Interface characterization of duplex metal-coated SiC fiber-reinforced Ti-15-3 matrix composites
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INTRODUCTION
CONTINUOUS SiC (SCS-6) fiber-reinforced titanium alloy matrix composites are of particular interest to the aerospace industry because of their high specific stiffness and strength, as well as good elevated-temperature properties.[1–7] Studies have shown the composites to be suited to components which are predominantly uniaxially stressed, such as a rotating ring, a turbine disc, etc.[1–7] Under these service conditions, improvement of the fatigue resistance of the composites is an important factor which should be realized before application. The fatigue properties of various types of SiC (SCS-6) fiber-reinforced Ti alloy matrix composites significantly depend on the properties of the matrix, fiber, and interface. Recently, it was reported that the fatigue properties are also highly dependent on the stress/strain conditions. Fiber bridging behind an advancing matrix crack tip was observed in a notched composite specimen, and a significant effect of this bridging on the deceleration of the matrix crack growth rate was reported.[8,9,10] In an unnotched composite, fiber fracture occurred at an early stage of fatigue and the fiber failure accelerated the fatigue crack growth rate.[11,12,13] Thus, prevention of early stage fiber fracture is essential in order to achieve the higher fatigue resistance of an unnotched composite. The early stage fiber fracture was correlated to the SiC fiber strength reduction (by '50 pct) by debonding of the SCS coating layer from the SiC fiber surface,[12,13,14] because S.Q. GUO, Postdoctoral Research Fellow, and Y. KAGAWA, Professor, are with the Institute of Industrial Science, The University of Tokyo, Tokyo 106-8558, Japan. A. FUKUSHIMA, Research Engineer, and C. FUJIWARA, Project Engineer, are with the Materials Research Section, Engineering Research Department, Nagoya Aerospace Systems, Mitsubishi Heavy Industries, Ltd., Nagoya 455, Japan. Manuscript submitted March 23, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
the debonding increases the stress concentration at the fiber surface, which originates from surface defects of the fiber.[14,15] The authors recently reported that the debonding of the SCS coating layer could be prevented by a duplex metal coating of Cu/Mo and Cu/W.[16] Thus, it is expected that the fatigue resistance of an unnotched composite is increased by the use of these duplex metal coatings. The characterizations of the interface reaction behavior of these composites and the effect of the duplex metal coatings on interface shear sliding stress have not yet been examined, however. As the composites are expected to be used at elevated temperatures, the stability of the interface after longterm thermal exposure is also important. The emphasis of this article is on the effect of duplex metal coatings on the interfacial reaction behavior and interfacial shear sliding stress of the composites before and after long-term thermal exposure conditions at high temperatures. II.
EXPERIMENTAL PROCEDURE
A. Composite Materials Ti-15V-3Al-3Sn-3Cr (hereafter denot
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