Microhardness and Macrohardness of as-Cast Nb-Ti-Si-B Alloys

In order to evaluate the application possibility of the Nb-Si based alloys as cutting tool materials, we investigated the microhardnesses and the macrohardnesses of three alloys (Alloy 1#: Nb-12Ti-16Si-16B, Alloy 2#: Nb-23.5Ti-16Si-10B, Alloy 3#: Nb-20Ti-16Si-1.5B) selected from NbB, Nb3B2 and Nb3Si primary phase regions, respectively, in liquidus projection of the Ni-Ti-Si-B quaternary system. The microhardnesses were measured at a load of 200 g for 15 s using Hx-1000 TM hardness tester. Macrohardness measurement was performed using a Rockwell C scale (HRc). High microhardnesses were obtained for the Nb-based intermetallics: NbB (16.58 GPa in Alloy 1#), Nb3B2 (10.08 GPa in Alloy 2#) and Nb3Si (12.12 GPa in Alloy 3#), which are attributed to strong covalent bonding and appreciable solid solution hardening in these phases. Furthermore, macrohardnesses (HRc) of these three alloys are all above 70, i.e. 77, 75 and 78, respectively, which are primarily provided by certain volume fractions of the hard intermetallic phases in the Nb-Ti-Si-B alloys. The macrohardness (HRc) values of these alloys are higher than those of carbon tool steels (HRc∼65) and high speed steel (HSS, HRc∼67).