1 D. Marechal, C. W. Sinclair Department of Materials Engineering, The University of British Columbia, Vancouver Canada Revisiting the TRIP effect with a “dynamic composite” mechanical model J.-D. Mithieux, V. Kostoj ArcelorMittal Stainless Steel Research Centre, Isbergues France

2 Austenitic Stainless Steels for Structural Applications -Many possible applications for austenitic stainless steels. -Main limitations: Cost and lack of predictive capability of mechanical response. Schmitt, 4th Stainless Steel Science and Market Congress, 2002

3 Dynamic Evolution of Microstructure Each phase contributes to the overall stresses. Contribution difficult to measure separately. Hindered the elaboration and validation of constitutive laws

4 Previous Approaches Mechanics-based models: –Tensorial expressions (strain extrapolation of yield surface) –Sophisticated homogenization schemes, e.g. secant, tangent, FEM –Many empirical parameters e.g. Iwamoto-Tsuta (24 fitting parameters) Materials-based models: – One-dimensional – Explicit microstructure evolution GND model (but no dynamic evolution of microstructure) [Talonen, PhD thesis, (2007), TKK Finland] Size evolution of  and  ’ (but no evidence for mechanical behaviour of  ’) [Bouquerel et al., Acta Materialia 54 (2006) ] Bouquerel et al., Acta Materialia 54 (2006)

5 Need for a new approach Want “simple” physical materials-based approach –consistent with microstructure. –with minimum fitting parameters. Need to account for addition of freshly formed  ’ at each step of strain. Need to be consistent with experimental measurements of load partitioning between austenite and martensite. Want to be predictive – explain effects of –Temperature –Microstructure (e.g. Grain size) –Strain path (not presented here)

6 Material Studied AISI 301LN stainless steel Initial microstructure: –Fully recrystallized, equiaxed grains. –Mainly focus on D = 28  m. 50 µm

7 Experimental Data: Tensile Response Bulk Tensile Response 1. Bulk response of alloy 2. Evolution of phase fraction 3. Behaviour at 80°C (guess for 

8 Experimental Data: Intrinsic Behaviour of  ’ Measured via neutron diffraction [Dufour, Master thesis (2007), UCL Belgium]. Measured via magnetomechanical effect effect [Marechal, Ph.D. thesis (2011), UBC Canada]. Results are self-consistent (  ’,  and bulk response)

9 A Dynamic Composite Model   -> Kocks-Mecking   ’ -> Elastoplastic behaviour Mechanical equilibrium for dynamic  ’: Strain homogenization:  ’ initially formed is under compression, consistent with volume expansion for  →  ’ [Delannay et al., Int. J. Sol. Struct. 45 (2008) ]. Old  ’ Fresh  ’

10 Average  ’ Behaviour Problem: each strain at which  ’ forms needs to be tracked individually. Average behaviour of  ’: Only need to track the average strain and deduce corresponding. Old  ’ Fresh  ’ <><>

11 Application of the Model Influence of grain size Need adjustment of   ’0 [8] Marechal, Ph.D. thesis, UBC Only parameters changed: austenite yield stress and evolution of  ’ volume fraction. Grade 301LN T = 25°C Influence of Temperature [9] Nanga et al., ICOMAT proc., 2008 Grade 301LN Grain size = 8  m

12 Predicting Tensile Unstabilities Tensile unstabilities known to appear in austenitic stainless steel deformed at low temperature. Effect of replacing austenite by softer  ’-martensite. Term 1 Term 2 Term 3 “Usual behaviour” Strain localization

13 Summary 1.A model of constitutive law for materials whose microstructure is dynamically evolving. 2.Model calibrated with experiments: Intrinsic mechanical properties of austenite. Load partitioning between austenite and  ’. 3.Model fitted to account for different conditions (Temperature, grain size, composition). 4.Few basic assumptions that remain to be checked (iso- strain,  ’ initially under compression).

Additional Material

Magneto-mechanical measurements 

Magneto-mechanical measurements: Reference sample

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. Magneto-mechanical measurements