On the formation mechanism of in-situ synthesized TiB-TiC reinforced Ti matrix composites, twomechanisms are prevalent:1) at a low temperature, the solid-state diffusion mechanism is predominant;2) at a high temperature,a solution and precipitation mechanism is dominant. The latter mechanism is principal reaction pattern in the synthesis process, evidenced by the obvious existing of TiB-TiC eutectic.
The detailed process could be explained as follows. Firstly, the molten Ti reacts with the solid BaC to form large aggregation of TiB and TiC . Subsequently, most of ceramics formed by solid reaction would undergo disintegration, dissolution to form Ti-Ni-B-C melting pool due to the high reaction temperature in the laser cladding process. Finally, solidification reaction for composites Ti-Ni-B-C takes place with the gradually decreasing temperature. According to the microstructure shown in Fig.1, it is suggested that nucleation and growth of TiC occur firstly, then TiB-TiC eutectic precipitates abundantly. The remaining Ti-Ni liquid solidifies as a dual-phase intermetallic matrix, where the dendritic TiNi primary phase precipitates with the flower-like particles as the heterogeneous nucleation sites, producing a unique encapsulated TiNi dendrite with a hard flower-like TiB-TiC core, and the interdendritic residual melt between the TiB-TiC cored TiNi dendrite solidifies as the harder Ti2Ni.
Fig. 1 SEM images showing typical microstructures of laser clad coating (a, b), electron back-scattering micrograph (c) and detailed SEM image of TiB-TiC eutectic (d)
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