Morphology, crystallography and defects of the intermetallic χ-phase precipitated in a duplex (δ + γ) stainless steel

The ferritic matrix in the Fe-22Cr-5Ni-3Mo-0.03C ferritic-austenic duplex stainless steel undergoes a variety of decomposition processes when aged in the temperature range 650–750°C. These processes involve the precipitation of the austenite and of the σ and χ Frank-Kasper phases. The intermetallic χ-phase is found at both the grains boundaries (homo and heterophase interfaces) and inside the ferritic grains where it adopts an unexpected hexagonal shape. At the early stage of its precipitation, it nucleates at the δ/γ and δ/σheterophase interfaces and then grows by expanding exclusively in the ferritic matrix. This study is basically focused on this intermetallic χ-phase. The crystal structure and the chemical composition are respectively studied by electron diffraction and energy dispersive X-ray spectroscopy. The χ-phase exhibits rational orientation relationships with the austenite and the σ-phase with which it is in contact and an invariably cube-on-cube orientation relationship with the ferritic matrix into which it grows. Based on the orientation relationship, the morphology and the number of variants of this χ-phase are understood in terms of the group theory. The planar defects present in a large density in the χ-phase, are roughly parallel to {0 1 1}χ//{0 1 1}δ. The fault vectors are determined as: \(\frac{1}{3}\)〈110〉χ and \(\frac{1}{4}\)〈111〉χ, the latter corresponding for a bcc structure to a π phase shift, the defects can be simply described as π boundaries. Based on the obtained results, a structural proximity between the χ-phase and a super-cell derived from the ferritic matrix has been brought to light. This super-cell is described as a stacking of corrugated and planar layers obeying the following parallelism {0 1 1}χ//{0 1 1}δ. Indeed this super-cell approach provides an interpretation for several microstructural features such as the χ/δ interface plane, the planar defects in the χ-phase and their related fault vectors.