1. Linkage between mechanical properties and phase transformations in a 301LN austenitic stainless steel Ph.D. candidate David Marechal Scientific Supervision Dr. Chad Sinclair (UBC) Industrial support Jean-Denis Mithieux (Aperam, R&D Center) Valerie Kostoj (Aperam, R&D Center)

2. Austenitic Stainless Steels for structural applications Schmitt, 4th Stainless Steel Science and Market Congress, 2002 • Many possible applications for austenitic stainless steels. • Main limitation: lack of predictive capability of mechanical response. 2

3. Nuclei of a’ martensite (bcc) Rousseau et al., Mem. Sci. Rev. Metallurgie, 67, 1970 Phase Transformations During Deformation Plates of e martensite (hcp) g -> e ->a’ g -> a’ Spencer, Ph.D. thesis, McMaster University, 2004 • Two strain-induced phase transformations • Difficulty: • 3 phases which co-deform • Kinetics of phase transformations • How are these linked to microstructure and plasticity ? 3

4. AISI 304 at -50°C, De et al., Met. Mat. Trans. A, 37, 2006 Influence of physical/microstructural parameters • Unlike other fcc metals, strong sensitivity to: • Temperature / strain rate • Stress state / strain path • Linked to phase transformations • Microstructure (grain size) less examined in relation to phase transformation, despite substantial grain size refinement (e.g. Poulon et al., ISIJ International, 49, 2009). • Mechanical behaviour strongly grain size dependent (Misra, Met. Trans. A, 41, 2010) • Missing link to phase transformations: • Contradictory results. • Existing models of the kinetics fail to accurately predict grain size dependence. AISI 301LN, Nanga et al., ICOMAT proc., 2008 AISI 304 at 20°C, Varma et al., J. Mat. Sci. Lett., 13, 1994 4

5. Scope and objectives • Scope: Study AISI 301LN with different grain sizes, deformed at low TH along two monotonic paths: • uniaxial tension • simple shear • Objective: • Build a quantitative model capable of predicting mechanical response. Data needed: • phase transformation kinetics, • microstructure evolution, • nucleation of martensites, • stresses carried by individual phases. 5

6. Starting Material and Microstructure • Material: 301LN, low stability of austenite • Grain size refinement: • Using literature, developed processing route for grain sizes from 0.5 mm-28 mm, fully recrystallized. • No previous study has systematically examined such a grain size range. 6

7. Characterization of tensile response • Transformation reduced with reduced grain size, reduces work-hardening. • But, for D < 1 mm, rate increases, work-hardening increases. • Change in nucleation mechanism suggested in one previous study (Yang et al., Acta Metallurgica Sinica, 45, 2009). 7

8. e = 41% e = 10% e = 5% a’2 e a’1 D=28mm e=15% Grey: austenite (g) Colour: a’ martensite • Low band contrast plates (e) • a’ forms along those plates. • Only 3-4 specific crystalline orientations. ND 20 m 20 m 20 m RD Deformed microstructure: Coarse Grain Size 8

9. Variant selection in coarse grain material • Variant selection linked to mechanism of formation. • Importance of macroscopic stress. • Tested 2 hypotheses about factors controlling nucleation: 1. Interaction energy(Humbert, Mat. Sci. Eng. A, 454, 2007) 2. Critical shear stress(Suzuki, Acta Metal., 25, 1977) • Both explained nucleation of e. • None explained nucleation of a’. • Despite recent claims that macroscopic stress dominates the selection of a’ variants, here such trends are not clearly observed. 9

10. Deformed microstructure: Fine Grain Size • Another mechanism of transformation compared to coarse grain size • no e-martensite • Grain boundary becomes dominant nucleation sites • Reduction of e with grain size can be explained. • Grain boundary nucleation of a’ could explain kinetics non-monotony. • Mechanism could be more complex: apparent growth of a’. 10

11. Intrinsic Stresses Carried by Phases • Transformation kinetics important for stress-strain response. • But, overall work-hardening cannot be predicted from the transformation kinetics alone. • Need to measure intrinsic stresses carried by a’ and g. ds/de dfa’/de ? 11

12. s Magneto-mechanical measurements 12

13. Magneto-mechanical measurements • Some limitations (e.g. Empirical approach with a polynomial fit) • Simple method, inexpensive equipment. • Good agreement to Neutron and X-ray diffraction on same material. • Unusually large hardening rate of a’. 13

14. A Dynamic Composite Model • Common simplifications: • Neglect e-martensite. • Both phases obey Voce hardening • Transformation kinetics taken from experiment • Equal strain increments • Key-assumptions of present model: • a’ formed under compression • Each a’ tracked independently df a’ 14

15. Results of the model D = 28mm Uniaxial Tension 15

16. Application of the model Influence of Temperature Influence of grain size • Only parameters varied: • Austenite yield stress • Kinetics for a’ Nanga et al., ICOMAT proc., 2008 Current work Influence of composition Influence of strain path D = 0.5mm D = 2.2mm D = 28mm • - Effect of deformation Texture Need adjustment of sa’0 16 Current work Spencer, PhD thesis, McMaster, 2004 Need adjustment of sa’0

17. Conclusion • First systematic analysis of grain size effect on both kinetics and tensile behaviour. • Non-monotonic effect, due to different nucleation mechanisms. • Design of new experimental method to capture stresses in the magnetic phase. • Physical explanation for the mechanical behaviour and tensile instabilities. 17

18. Perspectives and future work Relevance for industry: Structural application can benefit of grain size strengthening.But, limited by strain localization upon forming. Internal stresses after deep drawing -> magneto-mechanical measurements. Model allows for improved formability / experience. Future work: Fine grain size: In-depth study of a’ nucleation -> better predictive capability of the a’ kinetics. Theoretical approach in magneto-mechanical measurements. Extension of present model to other grades. 18

19. 19

20. ADDITIONAL MATERIAL 20

21. Tensile response

22. Characterization of shear response

23. Behaviour in Simple Shear • Agreement not as good as in tension • Texture effects • Von Mises equivalents may not be adapted • Affect both Kinetics and Mechanical response. • Crystal plasticity D = 0.5mm D = 2.2mm D = 28mm Simple Shear 17

26. Jiles and Atherton, J. of Phys. D, vol. 17, 1984

31. Uniaxial tension Simple shear VPSC simulations