Stabilizing nanograined austenitic stainless steel with grain boundary relaxation

Grain boundary (GB) relaxation can be triggered by interaction of GB with partial dislocations during plastic deformation, which is effective to stabilize nanograined structures in a number of face-centered cubic (FCC) metals and alloys. But this mechanism is interfered by deformation-induced martensitic transformation in 304 stainless steels with a metastable FCC structure. In this work, gradient FCC nanograined structures were prepared in a 304 stainless steel by using warm surface mechanical grinding treatment. GB relaxation was triggered in the samples with grains below 60 nm in size, in which partial dislocation dominates the plastic deformation. With the GB relaxation, thermal stability increases with decreasing grain sizes, contradictory to the conventional smaller less stable trend. The onset temperature for coarsening of nanograins of-30 nm in size is as high as-896 degrees C (0.7 Tm), about 250 degrees C above the reported value of the steel. The GB relaxation was not detected in martensitic nanograins of the same composition, of which the instability temperatures is around 650 degrees C (0.54 Tm), independent of grain size from 26 nm to the submicron scale, as it is controlled by the reverse martensitic transformation.

Automated summary

Grain boundary relaxation can stabilize nanograined structures in many FCC metals and alloys, but this mechanism is interfered by deformation-induced martensitic transformation in 304 stainless steels. In this study, gradient FCC nanograined structures were prepared in a 304 stainless steel and triggered GB relaxation was observed in samples with grain sizes below 60 nm. Thermal stability increased with decreasing grain size, contrary to the conventional trend. Martensitic nanograins of the same composition did not exhibit GB relaxation, as their instability temperature was controlled by reverse martensitic transformation.