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

Abstract

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 ∼250 °C, somewhat higher than previously reported. The discrepancy is suggested to be due to carbon-vacancy interactions, because carbon (impurities) was observed to diffuse out of all samples at or above ∼350 °C. The high-temperature annealing stage in boron-doped Ni76Al24 (which is ductile) starts at 700–750 °C and is complete at 1000 °C. This stage was attributed to migration of dislocations to various sinks. In pure Ni76Al24, Ni74Al26, and boron-doped Ni74Al26 (which are brittle) the intermediate and high-temperature annealing stages occur at ∼750–800 and 1000 °C, respectively. These stages were attributed to the migration of dislocations (750–800 °C) and recrystallization (∼1000 °C) with incomplete annealing of dislocations at 1000 °C. This being the case, it is clear that dislocations interact with grain boundaries far more strongly in brittle than in ductile Ni3Al alloys. Thus, these data strongly suggest that the ductilization of Ni76Al24 by boron is 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 support this interpretation.