Microstructural and analytical study of heavily faulted Frank-Kasper R-phase precipitates in the ferrite of a duplex stainless steel

The ferritic matrix in the Fe-22Cr-5Ni-3Mo-0.03C ferritic-austenitic duplex stainless steel can undergo a variety of decomposition processes when aged in the temperature range 550–650°C. These processes are the precipitation of the γs austenitic bi-crystal, the α′-BCC ferrite, the τ-phase and the heavily faulted R-phase. The latter is a Frank-Kasper phase, which nucleates on the dislocations in the ferritic δ-matrix and adopts a lenticular shape. This study is basically focused on this intermetallic R-phase. The crystal structure and the chemical composition are respectively studied by electron microdiffraction and energy dispersive X-ray spectroscopy. The R-phase is oriented with respect to the surrounding ferritic matrix by developing a rational orientation relationship such that: (0 0 0 1)R//(1 1 1)δ with [1 $$\bar 3$$ 2]δ//[2 $$\bar 1\bar 1$$ 0]R. This orientation relationship is here examined in term of lattice matching. Based on this orientation, the morphology and the variant number of this R-phase are understood in terms of the group theory. The defects, present in a large density in the R-phase, are identified as planar faults, which are grouped in two families parallel respectively to {1 3 9}δ and {11 13 23}δ lattice planes of the ferritic δ-matrix. Based on the obtained results, a structural proximity between the R-phase and a supercell derived from the ferritic matrix has been brought to light. It has been stated that Mo is an efficient R-phase forming element. The results provide valuable insights into the precipitation mechanism associated with the R-phase formation.