Kristi án Máthis 1 , Přemysl Beran 2 , Petr Harcuba 1 , Jan Čapek 1 , Petr Lukáš 2

1. In-situ neutron diffraction and acoustic emission investigation of twinning activity in cast magnesium Kristián Máthis1, Přemysl Beran2, Petr Harcuba1, Jan Čapek1, Petr Lukáš2 1Department of Physics of Materials, Faculty of Mathematics and Physics, Charles University, Prague, Czech Republic 2Nuclear Physics Institute, Řež, Czech Republic

2. Twinning in magnesium – one of the most important deformation mechanism Tension – compression asymmetry – different evolution of twinning in tension and compression, respectively Frequently studied for textured materials – limited number of data for random textured materials Our goal Study of the loading mode dependence of the twinning evolution in the entire volume of the texture free magnesium Motivation

3. Mg100 AE in cast polycrystalline magnesium Specimen – polycrystalline magnesium as-cast, random texture Mg + 1.00 wt.% Zr – grain size: 110 µm

4. Acoustic emissions are transient elastic waves generated by the rapid release of energy from localized sources within the material. (ASTM E610-82) What is acoustic emission? • Information about the dynamic processes involved in plastic deformation Source: http://www.ndt-ed.org/ • Major sources of AE in magnesium: • collective motion of high number of dislocations • deformation twinning

5. Advantages Real-time, non-destructive method Suited for global monitoring – information from the entire volume Detects movement/growth of defects (e.g. dislocations, twins, cracks) Intimate relationship to material failure Limitations Dependence on stress history Unstressed defects will not emit Wave attenuation and noise Why acoustic emission?

6. Measurement of surface displacement u caused by waves by means of a piezo-crystal detector Basic principles of AE measurements Hit-based processing, a hit is defined by threshold and dead-time Parameters: Amplitude, risetime, duration, energy, counts, count rate

7. AE response of twinning Twinning – first observed source of AE Hexagonal closed packed (hcp) structure – Thompson-Millard source Modification of the Frank-Read source;  = (1/12;1/4) Twin nucleation – collective motion of several hundred dislocations – u ~ 10-7 m, well detectable Twin growth – u ~ 10-22 m, not detectable

8. Loading mode dependence of AE Dependence of def. curves on loading mode. (a) compression; (b) tension Asymmetry in deformation curves – difference in yield stress, hardening Compression – S-shaped curve, lower hardening rate at the beginning

9. Loading mode dependence of AE Dependence of def. curves on loading mode. (a) compression; (b) tension Significant asymmetry also in AE – Why? Different development of twinning

10. Loading mode dependence of AE AE – info only about twin nucl. We need an additional method that give information about twinning growth from the entire volume Solution? Neutron diffraction measurements Getting complementary data: observation of twin nucleation (AE) + twin growth (ND)

11. Loading mode dependence of AE Neutron diffraction – in-situ observation of twin growth Active {10-12} twinning  change of intensity of {00.2} and {10.0} peaks

12. Loading mode dependence of AE Neutron diffraction – in-situ observation of twin growth Active {10-12} twinning  change of intensity of {00.2} and {10.0} peaks

13. Loading mode dependence of AE • Compression • Maximum of AE signal @ 1% of strain  above this limit mainly rapid twin growth  AE signal decreases • Higher hardening rate part  activation of non-basal slip systems increases the forest dislocation density  reduced mean free path of dislocations  AE signal under detectable limit

14. Loading mode dependence of AE • Compression • Maximum of AE signal @ 1% of strain  above this limit mainly rapid twin growth  AE signal decreases • Higher hardening rate part  activation of non-basal slip systems increases the forest dislocation density  reduced mean free path of dislocations  AE signal under detectable limit

15. Loading mode dependence of AE • Tension • Burst signal during the entire test • Twin growth is limited  plastic deformation requires nucleation of new twins Is the number of twins higher in tensile samples?

16. Loading mode dependence of AE • Tension • Burst signal during the entire test • Twin growth is limited  plastic deformation requires nucleation of new twins Is the number of twins higher in tensile samples?

17. Compression Tension Loading mode dependence of AE Micrographs of samples after 4% of deformation Compression – large twins Tension – high number of small twins

18. Compression Tension Loading mode dependence of AE Compression – large twins Tension – high number of small twins Difference in the amplitude of AE signals

19. Compression Tension Loading mode dependence of AE Micrographs of samples after 4% of deformation Compression – large twins Tension – high number of small twins What about the overall twinned volume?

20. Loading mode dependence of AE • The change of integrated intensities – estimation about the twinned volume • Different number of twins and twin size • BUT • NO DIFFERENCE in the overall twinned volume Comparison of normalized integrated intensities

21. Conclusions • In compression – twin nucleation only at the beginning of the deformation followed by twin growth • In tension – significant twin nucleation during the entire test, higher number of twins • Larger AE amplitudes in compression – larger twin size • ND measurements – no difference in overall twinned volume Acknowledgement The authors are grateful for financial support of the Czech Science Foundation, Grants P108/11/1267 and P204/12/1360