SFB-761 “Stahl – ab initio” Sub-project A2 “ Ab initio thermodynamics and kinetics”

1. SFB-761 “Stahl – ab initio” Sub-project A2 “Ab initio thermodynamics and kinetics”

2. Contents • Motivation • Methodology • ANNNI model • Explicit approach • Structures • Results • Magnetic phase diagram of iron • γ-surface • twins • Summary

3. Motivation

4. Motivation • 99% of the passenger cars have a steel body • 60-70% of the car weight consist of steel or steel-based parts • Passive safety and Protection from • Frontal crash • Rear collision • Side impact • Roll over Weight increasing • Decreasing of • Engine efficiency and economy • Environmental pollution • Steels with • low weight density • high Energy-absorbtion and • strength Tend to be mutually exclusive

5. Motivation Material should react only locally to failures induced by high external stress Strength + Plasticity • On the atomic scale this means • Changes in the atomic layers stacking sequence under the stress and gradual twin formation (TWIP) during deformation (“dynamical” Hall-Petch effect) B.C. De Cooman, Kwang-geun Chin, Jinkyung Kim

6. Motivation Atomic layers shear can take place only if the stacking fault energy (SFE), i.e. energy required to change the sequence in atomic layer stacking, lies in a specific range SFE is the crucial quantitative parameter that characterizes the type of plasticity mechanism in crystalline materials A. Saeed-Akbari, et al., Meta. and Mater. Trans. A 40, 3076 (2009) The idea that changing the SFE one can tune the mechanical properties opens an attractive way to the design of new high-strength lightweight steels

7. Motivation • High-Mnsteels (austenitic fccstructure stabilized by Mn) • exceptional mechanical properties • relatively lightweight • SFE trends • different influence of alloyingelements • chemicalandmagneticdisorder • dependenceofpropertiesonsamplepreparationandmicrostructure Ab initio calculations allow independent consideration of the influence of different factors on the SFE Figure from: J. Nakano and P. J. Jacques, CALPHAD 34 (2010) 167

8. Motivation Local magnetism can influence different properties of a material Magnetic contribution to SFE Phonon spectrum in PM fcc Fe, F.Körmann (SFB-761) Vibrational, electronic, and magneticcontributions to the heat capacity of FM bcc Fe F.Körmann (SFB-761) But due to computational challenge, often the simpler NM calculation are performed. Reliability may be questionable in this case.

9. Motivation • The impact of local magnetism on the SFE in pure iron is considered because • Iron is a basis element in steels • To avoid the influence of alloying elements

10. Contents • Motivation • Methodology • ANNNI model • Explicit approach • Structures • Results • Magnetic phase diagram of iron • γ-surface • twins • Summary

11. Methodology. ANNNI model

12. Methodology. Explicit approach • More general approach with the supercellexplicitly containing the defect • Shear of one part of the crystal with respect to another => concept of the generalized stacking fault energy (γ-) surface, where ISF is a particular point (minimum)

13. Methodology. fcc = …ABCABC… Construct the cell explicitly containing …ABCABC… stacking: Face centered cubic => hexagonal with 3 atoms per cell C C B B A A

14. Methodology. Supercell structures NM, FM, PM AFMS AFMD [111] [112]

15. Methodology. Supercell structures NM, FM, PM AFMS AFMD [111] [112]

16. Contents • Motivation • Methodology • ANNNI model • Explicit approach • Structures • Results • Magnetic phase diagram of iron • γ-surface • twins • Summary

17. Phase diagram of iron High-Mn steels NM FM AFMS AFMD DLM NM@hcp

18. Phase diagram of iron NM FM AFMS AFMD DLM NM@hcp

19. Phase diagram of iron NM FM AFMS AFMD DLM NM@hcp

20. Phase diagram of iron NM FM AFMS AFMD DLM NM@hcp

21. Phase diagram of iron NM FM AFMS AFMD DLM NM@hcp

22. Phase diagram of iron >0 NM FM AFMS AFMD DLM NM@hcp

23. Contents • Motivation • Methodology • ANNNI model • Explicit approach • Structures • Results • Magnetic phase diagram of iron • γ-surface • twins • Summary

24. γ-surface of fcc Fe

25. γ-surface of fcc Fe FMLS FMHS

26. γ-surface of fcc Fe FMLS FMHS AFMS

27. γ-surface of fcc Fe FMLS AFMD FMHS AFMS

28. γ-surface of fcc Fe FMLS • The GSFE surface topology as well as the ISFE are affected by magnetism • Magnetism changes the ISF energetic barrier AFMD FMHS AFMS DLM

29. ISF Energy • Absolute value of the ISFE changes considerably in magnetic phases

30. Contents • Motivation • Methodology • ANNNI model • Explicit approach • Structures • Results • Magnetic phase diagram of iron • γ-surface • twins • Summary

31. Twin Structure ( + magnetic mirror symmetry) NM, FM, DLM AFMS AFMD [111] [112]

32. ISF and Twin Energy • The general rulethat SFE = 2Twin is fulfilled. • The difference is mainly due to the decreasing of the distance between defects in the same size supercell

33. Magnetism vs. Chemical trends of SFE (carbon) T. Hickel, S. Sandlöbes, R.K.W. Marceau, A. Dick, I. Bleskov, J. Neugebauer, and D. Raabe, ActaMaterialia, SUBMITTED • The influence of magnetism on the SFE is not relevant for chemical trends

34. B 1.8 Å Atomic planes mismatch on the twin boundary 2.2 Å (002) (-111) 33

35. Twin in fcc [111] [112]

36. Twin γ-surface [111] [112]

37. Twin γ-surface

38. Twin γ-surface

39. Twin γ-surface

40. Twin γ-surface

41. Twin γ-surface

42. Twin γ-surface • Twin γ-surface is not strongly affected by local magnetism except FMHS phase, where the minimum exists. • “Agreement” with exp. => local ordering near the twin boundary

43. Twin FMHS local magnetic moments Only local magnetic moments of the atoms lying in the vicinity of the defect are affected

44. Twin γ-surface [111] [112]

45. AB’ stacking [111] [1-21]

46. AB’ stacking FM FM@AB’

47. AB’ stacking FM FM@AB’

48. Contents • Motivation • Methodology • ANNNI model • Explicit approach • Structures • Results • Magnetic phase diagram of iron • γ-surface • twins • Summary

49. Summary • Detailed magnetic phase diagram for different structures in pure Fe (fcc, hcp, AB’) => PM/AFM magnetic phase is relevant for TWIP-steels • Topology of GSFE surface strongly depends on local magnetic structure • Magnetism strongly influences the SFE and twin energy in pure Fe • Topology of twin γ-surface doesn’t suffer strong changes subject to local magnetism changes but • discovers an unexpected minimum corresponding to AB’ stacking sequence which can shad a light to atomic planes mismatch in twin and matrix observed experimentally

50. Thank you for your attention!