Formation Mechanism of Spheroidal Graphite in Nickel Alloy Cooled Ultra-Rapidly

Abstract

The formation mechanism of spheroidal graphite (S.G.) in high purity Ni–C and Ni–C–Mg alloys cooled ultra-rapidly from the liquid state at four different rates (Exp. 1–Exp. 4) was investigated. The cooling rate increased from 2×10⁴ to 5×10⁵ (K/s) in the order of the experimental number. Contrary to the previous result on S.G. in Ni–C alloy with impurities, no core of the different phase was found at each center of S.G. (1–3 μm in dia.) in Ni–C alloy in Exp. 1 and a number of the S.G. appeared to have collapsed or burst during the process of growing. In Ni–C–Mg alloy samples of Exp. 1 and Exp. 2, the S.G. with various sizes (1–10 μm in dia.) were observed; some had hallow in each center and some appeared to have collapsed or burst. In Ni–C samples of Exp. 2, Exp. 3 and Exp. 4, and Ni–C–Mg samples of Exp. 3 and Exp. 4, only a few S.G. was found.
The results could provide two growing mechanisms of the S.G., namely, a heterogeneous nucleation of graphite on a core of the impurity phase formed during cooling, and a nucleation of graphite on the interface between a gas bubble and supercooled liquid nickel. The gas bubble came from vaporized Mg or Ar atmosphere; the Ar gas had been contained in porous alloy sample made of powder of Ni and graphite by pressing. At high cooling rate, the increase or decrease in gas pressure in the bubble would cause the collapse and burst, thus results in a few S.G.