MICROSTRUCTURE, HARDNESS, AND FRACTURE TOUGHNESS OF SURFACE COMPOSITES FABRICATED BY HIGH-ENERGY ELECTRON-BEAM IRRADIATION OF FE-BASED METAMORPHIC ALLOY POWDERS AND VC POWDERS

Abstract

In this study, surface composites were fabricated with Fe-based amorphous alloy powders and VC powders by high-energy electron beam irradiation, and the correlation of their microstructure with hardness and fracture toughness was investigated. Mixture of Fe-based metamorphic powders and VC powders were deposited on a plain carbon steel substrate, and then electron beam was irradiated on these powders without flux to fabricate surface composites. The composite layers of 1.3 similar to 1.8 mm in thickness were homogeneously formed without defects and contained a large amount (up to 47 vol.%) of hard Cr2B and V8C7 crystalline particles precipitated in the solidification cell region and austenite matrix, respectively. The hardness of the surface composites was directly influenced by hard Cr2B and V8C7 particles, and thus was about 2 to 4 times greater than that of the steel substrate. Observation of the microfracture process and measurement of fracture toughness of the surface composites indicated that the fracture toughness increased with increasing additional volume fraction Of V8C7 particles because V8C7 particles effectively played a role in blocking the crack propagation along the solidification cell region heavily populated with Cr2B particles. Particularly in the surface composite fabricated with Fe-based metamorphic powders and 30 % of VC powders, the hardness and fracture toughness were twice higher than those of the surface composite fabricated without mixing of VC powders.