Abstract
The mechanism of retardation of mottling tendency in white iron castings by a small addition of bismuth has not been clarified yet. With a view to analysing this mechanism, the effect of bismuth addition on the morphological change of primary graphite in hypereutectic Ni-C alloy was studied using scanning electron microscope, electron probe microanalyser and quantitative television microscope.
Primary graphite consisting of hexagonal stacking structure was obtained in this alloy when cooled slowly at the cooling rate of 15 and 5°C/min. From the morphological study of graphite structure which appeared in hypereutectic Ni-C alloy, it is obvious that there is adsorption of bismuth on the plane of graphite structure which is perpendicular to the basal plane. This behaviour affects the formation of hexagonal structure resulting from the appearance of anisotropy in the graphite crystal, retards the growth of graphite, and then leads to the formation of stacking structure which may be attributed to two dimensional nucleation.
