Creep characterization of a Ni-based Hastelloy-X alloy by using theta projection method

The θ projection method was applied to characterize the creep behavior of the Hastelloy-X alloy at 950 °C. Four θ parameters were established by a nonlinear least square fitting (NLSF) to the creep curves. In the NLSF of the full creep curves, the θ₁ and θ₂ parameters were not defined with a large error, but the θ₃ and θ₄ parameters were defined well without an error. An optimum cutoff strain range for defining the four θ parameters was found to be a 3% strain. Four θ parameters revealed a good linearity as a function of stress. The predicted minimum creep rate showed a good agreement with the experimental data. At 950 °C of the Hastelloy-X alloy, the creep curves, the creep rate, and the time to reach a limiting strain were estimated with a wide range of stresses.     

On the tensile creep behaviour of a directionally-solidified Ni3Al-based alloy

High temperature tensile creep behaviour of a directionally-solidified Ni₃Al-based alloy is presented. The study involved selection of nine alloy systems based on Ni₃Al. The alloys contained varying amounts of Cr and Ta, fixed amounts of 1·5 at.% Hf and 0·5 at.% Zr and doped with 0·2 at.% each of C and B. The alloys were vacuum arc-melted into buttons and homogenized at 1050°C for 68 h. The test pieces of the alloys were hot compression tested at 600, 700, 800 and 900°C. The yield strength data of some of the alloys were superior to conventionally cast Mar-M 200, a cast nickel-base superalloy widely used in gas turbine structural applications. The best alloy system was chosen based on consistent performance in the hot compression studies. The alloy so chosen was directionally solidified and vacuum-homogenization-treated for 20 h at various selected temperatures. Optimum creep properties were observed at 1120°C, 20 h treatment. The minimum creep rate data of the DS alloy showed relatively higher values even at lower temperatures and stress levels as compared to Mar-M 200. Hence, the alloy is less promising in replacing nickel-based superalloys used as structural materials in gas turbine applications.     

Dependence of accelerated creep rate on carbide particle distribution in martensite for 0·18C–11·5Cr–0·29V steel

Abstract

The number of carbide particles stringers at boundaries and subboundaries of martensite per unit of surface decreases with the third root of tempering time at 1073 K. The accelerated creep rate at 853 K depends on the number of stringers of carbide particles per unit of surface, and below an inflection point, the creep rate increases strongly. Equations relating for creep rate are discussed in terms of experimental creep rate, number of carbide particles stringers and average particles spacing.     

Issues relating to numerical modelling of creep crack growth

In this paper creep crack growth behaviour of P92 welds at 923 K are presented. Creep crack growth behaviour for P92 welds are discussed with C^* parameter. Creep crack growth behaviour of P92 welds has been compared with that of P91 welds with C^* parameter. NSW and NSW-MOD model were compared with the experimental creep crack growth data. Plane strain NSW model significantly overestimates the crack growth rate, and plane stress NSW model underestimates it. Whilst, NSW-MOD model for plane stress and plane strain conditions gives lower and upper bound of the experimental data, respectively.FE analysis of creep crack growth has been conducted. Constrain effect for welded joints has been examined with C^* line integrals of C(T) specimens. As a result, constant C^* value using the material data of welded joint gives 10 times lower than that of only HAZ property. Whilst, the predicted CCG rates for welded joint are 10 times higher than those for only HAZ properties. Compared with predicted CCG rate from FE analysis and the experimental CCG rate, it can be suggested that creep crack growth tests for lower load level or for large specimen should be conducted, otherwise the experimental data should give unconservative estimation for components operated in long years.     

Constitutive modelling and creep life prediction of a directionally solidified turbine blade under service loadings

Abstract

The high pressure turbine (HPT) blade of a heavy duty gas turbine operates under the interaction of complex aerodynamic, centrifugal and thermal loadings. The reliability of continuous working at elevated temperatures is a major limitation in service application of these materials. Therefore, it is essential to build the constitutive equations for predicting and analysing the creep deformation and creep lifetime of the blade. In present work, the creep deformation and lifetime of a HPT blade made of a nickel-based directionally solidified (DS) superalloy was numerically predicted. The θ-projection method was used to characterize the creep deformation of DZ125 under different temperatures and stress levels. The uniaxial equations of the θ-projection method were expanded into multi-axial form which was implemented into ABAQUS/UMAT (User MATerial subroutine) by an Euler method. A modified θ-projection method was employed to promote the adaptability of the original model to both steady state and transient temperature fields. Transient stress, strain and displacement distribution of the critical position inside the blade were obtained for service loading. The Larson–Miller parameter was employed to predict the creep lifetime of the blade. Simulation and results of the θ-projection method may also provide suggestions for the safety and life evaluation of HPT blade and other turbine blades.     

铸造Mg-RE合金的显微结构及其蠕变行为

研究了铸造Mg-RE合金的高温蠕变特性,通过SEM和TEM观察,讨论与分析了该合金的蠕变机制.结果表明:在473K、应力低于100MPa条件下,该合金具有良好的蠕变抗力,其稳态蠕变速率低于2.66×10~(-6)s~(-1);富含Ce、La、Nd等稀土元素的共晶β相和蠕变期间动态沉淀产生的晶界强化及沉淀强化是该合金的主要强化机制,而孪晶切变和共晶β相开裂致使其高温蠕变抗力显著降低。     

Effects of creep properties of materials on unified creep constraint parameter Ac for cracked pipes

Extensive finite element analyses of cracked pipes with different crack sizes and orientations have been conducted to investigate effects of creep properties of materials on the unified creep constraint parameter A_c. The results show that the constraint parameter A_c is independent on Norton’s coefficient A, and it is only affected by the creep exponent n of materials. For a given crack size, with increasing n, A_c decreases and constraint level increases. The A_c of lower constraint cracks is more sensitive to n. The unified correlation equations between A_c and n have been obtained for cracked pipes with a wide range of crack sizes and constraint levels. They may be used to estimate the constraint parameter A_c at different positions along the crack fronts in cracked pipes made of materials with different n values. The two-parameter C*-A_c approach for assessing creep life of cracked pipes has also been discussed.     

Proposal of a new concept on creep fracture remnant life for a precracked specimen

Abstract

The creep life time of a smooth specimen can be predicted using existing laws for creep deformation and steady state creep rate. When crack growth behaviour is involved, it is necessary to construct a law of creep crack growth rate to predict creep fracture life. Creep fracture life can be measured by integrating the law of creep crack growth rate. One example is the creep crack growth rate, represented by the parameter Q*. In this study, we investigated the applicability of this prediction method to creep fracture remnant life for a cracked specimen. The Ω criterion is proposed to predict creep fracture remnant life for a smooth specimen for creep ductile materials. In this study, the correlation between Q*_L derived from the paremeters Q* and Ω is investigated. The correlation between Q_L* and Ω provided a unified theoretical prediction law of creep fracture remnant life for high-temperature creep-ductile materials in the range from smooth to precracked specimens.     

形变热处理工艺对2519A铝合金动态变形行为的影响

通过霍普金森压杆实验研究2519A铝合金T87,T8,T9和T9I64种形变热处理状态在1040~5900s^-1应变率范围的动态冲击变形行为,并利用金相、透射电镜等手段分析在动态变形中合金微观组织的演变规律,研究不同形变热处理工艺对2519A铝合金动态变形行为的影响。结果表明:与T87态合金相比,强冷变形的T8和T9态合金高速冲击的动态屈服强度大幅提升,但是合金的绝热剪切敏感性也显著增加,更容易发生绝热剪切开裂。断续时效T9I6工艺可以提高2519A合金强化析出相的密度,使θ'相(Al₂Cu)更细小弥散分布。这样降低了θ'析出相在高应变率下被位错切割分解的速率,提高合金在高速变形过程中的稳定性。2519A-T9I6铝合金在高应变率下拥有较高的动态屈服强度和较低的绝热剪切敏感性,在高速变形过程中表现最佳。     

Effect of metakaolin on creep and shrinkage of concrete

The effect of metakaolin (MK) on the creep and shrinkage of concrete mixes containing 0%, 5%, 10%, and 15% MK has been investigated. The results showed that the early age autogenous shrinkage measured from the time of initial set of the concrete was reduced with the inclusion of MK, but the long-term autogenous shrinkage measured from the age of 24 h was increased. At 5% replacement level, the effect of MK was to increase the total autogenous shrinkage considered from the time of initial set. While at replacement levels of 10% and 15%, it reduced the total autogenous shrinkage. The total shrinkage (autogenous plus drying shrinkage) measured from 24 h was reduced by the use of MK, while drying shrinkage was significantly less for the MK concretes than for the control concrete. The total creep, basic creep as well as drying creep were significantly reduced particularly at higher MK replacement levels. Compared with estimated values by the CEB 90 model, total creep of all concretes was overestimated, especially in the mixes containing the higher levels of MK. For basic creep, estimates for low levels of MK were acceptable but, for the higher levels, creep was overestimated.     

Effect of electric current on the creep deformation of lead

Using the impression test, the effect of electric current on the creep deformation of Pb was studied in the temperature range of 343-403 K and under the punching stress of 11.1-30.5 MPa. During the impression test of constant load, a direct electric current in a range of 0-5 A passed through the punch into the Pb-sample. A steady state creep with sudden jumps in the impression curves was observed. A power-law relation was used to describe the dependence of the steady state impression velocity on the punching stress. The average stress exponent increased from 3.2 to 4.0 with increasing electric current and the apparent activation energy was 36.0 kJ/mol for small punching stresses. For large punching stresses, the average stress exponent decreased with increasing electric current and the apparent activation energy decreased from 63.5 kJ/mol to 49.7 kJ/mol with increasing electric current from 0 to 5 A.     

Estimation of the transient creep parameter C(t) under combined mechanical and thermal stresses

This paper reports finite element analyses for an axially loaded cylinder with three different types of thermal stress. The magnitudes of the thermal and mechanical loadings are varied and creep crack tip parameters are evaluated for part-through-wall and fully penetrating defects with a range of sizes. The relaxation rate for the creep crack tip parameter is examined in detail and is found to depend on the magnitudes of the mechanical and thermal loads and on whether or not there is plasticity on initial loading. Simplified formulae are developed for describing the rate of relaxation by a modified redistribution time.     

Interpretation of creep crack initiation and growth data for weldments

Creep crack growth testing of macroscopically homogeneous materials is well established and standardised test procedures are detailed in ASTM E1457. In ASTM E1457 the use of the compact tension C(T) specimen is specified and crack growth data are interpreted using the fracture mechanics parameter C^∗. The resulting benchmark crack growth data are used in assessment procedures, together with estimates of the value of C^∗ in the component, to predict creep crack growth behaviour. In this work, the results of a series of creep crack growth tests performed on a Type 316 stainless steel weldment at a temperature of 550 °C are presented. The initial crack is located within the heat affected zone (HAZ) of the weldment. Since there are currently no agreed methods for determining C^∗ in inhomogeneous laboratory specimens, this paper examines the application of procedures in ASTM E1457 for the characterisation of crack growth in weldments. In addition, the creep toughness parameter is evaluated for the material. It is shown that the creep crack growth rates in the weldment may be described by the C^∗ values obtained following ASTM E1457 in conjunction with parent material properties. Comparison of the results with similar data for Type 316H stainless steel parent material shows that the crack growth rates for the crack located within the HAZ are higher and the initiation times lower than the parent values, for the range of test conditions examined. Simple analytical models based on ductility exhaustion arguments have been shown to bound the crack initiation and growth data for the weldment.     

Effect of initial temper on the creep behavior of a Mg–Gd–Nd–Zr alloy

The effect of initial temper on the tensile creep behavior of a cast Mg–Gd–Nd–Zr alloy has been investigated. Specimens in unaged, underaged and peak-aged conditions exhibit a sigmoidal creep stage between the primary and steady-state creep stage, while the overaged specimens have no such creep stage. Transmission electron microscope observations revealed that sigmoidal creep stage was induced by the dynamic precipitation in the microstructure, and the rapid formation of β₁-phase and β-phase plates takes responsibility for the softening of material in this stage. Comparative evaluation of creep properties of the specimens showed that alloy in overaged condition had creep resistance superior to those in other conditions. Stress and temperature dependence of the steady-state creep rate were studied over a temperature range of 250–300 °C and stress range of 50–100 MPa, and a dislocation creep mechanism was proposed for the alloy.     

膨胀剂和钢管侧限对混凝土徐变应变的影响

为了研究不同膨胀剂质量分数和钢管侧限对混凝土徐变应变的影响,共进行了12组有、无钢管侧限的混凝土徐变试验。结果表明:加载龄期到480天的试验试件中,有钢管侧限的混凝土徐变应变约是无钢管侧限徐变应变的0.175~0.181倍,钢管侧限大大降低了混凝土的徐变应变。同时进行了三种不同质量分数膨胀剂(质量分数分别为4%、8%和12%)的混凝土徐变试验。结果表明:无论有、无钢管侧限的混凝土结构,膨胀剂质量分数对混凝土徐变应变的影响关系为:质量分数4%时,结构徐变应变均最大;质量分数为8%时,结构徐变应变次之;质量分数为12%时,结构徐变应变最小。通过钢管侧限对混凝土的套箍效应计算分析表明,在混凝土结构加载应力相同情况下,有钢管侧限的混凝土结构应力比小于无侧限的混凝土结构应力比;钢管侧限对混凝土的侧向约束增加,限制了混凝土纵向变形;随着应力比的减小,结构徐变变形减小。同时应用扫描电镜二次电子成像分析膨胀剂的作用机制得出:水泥浆中膨胀剂水化产生粗棒状、互相交叉的三硫型硫铝酸钙(AFt)结晶体,填充在原来被溶液占据的小孔里,使水泥石更密实。     

锰对ZA27合金压缩蠕变的影响

为了解质量分数为0.5%Mn对铸态ZA27合金压缩蠕变行为的影响,采用自制的试验装置研究了ZA27-Mn合金和ZA27合金第一阶段的压蠕变量和稳态蠕变速率.结果表明:ZA27-Mn合金第一阶段的压蠕变量和稳态蠕变速率低于ZA27合金,而两种合金的压蠕变均符合于lnt=C-nlnσ Q/RT,ZA27-Mn合金的应力指数(n)和蠕变激活能(Q)分别为3.83和82.76 kJ·mol-1,而ZA27合金的应力指数和蠕变激活能分别为3.46和81.09kJ·mol-1,材料结构常数C不同,合金压蠕变不同,ZA27-Mn合金的压蠕变抗力高于ZA27合金;加入0.5%Mn对ZA27合金蠕变机制并无影响,均是锌的点阵自扩散和位错的攀移机制.     

Effects of component size, geometry, microstructure and aging on the embrittling behavior of creep crack growth correlated by the Q parameter

The driving force for creep crack growth is dominated by local elastic-plastic stress in the creep damage zone around a crack tip, temperature and microstructure. In previous work, C^∗, C_t, load line displacement rate dδ/dt and Q^∗ parameters have been proposed as formulations of creep crack growth rate (CCGR). Furthermore, using parameters mentioned above, the construction of the algorithm of predictive law for creep crack growth life is necessary for life assessment procedures. The aim of this paper is to identify the effects of component size, geometry, microstructure, aging and weldment on the embrittling behavior of creep crack growth and incorporate these effects in a predictive law, using the Q^∗ parameter. It was found that for specimen size (width and thickness) and of material softening due to aging the values of the activation energy were the same whereas for grain size change and structural brittleness, which affected crack tip multi-axial stress state the values for the activation energy for CCGR differ.     

Investigating creep rupture and damage behaviour in notched P92 steel specimen using a microscale modelling approach

Idealized random grains separated by pseudo grain boundaries were generated by using Voronoi tessellation to simulate the polycrystalline microstructure. Combined with finite element analyses, this approach made it possible to addressing crack initiation and progressive failure due to crack growth in notched bar geometries of P92 steel at high temperature. The calculations provided good predictions for creep rupture lives of notched specimen with different notch radii and external stress. Simultaneously, irregular crack growth shape, intergranular crack mode, and wedge cracks at triple grain interaction were captured in the model. The crack initiation positions were found to be influenced by notch radius and applied stress causing high stress triaxiality at the subgrain level. Furthermore, the preferential crack growth directions were changed as the notch varied from sharp to blunt.     

Microstructure and corrosion resistance of Ni-based alloy laser coatings with nanosize CeO2 addition

Abstract

Micron-size Ni-base alloy (NBA) powders were mixed with both 1.5 wt.% (hereinafter %) micron-size CeO₂ (m-CeO₂) and also 1.5% and 3.0% nano-size CeO₂ (n- CeO₂) powders. These mixtures were coated on low-carbon steel (Q235) by 2.0 kW CO₂ laser cladding. The effects on the microstructures, phases and electrochemical corrosion of the coatings upon the addition of m- and n- CeO₂ powders to NBA (m- and n- CeO₂ /NBA) have been investigated. The results showed that a smooth coating was prepared under suitable processing parameters (P= 2.0 kW, V= 180 mm min^{- 1}) by adding 1.5% n- CeO₂. In addition to the primary phases of γ-Ni, Cr₂₃ C₆ and Ni₃ B in the Ni-base alloy coating, CeNi₃ was formed in Ni-base alloy coatings with both n- CeO₂ and m-CeO₂ particles, and CeNi₅ appeared in the coating upon decreasing the size of CeO₂ particles. Well-developed dendrites were observed in the Ni-base alloy coating; directional dendrites grew at the interface in the coating upon the addition of m-CeO₂, whereas fine and multioriented dendrites grew upon decreasing the size of CeO₂ particles to the nanoscale. Actinomorphic dendrites and compact equiaxed dendrites grew from the interface to near the surface upon increasing the content of n- CeO₂ from 1.5 to 3.0%. In strongly acidic HNO₃ solution, the severe corrosion of dendrites occurred and there were many corrosion pits in the Ni-base alloy coating; intercrystalline corrosion also has a dominant role upon the addition of m-CeO₂, whereas uniform corrosion occurs in the coating as the size of CeO₂ particles is decreased to nanoscale.