Superplastic Deformation of Continuous Fiber Reinforced Titanium Matrix Composites

Abstract

Continuous fiber reinforced titanium matrix composites (TMCs) are attractive as potential structural materials for aerospace applications, because of their high specific strength and stiffness. However, TMC parts have not yet been put into practical use due to their limited damage tolerance and the enormous production cost resulting from preliminary forming and elaborate tooling for consolidation. In order to reduce the production cost, superplastic TMC sheets (SiC/Ti-4.5Al-3V-2Mo-2Fe) were developed, and the deformation characteristics and cavitation behavior were investigated. In this study, plasma-sprayed preforms, which replace conventional woven preforms, were newly developed to improve the cavitation resistance. The superplastic deformation characteristics and the cavitation behavior of SiC/Ti-4.5Al-3V-2Mo-2Fe composites made of sprayed preforms (spray composites) were investigated. The fiber spacing in the spray composites was controlled more uniformly than that in composites fabricated from woven preforms (woven composites), and two closely spaced fibers were rarely found in the spray composites. The spray composites showed cavitation-induced superplasticity with the strain rate sensitivity exponent (m) being 0.58 at 1048 K, over the strain rate range of 5 x 10 -5 s -1 to 1 x 10 -3 s -1 . This indicated greater elongation than that in the case of the woven composites, which may be due to the improved uniformity of fiber distribution. Most of the defects observed in the deformed spray composites were defects due to fiber/matrix interface separation (type-1). On the other hand, those observed in the deformed woven composites were defects caused by weaving ribbons (type-2) or narrow fiber spacing (type-3). They were less related to the deformation conditions. This suggested that the sprayed preforms were effective in preventing the occurrence of type-2 and type-3 defects. The occurrence of type-1 defects was strongly related to the maximum flow stress during deformation, and the occurrence of such defects rapidly increased with the maximum flow stress exceeding the threshold stress of 18 MPa.