镍基单晶高温合金热机械疲劳本构模型研究

在晶体塑性理论的基础上,提出了一种适用于镍基单晶高温合金热机械疲劳的本构模型,并采用该模型对单晶材料不同晶体取向的热机械疲劳力学响应进行有限元模拟。结果表明,该本构模型可以较好地模拟镍基单晶合金的热机械疲劳行为。对于同相位热机械疲劳,压缩应力幅大于拉伸应力幅,循环平均应力小于零;对于反相位热机械疲劳,拉伸应力幅大于压缩应力幅,循环平均应力大于零。随着循环次数的增加,材料呈现出在高温半周为初始软化,低温半周为初始硬化的特征。晶体取向对于材料的热机械疲劳性能具有显著的影响。     

Low-Cycle Fatigue and Creep-Fatigue Behaviors of a Second-Generation Nickel-Based Single-Crystal Superalloy at 760 ℃

Low-cycle fatigue (LCF) behaviors of a second-generation nickel-based single-crystal superalloys with [001] orientation at 760 °C have been investigated. Different strain amplitudes were introduced to investigate the creep-fatigue effects. The LCF life of none tensile holding (NTH) was higher than that of the 60-s tensile hold (TH) at any strain amplitude. As the strain amplitude was 0.7%, the stacking and cross-slip dislocations appeared together at the γ/γ’ coherent microstructure in both TH and NTH specimens. At the strain amplitude of 0.9%, plenty of the cross-slip dislocations appeared in γ channel and other dislocations were stacking at γ/γ’ interfaces. However, the SFs still appeared in γ’ phase with 60-s TH which caused cyclic softening. As the strain amplitude increased up to 1.2%, the dislocations are piling up at the γ/γ’ interfaces and cutting through the γ’ phase in both TH and NTH tests, which caused cyclic hardening. The influences of strain amplitude and holding time were complicated. Different stress response behaviors occurred in different loading conditions. The surface characteristic and fracture mechanism were observed by scanning electron microscopy. This result is helpful for building the relationship of various blade fatigue failure modes, cyclic stress response and microstructure deformation under different strain amplitudes.     

取向偏离度对镍基单晶高温合金DD10低周疲劳行为的影响

本文研究了特定试验条件下,镍基单晶DD10取向偏离[001]的程度对其低周疲劳行为的影响。结果表明：取向偏离度对DD10合金的低周疲劳行为影响明显。疲劳寿命随取向偏离度的增加而明显降低,在同等偏离度下,晶体取向靠近[001]-[011]的合金低周寿命优于取向靠近[001]-[11]的合金;分析发现：不同取向偏离度合金DD10低周疲劳寿命的差异源于其每循环周次的可累计塑性变形的差异。另一方面,取向偏离度对疲劳总应力幅和塑性应变幅的影响却截然相反,但对疲劳裂纹萌生和扩展方式影响不大。     

基于损伤力学的粉末高温合金FGH96疲劳寿命预测

以粉末高温合金FGH96为研究对象,提出采用损伤力学理论来建立寿命预测模型.对于不同夹杂物特征,粉末高温合金裂纹萌生有不同的表征参量,其数值变化为裂纹萌生的寿命预测提供思路.对粉末高温合金寿命预测的研究现状进行分析,然后利用损伤演变方程建立寿命预测模型;使用有限元软件(ANSYS)分别模拟夹杂物的不同位置、不同尺寸以及...     

Metallurgical control of fatigue crack propagation in superalloys

Low-cycle fatigue life of turbine engine disk alloys is determined by the initiation and propagation of fatigue cracks. Performance improvements can be achieved through the combination of clean melting technology, to reduce the defect size, and a new generation of high-strength superalloys with fatigue cracking resistance. Metallurgical control of fatigue crack propagation in high-strength superalloys becomes feasible only through a clear understanding of the fatigue cracking mechanism, as well as the micro-structure/property relationships. Many metallurgical parameters have been identified to control the fatigue cracking resistance at high temperatures. One of the most effective methods, applicable to all high γ′ content superalloys, is to modify the grain boundary structure by means of a controlled cooling from a supersolvus solutioning. The precipitation reaction occurring on the grain boundaries during cooling generates a serrated structure that exhibits a good stress oxidation resistance for fatigue cracking.     

Tool wear characteristics in machining of nickel-based superalloys

Nickel-based superalloy is widely employed in aircraft engines and the hot end components of various types of gas turbines with its high strength, strong corrosion resistance and excellent thermal fatigue properties and thermal stability. However, nickel-based superalloy is one of the extremely difficult-to-cut materials. During the machining process, the interaction between the tool and the workpiece causes the severe plastic deformation in the local area of workpiece, and the intense friction at the tool–workpiece interface. The resulting cutting heat coupled with the serious work hardening leads to a series of flaws, such as excessive tool wear, frequent tool change, short tool life, low productivity, and large amount of power consumption etc., in which the excessive tool wear has become one of the main bottlenecks that constraints the machinability of nickel-based superalloys and its wide range of applications.In this article, attention is mainly focused on the tool wear characteristics in the machining of nickel-based superalloys, and the state of the art in the fields of failure mechanism, monitoring and prediction, and control of tool wear are reviewed. The survey of existing works has revealed several gaps in the aspects of tool self-organizing process based on the non-equilibrium thermodynamics, tool wear considering the tool nose radius, thermal diffusion layer in coated tools, tool life prediction based on the thermal–mechanical coupling, and industrial application of tool wear online monitoring devices. The review aims at providing an insight into the tool wear characteristics in the machining of nickel-based superalloys and shows the great potential for further investigations and innovation in the field of tool wear.     

Research Progress on Surface Impact Strengthening Mechanisms and Application of Nickel-Based SuperalloysEI北大核心CSCD

为满足不断攀升的两机涡轮动力系统的快速发展,表面冲击强化技术在涡轮转子用高温合金表面强化的应用及相应机制的研究受到了广泛关注。然而,高温合金表面硬化层在高温服役环境下的回复、再结晶行为难以避免,由此引起的表面强韧化、抗疲劳效果的退化,成为制约表面冲击强化技术在先进高温合金关键部件深入应用的瓶颈。本文总结了近年来镍基高温合金表面冲击强化机制及应用研究进展,分析了表面冲击强化对镍基高温合金表面强韧性及抗疲劳的作用规律,探究了高温合金表面冲击硬化层在高温及长期时效过程中的显微组织、微结构演化及其对高温稳定性的作用机理。以期为发展镍基高温合金表面冲击强化、提高两机涡轮转子疲劳抗力提供基础。     

镍基粉末冶金FGH95高温合金的拉伸及低周疲劳性能研究

在420～650℃的温度范围内,研究了PM FGH95高温合金在应变率0.0001～0.01 s-1范围内的拉伸性能及在不同应力比R=-1及R=0下的低周疲劳性能.拉伸试验结果表明:在此温度范围内,应变率对弹性模量、拉伸屈服强度及塑性模量的影响可以忽略不计;并且应变率自0.0001 s-1增大至0.01 s-1时,弹性模量、拉伸屈服强度及塑性模量受温度的影响也不明显.PM FGH95合金受材料微结构的影响很大,材料含有的微缺陷对其力学性能有非常不利的影响.LCF试验结果表明:PM FGH95合金是循环硬化材料.当应力比R=-1时,温度对LCF寿命的影响很小;但在应力比R=0时,LCF寿命受温度的影响很大,且随着温度的增加材料的低周疲劳寿命减小.SEM断口扫描发现,断裂表面有很多的解理面,但没有疲劳条带,且此断裂模式下没有明显的塑性变形,所以FGH95合金的低周疲劳寿命几乎是由裂纹萌生阶段决定的.     

粉末冶金高温合金FGH97的低周疲劳断裂特征

研究了粉末冶金镍基高温合金FGH97在650℃,30—980 MPa,1 Hz实验条件下的低周疲劳断口的宏观及微观特征,裂纹源的类型及其形貌特征,以及裂纹源的位置、缺陷类型、形状和尺寸对低周疲劳寿命的影响.结果表明,在本次实验条件下该合金的低周疲劳寿命均超过了5000 cyc;统计得出,低周疲劳断口裂纹源在表面的试样占23%,在亚表面的占47%,在试样内部的占30%;裂纹源分平台、粉末颗粒、夹杂物3种类型,其中平台类型约占5%,粉末颗粒间断裂占15%,夹杂物占80%.由统计分析和计算得出,不同类型裂纹源对疲劳寿命的影响程度不同:夹杂物最严重,其次为异常粉末颗粒,再次为局部塑性变形.     

复杂应力状态下镍基单晶合金低周疲劳寿命预测模型

镍基单晶合金的低周疲劳（ＬＣＦ）晶体取向相关性和其弹塑性的晶体取向相关性有密切关系,基于所建晶体取向各向异性弹塑性晶体滑移模型,本文推导了复杂应力状态下镍基单晶合金低周疲劳寿命预测模型,结论为ＬＣＦ寿命与修正的滑移应变呈指数关系。在详细分析薄壁圆筒试样受拉－扭载荷下应力应变分布规律的基础上,利用不同拉－扭载茶下的低周疲劳寿命数据,对本文所提模型进行了成功的考核。     

Modelling the optimum grain size on the low cycle fatigue life of a Ni based superalloy in the presence of two possible crack initiation sites

LCF experiments, in situ tensile tests and small fatigue crack growth rate measurements on different heats of Inconel 718 alloy with widely different grain sizes have been performed. A fatigue life prediction model including a transition in the crack initiation mechanism (particle/Stage I) is identified. This model is essentially based on Tanaka and Mura model [J Appl Mech 48 (1981) 97] for Stage I initiation and Tomkins model [Philos Magazine 18 (1981) 1041] for fatigue crack growth rate.     

Low-cycle fatigue lifetime estimation of Ti–6Al–4V welded joints by a continuum damage mechanics model

The low-cycle fatigue(LCF)behavior of directionally solidified nickel-based superalloy Ti–6Al–4V was studied under bare and electron beam welding conditions at room temperature.Results show that:(1)under the same test conditions,all the joints exhibit lower LCF lifetime than Ti–6Al–4V;(2)the failure of welded structures is mainly ascribed to the welding defect.A novel lifetime prediction methodology based on continuum damage mechanics is proposed to predict the lifetime of Ti–6Al–4V and its welded joints.     

Effect of Ultrasonic Shot Peening on Microstructure and Mechanical Properties of High-Nitrogen Austenitic Stainless Steel

The effect of ultrasonic shot peening (USSP) was studied on microstructural modification and mechanical properties such as microhardness, yield strength, tensile strength, and low cycle fatigue (LCF) life of a nitrogen stabilized austenitic stainless steel, at room temperature. There was grain refinement up to nano scale in surface region of the shot peened specimens and the microhardness was increased markedly up to the depth of approximately 100 µm. There was insignificant increase in yield and tensile strength, but drastic reduction in LCF life, particularly at low strain amplitude, from USSP. The nominal increase in yield and tensile strength was due to grain refinement in the surface region and drastic fall in LCF life was due to surface cracking resulting from USSP.     

一种新的低周疲劳损伤模型及实验验证

通过316L钢在420℃环境下应力控制的低周疲劳实验,基于连续损伤力学,提出一种新的低周疲劳损伤模型,采用间接反映循环塑性应变能的应力-位移曲线面积的变化作为损伤变量,实验结果与该模型显示的疲劳损伤演变规律符合较好。     

Experimental Investigation on the LCF Behavior Affected by Manufacturing Defects and Creep Damage of One Selective Laser Melting Nickel-Based Superalloy at 815℃

Uniaxial tensile tests and stress-controlled low-cycle fatigue(LCF) and creep-fatigue interaction(CFI) tests of Inconel 625 alloy manufactured by selective laser melting(SLM) were performed at 815℃ in air environments.The microstructure was characterized by optical microscopy and scanning electron microscopy after testing.The results confirmed that significant embrittlement and large scatter in LCF life are resulted from manufacturing defects.The CFI life is decreased sharply to approximately dozens of cycles with the accumulated creep strain;however,the selected dwell time(i.e.,60 s and 300 s)exhibits low sensitivity to the fracture time and elongation to failure.The embrittlement of SLM Inconel 625 was proposed to be due to the low grain uniformity and precipitation of carbides at the grain boundaries.Due to the quality of the SLM process,the accelerated initiation and propagation of fatigue crack are caused by the present unmelted powder particles,which result in the large dispersion of LCF life.Meanwhile,due to the accumulation of creep damage,cracks in the CFI test are initiated along the grain boundaries and then linked together,contributing to a significant decline in fatigue life.     

GH3044的低周疲劳行为研究

研究了镍基高温合金GH3044在室温和600℃的低周疲劳行为,对循环应力-应变和应变寿命数据进行了分析,给出了GH3044合金在此温度下的疲劳参数.合金的循环应力响应行为在室温下呈现循环硬化而后软化的特征,而在600℃时呈现循环硬化的特征,原因在于循环变形过程中位错之间以及位错与析出相之间的相互作用.Coffin-Ma...     

Recrystallization During Thermo-Mechanical Fatigue of Two High-Generation Ni-Based Single Crystal SuperalloysEI北大核心CSCD

通过SEM和TEM等手段研究了经热机械疲劳变形后的第三代和第四代单晶高温合金的显微组织,了解高温合金在近服役条件下的变形组织,分析单晶高温合金近服役条件下的变形机制。结果表明,第三代和第四代单晶高温合金样品中在不同{111}面上产生了大量的变形孪晶,且在平行的孪晶片层中或者孪晶片层交截周围发现大量再结晶晶粒。再结晶晶粒的界面主要由变形后的孪晶界、小角度晶界以及孪晶相交产生的大角度晶界组成。借助像差校正透射电镜解析了变形后的孪晶界结构以及孪晶诱发动态再结晶的过程,揭示了单晶高温合金热机械疲劳断裂机制。     

Low Cycle Fatigue Evaluation of NiTi SESMA Thin Wires

This paper presents experimental studies on low cycle fatigue (LCF) life of super-elastic shape memory alloy (SESMA) wires. The effect of frequency of the loading and amplitude of the strain on the fatigue life has been studied individually. Various loading frequencies have been considered to study the effect of frequency, by keeping the amplitude constant. From the experimental data, it was found that the LCF life of the SESMA reduces with increase in the frequency. The effect of amplitude on the LCF life of SESMA has also been studied, and it was found that the SESMA cycled at lower net strain has more fatigue life than the one cycled at higher net strain. Further, the plastic strain accumulation is also more in the samples tested at the higher net strain loadings. The modulus of austenite is found to be by and large independent of the frequency and amplitude of the loading. Further, martensitic unloading modulus is same for all the minimum strain amplitudes.     

Thermomechanical analysis of multi-bead pulsed laser powder deposition of a nickel-based superalloy

In laser powder deposition (LPD) repair of nickel-based turbine blades, hot cracking is the most common defect. The cracking has been found to be associated with thermal stress concentration and low-melting constituents on the grain boundaries. For directionally solidified and single-crystal blades, a positive correlation is established between cracking and the “stray grain” formation. Control of the deposit molten pool shape has been proposed as an effective method to limit the stray grains. However, in multi-bead, multi-layer LPD with a pulsed laser, due to a much more complex bead geometry, appearance of stray grains seems to be random. To obtain insight into cracking and stray grain formation during multi-bead LPD process on nickel-based superalloys, a 3-D transient finite element (FE) model has been developed and a thermomechanical analysis is performed. Time-dependent temperature and thermal strain fields have been predicted. The developed thermal model has been applied to predict and analyze the distribution of thermal stress concentration and the tendency of stray grain formation. The reliability and accuracy of the model are verified experimentally by the measured temperature field profile and the observation of microstructure.     

GH4133镍基高温合金激光冲击强化研究

为了研究激光冲击强化技术在高温部件上应用的可行性,研究了GH4133镍基高温合金激光冲击后强化效果的热稳定性。分别采用激光冲击强化、激光冲击强化加保温的方法进行处理,并利用SEM、显微硬度和残余应力的测试方法分析了温度对激光冲击处理后GH4133材料微观组织和力学性能的影响。通过冲击强化后涡轮叶片的高温疲劳试验验证强化效果的热稳定性,并分析其高温下的强化机制。结果表明,激光冲击强化可以在GH4133镍基高温合金表层产生较大残余压应力,细化晶粒;并且在温度作用下,激光冲击GH4133合金形成的细化晶粒在析出相的钉扎作用下具有较好的热稳定性。另一方面残余压应力的应力集中减小,分布均匀。两者的共同作用提高了强化效果的热稳定性,有利于疲劳性能的提高。