Kappa carbides in Fe-(4 6)Mn-(6 8)Al-0.3C steel

Figure 1: A back-scattered electron image of the tensile specimen in the vicinity of fracture surface of the Fe-29Mn-9Al-2.6C (in mass%) alloy (Kimura et al., 2004). Scale bar: 20 µm.

Figure 2: Scanning Electron Microscope (SEM) micrographs of the crosssectional area beneath the tensile fracture surface of the Fe-(4 6)Mn-(6 8)Al- 0.1C (mass%) alloy. Crack was initiated in kappa-carbide and rapidly propagated into ferrite (Han et al., 2010). Scale bar: 5 µm.

Figure 3: SEM micrographs of the cross-sectional area beneath the tensile fracture surface of the Fe-(4 6)Mn-(6 8)Al-0.3C (mass%) alloy. Crack in the kappa-carbide band is short and discrete (Han et al., 2010). Scale bar: 1 µm.

Carbide name: Kappa carbide
Record No.: 1232
Carbide formula: (Fe,Mn)3AlC
Carbide type: No data
Carbide composition in weight %: No data
Image type: SEM
Steel name: Fe-(4 6)Mn-(6 8)Al-0.3C
Mat.No. (Wr.Nr.) designation: No data
DIN designation: No data
AISI/SAE/ASTM designation: No data
Other designation: No data
Steel group: High manganese and high aluminum steels
Steel composition in weight %: Fe-29Mn-9Al-2.6C
Heat treatment/condition: No data
Note: (Fe,Mn)3AlC kappa-carbides are important substance in high strength light-weight steels. Kappa-carbide is known to initiate crack and propagate the crack or, otherwise, pin the slips and make uniform shear bands. These opposite properties was decided by environment of the system. Therefore phase diagram of Fe-Mn-Al-C quaternary system and kappa-carbide is vital for this kind of steels. However, there is no solid thermodynamic value and stability of kappa-carbide. To work towards this goal, the all-electron full potential linearized augmented plane-wave method(FLAPW) was used within the generalized gradient approximation. The formation enthalpies of various kappa-carbides are calculated. All of kappa-carbides have negative formation enthalpy. The lowest kappa-carbide formation was Fe2MnAlC which is 9.5 kJ atom-mol.1 lower than the highest formation Fe3AlC. When the carbon position was changed to another octahedral position in Fe2MnAlC, the formation energy becomes positive but magnetic moment was increased. In this research, first-principles calculation result was reassessed using Monte-Carlo cell gas model. The result of Monte-Carlo simulation showed smaller entropy value than configurational entropy caused by implementation problem. However, general temperature dependence of free energy, entropy, specific heat and internal energy is well predicted by simulation. In the future work, we hope to incorporate the calculated energies in to phase diagram calculation methods and modify cell gas model to improve implementation problem.

In general, carbide is harder than pure iron and strengthens steels. However, it can also be brittle, therefore, it initiates cracks or helps the propagation of cracks. Also, the phase which is surrounding kappa-carbide is important. For example, coarsened kappa-carbide in a phase boundary can easily initiate cracks which propagate (Fig. 1.4) when austenite (gamma) coexists with kappa (Kimura et al., 2004). Indeed, if there is ferrite, the crack will go through the ferrite (Fig. 1.5) and non-work hardened ferrite in the kappa-carbide inhibits crack propagation or changes the direction of crack (Fig. 1.6) (Han et al., 2010). The experiments on fine kappa-carbide was reported by Frommeyer and Bršux (2006). In their work, nano-size kappa-carbide was regularly distributed and coherent with austenite and it sustained homogeneous shear band acquired by dislocation glide. As a result, ductility is remarkably improved and the specific energy absorption is as high as that of TWIP steels. Scattered kappa-carbide on phase boundaries is usually the initial point of cracks. When austenite coexists with kappa-carbide, cracks propagate along kappa-carbide or along the boundary between kappa and austenite. However, when kappa is in ferrite, the crack rapidly moves into the ferrite. With high manganese and high aluminum contents, austenite is highly stable so phase transformation to martensite would not happen and the stacking fault energy is too high to induce mechanical twinning. So shear band induced plasticity will be the primary deformation mode. Kappa-carbide sustains shear bands so, it is possible for those to be uniformly dispersed. However, to obtain good ductility via the SIP effect, kappa-carbide should be fine and coherent with the austenite (Frommeyer and Bršux, 2006). It clearly is necessary to strictly control the precipitation of kappa-carbide.
Links: No data
Reference: Not shown in this demo version.

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