(AHSS) Development of Nano-acicular Duplex Steels- CMMI David C. Van Aken, Missouri University of Science and Technology, DMR Overview: Recent developments in steels have resulted in first generation advanced high strength steels (AHSS) with ferritic microstructures and second generation high strength steels with austenitic microstructures. The research proposed here will develop steels with duplex ferrite and austenite microstructures that will be analogous to acicular microstructures developed in  +  titanium alloys developed for high fracture toughness. The proposed steels will have strengths greater than first generation advanced high strength steels, but cost less than the ultra high manganese second generation steels. Two lightweight Fe-Mn-Al-C steels have been produced at Missouri S&T that meet the property goals for 3 rd generation AHSS (see circles in above property plot). These steels are 4-5% lighter than traditional 1 st generation AHSS (shown as the lower band of various steels in the property plot). Highlight: A lightweight steel with composition Fe-14Mn-3.5Al-1.4Si-0.14C has been produced at Missouri S&T that consists of a austenite and ferrite microstructure.

Intellectual Merit: A physics based approach to alloy design is being pursued to understand the role of Mn and Al in the defect formation with interstitial atoms and the stabilization of austenite. Systematic calculations of the electronic structure and lattice distortions are being performed using the highly precise first-principles full potential linearized augmented plane wave method (FLAPW) with the structure optimization capabilities. This will allow us to calculate the dependence of the lattice parameters on carbon and nitrogen concentrations for austenite and ferrite. Similar calculations will be performed for the transition metal additions including Mn, Cr and Mo. The interaction between solutes and interstitials will be the most important part of our investigations. The total and formation energy calculations will allow us to analyze the possibility of clustering and the effect of solutes on the diffusion of interstitial impurities (C, N, and B). Mn Fe C Highlight: A charge density distribution plot for (110) revealing the formation of a Mn-C-Fe (180°) defect structure is shown. Other Mn positions result in higher total energy by ~ meV. Previously reported Mn-C complexes based on a solid mechanics model place the carbon atom in the next nearest octahedral position relative to the Mn atom. (AHSS) Development of Nano-acicular Duplex Steels- CMMI David C. Van Aken, Missouri University of Science and Technology, DMR

Highlight: [100] projection of  -carbide (Fe 3 AlC- perovskite crystal structure) after phosphorous substitution and relaxation. First principles calculations show that phosphorous substitutes for aluminum and the relaxed structure is easily cleaved on (001). Broader Impact: The NSF funded first principle studies are providing significant insight as to the role of Mn and Al alloying in steel. The Missouri S&T team has also been studying a 2 nd generation Fe-Mn-Al-C steel with composition Fe-30Mn-9Al-1Si-0.9C-0.5Mo. This lightweight steel is 12-14% less dense than current 1 st generation AHSS and has application as military armor. A fully austenitic microstructure is obtained by solution treatment and age hardening is produced by precipitation of  -carbide. The Fe-Mn-Al-C steels are prone to phosphorous embrittlement and {001} cleavage is observed. This study was a synergistic activity by the Missouri S&T team that was partially funded by both NSF and ARL-APG. (AHSS) Development of Nano-acicular Duplex Steels- CMMI David C. Van Aken, Missouri University of Science and Technology, DMR