Deformation microstructures as well as strengthening and toughening mechanisms of low-density high Mn steels for cryogenic applications

Abstract

The three Al-bearing high Mn steels were prepared to investigate the effect of Al on microstructures as well as tensile and cryogenic impact properties. The Al addition leads to three strengthening modes of solid-solution, grain refinement and δ-ferrite. The former two strengthening modes can increase yield strength from 406 to 467 MPa with increasing Al content from ~3.0 to ~5.0 wt%. Once the Al content reaches ~8.0 wt%, the yield strength reaches 588 MPa owing to the formation of δ-ferrite. The changes in major deformation mechanisms are sufficient twinning plus deformation bands → a little twinning plus deformation bands → nearly no twinning plus highly dense dislocation walls as the Al content increases from ~3.0 to ~8.0 wt%. A loss in total elongation is mainly due to the suppression of twinning in the 5Al and 8Al steels. However, under an impact loading at −196 °C, the major deformation mechanisms are cross twinning and highly dense dislocation walls in the 3Al and 5Al steels, leading to ductile dimpled fracture as well as high impact absorbed energies of ~122 and ~138 J for the 3Al and 5Al steels, respectively. The toughening modes contain high angle grain boundaries induced deflections of a crack, cross planar defects induced tortuousness of a crack in grain interior and a large plastic deformation dissipating energy. In addition, it is due to the formation of δ-ferrite that the 8Al steel becomes a brittle material at −196 °C. The δ-ferrite induced cryogenic brittlement (δ-ICB) effect was sufficiently discussed.

Keywords

• High Mn steel
• Low density
• Planar defects
• Strength
• Cryogenic impact toughness