Effect of Annealing on Microstructure and Microhardness of Co-Fe-Ni-Ta-B-Si Bulk Metallic Glass

Effects of annealing temperature and time on microstructure and microhardness of Co41Ni2Fe20Ta5.5B26.5Si5 amorphous alloy have been investigated. Microhardness of the as-cast alloy is found to be 1170 HV. The maximum microhardness, 1798 HV, is achieved after annealing the amorphous alloy at 1250 K for 10 min. Examination of the microstructure reveals that (Co, Fe)(2)B, (Co, Fe)(21)Ta2B6 and (Co, Fe)(16)Ta6Si7 phases precipitate in the matrix after annealing. Intrinsic hardness and Hall-Petch coefficients of the precipitates indicate that high hardness values result from precipitation of (Co, Fe)(2)B and (Co, Fe)(21)Ta2B6 phases. It is also observed that increasing the annealing time results in the decrease in microhardness values because of the increase in particle sizes of the precipitates.