Thermodynamics of Fe-Mn-C TRIP/TWIP steels

Transcription

1 THERMEC 2009 International Conference on PROCESSING & MANUFACTURING OF ADVANCED MATERIALS Processing, Fabrication, Properties, Applications Berlin, Germany August 25-29, 2009 Thermodynamics of Fe-Mn-C TRIP/TWIP steels Dejan Djurovic, Bengt Hallstedt

2 1 Content 1. Introduction 2. Binary Systems 3. Results 4. Summary

3 1. Introduction 2 The stacking fault energy (SFE) Planar slip together with carbides-sbip-steels (SFE mj/m²) twinning and slip-twip steels (SFE mj/m ²) martensite formation and slip-trip steels (SFE < 20 mj/m²) SFE =f( G γ ε )

4 Thermodynamic models (Regular solutions, Sublattice model, ) Experimental techniques (Calorimetry, electromotive forces, ) Theory (Quantum machanics, Statistical thermodynamic, ) 3 Model equations for G φ (T,x) Crystal structure data Thermodynamic and phase equilibrium data Thermodynamic optimization (Least square method) Thermodynamic properties G φ (T,x) CALPHAD-type ThermoCalc software tools Phase diagrams Applications

5 2. Binary Systems 4 Fe-C P. Gustafson, Scand. J. Metall., 14 (1985) 259 Fe-Mn V. T. Witusiewicz et al., J. Phase Equilib., 25 (2004) 346 Mn-C D. Djurovic and B. Hallstedt 2008 Fe 23 C 6 Fe 5 C 2 Fe 7 C 3 G(Fe n C m,t)- G(Cementite,T)- G(Graphite,T)=a

6 5 a) 3. Results b) Heat capacity, Jmol -1 k Temperature, K Temperature, K L.S. Darken and R.W. Gurry, Trans AIME J. Metals, , vol. 3, pp R.V Andes, Iowa State College δ LiqJ. sci., 1936., vol. 11, 1600 pp G Naeser, Mitt. γ Kaiser Whilhem Inst. Eisenfoes , vol. 16 pp H Selz, H.J. McDonald and C. Wells, Trans AIME, 1940., vol. 140, pp α 800 Fe 3 C J. Mazur, and W. Zacharko, Acta Phys. Polon., , vol. 35 pp M. Umemoto, K. Tsuchiya, Tetsu-to-Hagane., 2002., 200vol. 88, pp Mole fraction C c) Heat capacity Heat of capacity a) Fe 3 C b) of α-fe 3 Cc) Graphite The metastable Fe-C phase diagram

7 6 Fe 23 C 6, Fe 5 C 2, FeX 7 C 3 Enthalpies of formation of metastable iron carbides Fe 3 C f f H K, K, J/mol of atoms Ab initio Present work Hua Fe 23 C Fe 5 C Fe 7 C

8 7 34 Fe 5 C 2 Fe7 C 3 32 The Debye-model entropy and predicted Debye temperature o S(298.15), JK -1 mol Fe 23 C 6 Fe 3 C Based on integration of the experimental heat capacity data of Fe 3 C Previous assessment 26 This work MOLE FRACTION C The room-temperature entropy of iron carbides

9 8 (a) (c) (b) Comparison of (a) the experimental isothermal section of Benz et al. with the calculated isothermal section using TDB from (b) Huang and (c) this work at 873 K.

10 9 Hua1990 this work The calculated Mn distribution ratio between the α phase and M 3 C in comparison with experimental data.

11 10 Hua1990 this work γ T 0 Ms + As 2 ε J.-C. Kim The et al., calculated Mater. Sci. T-zero Eng., line A 378, in comparison (2004) with experimental data for Fe-17wt%Mn-C alloys.

12 11 (a) (c) (b) Comparison of (a) the experimental isothermal section of Benz et al. with the calculated isothermal section using TDB from (b) Huang and (c) this work at 1373 K.

13 12 this work Hua1990 The calculated isocarbon activity lines in the γ phase at 1420 K and 1273 K in comparison with experimental data.

14 13 this work Hua1990 The calculated Mn distribution ratio between The calculated graphite solubility in the γ phase and M 3 C in comparison the liquid phase in comparison with experimental data. with experimental data..

15 14 Schürmann and Geissler's data extrapolated to the Fe-Mn side give liquidus temperature about 10 to 30 K higher than accepted values. The liquidus by Schürmann and Geissler is not in agreement with accepted data on the graphite solubility in the liquid Hua1990 Sch1977 this work Comparison between the calculated and liquidus The experimental experimental projections liquidus liquidus in the projection direction projections in the of in the direction direction u(c). of of u(c). u(c).

16 15 6. Summary The CALPHAD description of Fe-Mn-C system is updated with a modified the description of Fe 3 C phase by the present authors, recently published reassessment of the Fe Mn system by Witusiewicz et al. and the reassessment of the Mn C system by the present authors. A consistent thermodynamic description of the entire Fe Mn C system has been obtained by considering ab initio calculatied enthalpies of formation of metastable carbides together with critically assessed literature data by Huang. The reevalution of the phase equilibria and thermodynamic functions leads to better fits to the available experimental data at lower temperatures.

17 16 Acknowledgement We want to thank DFG for financial support through the collaborative research centre SFB 761 Steel - ab initio; quantum mechanics guided design of new Fe based materials"