Nucleation And Growth Of Graphite And The Evolution Law In The Ni-C Alloy

Various morphologies of graphite including spherical graphite, flake graphite, compacted graphite, star-like graphite and the recently developed single-layer graphene have been observed in metallic solutions. As the formations of different graphite morphologies may significantly affect the properties of metal-carbon alloys, scholars did a lot of research involving the nucleation and growth of graphite in the metal melt. When the research achievement applied in the industrial production, cast iron may get better performance, such as the heat conducting property, the wear resistance, the fatigue property, the impact toughness and the strength, producing enormous economic benefits. Meanwhile, the research results not only enrich the nucleation and growth theory greatly, but also make a better understanding of solidification, promoting the study and application of casting alloy in a broader field.The article discusses the nucleation and growth regulation of graphite during the solidification in eutectic Ni-C alloy, and the precipitation and morphology evolution of graphite in virtue of carbon diffusion. The main research contents in the paper contain three parts:(1) Nucleation and growth law of graphite under different cooling rates in eutectic Ni-C alloyDifferent morphologies of graphite are obtained by using vacuum induction furnace, copper mold and single-roller melt spinning equipment. At moderate cooling rates, graphite exhibit lamellar structure. At fast cooling rates, spherical graphite is obtained. However, at ultra-fast cooling rates, there are graphite film on the surface of alloy ribbon and sphere in it. Through characterization of field emission scanning electron microscopy, high resolution transmission electron microscope and raman spectrometer, the micro-structure, initial morphology and growth process of graphite are researched.(2) Effects for nodulizing of SiBy adding eutectic Ni-Si alloy or pure silicon into the eutectic Ni-C melt, the effects to the quantity, distribution and morphology of graphite and the action mechanism are discussed. Via adding different inoculate such as, hypoeutectic Ni-Si, eutectic Ni-Si, hypereutectic Ni-Si and 75SiFe, the influence of Ni-Si alloy for inoculation and growth of graphite in casting iron is researched.(3) Diffusion behavior of carbon in Ni-C alloyThe Ni-C alloy, which is cooled by copper mold, is heat-treated in a vacuum resistance furnace for different time intervals. The quantity, distribution and morphology of graphite are all changed at different period of time. At last, graphite spheres with excellent roundness are obtained. Field emission scanning electron microscopy and Image-Pro Plus software are used to record and analyze the evolution process. The precipitation and evolution law, which is based on the diffusion of carbon atom, is described based on the characterization results.