Steady State Creep Rate Equation of Inconel 718 Superalloy

The steady state creep rate equdtion ofa nickel base superalloy Inconel 718, strengthenedby coherent ordered disc-shaped bct γ11 phaseand coherent spherical fcc γ1 phase precipitates,has been established in the stress and temperatureranges of 620-840 MN m-2 and 853-943K, respecti-vely. Constant stress tensile creep tests wereused to medsure the values of steady state creeprate, εs, and the consecutive stress reductionmethod was used to measure the back stress duringcreep deformation. The values of effective stressexponent, ne, were detemined from the slopesof the lgεs vs. lg（σa-σ0）/G plots. The effectof grain size, d, on steady state creep rdtehas been also studied in this investigation,and the grain size sensitive exponents m weredetemined from the slopes of lgεs vs. lg（b/d）plots. The creep rate equations of Inconel 718,in the above stress and temperature ranges,have been proposed to beεs=1.6×10-5（D1Gb/KT） （b/d ）0.19[（σa-σ0）/G]1.35in diffusional creep region, andεs =75（D1Gb/KT） （b/d）-0.42[（σa-σ0）/G]5.5in dislocation power law creep region.

Creep rate sensitivities of materials by a depth-sensing indentation technique

The strain rate sensitivity to creep of single crystal Cu(110), metal tantalum, and 128oY-X LiNbO3 piezoelectric single crystal were measured at room temperature by MTS Nanoindenter XP. Among the three kinds of materials studied, Cu showed the highest degree of resistance to creep-induced deformation, which is followed by Ta, while the LiNbO3 single crystal deformed more readily than the others. The values of the steady-state strain rate sensitivities determined by the indentation methods are in the range of 0.002-0.006, 0.02-0.06 and 0.02-0.03 for Cu, Ta, and LiNbO3, respectively. The mechanisms for the indentation-induced creeping behavior and the factors that influenced the creeping are discussed.

Effect of cooling rate during quenching on the microstructure and creep property of nickel-based superalloy FGH96

The effect of cooling rate during quenching on the microstructure and creep property of nickel-based superalloy FGH96 was investigated. Three groups of samples were quenched continuously with three fixed cooling rates, respectively, then subjected to a creep test under a constant load of 690 MPa at 700℃. Clear differences in size of secondary γ′ precipitates, creep properties and substructure of creep-tested samples were observed. The quantitative relationship among cooling rate, the size of secondary γ′ precipitates, and steady creep rate was constructed. It was found that with increasing cooling rate, the size of secondary γ′ precipitates decreases gradually, showing that the relationship between the size of secondary γ′ precipitates and the cooling rate obeys a power law, with an exponent of about –0.6, and the creep rate of steady state follows a good parabola relationship with cooling γ′ precipitate size. For 235℃/min, FGH96 alloy exhibited very small steady creep rate. The density of dislocation was low, and the isolated stacking fault was the dominant deformation mechanism. With decreasing cooling rates, the density of dislocation increased remarkably, and deformation microtwinning was the dominant deformation process. Detailed mechanisms for different cooling rate were discussed.

Uniqueness of rate-dependency, creep and stress relaxation behaviors for soft clays

This work focuses on the uniqueness of rate-dependency, creep and stress relaxation behaviors for soft clays under one-dimensional condition. An elasto-viscoplastic model is briefly introduced based on the rate-dependency of preconsolidation pressure. By comparing the rate-dependency formulation with the creep based formulation, the relationship between rate-dependency and creep behaviors is firstly described. The rate-dependency based formulation is then extended to derive an analytical solution for the stress relaxation behavior with defining a stress relaxation coefficient. Based on this, the relationship between the rate-dependency coefficient and the stress relaxation coefficient is derived. Therefore, the uniqueness between behaviors of rate-dependency, creep and stress relaxation with their key parameters is obtained. The uniqueness is finally validated by comparing the simulated rate-dependency of preconsolidation pressure, the estimated values of secondary compression coefficient and simulations of stress relaxation tests with test results on both reconstituted Illite and Berthierville clay.

Correct Interpretation of Creep Rates: A Case Study of Cu

Traditionally the deformation resistance in creep is characterized by the minimum creep rate εminand its sensitivity to stress(stress exponent n) and temperature(activation energy Q). Various values of constant n have been reported in the literature and interpreted in terms of specific mechanisms. The present case study of coarse-grained Cu at 573 K yields a stress exponent n = 9 for εminin tension and a relatively low activation energy. The evolution of the deformation resistance with strain at constant tensile creep load and comparison with creep in compression without fracture indicates that the tensile εminresult from transition from uniform deformation to strain localization during fracture. This is confirmed by the results of creep in compression where fracture is suppressed. Both the tensile εminand the compressive creep rate at strains around 0.3 can be described using existing equations for quasi-stationary deformation containing the subgrain boundary misorientation θ as structure parameter. While in the latter case constant θ leads to monotonic increase of n with stress, the tensile nine-power-law results from variable θ, and has no simple meaning. The result of this case study means that uncritical interpretation of minimum tensile creep rates as stationary ones bears a high risk of systematic errors in the determination of creep parameters and identification of creep mechanisms.

Instantaneous Creep in Face-centered Cubic Metals at Ultra- low Strain Rates by a High-resolution Strain Measurement

Instantaneous creep in face-centered cubic metals, 5N Al(99.999%), 2N Al(99%) and 4N Cu(99.99%) with different grain sizes, was firstly investigated by sudden stress-change experiments at ultra- low strain rates 10 10s 1and temperature T < 0.32 T m. The experimental results indicate that the observed instantaneous creep is strongly dependent on grain size, the concentration of impurity, and stacking fault energy. Creep in high-purity aluminum, 5N Al, with a very large grain size, d g > 1600 m, shows non-viscous behavior, and is controlled by the recovery of dislocations in the boundary of dislocation cells. On the other hand, for 5N Al with a small grain size, d g =30 m, and low-purity aluminum, 2N Al, with d g = 25 m, creep shows viscous behavior and may be related to 'low temperature grain boundary sliding'. For high-purity copper, 4N Cu, with d g = 40 m and lower stacking fault energy, creep shows a non-viscous behavior, and is controlled by the recovery process of dislocations. For all of the samples, creep shows anelastic behavior.

协同Orowan绕过和攀移机制的镍基高温合金蠕变新模型

提出一种用于准确便捷预测析出相增强镍基高温合金稳态蠕变速率的蠕变新模型。基于析出相的空间分布,采用概率依赖的方法将位错-析出相交互作用中Orowan绕过和位错攀移机制的协同模式耦合到新的蠕变模型中。结果表明,新模型预测的稳态蠕变速率与实验数据吻合很好。此外,当前模型不再依赖于可调参数;同时,确定了阈值应力与温度的定量关系。这项工作为蠕变变形过程中位错-析出相交互作用提供新思路,并为设计具有更优异蠕变性能的析出强化合金提供有效模型。

宽幅温度和应力范围下蠕墨铸铁的蠕变试验及本构模型对比研究

随着发动机功率密度的增加,气缸盖在运行过程中所承受的温度和应力也增加,因此蠕变损伤预测需要考虑准确的蠕变本构模型和参数。针对蠕墨铸铁(Compacted Graphite cast Iron,CGI)气缸盖大范围的温度和应力工作条件,开展温度450~550℃、试验应力100~300 MPa条件下的CGI蠕变试验,进行表征宽幅温度和应力下CGI最小蠕变速率的蠕变本构模型对比研究。研究表明:与较小的温度和应力范围相比,在宽幅条件下CGI更容易发生蠕变损伤,且温度相比应力更能促使CGI发生蠕变变形;分别基于Norton⁃Bailey幂律模型、Garofalo双曲正弦模型和变形机制的真应力(Deformation Mechanism⁃based True Stress,DMTS)模型,结合多目标优化方法,构建了CGI蠕变本构模型。其中,基于DMTS模型的CGI蠕变模型有73%的预测值在两倍误差范围内,吻合效果在三种模型中为最佳。

温度及预紧载荷对氧化石墨烯无石棉垫片蠕变松弛和泄漏率的影响

垫片在高温长期使用过程中发生蠕变松弛导致垫片应力下降,从而影响密封效果。因此,采用理论模型预测垫片的蠕变松弛与确定垫片在高温环境长期使用过程中泄漏率的变化规律非常重要。通过实验测试氧化石墨烯无石棉垫片在不同温度和不同预紧载荷下的垫片应力值,并与根据Burgers模型计算出的垫片应力值进行对比。然后,通过建立泄漏率与使用时间的关系,提出氧化石墨烯无石棉垫片在不同温度和不同预紧载荷下长期使用时的泄漏率计算方法,并计算出该垫片随时间变化的泄漏率变化规律。结果表明:垫片应力的松弛量随着预紧载荷的增加而增加;随着使用时间的增加,垫片的应力松弛变得更加平缓;当预紧载荷相同的情况下温度越高垫片应力下降越明显;Burgers模型对垫片应力的预测值与实验值吻合度很好,最大误差为4.4728%;当温度一定时,氧化石墨烯无石棉垫片计算的泄漏率随着预紧载荷的增加而降低;当预紧载荷相同时,垫片的泄漏率随着温度的增加而增加,且泄漏率均小于密封等级T2的泄漏率2×10^(-3) mg·s^(-1)·mm^(-1)。研究结果为氧化石墨烯无石棉垫片在高温环境长期使用过程密封性能应用提供依据。

电缆用Al-Fe系铝合金导体材料的恒应力高温压缩蠕变行为

对电缆用Al-Fe系铝合金导体材料进行恒应力(63.6,72.6,78.3,84.7 MPa)高温(90℃)压缩蠕变试验,研究了压缩应力σ对稳态蠕变速率ε·的影响,建立了压缩蠕变本构方程,分析了压缩蠕变机制。结果表明:铝合金试样的稳态蠕变速率随着压缩应力的增加成倍增大;90℃下铝合金试样的蠕变本构方程为ε·=σ^(9.27)e^(-52.2),拟合曲线和试验结果的相关系数达0.9976;铝合金蠕变过程中存在门槛应力,其值为34.2 MPa,引入门槛应力对蠕变方程进行修正,修正方程计算所得稳态蠕变速率与试验值的相对误差为8%,证明模型准确,得到的真应力指数为5,说明电缆用Al-Fe系铝合金导体材料在90℃下的压缩蠕变行为是由位错攀移机制主导的第二相粒子强化类蠕变。

注浆黏结锚杆荷载试验判稳方法的综合研究

对注浆黏结锚杆各种荷载试验的位移稳定判定方法研究表明:(1)锚固体与地层界面蠕变遵循蠕变机理,判断锚头位移稳定本质上就是判定界面蠕变是否稳定;(2)荷载较小时界面发生稳定蠕变,超过临界荷载后发生蠕变破坏,完整破坏过程可分为初始、持续及加速3个阶段,判稳不能采用初始阶段数据、应主要依据持续阶段的蠕变特性;(3)蠕变破坏形式约2/3锚杆为突变型,1/3为缓变型,对应着3类蠕变曲线形态;(4)用于判稳的位移阈值不应小于0.1 mm,单元时长不应短于10min,位移增量不能直接用于判稳;(5)判稳应定性判断蠕变速率总体上在减速,应确定定量指标使速率维持在较低水平从而使位移收敛;(6)主要应依据具有位移与时间双重属性的参数判稳,单元时长位移增量的判稳准确度较高,而蠕变率2.0 mm任何情况下都适用;(7)以蠕变率2.0 mm为内在原则的推荐判稳方法,适用于各种应用场合及各种类型的注浆黏结锚杆,适用于各种荷载试验及快速法等试验加卸载方法。

基于多目标粒子群算法的A356铝合金蠕变本构模型的构建

在150,250,300℃温度和20~150 MPa应力下,对A356铝合金进行单向拉伸蠕变试验,研究了其蠕变特性;采用多目标粒子群算法得到Norton-Bailey幂律模型、Garofalo双曲正弦模型和基于变形机制的真应力蠕变模型的参数,并采用这3种模型对A356铝合金的最小蠕变速率-应力曲线进行预测,分析3种模型的预测精度。结果表明:A356铝合金的最小蠕变速率与试验温度和应力成正相关,但温度对最小蠕变速率的影响程度大于应力;基于变形机制的真应力蠕变模型具有最高的预测精度,对最小蠕变速率的预测结果中90%数据点均位于2倍误差带内。结合多目标粒子群算法得到的基于变形机制的真应力蠕变模型可以用来准确模拟宽幅温度和应力范围下A356铝合金的蠕变特性。

X12转子钢蠕变-疲劳裂纹扩展行为的研究

在600℃条件下进行蠕变疲劳裂纹扩展速率实验,施加不同的载荷和保载时间,研究力值和保载时间对蠕变疲劳裂纹扩展速率的影响,并通过金相检测观察裂纹扩展路径。结果表明:在低应力条件下,蠕变疲劳裂纹扩展速率随着保载时间缩短而增大;在高应力条件下,蠕变疲劳裂纹扩展速率随保载时间延长而减小;相对含有疲劳成分裂纹而言,蠕变裂纹的扩展路径显得更加圆滑。

Creep resistance of as-cast Mg-5Al-5Ca-2Sn alloy

In the present study, creep properties of as-cast Mg-5Al-5Ca-2Sn(AXT552) alloy were investigated by means of a GWT304 creep testing machine at temperatures of 175 °C and 200 °C in the stress range of 35-90 MPa. Results show that creep rates increase with applied stress at an identical temperature. Creep strain at 100 hours is 0.0518% and 0.083% at creep conditions of 175°C/75 MPa and 200°C/60 MPa, respectively, which is comparable to MRI230 D and much lower than most of AX series alloys. By the observation and analysis for samples before and after creep tests using a Shimadzu XRD-7000 type X-ray diffractometer(XRD) and a Hitachi S-3400 N type scanning electron microscope(SEM), it was found that Al_2Ca(C15) phase precipitated out of C36 phase or matrix. The cavity formation and connection at the interface of soft matrix and hard intermetallics caused the propagation of cracking along the eutectic phase during creep process and dislocation accommodated grain/phase boundary sliding is expected to be the dominant creep mechanism.

INFLUENCE OF GRAIN SHAPE AND CARBIDE ON CREEP CRACK GROWTH IN HK40

The influence of two different grain shapes and carbides in the HK40 alloy,the material foruse of furnace tube,on the rate of creep crack growth has been investigated.The resistance tocreep crack growth of the material with columnar grains is inferior than that with equiaxialgrains when the load line is perpendicular to the columnar grain axis.The influence of secon-dary carbide on the rate of creep crack propagation depends upon the Brain shape.

THE CREEP AND FRACTURE BEHAVIOR OF TWO NICKEL-BASED SUPERALLOYS

The creep and fracture behavior of the cast K417 and forged GH4049 nickel-based superalloys were investigated in the temperature range of 700-900℃ C. Within the ranges of stress and temperature studied, the steady state creep rates exhibited a power law relationship with the applied stress and temperature. The time to rupture is inversely proportional to the steady state creep rate. Under all testing conditions, the creep fracture process was mainly controled by crack initiation and growth of the intergranular oxidation. Casting porosities, pores and carbides were also prefecentral locations of creep crack initiation in the cast K417 alloy. In addition, the intergranular fracture feature in the forged GH4049 alloy was apparently associated with the formation and coalescence of the cavitations on the grain boundaries.

Creep Function Parameter Analysis for Optimum Design with Calcium Carbonate Nanofiller – Polypropylene Composite

This paper is about the use of power law model to fit experimental creep data of PP reinforced with calcium carbonate nanofiller at 10% optimum volume fraction with a view to characterizing the new material (PPC2) parametrically. The creep parameters were evaluated for neat (PPC0) and reinforced PP (PPC2) to establish the influence of reinforcement on the creep variables like creep rate and creep activation energy. The coefficient parameter A estimated within the stress level range 13.08MPa-22.88MPa has the range 0.0165-0.0651 while the exponent parameter n has 0.299-0.370. The creep stress coefficient K and exponent m has the respective values 161.495 and 0.3288 for PPC0 and 1881.4965 and 0.5448 for PPC2. The value of the parameter p similar to Larson-Miller has the value 4014.1871. Two creep function models used found that PPC0 has higher activation energy with value 9.3642E-20 J/mol for the stress 13.08MPa and PPC2 has values for the stresses 13.08MPa, 19.61MPa and 22.88MPa as 5.55998E- 20J/mol,5.4573E-20J/mol and 4.845E-20J/mol respectively. Of the two master curves produced, that following Larson-Miller parameter is recommended as the relationship between lnσ and parameter f(σ) is relatively linear and will give better results than the curve assumed to follow Sherby-Dorn that will give average result.

CREEP CRACK GROWTH BEHAVIOR OF ALLOY 718 DURING HIGH TEMPERATURE EXPOSURE

Alloy 718 is a precipitation strengthened nickel-based superalloy based on the precipitation of γ″-Ni3Nb (DO22 structure) and γ′-Ni3(Al,Ti) (L12 structure) phases. Creep crack growth rate (CCGR) was investigated after high temperature exposure at 593, 650 and 677℃ for 2000h in Alloy 718. In addition to the coalescence of γ′/ γ″ and the amount increasing of δ phase, the existence of a bcc chromium enriched α-Cr phase was observed by SEM, and the weight fraction of α-Cr and other phases were determined by chemical phase analysis methods. The CCGR behavior and regulation have been analyzed by means of strength and structure analysis approaches. The experimental results show higher the exposure temperature and longer the exposure time, lower the CCGR. This is probably attributed to the interaction of material softening and brittling due to complex structure changes during high temperature exposure. Therefore, despite α-Cr phase formation and amount enhancement were run in this test range. It seems to us a small amount of α-Cr will be not harmful for creep crack propagation resistance, which is critical for disk application in aircraft and land-based gas turbine.

EFFECT OF Mg AND Zr ADDITION ON CREEP CRACK GROWTH BEHAVIOUR OF A Ni-BASE SUPERALLOY

The creep crack growth（CCG）and the time to carck initiation and rupture of specimen(trcand tr)were measured by means of electrical potential method on single edge notchedspecimens at 700℃.The field near the crack tip under steady-state creep was represented byenergy rate integral（C*）,and the CCG rate as a function of C* has been obtained.Theagreement between the predicted and observed trcvalues is quite good.It was found that theaddition of small amount of Mg and Zr in the alloys causes trcand tr of the specimens to in-crease significantly.

Fe-30Ni-15Cr-0.6Nb-3Al-2Mo-3.5Cu奥氏体耐热钢高温蠕变行为

研究了一种含铝奥氏体耐热钢在温度为750℃,蠕变应力为120、170、210和250 MPa下的高温蠕变行为,运用扫描电子显微镜、电子背散射衍射技术研究了蠕变试样的微观结构演变。结果表明,蠕变后第二相主要是B2-NiAl相与σ-FeCr相,没有发现Laves-Fe_(2)Nb相的析出;随着蠕变应力从250 MPa下降到120 MPa,最小蠕变速率从5.72×10^(-6)s^(-1)下降到3.11×10^(-7) s^(-1)。在蠕变过程中,B2-NiAl相初期在晶界析出,随后逐渐在晶内析出并有轻微长大现象,B2-NiAl相的析出有助于提高材料的抗蠕变性能。通过包含双曲正弦函数的方程计算得到应力指数n=3.01,确定相关的蠕变变形机制是位错滑移。根据蠕变断口微观形貌与蠕变损伤容限因子表明,确定蠕变失效是由析出相粗化导致的显微组织退化引起的。