Revisiting the formation mechanism of intragranular κ-carbide in austenite of a Fe-Mn-Al-Cr-C low-density steel

Abstract

It was usually believed that the formation of intragranular κ-carbide in γ-austenite was attributed to spinodal decomposition followed by ordering reaction. In this work, near-atomic scale characterization of an austenite-based Fe-20Mn-9Al-3Cr-1.2C (wt. %) low-density steel, using (high-resolution) scanning transmission electron microscopy and atomic probe tomography, reveals that the initially-formed κ-carbides (2-3 nm in particle size) are featured with an ordered L′1 2 structure but without detectable chemical partitioning. Further, the Gibbs energy of the FCC phase obtained by thermodynamic calculations always shows a positive curvature (i.e. < 0) with the variable contents of Al and C in the temperature range 400-800 °C. Both the results demonstrate that the ordered nuclei of κ-carbide can form directly in the disordered γ-austenite rather than through the well-known spinodal decomposition-ordering mechanism. The extremely low nucleation barrier is due to the similar lattice structure, same composition and complete coherency between the γ-austenite matrix and κ-carbides.

An accelerated aging assisted by electric current in a Fe-Mn-Al-C low-density steel

“…The average size of κ-carbides was obtained by measuring 100 κ-carbide particles using IPP software. The number density of κ-carbides was determined by counting the number of carbides in 100 nm 2 . As shown in the selected area electron diffraction pattern taken from [110] γ+κ axis in Fig.…”

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Owing to the demand for energy saving and consumption reduction, austenite-based Fe-Mn-Al-C steel with lowdensity and excellent mechanical properties has been attracting a lot of attention [1][2][3][4][5] . In this low-density steel, precipitation strengthening by nano-scale intragranular κ-carbides is an important method to increase the yield strength [6][7][8][9][10] , but the ductility would be significantly reduced if coarse κ-carbides are formed at grain boundaries [11][12][13] .

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Microstructural Refinement and Strain Hardening Behavior in Low‐Density Ultrafine Pearlite Steel

“…The conventional pearlite is composed of ferrite and cementite (θ-pearlite), and compared to κ-pearlite (ferrite and κ-carbide), it has lower strength, but better plasticity and toughness. [10][11][12][13] During the precipitation of carbide from austenite, the lattice misfit of κ-carbide/austenite leads to the replacement of Al by Mn atoms and partially changes the Al concentration. Relevant research [14] shows that the high strain hardening ability of ferrite layers in θ-pearlite suppresses the localization of plastic deformation in cementite layers and then stabilizes the elastoplastic deformation of the pearlite phase.…”

Effect of Cr on Phase Transformation Behavior of Austenite in Fe-20Mn-9Al-1.2C-xCr Low-Density Steels During Isothermal Aging

“…The common austenite decomposition reactions include: discontinuous precipitation, precipitation 1 3 transformation, cellular transformation and spinodal decomposition [11]. Nanoscale intragranular κ-carbide could directly form in austenite by spinodal decomposition or nucleation-growth mechanisms [1,12], which could significantly improve strength and maintain good elongation [13][14][15]. A lamellar discontinuous precipitation (ferrite + κ-carbide) was observed along austenite grain boundaries in a Fe-27Mn -8.5Al-1.0Si-0.92C (wt%) steel after aging at the temperatures ranging from 627 to 777 °C [16].…”

“…Fig 5. Microstructure analysis of the 3Cr sample after the aging at 650 °C for 0.5 h: a and b are the SEM micrographs; b is the magnified SEM micrograph from the rectangular region indicated by red dotted line in a; c is TEM BF micrograph of ferrite; d is the SADP taken from the ferrite and spherical precipitates along with [111]α zone axis; e is the NBD taken from irregularly shaped precipitates along with [1(12) ] zone axis; f is the TEM DF micrograph of the κ-carbides in the austenite and the inset is the corresponding SADP pattern…”