ENVIRONMENTAL EMBRITTLEMENT IN L12-ORDERED (CO, FE)3V ALLOYS

Environmental effects on mechanical properties and fracture behavior of L1(2)-ordered (Co, Fe)3V alloys with compositions (Co78Fe22)3V and (Co85Fe15)3V were studied by tensile test in the strain range of 3.3 x 10(-5) to 3.3 x 10(-1) s-1 at ambient temperatures. The (Co, Fe)3V alloys were found to be susceptible to environmental embrittlement. The yield and flow stresses were insensitive to the test environment, while the ductility and ultimate tensile strength decreased according to the sequence of dry oxygen, vacuum, air, and distilled water. The ductility loss was closely associated with intergranular fracture, and the propensity for intergranular fracture increased in the same environmental sequence. Lower strain rate resulted in more intergranular fracture and hence lower ductility. A beneficial effect of grain refinement was observed. All these results suggest that the embrittlement was caused by moisture-induced hydrogen which diffused to grain boundaries, resulting in reduced grain-boundary cohesion and increased intergranular fracture. Grain boundaries were more intrinsically brittle in (Co85Fe15)3V than in (Co78Fe22)3V. A possible reason was suggested to explain the grain-boundary brittleness in (Co, Fe)3V alloys, based on consideration of phase stability.