一种含4.5%Re/3.0%Ru的单晶镍基合金的高温蠕变行为

通过对含4.5%Re/3.0%Ru单晶镍基合金进行高温蠕变性能测试,并采用扫描电镜(SEM)、透射电镜(TEM)对不同蠕变期间的试样进行组织形貌观察,研究了该合金的高温蠕变行为。结果表明,本实验所选用的单晶合金在高温蠕变期间具有良好的蠕变抗力,在1040℃/160MPa的蠕变寿命达到725h。高温蠕变初期,合金中γ′相沿垂直于应力轴方向转变成筏状结构,其稳态蠕变期间的变形机制是位错在基体中滑移和攀移越过筏状γ′相。高温蠕变后期,合金的变形机制是位错在基体中滑移和剪切筏状γ′相。位错的交替滑移使筏形γ′相扭曲,并在γ/γ′两相界面发生裂纹的萌生与扩展直至断裂,是合金在高温蠕变后期的断裂机制。     

一种高W单晶镍基合金的组织演化与蠕变行为

通过蠕变曲线测定及组织形貌观察,研究了一种9％W单晶镍基合金的组织演化与蠕变行为.结果表明,当在1040℃施加应力大于160MPa时,合金表现出明显的施加应力敏感性,在试验的温度范围内,合金具有较低的应变速率和较长的蠕变寿命,测定出合金的表观蠕变激活能为465kJ/mol.在蠕变初期,合金中γ'相沿垂直于应力轴方向,转变成N-型筏状结构,在稳态蠕变期间合金的变形机制是位错攀移越过筏状γ'相,而蠕变后期合金的变形机制是位错剪切进入筏状γ'相.蠕变断裂后,在试样不同区域筏状γ'相具有不同的形貌,在远离断口区域,筏状γ'相与应力轴方向垂直,而在近断口区域筏状γ'相尺寸及扭曲程度增加的原因是承受的应力及变形程度增大所致.     

DD3合金蠕变寿命预测方法分析

基于金属高温蠕变理论,结合单晶镍基高温合金蠕变过程的微观特点,从蠕变曲线方程的选择、截断应变点的确定以及强化项和弱化项的影响等方面,对采用修正θ函数影射法提高DD3单晶镍基高温合金蠕变寿命预测精度的原因进行了研究.结果表明:θ函数及其修正式将蠕变视为应变强化和应变弱化过程的综合,符合单晶镍基高温合金蠕变过程的实质,从根本上保证了预测的精度;修正的θ函数比早期的θ函数减少一个待定参数,统一了截断应变的确定方法,不仅减少了计算量,而且降低了因参数确定所产生的误差对蠕变寿命预测精度的影响;采用修正的θ影射方法可以很好地描述DD3合金整个蠕变过程,其预测结果完全能够满足实际工程设计的需要.     

偏离<001>取向15°的三种镍基单晶高温合金试样的中温蠕变变形行为

研究了一种偏离001取向15°的镍基单晶高温合金的中温蠕变(760℃/793 MPa)性能和变形组织。结果表明:取向靠近001-101边界合金的蠕变寿命最长,而取向靠近001-111边界合金的蠕变寿命最短。虽然三种试样取向偏离001的夹角都约为15°,但是其微观变形组织明显不同。取向靠近001-101边界试样的变形主要受控于{111}110滑移系,而取向靠近001-111边界试样变形主要受控于{111}112滑移系。     

一种含Re单晶镍基合金的蠕变行为

通过组织形貌观察及蠕变曲线测定,研究了一种含Re镍基单晶合金的高温蠕变行为.结果表明,4.2％Re单晶合金在1060～1100℃温度区间具有较好的承温能力,但表现出较强的施加应力敏感性.经高温蠕变断裂后,在试样不同区域γ'相具有不同的组织形貌,在远离断口区域γ'相形成的筏状组织与施加应力轴方向垂直,而在近断口区域,筏状γ'相的粗化及扭曲程度增加为该区域发生较大塑性变形所致.在蠕变后期,合金的变形机制是迹线方向为[011]和[011]的滑移位错切入筏状γ'相,主次滑移系交替开动,使筏状γ '相发生扭折形成不规则的扭曲形态,直至发生断裂是合金的蠕变断裂机制.     

一种GH4169镍基合金的组织结构与蠕变性能

通过对一种等温锻造GH4169镍基合金进行直接时效处理,蠕变性能测试及组织形貌观察,研究了该合金的组织结构与蠕变行为。结果表明,GH4169合金的组织结构由γ基体,γ′相、γ″相和δ相组成,且各相之间保持共格界面。测定出合金在660℃/700MPa条件下的蠕变寿命为123h。合金在680℃/700MPa的蠕变寿命为39h,在实验温度和应力范围内,计算出直接时效合金的蠕变激活能为588.0kJ/mol。合金在蠕变期间的变形机制是位错滑移和孪晶变形,其中,沿晶界析出的粒状碳化物,可抑制晶界滑移,是使合金具有较好蠕变抗力的主要原因。随蠕变进行,开动的滑移系中位错运动至晶界受阻,并塞积于该区域引起应力集中,当应力集中值大于晶界的结合强度时,可促使其在与应力轴垂直的晶界处发生裂纹的萌生与扩展,直至断裂,是合金在蠕变期间的断裂机制。     

镍基单晶高温合金的再结晶

镍基单晶高温合金作为先进发动机叶片的主要用材,其再结晶问题日益受到重视.本文综述了热处理温度、热处理时间、变形程度及合金成分等多种因素对镍基单晶高温合金再结晶的影响规律,分析了镍基单晶高温合金再结晶对其蠕变和疲劳性能的影响,并讨论了回复处理及浸蚀直接去除表面变形层、渗碳和表面涂层等控制再结晶的方法.最后,指出了镍基单晶...     

DD6单晶合金蠕变特性及断裂机理

在500MPa、900℃和300MPa、1000℃两种环境条件下,通过实验研究了[001]、[011]、[111]三种取向的镍基单晶合金DD6的蠕变特性及断裂机理。结果表明:DD6单晶高温合金蠕变性能具有明显的晶体取向相关性,在相同的温度和应力条件下,三种取向的单晶合金寿命差别很大,温度是影响[001]取向单晶合金蠕变寿命的主要因素,而应力则是影响[011]和[111]取向单晶合金蠕变寿命的主要原因。同时,晶体取向和实验条件的不同,都会使最小蠕变率发生变化,随着温度的升高,不同取向晶体的各向异性减弱,相同条件下,[111]取向最大蠕变量最大,[001]取向次之,[011]取向最小。而晶体取向对断裂机理有直接的影响,[001]、[111]取向DD6单晶合金的断裂是由微孔引起的断裂,[011]取向的DD6单晶合金在900℃、500MPa条件下的蠕变断裂为滑移断裂,1000℃、300MPa条件下的断裂为滑移面断裂和韧窝断裂二者兼有的混合型断裂。     

偏离〈001〉取向15°的三种镍基单晶高温合金试样的中温蠕变变形行为

研究了一种偏离<001>取向15^o的镍基单晶高温合金的中温蠕变(760℃/793 MPa)性能和变形组织。结果表明：取向靠近<001>-<101>边界合金的蠕变寿命最长,而取向靠近<001>-<111>边界合金的蠕变寿命最短。虽然三种试样取向偏离<001>的夹角都约为15^o,但是其微观变形组织明显不同。取向靠近<001>-<101>边界试样的变形主要受控于{111}<110>滑移系,而取向靠近<001>-<111>边界试样变形主要受控于{111}<112>滑移系。     

DD3单晶合金蠕变性能的实验研究

对三种不同试验条件下DD3合金的蠕变寿命、蠕变曲线、断口及不同蠕变阶段的显微组织和对应的筏化宽度进行了分析.结果表明:中温条件下应力对DD3合金蠕变寿命影响显著;较高温度时,温度或应力的增加均可促进微孔和韧窝的长大,此时的断裂模式为微孔聚集型断裂;蠕变过程是γ'相发生定向粗化、形成筏排和解筏的过程,筏化形成的快慢和解筏过程直接影响合金的蠕变寿命.     

Self-consistent Constitutive Modeling of the Creep Damage Behaviour of Nickel-base Directionally Solidified Superalloys with Different Grain Orientations

A self consistent creep damage constitutive model is developed for nickel-base directionally solidified superalloys. Grain degradation and grain boundary voiding are considered. The model parameters are determined from the creep test data of single crystal and directionally solidified superalloy with a special grain orientation. The numerical analysis shows that the model creep damage behaviours of nickel-base directionally solidified superalloys with difFerent grain orientations are in good agreement with the experimental data.     

Effects of refractory elements on the microstructure and creep behaviour of nickel-base cast superalloys strengthened with high volume fraction gamma-prime phase

Microstructural observations and creep measurements at elevated temperatures were carried out on nickel-base cast superalloys strengthened by 0.65 atomic fraction gamma-prime (y'J phase with varied concentrations of Ta, Wand Mo and with elements commonly used in practice. The variations in the amounts of the refractory elements considerably altered the morphologies and the quantities of carbides as well as γ and γ’ eutectics. The steady-state creep rate for these alloys was related to lattice mismatch between the γ and γ’ phases as well as to the aluminium concentration in the γ’ phase. The effect of the elements on creep rupture life was in the order W>Ta>Mo.     

Mechanisms and modelling of creep in superalloys

The characteristics of creep deformation of nickel-base superalloys are reviewed and the implications for the micromechanisms controlling the behaviour are considered. The development of a model of the creep deformation that is compatible with the physical mechanisms is traced, first in an isotropic form and later incorporating full crystallographic anisotropy. The validity of the model and its ability to be extrapolated to more complex loading conditions are evaluated against a wide range of experimental measurements. Much of the work described in this review has been funded by the Defence Research Agency and creep data and specimens for examination were supplied by Dr M R Winstone of DRA.     

Modelling creep-corrosion interactions in nickel-base superalloys

Abstract

Some of the more recent experiments concerning the interactions between corrosion and creep in nickel-base superalloys have been reviewed and placed into three categories. The most systematic work, from the viewpoint of model development for the prediction of the service life, has concerned the exposure to a damaging environment before creep testing; but there is still insufficient work of this type. The ‘sharp interface’ composite model of corrosion-creep interactions has been developed to unify and extend the few attempt that have been made in the literature to provide predictive equations in this area. The virtue of such an approach is that it keeps the physics of the various processes well to the fore, but it is approximate and its success can only be gauged when well-defined systematic experiments have been performed in this important area. MST/446     

Time-Temperature Dependence of Anisotropy of Creep-Rupture Strength Characteristics of Single Crystals of Nickel-Base Superalloys

The paper presents the results of processing experimental data on creep-rupture strength of >110<- and >111<-oriented single crystals of ZhS36 and CMSX-4 nickel-base superalloys in the temperature range from 1023 to 1373 K. We give numerical values of the coefficients in equations of the time-temperature dependence of creep-rupture life and plot the creep-rupture strength curves for single crystals of the same orientation in the creep-rupture life range up to 105 h. We have revealed three various time-temperature regions of variation of the creep-rupture strength anisotropy factor, where the mode of anisotropy is governed by various creep mechanisms and depends on the microstructure evolution of single crystals.     

Variation in Creep Rupture of γ′ Strengthened Superalloys with Stacking Fault Energy of Matrices

The correlation between the creep rupturebehaviour and the stacking fault energy of matricesof γ′strengthened superalloys has been studied dur-ing constant load creep.At high temperature andintermediate stress,the creep rupture time andstrain strongly depend on the stacking fault energyof matrices rather than the creep friction stress,butat higher stress,the role of grain boundary carbidesbecomes more obvious.However,in the considerably extensive stressrange investigated here,the mean creep rate is apower function of the stacking fault energy ofmatrices and the power index decreases with in-creasing initial applied stress.Particularly,at inter-mediate stresses the product of this index and theinitial applied stress compensated by the shearmodulus is same for two series of superalloys.Hence,this product may be a criterion predictingthat the matrix deformation controls high tempera-ture creep rupture.     

Development of a Nickel-base Cast Superalloy with High Strength and Superior Creep Properties

Derived from Russian alloy CHS88U, six experimental Ni-base alloys named as A to F in the Ni-Cr-Co-W-Ti-Al-Hf system are designed, evaluated and processed. One of these alloys, F, shows excellent high temperature tensile strength and ductility with superior creep rupture properties. As predicted by using modeling tools such as PHACOM and NEW PHACOMP, there is hardly the tendency for formation of topologically close-packed phase (TCP) phase in alloy F. Furthermore, through microstructural observation, it is also found that no TCP phase is formed in alloy F after long-time exposure at high temperature. So alloy F has well balance of phase stability and mechanical properties in view of application for gas turbines. It is proved that d-electron approach can be applied for design and development of nickel-base superalloys for gas turbine application.     

A NEW METHOD TO PREDICT THE LIFE UNDER HIGH-TEMPERATURE LOW CYCLE FATIGUE CONDITIONS

The low cycle fatigue lives of some nickel-base alloys tested at high temperature are predicted using a linear time dependent damage accumulation rule. Based on stress rupture data, the rule correctly predicts the cyclic life provided that the ratio of the inelastic strain rate in the cycle to the minimum creep rate in a stress rupture test (performed at the maximum cycle stress and at the same temperature) is near to one. If this ratio is much larger than one, the rule gives an upper limit to the cyclic life time. For that case a modified rule is proposed with a single fitting parameter which is adjusted to high frequency low-cycle fatigue test data. Using the nickel-base alloy data it is shown that life extrapolations to (i) low testing frequencies, (ii) a higher temperature and (iii) different cycle shapes, are possible too within a factor of two.     

CRACK GROWTH IN A TITANIUM ALUMINIDE ALLOY UNDER THERMAL MECHANICAL CYCLING

Abstract— The fatigue crack growth behavior in a titanium aluminide (Ti₃Al) alloy under thermo-mechanical loading as well as under elevated temperature conditions was investigated. The thermal mechanical fatigue crack growth behavior appears, in a general sense, to follow the same trends observed in similar data obtained in tests on nickel-base superalloys. However, crack growth in Ti₃Al appeared to be influenced by blunting of the crack tip due to creep in addition to a cyclic-dependent contribution together with time dependent or environmental enhanced degradation. This complex phenomenon in Ti₃Al is unlike that in nickel-base superalloys where crack growth was found to be due to a linear combination of cycle and time dependent contributions. Thus, the linear cumulative modelling technique is not applicable to the tested Ti₃Al.