Martensite transformation in antimony implanted stainless steel

Abstract

Recent investigations have shown that P⁺ implantation of stainless steel at room temperature induces diffusionless transformations resulting in either a martensitic-like phase formation or in amorphization. In the work presented here we have used Rutherford backscattering analysis (RBS) and transmission electron microscopy (TEM) and diffraction to investigate austenitic stainless steel crystals implanted at room temperature with 80 keV Sb⁺ ions to a fluence of 5 × 10²⁰ ions m⁻², thus providing implantation with a heavy group-V element. RBS channelling spectra from implanted 〈110〉 crystals show a damage peak which approaches the height of the random level and therefore indicates a very high degree of disorder in the implanted layers. The distribution of the disorder extends to a depth three to five times the depth of the primary radiation damage. The Sb peaks under channelling as well as random conditions are indistinguishable, confirming that, substitutionality during implanatation is negligible.

To establish the nature of the disorder which cannot be assessed from the RBS analysis alone, and in particular to determine whether an amorphous alloy is formed in the implanted layer as indicated from the RBS spectra, samples implanted under similar conditions were investigated in the TEM. Significant extra spots in the diffraction patterns can be ascribed to the presence of a radiation induced b.c.c. phase of martensitic origin with the orientation relationship given by the Nishiyama—Wassermann rule. Dark-field microscopy using these spots for imaging show the transformed zones to be ∼0.1–0.15 μm in size.

The result that a significant amount of martensite can be induced by antimony implantation seems in the light of earlier observations on P⁺ implantations to indicate that the main driving force for the transition is due to damage-induced stress concentrations rather than to chemical or alloying effects. This is in agreement, with recent observations of ferrite formation in neutron-irradiated stainless steels.