Effects of Cobalt on the Properties and Phase Formation of Mo₂FeB₂ Base Hard Alloys

Abstract

Effects of Co content on the phase formation and the mechanical properties of Mo₂FeB₂ base hard alloys were studied in Fe-5B-44.4Mo-XCo alloys with six levels of Co content from 0 to 10 mass%. The transverse rupture strength (TRS) and hardness of the alloys depend strongly on their microstructure which varies significantly with Co content. TRS keeps about the value of 2.0 GPa up to 5 mass% Co; it then decreases drastically at 7.5 mass% Co and increases slightly again at 10 mass% Co. On the other hand, hardness keeps approximately 83 R_A up to 5 mass% Co, thereafter, it increases to 87.5 R_A with increasing Co content. X-ray diffraction results indicate that the changes of TRS and hardness from 7.5 mass% Co up are attributed to the formation of a new boride phase which is considered to be Mo(Fe, Co)B.
The TRS drop and the hardness increase at 7.5 and 10 mass% Co are attributed to the increase of area frac-tion of the boride phases from 0.66 to 0.78 associated with the formation of the new Mo(Fe, Co)B boride phase.
Moreover, the TRS drop at 7.5 mass% Co is also caused by coalescence of the boride grains, namely Mo₂FeB₂ and Mo(Fe, Co)B, and coarsening of the Mo₂FeB₂ grains, and a small increase of the TRS at 10 mass% is due mainly to the re-improved dispersion of Mo(Fe, Co)B boride particles despite its lager grains.