Stacking Fault Energies In Austenitic Stainless Steel: A Theoretical Study

Stacking fault energies (SFE) is a crucial intrinsic parameter of materials plastic deformation, and has significient effect on brittle-to-ductile transition (BDT). At normal temperature, two of the most common and important plastic deformation modes of materials are dislocation slipping and deformation twinning, which will result in glide band (GB) and deformation twin (DT), respectively. GB and DT will not result in disorder of crystal obviously, however, the SFE, especially intrinsic stacking fault energies (γisf) and unstable stacking fault energies (γus), caused by crystal slipping, influence nucleation, movement, bundle, across glide, dissociation of dislocation. Reducing stacking fault energy of materials is conducive to further stimulate dislocation slipping and twinning, thus improving mechanical properties.Nitrogen and nickel belongs to key alloying elements in austenitic stainless steel, and has significient influence on texture and properties of austenitic stainless steel. Throughγisf had been got from experimental method, the criterion for experimental equipment is high, and cannot be measured the whole SFE curve except forγisf. In addition, the difference of results between various experimental methods is large. The development of calculational science remedy imperfection of experiment. At present, computational science has been successfully used to calculate SFE such as aluminium, iron, copper, nickel etc.Therefore, the first-principle, which based on density functional theory (DFT), is used to study the effect of nitrogen and nickel on SFE from atom and electron level. The main contents are as follows.(1) Effect of nickel and nitrogen on the stability of austenitic stainless steel was studied. The results show that nickel and nitrogen can stabilize austenitic stainless steel, and atomic position influence on stability is not obvious.(2) The effect of nitrogen and nickel on austenitic stainless steel was studied from electron level. Nickel doped in austenitic stainless steel can optimize density of electron around iron and chromium and imprive binding capacity of iron and chromium. Nitrogen doped in 304 austenitic stainless steel create covalence with iron, chromium and nickel.(3) The influence of nickel and nitrogen on yus,γisf of austenitic stainless steel was studied. With nickel content increase, the potential barrier which must be overcome for dislocation slipping increase. This will result whole dislocation dissociate partial dislocation difficulty. By contrast, nitrogen will decrease potential barrier, and result whole dislocation dissociate partial dislocation easy. For nickel content increase,γus/γisf will reduce which is beneficial for Frank whole dislocation formation. However, nitrogen will increaseγus/γisf and is beneficial for Shockley partial dislocation and twinning forming.(4) The comprehensive analyses of the effect of nickel and nitrogen on austenitic stainless steel indicate that nickel can reduceγus/γisf in small range. By contrast, a minute of nitrogen can increaseγus/γisf evidently compared with nickel.