DISLOCATION PINNING IN HIGH TEMPERATURE DEFORMED Ni3(Al,Ti) SINGLE CRYSTALS CONTAINING DISORDERED γ PRECIPITATES

A transmission electron microscopy (TEM) investigation has been performed on the dislocation pinning in LI2-ordered Ni3(Al, Ti) containing disordered γ precipitates. The morphology of deformation induced dislocations in the γ' base alloys containing fine dispersion of disordered γ was investigated by means of weak-beam electron microscopy. The superdislocations are strongly attracted to the disordered particles and dissociate on the (111) plane in the γ particles, while they dissociate on the (010) plane in the γ' matrix. The disordered γ precipitates play an important role as a pinning point during the cross-slip of superdislocations from (111) to (010) planes in the γ'matrix and restrain the cross-slip of superdislocations. The interaction of superdislocations with disordered particles causes the formation of superkinks, jogs and closed loops.     

RETRACTED ARTICLE: Dislocation-particle interaction in precipitation strengthened Ll<Subscript>2</Subscript>-ordered Ni<Subscript>3</Subscript>Al

Transmission electron microscope investigation has been performed on the particle-dislocation interactions in Ni₃Al-based intermetallics containing various types of fine precipitates. In an Ll₂-ordered Ni₃Al alloy with 4 mol.% of chromium and 0.2–0.5 mol.% of carbon, fine octahedral precipitates of M₂₃C₆ type carbide, which has a cube-cube orientation relationship with the matrix, appear during aging. Typical Orowan loops are formed in Ni₃Al containing fine dispersions of M₂₃C₆ particles. In the alloys with appropriate titanium content, fine precipitates of coherent disordered γ are formed during aging. The γ precipitates are initially spherical or rounded cubic in shape and grow into platelets as aging proceeds. Loss of coherency is initiated by the introduction of dislocations at the γ/γ′ interface and results in step formation at the dislocations. The γ precipitates become globular after the loss of coherency. In the γ′ phase hardened by the precipitation of the disordered γ phase, dislocations are attracted into the disordered γ phase and cut through the particles during deformation at any stage of aging. In Ni₃Al containing a fine dispersion of disordered γ, superdislocations are strongly attracted to the disordered particles and dissociate on the (111) plane in the γ particles, while they dissociate on the (010) plane in the matrix. It is shown by comparison that the strengthening due to attractive interaction is more effective than that due to repulsive interaction. The roles of the variation of the interaction modes and of the dissociation of superdislocations in the matrix and particles are discussed in connection with the optimum microstructures of Ll₂-ordered intermetallics as high temperature structural materials.     

Cyclic deformation behaviour of γ′-Ni3(Al,Ti) single crystal containing disordered γ precipitates

Effect of disordered γ precipitates on the cyclic deformation behaviour of γ′-Ni₃(Al,Ti)-based single crystals with L1₂ structure was investigated. The γ precipitates with fine spherical and coarse plate-like shapes were formed in the γ′-based single crystals by annealing at 1073 K for 3 h (peak-aged) and 100 h (over-aged), respectively. The over-aged crystal shows stronger cyclic hardening than does the peak-aged crystal, although the yield stress of the peak-aged crystal is higher than that of the over-aged one. In these two crystals the precipitates were sheared by superlattice dislocations dissociated into superpartials. The separation distance between superpartials increased in the precipitates because there was no APB in the disordered phase and the cyclic hardening was enhanced in the aged crystal. The size of the precipitates in the over-aged crystal was reduced by shear deformation resulting in acceleration of the cyclic hardening. Dissolution of γ phase into γ′ matrix did not occur during cyclic loading and cyclic softening did not appear.     

Phase field modeling of channel dislocation activity and γ′ rafting in single crystal Ni–Al

Dislocation-precipitate interaction and directional coarsening (rafting) of γ′ precipitates in single crystal Ni–Al alloys under external load are investigated by three-dimensional computer simulations. The simulation technique is based on an integrated phase field model that characterizes simultaneously the spatiotemporal evolution of both precipitate microstructure and dislocations. The initial configurations consisting of cuboidal γ′ particles and dislocations in γ channels are constructed according to experimental observations and phase field simulations of the dislocation filling process in the γ channels. For a given state (sign and magnitude) of the lattice misfit and external load commensurate with experimental values, the predicted morphologies of the rafted γ′ precipitates and the rafting kinetics agree well with experimental observations. This indicates that plasticity plays a dominant role in the rafting process as compared to elastic modulus mismatch between γ and γ′ phases, which has been ignored in the present simulations. The spatial variation of chemical potential of solute atoms caused by the coupling between channel dislocations and misfit stress is evaluated from concentration and stress distributions, from which diffusion fluxes in the γ channels are analyzed.     

[111]取向镍基单晶合金的组织结构与蠕变行为

通过蠕变曲线的测定及微观组织形貌观察,研究[111]取向镍基单晶合金在高温低应力条件下的组织结构与蠕变行为。结果表明:经完全热处理后,[111]取向单晶合金的组织结构是立方γ′相以共格方式嵌镶在γ基体中,并沿100方向规则排列;在1 040~1 080℃、137~180 MPa的温度和应力范围内,该取向单晶合金表现出明显的温度和施加应力敏感性。蠕变期间,γ′相沿与应力轴呈一定角度形成筏状组织,蠕变后期在近断口区域筏状γ′相发生粗化和扭折。[111]取向单晶合金在蠕变期间的变形特征是位错在γ基体通道中运动和剪切筏状γ′相,由于形变量较大,较多位错切入筏状γ′相后使其形成亚晶结构,其中,蠕变后期大量位错切入筏状γ′相导致合金的蠕变抗力降低,是合金发生蠕变断裂的主要原因。     

Inconel783高温合金螺栓的组织与织构

研究了Inconel783高温合金螺栓的组织结构与织构特点,重点关注β相的分布与析出特征,并研究了高温应力松弛前后组织与织构的变化情况。结果表明,螺栓试样γ相晶粒分布较为均匀,同时存在弥散分布的γ′相析出。大颗粒的β相沿螺栓轴向成串分布,其分布特征与γ相晶粒取向及尺寸无关,主要由冶炼方式与热加工过程决定;较小的β相在γ相晶界与晶内、连续或孤立析出。织构方面,γ相与β相均未表现出强织构,与热加工过程中变化的多向应力状态有关。以螺栓轴向(x方向)为试样参考方向,γ相存在一定的<111>//x与<100>//x择优,其中前者强度稍高。高温应力松弛试验后,γ相组织、β相分布与各相织构均较为稳定。对该合金中发挥重要作用的小尺寸β相分析发现,其析出行为受到γ相基体取向与晶界特征影响,其中<111>取向晶粒内析出较多,孪晶界上β相析出不占优势。因此,通过控制热加工与固溶处理过程中γ晶粒尺寸与取向择优,有望调控其析出特征。     

Discontinuous Precipitation of γ′ Phase in Ni-Co-Al Alloys

Decomposition of nickel-base alloys by precipitation of the γ′ phase occurs either continuously (homogeneously) or discontinuously. Under certain conditions of solute content and temperature, discontinuous precipitation is observed. Ni-Al-Co alloys have been characterized by coupling atom probe tomography with transmission electron microscopy studies. The primary focus was to investigate the discontinuous precipitation of γ and γ′ phases. When subjected to fast quenching after solution treatment, the γ′ precipitates exhibit a near-spherical shape and monomodal size distribution with an average size of less than 5 nm. After early stage annealing at 600°C for 10 min, discontinuous precipitation nucleated near the grain boundaries while some regions of homogeneous γ′ precipitates were observed. Discontinuous γ + γ′ product was completely transformed throughout the grain after 600°C/1 h. On long-term annealing (600°C/256 h), coarser γ + γ′ lamellae products replaced the fine discontinuous products that exhibited after 1 h annealing at 600°C. Equilibrium compositions of the γ and γ′ phases were achieved in this coarsening stage. The γ′ phase has an Al content of 25 at.%, which is consistent with the as-quenched condition where the (Ni + Co)₃Al stoichiometry is maintained after 600°C/256 h annealing.     

HARDENING OF L1_2-ORDERED Co_3Ti BY PRECIPITATION OF FCC-Co PHASE

1.IntroductionLlz-orderedC03Tihasat6ractedm1lchattentionbecauseofitsp0sitivetemperaturedependenceofstrengthI1t2],thehighesttemperatureatwhichthestrellgthattainsitshigh-estval..[2]andhighductilityoverawiderangeoftemperature[3].Thestrellgthlevel,however,isnotashighasthat0fNi3ALbasedall0ys[2].Acc0rdingt0thereI,ortedCo-Tibinaryphasediagram[']?theryi-C.,Tiphaseexistsoveracompositi0nrangeapproximatelyfrom2Ot025m0l%TiasshowninFig.l.Thes0lubility0fTiinCohasbeenwellestab-lishedbecausetheprecipit…     

Creep Properties and Deformation Mechanisms of a FGH95 Ni-based Superalloy

By means of full heat treatment, microstructure observation, lattice parameters determination, and the measurement of creep curves, an investigation has been conducted into the microstructure and creep mechanisms of FGH95 Ni-based superalloy. Results show that after the alloy is hot isostatically pressed, coarse γ′ phase discontinuously distributes along the previous particle boundaries. After solution treatment at high temperature and aging, the grain size has no obvious change, and the amount of coarse γ′ phase decreases, and a high volume fraction of fine γ′ phase dispersedly precipitates in the γ matrix. Moreover, the granular carbides are found to be precipitated along grain boundaries, which can hinder the grain boundaries’ sliding and enhance the creep resistance of the alloy. By x-ray diffraction analysis, it is indicated that the lattice misfit between the γ and γ′ phases decreases in the alloy after full heat treatment. In the ranges of experimental temperatures and applied stresses, the creep activation energy of the alloy is measured to be 630.4 kJ/mol. During creep, the deformation mechanisms of the alloy are that dislocations slip in the γ matrix or shear into the γ′ phase. Thereinto, the creep dislocations move over the γ′ phase by the Orowan mechanism, and the $ \left\langle { 1 10 } \right\rangle $ 〈 1 10 〉 super-dislocation shearing into the γ′ phase can be decomposed to form the configuration of (1/3) $ \left\langle { 1 12 } \right\rangle $ 〈 1 12 〉 super-Shockleys’ partials and the stacking fault.     

Mechanism of α-Cr Formation in Nickel-Base Superalloy INCONE1718 after Long Time Thermal Exposure

It is commonly accepted that the drop in mechanical properties of INCONE1718(IN718) after long time exposure at high temperature is a result of coalescence ofγ", γ and the transformation ofγ′ to δ-Ni3Nb phase. Besides the γ′, γ and δ,the precipitation of B.C.C. α-Cr is another factor that affects the mechanical properties of IN718. In this paper, the Thermal-Calc software is used to reveal the thermodynamic condition of α-Cr formation. The calculated sections of the pseudo ternary diagrams of Cr-Nb-IN718 and Cr-Fe-IN718 explain why α-Cr particles do not precipitate dispersedly in γmatrix but in the vicinity of δ phase. The selected area diffraction (SAD) of TEM confirms the crystallography relationship of α-Cr with δ is (010) δ// (-110) α-Cr, [100] δ//[111] α-Cr° In combination of TTT diagram with the Thermal-Calc and TEM results, the mechanism of α-Cr formation in IN718 is suggested.     

TENSILE CREEP DEFORMATION AND DAMAGE BEHAVIOR IN A NICKEL-BASE SUPERALLOY AT 900℃

The tensile creep deformation and damage evolution in a Ni-base superalloy at 900℃/170MPa were investigated. At the first creep stage, abnormal creep occured due to the resolution of fine particles, and the deformation initiated from grain boundary areas. It is evident that nearly all of the dislocations were in γ matrix channels in form of dislocation pairs and the dislocations were impeded at γ/γ′ interfaces, thus the dislocation networks developed deformation. At the steady creep stage, impeded dislocations at γ /γ′ interfaces climbed over γ′ phases by diffusion-dominant mechanism.At the last creep stage, voids were formed around carbides at grain boundary which leaded to accumulated damage and caused creep rate accelerated. With the dislocation networks being broken, the voids connected and grew into micro-cracks gradually.Finally the cracks propagated along grain boundary area and resulted in failure.     

A new phase-field method for simulating γ′ precipitation in multicomponent nickel-base superalloys

The phase-field method is used to develop a new technique to simulate γ′ precipitation in multicomponent Ni-base superalloys. This technique generates the Gibbs free energy for both the γ and γ′ phases from the single function formalism of both phases in the CALPHAD method without using conventional CALPHAD software. The Gibbs free energy surfaces for eight-element multicomponent systems are used to confirm that there is an accurate coupling with the CALPHAD method, because they compare favorably with calculated results from Thermo-Calc. In addition, γ′ precipitation in a third-generation Ni-base superalloy containing eight components is simulated in one dimension with anti-phase effects of the γ′ phase on the precipitate interactions. The simulation results reveal late-stage enrichments of Re and W in the γ phase during coalescence of the γ′ precipitates.     

Ni75Al15Mn10合金γ′相沉淀过程以及原子占位的微观相场模拟

基于微观相场模型和微观弹性理论,对Ni75Al15Mn10合金γ′相沉淀过程以及原子占位进行了原子层面的计算机模拟。结果表明:合金在1273K进行时效,沉淀早期先析出L10结构,之后随着有序度的增加,逐渐转变为L12结构;原子的有序化早于成分簇聚,γ′相的沉淀机制为等成分有序化+失稳分解的混合机制;γ′有序相的体积分数比γ无序相小,且γ′和γ相的体积分数比值约为60%;Al原子主要占据β格点(γ′相顶角位置),αⅡ和αⅠ格点主要由Mn原子占据,且在αⅡ格点占位几率高于αⅠ格点,Mn原子主要占据Ni位,形成的γ′相为单一的(Ni,Mn)3Al相。     

喷射沉积Ni3Al-Mo合金的位错结构分析和断裂特性

利用多功能雾化沉积装置,制备了Ni3Al-Mo高温合金.拉伸性能测试表明:喷射沉积Ni3Al-Mo合金的屈服强度有明显的"R"特性,即喷射沉积合金的屈服强度随着温度的增加而增加,在760 ℃时达到最大值;随着温度的继续增加,屈服强度会逐渐降低.利用TEM对位错结构分析,阐明了合金屈服强度与微观结构之间的关系.研究结果表明,试验合金所具有的屈服强度随温度的变化规律主要是由不同温度下γ′及γ相内的位错缠结状况、位错对的形态及数量和六面体滑移系的开动程度决定的.裂纹的TEM分析表明,随着变形应力的增加,裂纹在晶界部位产生,然后迅速扩展,而且在不同的晶面内存在一些不同的滑移带.喷射沉积合金在室温、中温及高温下裂纹严格沿八面体滑移系(111)面开裂.     

时效温度对0Cr15Ni70Ti3AlNb合金组织和性能的影响

采用相分析、SEM、万能拉伸试验机等手段,研究了不同时效温度对0Cr15Ni70Ti3AlNb合金组织和性能的影响。结果表明:0Cr15Ni70Ti3AlNb合金在不同的时效温度下析出相有MC相、M23C6相和γ′相。MC和M23C6碳化物在650、670、690℃时的含量基本保持不变。随着时效温度的升高,γ′相的含量和尺寸不断增加。时效温度从650℃升高到720℃,γ′相质量分数由4.971%增加至10.744%,γ′相晶粒尺寸由11.0 nm增大到38.8 nm。在650℃保温14 h后,基体内部析出细小的球状γ′相,当时效温度为750℃时,合金内部出现链状的γ′相,当时效温度为810和840℃时,合金中存在方形的γ′相。随着时效温度的升高,合金室温抗拉强度和屈服强度呈现先增高后减小的趋势,当时效温度高于750℃后,室温抗拉强度和屈服强度均迅速下降,时效温度为720℃时,合金的冲击韧性值最小。     

GH4096合金在长期热暴露过程中的组织演化

基于粉末冶金René88DT合金的成分,采用新型定向凝固铸锭变形工艺研制了GH4096合金,并研究了其固溶时效热处理后在700~900 ℃长期热暴露过程中γ′相的粗化和晶界析出相的析出行为。结果表明:GH4096合金在700 ℃热暴露时具有良好的组织稳定性,晶内γ′相的尺寸稳定在70 nm左右,未发现明显的长大行为,晶界上未发现析出相;750 ℃热暴露500 h以上时,二次γ′相开始出现聚集长大现象,晶界开始出现不连续析出相,但γ′相的长大速度和晶界析出相的析出速度均比较低;800 ℃以上长期热暴露时,γ′相的粗化速度和晶界析出相的析出长大速度明显加快。长期热暴露后晶内二次γ′相的尺寸与Larsen-Miller参数近似呈线性关系,晶界析出相以M₃B₂和μ相为主。     

Phase-field simulation of coarsening of γ precipitates in an ordered γ′ matrix

The evolution of the microstructure and coarsening kinetics of disordered γ precipitates in an ordered monovariant γ′ matrix was studied using a diffuse-interface phase-field model in two dimensions. Specifically, we studied the morphological evolution, average precipitate size and size distribution as a function of time for a given temperature and for different concentrations. It was found that the coarsening kinetics for the average particle size are described by a t^1/3 power law with a rate constant that increases with the volume fraction of the γ phase. The particle size distribution scaled by the average particle size is time invariant. We find that the particle shapes are perturbed by elastic interaction and local distributions of γ particles. Comparison of the phase-field simulation results with experiments shows good qualitative agreements.     

Stability of γ′ precipitates in a PWA1480 alloy

In the nickel-based superalloy PWA1480, 5–10-nm diameter γ precipitates form during heat treatment at 871 °C within the primary cuboidal γ′ precipitates that formed at high temperature (1079 °C). The stability of these γ precipitates during extended aging at 871 °C was investigated by atom probe tomography, computational thermodynamics, and diffusion-controlled growth kinetic modeling. The results show that these γ precipitates are not in thermodynamic equilibrium in the alloy and dissolve during extended aging. Thermodynamic and kinetic calculations indicate that the formation of these non-equilibrium precipitates is related to local concentration fluctuations in the primary γ′ phase that developed during diffusion-controlled growth of γ′ at high temperature.     

Deformation behaviour of an advanced nickel-based superalloy studied by neutron diffraction and electron microscopy

Deformation mechanisms under tensile loading at room temperature have been studied in a polycrystalline nickel-based superalloy containing close to 50 vol.% γ′. In order to identify the effect of γ′ particle size on deformation mechanisms, model microstructures with unimodal γ′ size distributions were developed. The investigations were carried out by combining in situ loading experiments using neutron diffraction and two-site elasto-plastic self-consistent plasticity modelling with detailed post-mortem electron microscopy. The microscopy work also includes results for samples strained at 500 °C. During early plastic deformation, the diffraction data demonstrate that γ and γ′ display the same elastic strain response, indicating that at this stage γ′ is cut by dislocations regardless of the γ′ particle size. Scanning electron microscopy studies showed an abundance of shearing processes in all three microstructures, hence supporting the conclusions drawn from the diffraction experiment. As the material is further deformed, elastic load transfer from γ to γ′ was observed in the medium (130 nm) and coarse (230 nm) γ′ microstructures but not in the fine (90 nm) γ′ microstructure. The load transfer can be explained by assuming that Orowan looping becomes an additional operative deformation mode. Transmission electron microscopy confirmed that in the fine γ′ microstructure deformation takes place by strongly coupled dislocations cutting the γ′, while the medium and coarse γ′ microstructures showed additional signs of Orowan looping.     

Dislocation decorrelation and relationship to deformation microtwins during creep of a γ′ precipitate strengthened Ni-based superalloy

The evolution of microtwins during high temperature creep deformation in a γ′ strengthened Ni-based superalloy has been investigated through a combination of creep testing, transmission electron microscopy (TEM), theoretical modeling, and computer simulation. Experimentally, microtwin nucleation sources were identified and their evolution was tracked by characterizing the deformation substructure at different stages of creep deformation. Deformation is highly localized around stress concentrators such as carbides, borides and serrated grain boundaries, which act as sources of a/2〈1 1 0〉 matrix-type dislocations. Due to fine channels between the γ′ particles, coupled with a low γ matrix stacking fault energy, the a/2〈1 1 0〉 matrix dislocations dissociate into a/6〈1 1 2〉 Shockley partials, which were commonly observed to be decorrelated from one another, creating extended intrinsic stacking faults in the γ matrix. Microtwins are common and form via Shockley partial dislocations, cooperatively shearing both the γ and γ′ phases on adjacent {1 1 1} glide planes. The TEM observations lead directly to an analysis of dislocation–precipitate interactions. The important processes of dislocation dissociation and decorrelation were modeled in detail through phase field simulations and theoretical analyses based on Orowan looping, providing a comprehensive insight into the microstructural features and applied stress conditions that favor the microtwinning deformation mode in γ′ strengthened Ni-based superalloys.