Linkage between mechanical properties and phase transformations in austenitic stainless steels

1. Linkage between mechanical properties and phase transformations in austenitic stainless steels Ph.D. candidate David Marechal Scientific Supervision Chad Sinclair (UBC) Industrial support Jean-Denis Mithieux (Ugine&ALZ) Valerie Kostoj (Ugine&ALZ)

2. ContextAustenitic Stainless Steels for structural applications reduce weight of cars and improve crashworthiness high mechanical strength and excellent formability

3. Context • Constitutional laws needed for forming. • These are not well identified for austenitic stainless steels • Partially due to the strain-induced g -> a’ transformation that occurs during forming. • Coupling between plasticity and phase transformation.

4. Current understanding of deformation mechanisms

5. Deformation mechanisms in austenite Plates of e martensite (hcp) Rousseau et al., 1970 g -> e ->a’ g -> a’ Nuclei of a’ martensite (bct) Spencer, Ph.D. thesis, McMaster University, 2004

6. Questions • Scale of microstructure • Mode of deformation ? • Kinetics • Nucleation • Growth • Mechanical properties

7. Influence of grain size FE-C-Cr-Ni-Mn deformed at 298K Jin et al., 2007 AISI 304 at -50°C, measured by XRD De et al., 2006 AISI 304 at 298K, measured by Ferritescope Varma et al., 1994 • In general, kinetics is accelerated for coarse grains. • However, lack of experiments below 50mm. • What happens for submicron grains ?

8. Influence of mode of deformation • Two components : • strain path • stress state • g -> a’ transformation accompanied by volume expansion. • High triaxiality favours formation of a’ martensite. (Stringfellow et al., 1991) • Tension assists g -> a’ transformation more than compression. • Shear components also important.

9. Influence of mode of deformation AISI 304 deformed at 77 K Lebedev et al., 2000. AISI 304 deformed at 298 K Iwamoto et al., 1998. • Limited amount of data. Does not allow full understanding. • Stringfellow theory based on hydrostatic component. • However, there are cases where a’ dominates in compression.

10. Summary • g -> a’ transformation contributes to increased W-H in austenitic stainless steels. • Nucleation of a’ motivated by intersection of e plates. • Lack of data for the growth of a’. • Besides temperature and strain rate, transformation affected by: • grain size • strain path • stress state

11. 2. Outline on current research

12. Material studied • Grade AISI 301LN, sheet samples • Low C, low Ni and high N reinforce low stability of austenite and low SFE. • High levels of a’ (up to 70%) formed upon Room Temperature straining. • Because of low C, a’ has a nearly bcc-structure.

13. Generation of grain size Annealing 750 to 1050°С 3 or 30 min Temperature Cryorolling Cooling down in air Time + +

14. Characterization of tensile response

15. Characterization of a’ content • a’ fraction measured with a Ferritescope. • Good reproductibility. • Non-monotonic trend of the rate of transformation towards grain size

16. Deformation microstructure1) High-resolution EBSD ND RD Austenite (grey) a’ martensite (colored)

17. Deformation microstructure2) TEM (110)a’ 1mm 1mm (211)a’

18. Preliminary ideas for modelling • Need: • 1. Behaviour of austenite • Vocce law from experiment • 2. Behaviour of a’ • Best fit assuming Vocce law • 3. Transformation kinetics • From experiment • 4. Law of mixtures • Equal strain in both phases • stot = f sa’ + (1-f) sg • 5. Need physical understanding: • Why is a’ behaviour independent of austenite grain size ?

19. Preliminary ideas for modelling

20. Work planned • What is the mechanical behaviour of a’ ? • Neutron diffraction • Need to understand the nucleation / growth of a’ • TEM / EBSD • Effect of strain path / stress state • Extension to other paths of deformation (i.e. pure shear & plane strain tension)

21. Conclusion • Poor knowledge of kinetics / mechanical response towards : • grain size • mode of deformation • Wide range of grain size has been generated (0.4 to 30mm) • Experiments in uniaxial tension • Preliminary simple model --> encouraging. • Provide physical understanding to problem. • Plan to extend to other deformation modes.