SiC_P/Al基复合材料在等径角挤扭变形中的界面原子扩散行为

为了研究金属基复合材料在剧烈塑性变形(SPD)过程中增强颗粒与金属基体的界面连接机制,通过等径角挤扭(ECAP-T)工艺在较低温度下制备块状10wt%SiCP/Al基复合材料,并对经过1、2和4道次ECAP-T变形的SiC颗粒与纯Al之间的界面反应以及元素扩散进行了研究。通过TEM和XPS研究了界面和元素扩散,结果表明:即使在较低的外界制备温度下,Al和SiC颗粒表面的SiO2层也能够发生反应,形成主要由Al2O3组成的界面层。相比理论计算值,ECAP-T变形可以将Al的扩散系数提高约1016倍,增强扩散的原因主要是ECAP-T变形促使界面温度升高,且在铝基体内产生空位、位错和晶界等高密度晶格缺陷。     

扭转圈数对高压扭转SiC_P/Al复合材料界面扩散行为和组织性能的影响

采用OM和EDS研究不同扭转圈数下高压扭转法制备SiC_P/Al复合材料的显微组织和界面扩散行为,并结合组织特点和界面特征分析扭转圈数对复合材料拉伸性能和断裂机理的影响。结果表明:扭转圈数的增加可以有效提高SiC颗粒分布的均匀性,闭合孔隙,界面处Al元素扩散能力增强,扩散距离增大,Al扩散系数实际计算值较理论值增大了10~(17)倍,形成以元素扩散和界面反应为主的强界面结合,试样抗拉强度和伸长率不断提高,少量的SiC颗粒均匀分布在断口韧窝中,断裂主要以基体的韧性断裂为主;当扭转圈数较大时,SiC颗粒在剧烈剪切作用下破碎加剧,颗粒"再生团聚"导致孔隙率增大,潜在裂纹源增多,形成大量结合强度较低的断裂新生界面,试样抗拉强度和伸长率显著降低,在团聚位置易形成尺寸较大的深坑韧窝,复合材料断裂呈现韧性断裂与脆性断裂的混合模式。     

Importance of interface chemistry in hipping diffusion bonding of precipitation hardened martensitic stainless steels

Abstract

The effect of processing route on the microstructure and properties of hot isostatically pressed diffusion bonds, manufactured from precipitation hardened martensitic stainless steels, is addressed in this paper. The quality of the diffusion bond was assessed using a range of analytical techniques and a programme of mechanical testing. It is shown that the microstructure and properties of the diffusion bond are strongly influenced by the prebonding processing route used in its manufacture. Using conventional encapsulation techniques the interface is contaminated by oxide particles, whose composition and morphology depends upon the precise chemistry of the steel. These particles prevent grain growth across the interface and frequently result in tensile fracture at the interface. Refinements to the prebonding processing route result in lower particle densities and grain growth across the interface. However, these measures result in the interface becoming weakened by the precipitation of copper at the interface.     

Model for growth and coarsening of two phase systems under diffusional control

Abstract

A model is described which is capable of simulating the two-dimensional evolution of microstructure in two phase systems undergoing diffusion controlled growth and surface tension driven coarsening. To solve the diffusion equation in the matrix phase, an integral equation method is employed. Thus, although it is necessary to describe the shapes of the second phase particles using a number of elements, it is not necessary to discretise the matrix phase as the particles evolve. This allows the computation times to be kept within reasonable limits. The boundary condition at the interface and the variation of the interfacial concentration with interface curvature are accounted for in a rigorous fashion. It is shown that the method can handle the ‘soft’ impingement of overlapping diffusion fields. A treatment of the ‘hard'’ physical impingement of particles is developed. To demonstrate the accuracy and stability of the method, the results from the model are compared with the exact solution for a spherical particle growing in a circular domain; it is shown that the agreement is reasonable. The results from a number of example computations are presented, which include (a) growth of a single particle in a finite domain, (b) soft and hard impingement of two particles in a finite domain, (c) coarsening of a significant number of particles at constant volume fraction, and (d) simultaneous nucleation, growth, and coarsening of second phase particles. Where appropriate, the results are compared with those from other models which have been published in the literature. The advantages and disadvantages of the present model are discussed.     

Growth of Ge nanoparticles on SiO2/Si interfaces during annealing of plasma enhanced chemical vapor deposited thin films

Multilayer germanosilicate (Ge:SiO₂) films have been grown by plasma enhanced chemical vapor deposition. Each Ge:SiO₂ layer is separated by a pure SiO₂ layer. The samples were heat treated at 900 °C for 15 and 45 min. Transmission electron microscopy investigations show precipitation of particles in the layers of highest Ge concentration. Furthermore there is evidence of diffusion between the layers. This paper focuses mainly on observed growth of Ge particles close to the interface, caused by Ge diffusion from the Ge:SiO₂ layer closest to the interface through a pure SiO₂ layer and to the interface. The particles grow as spheres in a direction away from the interface. Particles observed after 15 min anneal time are 4 nm in size and are amorphous, while after 45 min anneal time they are 7 nm in size and have a crystalline diamond type Ge structure.     

Theoretical solutions for mixed control of solid state reactions

The kinetics of solid state reactions has been reexamined to include solutions for mixed control by an interfacial process and diffusion. The Valensi-Carter, Ginstling-Brounshtein, and other quasi steady state models have been extended to include mixed control. Solutions were obtained for planar, cylindrical and spherical particles. Mixed control may occur at either the inner or outer interface for cylindrical or spherical particles, for which significantly different solutions are valid. Mixed control at the outer interface usually gives faster reaction than mixed control at the inner interface. The kinetics of these solid state reactions with mixed control is usually nearly independent of the product to reagent volume ratio. This revised version was published online in November 2006 with corrections to the Cover Date.     

双屏蔽(B4C-W)/6061Al层状复合板设计与性能

基于B₄C和W良好的屏蔽中子和γ射线性能,采用6061铝合金作为基体,设计了一种新型双屏蔽(B₄C-W)/6061Al层状复合材料,通过放电等离子烧结后加热轧制成板材,对制备的复合材料微观组织和力学性能进行了研究。结果表明,屏蔽组元B₄C和W颗粒均匀地分布在6061Al基体中,层界面、B₄C/Al、W/Al异质界面之间结合良好,无空隙和裂纹。在颗粒与基体界面处形成扩散层,扩散层的厚度约为6 μm (W/Al)和4 μm (W/Al)。轧制态的(B₄C-W)/6061Al层状复合板的屈服强度(109 MPa)和极限抗拉强度(245 MPa)明显优于烧结态的复合材料,但断裂韧性降低。强度提高的原因主要是轧制后颗粒的二次分布、均匀性及界面结合强度提高,基体合金的晶粒尺寸减小,位错密度增加。层状复合板的断裂方式为基体合金的韧性断裂和颗粒的脆性断裂。      

Reactions of Hf-Ag and Zr-Ag alloys with Al2O3 at elevated temperatures

We are studying reactions of Ti, V, Zr, and Hf with ceramics as part of a program to understand fundamental reaction and bonding mechanisms in active metal brazing of ceramics. In this paper we present results of experiments with model systems comprising Ag alloys that contain different amounts of Hf or Zr that were reacted with sapphire or 99.6% alumina for different times and temperatures in a controlled atmosphere furnace. In these alloys the Ag functions as an inert solvent, which allowed us systematically to determine the effects of changes in concentration of the active element. We observed qualitative wetting and spreading tendencies of the alloys during heating and examined cross sections after cooling using electron analytical techniques. For all reaction times studied, the Hf/Ag alloys formed a discontinuous reaction layer, which was consistent with earlier high-resolution electron microscopy that showed sub-micrometer HfO₂ particles embedded in the surfaces of the Al₂O₃ grains. By contrast, initial reaction of the Zr/Ag alloys with Al₂O₃ produced a continuous interface layer. With longer reaction times, the ZrO₂ reaction product became much thicker and exhibited three distinct zones at the interface. The results suggest that the rate limiting step in the Zr/Ag reaction is the chemical reaction at the interface, whereas with Hf/Ag reaction diffusion of products away from the interface is rate limiting.     

Microstructural characterization of the failures of thermal barrier coatings on Ni-base superalloys

Abstract

Typical thermal barrier coating (TBC) systems consist of a nickel-base superalloy substrate coated with a MCrAlY or diffusion aluminide bond coat, onto which is deposited a yttria-stabilized zirconia (YSZ) TBC. The bond coats are usually deposited via diffusion aluminizing processes or low pressure plasma spray processes (LPPS). The YSZ can be deposited by air plasma spraying (APS) or electron beam physical vapor deposition (EBPVD). A layer of thermally-grown oxide (TGO), which is usually alumina, forms between the bond coat and YSZ during TBC deposition and subsequent high-temperature exposure. The conventional wisdom is that APS coatings tend to fail in the YSZ and that EBPVD coatings tend to fail at the interface between the TGO and bond coat. However, current research has shown that the situation is much more complex and that the actual fracture path can be a function of the type of bond coat, the type of high-temperature exposure, and coating process parameters. This paper describes the results of a study of the failure of state-of-the-art EBPVD TBCs deposited on NiCoCrAlY and platinum-modified diffusion aluminide bond coats. The failure times and fracture morphology are described as a function of bond coat type. The failure times were found to be a strong function of temperature for both bond coats. The failure for NiCoCrAlY bond coats was found to initiate at defects in the coating, particularly at the TGO/YSZ interface, but the fracture propagated primarily along the TGO–bond coat interface. The failure times and morphologies for platinum-modified diffusion aluminide bond coats depended strongly on bond coat surface preparation. The mechanisms for failure of the two bond coats are described. Also, the effects of modifications to the bond coats and variations in processing parameters on these mechanisms are presented.     

高压扭转对SiC_P/Al复合材料微观组织和力学性能的影响

为研究大塑性变形对金属基复合材料微观组织和力学性能的影响,利用高压扭转工艺(HPT)在200℃下将纯Al粉末和经氧化处理的SiC粉末混合固结成10wt%SiC_P/Al复合材料。采用TEM观察HPT变形后不同圈数试样的SiC-Al界面及Al基体微观组织,采用EDS能谱仪分析界面处原子扩散现象,采用万能拉伸试验机测试研究不同扭转圈数试样的力学性能。结果表明:不同圈数试样Al基体内出现大量位错、非平衡晶界等晶格缺陷;组织内存在两种SiC-Al界面,含SiO_2层的原始界面和因颗粒破碎而新生成的界面。两种界面结合良好,界面处元素相互扩散;随着扭转圈数的增加,10wt%SiC_P/Al复合材料抗拉强度增加,延伸率得到较大提高。分析发现高压扭转后不同圈数组织内产生的大量晶格缺陷和细小晶粒,促进界面处元素的相互扩散,使界面结合良好,同时大量晶格缺陷和细小晶粒的产生以及结合良好的SiC-Al界面是SiC_P/Al复合材料力学性能大幅提升的主要原因。     

Particle drag retardation of surface smoothing and sintering

Diffusion-controlled capillarity phenomena in solids, including sintering and surface profile changes, can be retarded by dispersed particles which are pinned at the surfaces. A model is proposed which predicts that dispersed particles which are more wetted by the substrate material, and subsequently partially imbedded, will tend to have a lower mobility than particles which are less wetted, if the force exerted on the particles by the moving interface is sufficiently low that the particles are dragged by the surface. On the other hand, less wetted immobile particles can provide a greater restraining force than more wetted particles. For a given volume of dispersoid per unit area of substrate, larger particles will provide a greater restraint to interfacial motion. Particle drag at the surface is probably not very effective in retarding sintering brought about by grain-boundary and volume diffusion. Surface diffusion-controlled sintering and surface-shape changes can be strongly retarded by dispersed particles.     

The conditions for realization of the Boltzmann distribution of negative ions in a plasma

The self-consistent electric field acting in a plasma retards the most mobile charged particles, which usually leads to a Boltzmann distribution of electrons. If negative ions cross the discharge volume several times during their lifetime in the volume processes, these particles also obey the Boltzmann distribution. It is demonstrated that this condition is usually satisfied when the characteristic time of electron attachment is small as compared to the time of ambipolar diffusion of the negative ions (ion diffusion at an electron temperature). In the opposite case, the profiles of electrons and negative ions are similar.     

A numerical model for the dissolution of spherical particles in binary alloys under mixed mode control

A general numerical model is described for the dissolution kinetics of spherical particles in binary systems for any combination of first order reactions at the particle-matrix interface and long distance diffusion within the matrix. The model is applicable to both finite and infinite media and handles both complete and partial particle dissolution. It is shown that interfacial reactions can have a strong effect on the dissolution kinetics, the solute concentration at the particle-matrix interface and the solute concentration profile in the matrix.     

Transient liquid phase diffusion bonding of 6061-13 vol.% SiCp composite using Cu powder interlayer: mechanism and interface characterization

Transient liquid phase diffusion bonding of an extruded 6061-13 vol.% SiCp composite at 560 °C, 0.2 MPa, using a 50-μm thick copper powder interlayer with 20 min, 1 h, 2 h, 3 h, and 6 h hold times has been investigated. The isothermal solidification and homogenization of the bond region occurred with 3 and 6 h holding, respectively. During isothermal solidification, smaller SiC particles (size range: 11–13 μm) were pushed by the moving solid/liquid interface and segregated around the bond centerline, whereas bigger SiC particles (size range: 24–33 μm) were engulfed. During isothermal solidification, the bigger SiC particles locally hindered the solidification front movement causing grain refinement. The kinetics of isothermal solidification, representing the displacement of the solid/liquid interface (y, in μm) as a function of time (t, in s), followed a power-law relationship: y = 35 t ^0.22. According to this kinetic equation, the effective diffusivity of copper in composite system was found to be about 10⁵ times higher than the lattice diffusivity indicating the dominance of short circuit diffusion through the defect-rich particle/matrix interface. Ultrasonic investigation of the bond interface indicated that the signal attenuation was strongly correlated with the width of the segregated layer-a feature that decreased with the increasing bonding time. The completion of isothermal solidification was indicated by a sharp rise in the received signal amplitude with a negligible attenuation.     

基于真空热压扩散法的金刚石/Ti界面生成机制

采用真空热压扩散法在聚晶金刚石表面制备Ti层,探究金刚石表面金属化过程中的界面生成机制。利用扫描电子显微镜和X射线衍射仪,分析了钛层的表面形貌、界面结构和界面间的物相组成,采用能谱仪对界面进行了元素分析,计算了聚晶金刚石与Ti层之间界面的扩散带宽度及生成TiC的化学反应吉布斯自由能变。研究结果表明:在聚晶金刚石表面形成了平整、致密的Ti层,在聚晶金刚石与Ti层界面之间存在C、Ti和Co元素的扩散,在结合界面处产生了一定宽度的元素扩散带,同时在金刚石表面生成了点状TiC。真空热压扩散法实现了金刚石与Ti层的化学结合,可以提高金刚石与Ti层的结合强度。      

金刚石表面Cr金属化的界面扩散反应研究

利用直流磁控溅射法在金钢石颗粒表面沉积了厚度为１５０ｎｍ的金属Ｃｒ薄膜。ＳＥＭ研究表明在金刚石表面形成的Ｃｒ膜基本均匀,但有小的金属聚集体存在。俄歇深度剖析研究发现,在镀膜过程中Ｃｒ膜和金刚石基底间发生了显著的界面扩工用作用。相应的俄歇线形分析表明,沉积过程中在界面上发生化学反应形成了部分Ｃｒ２Ｃ３物种。溅射沉积功率对金刚石颗粒与金属Ｃｒ膜的界面反应有较大的影响,提高溅射功率可大大促进Ｃｒ元素的扩     

Fixed mesh front‐tracking methodology for finite element simulations

A direct front‐tracking method using an Eulerian–Lagrangian formulation is developed in two space dimensions. The front‐tracking method is general in that it can track any type of interface once its local velocity is specified or has been determined by calculation. The method uses marker points to describe the interface position and tracks the interface evolution on a fixed finite‐element mesh, including growth, contraction, splitting and merging. Interfacial conditions are applied directly at the interface position. The method is applied to three scenarios that involve different interface conditions and are based on energy and mass diffusion. The three calculations are for the dendritic solidification of a pure substance, the cellular growth of an alloy, and the Ostwald ripening of silica particles in silicon. Numerical results show that very complicated interface morphologies and topological changes can be simulated properly and efficiently. Copyright © 2004 John Wiley & Sons, Ltd.     

Friction and wear reduction with tribological coatings

The basic requirements for a good tribological surface are (1) low sliding friction, (2) good resistance to scuffing, wear and abrasion, (3) long contact fatigue life and (4) adequate subsurface strength to provide dimensional stability. Coatings have inherent deficiencies. The major problem is failure at the interface between the coating and the substrate, which results in flaking, peeling or spalling of the coating under the repetitively applied contact stresses.Three types of coatings which employ different mechanisms to improve the tribological properties and to maintain coating integrity are described in this paper. Nitrocarburizing represents a class of coatings in which the elements are allowed to diffuse into the surface of the structural material to form an alloy with the substrate. Diffusion provides compositional gradients which result in hard wear-resistant surface and which at low shear strengths avoid the interfaces that frequently exist between coatings and substrates. Chemically vapor-deposited chromium and titanium carbides represent a class of coatings in which a chemically distinct layer is grown on top of the substrate and is bonded to the substrate by diffusion. In the third type of coating, hard particles are suspended in a soft matrix. The hard particles provide the wear and abrasion resistance and the soft matrix both bonds the particles together and provides the low friction. Although the bond strength of this coating to the substrate is lower than that provided by diffusion in the other coatings, the soft matrix will yield without flaking under the shear stresses which are developed at the interface.     

WC-氧化石墨烯/Ni复合熔覆层的制备及形成机制

采用真空熔覆技术制备了WC-氧化石墨烯(GO)/Ni复合熔覆层,运用扫描电镜、能谱仪、X射线衍射仪观察并分析在不同温度下熔覆层内显微形貌的变化与物相组成。结果表明:在ZG45表面制备了组织致密、与基体形成良好冶金熔合的WC-GO/Ni复合熔覆层;熔覆层的微观结构组成从表面至基体依次是约1.5 mm厚的复合层、360 μm左右的过渡层、50 μm左右的扩散熔合层和100 μm左右的扩散影响层,其主要组成相有Cr₇C₃、FeNi₃、WC、Cr₂₃C₆、Ni₃Si、C、Fe₇W₆、γ-Ni固溶体等,FeNi₃、Fe₇W₆分散在冶金熔合带,扩散影响区主要组织为珠光体;复合区的物相尺寸小于界面区的物相尺寸,熔覆层形成过程中复合区的金属颗粒变化先于界面区,凝固时熔化不完全的颗粒表面长出团簇物(Cr₇C₃/Cr₂₃C₆),随着保温长大逐渐变成针状物镶嵌在Ni基固溶体中。      

Prediction of Austenitization and Homogenization of Q235 Plain Carbon Steel during Reheating Process

In this paper,the austenitization and homogenization process of Q235 plain carbon steel during reheating is predicted using a two-dimensional model which has been developed for the prediction of diffusive phase transformation(e,g.αto γ).The diffusion equations are solved within each phase(αand γ) and an explicit finite volume technique formulated for a regular hexagonal grid are used.The discrete interface is represented by special volume elements α/γ,an volume element α undergoes a transition to an interface state before it becomes γ.The procedure allows us to handle the displacement of theinterface while respecting the flux condition at the interface.The simulated microstructure shows the dissolution of ferrite particles in the austenite matrix is presented at different stages of the phase transflrmation.Specifically,the influence of the microstructure scale and the hwating rate on the phase transformation kinetics has been investigated.The experimental results agree well with the simulated ones.