METAL MATRIX COMPOSITES REINFORCED WITH INTERMETALLIC RIBBONS

The design procedure for low melting point alloy composites reinforced with melt-spun intermetallic ribbons for elevated temperature applications is presented. Long-range ordered Ni₃Al intermetallic is selected as a candidate reinforcement by virtue of its ease of being ductilized by boron and an increase in yield strength with increasing temperature. A certain composition of Ni₃Al alloyed with cobalt, tantalum, hafnium and boron for the maximum reinforcing effect is formulated. The rule of mixtures calculations of elastic modulus and yield strength are performed for Al-Si and Mg-Al matrices. The results of the structural investigation of a model composite system consisting of a 1100 Al matrix with embedded Ni₇₅Al₂₃Zr₁B₁ (at.%) ribbons, obtained by casting, are presented. Long-term stability of the reinforcing ribbons is investigated by annealing of “as-cast” specimens at elevated temperatures up to 100 h.     

玻璃陶瓷基复合材料制造工艺的探讨

本文研究了碳纤维增强微晶玻璃复合材计的制造工艺。基体采用三种材料:金云母可切削微晶玻璃,锂云母和LAS微晶玻璃。在实验室内采用粉末烧结法研制出这种材料样品。本文在成型工艺上作了改进,对关键工艺——粘接成型引入溶胶凝胶法。本文探讨了适合复合材料制造工艺的溶胶配方和工艺,改进了其使用方法。本文还利用物化检测手段对这三种微晶玻璃复合材料进行了分析对比,其性能仍保持了原基体材料的优良性能。     

PROCESSING OF METAL AND CERAMIC MATRIX COMPOSITES WITH THREE-DIMENSIONAL BRAIDED REINFORCEMENT

This paper summarizes some of the pioneering work done in developing the processing techniques to consolidate metal and ceramic matrix composites with 3-D braided fiber architecture. The 3-D braided fiber architecture features a fully integrated structure with multidirectional reinforcement and allows for the braiding of complex shapes. For metal matrix composite, a 3-D braided AI₂O₃ fibers preform reinforced Al-Li matrix composite has been successfully fabricated by liquid vacuum infiltration methods. For ceramic matrix composites, chemical vapor infiltration technique has been used to densify a 3-D braided Nicalon fibers preform with SiC matrix composite. A significant improvement in composite performance can be achieved through the architecture of 3-D integrated reinforcement geometry. The future needs and directions for developing viable methods for fabricating strong, tough high temperature composites with 3-D reinforcements is also discussed.     

PROCESSING AND FRACTURE TOUGHNESS OF SINTERED FIBER REINFORCED CERAMIC MATRIX COMPOSITES

ABSTRACT

The paper discusses the processing and the resulting mechanical properties of sintered fiber reinforced ceramic matrix composites. In situ observations of the sintering process revealed that stresses which develop due to the differential shrinkage between the fibers and the matrix initiate already during the heating cycle and are of sufficient magnitude and duration that crack like damage forms. Successful methods were employed for reducing and avoiding these stresses during the sintering process. Coarse grained alumina coatings deposited onto the fibers with a coating thickness of up to 10 µm delayed and reduced the stress development. Polymer coated fibers produced fully dense composites on which fracture toughness measurements were performed. Crack propagation and crack/fiber interaction was observed in situ inside a scanning electron microscope. The importance of studying both the crack front and the crack wake phenomena in fiber reinforced composites is illustrated. In specimens where the cracks are already bridged by 10% area fraction of fibers a toughness of 7 MPa √m was obtained. However, in samples where the cracks are not bridged yet by fibers, the crack becomes unstable before reaching the fiber positions and the fibers had no effect in resisting the crack propagation.     

PROCESSING AND FRACTURE TOUGHNESS OF SINTERED FIBER REINFORCED CERAMIC MATRIX COMPOSITES

The paper discusses the processing and the resulting mechanical properties of sintered fiber reinforced ceramic matrix composites. In situ observations of the sintering process revealed that stresses which develop due to the differential shrinkage between the fibers and the matrix initiate already during the heating cycle and are of sufficient magnitude and duration that crack like damage forms. Successful methods were employed for reducing and avoiding these stresses during the sintering process. Coarse grained alumina coatings deposited onto the fibers with a coating thickness of up to 10 µm delayed and reduced the stress development. Polymer coated fibers produced fully dense composites on which fracture toughness measurements were performed. Crack propagation and crack/fiber interaction was observed in situ inside a scanning electron microscope. The importance of studying both the crack front and the crack wake phenomena in fiber reinforced composites is illustrated. In specimens where the cracks are already bridged by 10% area fraction of fibers a toughness of 7 MPa √m was obtained. However, in samples where the cracks are not bridged yet by fibers, the crack becomes unstable before reaching the fiber positions and the fibers had no effect in resisting the crack propagation.     

PROCESSING ASPECTS OF SQUEEZE CASTING FOR SHORT FIBER REINFORCED METAL MATRIX COMPOSITES

This paper deals with the fabrication processes of short fiber reinforced metal matrix composites and controlling parameters for squeeze casting. It refers to the preparation of fiber preforms, heat-exchange between fiber and molten metal, infiltration pressure, optimum fabrication conditions and die design.     

PROCESSING AND PERFORMANCE OF SEVERAL SiC WHISKER-REINFORCED Al2O3 MATRIX COMPOSITES

The influence of different SiC whiskers on the processing and performance of Al₂O₃ matrix composites were investigated. Three types of commercial SiC whiskers produced in the United States were used. The composites contained 30 v/o of SiC whisker and were consolidated by both hot pressing (HP) and hot pressing followed by hot isostatic pressing (HP/HIP) techniques. The microstructure, mechanical properties, fracture behavior and toughening mechanisms of the composites were evaluated. The results show that the whisker types and processing techniques have profound effect on the densification and the properties of the composites.     

LASER PROCESSING OF METALLIC AND INTERMETALLIC SURFACES

The most common applications of the laser processing of metallic and intermetallic surfaces are presented. The advantages of this technology are deduced from the basic principles of the Interaction of laser light with metallic materials. Applications, like superficial hardening and surface cleaning and alloying, are extensions of existing processes benefiting by the specific advantages of laser annealing. Other ones, like deposition of passivating or wear-resistant coatings, exploit photochemical reactions induced in gas-phase or at metallic and semiconducting surfaces by UV or high intensity laser beams.     

PTC型炭黑／聚烯烃导电复合材料加工工艺的研究

归纳了影响炭黑／聚烯烃复合材料ＰＴＣ效应的诸因素，认为归根结底取决于炭黑在聚合物的基体中的分散程度与分布状态及其随外部条件变化的结果，加工方法和工艺条件在很大程度上决定着制品性能的优劣。文中详细讨论了混合、成型及后处理方法戌相应工艺参数对复合材料导电性能的影响，更好地解这类材料的结构－加工－性能之间的关系、为制备更高性能的ＰＴＣ材料提供了科学依据。     

金属基复合材料界面问题

金属基复合材料都要在基体合金熔点附近的高温下制备。在制备过程中，纤维，晶须、颗粒等增强体与基体净发生程度不同的相互作用和界面反应，形成各种结构的界面。界面结构和性能对金属基复合材料的的性能起着决定性作用，深入研究的掌握界面反应和界面影响性能的规律，有效地控制界面的结构和性能，是获得高性能金属基复合材料的关键。     

三维整体编织物增强陶瓷基复合材料的制备工艺及性能表征

本文研究了以连续碳纤维(C_f)三维整体编织物(3D-B)为增强体,以聚碳硅烷(PCS)裂解转化成碳化硅(SiC)为基体的陶瓷基复合材料(CMC)的制备工艺,探讨了不同工艺方法对复合材料微观和宏观性能的影响,并提出进一步改善制各工艺,提高复合材料性能的途径。研究结果表明,采用聚碳硅烷/二乙烯基苯(PCS/DVB)体系浸渍经交联固化后可以大大缩短裂解时间,致密化效率较聚碳硅烷/二甲苯溶液(PCS/Xylene)体系有着显著提高。对于3D-B C_f/SiC CMC用PCS/DVB体系浸渍经7个浸渍裂解周期后试样密度为1.65g/cm3,弯曲强度达326MPa,断裂韧性K_IC为13.72MPa·m1/2;用PCS/Xylene体系浸渍的试样密度为1.54g/cm3,弯曲强度为243MPa,断裂韧性K_IC为8.19MPa·m1/2。研究中利用扫描电镜观察了弯曲试样的断口以分析材料破坏时纤维的断裂、拨出形貌。     

COMPUTER CONTROLLED CONTAINERLESS PROCESSING OF HIGH TEMPERATURE INTERMETALLIC COMPOUNDS

A containerless automated system for processing high temperature reactive intermetallic compounds from the liquid state has been developed. Single crystals 2.5 cm in diameter have been processed in various atmospheres from nearly turbulent liquids. The liquid is electromagnetically contained in such a fashion as to permit the propagation of the solidification interface to control the resulting dimensions and structure. The operating temperature reaches nearly 2000°C depending upon the material and the resulting diameter control is within ± 0.08 mm

Several operational characteristics that mandate the need for a dedicated computer control and data acquisition system are its fast response to changes in inputs, sensitivity to changes in inputs, highly interactive nature, inherent transient behavior and small parameter region of stable operation. The computer control system is built around a DEC 11/23⁺computer and an interfaced image camera. The control system is contactless as all measurements are from on-line image analysis and remote sensors. Two control configurations, both of which employ three control loops are used to control three process outputs by manipulating two process inputs. In order to do this two controllers are cascaded. One control configuration is based totally on image analysis. The controllers are standard PI or I mode controllers. Several ingots processed using the flexible computer control system are shown. Samples are presented of processed specimens that show the degree of dimensional control possible. The systems TiAI, TiAI × X, and Ni₃AI are discussed as examples of the control of structure and properties that are attainable utilizing the processing techniques described.     

EFFECTS OF MICROSTRUCTURE ON RESISTANCE TO SHEAR LOCALIZATION FOR A CLASS OF METAL MATRIX COMPOSITES

The development of shear bands in a class of materials with inhomogeneous and composite microstructures is analysed using dynamic finite element calculations. The analysis is based on experimental studies of the deformation localization and constitutive behaviour of tungsten composites having two-phase microstructures consisting of hard tungsten grains and a soft alloy metal matrix. The calculations concern the effects of variations in microstructures on the resistance of the materials to shear banding. Specifically, three variations from the currently used microstructure are considered: (1) the alteration of the matrix phase to one that has a lower thermal conductivity; (2) the introduction of an interfacial layer between the grains and the matrix which limits microscopic heat exchange between the phases by having a low thermal conductivity; and (3) the modification of the matrix to one having a lower strength at high temperatures, but otherwise the same thermal and mechanical properties as the nickel–iron–tungsten matrix currently used in many tungsten composites. Numerical results show that the introduction of the low-conductivity interfacial layer significantly enhances the formation of shear bands and reduces the overall flow stress of the composite throughout the deformation. On the other hand, accelerated thermal softening of the matrix phase at high temperatures expedites localization in later stages of deformation while allowing the material to maintain a strong stress–strain response in the early part of the deformation. The results point out possible directions for the improvement of advanced composites in applications where the localization of deformation plays an important role.     

新型耐热树脂基复合材料的性能研究

以石墨、MoS2和含二氮杂萘联苯型聚醚砜酮（PPESK）为原料，用溶液共混共沉淀、热压模塑方法研制出PPESK基减摩复合材料，摩擦磨损实验结果表明摩擦系数与PTFE的相近，磨损率比纯树脂降低1个数量级。利用KYKY100B扫描电镜观察材料磨损表面，分析了材料的磨损机理。复合材料的摩擦磨损性能良好，且具有优异的耐热性能，是一类新型无油润滑的耐高温低摩擦材料。     

MgO DIFFUSION BARRIER FOR TITANIUM MATRIX COMPOSITES

The feasibility of developing an effective diffusion barrier for titanium matrix composites was investigated. MgO was identified as a potential barrier and was applied to silicon carbide and B₄C-coated boron filaments. The interface microstructure and fracture behavior of the resulting composites were characterized using ion microprobe mass analysis, optical microscopy, and scanning electron microscopy. The results indicate that MgO is a viable diffusion barrier for titanium matrix composites.     

PROGRESSIVE MICROCRACKING IN UNIDIRECTIONAL BRITTLE MATRIX COMPOSITES

Abstract

In this work we investigate the initiation of microcracking in a unidirectional Nicalon^TMfibre, glass-ceramic (CAS) matrix composite under flexural loading. We find that damage develops and grows in these composites as a continual initiation of new transverse cracks in a more-or-less random manner, as well as growth of existing cracks, within the matrix. Initial cracks are usually of limited extent, being stopped when they encounter the nearest fibres, in a mode idealized by the so-called full-cell cracking model. Each new crack that initiates under increasing load yields another data point, provided the precracked state of stress and crack geometry at the initiation site can be determined. The latter conditions are satisfied since the zone of influence of each crack can be determined analytically and the full-cell cracking mode geometry receives at least partial validation. Analysis is accomplished by use of an axisymmetric micro mechanical model based on Reissner's variational principle in which variable fibre spacing can be recognized. In a semi-empirical failure model, the matrix axial stress just prior to crack initiation is computed and assumed to act on an unbounded matrix containing a penny-shaped crack having a radius dictated by the local fibre spacing, s. This model is geometrically appropriate for the case of large localized fibre spacings. Statistical information on s and the critical matrix mode I stress intensity factor is presented, and the average value seems to be consistent with expectation based upon measurements on monolithic samples of the matrix material. Application of the micromechanical model shows that, at least for the predominant range of s, sufficient energy is available to mobilize annular matrix cracks at stresses predicted by the semi-empirical model, and also that interfacial crack deflection is expected rather than fibre penetration. In accordance with this observation, very few fibre breaks were seen in the test programme. Finally, some comments regarding the practical consequences of this work are offered and the conventional view of a constant microcracking stress level in a steady-state model is dismissed, at least for the class of composites considered.     

聚合物基复合材料的湿扩散参数研究

通过系统地研究几种国产复合材料的吸湿行为,获得了描述湿气在其中扩散的基本参数,如平衡湿含量M_m,M_m与相对湿度φ关系中的常数a和b,沿板材厚度方向上的一维湿扩散系数D_x以及D_x与温度关系中的常数ΔE(湿扩散活化能)和D₀等。      

晶须增强氮化硅陶瓷自生复合材料

本文提出了两种制备氮化硅陶瓷自生复合材料的方法:以氟化钙为添加剂,对硅坯体直接进行氮化;经预氮化后的反应烧结氮化硅多孔体,在含酚醛树脂和硅酸乙酯的无水乙醇溶液中浸渍处理后,再经过高温热处理,分别可得到以β-Si₃N₄和β-SiC晶须增强的氮化硅陶瓷复合材料。同时,还对生长条件对晶须的形态,以及自生晶须对氮化硅陶瓷力学性能的影响进行了探讨。     

界面相性态对纤维增强复合材料内应力传递的影响

本文用有限元法研究了具有基体裂纹的纤维增强复合材料内的应力传递问题.假设纤维与基体的界面为非理想的,文中运用“弹簧层”模型首先分析了在不同的组分弹性模量比、纤维体积含量与边界约束条件下,界面相性态对复合材料内的应力传递的影响,然后进一步考察了在几种典型的损伤模式下界面附近的应力分布情况.     

金属基铸造颗粒复合材料

金属基铸造颗粒复合材料是复合材料中的一个新兴的研究领域,具有十分广阔的应用前景。本文在对铸造颗粒复合材料的制造工艺方法、润湿及其改善途径、金属液的物理、化学特性、界面结构及形态等重要问题进行综合评述的基础上,对目前存在的问题、今后的研究课题和发展趋势提出了看法。认为不断完善和开发各种工艺方法,深入开展基础理论研究工作,开发新材料是今后铸造颗粒复合材料研究的中心任务。