Formation of core-rim structure and interface toughening mechanism in cast WC- FeCoNiCrMo (HEA) based composite materials

Abstract

The aim of the work is to improve the toughness of cast WC-based composite material by introducing FeCoNiCrMo high-entropy alloy (HEA) into the system. The results indicated that a circular ‘diffusion layer’ formed on the surface of cast WC after HEA addition, resulting in a ‘core-rim’ structure. During the pressureless infiltration process, decomposition of cast WC occurs and the carbon element reacts with Mo in HEA forming Mo2C, and the other elements such as Co, Fe and Ni react with Mo and W forming (Fe, Co, Ni)3Mo3C type carbon-deficient phase. The composition of the ‘diffusion layer’ is mainly HEA content, but the binder is enriched in Ni and rare in Mo regardless of HEA addition level. With the increase of HEA addition, formation of irregular island-shaped structure can be seen in the Cu binder phase. Due to the core-rim structure, the grain size of cast WC can be reduced and the hardness of the composite is improved. The bonding energy of WC (0001) -Mo2C (0001) and WC (101¯0) -Mo2C (0001) interfaces is evaluated using DFT calculation, which 7.975 eV and 6.480 eV and higher than that of WC-Cu interface in composite without HEA addition. FEM analysis reveals that the interfacial residual stress is alleviated due to the existence of the diffusion layer. In composite without HEA addition, the cracks initiated in the cast WC phase mainly propagates along the ceramics-metal interface, resulting in debonding and early fracture. The crack propagation mode shows much difference after the formation of core-rim structure, which indicate the toughening by HEA addition. When the crack propagates towards the interface, the crack does not propagate along the interface, but passes through the interface area and quickly stops propagating. Therefore, the interface toughness is improved after HEA addition.