Microstructural development of SCS-6 SiC fibers during high temperature creep
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Microstructural development of SCS-6 SiC fibers during high temperature creep Lucille A. Giannuzzi,a) Charles A. Lewinsohn,b) Charles E. Bakis, and Richard E. Tressler Center for Advanced Materials, The Pennsylvania State University, University Park, Pennsylvania 16802 (Received 27 February 1997; accepted 10 September 1997)
Microstructural development of SCS-6 SiC fibers induced by creep deformation at 1400 ±C is presented. Grain growth occurs in all SiC regions of the fiber during creep. Portions of the SiC4 region transform from bSiC to aSiC growing at the expense of the bSiC. The SiC1 through SiC3 regions of the fiber consist of a distinct (C 1 bSiC) two-phase region. The grain growth of the bSiC grains in the two-phase region is not as extensive as in the SiC4 region, suggesting that the presence of excess carbon may inhibit the growth of bSiC.
I. INTRODUCTION
The SCS-6 SiC fiber (CVD Fibers manufactured by Textron Specialty Materials, Inc., Lowell, MA) is commercially produced by the chemical vapor deposition (CVD) of bSiC on a carbon core and is used as a reinforcement in both metal matrix and ceramic matrix composites. The critical issues relevant to the development and use of these composite materials are related to the thermal and mechanical stability of the reinforcement phase, and the properties of the fiber-matrix interface. In particular, the oxidation, stress rupture, and creep behavior of the fibers limits the environmental conditions for use of composite materials. Research in our laboratory has focused on the creep behavior of SiC fibers fabricated by chemical vapor deposition.1 The creep conditions included testing at stress levels ranging from 190–500 MPa, in air, over a temperature range between 1200 and 1400 ±C, for a duration of up to 500 h. It was noted that the SCS-6 fiber (as well as the SCS-9 and the developmental SCS-50 mm fiber) undergoes primary creep (i.e., the creep rate constantly decreases as a function of time) throughout the lifetime of the creep test. For constant test conditions (i.e., time, stress, and temperature), the SCS-9 fiber exhibited the largest creep strain value, the SCS-50 mm fiber exhibited the lowest creep strain value, and the SCS-6 fiber exhibited an intermediate creep strain value.1 The differences in creep rates between the SCS-6 and the SCS-9 and SCS-50 mm fibers could not be explained by differences in initial grain sizes alone.2 In addition, creep testing of the SCS-50 mm fiber both with and without the carbon core showed that the core had a negligible a)
Currently at the Department of Mechanical, Materials & Aerospace Engineering, University of Central Florida, Orlando, Florida 328162450. b) Currently at Pacific Northwest Laboratory, Battelle Blvd., Richland, Washington 99352-0999. J. Mater. Res., Vol. 13, No. 7, Jul 1998
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effect on the creep behavior. Hence, the primary nature of the creep behavior is unlikely to be a result of a shift in load sharing b
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