Microstructure formation of rapidly/sub-rapidly solidified Gd-Co-Ti immiscible alloys

Solidification experiments were conducted for ternary Gd-Co-Ti immiscible alloys by copper mold casting. The impacts of the composition and cooling rate on the microstructure features and the properties were investigated. The samples with a composite microstructure were acquired, in which the subglobose TiCo-rich particles homogeneously distribute in the Gd matrix. The microhardness of Gd-Co-Ti alloys increases with the content of Co and Ti and the cooling rate during solidification. To describe the microstructure formation of Gd-Co-Ti immiscible alloys during rapid/sub-rapid solidification, a theoretical model was proposed, and it takes into consideration the comprehensive effects of the nucleation, diffusional growth and spatial motions of the TiCo-rich phase droplets. The solidification microstructure evolution was simulated. The numerical results are well consistent with the experimental data, which verifies the validity of the developed model. Calculations demonstrate that the nucleation of the TiCo-rich droplets only takes place in the liquid-liquid transformation stage, and no nucleation events of the TiCo-rich droplets occurs in the liquid-liquid-solid three-phase transformation stage. The number density (N) and maximum nucleation rate (I-max) of the TiCo-rich droplets/particles is exponentially associated with the cooling rate (T-nuc) of the alloy during the nucleation period of the TiCo-rich droplets. Besides, the Ostwald coarsening of TiCo-rich phase droplets can be ignored under the current cooling conditions.