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

CHARLES UNIVERSITY PRAGUE

Department of Physics of Materials

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



Motivation

• 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



AE in cast polycrystalline magnesium

Specimen – polycrystalline magnesium

as-cast, random texture

Mg + 1.00 wt.% Zr – grain size: 110 µm

Mg100Mg100

Strain rate 210-3 s-1

Testing temperature

20ºC

Methods AE, microscopy, neutron diffraction



What is acoustic emission?

• Acoustic emissions are transient elastic waves generated by the

rapid release of energy from localized sources within the material.

(ASTM E610-82)

Source: http://www.ndt-ed.org/

• Major sources of AE in magnesium:

collective motion of high number of dislocations

deformation twinning

• Information about the dynamic processes involved in plastic deformation



Why acoustic emission?

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



Basic principles of AE measurements

Measurement of surface displacement u caused by waves by means

of a piezo-crystal detector

Hit-based processing, a hit is

defined by threshold and dead-

time

Parameters: Amplitude, risetime, duration, energy, counts, count rate



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

01100110]0001[



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




Dependence of def. curves on loading mode. (a) compression; (b) tension

Significant asymmetry also in AE – Why?

Different development of twinning 2110




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)




Neutron diffraction – in-situ observation of twin growth

Active {10-12} twinning change of intensity of {00.2} and {10.0} peaks




Neutron diffraction – in-situ observation of twin growth

Active {10-12} twinning change of intensity of {00.2} and {10.0} peaks




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




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




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?




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?




Micrographs of samples after 4% of deformation

Compression Tension

Compression – large twins Tension – high number of small twins




Compression Tension


Difference in the amplitude of AE signals




Micrographs of samples after 4% of deformation

Compression Tension


What about the overall twinned volume?




Comparison of normalized integrated intensities

• 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



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

Documents

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