Author： Redjaïmia A. Morniroli J.P. Donnadieu P. Metauer G.
Publisher： Springer Publishing Company
ISSN： 0022-2461
Source： Journal of Materials Science, Vol.37, Iss.19, 2002-10, pp. : 4079-4091
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Abstract
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 2]δ//[2 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.
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