Compositional changes associated with periodic antiphase boundaries in the initial stages of ordering in Ni₃Mo¹. I. Crystallographic Analysis

The early stages of ordering in Ni₃Mo have been studied by electron diffraction and microscopy. A long period superlattice of 13.9 Å is observed which can be attributed to a periodic array of non‐conservative antiphase boundaries in the Ni₃Mo‐structure. This superlattice disappears by coarsening of the array after longer annealing times. From the shift of the superlattice reflections the displacement vector associated with the antiphase boundaries could unambigeously be determined as [102] using a method developed earlier for shear structures. This result is in agreement with the structural model and with the displacement vector as derived from image extinctions. Direct evidence is found for the shear transformation from the f.c.c. high temperature phase to the ordered Ni₃Mo‐structure, which is a superstructure of the h.c.p. structure.