Microstructural and corrosive interactions in phosphorus ion implanted 304L stainless steel—I. Alterations in microstructure by implantation

Abstract

Ion implanting phosphorus (P +) ions into 304L stainless steel is shown to improve its corrosion resistance (Part II). Pivotal to this improvement is the production of an amorphous phase throughout the implanted layer. The present paper addresses the microstructures induced by P + implantation, particularly the role of phosphorus concentration and the influence of the P + ions' energies in promoting amorphous phase. Phosphorus ions accelerated to 50, 100, 150, 175 and 200 keV were implanted into electropolished 304L stainless steel specimens near room temperature to fluences between 0.16 and 19.2 × 10 17 P + cm −2. Initial implantations produce an f.c.c. to b.c.c. transformation at ⋍ 8% P, followed by amorphous phase formation from the b.c.c. matrix, particularly at concentrations beyond 20% P. Fully amorphous specimens are observed at concentrations near 35% P. Further P + implantation precipitates a phase isomorphous to hexagonal Fe 2P from the amorphous matrix, with a phase isomorphous to FeP present at even higher fluences. The role of radiation damage in determining phase formation is addressed through the use of calculated Fe-P free energy/concentration curves. At all the accelerating energies investigated, the presence of the amorphous phase is concentration dependent rather than atomic displacement 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 phase transformations and to control the ion depth of penetration. Crystallization of the amorphous phase, induced by annealing at 300–600°C, generates FeCrP and the phase isomorphous with Fe 2P. The formation of these phases requires diffusion of chromium and phosphorus as observed by the experiments.