Defect structures in cold worked and small grain pure and boron-doped Ni3Al alloys

Positron lifetime spectroscopy was used to study the isochronal annealing of cold worked Ni3Al samples. In pure Ni/sub 76/Al/sub 24/, Ni/sub 74/Al/sub 26/, and boron-doped Ni/sub 74/Al/sub 26/ three annealing stages were observed. Boron-doped Ni/sub 76/Al/sub 24/ showed only two annealing stages. Vacancy annealing (stage III) was identified in all cases to start at /similar to/250 0C, somewhat higher than previously reported. The discrepancy is suggested to be due to carbon-vacancy interactions, because carbon (impurties) was observed to diffuse out of all samples at or above /similar to/350 0C. The high-temperature annealing stage in boron-doped Ni/sub 74/Al/sub 24/ (which is ductile) starts at 700--750 0C and is complete at 1000 0C. This stage was attributed to migration of dislocations to various sinks. In pure Ni/sub 76/Al/sub 24/, Ni/sub 74/Al/sub 26/, and boron-doped Ni/sub 74/Al/sub 26/ (which are brittle) the intermediate and high-temperature annealing stages occur at /similar to/750--800 and 1000 0C, respectively. These stages were attributed to the migration of dislocations (750--800 0C) and recrystallization (/similar to/1000 0C) with incomplete annealing of dislocations at 1000 0C. This being the case, it is clear that dislocations interact with grain boundaries fare more strongly in brittle than in ductile Ni3Al alloys. Thus, these data strongly suggest that the ductilization of Ni/sub 76/Al/sub 24/ by boron is largley due to a change in grain boundary structure which inhibits the pinning of dislocations at grain boundaries that occurs in pure Ni/sub 76/Al/sub 24/ (i.e., boron increase the susceptibility of grain boundaries to slip). Similar results for small-grain samples support this interpretation