WC-Co合金の破壊に関する一観察

Abstract

The purpose of this experiment was to make clear the reason why WC-Co alloy is superior in the strength to other WC-Ni and WC-Fe alloys. The observation on the surface cracks and the fracture surfaces have been carried out for various cemented carbides, using optical and electron microscope, respectively. Specimens having 20% binders of Co, Ni and Fe were sintered. Their grain size was fixed at 75∼149 μ for the study of surface cracks, and at 75∼149,∼44 and 1∼2 μ for the study of fracture surfaces. Surface cracks were produced by the Vikers-indentor, and fracture surfaces, by bending-rupture. The results obtained were as follows:(1) The following phenomena were commonly observed in each alloy having the coarrest grain size. On the portion of the surface subjected to tensile stress, initial cracks formed mainly in carbide grains, accompanied by almost no slip lines in both carbide and binder phases. On the sheared portion, however, cracks formed preferentially in the binder, with numerous slip lines in both phases of the alloys excepting WC-Fe alloy.(2) As for the surface or interior cracks formed under tensile stress, various types of propagation were observed. In WC-Ni alloy, the propagation was very easy to occur along the carbide-binder interface; In WC-Fe alloy, along the double carbide-binder and-carbide interfaces, or through the double carbide; In WC-Co alloy, generally through the binder and scarcely along the carbide-binder interface. Therefore, the resistance to crack propagation should be largest in WC-Co alloy considering the plastic deformation of the binder, leading to the superior strength of that alloy.(3) In WC-Co alloys, cleavage fracture of the ε′(hcp) phase, transformed from the γ(fcc) binder phase before the occurrence of fracture, was not often observed, probably due to the small amount of the ε′ phase.