METASTABLE PHASE-FORMATION AND ENHANCED DIFFUSION IN FCC ALLOYS UNDERHIGH-DOSE, HIGH-FLUX NITROGEN IMPLANTATION AT HIGH AND LOW ION ENERGIES

The use of elevated target temperatures near 400-degrees-C during high flux ion implantation of N2+ at energies ranging from 60 keV to 0.4 k eV leads to a metastable, f.c.c. high nitrogen solid solution phase in duced in austenitic (f.c.c.) Cr-containing stainless steels. This phas e has not been produced in an f.c.c. Ni-Fe alloy containing no Cr. Pen etration depths of the N are significantly larger than expected on the basis of known diffusion coefficients of N in Cr-containing stainless steels or in pure fc.c.-Fe. X-ray diffraction data suggest unusual di fferences in N penetration and concentration depending on the fc.c. gr ain orientation. Consideration is given to possible residual stresses induced by N expansion of the lattice and to the anisotropic elastic c onstants for austenitic stainless steels. The amount of N in interstit ial solid solution approaches 40 at.% under the lower energy, higher f lux conditions, but only in the (200) crystallographic planes parallel to the surface. The N-expanded f.c.c. phase with the highest amount o f N is found by conversion electron Mossbauer spectroscopy to be magne tic in AISI 304 and 310 stainless steels. A comparison of our results with those from related methods such as conventional plasma ion nitrid ing and pulsed plasma ion implantation is made to demonstrate that the observed metastable solid solution phase is often produced by other m ethods provided that appropriate temperatures and processing times are used. The similar penetration depths of N in the fc.c. Cr-containing stainless steels for the various methods are consistent with thermal d iffusion and metastable solubility that are enhanced by the Cr. The me tastability is associated with the low Cr mobility below about 450-deg rees-C and the strong N-Cr bond. Evidence for vacancy-enhanced diffusi on is not found and we see at present no advantage to using ion energi es higher than about 2 keV at these elevated temperatures for producin g surfaces with optimized tribological behavior based on the extremely high strength, metastable, N solid solution phase.