Effect of boron on the vacancy hardening of FeAl

Thermal vacancies harden Fe-45 at.% Al, and the hardening was observed even with the addition of boron. A similar phenomenon was reported earlier in the case of FeAl with substitutional elements such as Ti, Ni, and Cu [8]. The addition of boron increased the room-temperature yield and ultimate tensile strengths, and this increase can be attributed to the grain-boundary strengthening. While the strengths of binary FeAl decreased after vacancy treatment, the strengths of boron-doped FeAl seem to be insensitive to the vacancy treatment as an alloy of binary FeAl. There may be bonding between B and vacancies such that the heat treatment doesn't annihilate all the vacancies. It is worth noting that an indentation hardness test in effect measures the equivalent of the flow stress after a mean strain of about 15%, and is therefore determined by a blend of true (zero-strain) yield stress and the rate of work hardening. This level of strain is attained around the hardness indentation [16] even though it is difficult to attain a high tensile ductility in these alloys. Also, the results suggest that the work hardening rate of FeAl (which is not accessible to direct measurements) is notably reduced due to alloying with boron. It is important to note that a 400°C vacancy anneal has a significantly higher effect on the hardness than on the yield stress, suggesting possibly that elimination of vacancies sharply reduces the rate of work -hardening, as one would expect. We are further investigating the role of vacancies in the strengthening of FeAl, and are examining the vacancy formation by thermal expansion and electrical resistivity measurements.