Electron Microscopy Structure Study of Laser-Clad TiC-Ni Particle-Reinforced Coating

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The microstructure of a laser-clad TiC-Ni particle-reinforced coating on 1045 steel was studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and ion microprobe mass spectroscopy (IMMS). The microstructural constituents of the clad layers (CLs) were analyzed to be TiC particles, γ-Ni primary dendrites, and interdendritic eutectics of γ e -Ni plus M23(CB)6 and M6(CB) carboborides. Three growth mechanisms of the original TiC particles were found: (1) stepped lateral growth at the edges, (2) radiated and cylindrically coupled growth at the edges, and (3) bridging growth of the clustered particles. Ordered and modulated structures were found in the original TiC particles. In addition to the original TiC particles, fine TiC particles precipitated from the liquid phase and γ-Ni solid solution during laser cladding. The microstructures of the bonding zones (BZs) were intimately associated with laser processing parameters. The BZs of the clad coatings can be categorized into three types according to the combination of the CL with heat-affected zone (HAZ): (1) straight interface combination, (2) zigzag connection, and (3) combination by partial melting of prior austenitic grain boundaries of the substrate. The straight interface BZ consists of the γ-Ni solid solution only, but in the other two types of combinations, the BZs consist of γ-Ni primary dendrites, γ-(Fe, Ni)/M23(C, B)6 eutectics, and martensite of the substrate. The microstructural evolution of the CLs was discussed. The formation and phase transformation models of the BZs were proposed.