Hisashi Suzuki1, Takaharu Yamamoto1 and Hitoyuki Sakanoue1
1Department of Metallurgy, Faculty of Engineering, University of Tokyo, Tokyo
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.
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