Effect of microstructural parameters on twinning activity of magnesium alloys

CHARLES UNIVERSITY PRAGUE

Department of Physics of Materials

Effect of microstructural parameters on twinning activity of magnesium alloys

K. Máthis, J. Čapek, Z. Zdražilová

Department of Physics of Materials, Faculty of Mathematics and Physics, Charles University Prague

CHARLES UNIVERSITY PRAGUE

Department of Physics of Materials

Motivation

• Magnesium alloys – twinning is one of the

main deformation mode

• Twinning has impact on e.g. ductility or tension-

compression asymmetry

• Investigation of influence of initial texture and loading

mode on twinning activity

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CHARLES UNIVERSITY PRAGUE

Department of Physics of Materials

Experimental

Continuous cast AZ31 alloy

Mg + 3 wt.% Al + 1 wt.% Zn

• Tensile and compression tests at RT, strain rate : 10-3 s-1

• Two orientations:

Specimen axis || RD, TD

• In-situ acoustic emission (AE) measurements

CHARLES UNIVERSITY PRAGUE

Department of Physics of Materials

ExperimentalAcoustic emission

In-situ technique – sensitive on twin nucleation

Information from the entire volume of the specimen

Two threshold levels: Count 1 – 302 mV – all effects

Count 2 – 600 mV – mostly twinning

CHARLES UNIVERSITY PRAGUE

Department of Physics of Materials

Initial microstructure

Elongated grains in RD

Strong initial texture

RD TD

CHARLES UNIVERSITY PRAGUE

Department of Physics of Materials

Influence of the initial texture

• Higher strength for TD direction

• AE indicates higher twinning activity

CHARLES UNIVERSITY PRAGUE

Department of Physics of Materials

Influence of the initial texture

• Micrographs show larger amount of twins for TD

RD TD

CHARLES UNIVERSITY PRAGUE

Department of Physics of Materials

Influence of the initial texture

• Twin boundaries are impenetrable for basal dislocations twin boundaries are non-penetrable obstacles

• Dislocation pile-ups at the twin boundaries stress concentration could initiate non-basal slip

• Interactions of non-basal dislocations with basal dislocation results sessile dislocations increased forest dislocation density

• As a consequence of above mentioned larger amount of twin boundaries in TD direction causes more significant hardening

CHARLES UNIVERSITY PRAGUE

Department of Physics of Materials

Influence of the loading mode

• Significant tension – compression asymmetry

• Compression – twin nucleation at the beginning than their growth

• Tension – twin growth is limited twinning during the entire test

CHARLES UNIVERSITY PRAGUE

Department of Physics of Materials

ConclusionsInfluence of the texture

• Larger amount of twins is observed for TD direction interaction

of basal dislocations with twin boundaries causes hardening

Influence of the loading mode

• Compression – twin nucleation at the beginning of the deformation

• Tension – twinning during the entire test

Good agreement between AE measurement and the micrographs

CHARLES UNIVERSITY PRAGUE

Department of Physics of Materials

AcknowledgementThis work received a support from the Grant 106/09/0712 by the Czech Science Foundation.