超急冷したNi-CおよびNi-C-Mg系合金と合金中の球状黒鉛の観察

抄録

Liquid Ni-C and Ni-C-Mg alloys (12.5 at% C) were solidified ultra-rapidly using twin rollers at various cooling speeds. A number of spheroidal graphite (S.G.) with a diameter of a few micron meters were observed by TEM in both of the alloys. At each center of S.G. in Ni-C alloy, a small different phase was recognized, in which some of the calcium, silicon, aluminum, potassium, magnesium and oxygen were detected. The result shows that S.G. in Ni-C alloy could be formed through heterogeneous nucleation on a nucleus formed from the impurities of graphite reagent. In contrast, S.G. in Ni-C-Mg alloy did not contain such kind of nucleus, having shown more roundish shape than that in Ni-C alloy. Supersaturated carbon in nickel has a tendency to crystallize at the surface of bulk nickel, which was recognized through carbon films precipitated on surface of all the samples. Accordingly, the S.G. in Ni-C-Mg alloy could be formed through nucleation at the interface between magnesium bubble and nickel, and growing up to the center. X-ray diffraction of both alloys proved two types of nickel containing 1.2-1.3% C and 7.5-8.5% C, respectively. In case of Ni-C alloy solidified at a rather slow cooling speed, Ni₃C in addition to the two nickel phases was confirmed. For the samples solidified at a most rapid cooling speed, only one type of nickel containing 1% C was shown. The electron diffraction of carbon film on nickel surface showed that basal plane of graphite preferentially covered the nickel.