Bend Deformation and Fracture of WC–Co Alloys at Elevated Temperatures

Abstract

To elucidate the nature of elevated temperature deformation and fracture of WC–(6–20)%Co alloys, the influence of the various factors such as the size and volume fraction of WC, defects (pores), and strain rate on the mechanical properties was investigated in bend testing over a temperature range up to 1000°C and macro- and micro-fractographic analyses were carried out. The results indicate that in each alloy, there exist three transition temperatures, TI, TII′, and TII(TI<TII′<TII) which mark a distinct change in the mode of deformation and fracture. The transition temperatures in general increase with decrease in mean free path (referred to as m.f.p.). In the temperature range I(TI-TII′), the transverse rupture strength (referred to as T.R.S.) of the fine-particled alloy drops less rapidly with increase in temperature than that of the coarse-particled alloy. On the other hand, in the temperature range II(>TII), T.R.S. of the coarse-particled alloy becomes larger than that of the fine particled alloy. Moreover, numerous microvoids originate from either WC/WC/Co or WC/Co boundaries. The microvoids which are located in the direction perpendicular to the tensile axis merge into parallel, interconnected cracks which propagate through the Co phase. The temperature range II′(TII′-TII) was found to be the transition range with reference to the fracture mode from the type specific for the range I to that for the range II. In regard to the effect of strain rate on deformation and fracture, it was found that the increase of strain rate gives an equivalent effect to the lowering of temperature. The mechanisms of the deformation and fracture in the respective temperature ranges were discussed.