A cta me tall. mater. Vol. 41, No. 8, pp. 2297-2316, 1993 0956-7151/93 $6.00 + 0.00 Printed in Great Britain. All fights reserved Copyright 1993 Pergamon Press Ltd

MODELING THE DENSIFICATION OF METAL MATRIX COMPOSITE MONOTAPE

D. M. ELZEY and H. N. G. WADLEY Department of Materials Science and Engineering, University of Virginia, Charlottesville,

VA 22903, U.S.A.

(Received 28 September 1992; in revised form 9 February 1993)

Almraet--The consolidation of monotapes consisting of a layer of continuous aligned ceramic fibers embedded in a plasma sprayed metal or intermetallic matrix is becoming a preferred approach for the processing of high performance composite systems. We present a first model that enables prediction of the density (and its time evolution) of a monotape lay-up subjected to a hot isostatic or vaceum hot pressing consolidation cycle. Our approach has been to break down the complicated (and probabilistic) consolidation problem into simple, analyzable parts and to combine them in a way that correctly represents the statistical aspects of the problem, the change in the problems interior geometry and the evolving contributions of the different deformation mechanisms. The model gives two types of output. One is in the form of maps showing the relative density dependence upon pressure, temperature and time for step function temperature and pressure cycles. They are useful for quickly determining the best place to begin developing an optimized process. The second gives the evolution of density over time for any (arbitrary) applied temperature and pressure cycle. This has promise for refining process cycles and possibly for process control. Examples of the models application are given for Ti3A1 + Nb, ), TiAI, Ti6AI4V and pure aluminum.

1. INTRODUCTION

Continuous fiber reinforced metal and intermetallic matrix composites are attracting increasing interest for high temperature aerospace applications because of their attractive specific stiffness and high tem- perature strength. Many methods are being devel- oped for their processing (for recent reviews see Mehrabian [1] and others [2, 3]). In one increasingly favored approach, foils (also called monotapes) con- sisting of uniformly spaced continuous fibers in a porous alloy matrix are produced using plasma spray methods [4]. Lay-ups of these porous, unidirectional reinforced monotapes are then consolidated to form near net shape composite components using pro- cesses such as hot isostatic pressing (HIP), vac- uum hot pressing (VHP) and roll bonding. Here we develop predictive models for the densification of these lay-ups. We seek models that predict the effect of process conditions (i.e. pressure, temperature histories) and foil attributes (geometry, matrix prop- erties . . . . ) upon densification. Our approach is inspired by that of Arzt, Ashby and others, who have developed a predictive densification model for metal, alloy and ceramic powders undergoing hot isostatic pressing (HIPing) [5].

The plasma spray process consists of winding 100-150tim diameter silicon carbide or aluminum oxide fibers onto a mandrel which is then rotated under a stream of (plasma melted) matrix alloy droplets with diameters of 100-200/~m [4]. The droplets, upon contact with the substrate, spread out

coating the fibers and filling the interstices between with metal. Usually, the droplets freeze before fully filling the interstices creating small unconnected pores. Thus, at the completion of the spray process one obtains a unidirectionally reinforced foil with one relatively smooth surface (the one in contact with the mandrel during deposition), one much rougher surface (determined by the spreading of successive droplets), and several percent of closed internal porosity. The foils are then stacked to achieve a desired lay-up and placed either in the die of a vacuum hot press or suitably shaped HIP tooling and subjected to heating and pressurizing cycles to both fully densify the lay-up and obtain a near net shape component.

During the consolidation process one seeks to utilize the deformation mechanisms of plasticity, power law creep and diffusional flow to close both interfoil pores (i.e. those formed by contacts between foils) and intrafoil (i.e. the closed) porosity. The model we seek should predict the evolution of lay-up density over time for any given (arbitrary) appli- cation of pressure and temperature, foil geometry (foil thickness, surface roughness, internal porosity), and matrix material properties. There are several important reasons for wanting to do this. First, one would like to find the most convenient process cycle to densify a component without resorting to costly and time consuming trial and error experiments. Second, other phenomena during processing may degrade the component's properties, i.e. fibers may be damaged or broken, the fiber-matrix interface may

2297

2298 ELZEY and WADLEY: MODELING OF METAL MATRIX COMPOSITE MONOTAPE

degrade, or the matrix may undergo phase trans- formations (because the foil matrix is often in a metastable, rapidly solidified state [6]). Ideally, one would use the model to design a process cycle that reaches the target density without adversely impact- ing the component's properties. It is quite difficult to do this purely by experiment because of the many things that must be optimized and their complicated dependence upon the process path. Third, one is interested in determining the role of matrix proper- ties on the feasibility of consolidation. For example, we are interested in discovering which matrices are most easily consolidated and how this is affected by previous processing steps. Fourth, the model should allow assessment of the influence of foil surface roughness/internal porosity upon consolida- tion in order to identify those that will give the best results. This can be used to help in defining a target for the plasma spray process step. Finally, there is increasing interest in using the model for feedback control of consolidation processing [7]. Here, we derive a first model that is capable of each of these tasks and show its application to the densification of several systems of current interest. Its appli- cation for understanding and controlling some of the other phenomena during processing mentioned above has been presented elsewhere [8, 9].

2. MODEL FORMULATION

The cross-section of a typical continuous fiber reinforced monotape produced by the Induction Coupled Plasma Deposition (ICPD) process is shown in Fig. l(a). One sees that the tapes produced by this process are characterized by internal porosity, one very rough surface (the other side in contact with a mandrel during deposition remaining rela- tively smooth) and a certain fiber volume fraction. Figure l(b) shows a fully consolidated lay-up of TiaA1 + Nb/SiC monotapes (volume fraction of fiber

Fig. 1. (a) Cross section of a plasma sprayed MMC mono- tape (fibers are 140#m diameter SCS-6 (SIC) within a Ti-24Al-llNb alloy matrix). (b) Ti-24-11/SCS-6 mono-

tapes following consolidation processing.

is 25%). Figure 2 illustrates schematically the physi- cal situation encountered in a lay-up at the begin- ning of consolidation. When the monotapes are stacked one on top of another, there is a large (30--40) volume percent of porosity. Most is associated with contacts. Typically, for a total initial void volume fraction of 0.35, the surface roughness contributes about 0.3 and internal porosity the remaining 0.05.

"~ Initial deformation at contact points

Interlaminar porosity due to surface roughness

Intermetallic Matrix

lib

I ~ B

Fiber (e.g. SiC, AI203)

HIP Cannist [._j v Intralaminar (closed) porosity

Fig. 2. Schematic view of the situation encountered during consolidation processing of a lay-up of plasma sprayed monotapes.

ELZEY and WADLEY: MODELING OF METAL MATRIX COMPOSITE MONOTAPE 2299

This obviously is determined by statistical proper- ties of the roughness and it is important that this statistical representation be included in a realistic model. A second important consequence of this physical situation is that even in the absence of reinforcing fibers and the application of hydrostatic stress, the aggregate will deform anisotropically. This is because deformation (at the contacts) occurs quite readily perpendicular to the laminate plane, whereas the compressive stresses in the two orthogonal direc- tions are well supported by the relatively dense (and thus more deformation resistant) monotapes. The totaling used for HIP of planar components also better supports the in-plane loads and encourages only strains in the through thickness direction. The presence of fibers also inhibits in-plane macroscopic deformations with the result that almost any uniform applied load combination leads to a state of con- strained uniaxial compression during most of the early stages of densification.

As the lay-up is raised to a high temperature and pressure, densification is accomplished by a complex sequence of deformations. Interfoil contacts, which initially support the highest stresses, begin to deform by plastic yielding and time-dependent creep. This allows more contacts to form (and the areas of previous ones to increase). As the applied stress rises, the stresses around the closed pores within the foil become sufficient to cause their collapse, again by a combination of plasticity and creep. The addition of these deformations results in a net shrinkage (confined almost totally to the through thickness direction) and densification of the laminate. The rate of densification will depend upon process variables (pressure, temperature and time), material proper- ties (which determine the extent of deformation by plasticity and creep processes) a