Transmission Electron Microscopy Study of Strengthening Precipitates in 18% Ni Maraging Steel

Abstract

Transmission electron microscopy and electron diffraction studies were carried out in order to make clear the crystallograsphic properties of strengthening precipitates in an 18% Ni maraging steel. The primary precipitates observed in specimens in the maximum hardness state were rod in shape, the size being about 25 Å in width and 500 Å in length. The length directions of the rod precipitates were parallel to the ⟨111⟩ direction of the matrix, and the crystal structure was identified as the orthorhombic Ni₃Mo intermetallic compound. The orientation relationship between the Ni₃Mo precipitate and matrix was proposed possibly to be
(010)_Ni3Mo⁄⁄(011)_α′, [100]_Ni3Mo⁄⁄[1\bar11]_α′,
that is, the closest packed plane and direction in precipitates were parallel to those in the matrix, as observed in many other precipitation phenomena. When the ordering of atoms in the Ni₃Mo crystal was disregarded, the crystal could be described in terms of the hcp lattice and the relationship was represented by the Burgers relationship which is well known in the bcc to hcp phase transformation. The length direction of Ni₃Mo precipitates, ⟨111⟩_α′, was verified to be the [1\bar11]_α′ direction involved in the above orientation relationship.
The interfacial coherency between the Ni₃Mo precipitate and the matrix was examined referring to the orientation relationship and the length direction. The hexagonal Ni₃Ti phase was also detected as the secondary precipitates.