MICROSTRUCTURAL AND CORROSIVE INTERACTIONS IN PHOSPHORUS ION-IMPLANTED 304L STAINLESS-STEEL .1. ALTERATIONS IN MICROSTRUCTURE BY IMPLANTATION

Ion implanting phosphorus (P+) ions into 304L stainless steer is shown to improve its corrosion resistance (Part II). Pivotal to this improv ement is the production of an amorphous phase throughout the implanted layer. The present paper addresses the microstructures induced by Pimplantation, particularly the role of phosphorus concentration and th e influence of the P+ ions' energies in promoting amorphous phase. Pho sphorus ions accelerated to 50, 100, 150, 175 and 200 keV were implant ed into electropolished 304L stainless steel specimens near room tempe rature to fluences between 0.16 and 19.2 x 10(17) P+ cm(-2). Initial i mplantations produce an f.c.c. to b.c.c. transformation at similar or equal to 8% P, followed by amorphous phase formation from the b.c.c. m atrix, particularly at concentrations beyond 20% P. Fully amorphous sp ecimens are observed at concentrations near 35% P. Further P+ implanta tion precipitates a phase isomorphous to hexagonal Fe2P from the amorp hous matrix, with a phase isomorphous to FeP present at even higher fl uences. The role of radiation damage in determining phase formation is addressed through the use of calculated Fe-P free energy/concentratio n curves. At all the accelerating energies investigated, the presence of the amorphous phase is concentration dependent rather than atomic d isplacement dependent. The amorphous phase is observed at all energies in the concentration range 35-41% P. The role of accelerating energy is to provide the atomic transport needed to accomplish the various ph ase transformations and to control the ion depth of penetration. Cryst allization of the amorphous phase, induced by annealing at 300-600 deg rees C, generates FeCrP and the phase isomorphous with Fe2P. The forma tion of these phases requires diffusion of chromium and phosphorus as observed by the experiments.