Processing, microstructure, and properties of co-continuous alumina-aluminum composites

MATERIALS SCIENCE & ENGINEERING ELSEVIER

Materials Science and Engineering A195 (1995) 113-119

A

Processing, microstructure, and properties of co-continuous aluminaaluminum composites M.C. Breslin a, J. Ringnalda a, L. X u a, M. Fuller a, J. Seeger a, G.S. Daehn a, T. Otani b, H.L. Fraser a aDepartment of Materials Science and Engineering, The Ohio State University, 2041 College Road, Columbus, 01443210, USA bDepartment of Mechanical Engineering, College of Industrial Technology, Nihon University, Tokyo, Japan

Abstract A novel co-continuous composite of AI203 and AI has been developed, consisting of approximately 65% (by volume) of the ceramic phase. It is formed by a liquid phase displacement reaction, involving the displacement of Si from S i O 2 and its replacement by Al. A model for the formation mechanism is presented, based on the reaction thermodynamics and the associated experimentally deter° mined transformation kinetics. It is shown that the process is essentially near-net shape, in which the features of the S i O 2 precursors are faithfully reproduced in the composie product. Various physical and mechanical properties that are exhibited by this composite have been determined and are presented.

Keywords: Aluminium; Composites; Oxygen

1. Introduction There has been an increasing interest in the production of novel composite materials, especially those involving ceramic matrices, or at least high volume fractions of these phases (e.g. Refs. [1,2]). Among the various types of novel composites, those formed by insitu means have generated interest because they offer many potential advantages over similar materials produced in traditional ways. Thus, traditional composites are formed by introducing second phases, usually reinforcing in nature, with the aim of enhancing particular properties. While many of these materials do indeed offer interesting properties, they are usually quite difficult to fabricate, especially if the reinforcing phase consists of whiskers or very refined particles. For example, there are often manifestations of deleterious interfacial reactions between thermodynamically incompatible phases. Alternatively, in situ composites may be more simple to fabricate because of the intrinsic thermodynamic compatibility of the product phases. Examples of in situ composites include systems exhibiting spinodal decomposition, second phase precipitation, and reaction synthesis. In each of the

above systems, the composite product is more stable than the parent phase(s), and often will exhibit a very attractive balance of mechanical and physical properties. Co-continuous composites are especially interesting since each phase is a continuous network penetrated by similar networks of the other constituents. Consequently, many of the more attractive properties of each of the constituent phases may be retained in the product composite [3]. Examples of such systems include Corning's VycorTM glass [3] and the Lanxide Corporation's DIMOX TM material [4]. Although in principle the fabrication of interpenetrating phase composites may be more simple than those processed by traditional methods, processing times may be inordinately long and involve extensive thermal excursions, both of which contribute to high costs. In addition, in many cases, microstructures are not homogeneous, which leads to wide variations in properties. These problems have served to limit applications of these interpenetrating phase composites. In this paper we describe a new method of producing a co-continuous composite structure which exhibits excellent physical and mechanical properties and

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relative ease in processing [5]. Formed as a result of a liquid phase displacement reaction between liquid A1 and a geometric SiO 2 precursor, the product material, consisting of approximately 65% (by volume) of A1203 and the remainder of AI, is a co-continuous ceramicmetal composite (C 4) material. This displacement reaction is characterized by near-net shape processing yielding a composite body of the same size and dimension as the SiO 2 precursor.

2. Experimental procedure The processing of C 4 is described in greater detail in the following sections; however, in the bulk of the experiments described in this paper, the following conditions were followed. Preforms of SiO 2 were produced either from rods of optical purity fused quartz glass or by slip casting a slurry containing relatively high purity fused SiO 2 powders. Transformation of the preforms has been performed over a range of temperatures, between 700 °C and 1300 °C, in liquid AI heated by electrical resistance. Transformation times varied and are indicated below. The microstructures of the resulting composites were characterized using optical metallography and also techniques involving scanning electron microscopy (SEM) and transmission electron microscopy (TEM), coupled with light element energy-dispersive X-ray spectroscopy (EDS)for compositional analysis. Elastic modulus measurements were made both dynamically with Grindosonic modulus measuring equipment (J.W. Lemmens, Inc.) and statically from stress-strain data in tension-compression tests. Fracture toughness measurements were performed in three-point bending, details of these tests being described elsewhere [6]. The thermal conductivity data were determined by laser flash method at the Thermophysical Properties Laboratory at Purdue University.

Fig. 1. SEM micrograph of the microstructure of C4. The phase exhibiting above-background contrast is the A1203,whereas that exhibiting below-background contrast is the Al-rich phase.

3. Results and discussion Fig. 2. SEM micrograph of the C4 composite, following leaching of the metallic phase. The continuity of the A1203 phase is evident.

3.1. D e v e l o p m e n t a n d microstructure

The displacemem reaction which results in the formation of C 4 materials involves the submersion of a preform of SiO 2 in liquid A1 held at a temperature above 1000 °C, whence the following reaction occurs: 3SiO2(s ) + 4AI(1) -" 2AlaO3(s ) + 3[Si]n 1

(1)

A micrograph which is representative of the microstructure of the C 4 composite is shown in Fig. 1. If the A1 is leached out of the structure, the continuity of the ceramic phase becomes evident, as shown in Fig. 2. In

the displacement reaction, the SiO 2 preform serves as both the geometric precursor which defines the final shape of the composite as well as the source of oxygen from which the A1203 is formed. This suggests that the volume fraction of the ceramic phase in the final composite is dependent on the density of the SiO 2 precursor, which of course may vary over a rather large range, both intrinsically and also in terms of the green

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Materials Science and Engineering A195 (1995) 113-119

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density of the slip. During the displacement reaction, provided that sufficient time is allowed, all the SiO2 is consumed, and the Si is dissolved in the liquid A1 within the composite. Following the displacement reaction, the Si diffuses out of the composite into the surrounding AI bath. The extent to which this diffusion occurs, and therefore the concentration of Si in A1 in the composite after the transformation, depends on the time and temperature of processing. Following the transformation, during cooling to room temperature, the Si becomes supersaturated in the AI both in the composite and also in the surrounding bath, and it precipitates out of solution. This precipitation is potentially useful in terms of increasing wear resistance of the composite, a property which enhances the applicability of this material. The processing temperature of C a materials is significant. If processing is conducted at temperatures at or above 1000 °C, an excellent balance of properties is exhibited (see Table 1). These properties are associated with the microstructure of the C 4 composite (described below), which consists in the main of a-Al:O 3 and A1. It is found that, when processing is effected at temperatures below approximately 1000 °C, the evolution of microstructure involves the formation of 0-A1203 on a very fine scale [7,8]. The properties associated with this form of microstructure are very much inferior to those of the a-A1203 phase [8]. The reason for this change in transformation characteristics is not well understood. The microstructure of the C 4 composite has been examined in detail. EDS spectra recorded from the ceramic and metallic phases are shown in Figs. 3(a) and 3(b) respectively. It may be seen that these spectra are relatively simple, the ceramic phase being essentially phase-pure AI203 and the metallic phase containing o ~ y small amounts of Si. A T E M micrograph of C 4 microstructure is shown in Fig. 4, from which it may be seen that the structure as formed is relatively defect

Fig. 4. TEM micrograph of a C4 sample showing the nature of the microstructure in greater detail.

free. An interesting observation involves the fact that the interfaces between the constituent phases are relatively robust, with little tendency for debonding to occur when thinned to a thickness of about 100 nm for T E M studies. The nature of these interfaces between the metallic and ceramic phases has been studied in some detail. Fig. 5(a) shows a micrograph recorded at higher magnification than that shown in Fig. 4, revealing the faceted nature of the interface. An H R T E M micrograph is shown in Fig. 5(b), again illustrating the faceted nature of the interfaces. The interfaces appear to be of high integrity with an additioanl effect of mechanical keying afforded by the development of the facets along the interfaces. There is of course no tendency for any reactions or instabilities to develop at

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MaterialsScience and Engineering A195 (1995) 113-119 "~ 5O

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