Process Modeling for Titanium Aluminide Matrix Composites

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PROCESS MODELING FOR TITANIUM ALUMINIDE MATRIX COMPOSITES

C.C. BAMPTON AND J.A. GRAVES Science Center, Rockwell International Corporation, 1049 Camino Dos Rios, Thousand Oaks, CA 91360 K.J. NEWELL, R.H. LORENZ North American Aircraft, Rockwell International Corporation, 201 North Douglas Street, El Segundo, CA 90245 ABSTRACT Consolidation of continuous fiber-reinforced titanium aluminide matrix composites (TMC) by the foil/fiber/foil method has traditionally taken an empirical approach utilizing processing and error. In an effort to reduce cycles derived by simple trial the empirical nature of producing TMC, a simple but effective analytical approach is employed. This approach analyzes the effect of fiber and foil geometries on consolidation parameters by combining a physical constitutive creep model with computational methods of interpreting raw materials characterization data. Examples of SCS-6/super a2(Ti-25AI-lONb-3Mo-lV) and Saphikon/7-TiAl composites consolidation are discussed by comparing the model predictions with equivalent validation specimen microstructures. 1.

INTRODUCTION

In the foil/fiber/foil consolidation process, heat and pressure are applied to a composite lay-up of metal foils and fiber mats usually by means of a hot isostatic press (HIP) for the purpose of: (1) embedding the fibers in the metal matrix by creep forming the metal around the fibers, and (2) providing suitable conditions for diffusion bonding the metal to metal and fibers to metal surfaces as they make contact. Readily apparent is that the selection of bonding parameters for a given combination of fiber and matrix is a principal criterion for the achievement of well consolidated titanium matrix composite. Equally important however, is that the consolidation take place in a reasonable amount of time, at practical temperatures and pressures while taking into account the effect of these parameters on the composite constituents and residual stresses. Issues which are specific to processing of TMC's are: (1) Consolidation temperatures which are close to or above phase transition temperatures are often used in attempts to facilitate metal flow and avoid fiber mechanical damage or displacement. This is not, however, generally desirable with regard to the final composite microstructure; (2) Titanium-based matrices have the ability to absorb (interstitially) surface oxides. This enhances foil-to-foil diffusion bonding but may adversely affect phase stabilities and mechanical properties in the oxygen-rich areas; (3) All titanium-based alloys are known to be reactive with currently available structural fibers. This may degrade the fiber integrity and embrittle the near-fiber matrix. Minimized temperatures and exposure times are therefore generally desirable. Mat. Res. Soc. Symp. Proc. Vol. 273. 01992 Materials Research Society

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Conventional titanium alloys, such as 0-21S, are generally "well-behaved" as TMC matrices, consolidating easily and predictably. Even in these cases, however, we have found successful consolidation cycles, usi

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