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The degradation of the mechanical properties of graphite fibers in intimate contact with nickel at high temperatures is due to the interdiffusion of carbon and nickel. There are two mechanisms by which the damage occurs: diffusion of carbon into the nickel and recrystallization of the fibers. There has been much disagreement in the literature and seemingly conflicting data. These apparent conflicts in data have lead to two different schools of thought, each favoring one mechanism over the other. On careful analysis of the entire body of the literature, however, a unified picture emerges. Below a critical temperature Tg the fiber is weakened by diffusion into the nickel matrix. Above Tg, the fiber recrystallizes at an appreciable rate, catalyzed by the diffusion of nickel into the fiber. The value of Tg varies with the type of fiber, and it is significantly higher for high modulus fibers than for high tensile strength fibers.
Research associate for American Cyanamid Company, National Institute of Standards and Technology, Gaithersburg, MD
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