Deformation mechanism of TWIP steels at high strain rates HUANG Mingxin LIANG Zhiyuan The University of Hong Kong Collaborators: HUANG Wen Shenzhen Un

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1 Deformation mechanism of TWIP steels at high strain rates HUANG Mingxin LIANG Zhiyuan The University of Hong Kong Collaborators: HUANG Wen Shenzhen University LIU Rendong WANG Xu Ansteel XIONG Xiaochuan General Motors Automotive steel workshop

2 Introduction Deformation twinning TWIP steels 1800 Fe17Mn0.95C Fe12Mn1.2C True stress (MPa) Fe22Mn1.2C Fe30Mn1C Fe22Mn0.6C Fe30Mn0.5C 1200 Fe30Mn True strain TWIP steels for automotive applications 2

3 Review on strain rates effects σ D TWIP steels ε Experimental data for pure Cu Gray 3

Experiments Material Fe Mn C Al Si wt (%) Bal. 18 0.6 1.5 0.

Split Hopkinson bar system High pressure gas http://what-whenhow.

4 Experiments Material Fe Mn C Al Si wt (%) Bal Mechanical tests Quasi-static tensile tests (10-3 to 10-2 s -1 ) =20 µm; IPF Z0; Step=0.5 µm; Grid200x um High strain rate tensile tests (10 1 to 10 3 s -1 ): Split Hopkinson bar system High pressure gas Microstructure characterization TEM for observing the dislocations and deformation twins Synchrotron X-ray diffraction for measuring the dislocation and twin density 3

5 Mechanical properties Strain rate: to 3750 s -1 Engineering stress (MPa) (a) m I Yielding stress Ultimate tensile stress 80 Uniform elongation dσ y = = 14.4MPa d lnɺ ε E-4 1E Strain rate (s -1 ) Significant instantaneous strain rate sensitivity mi = 14.4MPa 30 Uniform elongation (%) True stress (MPa) (b) 5.7*10-4 s *10-3 s s s True strain Similar UTS obtained regardless of the strain rate. Possible explanations for similar UTS: 1. Instantaneous effect weakens with strain 2. Weakened work-hardening at higher strain rate 4

6 Strain rate jump tests Necking Prestrain at s -1 to different levels Unload and reload at 1700 s -1 until fracture Prestrain 0% 5% 15% 25% σ (MPa) Instantaneous effect stay relatively constant with strain Weaker work-hardening rate at higher strain rate Lower density of defects developed at higher strain rate 5

7 10-4 s -1 TEM bright field (d) 1700 s -1 TEM bright field Massive deformation

7 TEM characterization C D (a) s -1 STEM dark field (b) 1700 s -1 STEM dark field (c) s -1 TEM bright field (d) 1700 s -1 TEM bright field Massive deformation twins and dislocation found in specimen strained at both low and high strain rate Difference in defects density is difficult to detect in TEM 6

8 XRD tests Defects in crystal will cause peak broadening in the X-ray diffraction Intensity % 15% 25% 40% Normalized intensity % 15% 25% 40% Theta Theta Modified Williamson-Hall method-the peak broadening can be linked to the crystal size, dislocation density and population of twin boundaries & stacking faults as: 2 2 1/ K = 0.9 / d + ( π A' b / 2) ρ K C + β ' W( K) + O( K C ) 7

9 Modified W-H plot 2 2 1/ K = 0.9 / d + ( π A' b / 2) ρ K C + β ' W( K) + O( K C ) 220 Bragg angle-θ K = cos θ ( 2 θ ) / λ FWHM- 2θ K = 2sin θ / λ h k + k l + l h C = Ch00 1 q h + k + l K β W K = + π A b ρ K C + O 2 2 1/ ' ( ) 0.9 / d ( ' / 2) ( K C ) 2 y = n + m K C K 2 C K W(K) *10-4 s m = ( π A' b / 2) ρ n = 0.9 / d 2 2 1/2 8

10 Synchrotron XRD Full profiles 5.7*10-4 s s s *10-4 s s s -1 Intensity I/I max Theta θ ( ο ) Modified Williamson-Hall method-the peak broadening can be linked to the crystal size, dislocation density and population of twin boundaries & stacking faults as: 2 2 1/ K = 0.9 / d + ( π A' b / 2) ρ K C + β ' W( K) + O( K C )

11 Modified Williamson-Hall plot (1/nm) *10-4 s s s C (1/nm 2 ) ρ (m -2 ) β' (%) d (nm) εɺ ρ & β ' Negative rate sensitivity of work-hardening

12 Modelling σ = σ + σ th ath Thermal and athermal stress ɺ ε ρ ν G V σ / M kt 0 th = mbλ 0 exp σ MkT M = lnɺ ( G kt ln b ) m ln V ε + V ρ Λ ν = ɺ ε + σ th 0 m 0 I 0 K σ ath = MαGb ρ + L HP 800 Yielding stress Engineering stress (MPa) m I dσ y = = 14.4MPa d lnɺ ε ( + ) dσ y d σ th σ ath dσ th ln = ln = ln = m d ɺ ε d ɺ ε d ɺ ε T, ψ I 400 1E-4 1E Strain rate (s -1 )

Thermal stress Engineering stress (MPa) 1600 1400 1200 1000 800 600 400 5.

13 Thermal stress Engineering stress (MPa) *10-4 s s -1 5% prestrain 15% prestrain 25% prestrain 40% prestrain Prestrain 0% 5% 15% 25% σ (MPa) Engineering strain Conclusion :Thermal stress does not change with dislocation and twin density, and should be constant during straining. 13

14 Athermalstress σ = MαGb ρ + ath K HP L (a function of strain because of dislocation density and twin volume fraction) 500 Fe18Mn0.6C1.5Al Fitting Yield stress (MPa) =634.7 MPa*um 1/ Grain size (µm)

15 Dislocation and Twin evolution d = M k dε b ρ a ρ ρ f F = 1 e χ ( ε ε ) init n Dislocation density 1E15 1E14 1E13 Dislocation 5.7*10-4 (Modelling) Dislocation 1700 (Modelling) Dislocation 5.7*10-4 (Experiment) Dislocation 1700 (Experiment) 1E True strain Twin 5.7*10-4 Twin Twin volume fraction 5.7* χ εɺ ka f ρ εɺ F

16 Stress-strain curves 1600 Experiment 5.7*10-4 Simulation 5.7*10-4 Experiment 1700 Simulation 1700 True Stress True Strain 16

17 Conclusions σ ɺ ε < ɺ ε εɺ σ A A B εɺ B θ Positive instantaneous strain rate sensitivity Negative rate sensitivity of workhardening Yield stress increases with strain rates. UTS remains the same for various strain rates. Higher strain rates leads to lower dislocation density and twin volume fraction. The average glide distance is higher at higher strain rate due to its higher stress. ε 17

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