Alümina fiber takviyeli Al-Si metal matriksli kompozitlerin üretimi ve mikroyapı-özellik ilişkilerinin incelenmesi

Abstract

Metal Matriks Kompozit (MMK) malzemelerin yüksek elastik modül, mukavemet, aşınma dayanımı ve yüksek sıcaklık özellikleri gibi üstün fiziksel ve mekanik özelliklerinden dolayı, son yıllarda üretimleri ve kullanımları yaygınlaşmıştır. Bu çalışmada, geleneksel piston malzemesi olarak kullanılan LM 13 (ETİ AL 145) Al-Si alaşımının kısa fiberler ile takviye edilerek mikroyapı ve mekanik özelliklerinde meydana gelen değişimlerin incelenmesi amaçlanmıştır. Takviye malzemesi olarak kullanılan Alümina kısa fiberleri son yıllarda yaygın ve ekonomik olarak üretilen ve üstün mukavemet özelliklere sahip olan 8-AI2O3 (Saffil) fiberlerdir. Kompozit malzemeler, 0.1 den 3 MPa' a kadar değişen basınçlarda, özgün bir sıvı infiltrasyon prosesi yöntemi; % 10, % 15, % 20, % 25 ve % 30 hacim oranlarında ki Saffil fiber preformlara metal emdirilmesi tekniği ile üretilmişlerdir. Yoğunluk ölçümleri, infiltrasyonun fiber hacim oranının artması ile kritik bir basınçtan sonra mey dana geldiğini göstermiştir. 1 MPa gibi düşük basınçta infiltrasyonun % 80-90' 1 ger çekleşirken infiltrasyonun 3 MPa' a çıkarılması geri kalan boşlukların dolmasını sağla maktadır. Bunun nedeni ise preformlarda ilkin büyük boşluklar dolmakta ve takiben ince fiberler arası bölgelere metal nüfuz etmektedir. 3 MPa basınçta üretilen takviye- siz alaşım ve MMK' lere, standart T6 ısıl işemi uygulanmış, elastik modül, mukavemet, aşınma ve yaşlanma çalışmaları yapılmıştır. Kompozit mikroyapı incelemelerinde fi berlerin, ötektik bölgelerde segregasyona uğrama eğilimi gösterdikleri ve hem matriksin hem de Si kristallerinin incelmesine sebebiyet verdikleri saptanmıştır. Fiber segregasyonunun fiber hacim oranı arttıkça azaldığı ve alaşımının ötektik altı bileşimde olmasına rağmen MMK mikroyapılannda primer Si kristallerinin oluştuğu belirlenmiştir. Bunun nedeni ise fiberlerin Si kristallerini çekirdeklemeleri ve fiber ile matriksteki Mg elementinin reaksiyonunun ortaya çıkardığı serbest Si' dan dolayı matriksin bölgesel olarak ötektik üstü bileşime kaymasına bağlanmıştır. Elastik modül değerlerinin artan fiber hacim oranı ile arttığı tesbit edilmiş ve sonuçların teorik modellerle son derece uyumlu oldukları tesbit edilmiştir. MMK mukavemetlerinin, artan fiber hacim oranı ile arttığı, ancak % 20 Saffil fiber takviyelerinin üzerindeki MMK' lerde zayıf fi- ber/matriks arayüzey bağ yapısından dolayı düştüğü sonucu ortaya çıkmıştır. Pin-on- disk yöntemi ile değişik aşınma yüklerinde yapılan aşınma deneylerinde artan fiber ha cim oranının hacimsel ve ağırlıkça aşınma hızlarının yanında sürtünme katsayısının da düşüşüne yol açtığı tesbit edilmiştir. Aşınma yüzeyi incelemeleri, artan fiber hacim oranı ile aşınma mekanizmasının değiştiğini göstermiştir. Yaşlanma kinetiği çalışmalarında, fiber hacim oranının artışı ile MMK malzemelerde yaşlanma hızının arttığı, takviyesiz alaşım için maksimum sertliğin elde edildiği yaşlanma sürelerinde kompozit malzemelerde aşın yaşlanma meydana geldiği saptanmıştır. Direnç ölçümleri, takviyesiz alaşımda GP zonlarının oluştuğunu, MMK' ler de ise su vermede oluşan boşlukların fiber/matriks arayüzeyinde yutulmasından dolayı GP zonlarının oluşamadığını ortaya çıkarmıştır.

There is no universally accepted definition of composite materials. Definitions in the literature differ widely. The problem is the level of definition. In the dictionary and in everyday usage the term composite refers something made up of various parts of elements. For starting to devise a definition for composite materials in accordance with this idea, it is quickly discovered that several definitions are possible. For starting a valid definition in terms of the constituents making up engineering materials at each of the several structural levels of matter, which materials are to be regarded as composites and which as monolithics depends upon the level chosen as the basis for definition. There are three main points to be included in a definition of an acceptable composite material for use in structural applications; (i) It consist of two or more physically distinct and mechanically separable materials (ii) It can be made by mixing the separate materials in such a way that the dispersion of one material in the other can be done in a controlled way to achieve optimum properties. (ii) The properties are superior, and possibly unique in some specific respects, to the properties of the individual components. One of the possible definition of composite materials which takes into account both the structural form and composition of the material is as follows: " A composite material is a material brought about by combining materials differing in composition or form on a macroscale for the purpose of obtaining specific characteristics and properties. The constituents retain their identity such that they can be physically identified and they exhibit an interface between one another". But even this definition needs clarification. Composite materials have been classified in many ways depending on the ideas and concepts that need to be identified. Several classification systems can be used, including classification, e.g., metal-organic or metal-inorganic; by bulk form characteristics, e.g., matrix systems or laminates, by distribution of the constituents, e.g., continuous discontinuous; and by function, e.g., electrical or structural. The classification system used in this work is based on the matrix systems. According to the classification system in terms of matrix materials which are the predominant phases that strictly effect the microstructure and mechanical and physical»properties of the resultant composite materials, there are three general classes of composites: -vu- 1) Ceramic Matrix Composites (CMC's). 1) Polymer Matrix Composites (PMC's). 1) Metal Matrix Composites (MMC's). Metal Matrix Composites (MMC's), in general, consist of at least two components: one obviously is the metal matrix (in most cases,, an alloy is the metal matrix), and the second component is a reinforcement (in general, an intemetallic compound, an oxide, a carbide or a nitride). The distinction of metal matrix composites from other two phase alloys comes about from the processing of the composite. In the production of composite, the matrix and the reinforcement are mixed together. This is to distinguish a composite from a two phase alloy, where the second phase forms as a particulate, eutectic or eutectoid reaction, etc. In other words, a composite initially begins as separate components, i.e., the metal matrix and the reinforcement. In all cases the matrix is defined as a metal, but a pure metal rarely used as the matrix; it is generally an alloy. Each type of MMC is defined as follows:.Dispersion Strengthened: This composite is characterized by a microstructure consisting of an elemental matrix within which fine particles are uniformly dispersed. The particle diameter ranges from about 0.01 um to 0.1 u.m, and the volume fraction of particles ranges from 1 to 15 %..Particle Reinforced: The composite is characterized by dispersed particles of greater than 1.0 um diameter with a volume fraction 5 to 40 %..Fiber (whisker) Reinforced: The reinforcing phase in fiber composite materials spans entire size range, from 0.1 to 250 mm in length to continuous fibers, and spans the entire range of volume concentrations from a few percent to greater than 70 %. The distinguishing microstructural feature of fiber reinforced materials is that the reinforcing fiber has one long dimension, whereas the reinforcing particles of the other two types do not. Continuous fiber reinforced metals are a special and sophisticated classes of composite materials. Fiber reinforced metals, unlike most metals and alloys, are anisotropic. The degree of anisotropy depends primarily on the degree of fiber orientation. The prime role of fiber is to carry the load, while the metal matrix serves to transfer and distribute the load to the fibers. The efficiency with which the loads are transferred from the matrix to the fibers depends on the bonding interface between them. Assuming high interface efficiency, the mechanical properties of the composite depend more on the properties of the fiber rather than the properties of the matrix. This means that the matrix can be selected on the basis of oxidation and corrosion resistance or other required properties. Applications of continuous fiber MMC are mostly limited to some of the primary and structural members of aerospace structures and military airplanes, except for an automobile connecting rod which is made of aluminium reinforced with continuous stainless steel fibers. -vui- MMC's have several advantages that are very important for their use as structural materials. These advantages include a combination of the following properties: - High strength - High elastic modulus - Low sensitivity to temperature changes or thermal shock - High surface durability and low sensitivity to surface flaws - High electrical and thermal conductivity - High vacuum environment resistance. In addition to conductivity of MMC's, the most obvious advantages of MMCs are their resistance to severe environments, toughness, and retention of strength at high temperatures. For a composite structure it is possible to emphasize environmental stability of the matrix at elevated temperatures, since the required mechanical strength and stifhess can be obtained from the reinforcement. The shear strength requirements of the matrix are nominal since the matrix serves only to transfer load into the fibers. Metal matrix composite (MMC) materials have been under development for more than 20 years. However, the initial emphasis was on continuous filament MMC's. They were first developed for applications in aerospace, followed by applications in other industries. The expansion non aerospace and nonmilitary fields came about slowly as the price of MMC materials was coming down. This due to mainly to development of new low cost fibers. In recent years, discontinuous MMCs have been investigated. Recent interest in discontinuous MMCs has been rekindled because it is more economical to produce economic production of silicon carbide (SiC) particles or whiskers, alumina (AI2O3) particles or short fibers and other low cost particles or short fibres such as carbon, B4C, Si02, graphite etc. One of the advantages of discontinuous composites is that they can be shaped by standard metallurgical processes such as forging, rolling, extrusion etc. Due to this ease of formability and relatively modest cost discontinuous MMCs have recently been used in various applications. Some of such applications are tennis rackets and heads of golf clubs, which are made of SiCp/Al composite, and automobile engine components, piston and connecting rod, which are made of randomly oriented short alumina Saffil (8-AI2O3) fibers/Al or SiCw/Al composites. The applications of MMCs into industrial is increasing progressively by the production of low cost discontinuous reinforcements. Over the last decade Imperial Chemical Industries (I.C.I) has developed the manufacture of short staple, polycrystalline alumina fibres which are sold under the trade name of Saffil. The main commercial application is the insulation of high temperature industrial furnaces. In recent years, however, improved grades of these fibers (RF and RG) have been developed for use in metal matrix composites. The alumina fibers are produced as short staple by a spinning process which controls the fiber diameter within tight limits around a median value of 3 u,m. Spinning process gives very short fibers in various controlled lengths ranging in aspect ratio from 100:1 down to 20:1. Contrary to methods employed to prepare whiskers and melt-spun ceramic fibers, this process results in a very low level of non-fibrous material. A fine grained microstructure is -IX- developed by incorporating about 4 % silica and by close attention to each heat treatment stage during fiber production. The silica is effective in enabling a controlled progression through the transition alumina forms, thus facilitating the removal of porosity, and by acting as a crystal growth inhibitor once transformation to be alpha alumina phase occurs. Fiber in delta alumina form (RF grade) has a tensile strength of 2000 MPa, a modulus of 300 GPa and a density of 3.3 g/cm3. Further processing at high temperature causes the gradual conversion of delta phase to the alpha alumina form with corresponding increase in hardness, modulus and density, and a decrease in strength. The loss of strength is due to a step increase in crystal size associated with the phase transformation. The flexibility of the production process allows the concentration of the alpha alumina form to be controlled and the properties of the fiber to be tailored to meet the demands of the reinforcement application. In recent years a considerable attention has been given to the various fabrication routes, and characterization of the mechanical properties of these composites. But little work has been carried out to reveal the effect of the fibres on microstructure of the matrix alloy upon solidification. However, sufficient experimental data is still not yet available to support a comprehensive analytical approach in predicting the elastic modulus and strength of such composites. While a limited experimental data is available in the literature on the friction behaviour of Saffil + LM 13 MMC's. Whereas, no experimental data was found in the literature on the effects of alumina fiber reinforcements of the aging characteristics LM 13 metal matrix composites. The matrix alloy chosen in the present work was an Al-12 wt % Si, 1.16 wt % Cu, 1.21 wt % Mg, 0.90 wt % Ni. In this study, the reason of the using of Aluminium-Silicon alloy as a matrix material is that Aluminium-Silicon alloys are the traditional choice for piston applications due to their high fluidity and good balance of thermal and mechanical properties. Saffil alumina fibres were supplied by ICI in the form of disc-shaped preforms 100 mm in diameter by 10 mm thick. These preforms contained 10 %, 15 %, 20 %, 25 % and 30 % by volume of Saffil fibres oriented predominately in the plane of the disc. The infiltration of the preforms was carried out by positioning the preforms in a metal die-cavity and then pressurizing the molten metal at 850 °C into die-cavity which was heated up to 450 °C and evacuated prior to the infiltration. The infiltration pressure applied varied from 0.1 MPa to 3 MPa for each material and their corresponding densities were determined by weighting discs in air and water. Matrix alloy and the composites produced by the highest infiltration pressure were subjected to a standard T6 heat treatment and consequently tensile tested and modulus measurements were carried out using an Instron machine operated at a cross-head speed of 0.016 mm/sec. Optical Metallography was carried out on the polished surfaces of the both radial and planar section of the composites and more detailed examination of the tensile test fracture surfaces was carried out using a scannig electron microscope (SEM). Wear tests were carried out using a pin-on disk machine under 5 N, 10 N, 20 N, 40 N and 60 N normal loads with lm/sec. wear rate and volumetric and mass losses were recorded with the corresponding friction of coefficients are measured. For the studying of the aging characteristics of the -x- composites, microhardness and resisitivity measurements and X-ray studies have been performed. The results of permeability of molten alloy through the preforms indicated that infiltration of the liquid metal took place over a range of pressures for each different volume fractions of fibres, and increased fiber volume shifted the pressure range for infiltration to higher pressures, and therefore decreased the amount of metal infiltrated. This was because of that the molten metal firstly infiltrated into the larger pores of the preforms and the pressures up to about 1 MPa were high enough for the liquid metal to fill in these pores. However, some additional amount of pressure was required for the further progress of the metal into the narrower inter-fibres channels due to capillarity effect. The results showed that approximately 90% of the total pores in the preforms were impregnated by the liquid metal under the pressures about 1 MPa by increasing the pressures up to 2 MPa provided only the rest of remaining spaces which counted about 10 % of the total pores in the preforms to be completely infiltrated by the liquid metal. Metallographic studies showed that the fibres remained predominantly in a planar-random arrangement in the composites with relatively few parallel to disc axis after the infiltration process, but segregated into inter-dendiritic eutectic regions modifying the a-Al dendrites and the Silicon particles of the matrix alloy. The average size of the a dendrites measured on the unreinforced alloy was about 46 um and gradually decreased as the fibre volume fraction increased, reaching to about 18 um in the composite with 30% by volume of Saffil. The gradual decrease in the size of the a dendrites could be explained in terms of the rapid cooling of the liquid metal in the presence of the fibres ensuing high nucleation rates in the matrix. The optical and SEM studies also showed that the fibres predominantly segregated into inter- dendiritic areas resulting in clusters of the non-homogeneously distributed fibres in these areas, but as the fibre volume increased the distribution of the fibres has become more uniform throughout the matrix. A Number of primary silicon particles which preferentially nucleated on the alumina fibres were also observed in the microstructure. The unusual development of these particles was thought to be due to presence of alumina fibres which acted as easy nucleation sites at high undercoolings and the chemical reactions which produce silicon from the silica layer of the fibers. The modulus of elasticity of the composites showed a gradual increase with the increasing volume fractions of the fibers. The increase, however, did not obey a rule of mixture behaviour, and the rate of the increase was much slower at high volume fractions. This behaviour was thought to result from the following reasons: a) The aspect ratio, i.e. fiber length to fiber diameter, gradually decreased with the increased volume fraction of the fibers in the preforms b) The response of the composites to the age-hardening heat treatment was different depending on the fiber volume fraction they contained. The presence of the fibers accelerated the aging, the rate of which increased with the increased volume fraction of the fibers. Thus, the composites containing high volume of fibers became quickly overaged. -xi- The results obtained from the modulus measurements, were compared with some theoretical models developed for the composites containing short fibers, and found to be well in accordance with them. The tensile test results of unreinforced alloy and composites were in well agreement with those predicted by ROM equations up to the Saffil volume fraction of 15%. However, the strength of the composites with volume fractions higher than 1 5% deviated sharply from the linear behaviour and decreased continuously as the fibre volume increased. The reason for this is not clear, but it was evident from the examination of the fracture surfaces using SEM that this was probably due to the poor bonding between fibres and matrix alloy. Besides, the presence of the short fibres accelerated the age-hardening of the matrix alloy and the composites containing high volume fractions of fibres became easily over-aged shortly after the aging was commenced. This may also have an effect on the observed decrease in the strength at high volume fractions. From the results for these composites the minimum fibre volume fraction (V^ ) was found to occur at about 7% by volume of Saffil, and the critical fibre volume fraction (V^ ) which must be exceeded was found to be at approximately 9% by volume of Saffil. Dry wear tests carried out using the pin-on-disc technique showed that both the rate of wear and coefficient of friction decreased with the increased volume fraction of fibers in the composites, and for the 30 % Saffil fiber contained composite, for example, the wear rate reductions were at levels of about 300 % compared with the matrix alloy. Increasing load, on the other hand, caused the wear rates to increase and the coefficients of friction to decrease. Metallographic examination of the wear surfaces showed that in the unreinforced alloy the wear was of an adhesive type in which the wear took place by an extensive subsurface plastic deformation, whereas in the composites a transition from the adhesive type wear to delamination type with increase in volume fraction of fiber was evident, and in those containing high volume of fiber the wear was primarily due to delemination of the surface layers. Age-hardening characteristics of the unreinforced alloy and composites were studied by microhardness and resisitivity measurements and the results showed that the rate of aging increased with increased volume fractions of fibers. The maximum (peak) hardness in the primary a - Al dendrites was reached at an aging time of about 18 hours, but at the same period of aging time the composites became already overaged. X-ray studies showed that the formation and the growth of precipitate phases in the matrix alloy were accelerated by the presence of fiber, with the rates directly related to the volume fractions of fibers. The resisitivity measurements showed that the GP zones formed in the matrix alloy, but not in the composites. The reason for this was thought to be due to the concentration of quenched-in vacancies which must be high in the matrix alloy and gradually lower in the composites since the fiber/matrix interface from an ideal regions for sinking of the vacancies.