Nano-mechanical properties of Fe-Mn-Al-C lightweight steels

High Al Low-density steels could have a transformative effect on the light-weighting of steel structures for transportation. They can achieve the desired properties with the minimum amount of Ni, and thus are of great interest from an economic perspective. In this study, the mechanical properties of two duplex low-density steels, Fe-15Mn-10Al-0.8C-5Ni and Fe-15Mn-10Al-0.8 C (wt.%) were investigated through nano-indentation and simulation through utilization of ab-initio formalisms in Density Functional Theory (DFT) in order to establish the hardness resulting from two critical structural features (κ-carbides and B2 intermetallic) as a function of annealing temperature (500-1050 °C) and the addition of Ni. In the Ni-free sample, the calculated elastic properties of κ-carbides were compared with those of the B2 intermetallic Fe3Al-L1₂ and the role of Mn in the κ structure and its elastic properties were studied. The Ni-containing samples were found to have a higher hardness due to the B2 phase composition being NiAl rather than FeAl, with Ni-Al bonds reported to be stronger than the Fe-Al bonds. In both samples, at temperatures of 900 °C and above, the ferrite phase contained nano-sized discs of B2 phase, wherein the Ni-containing samples exhibited higher hardness, attributed again to the stronger Ni-Al bonds in the B2 phase. At 700 °C and below, the nano-sized B2 discs were replaced by micrometre sized needles of κ in the Ni-free sample resulting in a lowering of the hardness. In the Ni-containing sample, the entire α phase was replaced by B2 stringers, which had a lower hardness than the Ni-Al nano-discs due to a lower Ni content in B2 stringer bands formed at 700 °C and below. In addition, the hardness of needle-like κ-carbides formed in α phase was found to be a function of Mn content. Although it was impossible to measure the hardness of cuboid κ particles in γ phase because of their nano-size, the hardness value of composite phases, e.g. γ + κ was measured and reported. All the hardness values were compared and rationalized by bonding energy between different atoms.