新型抗热腐蚀高温合金K44热疲劳性能的影响.结果表明:新料和返回料合金试样V型缺口尖端主裂纹扩展长度与热循环次数之间遵循L=bNa规律.新料合金热疲劳裂纹萌生和扩展速率最低,随着合金中返回料比例的增大,热疲劳裂纹萌生速率和扩展速率也增大.热疲劳裂纹萌生于V型缺口尖端附近区域,沿枝晶间、晶界和开裂的碳化物扩展,主裂纹扩展以裂纹尖端连续开裂的形式进行.返回料合金由于氮含量增加导致共晶和夹杂物增多,碳化物聚集块化,加速了热疲劳裂纹的萌生与扩展.合金经热疲劳实验后,裂纹两侧产生氧化带和γ'相贫化带.     

直流电位法检测高温合金的疲劳裂纹扩展性能

介绍了在高温下使用直流电位法测定疲劳裂纹长度的基本原理、试验方法及所需配置的仪器,并对有关影响因素进行了分析;并利用Johnson的分析型关系式,研究了一种适用于自动测定材料高温疲劳裂纹长度的直流电位法,该方法已成功应用于高温合金疲劳裂纹扩展试验,其测得的疲劳裂纹扩展速率da/dN-ΔK数据与长焦聚显微镜法测得的数据完全一致。     

沉积颗粒对7N01-T6铝合金疲劳裂纹扩展行为的影响

利用自主设计的实验方法,结合疲劳裂纹扩展速率测试以及断口微观形貌观察研究了R=0.1和R=0.5两种应力比下,石墨和氧化铝沉积颗粒对7N01-T6铝合金疲劳裂纹扩展行为的影响。结果表明:在两种应力比条件下,裂纹扩展Ⅰ、Ⅱ阶段中,合金在石墨颗粒环境下的疲劳裂纹扩展速率均最快。当R=0.5时,在裂纹扩展Ⅰ、Ⅱ阶段,合金在氧化铝颗粒环境下的疲劳裂纹扩展速率最慢。当R=0.1时,在应力强度因子ΔK15 MPa·m~(1/2)阶段,合金在氧化铝颗粒环境下的疲劳裂纹扩展速率最慢,在ΔK=15～30 MPa·m~(1/2)阶段,合金在氧化铝颗粒环境和大气环境下的疲劳裂纹扩展速率相当。石墨颗粒环境下合金疲劳裂纹扩展速率的增加是由于石墨颗粒的润滑作用降低了疲劳卸载过程中的裂纹闭合效应。氧化铝颗粒环境下合金疲劳裂纹扩展速率的降低是由于氧化铝颗粒在断面的沉积增强了疲劳卸载过程中的裂纹闭合效应。     

GH33A合金在蠕变-疲劳交互作用下的裂纹扩展特性

通过对 GH33A 合金在蠕变与疲劳复合加载条件下的系列试验,发现拉伸保时使蠕变与疲劳发生了交互作用,加快了疲劳裂纹扩展速率,加速裂纹早期进入失稳扩展,大大降低了疲劳寿命。GH33A 合金具有良好的抗蠕变裂纹扩展能力,但疲劳裂纹扩展阻力较低。由此讨论了拉伸保时对裂纹扩展的影响,并对在蠕变-疲劳交互作用下的裂纹扩展模型作了探讨。     

镍基单晶高温合金热机疲劳断裂特征

为了进一步提高镍基单晶高温合金的热机疲劳性能,通过微观结构解析研究了合金热机疲劳断裂特征.通过金相和扫描电子显微镜研究了热机疲劳断裂的断口特征和微观结构.研究表明:裂纹起源于形变孪晶与试样外表面的交截处,过程中的氧化有助于裂纹的长大;裂纹尖端的应力场诱发出大量形变孪晶,而形变孪晶的存在为裂纹进一步沿着孪晶界扩展提供了便利条件;镍基单晶高温合金的疲劳断裂主要是由于形变孪晶的形成以及裂纹沿孪晶界的扩展造成的.形变孪晶与高温合金疲劳断裂密切相关.     

高温合金热疲劳实验机的研制及实验研究

介绍了用于测试高温合金热疲劳性能的热疲劳实验机。该实验机主要用于定性地测量合金在某一条件下循环规定的次数后热疲劳裂纹扩展的长度,或热疲劳裂纹在扩展一定长度下循环的次数。该实验机的优点是:完全自动化、运行灵活、定位准确。通过对DZ951定向凝固镍基高温合金进行实验研究,表明该热疲劳实验机具有较好的稳定性和可靠性。DZ951合金主要通过铝和铬元素氧化,产生氧化孔洞。热疲劳裂纹通过孔洞相互连接萌生和扩展。合金具有两条热疲劳裂纹,且扩展具有一定方向,与枝晶生长方向成45°扩展。     

The effect of cerium on high-cycle fatigue properties of die-cast magnesium alloy

The effect of cerium (Ce) on high‐cycle fatigue behaviour of die‐cast magnesium alloy AZ91D was investigated. Mechanical fatigue tests were conducted at the stress ratio, R= 0.1 on specimens of AZ91D alloys with different Ce additions. The microstructure and fatigue fracture surfaces of specimens were examined using a scanning electron microscope (SEM) to reveal the micromechanisms of fatigue crack initiation and propagation. The results show that the grain size of AZ91D is refined, and the amount of porosity decreases and evenly distributes with the addition of Ce. The fatigue strength of AZ91D evaluated by the up‐and‐down load method increases from 96.7 MPa to 116.3 MPa (1% Ce) and 105.5 MPa (2% Ce), respectively. The fatigue cracking of AZ91D alloy initiates at porosities and inclusions of the alloy's interior, and propagates along the grain boundaries. The fatigue fracture surface of test specimens shows the mixed fracture characteristics of quasi‐cleavage and dimple.     

Biodegradable behaviour and fatigue life of ZEK100 magnesium alloy in simulated physiological environment

The fatigue life of ZEK100 magnesium alloy in the phosphate buffered solution for various immersion intervals was investigated by experiments and theoretical predictions. The biodegradable behaviours of ZEK100 magnesium alloy were also studied. Microstructure observation showed that the corrosion behaviours were characterized by pitting corrosion. The corrosion rate decreased a lot in the initial 7 d and then almost stayed unchanged. After 28 d immersion, the elastic modulus almost kept stable, while the yield strength and the ultimate strength decreased a lot, which indicated that corrosion had important effects on the tensile mechanical properties. It showed that the fatigue life of the samples under the same stress conditions decreased with increasing immersion time under the asymmetric stress‐controlled cyclic loading. Considering the effect of corrosion on the material failure, a modified fatigue life model was proposed for magnesium alloy under corrosion.     

Effects of humidity and temperature on the fatigue behaviour of an extruded AZ61 magnesium alloy

Load‐controlled fatigue tests were performed at 20 and 50 °C using two relative humidity levels of 55 and 80% to characterize the influence of humidity and temperature on the fatigue behaviour of an extruded AZ61 magnesium alloy. Fatigue tests were also conducted at 150 °C. No significant variation in fatigue properties was noticed with respect to temperature over the range from 20 to 50 °C for both the humidity levels. Fatigue limits in the range 140–150 MPa were observed for relative humidity of 55%. Fatigue strength decreased significantly with increase in temperature to 150 °C. Further, a significant reduction in fatigue strength with a fatigue limit of ～110 MPa was observed with increase in relative humidity to 80% at 20 and 50 °C. The crack initiation and propagation remained transgranular under all test conditions. The fatigue fracture at low stress amplitudes and high relative humidity of 80% results from the formation of corrosion pits at the surface and their growth to a critical size for fatigue‐crack initiation and propagation. The observed reduction in fatigue strength at high humidity is ascribed to the effects associated with fatigue–environment interaction.     

残余应力和粗糙度对叶片振动疲劳性能的影响

为了研究叶片表面完整性对其振动疲劳性能的影响,本文模拟分析了某型高温合金叶片在振动疲劳实验过程中的动力学应力响应,获得叶片共振时应力幅值随时间的变化规律,分析了残余应力和粗糙度对叶片振动疲劳寿命和疲劳极限的影响规律.结果表明:叶片共振过程中的应力响应幅值先增大后减小呈周期性变化,属于"拍"现象,满足关系σ=1 046sin(242.83t)sin(5 828t);叶片的振动疲劳极限和疲劳寿命均随残余应力和粗糙度的增大而减小,振动疲劳极限和残余应力之间的关系满足σfat=510.9-0.31-70.93σrest;而疲劳极限和粗糙度之间的关系则满足σfat=9.67R2roughness-70.93Rroughness+713.23.     

Effects of extrusion and equal channel angular pressing on the microstructure,tensile and fatigue behaviour of the wrought magnesium alloy AZ80

The microstructure, mechanical properties, fatigue life and fatigue crack propagation rate of Mg‐8Al‐0.5Zn‐0.3Mn (AZ80) magnesium alloy were investigated after extrusion and equal channel angular pressing (ECAP). The highest ultimate and yield strengths and a large enhancement in the fatigue lifetime were obtained after two passes of ECAP. These were decreased with further pressing, although the grain size became finer. There was a correlation between the fatigue and ultimate strengths of AZ80 alloy. The transition from twinning to dislocation slip has also occurred at an average grain size of 7.9 μm. Simultaneous influences of the grain size and the yield strength caused an almost the same threshold of the stress intensity ratio for different process conditions. Moreover, the enhanced ductility of the ECAPed alloy resulted in an increase in the crack growth resistance because of its better ability to accommodate plastic strains during cycling.     

Low cycle fatigue properties of friction stir welded joints of a semi-solid processed AZ91D magnesium alloy

A semi-solid processed (thixomolded) Mg–9Al–1Zn magnesium alloy (AZ91D) was subjected to friction stir welding (FSW), aiming at evaluating the weldability and fatigue property of the FSW joint. Microstructure analysis showed that a recystallized fine-grained microstructure was generated in the nugget zone (NZ) after FSW. The yield strength, ultimate tensile strength, and elongation of the FSW joint were obtained to be 192 MPa, 245 MPa, and 7.6%, respectively. Low-cycle fatigue tests showed that the FSW joint had a fatigue life fairly close to that of the BM, which could be well described by the Basquin and Coffin-Manson equations. Unlike the extruded magnesium alloys, the hysteresis loops of FSW joint of the thixomolded AZ91D alloy were basically symmetrical, while the non-linear or pseudoelastic behavior was still present. The FSW joint was observed to fail in the BM section rather than in the NZ. Fatigue crack initiated basically from the pores at or near the specimen surface, and crack propagation was mainly characterized by fatigue striations along with the presence of secondary cracks.     

Fatigue lifetime of AZ91 magnesium alloy subjected to cyclic thermal and mechanical loadings

In the present paper, thermo-mechanical fatigue (TMF) and low cycle fatigue (LCF) or isothermal fatigue (IF) lifetimes of a cast magnesium alloy (the AZ91 alloy) were studied. In addition to a heat treatment process (T6), several rare elements were added to the alloy to improve the material strength in the first step. Then, the cyclic behavior of the AZ91 was investigated. For this objective, strain-controlled tension–compression fatigue tests were carried out. The temperature varied between 50 and 200 °C in the out-of-phase (OP) TMF tests. The constraint factor which was defined as the ratio of the mechanical strain to the thermal strain, was set to 75%, 100% and 125%. For LCF tests, mechanical strain amplitudes of 0.20%, 0.25% and 0.30% were considered at constant temperatures of 25 and 200 °C. Experimental fatigue results showed that the cyclic hardening behavior occurred at the room temperature in the AZ91 alloy. At higher temperatures, this alloy had a brittle fracture. But also, it was not significantly clear that the cyclic hardening or the cyclic softening behavior would be occurred in the material. Then, the high temperature LCF lifetime was more than that at the room temperature. The OP-TMF lifetime was the least value in comparison to that of LCF tests. At the end of this article, two energy-based models were applied to predict the fatigue lifetime of this magnesium alloy.     

High cycle fatigue behavior of as-extruded ZK60 magnesium alloy

Tensile and high cycle fatigue properties of hot extruded ZK60 magnesium alloy have been investigated, in comparison to that of hot-extruded plus T5 heat-treated ZK60 magnesium alloy which was named as ZK60-T5. High cycle fatigue tests were carried out at a stress rate (R) of −1 and a frequency of 100 Hz using hour-glass-shaped round specimens with a gage diameter of 5.8 mm. The results show that tensile strength greatly improved and elongation is also slightly enhanced after T5 heat treatment, and the fatigue strength (at 10⁷ cycles) of ZK60 magnesium alloy increases from 140 to 150 MPa after T5 heat treatment, i.e., the improvement of 7% in fatigue strength has been achieved. Results of microstructure observation suggest that improvement of mechanical properties of ZK60 magnesium alloy is due to precipitation strengthening phase and texture strengthening by T5 heat treatment. In addition, fatigue crack initiations of ZK60 and ZK60-T5 magnesium alloys were observed to occur from the specimen surface and crack propagation was characterized by striation-like features coupled with secondary cracks.     

High‐cycle fatigue and fracture behaviours of Cu‐Be alloy with a wide strength range

The high‐cycle fatigue and fracture behaviours of Cu‐Be alloy with tensile strength ranging from 500 to 1300 MPa acquired by different treatments were studied. Fatigue crack initiation, fracture surface morphologies, S‐N curves and fatigue strength show obvious differences due to the change of microstructure. At relatively low‐strength level, some fatigue cracks originated from defects; while at high‐strength level, all the fatigue cracks initiated from cleavage facets. It was found that the fatigue ratio increases linearly and fatigue strength changes quadratically with increasing tensile strength, only considering one strengthening mechanism. Finally, the fatigue strengths of various Cu‐Be alloys were summarized.     

Fatigue strength of die-cast magnesium components

There is a commercial interest to extend the use of die‐cast magnesium from low stress applications, such as interior components of motor vehicles, to components carrying significant loads. In high stress applications it is the strength and fatigue properties of die‐cast magnesium alloys that limit their use. Manufacturing defects, such as microscopic shrink holes, pores and oxide inclusions, impair the strength of components under fatigue loads, but are unavoidable with present‐day magnesium casting technology. In the present study, the effects of different rib thicknesses and notch radii on the fatigue strength were investigated on realistic cast specimens with unmachined surfaces. The tests were performed on ribbed specimens of magnesium alloys AZ91 and AM60 under pulsating bending stress with a constant amplitude at a stress ratio R = 0. As indicated by the results of the investigation, the real material must be considered together with its defects in designing die‐cast magnesium components. For this purpose, the influence of defects must therefore be given a higher priority than the local stresses at the surface.