基于微结构动态演化机制的单晶镍基高温合金晶体塑性本构及其有限元模拟

因其优异的高温力学性能,镍基单晶高温合金在航空航天和能源等领域得到了广泛的应用.镍基单晶高温合金优异的高温性能来源于其特有的两相微结构.基于代表体胞模型及分块均匀化方法,以位错密度为主要内变量,发展了一个包含两相微结构和位错演化信息的单晶镍基高温合金塑性行为的本构模型.该本构模型充分考虑了镍基单晶合金中位错在基体相和沉淀增强相中的多种演化机制,例如,基体位错八面体滑移、立方滑移、位错攀移、交滑移、位错弓出、位错切过沉淀增强相以及位错Kear-Wilsdolf(K-W)锁形成与解锁等.在商用有限元软件ABAQUS的框架下,编制了UMAT用户材料子程序.利用该用户子程序,对单晶和多晶镍基高温合金在不同温度、不同加载方向下的单调塑性、循环塑性、蠕变等典型行为进行了计算模拟.结果表明:该晶体塑性本构模型能"统一地"刻画镍基高温合金在不同温度、不同方向下的多种变形行为,并与实验结果具有良好的一致性.     

镍基单晶合金蠕变变形过程的描述

基于强化相γ’的筏化－解筏和粗化的细观过程,本文推导了一组用于描述基单在合金蠕变变形过程的蠕变方程,并在９５０℃和７６０℃标定了国产ＤＤ３镍基单晶合金不同晶体取向的模型参数,与实验结果比较,所提模型可以准确地描述蠕变的主要阶段。     

沉淀相的尺寸与形态对镍基合金蠕变行为的影响

镍基合金具有优良的高温力学性能,广泛应用于涡轮叶片等热端部件。沉淀相的尺寸和形态是影响镍基合金力学性能的重要因素。本文在考虑应变梯度的镍基合金晶体塑性本构模型的基础上,引入了各向异性损伤张量,研究了包含两种不同尺寸和三种不同长细比的沉淀相形态的镍基合金蠕变行为。结果表明,该模型能够很好地反映沉淀相的尺寸对镍基合金蠕变行为的影响,与实验结果符合较好。同时,沉淀相的形态也对镍基合金的力学性能产生重要影响,随着沉淀相长细比的增加,镍基合金的蠕变寿命延长,这体现了粗化和形态对镍基合金蠕变行为影响的一种竞争的机制。     

锆基非晶合金的动态弛豫机制和高温流变行为

非晶合金的动态弛豫机制对于理解其塑性变形, 玻璃转变行为, 扩散机制以及晶化行为都至关重要. 非晶合金的力学性能与动态弛豫机制的本征关联是该领域当前重要科学问题之一. 本文借助于动态力学分析(DMA), 探索了Zramorphous alloy,dynamic mechanical analysis,high temperature deformation,structural relaxation,quasi-points defects,1)国家自然科学基金(51971178);陕西省自然科学基金(2019JM-344);中央高校基本科研业务费专项资金(3102019ghxm007);中央高校基本科研业务费专项资金(3102017JC01003)2020-01-062020-04-10非晶合金的动态弛豫机制对于理解其塑性变形, 玻璃转变行为, 扩散机制以及晶化行为都至关重要. 非晶合金的力学性能与动态弛豫机制的本征关联是该领域当前重要科学问题之一. 本文借助于动态力学分析(DMA), 探索了Zr$_{50}$Cu$_{40}$Al$_{10}$块体非晶合金从室温到过冷液相区宽温度范围内的动态力学行为. 通过单轴拉伸实验, 研究了玻璃转变温度附近的高温流变行为. 基于准点缺陷理论(quasi-point defects theory), 对两种力学行为的适用性以及宏观力学行为变化过程中微观结构的演化规律进行描述. 研究结果表明, 准点缺陷理论可以很好地描述非晶合金损耗模量$\alpha$弛豫的主曲线. 基于非晶合金的内耗行为, 玻璃转变温度以下原子运动的激活能$U_\beta$为0.63 eV. 与准点缺陷浓度对应的关联因子$\chi $在玻璃转变温度以下约为0.38,而在玻璃转变温度以上则线性增大. Zr$_{50}$Cu$_{40}$Al$_{10}$块体非晶合金在玻璃转变温度附近, 随温度和应变速率的不同而在拉伸实验中显示出均匀的或不均匀的流变行为. 非晶合金的高温流变行为不仅可以通过扩展指数函数和自由体积理论来描述, 还可以通过基于微剪切畴(shear micro-domains, SMDs)的准点缺陷理论来描述.     

基于准点缺陷理论探索非晶合金蠕变机制

作为典型多体相互作用非平衡体系,如何明晰非晶合金多场耦合激励下变形机制,建立非晶合金变形行为、流动特性与微观结构特征本征关联始终是非晶合金力学性能的重要研究内容.本文以具有显著β弛豫行为的La_56.16Ce_14.04Ni_19.8Al₁₀非晶合金作为研究载体,通过开展宽温度应力窗口蠕变实验,着重考查了蠕变柔量、准稳态蠕变速率、特征弛豫时间、蠕变应力指数及蠕变激活能演变规律,系统研究了非晶合金蠕变行为与蠕变机制.基于准点缺陷理论分析了非晶合金蠕变行为由弹性向黏弹性及黏塑性逐步转变的过程,从微观结构演化角度构建了非晶合金蠕变行为完整物理图像.研究结果表明,非晶合金高温蠕变行为是一典型热力耦合、非线性过程,其潜在蠕变机制受控于温度、应力与加载时间.应力较低时,非晶合金蠕变机制对应于热激活单粒子流动.应力较高时,蠕变机制则对应于应力诱导局部剪切变形增强与温度诱导原子扩散等复杂耦合过程.非晶合金蠕变变形过程所涉及弹塑性转变源于非晶合金准点缺陷激活、微剪切畴经热力耦合激励形核长大、扩展与不可逆融合.     

高温条件下材料与结构力学行为的研究进展

本文回顾了最近10年来材料与结构高温力学行为若干热点问题的研究进展,包括高温下材料力学行为的多尺度特性、多组元材料的蠕变力学性能、拘束条件下的高温断裂理论、高温下的结构安全评定理论等。指出：为了保障重大高温装备的安全运行,并为高技术工艺装备的实现提供理论支撑,必须实现时间尺度与空间尺度上的事件的转换与关联,解决高温材料老化的物理化学动力学问题,理清应力应变场与化学动力学过程的相互作用;应强化材料界面缺陷及高温结合强度的微观测量方法与理论分析的研究;需要进一步研究损伤影响下的实际蠕变裂尖应力场,精确描述拘束效应的蠕变裂纹扩展速率模型,以进一步建立拘束下的高温断裂理论;在高温结构的安全评定方面,在目前的失效评定图基础上引入与材料、结构同时关联的损伤维度（时间相关）,可望实现较为复杂损伤机制下的失效评定。     

高温氧化处理前后Inconel 718高温合金摩擦学性能的探究

为了研究高温氧化处理前后Inconel 718高温合金摩擦学性能的差异,对Inconel 718高温合金在1 000℃大气环境下进行高温热处理,并进行25~800℃宽温域摩擦试验.利用扫描电子显微镜、三维轮廓仪、X射线衍射仪对Inconel 718高温合金的表面形貌、磨痕形貌、物相组成和结构进行分析.Inconel 718高温合金经热处理后物相组成发生了明显变化,晶化程度提高.在表面形成了类网状凸起结构,其组成主要为Cr_2Ti_7O_(17)和Cr_2O_3混合相,这是由于高温热处理使Inconel 718高温合金的Ti和Cr元素沿晶界发生了扩散,至表面后与大气中的氧反应,生成Cr_2Ti_7O_(17)和Cr_2O_3混合相.高温氧化后的Inconel 718高温合金摩擦系数和磨损率与未处理时相比有所降低,磨痕形貌有了明显变化,将这种变化归功于类网状凸起结构的减摩抗磨作用和其组成相中Cr_2Ti_7O_(17)的润滑作用.     

纳米SiO2颗粒增强镍基复合镀层的组织与微动磨损性能研究

采用电刷镀技术在45#钢表面制备了纳米SiO2颗粒增强镍基复合镀层,用扫描电子显微镜和透射电子显微镜观察分析了复合镀层的表面形貌和微观组织形貌,用纳米压痕仪测试了复合镀层的微观力学性能,并采用PLINT型高温微动疲劳试验机考察了复合镀层在室温至500 ℃下的微动磨损行为.结果表明:纳米SiO2颗粒促进了镀层的晶粒细化,提高了镀层的力学性能,复合镀层的硬度和弹性模量分别比镍镀层提高了2.01GPa和5 GPa,从而改善了镀层的微动磨损性能;复合镀层的耐磨性能约为镍镀层的2倍,这是由于纳米SiO2颗粒对复合镀层具有超细晶强化、硬质点弥散强化以及高密度位错强化机制所致.     

锆基非晶合金的动态弛豫机制和高温流变行为

非晶合金的动态弛豫机制对于理解其塑性变形,玻璃转变行为,扩散机制以及晶化行为都至关重要.非晶合金的力学性能与动态弛豫机制的本征关联是该领域当前重要科学问题之一.本文借助于动态力学分析(DMA),探索了Zr_(50)Cu_(40)Al_(10)块体非晶合金从室温到过冷液相区宽温度范围内的动态力学行为.通过单轴拉伸实验,研究了玻璃转变温度附近的高温流变行为.基于准点缺陷理论(quasi-point defects theory),对两种力学行为的适用性以及宏观力学行为变化过程中微观结构的演化规律进行描述.研究结果表明,准点缺陷理论可以很好地描述非晶合金损耗模量α弛豫的主曲线.基于非晶合金的内耗行为,玻璃转变温度以下原子运动的激活能U_β为0.63 eV.与准点缺陷浓度对应的关联因子χ在玻璃转变温度以下约为0.38,而在玻璃转变温度以上则线性增大.Zr_(50)Cu_(40)Al_(10)块体非晶合金在玻璃转变温度附近,随温度和应变速率的不同而在拉伸实验中显示出均匀的或不均匀的流变行为.非晶合金的高温流变行为不仅可以通过扩展指数函数和自由体积理论来描述,还可以通过基于微剪切畴(shear micro-domains, SMDs)的准点缺陷理论来描述.     

纳晶金属的力学行为

纳晶金属特指晶粒尺寸在($1 \sim100)$\,nm块体金属材料,其在力、热、声、电、磁等方面有着潜在应用,对它的制备、表征和模拟是材料科学及相关领域的重要前沿.由于纳晶金属结构简单,影响性能的因素相对单一,因而对结构与性能之间关系的理论研究具有深刻的意义.纳晶金属三维细观拓扑结构与常规多晶体类似,但由于晶粒尺寸减小,晶界原子体积比增加,因此呈现出与粗晶金属不同的性质,并且当微观物理过程的特征尺度大于晶粒尺寸时,与其对应的性质也将受到晶粒或者晶界的调制作用.本文从制备、力学性能和塑性变形机制3个方面介绍了纳晶金属力学的部分最新进展,并讨论了结构特征与力学性能之间的关系.      

Creep,plasticity, and fatigue of single crystal superalloy

Single crystal components in gas turbine engines are subject to such extreme temperatures and stresses that life prediction becomes highly inaccurate resulting in components that can only be shown to meet their requirements through experience. Reliable life prediction methodologies are required both for design and life management. In order to address this issue we have developed a thermo-viscoplastic constitutive model for single crystal materials. Our incremental large strain formulation additively decomposes the inelastic strain rate into components along the octahedral and cubic slip planes. We have developed a crystallographic-based creep constitutive model able to predict sigmoidal creep behavior of Ni base superalloys. Inelastic shear rate along each slip system is expressed as a sum of a time dependent creep component and a rate independent plastic component. We develop a new robust, computationally efficient rate-independent crystal plasticity approach and combined it with creep flow rule calibrated for Ni-based superalloys. The transient variation of each of the inelastic components includes a back stress for kinematic hardening and latent hardening parameters to account for the stress evolution with inelastic strain as well as the evolution for dislocation densities. The complete formulation accurately predicts both monotonic and cyclic tests at different crystallographic orientations for constant and variable temperature conditions (low cycle fatigue (LCF) and thermo-mechanical fatigue (TMF) tests). Based on the test and modeling results we formulate a new life prediction criterion suitable for both LCF and TMF conditions.     

Evaluation of creep damage behavior of nickel-base directionally solidified superalloys with different crystallographic orientations

A crystallographic creep damage constitutive model is developed for nickel-base directionally solidified superalloys. The rates of material degradation and grain boundary void growth are considered. The governing parameters are determined from the creep test data of single crystals and directionally solidified superalloys with a crystallographic orientation. A finite element program is used to analyze the creep damage behavior of nickel-base directionally solidified superalloys for different crystallographic orientations. The results depend on the number of grains modelled and compare well with the experimental data.     

Meso approaches for the creep damage behavior of nickel-base directionally solidified superalloys

IntroductionNickel-basehightemperatureresistancesuperalloysarsewidelyusedingasturbinesandjetengines.DireetionallysolidificationwasintroducedtoenhancecreepsbengthbyelindnahnggrainboundariesnormaltotheappliedstresswherevoidsarelikelytOoccurundercreepload.Inpractice,however,theappliedstressmaynotbeuniaxialnordirectedparalleltOthegrainboundaries.Singlecrystalswerethusdeveloped.CongregationoflowtempefAnremeltingelementsongrainboundaries,grainboundaryoxidation)etc.,areadditionalfactorsthatwoulddeg…     

A model for high temperature creep of single crystal superalloys based on nonlocal damage and viscoplastic material behavior

A model for high temperature creep of single crystal superalloys is developed, which includes constitutive laws for nonlocal damage and viscoplasticity. It is based on a variational formulation, employing potentials for free energy, and dissipation originating from plasticity and damage. Evolution equations for plastic strain and damage variables are derived from the well-established minimum principle for the dissipation potential. The model is capable of describing the different stages of creep in a unified way. Plastic deformation in superalloys incorporates the evolution of dislocation densities of the different phases present. It results in a time dependence of the creep rate in primary and secondary creep. Tertiary creep is taken into account by introducing local and nonlocal damage. Herein, the nonlocal one is included in order to model strain localization as well as to remove mesh dependence of finite element calculations. Numerical results and comparisons with experimental data of the single crystal superalloy LEK94 are shown.     

含冷却孔镍基合金次级取向效应的应变梯度晶体塑性有限元研究

单晶镍基合金具有优异的耐高温、高强、高韧等性能, 这些力学性能受制造过程引入的次级取向和冷却孔的影响. 已有研究大多关注单孔薄板的变形机理和力学性能, 而工程中应用的往往是多孔薄板, 当前亟需阐明多孔的塑性滑移带变形机理、次级取向效应以及冷却孔引起的应变梯度效应. 文章采用基于位错机制的非局部晶体塑性本构模型对含冷却孔镍基单晶薄板的单拉变形进行了数值模拟. 此模型基于塑性滑移梯度与几何必需位错的关系引入了位错流动项, 因此可有效刻画非均匀变形过程中的应变梯度效应. 为了全面揭示含孔镍基薄板的次级取向效应, 系统研究了[100]和[110]取向(两种次级取向)下镍基薄板的单拉变形行为, 并重点探究了在两种次级取向下冷却孔数量对薄板塑性行为的影响. 此外, 还分析了镍基合金板变形过程中各个滑移系上分切应力变化、主导滑移系开动以及几何必需位错密度的演化过程, 并讨论了塑性滑移量及其分布特征对不同次级取向镍基合金板强度的影响. 研究表明, 单孔和多孔的[110]薄板抗拉强度均低于[100]薄板, 多孔薄板的塑性变形过程比单孔薄板更为复杂且受次级取向影响更大, 并且发生滑移梯度位置主要位于冷却孔附近以及塑性滑移带区域. 研究结果可为工程中镍基合金的设计和服役提供理论指导.      

Superplastic characteristics of high purity yttria-stabilized zirconia polycrystals

The mechanical characteristics of superplastic yttria-stabilized zirconia polycrystals have been analyzed as a function of stress, temperature and grain size. The evolution of the stress exponent n with stress found in high purity materials is similar to that observed in superplastic metals. True creep parameters can be ascribed to the deformation mechanism at high stresses. By contrast, the creep parameters exhibit a continuous evolution with stress, temperature and grain size at low stresses. The threshold stress formalism used in conventional and high strain rate superplastic metals accounts for the mechanical characteristics observed in fine-grained zirconia polycrystals.     

Anisotropic continuum damage modeling for single crystals at high temperatures

In single crystals, the process of creep damage is generally anisotropic. Indeed, the damage evolution does not only depend on the loading conditions, but also on the lattice orientation. And the current state of damage has an anisotropic influence on the effective stress state, so that it is represented by a tensorial damage variable. Based on the continuum damage mechanics theory, a creep damage model for F.C.C. single crystals has been developed and implemented in a three-dimensional anisotropic creep model. It is shown that the resulting material model is capable of describing the orientation dependence of the creep and damage evolution of nickel-based superalloys in the high temperature regime.     

A phase field model incorporating strain gradient viscoplasticity: Application to rafting in Ni-base superalloys

The first formulation of a phase field model accounting for size-dependent viscoplasticity is developed to study materials in which microstructure evolution and viscoplastic behavior are strongly coupled. Plasticity is introduced using a continuum strain gradient formalism which captures the size effect of the viscoplastic behavior. First, the influence of this size effect on the mechanical behavior of the material is discussed in static microstructures. Then, the dynamic coupling between microstructure evolution and viscoplastic activity is addressed and illustrated by the rafting of the microstructure observed in Ni-base superalloys under creep conditions. It is found that the plastic size effect has only a moderate impact on the shape of the rafts but is crucial to reproduce the macroscopic mechanical behavior of that particular material.