Aloi Mg-Al-Zn

Aluminium Terhadap Sifat

J Syarif

Kesan Kandungan

N Abdul Latif

Tegangan Bagi

Z Sajuri

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Jurnal Kejuruteraan 26(2014): 35-39
Effect of Aluminium Content on the Tensile Properties of Mg-Al-Zn Alloys
(Kesan Kandungan Aluminium Terhadap Sifat Tegangan Bagi Aloi Mg-Al-Zn)
N. Abdul Latif*, Z. Sajuri & J. Syarif
ABSTRACT
The aim of this study is to investigate the effect of aluminium content on microstructure, tensile properties and work 
hardenability of Mg-Al-Zn alloys. Two types of magnesium alloys were investigated i.e. AZ31 and AZ61, where the 
aluminium contents were 3% and 6%, respectively. Microstructure observation revealed that higher aluminium content 
decreases the grain size and increases the volume of Mg17Al12 precipitations. From the tensile test, AZ61 demonstrated 
higher yield stress and tensile strength while maintaining the elongation as compared to AZ31. The work hardening rate 
for AZ61 was also greater as compared to that of AZ31.
Keywords: Magnesium alloys; aluminium content; microstructure; tensile properties; work hardening
ABSTRAK
Tujuan kajian ini adalah untuk mengkaji kesan kandungan aluminium pada mikrostruktur, sifat tegangan dan kebolehkerasan 
kerja bagi aloi Mg-Al-Zn. Dua jenis aloi magnesium telah dikaji iaitu AZ31 dan AZ61, di mana kandungan berat aluminium 
masing-masing adalah 3% dan 6%. Pemerhatian mikrostruktur menunjukkan bahawa kandungan aluminium yang tinggi 
dapat mengurangkan saiz ira dan meningkatkan jumlah mendakan Mg17Al12. Daripada ujian tegangan, AZ61 menunjukkan 
tegasan alah dan kekuatan tegangan yang lebih tinggi serta mengekalkan pemanjangan apabila dibandingkan dengan 
AZ31. Kadar kerja pengerasan bagi AZ61 juga lebih tinggi berbanding AZ31.
Kata kunci: Aloi magnesium; kandungan aluminium; mikrostruktur; sifat tegangan; kerja pengerasan
INTRODUCTION
In recent years, magnesium alloys are widely used as 
structural materials in automotive industry due to the 
combination of low density, high strength properties and 
good machinability (Blawert et al. 2004; Gaines et al. 1995). 
Excellent mechanical properties of magnesium alloys have 
attracted engineers’ attention to choose these alloys for 
engineering structures but so far the application in major 
load bearing components in which reliability, durability 
and safety are of major concern is still very limited. One of 
the most popular magnesium alloys used in automotive and 
industry are the Mg-Al-Zn alloy series i.e. AZ31, AZ61 and 
AZ90. These alloys are now becoming the premier choices 
for light weight application due to their improved mechanical 
properties with addition of certain percentage of aluminium 
and zinc (Marya et al. 2006). 
Addition of aluminium as an alloying element in 
magnesium alloys is very important for high precipitation 
density of Mg17Al12 phase. This β-Mg17Al12 will act as 
strengthening phases that increase the tensile strength of 
magnesium alloys. Beside aluminium, the addition of zinc 
will also affect the strengthening of magnesium alloy (Cheng 
et al. 2009; Kainer 2003; Marya et al. 2006). However, the 
different composition of aluminium and zinc elements in 
Mg-Al-Zn alloy series could results in different mechanical 
properties especially the work hardenability and the tensile 
properties. In present study, effect of aluminium content on 
microstructure, tensile properties and work hardenability of 
constant Zn content in Mg-Al-Zn alloys were investigated.
EXPERIMENTAL PROCEDURE
The materials used in this study were extruded AZ31 and 
AZ61 of the Mg-Al-Zn alloy series. The two letters followed 
by two numbers designate the name of the magnesium alloys. 
The two letters tell the main alloying elements (A is for 
aluminium and Z is for zinc). The numbers tell the nominal 
compositions of main alloying elements respectively. Detail 
chemical compositions of  AZ31 and AZ61 being used in this 
study are listed in Table 1.
TABLE 1. Chemical composition of AZ31 and AZ61
 Al Zn Mn Fe Si Cu Ni Mg
AZ31 3.35 0.88 0.33 0.003 0.01 <0.002 <0.002 Bal.
AZ61 5.84 0.65 0.29 0.002 0.01 0.001 0.0003 Bal.
Both alloys were polished and etched to reveal their 
microstructure. They were then observed under the optical 
microscope. To obtain the mechanical properties of the 
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magnesium alloys, tensile tests were performed on dumb-bell 
shaped specimens with a gage length and gage diameter of 
10 mm and 3 mm, respectively. Tensile test was conducted 
using the Universal Testing Machine with a capacity of 100 
kN at strain rate of 1 × 10-3 s-1 in room temperature. Detail 
stress-strain responses were recorded using a data acquisition 
system. Further analyses of work hardening behavior for both 
alloys were performed and identifi ed by using the following 
equations (1) to (3).
 
T
F
A
s =  (1)
where, σ is the true stress, F is the force and AT the area of 
cross section at the force, F and the true strain ε is defi ned 
as:
 
dln 1
O
L
L
e
⎛ ⎞
= +⎜ ⎟
⎝ ⎠
 (2)
where dL is the elongation and LO the original gage length. 
The work hardening rate is given as:
 2 1
2 1
d
d
s ss
e e e
−
=
−
 (3)
where dσ is the increment of true stress and dε the increment 
of true strain.
RESULT AND DISCUSSION
MICROSTRUCTURE
Figure 1 shows the microstructures of AZ31 and AZ61 
observed under the optical microscope. From the fi gure, it 
shows that both alloys have an equiaxed grain structure. The 
average grain size of AZ31 and AZ61 were 24 μm and 15 μm 
as shown in Figure 2.
FIGURE 2. Average grain sizes of AZ31 and AZ61
FIGURE 1. Microstructure of (a) AZ31 and (b) AZ61
(a) (b)
Twinning
Twinning
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From Figure 1, it is clearly seen that the AZ31 has a larger 
grain size compared to that of the AZ61. Microstructure of 
AZ31 as shown in Figure 1(a) indicated more twinning, as 
indicated by the arrow, which probably due to the extrusion 
process while, less twinning for AZ61 as shown in Figure 1(b). 
Higher density of twinning was observed in AZ31 which had 
a larger grain size compared to that of the AZ61. Twinning 
formation is assumed to be resulted from the extrusion 
process as stated by Barnett (2007a) (Barnett 2007b). For 
AZ61 as in Figure 1(b), it is assumed that more precipitates in 
the alloy pinned the twinning formation so that less twinning 
was observed compared to that in AZ31.
The smaller grain size and higher density of Mg17Al12 
precipitation in AZ61 are believed to be caused by higher 
aluminium content that formed the solid solution in the alloy 
(Kainer 2003, St John et al. 2005). The Mg17Al12  precipitation 
observed in AZ61 is shown in Figure 3. Sajuri (2005) 
mentioned that during the extrusion process of magnesium 
alloys, the casting defects will almost disappear when as-cast 
alloy heated to 350-400°C and squeezed through an extrusion 
die. SEM observation showed that after the extrusion process, 
the β-Mg17Al12 phase in AZ61 alloy was almost completely 
dissolved and there was no casting defects presented in the 
matrix (Sajuri 2005). On the other hand, the existence of 
twinning in a material would also contribute to higher fl ow 
stress and better work hardening (Yablinsky et al. 2006). 
However, in most magnesium alloy series, fi ner grain size and 
Mg17Al12 precipitation have more infl uenced to improve the 
strength, work hardening and ductility despite of the smaller 
grain size would be suppression the twinning (Barnett 2007; 
Khan et al. 2006).
precipitation (Khan et al. 2006, Ryu et al. 2000). The tensile 
properties of AZ31 and AZ61 were summarized in Table 2. 
Yield stress and ultimate tensile strength of AZ61 were higher 
than that of AZ31 with 14.7% and 36.3%, respectively. More 
work hardening of AZ61 could be recognised by the large 
increment of differences between yield stress and ultimate 
tensile strength in nominal stress-strain curve. On the 
contrary, AZ31 clearly possessed less work hardening compare 
to AZ61 (Latif et al. 2014). However, the elongation for both 
AZ31 and AZ61 were found identical. 
FIGURE 3. The Mg17Al12 precipitation of AZ61
FIGURE 4. Nominal stress-strain curves of AZ31 and AZ61
FIGURE 5. Work hardening rate of AZ31 and AZ61
TENSILE TEST AND WORK HARDENING
Figure 4 shows nominal stress-strain curves of AZ31 and 
AZ61 magnesium alloys. The result shows that AZ61 exhibited 
higher tensile strength and better work hardening as compared 
to that of the AZ31, which caused by the grain size reduction 
and Mg17Al12 precipitation. Khan et al. (2006) and Ryu 
et al. (2000) have reported that the higher tensile strength was 
due to the effects of smaller grain size and higher density of 
Work hardening rate for AZ31 and AZ61 were clearly 
shows in Figure 5. It is noted that the work hardening rate 
for AZ61 was greater compared to that of AZ31 due to the 
existence of precipitation and fi ner grain size of alloy. These 
TABLE 2. Tensile properties of AZ31 and AZ61
 σy (MPa) σUTS (MPa) ε (%)
AZ31  191 240 16.2
AZ61  219 327 16.6
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CONCLUSION
Aluminium content in Mg-Al-Zn alloy series infl uenced 
the microstructure, grain size and precipitation of Mg17Al12 
phase. The grain size refi nement and precipitation increases 
tensile strength and allow for more work hardening of the 
alloy due to more dislocation piles up and blocked at the 
grain boundaries before further deformation of the material. 
AZ61 exhibited better tensile properties than that of AZ31 with 
the yield stress and ultimate tensile strength of 14.7% and 
36.3%, respectively. However, both AZ31 and AZ61 fractured 
in ductile manner with more dimples and microvoids in 
fracture surfaces.
ACKNOWLEDGEMENT
The authors acknowledge the supports from the Universiti 
Kebangsaan Malaysia, Ministry of Higher Education 
Malaysia (ERGS/1/2011/TK/UKM/02/9), Nagaoka University 
of Technology, Universiti Tun Hussein Onn Malaysia and 
Fuel cell Institute, Universiti Kebangsaan Malaysia. 
REFERENCES
Barnett, M. R. 2007b. Twinning and the ductility of 
magnesium alloys Part II. “Contraction” twins. Materials 
Science and Engineering A 464: 8-16.
Barnett, M. R. 2007a. Twinning and the ductility of 
magnesium alloys. Part I: “Tension” twins. Materials 
Science and Engineering A 464 1-7.
Blawert, C., Hort, N. & Kainer, K. U. 2004. Automotive 
applications of magnesium and its alloys. Trans. Indian 
Inst. Met. 57: 397-408.
Cheng, Y.-L., Qin, T.-W., Wang, H.-M. & Zhang, Z. 2009. 
Comparison of corrosion behaviors of AZ31, AZ91, 
AM60 and ZK60 magnesium alloys. Transactions of 
Nonferrous Metals Society of China 19: 517-524.
Gaines, L., Cuenca, R., Stodolsky, F. & Wu., S. 1995. 
Potential applications of wrought magnesium alloys for 
passenger vehicles. ATD-CCM 
Kainer, K. U. 2003. The current state of technology and 
potential for further development of magnesium 
applications. In Magnesium-Alloys and Technologies, 
edited by K. U. Kainer. Germany: Wiley-vch, 
Germany.
Khan, S. A., Miyashita, Y., Mutoh, Y. & Sajuri, Z. B. 2006. 
Infl uence of Mn content on mechanical properties and 
fatigue behavior of extruded Mg alloys. Materials 
Science and Engineering: A 420: 315-321.
Latif, N. A., Sajuri, Z., Syarif, J. & Mutoh, Y. 2014. Effect 
of strain rates on tensile strength and work hardening 
for Al-Zn magnesium alloys. In Recent Trends in 
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Lumban and W. Jeffrey, 77-91. Springer.
factors have infl uenced to the dislocation piles up against 
the microstructure for delayed fracture. Work hardening rate 
for AZ61 was considered after yielding up to the ultimate 
tensile strength, while the hardening process for AZ31 was 
diminished after yielding. The diffi culty of deformation 
would be induced by the presence of dislocation density to 
allow more hardening of the alloy (Latif et al. 2014).
FRACTURE SURFACE
Figure 6 shows the fracture surface of AZ31 and AZ61. 
Both alloys clearly show ductile fracture behaviour where 
many dimples observed on the fracture surface. Similar 
observation was reported by Marya et al. (2006). However, 
fracture surface of AZ61 as shows in Figure 6(b) indicated 
more ductile fracture behaviour where more dimples and 
micro voids present in the fracture surface than that of AZ31 
at Figure 6(a). 
FIGURE 6. Fracture surface of (a) AZ31 and (b) AZ61
(a) (b)
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39
Marya, M., Hector, L. G., Verma, R. & Tong, W. 2006. 
Microstructural effects of AZ31 magnesium alloy on 
its tensile deformation and failure behaviors. Materials 
Science and Engineering A 418: 341-356.
Ryu, H. J., Baik, H. K. & Hong, S. H. 2000. Effect of 
thermomechanical treatments on microstructure and 
properties of Cu-base leadframe alloy. Journal of 
Materials Science 35: 3641-3646.
Sajuri, Z. 2005. Study on Fatigue Behavior of Magnesium 
Alloys. Nagaoka: Nagaoka University of Technology, 
Japan, Nagaoka.
St. John, D. H., Qian, M., Easton, M. A., Cao, P. & 
Hildebrand, Z. 2005. Grain refi nement of magnesium 
alloys. Metallurgical and Materials Transactions A 
36A: 1669-1679.
Yablinsky, C. A., Cerreta, E. K., Gray, G. T., III, Brown, D. 
W. & Vogel., S. C. 2006. The effect of twinning on the 
work-hardening behavior and microstructural evolution 
of hafnium. Metallurgical and materials transactions A 
37: 1907-1915.
N. Abdul Latif*, Z. Sajuri & J. Syarif
Department of Mechanical and Materials Engineering
Faculty of Engineering and Built Environment
Universiti Kebangsaan Malaysia
43600 UKM Bangi, Selangor D.E.
Malaysia
*Corresponding author; email: noradila.abdlatif@gmail.
com
Received date: 21st April 2014
Accepted date: 22nd August 2014
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Effect of Aluminium Content on the Tensile Properties of Mg-Al-Zn Alloys

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Effect of Aluminium Content on the Tensile Properties of Mg-Al-Zn Alloys

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