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. 2018 Jul 25;8(1):11200.
doi: 10.1038/s41598-018-29632-y.

Superior Strength and Multiple Strengthening Mechanisms in Nanocrystalline TWIP Steel

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Free PMC article

Superior Strength and Multiple Strengthening Mechanisms in Nanocrystalline TWIP Steel

Jung Gi Kim et al. Sci Rep. .
Free PMC article

Abstract

The strengthening mechanism of the metallic material is related to the hindrance of the dislocation motion, and it is possible to achieve superior strength by maximizing these obstacles. In this study, the multiple strengthening mechanism-based nanostructured steel with high density of defects was fabricated using high-pressure torsion at room and elevated temperatures. By combining multiple strengthening mechanisms, we enhanced the strength of Fe-15 Mn-0.6C-1.5 Al steel to 2.6 GPa. We have found that solute segregation at grain boundaries achieves nanograined and nanotwinned structures with higher strength than the segregation-free counterparts. The importance of the use of multiple deformation mechanism suggests the development of a wide range of strong nanotwinned and nanostructured materials via severe plastic deformation process.

Conflict of interest statement

The authors declare no competing interests.

Figures

Figure 1
Figure 1
Mechanical properties of the HPT-processed samples. (a) Microhardness distributions as a function of εs, i.e. distance from the center. Hardness evolution of the HPT-process samples is classified: for εs < 92, the sample strengths of HT- (open circles) and RT-HPT (closed circles) increase with increasing εs; for 92 ≤ εs ≤ 125, a strength difference evolves; for εs = 125, a significant strength enhancement occurs in HT-HPT. (b) The stress-strain curves of the RT-92, RT-125, HT-92, and HT-125 samples.
Figure 2
Figure 2
RT-HPT microstructures showing solid phase transformation with respect to applied shear strains. (a) Low-magnification bright-field micrograph of RT-HPT samples. (b) Grain size distribution (average grain size 45 nm) determined from HRTEM images. (c) HRTEM images and corresponding selected area electron diffraction patterns with [011]fcc zone axis shown as inset for RT-92 samples. Solid phase transformation occurs from γ to ε-martensite. (d) The HRTEM micrograph for the RT-125 samples shows phase transformation from γ to ε- and ε- to α′-martensite at the intersection point of two ε-crystals (for detailed analysis, see the Supplementary Fig. 2).
Figure 3
Figure 3
HT-HPT microstructures, showing the formation of deformation twins. (a) Low-magnification bright-field micrograph of the HT-HPT sample. (b) HRTEM images and corresponding selected area electron diffraction patterns with a [011]fcc zone axis shown as an inset for HT-92 samples. The formation of nanometer-thick twin. (c) HRTEM micrograph for the HT-125 sample. Both nanometer-thick twins and nano-sized particle on grain boundary, highlighted by a yellow circle. The distributions of (d) grain size and (e) lamellar twin thicknesses, determined from HRTEM images, revealing an average grain size and twin thickness of 45 and 3 nm, respectively.
Figure 4
Figure 4
C segregation to grain boundaries in HT-125 sample. (a) Bright-field TEM image and correlative APT results of C (red) obtained from the same tip. Red arrows and dotted lines mark grain boundaries visible in both TEM micrograph and 3D map. (b) 3D C atom map and corresponding 2D contour map for visual correlation with the APT map of segregation to four different grain boundaries. (c) 1D compositional profiles across matrix-boundary interfaces highlighted in the atom map reveal that not all grain boundaries contain the same chemical composition of C. (d) The sectional Mn (cyan) and C (red) distributions (measured volume = 50 nm X 50 nm X 150 nm). (e) 2D contour map of Mn that is superimposed to C atoms near the line dislocation (the magnified area is marked as a solid line; measured area = 8 × 16 nm2; red: high-quantity of Mn, blue: low-quantity of Mn). (f) 1D compositional profiles show both Mn and C are segregated at the line dislocation.
Figure 5
Figure 5
Schematic diagram showing dominating strengthening mechanism evolution steps of HT- and RT-HPT samples. For εs < 92 (Stage I), grain refinement occurs in all HPT-treated samples and grain size remains unchanged at a given HPT temperature. (ii) For εs = 92 (Stage II), the RT-HPT sample achieves plasticity by means of solid phase transformation, while nanometer-thick twins occur in the HT-HPT sample. (iii) For εs = 125 (Stage III), multiple phases coexist by means of two-step paths of solid phase transformation for RT-HPT sample, while nano-twins, nano-grains, and solute segregation occur for the HT-HPT sample.

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