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.

IN SITU OBSERVATION OF DEFORMATION AND FRACTURE FOR SUPERALLOY GH169 UNDER COMBINED FATIGUE-CREEP ACTION

In Situ observation of deformation and fracture for superalloy GH169 under combined fatigue-creep action is made by using high temperature metalloscope,it is shown that under the test conditions the deformution takes place by merely of slipping,twinning and grain houndary sliding,and the mode of failure depends on the microstructure of specimen. lntergranular cracks arise.from W-type voids produced by the stress concentration at triple point which could not be relaxed by the interior deformation of grains and the local deformathm region along grain bounaries.And the crack propagation mechanism is the nucleation,growth and linkage of carities at the grain boundaries.Transgranular cracks form from deformatiom damages within the grain,and its propagation mechanism is shear rupture along the slip phme.

CRACK GROWTH BEHAVIOR AND FRACTURE FEATURE OF GH2132 UNDER CREEP-FATIGUE CONDITIONS

The superalloy GH2132 is equivalent to A286. The tests were carried out at 550°C under various cyclic frequencies (hold time) and load levels, and the fracture surfaces were examined by using a scanning electron microscope. It was shown that the fracture mode of creep-fatigue and the effect of cyclic loading on crack growth change with the growth of crack and the increase of net-section stress, and both are reversed when the net-section stress is up to the yield stress of material. When σn0.2, cracking is predominantly cyclic-dependent transgranular and cyclic loading accelerates creep crack growth, whereas when σn>σ0.2, the case is reversed.

High temperature creep-rupture features of nickel-base single crystal superalloy

The creep and rupture behavior of a nickel-base single crystal superalloy with [001] orientation was investigated at temperature of 10001040℃ and stress in the range of 150320MPa. The creep features and microstructure were studied by means of the measurement of creep curves and TEM observation. The results show that all creep curves exhibit a short primary and a dominant accelerated creep stage. From the creep parameters and TEM observations, it is suggested that the primary deformation mechanism has a change from precipitatation shearing by pairs of dislocation in the high applied stress region to dislocations climb around the γ′ particles in the low applied stress region. Furthermore, the detailed failure process and fracture surfaces were analyzed by SEM observation.

A CRYSTALLOGRAPHIC MODEL FOR THE ORIENTATION DEPENDENCE OF LOW CYCLIC FATIGUE PROPERTY OF A NICKEL-BASE SINGLE CRYSTAL SUPERALLOY

Fully reversed low cyclic fatigue (LCF) tests were conducted on [0 0 1], [0 1 2], [(1) over bar 1 2], [0 1 1] and [(1) over bar 1 4] oriented single crystals of nickel-bared superalloy DD3 with different cyclic strain rates at 950 degrees C. The cyclic strain rates were chosen as 1.0 x 10(-2), 1.33 x 10(-3) and 0.33 x 10(-3) s(-1). The octahedral slip systems were confirmed to be activated on all the specimens. The experimental result shows that the fatigue behavior depends an the crystallographic orientation and cyclic strain rate. Except [0 0 1] orientation specimens, it is found from the scanning electron microscopy(SEM) examination that there are typical fatigue striations on the fracture surfaces. These fatigue striations are made up of cracks. The width of the fatigue striations depends on the crystallographic orientation and varies with the total strain range. A simple linear relationship exists between the width and total shear strain range modified by an orientation and strain rate parameter. The nonconformity to the Schmid law of tensile/compressive flaw stress and plastic behavior existed at 95 degrees C, and an orientation and strain rate modified Lall-Chin-Pope ( LCP) model was derived for the nonconformity. The influence of crysrallographic orientation and cyclic strain rate on the LCF behavior can be predicted satisfactorily by the model. In terms of an orientation and strain rate modified total strain range, a model for fatigue life was proposed and used successfully to correlate the fatigue lives studied.

Effect of solution cooling rate on the microstructure and creep deformation mechanism of a rhenium-free second-generation single crystal superalloy

Various cooling scenarios(water,oil,air and furnace)were employed to study the impacts of the solution cooling rate(SCR)on the microstructure and creep behavior of a novel single-crystal(SX)superalloy.The results showed that the cubic degree and size of theγphases were inversely proportional to the SCR.The creep life first increased and then dropped dramatically with a reduction in the SCR.The creep life of the sample cooled with air cooling(AC)was the highest,up to 144.90 h at 800℃/750 MPa and160.15 h at 1100℃/137 MPa.During creep at 800℃/750 MPa,the improved creep life of the AC sample was mainly attributed to the fine cubicγphases,which decreased the rate ofγ-phase coarsening and favoured plastic deformation by promoting the active movement of dislocations.The AC helped theγphases become rich in Al,Ti and Ta while depleted in Co and Cr,which enhanced its stacking fault energy,thus promoting the formation of dislocation locks.Meanwhile,the largest negative lattice misfit caused by AC induced denserγ/γinterface dislocation networks at 1100℃/137 MPa,which efficiently reduced the minimum creep rate.The calculated average dislocation spacing results indicated that the smallest density of excess dislocations corresponded to the AC sample,proving its greatest creep resistance.Interestingly,the size of the secondaryγphases first decreased and then increased sharply with decreasing SCR during creep at 1100℃/137 MPa,when fine secondaryγphases had a positive role in the blockage of dislocation movement in the matrix.Eventually,the comprehensive SCR effect was explored to provide more guidance in the design of Re-free SX superalloys.

Effect of Solidification Parameters on the Microstructure and Creep Property of a Single Crystal Ni-base Superalloy

A single crystal Ni-base superalloy was processed with withdrawal rates between 2 and 7 mm/min.The ascast microstructures,heat treatment response and creep property have been characterized as a function of the withdrawal rate.As expected,the primary and secondary dendrite arm spacing decreased with increasing withdrawal rate; microsegregation degree and porosity distribution were also varied with different withdrawal rates.The withdrawal rate of 2 mm/min resulted in a noticeable residual microsegregation even after full heat treatment.The samples solidified at 7 mm/min exhibited a high density of cast porosities,and this led to a dramatical decline of the creep strain.4 or 6 mm/min appeared to be the optimum withdrawal rate in the present study,which resulted in a uniform microstructure and an optimum density of cast porosity.

The dependence of stress and strain rate on the deformation behavior of a Ni‐based single crystal superalloy at 1050°C

Ni‐based single crystal(SX)superalloys are important high‐temperature materials used for manufacturing turbine blades in aero‐engines.During service under combinational impacts of temperature and stress,the SX superalloy may reach its life due to plastic deformation,which normally accompanies time‐dependent microstructural degradation.To reveal this dynamically mechanical response,tensile tests at 1050°C are carried out to record stress‐strain curves at five stain rates as well as creep curves at four applied stresses.Deformed microstructures and defects have been analyzed to understand mechanical behaviors and the underlying mechanism by using advanced scanning electron and scanning transmission electron microscopes.Results show that the de-formation mode of the alloy strongly depends on the strain rates/applied stresses under mechanical loading.The dislocation density inside theγphase is extremely low at all tests,indicating that theγphase is relatively weak and ready to flow at this temperature even at a very fast strain rate.The deformation behavior of theγ′phase is much complicated.At fast strain rates or high applied stresses,the dislocation density in theγ′phase is very high,contributing to high‐stress requirements to deform the material.At slow strain rates or low applied stresses,rafting microstructures develop and the de-formation mode becomes directional coarsening/diffusion‐dominated.Our results de-monstrate a comprehensive understanding of the deformation mechanism of Ni‐based SX superalloys,which may provide lifetime prediction of the mechanical fail-ure,as well as the database for superalloy applications in mechanical systems.

A modern and robust methodology for modeling anisotropic creep characteristics of Ni-based DS and SC superalloys

According to more recent work,the Wilshire equations have shown good prediction accuracy in a wide range of materials and stress-temperature conditions,particularly in extrapolation of short term results to long term predictions.In the current paper,this methodology was further developed for modeling anisotropic creep characteristics(i.e.minimum creep strain εmin ,stress rupture life tf and time to a specified strain tε)of four typical Ni-based directionally solidified(DS)and single crystal(SC)superalloys,where a simple orientation factor related to the ultimate tensile strength(UTS)was introduced.The application of these simplistic approaches showed that the anisotropic creep characteristics in a wide range of stress-temperature conditions can be accurately simulated.Meanwhile,during the application of the modified Wilshire equations,break points occurring at the specified stress levels agree well with the transition of creep deformation mechanisms occurring in different stress regions,which provides confidence for using this method.

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。

Uncovering the creep deformation mechanism of rock-forming minerals using nanoindentation

The creep phenomenon of rocks is quite complex and the creep mechanisms are far from being well understood.Although laboratory creep tests have been carried out to determine the creep deformation of various rocks,these tests are expensive and time-consuming.Nanoindentation creep tests,as an alternative method,can be performed to investigate the mechanical and viscoelastic properties of granite samples.In this study,the reduced Young’s modulus,hardness,fracture toughness,creep strain rate,stress exponent,activation volume and maximum creep displacement of common rock-forming minerals of granite were calculated from nanoindentation results.It was found that the hardness decreases with the increase of holding time and the initial decrease in hardness was swift,and then it decreased slowly.The stress exponent values obtained were in the range from 4.5 to 22.9,which indicates that dislocation climb is the creep deformation mechanism.In addition,fracture toughness of granite’s rock-forming minerals was calculated using energy-based method and homogenization method was adopted to upscale the micro-scale mechanical properties to macro-scale mechanical properties.Last but not least,both three-element Voigt model and Burgers model fit the nanoindentation creep curves well.This study is beneficial to the understanding of the long-term mechanical properties of rock samples from a microscale perspective,which is of great significance to the understanding of localized deformation processes of rocks.

Design and Performance Evaluation of a Sustained Load Dual Grip Creep Testing Machine

The design and performance evaluation of a sustained load creep testing machine was undertaken in this research. The design was motivated by the need to make locally available, a cost effective, technically efficient, and easily operated creep testing facility;for creep behaviour studies of materials. Design drawings and purchase of materials and components for the design were undertaken after thorough evaluation of the following design and materials selection criteria: design principle and theory, local availability of raw materials and components required for the design, material properties, cost of materials and design, ease of utilization and maintenance, and basis of testing and data capture. The machine casing and frame, heating chamber (consisting of the furnace and a dual specimen mounting stage), load lever and hanger system, and the electro-technical components;were fabricated and coupled following the produced design specifications. The machine was tested and its performance was assessed using its heating efficiency, repeatability and reproducibity of experimental test results, maintainability and cost-effectiveness as criteria. It was observed from repeat tests that the machine has the capacity of generating reliable data for computing creep strain-time results. The efficiency and temperature regulating capacity of the heating unit of the machine were also observed to be very satisfactory. The cost of the design was about 112,000 Naira ($700.00) which is cheaper in comparison to similar commercial creep testing machines from abroad. The machine was also found not to pose maintenance or repairs challenges.

某镍基粉末合金高温疲劳裂纹扩展行为与模型研究

针对涡轮盘用某镍基粉末高温合金材料,开展了高温疲劳裂纹扩展试验,分析了环境温度、载荷比、紧凑拉伸(CT)试件厚度等因素对于疲劳裂纹扩展行为的影响规律,建立和评估了考虑影响因素的裂纹扩展速率模型。结果表明:随着温度的升高和载荷比的增大,疲劳裂纹扩展速率会显著增大,而CT试件厚度对于扩展速率的影响较小;通过观测断口截面发现,在500~700℃内,疲劳裂纹的扩展机制与温度之间没有显著关联,而在700℃下,高载荷比的条件使得裂纹扩展断面更为粗糙;提出了一种温度相关的疲劳裂纹扩展速率模型,可较好地描述不同温度下的疲劳裂纹扩展曲线;对比了Walker模型和Newman裂纹闭合模型的应用效果,发现Walker模型在描述不同载荷比下的裂纹扩展速率方面更为优秀。

铸造镍基合金K44的高温蠕变行为

研究了铸造镍基高温合金K44在850—900℃,225—380MPa的高温拉伸蠕变行为.结果表明,实验合金蠕变曲线具有较短的初始阶段和较长的加速阶段,加速阶段较长是由于筏形化的γ′粒子和位错相互作用促成的.加速蠕变为应变软化阶段时,可以由关系式ε=ε_(min)(1+c′Δε)exp(n′Δε)来描述.蠕变断裂数据遵守Monkman-Grant规律,裂纹起源于晶界或枝晶界的空洞.

抽拉速率对定向凝固DZ483合金的微观组织及力学性能的影响

研究抽拉速率对DZ483合金微观组织及力学性能的影响。结果表明:随着抽拉速率的提高,合金的一、二次枝晶间距逐渐减小,γ′相尺寸减小,但γ′相的数量增多;共晶组织及枝晶间碳化物的数量增多。碳化物形貌由块状转变成长条状,类型不随抽拉速率的提高而改变。随着抽拉速率的升高,除Cr元素偏析系数在1左右波动外,其余元素的偏析程度都增加,其中W、Mo元素偏析程度明显加重;DZ483合金的持久性能随抽拉速率的提高先升高后降低。不同抽拉速率下,合金断口均呈现出韧性断裂和脆性断裂混合特征,γ′相为合金的主要强化相。随着抽拉速率的提高,一方面,合金中γ′相的数量逐渐增多,尺寸逐渐减小,提高了合金的持久性能;另一方面,由于元素偏析程度加重、碳化物和共晶数量增多及γ′相筏形化规则程度的下降,降低了合金的持久性能。

施载方向对定向凝固镍基高温合金高温蠕变行为的影响

研究了施载方向对定向凝固镍基高温合金980℃130—190MPa蠕变及断裂性能的影响结果表明三种取向试样的蠕变曲线相似，稳态蠕变均由位错的攀移过程所控制因蠕变断裂机制不同，纵向试样比横向及倾斜试样表现了高得多的啼变塑性，前者为完全穿晶断裂。

定向凝固镍基高温合金的高温蠕变

研究了一种定向凝固镍基高温合金在870℃，330—420MPa应力下的高温拉伸蠕变性能．结果表明：蠕变曲线表现了较短的减速蠕变阶段和较长的加速蠕变阶段，其蠕变变形机制为位错在第二相粒子间的Orowan弯曲过程，而不是由扩散过程所控制加速蠕变阶段蠕变速率的起始增加是显微组织发生变化（γ’相的定向粗化）的结果，而不是蠕变裂纹的形成与扩展．蠕变断裂数据符合MonkmanGrant关系．

含铪高钨K416B镍基铸造高温合金的组织与蠕变行为

通过组织形貌观察和蠕变性能测试,研究了含铪高钨K416B镍基铸造合金的组织及蠕变机制。结果表明:合金铸态组织由γ基体、γ'相、MC和M6C型碳化物组成;其中,MC碳化物主要以链状和汉字型结构分别在晶界和枝晶间析出,而大块状M6C碳化物镶嵌在共晶处;蠕变期间,合金的变形机制是位错在基体中可沿不同方向发生滑移,且位错可绕过或剪切γ′相;蠕变后期,高密度位错在碳化物和晶界处塞积,产生应力集中,致使裂纹沿晶界和碳化物/共晶界面处萌生及扩展是合金的断裂机制。

掺杂硫对IN718合金性能的影响

研究了掺杂硫对ＩＮ７１８合金的影响结果表明，硫使蠕变抗力和持久寿命降低．硫对 ＩＮ７１８合金的有害作用具有明显的表面效应。

热连轧GH4169合金组织结构与蠕变变形及断裂机制

为了研究直接时效对热连轧GH4169合金组织结构及蠕变行为的影响,通过对热连轧合金直接时效、蠕变性能测试和组织形貌观察,分析了热连轧直接时效合金的组织结构与蠕变变形及断裂机制.结果表明:经直接时效热连轧合金组织由细小晶粒构成,且具有明显的孪晶特征,细小γ″相在合金中弥散析出;在蠕变期间,合金的变形特征是形成三取向孪晶变形和位错在基体中发生单双取向滑移、起形变强化作用、使合金具有较高蠕变抗力和较长蠕变寿命的原因;随着蠕变的进行,位错逐渐在晶界处堆积并产生应力集中,促使裂纹在晶界处萌生;在蠕变后期,合金的蠕变断裂机制表现为裂纹沿晶界扩展并发生断裂.