Abstract
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.
Highlights
In order to reduce energy consumption and greenhouse gas emissions in the generation automobiles, there is an increasing demand for the development of advanced high-strength steels with high strength to weight ratio[1,2]
Nano-indentation test results have been observed as function of four annealing temperatures (500 °C, 700 °C, 900 °C and 1050 °C) and two alloys compositions (Fe-15Mn-10Al-0.8C-5Ni and Fe-15Mn-10Al-0.8 C)
It is important to note that the nanoindentation technique as performed in this study is unable to measure the hardness of isolated nano-sized precipitates, but measures instead the hardness of the nanosized features within a surrounding matrix, and yield a compound hardness value of the matrix with particles
Summary
In order to reduce energy consumption and greenhouse gas emissions in the generation automobiles, there is an increasing demand for the development of advanced high-strength steels with high strength to weight ratio[1,2]. Our previous study[11] mapped out the phases present in two duplex low-density steels, one with 5 wt.% Ni and the other being Ni-free, in the temperature range of 500 to 1050 °C For both the grades at temperatures from 900 to 1050 °C9–11 B2 formed as micron-sized particles and grain boundary precipitates in the austenite phase and as nano-sized discs in the ferrite (see Fig. 1 (blue circle) and Fig. 2b and e). This study demonstrated that the room temperature mechanical properties of the samples remarkably varied with any change in the composition of precipitates These findings clearly showed that the determination of the properties of nano and micron-sized precipitates is of scientific and technological importance and can be utilized for the future developments of low-density steels. This scientific knowledge and experimental trends were used as the basis for further computational modelling efforts to explain the differences in properties based on precipitation composition and type
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