It was noted that the ferritic matrix of this Fe-22Cr-5Ni-3Mo-0.03C ferritic-austenitic duplex stainless steel underwent decomposition when aged at 650 to 750C. This involved the precipitation of austenite, and of σ and χ Frank-Kasper phases. The intermetallic χ-phase was found at the grain boundaries (homo and heterophase) and within the ferritic grains; where it exhibited an hexagonal shape. In the early stages of precipitation, it nucleated at δ/γ and δ/σ heterophase interfaces and then grew into the ferritic matrix. Its crystal structure and composition were studied by using electron diffraction and energy dispersive X-ray spectroscopy. It was found that the χ-phase exhibited rational orientation relationships with respect to the austenite, and to the σ-phase with which it was in contact. It had a cube-on-cube orientation relationship with respect to the ferritic matrix into which it grew. Based upon the orientation relationship, the morphology and number of variants of the χ-phase could be understood in terms of group theory. The large numbers of planar defects in the χ-phase were roughly parallel to {011}χ||{011}δ. The fault vectors were deduced to be: 1/3<110>χ and ¼<111>χ. The latter corresponded, for a body-centered cubic structure, to a π phase-shift. The defects could be described simply as π boundaries. A structural similarity between the χ-phase and a super-cell, derived from the ferritic matrix, was revealed. This super-cell was described as being a stacking of corrugated and planar layers which obeyed: {011}χ||{011}δ. This super-cell approach provided an explanation for microstructural features; such as the χ/δ interface plane, planar defects in the χ-phase, and their related fault vectors.

Morphology, Crystallography and Defects of the Intermetallic χ-Phase Precipitated in a Duplex (δ + γ) Stainless Steel. A.Redjaïmia, A.Proult, P.Donnadieu, J.P.Morniroli: Journal of Materials Science, 2004, 39[7], 2371-86