抄録
The binder phase of WC-Co alloys has been thought to be the γ(fcc) phase. But, recently it was found by the present authors that a γ(fcc)→ε′(hcp) martensitic transformation occurs at low temperature under some external stresses.
In this experiment, the amount, distribution and thermal stability of the ε′ phase transformed in deformed superficial layers of the alloys by abrasion were mainly studied. The electron-micrographs of a thin film of the deformed surface including the fracture surface were also studied. The two-phase specimens containing 10∼20%Co and having the average grain size of about 1.4 μ were vacuum-sintered at 1345°∼1375°C for 1.5 hr.
The results obtained were as follows: (1) The amount of the ε′ phase increased with increasing abrasion time, and it reached a maximum constant value after the abrasion of more than 4∼5 min. (2) This maximum value increased further in the alloy having higher cobalt and carbon contents, and also in the case of abrasion by a coarser diamond wheel. (3) As for distribution of the amount of the ε′ phase from the surface to the inside, peaks were found in each alloy at the depth of about 5 μ, with the sharp decrease at the further depth. (4) A (ε′→γ) reverse transformation was observed to begin at temperatures above about 400° and 600°C in high and low carbon alloys, respectively. The transformation was completed at about 800°C in each alloy. (5) The ε′ phase formed on the abrasion or fracture surface was confirmed using transmission-electron microscope.
