Search

• We describe a feasible method of avoiding the strength–ductility dilemma of typical ferrite-based lightweight steels by applying low-temperature tempering-induced partitioning (LTP) treatment. • We show that the simultaneous increments of strength and total elongation of widely known low-Mn lightweight steel with a composition of Fe–2.8Mn–5.7Al–0.3C by wt.%, were achieved by strength of 1,118 MPa and total elongation of 47%, through the facile route of LTP treatment. • This size-dependent partitioning results in slip plane spacing modification and resultant lattice strain, which can act through dislocation engineering. • The additional ductility and strengths, which result from the small austenite grains, is attributed to the easy passage of dislocations along slip planes (upon the early deformation), interface shielding effect of C-dislocations, delayed TRIP effect, and the cross-slip motions of mobile dislocations (at the high strain level). • This size-dependent dislocation activity in austenite grains, as proposed here by LTP, offers a low raw-materials cost than that of high-Ni maraging steels as well as a low thermomechanical-processing cost than that of high-Mn TWIP steels, while maintaining an analogous strength–ductility balance for light weighting approach. High- and medium-Mn (H/M-Mn) base lightweight steels are a class of ultrastrong structural materials with high ductility compared to their low-Mn counterparts with low strength and poor ductility. However, producing these H/M-Mn materials requires the advanced or high-tech manufacturing techniques, which can unavoidably provoke labor and cost concerns. Herein, we have developed a facile strategy that circumvents the strength–ductility trade-off in low-Mn ferritic lightweight steels, by employing low-temperature tempering-induced partitioning (LTP). This LTP treatment affords a typical Fe-2.8Mn-5.7Al-0.3C (wt.%) steel with a heterogeneous size-distribution of metastable austenite embedded in a ferrite matrix for partitioning more carbon into smaller austenite grains than into the larger austenite ones. This size-dependent partitioning results in slip plane spacing modification and lattice strain, which act through dislocation engineering. We ascribe the simultaneous improvement in strength and total elongation to both the size-dependent dislocation movement in austenite grains and the controlled deformation-induced martensitic transformation. The low-carbon-partitioned large austenite grains increase the strength and ductility as a consequence of the combined martensitic transformation and high dislocation density-induced hardening and by interface strengthening. Additionally, high-carbon-partitioned small austenite grains enhance the strength and ductility by planar dislocation glide (in the low strain regime) and by cross-slipping and delayed martensitic transformation (in the high strain regime). The concept of size-dependent dislocation engineering may provide different pathways for developing a wide range of heterogeneous-structured low-Mn lightweight steels, suggesting that LTP may be desirable for broad industrial applications at an economic cost. [ABSTRACT FROM AUTHOR]

• We experimentally determined the P sf and SFEs of austenite/ε-martensite dual-phase Fe–17Mn– x C (x = 0–0.2 wt.%) steels through electron diffraction analysis. • The role of carbon-doping contents on the structural properties, such as the segregation free energy of carbon at grain boundaries, austenite lattice parameter, and mean width of intrinsic stacking faults, was uncovered via atom probe tomography and transmission electron microscopy. • We find that increasing amounts of carbon doping give rise to the increased SFE from 8.6 to 13.5 mJ/m2 with non-linear variation • The SFE trend varies, described here, inversely with the mean width of localized stacking faults, which pass through both other stacking faults and preexisting ε-martensite plates without much difficulty at their intersecting zones. • Localized intrinsic stacking-faults and their intersecting zones with thermally induced ε-martensite are insufficient for strengthening undoped and carbon-doped 17 wt.% Mn steels with low SFEs. • The large amount of carbon doping in the steel yields two effects: increasing the segregation free energy (due to more carbon at grain boundaries) and larger lattice expansion (due to increased soluble carbon at internal grains). Assessing the stacking fault forming probability (P sf) and stacking fault energy (SFE) in medium- or high-Mn base structural materials can anticipate and elucidate the microstructural evolution before and after deformation. Typically, these two parameters have been determined from theoretical calculations and empirical results. However, the estimation of SFE values in Fe–Mn–C ternary systems is a longstanding debate due to the complicated nature of carbon: that is, whether the carbon doping indeed plays an important role in the formation of stacking faults; and how the amount of carbon atoms exist at grain boundaries or at internal grains with respect to the nominal carbon doping contents. Herein, the use of atom probe tomography and transmission electron microscopy (TEM) unveils the influence of carbon-doping contents on the structural properties of dual-phase Fe–17Mn– x C (x = 0–1.56 at%) steels, such as carbon segregation free energy at grain boundaries, carbon concentration in grain interior, interplanar d -spacings, and mean width of intrinsic stacking faults, which are essential for SFE estimation. We next determined the P sf values by two different methods, viz., reciprocal-space electron diffraction measurements and stacking fault width measurements in real-space TEM images. Then, SFEs in the Fe–17Mn– x C systems were calculated on the basis of the generally-known SFE equations. We found that the high amount of carbon doping gives rise to the increased SFE from 8.6 to 13.5 mJ/m2 with non-linear variation. This SFE trend varies inversely with the mean width of localized stacking faults, which pass through both other stacking faults and pre-existing ε-martensite plates without much difficulty at their intersecting zones. The high amount of carbon doping acts twofold, through increasing the segregation free energy (due to more carbon at grain boundaries) and large lattice expansion (due to increased soluble carbon at internal grains). The experimental data obtained here strengthens the composition-dependent SFE maps for predicting the deformation structure and mechanical response of other carbon-doped high-Mn alloy compositions. [ABSTRACT FROM AUTHOR]