mechanical properties. This is mainly caused by the hexagonal closed packed crystal (HCP)

this reason, special attention must be given to the understanding of their activity.

The present work has been carried out to study the extrusion processing of the Mg-Zn, Mg-Zn-

Ca and Mg-Zn-Nd alloys. A focus was given to the influence of the Neodymium (Nd) and

recrystallization behavior of the Mg-Zn based alloy. In that regard, the crystallographic texture

formed in the wrought Mg alloys depends on the active deformation mechanisms during the

(VPSC) model and by following the slip trace methodology combined with electron backscatter

During processing, the binary Mg-Zn alloy showed a homogenous, coarse and completely

recrystallized microstructure, whereas the ternary Ca containing alloy also exhibited a

observed in the Ca containing alloy. This result indicates that the Ca and Nd additions not only

and Ca results in a significant increase ductility at room temperature. Additionally, there is a

remarkable reduction of the tension-compression yield asymmetry. These properties are due to

non-basal crystallographic textures and the activity of non-basal deformation modes.

tension deformation of the Mg-Zn alloy, whereas extension twining and basal <a> slip control

the deformation in compression. In Mg-Zn-Ca and Mg-Zn-Nd alloys, additionally to basal <a>

verbessert werden, da sie bei der Herstellung oft starke kristallografische Texturen aufweisen,

die zu schlechten mechanischen Eigenschaften führen können. Dies ist vor allem auf die

hexagonale Kristallstruktur (HCP) und das Fehlen von ausreichend unabhängigen

Gleitsystemen zurückzuführen. Die Verbesserung der mechanischen Eigenschaften kann durch

Verständnis ihrer Aktivität besondere Aufmerksamkeit geschenkt werden.

Die vorliegende Arbeit wurde durchgeführt, um die Strangpressbarkeit der Legierungen Mg-

während der thermomechanischen Verarbeitung statt, während SRX nach dem Prozess erfolgt.

Beide Rekristallisationsprozesse wurden in dieser Arbeit untersucht. Darüber hinaus wurde

In first place, I would like to thank to my academic supervisor Prof. Dr.-Ing. Karl Ulrich Kainer

and to my group leader Dr. Dietmar Letzig, who have given me the opportunity to do this work

her supervision at the Technical University of Berlin.

My deep gratitude to Dr. Jose Victoria-Hernandez who supported, advised, and motivated me

thank to Dr. Jan Bohlen for the interesting discussions as well as suggestions and motivating

I would like to thank to Mrs. Petra Fischer, Mrs. Yukyung Shin and Mr. Gerd Wiese for their

V

1.1 Objectives ........................................................................................................................ 2

1.1.1 Specific objectives .................................................................................................... 2

2. Background ............................................................................................................................ 3

2.1 Magnesium ....................................................................................................................... 3

2.2 Influence of alloying additions on Magnesium ............................................................... 4

2.2.1 Aluminium ................................................................................................................ 4

2.2.2 Zinc ........................................................................................................................... 4

2.3.1 Slip systems .............................................................................................................. 6

2.3.2 Deformation by twinning .......................................................................................... 9

2.4 Extrusion process ........................................................................................................... 11

2.5 Extrusion of magnesium alloys ...................................................................................... 11

2.8 Mechanical properties on magnesium alloys ................................................................. 17

2.9 The Visco-Plastic Self-Consistent model ...................................................................... 19

2.10 Summary and motivation of the work ......................................................................... 22

3. Experimental procedure ....................................................................................................... 23

3.7 Mechanical characterization .......................................................................................... 25

3.8 Slip trace analysis .......................................................................................................... 26

4. Results .................................................................................................................................. 28

4.1 Extrusion processing ...................................................................................................... 28

4.2 Microstructure evolution ................................................................................................ 32

4.2.1 Z1 alloy ................................................................................................................... 32

4.2.2 ZX10 alloy .............................................................................................................. 33

4.2.3 ZNd10 alloy ............................................................................................................ 34

4.3 Crystallographic texture evolution ................................................................................. 36

4.3.1 Z1 alloy ................................................................................................................... 36

4.5 Yield asymmetry on the extruded bars .......................................................................... 48

5. Discussion ............................................................................................................................ 50

5.1 The importance of the alloy composition on the extrusion processing ......................... 50

5.2 Effect of extrusion conditions on the microstructure and texture evolution. ................. 52

5.3 Recrystallization on the extruded alloys ........................................................................ 54

5.3.1 Effect of dynamic recrystallization on the texture .................................................. 56

5.4 Correlation between microstructure and mechanical properties .................................... 59

5.5 Plastic deformation of the extruded alloys ..................................................................... 62

5.5.1 Microstructure evolution during tension and compression ..................................... 63

8. References ............................................................................................................................ 89

Appendix .................................................................................................................................. 98

1

for diverse components in automotive and aerospace industries owing to its low density (1.74

g/cm), high specific strength and stiffness. Furthermore, the use of magnesium for

manufacturing diverse devices applicable to biomedical industry is also possible; this is due to

Differently from the magnesium castings alloys, which are more commonly used, the wrought

because they usually exhibit better mechanical properties such as higher strength and ductility

and rolling as well as non-conventional processes like equal channel angular extrusion (ECAE)

issues such as low strength and high anisotropy still limit the extensive use of the wrought Mg

microstructural evolution and the crystallographic texture development [4]. The

crystallographic texture promotes or hinders the activation of individual deformation modes

due to the changing of critical resolved shear stress (CRSS) or Schmid factor (SF). To alter the

grain size and tailor the crystallographic texture, the extrusion process is one of the most

commonly applied thermomechanical processes. Processing conditions such as extrusion ratio,

microstructure and therefore have a strong effect, first on the crystallographic texture and

development of the extruded Mg-Zn based alloys.

As a processed magnesium product, the microstructure and crystallographic texture developed

during the processing has a great impact on several features of the mechanical properties such

crystallographic texture and microstructure affect the plastic flow of the extruded products. It

8

9

The use of alloying elements may also affect the balance of slip systems through different ways,

2.3.2 Deformation by twinning

does not come from the strain produced by the twinning process but from the fact that

Figure 2.5. Schematic presentation of how twinning results from shear stress τ [46].

Contrasting to slip, due to the polar nature of twinning, the shear can take place only in one

direction. According to the amount of twinning shear as a function of the c/a ratio, the twinning

extension twin since it provides extension along the c-axis [57, 58]. Thus, the {10-11} twining

is known as compression or contraction twin.

10

Figure 2.7. Schematic representation of twinning, a) tension twinning �1012�<101 1 > and

11

The extrusion processing is usually subdivided according to the billet temperature, i.e., cold or

extrusion process.

container through a tool of the desired shape, the die. Relative movement takes place between

extrudability of the material and the resulting conditions of the produced bars.

cracking (right lines) determine the extrusion window as is shown in figure 2.9. In comparison

be extruded at lower speeds and within a narrower range of extrusion temperatures [62].

and higher cost than aluminum extrusion alloys.

Magnesium alloys such as AZ61, ZK60 and alloys with high amount of rare earth elements

have a low extrudability owing to these alloys are hard and the extrusion press machine cannot

12

press them at low extrusion temperature. Thus, they must be extruded at high temperature with

The alloy design has been applied to enhance the extrudability. The low alloying concentration

on the magnesium alloys is related to a softer deformation during extrusion and thus a lower

[12]. Then, a lower alloy content can result in better extrudability.

crystallographic texture. Of these two alloy characteristics, the investigation of the basal texture

13

It is well accepted that in the evolution of microstructure and crystallographic texture, the

as Ca and RE elements.

greater than 1 wt. % promoted the occurrence of hot-cracking. Yan and Chen [66] reported that

Ca additions promotes an increase in the solidus temperature, they found that the excellent

14

15

The recrystallization process inside the twins is governed by local recovery, which can be

and growth of new, strain-free grains, i.e., the static recrystallization (SRX) occurs [67]. During

[81, 83-87].

Due to of the effects of deformation and recrystallization during the processing, the magnesium

crystal are positioned relative to a fixed reference [88].

deformation twinning [89], the crystallographic texture of a polycrystalline material plays an

16

In the goniometer, the source and counter are arranged in a fixed geometry, depending only on

17

in terms of an inverse pole figure (IPF). In such inverse pole figure, the distribution of a selected

b) inverse pole figure (I.P.F) used in this study for the representation of textures in extruded

bars [91].

2.8 Mechanical properties on magnesium alloys

be complex, but in many cases, they lack the desired mechanical properties. It is well known

and texture. In general, the mechanical properties shown by the wrought magnesium alloys are

better compared to the as-cast alloys; this is mainly because of the small grain size, the