Figure (a) shows the overall metallographic structure (after heat treatment) of ztap / HCCI composite under optical microscope. The microstructure of the composite can be divided into HCCI matrix sintered in the composite zone and HCCI matrix formed by casting ztap and columnar zone. In the figure, the light color area is carbide, the dark area is HCCI matrix, and the black large area in the upper right corner is ZTA ceramic.
In casting HCCI, the main form of carbides is fine eutectic carbides distributed on the matrix, and the shape is round or short rod. The average equivalent diameter of carbides is 3 ~ 5 μ m measured by the cut-off method. The amount of primary carbides is small and isolated, which has little effect on the matrix cleavage. It can improve the wear resistance and ensure a certain toughness at the same time.
Figure (b) is a local enlarged view of the interface between cast and sintered HCCI in figure (a). It can be seen that the matrix is connected with each other, and the carbide growth on the interface has certain directionality, which is closer to the direction perpendicular to the interface. Moreover, the carbide growth on both sides of the interface crosses the interface and is embedded in the opposite matrix, which indicates that the sintered HCCI has remelted during the casting process and is formed with the HCCI formed by casting The matrix forms metallurgical bonding. Sintering HCCI has experienced three stages of “sintering remelting heat treatment”. The carbides on the matrix grow up and connect into blocks during the precipitation process. The average equivalent diameter of carbides is 20 μ m, which is distributed around the grain boundary. Although it can increase the wear resistance, it will reduce the toughness of the matrix. Figure (c) shows a local enlarged view of the interface between ztap and sintered HCCI matrix in figure (a). The metal matrix and ztap are well bonded, and no defects such as cracks and pores are observed. Sintered HCCI matrix can provide good fixation and support for ztap, and enhance the impact resistance of ztap.