Effects of silicon and chromium additions on glass forming ability and microhardness of Co-based bulk metallic glasses

Effects of silicon and chromium additions on glass forming ability (GFA) and microhardness of a Co-Fe-Ta-B bulk metallic glass are investigated by using differential scanning calorimetry (DSC), X-ray diffractometry (XRD) and scanning electron microscopy (SEM). It is found that partial substitution of boron by silicon promotes the GFA of the alloy. Fully amorphous rod of 4 mm is fabricated by suction casting Co43Fe20Ta5.5B26.5Si5 alloy.. However, partial replacement of cobalt by chromium decreased the GFA significantly. In fact, critical casting thickness of Co39Cr4Fe20Ta5.5B26.5Si5 alloy is determined as 2 mm. It is also determined that microhardness values of the amorphous alloys are about 1200HV(300). This value is lower than the hardness of the base alloy,1455HV(300), and it is believed that decrease in hardness results from the reduction in boron contents of the alloys.