Positron lifetime spectroscopy was used to study the isochronal annealing of cold worked Ni3Al samples. In pure Ni76Al24, Ni74Al26, and boron-doped Ni74Al26 three annealing stages were observed. Boron-doped Ni76Al24 showed only two annealing stages. Vacancy annealing (stage III) was identified in all cases to start at 250C, somewhat higher than previously reported. The discrepancy was due to carbon-vacancy interactions, because carbon (impurities) was observed to diffuse out of all samples at or above 350C. The high-temperature annealing stage in boron-doped Ni76Al24 (which was ductile) starts at 700-750C and was complete at 1000C. This stage was attributed to migration of dislocations to various sinks. In pure Ni76Al24, Ni74Al26, and boron-doped Ni74Al26 (which were brittle) the intermediate and high-temperature annealing stages occurred at 750-800 and 1000C, respectively. These stages were attributed to the migration of dislocations (750-800C) and recrystallization (1000C) with incomplete annealing of dislocations at 1000C.Dislocations interact with grain boundaries more strongly in brittle than in ductile Ni3Al alloys. Thus, these data suggest that the ductilization of Ni76Al24 by boron was largely due to a change in grain boundary structure which inhibits the pinning of dislocations at grain boundaries that occurs in pure Ni76Al24 (i.e., boron increases the susceptibility of grain boundaries to slip). Similar results for small-grain samples supported this interpretation.

Defect Structures in Cold Worked and Small Grain Pure and Boron-Doped Ni3Al Alloys. Usmar, S.G., Lynn, K.G.: Journal of Materials Research, 1989, 4[1], 55-61