INELASTIC CONTACT DEFORMATION OF METAL COATED FIBERS D. M. ELZEY, R. GAMPALA* and H. N. G. WADLEY{ Department of Materials Science and Engineering, University of Virginia, Charlottesville, VA 22903 (Received 16 April 1997; accepted 23 June 1997) AbstractÐMetal matrix composites (MMCs) can be synthesized by aligning metal coated ceramic ®bers in a shaped container and applying pressure at elevated temperatures. The high stresses created at the contacts between neighboring ®bers cause inelastic matrix ¯ow that ®lls inter®ber voids, reduces inter®- ber separations and results in composite densi®cation. The rate of densi®cation depends on the con- tact's resistance to ¯ow. Current contact mechanics models are unable to adequately predict this resistance because they do not account for the eect of the (elastic) ®ber. Closed-form solutions for contact stress±displacement rate and contact area±strain relationships are used to describe metal coated ®ber blunting as a function of ®ber volume fraction, matrix material non-linearity (i.e. creep stress exponent) and ®ber packing geometry. The solutions contain two unknown coecients (c and F) which are evaluated using the ®nite element method. A simple model for the consolidation of coated SiC ®bers is developed in terms of the coecients, c and F. The model indicates that materials with a low creep stress exponent are more dicult to densify as the ®ber volume fraction increases whereas, per- fectly plastic materials exhibit a relatively weak dependence on the ®ber volume fraction. # 1997 Acta Metallurgica Inc. 1. INTRODUCTION Physical vapor deposition (PVD) processes are beginning to be used to produce metal coated cer- amic ®bers which can be used to create continuous ®ber-reinforced metal matrix composites (MMCs) [1±4]. By applying reactive metals such as titanium via a vapor phase process allows deposition of the matrix onto a ceramic reinforcing ®ber (e.g. SiC, Al 2 O 3 ) held at a relatively low temperature ( 0 6008C), thus avoiding chemical attack of the ®ber or its coating. By rotating the ®ber during depo- sition, a relatively uniform thickness coating can be applied [3]. Once a suitable thickness of alloy coat- ing has been deposited onto the ®ber, a fully dense composite component can be synthesized by align- ing a bundle of the ®bers and subjecting them to a consolidation process such as hot isostatic or vac- uum hot pressing (HIP or VHP) [5]. During the consolidation step, pressure is nor- mally applied at elevated temperatures (T 00.6 T m , where T m is the absolute melting temperature) caus- ing compressive stresses to develop at contacts. This results in creep ¯ow into the voids between ®bers and densi®cation of the composite [6]. Creep ¯ow is enhanced by the retention of a ®ne grain size in the metal phase. An optimal consolidation process sche- dule should exploit this and result in complete den- si®cation while minimizing ®ber/matrix reactions and bending or fracture of ®bers [7]. The retention of matrix voids in incompletely densi®ed com- ponents, excessive interfacial reaction or large ®ber bending can all lead to potentially serious degra- dation of the tensile, creep and cyclic strengths of fabricated components [8, 9]. The identi®cation of an optimal processing path is dicult by experimen- tation alone because of the competing phenomena (grain growth, matrix creep, ®ber/matrix reaction and ®ber bending) activated by a consolidation pro- cess. This has stimulated an interest in the develop- ment of process models [10]. For example, models have recently been developed for predicting the evolution of density, interfacial reaction zone thick- ness and ®ber microbending/damage during the consolidation of titanium matrix composite (TMC) monotapes produced by plasma spray deposition [11±13]. After experimental testing, these models have been used to successfully design optimal pro- cesses [14] and even new feedback control algor- ithms [15, 16] that expand the combinations of metal alloy and ®ber that can be successfully com- posited. This paper deals with the development of an analogous model for early stage consolidation of metal coated ®bers. During the consolidation of metal alloy coated ®bers by either HIP or VHP, the externally applied pressure is transmitted internally throughout the body via points where the coated ®bers contact (Fig. 1). As material is displaced at the contacts, the centers of the coated ®bers approach, resulting Acta mater. Vol. 46, No. 1, pp. 193±205, 1998 # 1997 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 1359-6454/98 $19.00 + 0.00 PII: S1359-6454(97)00215-2 *Now at Concurrent Technologies Corporation, Johnstown, PA, U.S.A. {To whom all correspondence should be addressed. 193