Oxidation of ZrB<Subscript>2</Subscript> and its composites: a review

The oxidation behavior and oxidation mechanisms of monolithic ZrB₂ and particulate-ZrB₂ matrix composites were reviewed. Dispersion of SiC particles into ZrB₂ was found to be an effective way to prevent extensive oxidation. However, the formation of a SiC-depleted layer can become a critical problem because it can lead to spallation and delamination of the protective surface layer. The addition of ZrC in conjunction with rapid heating to temperatures higher than 2000 °C effectively reduced the porosity of the SiC-depleted layer. The formation of a dense surface layer was attributed to large volumetric expansion during the conversion from ZrC to ZrO₂. The effect of the ZrC addition depended on the temperature, heating rate, and composition. This review showed that material design for specific applications is required for high-temperature applications to maximize the oxidation resistance of ZSZ composites.     

A technique for ultrahigh temperature oxidation studies of ZrB2-SiC

A new oxidation method, high frequency induction heating, was introduced to investigate the rapid oxidation characteristics of ZrB₂-SiC at 1800 °C up to 50 min. A high temperature was achieved within 30 s by adjusting the current or voltage. According to the isothermal test at 1800 °C, the oxidation kinetics for the composite followed a para-linear law. The specific weight gains increased from 4.86 to 12.89 mg/cm² when the exposure time increased from 10 to 50 min. When exposure for 30 min, a thin outmost layer of SiO₂, a ZrO₂ layer and a thick SiC-depleted layer formed on the surface of specimens. The structural characteristic of oxidation layer obtained by high frequency induction heating was similar to those formed by standard oxidation furnace.     

高温氧化对超高温陶瓷材料耦合传热的影响

超高温陶瓷材料暴露于极端高温飞行环境中会导致其发生氧化,表面生成的氧化物具有不同的热物性从而对传热过程造成影响。针对预氧化的ZrB₂和ZrB₂-SiC,基于氧化模型预测氧化层(ZrO₂、B₂O₃、SiO₂和SiC耗尽层)厚度,利用有限元建立圆柱形代表性体积单元,并与外部高超声速流场的CFD (Computational Fluid Dynamics)求解器相耦合,研究了高温氧化对超高温陶瓷材料的耦合传热的影响。计算中采用分区求解方法,通过耦合界面处非匹配网格间的插值完成实时数据交换,实现了基于Navier-Stokes方程的流动求解器与有限元求解器的多场耦合计算。ZrB₂、ZrB₂-SiC以及氧化生成物的热物性均为温度相关,通过理论计算给出了B₂O₃挥发及SiC耗尽导致的多孔结构的有效热导率和有效比热容。瞬态耦合传热分析的结果表明:ZrB₂在预氧化后其热阻能力略有提高, ZrB₂-SiC氧化前后的热阻变化很小,并且在相同流动环境条件下,氧化后ZrB₂的热阻能力高于氧化后ZrB₂-SiC的热阻能力。      

Cyclic oxidation of high-temperature alloys

Abstract

Many applications of high-temperature alloys involve high-temperature oxidation under thermal cycling conditions. The oxide scales formed have lower coefficients of thermal expansion than the metallic alloys and significant thermal stresses can arise during temperature variations. These thermal stresses add to the residual growth stresses which accompany oxide formation. Under certain conditions, stresses in oxide scales may be partly relieved by plastic deformation of scale or alloy. However, when these mechanisms cannot be operative, scale buckling or cracking occur depending on interfacial and oxide fracture strengths. Eventual oxide spallation causes rapid degradation since depleted regions of alloy are in contact with the oxidizing atmosphere. Incorporation of ‘active elements’ such as yttrium, in reducing the residual growth stresses significantly improves the cyclic oxidation resistance of high-temperature nickel-, cobalt-, and iron-base alloys. The present paper attempts to review briefly the mechanisms involved in these phenomena and the tentative cyclic oxidation models. MST/443     

基于梯度降温的叠层制备热残余应力

针对叠层制备工艺的热残余问题,为消除传统的基于同步降温假设的理论解与实际热残余现象的差异,本文在充分考虑成形过程中沿长度和厚度方向形成的温度梯度的基础上,分别建立在层平面和厚度方向引起的热残余变形和应力的解析解,并根据不同叠层制备工艺,将降温梯度概括为同步降温、均等梯度降温、非均等梯度降温、瞬态降温的4种模式.算例表明,梯度降温会造成在层平面和厚度方向均产生热残余现象.讨论了4种梯度降温模式对热残余程度的影响,梯度越大影响越大;合理解释了同一种材料制备的工件也会因降温梯度而产生明显的弯曲变形;对于梯度材料,叠层制备顺序会显著影响热残余的程度.研究表明,梯度降温假设符合实际制备、工艺,更准确地揭示了叠层制备热残余现象产生的机理,优化制备工艺缩小降温梯度是解决热残余问题的有效途径.     

Geometry effects of substrate and coating layers on the thermal stress response of FGM structure

Summary Due to transient temperature change, the plane strain elastic-plastic problem for a functionally graded material (FGM) bonded to a homogeneous coating layer and a metal substrate is considered by the use of the finite element method (FEM). The substrate and the coating are assumed to be aluminum and partially stabilized zirconia, respectively. The FGM layer is a particulate composite of aluminum and partially stabilized zirconia with volume fractions continuously varying through the thickness. Generally in high temperature applications, the FGM system is sandwiched between a substrate layer and a coating layer. The coating layer increases the protection from heat but decreases the thermal shock resistance while the substrate layer increases the rigidity of the structure and decreases strength-related properties at high temperature. In order to compromise the thickness of both the coating and substrate layers, different values of the substrate and coating thickness are studied in order to evaluate their effects on the thermal stress response of the FGM structure. Since the main objective of the FGMs is using them in different applications with severe thermal loading conditions, the thermal stresses may be so high that some reinforcements may be fractured and/or debonded from the matrix giving a weakening effect instead of a reinforcing one. Hence, the behaviors of the reinforcements and the matrix are essential to be studied. In this regard, microscopic constitutive equations along with the temperature-dependent properties of the constituent materials are considered to enable us obtaining more realistic results of thermal stresses. Since the FGM structures are fabricated at high temperatures, thermal residual stresses are produced. In order to find out the importance of the consideration of the residual stresses arising from the fabrication process, the FGM structure with stress-free conditions is heated to the operating temperature, and its thermal stress response is compared with that one where the residual stresses are taken into account. Also, several functional forms of gradation of the constituents in the FGM layer are examined to reach the optimum profile giving the minimum stress level for the FGM structure under thermo-elasto-plastic behavior.     

Determination of residual stresses in three-layer alumina composites using an indentation technique

The distribution of residual stresses in three-layer reaction bonded alumina (RBAO) composites with 17.5 vol % zirconia in the inner layer and 40 vol % mullite in the outer layers was determined by an indentation technique. It could be shown that the outer layers are subject to a residual compressive stress while the inner layer is under a tensile stress. With increasing outer layer thickness, the residual compressive stress decreases, and the residual tensile stress increases. Compressive stresses up to 550 MPa were measured in this study. It was found that the residual stresses are not uniform throughout the layers. The magnitude of the stresses decreases with increasing distance from the interfaces. This is attributed to creep during cooling from the sintering temperature. This revised version was published online in September 2006 with corrections to the Cover Date.     

Finite element analysis of the effect of welding heat input and layer number on residual stress in repair welds for a stainless steel clad plate

Stainless steel clad plate is widely used in petroleum, chemical and medicine industries due to its good corrosion resistance and high strength. But cracks are often formed in clad layer during the manufacture or service, which are often repaired by repair welding. In order to ensure the structure integrity, the effects of residual stress need to be considered. The objective of this paper is to estimate the residual stress and deformation in the repair weld of a stainless steel clad plate by finite element method. The effects of heat input and welding layer number on residual stresses and deformation have been studied. The results show that large residual stresses have been generated in the repair weld. The heat input and layer number have great effects on residual stress distribution. With the heat input and welding layer number increasing, the residual stresses are decreased. Using multiple-layer welding and higher heat input can be useful to decrease the residual stress, which provides a reference for optimizing the repair welding technology of this stainless steel clad plate.     

Prediction of residual stresses in thermally sprayed steel coatings considering the phase transformation effect

The present study is aimed to propose an elastoplastic bilayer model for prediction of residual stresses in thermally sprayed coatings, in which the effect caused by martensite phase transformation for steel coating materials was taken into account. Closed-form solutions of the curvature and stresses within the substrate and coating are obtained for the plastically deformed structures. Applications of the model for prediction of the twin-wire electric arc sprayed high carbon steel coatings were investigated subsequently. In the application case that a thin coating layer deposited with varying temperatures, the martensite phase transformation has a significant effect on the residual stress, e.g. a lower deposition temperature leads to more amount of martensite transformation and then to a lower level of final stress. The model was also used to predict the stress distribution of high carbon steel coating after quenching heat treatment, and the calculation results were compared with the X-ray residual stress measurements. It is found that the residual stresses on the coating surface obtained from the analytical model are closed to that obtained from the experiments.     

Parametric studies of multi-material laser densification

Fabrication of multi-material components via a laser-assisted layer-by-layer fabrication process has been numerically simulated and analyzed using a three-dimensional thermo-mechanical model. Effects of the chamber preheating temperature, laser scanning rate, initial porosity and thickness of each powder layer on the out-of-plane warping and residual thermal stresses of a nickel/porcelain workpiece have been investigated. It is found that warping and residual thermal stresses of the laser-densified multi-material workpiece are more sensitive to the chamber preheating temperature and the thickness of each powder layer than to the laser scanning rate and the initial porosity of the powder layer. The major mechanism responsible for these phenomena is identified to be related to the change of the temperature gradient induced by these laser processing parameters.     

挤压管状SiCw/Al复合材料中残余应力及其高温松弛行为

利用X射线应力测量方法,研究挤态20vol%SiCw/6061Al复合材料中残余应力,发现复合材料中存在较大的残余应力,而且各方向的残余应力分布很不均匀,动态测量去应力退火期间复合材料残余应力的高温松弛过程,证实高温状态下残余应力按幂指数的方式发生松弛,基于蠕变机制,分析残余应力搞温松弛行为,结果表明复合材料应力指数及应力松弛激活能明显高于基体合金。     

K4104镍基高温合金Al-Si涂层高温氧化过程中的元素扩散分析

采用无机盐料浆法在K4104镍基高温合金表面制备Al-Si涂层。依据GB/T13303-91《钢的抗氧化性能测定方法》标准,采用静态增重法对有涂层试样和无涂层试样进行了1000℃×200h抗高温氧化性能试验,并绘制了氧化动力学曲线。用带能谱扫描电镜对氧化膜的表面形貌和截面组织进行分析,研究有无涂层试样在高温氧化过程中的元素扩散。结果表明:Al-Si涂层和基体合金之间在高温氧化过程中的互扩散形成了厚度为120~140um的渗层。随着氧化时间的延长,外层铝含量逐渐降低,但仍能保持稳定的β-NiAl相。Si在扩散作用下形成内高外低的分布形式,形成的Cr3Si和富Si的M6C相有利于阻止涂层和基体元素之间的互扩散,降低化合物层的形成速度,体现了Al-Si涂层良好的抗高温氧化能力。     

面齿轮磨削齿面力热耦合及残余应力研究

根据面齿轮磨削残余应力的产生机理和Prandtl-Reuss方法,建立磨削表层热弹塑性力学本构关系;基于面齿轮磨削方法和Gleason接触原理,得出碟形砂轮磨削点接触椭圆方程参数、磨削力和磨削热流量的数学模型。构建面齿轮磨削单齿3D有限元模型,采用小步距移动法模拟磨削载荷的移动,仿真磨削温度场,得到磨削瞬态最高温度位于...     

层状ZrB2-SiC-G陶瓷在1300℃下的抗氧化性及其残余强度的研

采用流延-层叠-热压的方法制备了不同石墨界面层的层状ZrB₂-SiC-G超高温陶瓷, 研究其在1300℃下氧化0.5~10 h的氧化行为以及氧化时间对室温残余强度的影响, 观察了增重量随氧化时间的变化, 并对其物相和形貌进行了分析。结果表明, 石墨界面层中添加40vol% ZrB₂和10vol% SiC制备的层状ZrB₂-SiC-G超高温陶瓷, 室温弯曲强度为670 MPa, 断裂韧性为13.7 MPa·m^1/2, 氧化10 h后弯曲强度仍达429MPa, 断裂韧性仍达10.25 MPa·m^1/2, 优异的残余强度是由于表面形成了致密的SiO₂玻璃层而阻止了材料内部被氧化。     

Synergistic Effects of Temperature and Oxidation on Matrix Cracking in Fiber-Reinforced Ceramic-Matrix Composites

In this paper, the synergistic effects of temperatrue and oxidation on matrix cracking in fiber-reinforced ceramic-matrix composites (CMCs) has been investigated using energy balance approach. The shear-lag model cooperated with damage models, i.e., the interface oxidation model, interface debonding model, fiber strength degradation model and fiber failure model, has been adopted to analyze microstress field in the composite. The relationships between matrix cracking stress, interface debonding and slipping, fiber fracture, oxidation temperatures and time have been established. The effects of fiber volume fraction, interface properties, fiber strength and oxidation temperatures on the evolution of matrix cracking stress versus oxidation time have been analyzed. The matrix cracking stresses of C/SiC composite with strong and weak interface bonding after unstressed oxidation at an elevated temperature of 700 °C in air condition have been predicted for different oxidation time.     

The influence of hygrothermal conditions on the damage processes in quasi-isotropic carbon/epoxy laminates

The effects of hygrothermal conditions on damage development in quasi-isotropic carbon-fiber/epoxy laminates are described. First, monotonic and loading/unloading tensile tests were conducted on dry and wet specimens at ambient and high temperatures to compare the stress/strain response and damage development. The changes in the Young's modulus and Poisson's ratio were obtained experimentally from the monotonic tensile tests. The critical stresses for transverse cracking and delamination for the above three conditions are compared. The delamination area is measured by using scanning acoustic microscopy (SAM) at various loads to discuss the effects of delamination on the nonlinear stress/strain behavior. Next, the stress distributions under tensile load including hygrothermal residual stresses are computed by a finite-element code and their effects on damage initiation are discussed. Finally, a simple model for the prediction of the Young's modulus of a delaminated specimen is proposed. It is found that moisture increases the critical stresses for transverse cracking and delamination by reducing the residual stresses while high temperature decreases the critical stresses in spite of relaxation of the residual stresses. The results of the finite-element analysis provide some explanations for the onset of transverse cracking and delamination. The Young's modulus predicted by the present model agrees with experimental results better than that predicted by conventional models.     

树脂基纤维复合材料的热残余应力数值分析

以环氧树脂R368-1/硼纤维复合材料为研究对象,采用柱体单胞结构,建立了三维有限元分析模型。考虑试样加工制备过程和常温使用时的温度差,对残余应力分布特点和应力水平进行了讨论,给出了应力分布云图和应力沿径向的分布规律。进一步考察了纤维体积分数、温度差和附加界面层对残余应力分布的影响,结果表明,基体主要受拉伸应力作用,纤维主要受压缩应力作用,纤维体积分数增加和附加界面层有助于改善复合材料中残余应力的分布,试样制备温度的升高对纤维中应力的增加具有较大影响。     

More on the effects of thermal residual and hydrostatic stresses on yielding behavior of unidirectional composites

The influence of manufacturing process thermal residual stresses and hydrostatic stresses on yielding behavior of unidirectional fiber reinforced composites has been investigated when subsequently subjected to various mechanical loadings. Three-dimensional finite element micro-mechanical models have been used. The results of this study reveal that the size of the initial yield surface is highly affected by the thermal residual and hydrostatic stresses. It was also found that effects of a uniform temperature change on the initial yield surface in the composite stress space is not equivalent to a solid translation of the surface in the direction of the hydrostatic stress axis. At the micro-level, magnitudes of various stress components within the matrix due to the thermal residual and hydrostatic stresses are different. However, at a macro-level, both temperature change and hydrostatic loading of composites show similar effects on the initial yield surface in the composite stress space. In an agreement with experimental data, results also show that residual stresses are responsible for asymmetric behavior of composites in uniaxial tension/compression in the fiber direction. This asymmetric behavior suggests that the existing quadratic yield criteria need modification to include thermal residual stress effects.     

Simulation of Second Moments of Fluctuations of the Velocity and Temperature of Particles in Equilibrium Turbulent Flows

An analysis is performed of equilibrium solutions of equations for the second moments of fluctuations of the velocity and temperature of particles in a homogeneous shear flow, in a homogeneous flow subjected to tensile or compressive strain, and in a wall layer. The stability of equilibrium solutions to small perturbations is investigated. Algebraic models are given for turbulent stresses in the dispersed phase.     

The influence of thermal residual stresses and thermal generated dislocation on the mechanical response of particulate-reinforced metal matrix nanocomposites

Due to thermal expansion mismatch between reinforcing particles and matrix, thermal induced dislocations are generated in metal matrix nanocomposites (MMNCs) during cooling down from the processing temperature. These dislocations have been identified as an important strengthening mechanism in particulate-reinforced MMNCs. In this study, the development of thermal residual stresses and thermal induced dislocations in MMNCs are predicted using discrete dislocation simulation, assuming the whole material is under uniform temperature change. Shear deformation is applied after the composites are cooled down to room temperature and the influence of thermal residual stresses and thermal generated dislocation on the overall response of particulate-reinforced MMNCs are investigated. The results show that the thermal residual stresses are high enough to generate dislocations and the dislocation density is higher in the interfacial region than the rest of the matrix. The predicted mechanical behavior of the MMNCs matches the experimental results better when thermal residual stresses are included in the simulations.