Tem evaluation of ion-implanted and fatigued metal surface layers

Abstract

The fatigue deformation was characterized by transmission electron microscopy in unimplanted and in ion-implanted metal surface layers. The metals investigated were austenitic 304-stainless steel and copper, annealed and implanted at 3 MeV with nitrogen or neon to a dose of 10 21m −2. The implanted surface layers contain a high density of lattice defects, most likely being Frank-dislocation loops. They are obstacles against glide dislocations. After flexural fatigue at ±0.3% cyclic strain, dislocation channels were formed heterogeneously along crystallographic {111} slip planes. Inside the channels the Frank loops were annihilated. These areas became thus mechanically soften relative to their surrounding volume. The channels assume the role of the well known persistent slip bands in unimplanted metals and lead to extrusions at the free surface. They are the precursors of fatigue crack nucleation. The fatigue lives of the as-implanted samples were not improved. However, an improvement by a factor of two to three was obtained after aging of the nitrogen-implanted stainless steel. Electron diffraction evidence suggests that this improvement was due to the formation of beginning precipitation ( = stronger obstacles against glide dislocations).Stress-induced martensitic transformation and twinning in the stainless steel were also modified by the implantation. Implanted nitrogen prevented the formation of α′-martensite, whereas in neon-implanted samples the martensite formed freely.