First-principles Study Of Phase Structure,Interface And Deformation Mechanism In Fe-Mn-Al-C Low-density Steel

With the higher and higher requirements of lightweight material in the areas of automotive,construction and military,low density steel with good plasticity is the future direction of the steel industry.As the most common low-density steel material,the main alloy elements in Fe-Mn-Al-C low-density steel have important significance for the phase structure of the material,the interfacial relationship between two phases and the deformation mechanism.It is difficult to use the experiment to study the above-mentioned problems at the atomic scale.Therefore,the Fe-Mn-Al-C low-density steel crystal structure model was constructed.The crystal structure stability,interfacial bonding properties and deformation mechanism of Fe-Mn-Al-C low-density steel were studied by using the first-principles calculation method combining the experimental results.The main research results are as follows:（1）Based on the designed composition,the Fe-17Mn-10Al,Fe-27Mn-10Al and Fe-30Mn-7Al low-density steels were prepared.The density of the three steels were6.73g/cm³,6.67g/cm³and 6.80g/cm³.In addition,it is found that the Al element can effectively expand the lattice parameter of the crystal and have a smaller atomic mass because of the first-principle calculation of the crystal structure model of the three kinds of materials.And the density calculated by the calculation is very close to the experimental value,which shows that the calculation result has high reliability.（2）The Fe-30Mn-7Al steel was found to be a single-phase austenite structure by XRD and micro-morphology.The other two kinds of steels with high Al content are biphasic structure.The second phases of Fe-17Mn-10Al and Fe-27Mn-10Al steels are B2 ordered phases which precipitates from theγmatrix,in the form of equiaxed or banded grains and ferrite phases,respectively.（3）It is found that the austenite binding energy decreases with the increase of Mn content by the first-principle calculation.The interaction between Fe atoms and the adjacent Mn atoms is enhanced,and the stability of austenite crystal structure increases.But with the increase of Al content,the austenite binding energy increases.The interaction between Fe atoms and Al atoms is weakened and the stability of austenite structure is significantly reduced.It is worth noting that the new bonding peak appear because of the addition of C,and the C atom has the strongest binding ability with the surrounding atoms and the metal atoms around C atom,which strengthen the bonding ability.（4）The quasi-static tensile test found that the Fe-27Mn-10Al steel had the highest strength of 1309.2MPa.By calculating the misfit of the austenite model by the first-principle calculation,it is found that as the Al content increases,the crystal activity increases and the second phase is easily formed.The Mn element can improve the austenite strength when the Mn mass fraction is between 17 and 27 by tensile simulation in the Fe-Mn-Al-C system.（5）The EBSD results show that the primary crystallographic orientation relationship betweenγand B2 is{111}γ//{111}_B2.The electronic properties of the interface model show that Mn and Al form a stronger covalent bond with Fe at the interface of the two phases,which increases the interface bonding strength.The simulation of the interface model shows that the two-phase interface is a weak part of the system,which further enhance the crystal structure stability.（6）The dynamic compression test results show that Fe-17Mn-10Al steel has lath martensite formation.The formation of dislocation microband in Fe-27Mn-10Al and Fe-30Mn-7Al steels shows that microband induced plastic deformation and Fe-30Mn-7Al also appears Twins have twins to induce plasticity.In addition,the martensitic transformation temperature decreases with the increase of Mn content but increases with the increase of Al content.With the increase of Mn and Al content,the stacking fault energies increases.Indicating that Mn is not conducive to the formation of martensite and the effect of Al on martensitic transformation has two sides.