Potency of Grain Boundaries as Martensitic Nucleation Sites

Abstract
The potency of grain boundaries as martensitic nucleation sites has been investigated by measuring the Ms temperatures of bicrystal specimens in an Fe-32%Ni alloy. The average Ms temperature of single-crystal specimens was 88K, while that of polycrystalline specimens was 154K. The free surfaces did not have high potency as nucleation sites, and the higher Ms temperature in the polycrystalline specimens was attributed to the existence of certain kinds of grain boundaries. The Ms temperatures of five kinds of bicrystal specimens having one of annealing twin boundaries and high-angle random boundaries were equivalent to that of the single-crystal specimens and lower than that of the polycrystalline specimens. These results indicate that both the annealing twin boundaries and the high-angle random boundaries do not have high potency as martensitic nucleation sites, and that the most potent nucleation site which defines the Ms temperature of the polycrystalline specimens is attributed to other kinds of grain boundaries.