MODIFICATION OF TRANSVERSE CREEP-BEHAVIOR OF AN ORTHORHOMBIC TITANIUMALUMINIDE BASED TI-22AL-23NB SICF COMPOSITE USING HEAT-TREATMENT/

The feasibility of improving the transverse creep properties of an ort horhombic titanium aluminide matrix composite reinforced with unidirec tional SCS-6 SiC fiber via modification of matrix microstructure was i nvestigated using post-consolidation heat treatments. A Ti-22Al-23Nb/S CS-6 4-ply composite was subjected to heat treatments which consisted of solutionizing at either above or below the beta-transus temperature of the matrix alloy, followed by controlled cooling and aging within the orthorhombic (O) phase field. Mechanical testing showed that the t ransverse (90 degrees) creep behavior of the titanium alloy composite in the temperature range 650-760 degrees C is significantly improved b y a supra-transus solution heat treatment followed by the aging, witho ut any adverse effect on the longitudinal (0 degrees) tensile strength of the composite. This improvement in the creep response was attribut ed to an increase in the volume fraction of the orthorhombic phase and the lath morphology of the (O + beta) two-phase regions in the matrix microstructure. The effect of sub-transus heat treatment was less pro nounced and the transverse creep resistance of the composite decreased with increasing volume fraction of the alpha(2) phase in the matrix. Examination of creep ruptured 90 degrees composite specimens showed th at transverse creep damage in the form of fiber/matrix interfacial deb onding occurs predominantly at very closely spaced or touching fibers. (C) 1998 Elsevier Science S.A. All rights reserved.