Abstract
A detailed study has been undertaken on the transformation of nickel lattices under the influence of nitrogen as an “insertion” impurity. For this study, thin polycrystalline and monocrystalline layers of evaporated nickel have been heated in an atmosphere of ammonia. This nitriding process has been followed kinetically by electron diffraction. Using these techniques, it has been shown that the “insertion” of nitrogen atoms takes place in successive steps. First, a considerable expansion of the initial f.c.c. lattice of nickel (from a=3,52 to a=3,72 Å) is observed with the formation of Ni4N. Then, if the nitriding is continued, this f.c.c. lattice of expanded nickel (Ni4N) is transformed into a hexagonal lattice of Ni3N.
The exact structure of these nitrides has been determined and a mechanism of the transformation from the expanded cubic nickel lattice (Ni4N) to the hexagonal nickel lattice (Ni3N) has been proposed. This mechanism consists, in part, of a series of microslips on the (111) plane and in the [11\bar2] direction of nickel atoms, similar to those produced in the martensitic transformation of cobalt, which transforms the stacking order of the {111} planes of the f.c.c. lattice into the stacking order of the {0001} planes of the hexagonal lattice. In addition, the diffusion of nitrogen results in another stacking transformation from the cubic {1\bar10} planes into the {1\bar210} planes of the hexagonal variety.
