Microscopic Examination on ε' Phase in Cemented Carbide

Abstract

The structure and the thermal stability of e' phase formed by the strain-induced transformation of binder phase near the ground surface-layer in WC-Co cemented carbide, have been investigated mainly by optical microscopy. For microscopic studies, two-phase WC-20%Co alloy specimens with high and low carbon contents and with coarse WC grains (-50μ) were sintered in vacuum at 1400°C for 1 hr. For X-ray studies, two-phase WC-45%Co alloy specimens with fine WC grains (-1.6μ) were sintered at low temperature around 1250°C to minimize the grain size of binder phase.Results obtained were as follows: (1) The depth of layer of the e' phase formed by grinding was naturally affected with cobalt contents, carbon contents and mean grain size of WC in the alloy. (2) The e' phase was formed in a plate shape along slip bands in binder phase, suggesting that the habit plane of e' phase is (111)γ. (3) The rapid growth of the phase due to annealing was observed at relatively high temperatures up. to about 400°C and 600°C in high carbon and low carbon alloys, respectively. The increased amount of the e' phase seemed to be affected by the amount of e' phase pre-existed and the grain size of binder phase. The above results were considered to have a close relation with mechanical properties of cemented carbide at these temperatures. (4) The precipitate of Co3W appearing by annealing low carbon alloy at high temperatures was usually observed along the e' phase pre-existed.