Effect of Solution Treatment on Fatigue Crack Propagation Behaviour of Magnesium Alloy.
Effect of Solution Treatment on Fatigue Crack
Propagation Behaviour of Magnesium Alloy
M. A. M. Daud1, Z. Sajuri2, J. Syarif3, M. Z. Omar4
Abstract - An investigation on the effect of solution treatment on fatigue crack propagation
(FCP) behaviour of AZ61 magnesium alloy was carried out. A centre cracked plate tension (CCT)
specimen was prepared from an extruded cylindrical AZ61 magnesium alloy rod. The solution
treatment was performed at 400oC for one hour to get homogeneous solid solution before quench
in water. The FCP test was conducted in a laboratory air environment under a constant amplitude
sinusoidal loading with a stress ratio of 0.1 and a frequency of 10 Hz. The FCP curve for solution
treated samples was then compared to that of the extruded AZ61 magnesium alloy. Results showed
that solution treatment shifted the FCP curve to the left and demonstrated a lower fatigue crack
propagation resistance at the high stress intensity region. The threshold value was recorded at
0.91 MPa√m.
Keywords: Fatigue crack propagation, solution treatment, threshold value, magnesium alloy
σN
σys
σuts
Pmax
B
W
a
Hv
R
o
C
K
ΔK
ΔKth
F(α)
da/dN
C
m
nominal stress
yield stress
ultimate tensile strength
maximum load
specimen thickness
width of gauge position
crack length
Vickers hardness
stress ratio
degree Celsius
stress intensity factor
stress intensity factor range
threshold stress intensity factor range
geometrical factor
fatigue crack propagation rate
constant
slope of the curve
I. Introduction
Magnesium alloys are being widely utilized
especially, in transportation and aerospace industry due
to their lightweight and with high specific strength [1][3]. For many years, magnesium alloys have been
attractive to engineer due to their low density (1.74
g/cm3) compared to counterparts such as aluminium and
titanium. Nowadays, magnesium alloys are mostly used
for static parts such as cases, housings, brackets, panels,
etc., but these materials also indicate that they have a
potential to be used as load-bearing components, e.g.
wheels, which will be subjected to fatigue loading [4],
[5]. In order to use magnesium alloys as a high strength
structural component, especially in automotive,
aerospace and other transportations industries, it is very
important to make their fatigue characteristics clear and
understand the fatigue crack propagation (FCP)
mechanism. Wrought magnesium alloy such as the
extruded AZ61 have good mechanical properties and
widely utilized especially for chassis in car and other
strategic applications. Until now, there has been
growing interest in studies on fatigue behaviour of
magnesium alloy. These include crack growth behaviour
in die cast AZ91D [6], low cycle fatigue behaviour of
die cast AZ91E-T6 [7], fatigue crack growth of rolled
plate AZ31 [8], fatigue of calibre rolled AZ91D [9],
fatigue of extruded AZ91D [10], fatigue behaviour of
die cast AZ91, AM60B and AZ91E-T4 in very high
cycle regime [11]-[13]. Hilpert and Wagner examined
the fatigue performance of extruded AZ80 in ambient
air and NaCl [14]. It was found that there was no
pronounced effect of NaCl solution on fatigue life at
higher stress but fatigue life was considerably reduced
at stress below 125 MPa. Shih et al. studied the fatigue
life of AZ61A and reported that cracks initiate from
subsurface or surface inclusions [15]. There is no clear
information or finding as to whether heat treatment
(aging) accelerates the FCP rate of magnesium alloy or
does not. Bag and Zhou pointed out that the aging
treatment reduces the FCG rate of as-cast AZ91 alloys
mostly, due to the larger deviation and branching of the
crack from the plane of maximum stress caused by the
inhomogeneous microstructure [16]. However, in
contrast, Kobayashi et al. proposed that the precipitates
have a negative effect on the FCP rate of AZ91 [17].
This study aims to investigate the influence of
solution treatment on the FCP rate of extruded AZ61
magnesium alloy, and the emphasis is on understanding
the influence of solution treatment on the FCP rate of
AZ61.
AZ80 magnesium alloys have been performed by
Uematsu et al. [18].
II. Experimental Procedure
To investigate the effect of heat treatment on fatigue
crack propagation rate of the AZ61 magnesium alloy;
all samples were firstly heated in furnace to 400oC and
held for one hour before quenching them in water.
Time and temperature range used in this study were
based on the ASTM Standard [19]. Figure 2 shows the
microstructure of the material used. The average grain
size was about 20 μm.
The material used in this study was AZ61
magnesium alloy. The chemical composition of the
material used is shown in Table 1. AZ61 magnesium
alloy has about 6% of aluminium and 1% of zinc as its
major alloying elements.
Prior to the investigation on effect of solution
treatment on fatigue crack propagation behaviour of
AZ61 magnesium alloy, series of heat treatment
processes were performed to identify the effect of each
process on the mechanical property, i.e. hardness of the
alloy after treatment.
Solution treatment was performed at 400oC for 60
minutes followed by quenching in water. To investigate
the effect of aging temperature, samples were heated to
100, 150, 200, 250oC for 30 minutes before quenched
in water. The hardness of these samples were then
measured and compared to the hardness of solution
treated sample. It was found that the aging treated
samples showed significantly lower hardness compared
to that of solution treated sample, which exhibited
hardness value of Hv 71 as shown in Fig.1 (a). The
increase in aging temperature improved the hardness
but it was then saturated at the temperature above
200oC at Hv 60. Based on these results, further detail
aging treatment was performed to investigate the effect
of aging time on the hardness of AZ61 magnesium
alloy. Aging treatment was applied on solution heattreated samples at different time intervals of 30, 60, 90,
120 and 150 minutes at aging temperature of 200 oC.
Figure 1(b) shows the result of effect of aging time on
hardness. From the result, it shows that AZ61
magnesium reached the peak aging after only 60
minute of aging time. The hardness at the peak aged
was Hv 61.
From the above initial test results, solution treated
sample demonstrated the best mechanical property in
hardness compared to that of aged samples. Based on
these results, it is worth to investigate the effect of
solution heat treatment on fatigue behaviour of AZ61
magnesium alloy before proceeding detail investigation
on the effect of aging treatment of the investigated
(a) Effect of aging temperature at aging time of 30 minutes
(b) Effect of aging time at 200oC
Fig. 1. Vickers hardness as a function of aging treatment
TABLE 1
THE CHEMICAL COMPOSITIONS OF AZ61
MAGNESIUM ALLOY (WT.%)
Al
Zn
Fe
Si
Mn
Mg
6.53
0.96
0.002
0.024
0.164
Bal.
alloy. Moreover, similar study on the effect of aging
treatment on fatigue behaviour in extruded AZ61 and
(a) As-extruded
(b) Solution treated
Fig. 2. The optical micrograph of (a) as-extruded and (b) solution
treated samples
(a)
(2)
Here, F(α) is a boundary correction factor which
depends on the ratio of the crack length a to the width
of the specimen W. For the CCT test specimen used in
this study, the boundary correction factor is given as
[19],
N BW (1Pmax
ys
2a / W )
F ( ) 1 0.025 2 0.06 4
(1)
K
a F
sec
2
(3)
2a
W
(4)
Here, σN is the nominal stress, σys is the yield stress,
Pmax is the maximum load, B is the specimen thickness,
W is the width of gauge position and a is the crack
length. A screw type fixture was used in the CCT
specimen. To avoid the excessive lateral deflection or
buckling of the CCT specimen during the test, the
gauge length and thickness of gage position was
limited to 12 mm and 2 mm, respectively. The gage
position was then polished with 500 to 1500 grit emery
papers to obtain a smooth surface.
where
The fatigue crack propagation rate test was
conducted by using a pneumatic fatigue testing
machine (14 kN maximum capacity) and to investigate
the effect of heat treatment on fatigue crack
propagation behaviour. The tests were performed at
frequency of 10 Hz by using sinusoidal loading form.
A stress ratio R = 0.1 was applied in the tests. The
loading direction was in the extrusion direction of the
material and the testing was carried out at room
temperature.
The procedure for introducing a pre-crack was
followed the ASTM standard [20]. The specimen was
aligned so that the load distribution is symmetrical. The
load ratio R during pre-cracking is the same as the load
ratio used in the fatigue crack propagation test. The
pre-cracking was interrupted after a pre-crack length
equal to 0.1 of specimen thickness was attained at
maintained pre-cracking propagation rates of about 10-8
m/cycle.
The crack propagation curve (crack propagation
rate da/dN versus stress intensity factor range ΔK) was
obtained by using K-decreasing and K-increasing test
procedures. The decreasing and increasing load steps
are 5 - 7% of the previous loading value. The stress
intensity factor value for the CCT specimen was
calculated using the following equation:
The crack length was measured using travelling
microscope. The threshold stress intensity factor ΔKth
was determined when a crack growth is not observed
for 106 cycles. A hole with a 1 mm diameter was drilled
in the centre of the specimen before introducing a 1.35
mm notch by EDM (electrical-discharge machining) to
facilitate fatigue pre-cracking.
III. Result and Discussion
The comparison of fatigue strength of solution
treated and extruded AZ61 magnesium alloy is shown
in Figure 4. The figure shows that fatigue strength of
the solution treated samples increase as that compared
to the fatigue strength of the as-extruded AZ61
samples. The fatigue limit for solution treated and asextruded AZ61 were 180 MPa and 150 MPa,
respectively. The higher fatigue strength observed for
300
Maximum stress (MPa)
(a)
The specimen used for fatigue crack propagation
rate test was centre cracked-plate tension (CCT)
specimen. Figure 3 shows the geometry of the
specimen according to ASTM E647-08 standard [20].
The dimension of the specimen was determined by
following equation according to the test standard:
250
Solution treated
As-extruded
200
150
100 3
10
104
105
106
107
108
Number of cycles to failure, Nf (cycles)
Fig. 3. Centre cracked-plate tension (CCT) specimen used in
the FCP tests
Fig. 4. Fatigue strengths of solution treated and as-extruded samples
TABLE 2
MECHANICAL PROPERTIES OF AZ61 MAGNESIUM ALLOY
Yield
Stress,
σy (MPa)
Material
type
Asextruded
Ave.
Solution
treated
Vickers
Hardness
(Hv)
244
265
270
309
329
308
67
(Ave. of
10 points)
268
315
308
292
288
381
324
322
296
342
As-received
(Low hardness matrix)
Ave.
Ultimate
Tensile
Strength,
σuts (MPa)
71
(Ave. of
10 points)
Surface
Fatigue crack initiation
the solution treated sample is believed due to higher
tensile strength and also higher hardness properties
compared to that of the as-extruded sample as shown in
Table 2.
After the solution treatment the increment in
hardness from Hv 67 to Hv 71 is believed due to the
solid solution strengthening. In the heat treatment
process, the solution treated samples were heated into
the solid solution zone where atoms of alloying
elements dissolved into the matrix. In this condition,
the samples were quenched in water, which limit the
time for precipitation to takes place.
Optical micrographs revealed that there is no
precipitation of second phase observed in the solution
treated sample. Further, the aging processes performed
Fatigue crack
propagation
Cyclic load
Matrix
Matrix
Inclusion
Crack
P.S.B
Δai
Δai+1
Δai< Δai+1
Cyclic load
Surface
Solution treated
(High hardness matrix)
Crack
Matrix
Inclusion
Crack
Crack
High stress
Concentration site
Matrix
Δai
Δai+1
Δai< Δai+1
Fig. 5. Mechanisms of crack initiation and propagation for as-extruded and solution treated AZ61 magnesium alloy
at different aging times and temperatures were unable
to achieve higher hardness compared to that of solution
treated sample due to limitation of second phase
precipitation. This result was in aligned with the results
obtained by Uematsu et al. who reported that
precipitation of Mg17Al12 in AZ61 magnesium alloy is
very limited due to low percentage of Al content as
compared to other magnesium alloy with higher Al
content such as AZ80 [22].
figure, the threshold value for solution treated AZ61
magnesium alloy is at 0.91 MPa√m. This value is
almost at par to that of the extruded magnesium alloy
where the threshold value is at 0.92 MPa√m. From the
result, it can be concluded that heat treatment does not
affect the threshold value of AZ61 magnesium alloy.
However, a slight difference in the fatigue crack
propagation resistance is been demonstrated at higher
ΔK region. Similar FCP behaviour of the AZ91D
magnesium alloy was also reported by Kobayashi et al.
[17].
The increased in hardness of solution treated
sample resulted in difficulty for the extrusion and
intrusion of persistence slip bands to occur and
consequently the fatigue crack tends to initiate at
higher stress concentration sites such as inclusions or
foreign particles at sub-surface. For the as-extruded
samples, the extrusion and intrusion play a prominent
role in initiating fatigue crack. Therefore, the increased
in hardness and yield strength in solution treated
samples delay the fatigue crack initiation and result in a
higher fatigue life. The initiation and propagation
mechanisms of fatigue crack in both samples are
illustrated in Fig. 5.
The FCP curve of solution treated AZ61
magnesium alloy as a function of stress intensity factor
range (ΔK) at room temperature is shown in Fig. 7. The
FCP curve for as-extruded AZ61 magnesium alloy is
also plotted in the same figure for comparison [8].
From the figure, it can be noted that there is not much
difference in the FCP resistance for both curves at a
low ΔK region. However the difference of fatigue crack
propagation resistance can be seen at a higher ΔK
region above 2.0 MPa√m. Fatigue crack propagation
resistance for solution treated samples is found lower
as compared to the as-extruded samples. It can be
considered that the crack in as-extruded AZ61 has
more frequent chances of encounter with grain
boundaries due to the smaller grain size, resulting in a
slower propagation rate. This argument is similar to the
FCP behaviour of AZ31B-L as mentioned by Uematsu
et al. [22].
The arrows in the figure indicate that the threshold
value of stress intensity factor range at ΔKth. From the
(5)
da
C K m
dN
where C is a constant and m is the slope of the curve on
the log-log plot. Values of constants C and m were
calculated using the least square method and the results
are shown in Table 3.
b
a
Fig. 6. Observations were done using SEM equipped EDX. (a)
Foreign particle (b) Flat surface
10-6
Fatigue crack propagation rate, da/dN (m/cycle)
Detailed fracture surface observations showed that
foreign particle was found at the fatigue fracture origin
of the solution treated samples especially for the
samples which exhibited higher fatigue life more than
105 cycles as shown in Fig. 6(a). The foreign particle
size observed was about 20 to 30µm. Pile-up of slips
deformation at near the foreign particle during fatigue
cycles contributed to high stress concentration at
around the foreign particle and resulted in fatigue crack
initiation. In contrast, SEM observation results on
fracture surface of as-extruded samples showed that
there was no evidence of foreign particle at the fatigue
fracture origin. The fatigue crack initiation site was
relatively flat as shown in Fig. 6(b).
The da/dN-ΔK curves obtained at the Paris regime
can be expressed as:
Solution treated
As-extruded
(Sajuri et al. [21])
10-7
10-8
10-9
10-10
10-11
10-12
0.1
0.5
1
5
10
Stress intensity factor range, Δ K (MPa.m 1/2)
Fig. 7. Fatigue crack propagation behavior of solution treated and asextruded AZ61 magnesium alloy
TABLE 3
CRACK PROPAGATION PARAMETER AT THE PARIS
REGIME FOR AZ61 MAGNESIUM ALLOY
ΔKth
(MPa)
m
C
(m/cycle)
As-extruded
0.92
2.1
2.6x10-9
Solution treated
0.91
2.4
3.7x10-7
Material type
slip fracture. The directions of the slips patterns are
almost the same as the crack propagation directions.
Some faceted surface of cleavage fracture was appears
together with the featureless slip fracture as shown in
Fig. 8 (a). The striations-like slip markings can be seen
from the fracture surface as shown in Fig. 8 (b).
V. Conclusion
Experimental investigation on the effect of solution
treatment to the fatigue crack propagation behaviour of
AZ61 magnesium alloy was carried out. Based on the
results the following conclusions are made:
1. Solution treated sample demonstrated superior
mechanical properties and higher fatigue limit as
compared to as-extruded AZ61 magnesium alloy.
(a)
Overview
2. Foreign particle in sub-surface served as the fatigue
fracture origin for solution treated samples
especially at high fatigue life region.
3. The solution treatment shifted the FCP curve at high
ΔK region to the left and demonstrated lower
fatigue crack propagation resistance as compared to
as-extruded sample which has more frequent
chances of encounter with grain boundaries due to
the smaller grain size, resulting in a slower
propagation rate.
(b)
Micrograph of fracture surface at ‘a’
in (a) at low ΔK region
4. The threshold stress intensity factor range ΔKth for
solution treated and as-extruded are almost identical
at the value of 0.91 MPa√m.
Acknowledgement
The author would like to thank Prof. Dr. Y. Mutoh of
Nagaoka University of Technology, Japan for providing
the extruded AZ61 magnesium alloy and the Ministry
of Science, Technology and Innovation (MOSTI),
Malaysia for sponsoring this project under the Science
Fund Research Grant (03-01-02-SF0048).
(c)
Micrograph of fracture surface at ‘b’
in (a) at high ΔK region
Fig. 8. Fracture surface observations of FCP test for solution treated
specimen.
IV. Observation of Fracture Surface
Figure 8 shows the fracture surface of fatigue crack
propagation test specimens tested at R = 0.1 for
solution treated AZ61 magnesium alloy. Figures 8(a),
(b) and (c) show the macroscopic and microscopic
fractograph at the low ΔK and at high ΔK regions,
respectively. The fracture surface appearance of the
entire fracture surface was predominantly a featureless
References
[1]
[2]
[3]
[4]
[5]
B.L. Mordika and T. Ebert, Magnesium. Propertiesapplications-potential, Material Science and Engineering.
A302, pp. 37-45, 2001.
L. Duffy, Magnesium Alloy: The light choice for aerospace,
Materials World, pp. 127-130, 1996.
J. F. King, G. A. Fowler and P. Lyon, Corrosion resistant
magnesium alloy for aerospace casting, Light Weight Alloys for
Aerospace application II, The Mineral, Metals & Materials
Society, pp. 423-438, 1991.
A. A. Luo, Recent magnesium alloy development for
automotive powertrain application, Mat. Science Forum, pp.
419-422, 2003.
S. Schumann and H. Friedrich, Current and Future Use of
Magnesium in the Automobile Industry, Material Science
Forum. Vol. 419-422, pp. 51-55, 2003.
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
[21]
[22]
T. Shibusawa, Y. Kobayashi and K. Ishikawa, Fatigue crack
propagation in Die cast AZ91D magnesium alloy, J. Japan Inst.
Metal, 61(4), pp. 423-438, 1997.
D. L. Goodenberger and J.R.I. Stephens, Fatigue of AZ91E-T6
cast magnesium alloy, Engineering Mat. Technology. 115, pp.
391-397, 1993.
K. Tokaji, M. Kamakura, Y. Ishizumi, and N. Hasegawa,
Fatigue behaviour and fracture mechanism of a rolled AZ31
magnesium alloy, Int. J. Fatigue, 26, pp1217-1224, 2004.
T. Fujii, N. Fuyama, and C. Masuda, Fatigue fracture
mechanisms for calibre rolled AZ91D magnesium alloys,
Materials Science Forum, 419-422, pp109-114, 2003.
Z. Sajuri, Y. Miyashita and Y. Mutoh, Effect of stress ratio on
fatigue crack groeth behaviour of magnesium alloy, Materials
Science Forum, 419-422, pp81-89, 2003.
H.R. Mayer, H. J. Lipowsky, M. Rosch, R. Stich, and S.
Stanzl-Tschegg, Application of ultrasound for fatigue testing of
light weight alloy, Fatigue Frac Engineering Mat. Struct., 22,
pp591-599, 2004.
K. Gall, G. Biallas, H.J. Maier, P. Gullett, M.F. Horstemeyer,
D.L. McDowell and J. Fan, In-situ observations of high cycle
fatigue mechanisms in cast AM60B magnesium in vacuum and
water vapor environments, Int. J. Fatigue, 26, pp59-70, 2004.
M. F. Horstemeyer, N. Yang, K. Gall, D. L. McDowell, J. Fan,
P. M. Gullett. High cycle fatigue of a die cast AZ91E-T4
magnesium alloy, Acta Mater, 52, pp. 1327-1336, 2004.
M. Hilpert and L. Wagner, Corrosion behaviour of highstrength magnesium alloy AZ80, J. Mat. Engineering and
Performance, 9(4), pp402-407, 2000.
T. S. Shih, W. S. Liu, Y. J. Chen. Fatigue of as-extruded AZ61A
magnesium alloy, Mater. Sci. Eng.,325, pp. 152-162, 2002.
A. Bag and W. Zhou. Tensile and fatigue behaviour of AZ91D
magnesium alloy, Journal of Materials Science Letters, 20,
pp. 457-459, 2001.
Y. Kobayashi, T. Shibusawa, K. Ishikawa. Environmental effect
of fatigue crack propagation of magnesium alloy, Mater. Sci.
Eng. A, 234-236, pp. 220-222, 1997.
Y.Uematsu, K. Tokaji, M. Matsumoto. Effects of aging
treatment on fatigue behaviour in extruded AZ61 and AZ80
magnesium alloy, Mater. Sci. and Eng. A, 517, pp. 138-145,
2009.
Standard practice for heat treatment of magnesium alloy
[B661]. Annual book of ASTM Standards, Vol. 02.02. 1999,
Philadelphia.
ASTM Standard E647-08, Annual Book of ASTM Standards,
Vol. 03.01, 2008, 685-686.
Z. Sajuri, T. Umehara, Y. Miyashita, Y. Mutoh. Fatigue-Life
Prediction of Magnesium Alloys for Structural Applications,
Advanced Engineering Materials, Vol. 5(12), pp. 910-916,
2003.
Y. Uematsu, K. Tokaji, M. Kamakura, K. Uchida, H. Shibata, T.
Bekku, Effects of extrusion conditions on grain refinement and
fatigue behavior in magnesium alloy, Mater. Sci. and Eng. A,
434, pp. 131-140, 2006.
Authors’ information
1,2,3,4
Dept. of Mechanical and Materials Engineering, Faculty of
Engineering & Built Environment, Universiti Kebangsaan Malaysia,
43600 UKM Bangi, Selangor, Malaysia.
1
Mohd Ahadlin Mohd Daud is a PhD student in Department of
Mechanical & Materials Engineering,
Universiti Kebangsaan Malaysia. He
received his Master Degree (Mechanical
Eng.) from Universiti Putra Malaysia in
2002. His research interest includes
fatigue and fracture mechanics, and
manufacturing process.
2
Dr Zainuddin
Sajuri is a senior
lecturer at the
Department
of
Mechanical &
Materials Engineering,
Universiti
Kebangsaan Malaysia.
He received his
PhD (Material Science)
from Nagaoka
University, Japan in
2005. He has
taught several courses
throughout the
undergraduate
and
postgraduate
curriculum.
His
research
interest includes fatigue
and
fracture
mechanics, advanced materials and powder metallurgy
3
Dr Syarif Junaidi is a senior lecturer at the Department of
Mechanical &
Materials Engineering,
Universiti Kebangsaan Malaysia. He
received his PhD (Material Science) from
Kyushu University, Japan in 2003. He has
taught several courses throughout the
undergraduate
and
postgraduate
curriculum. His research interest includes
materials processing, advanced materials
and powder metallurgy.
4
Dr Zaidi Omar is an Associate Professor
at the Department of Mechanical &
Materials
Engineering,
Universiti
Kebangsaan Malaysia. He received his
PhD (Mechanical Eng.) from Sheffield
University, UK in 2005. He has taught
several
courses
throughout
the
undergraduate
and
postgraduate
curriculum. His research interest includes
semisolid processing, advanced materials
and metal matrix composite.
Propagation Behaviour of Magnesium Alloy
M. A. M. Daud1, Z. Sajuri2, J. Syarif3, M. Z. Omar4
Abstract - An investigation on the effect of solution treatment on fatigue crack propagation
(FCP) behaviour of AZ61 magnesium alloy was carried out. A centre cracked plate tension (CCT)
specimen was prepared from an extruded cylindrical AZ61 magnesium alloy rod. The solution
treatment was performed at 400oC for one hour to get homogeneous solid solution before quench
in water. The FCP test was conducted in a laboratory air environment under a constant amplitude
sinusoidal loading with a stress ratio of 0.1 and a frequency of 10 Hz. The FCP curve for solution
treated samples was then compared to that of the extruded AZ61 magnesium alloy. Results showed
that solution treatment shifted the FCP curve to the left and demonstrated a lower fatigue crack
propagation resistance at the high stress intensity region. The threshold value was recorded at
0.91 MPa√m.
Keywords: Fatigue crack propagation, solution treatment, threshold value, magnesium alloy
σN
σys
σuts
Pmax
B
W
a
Hv
R
o
C
K
ΔK
ΔKth
F(α)
da/dN
C
m
nominal stress
yield stress
ultimate tensile strength
maximum load
specimen thickness
width of gauge position
crack length
Vickers hardness
stress ratio
degree Celsius
stress intensity factor
stress intensity factor range
threshold stress intensity factor range
geometrical factor
fatigue crack propagation rate
constant
slope of the curve
I. Introduction
Magnesium alloys are being widely utilized
especially, in transportation and aerospace industry due
to their lightweight and with high specific strength [1][3]. For many years, magnesium alloys have been
attractive to engineer due to their low density (1.74
g/cm3) compared to counterparts such as aluminium and
titanium. Nowadays, magnesium alloys are mostly used
for static parts such as cases, housings, brackets, panels,
etc., but these materials also indicate that they have a
potential to be used as load-bearing components, e.g.
wheels, which will be subjected to fatigue loading [4],
[5]. In order to use magnesium alloys as a high strength
structural component, especially in automotive,
aerospace and other transportations industries, it is very
important to make their fatigue characteristics clear and
understand the fatigue crack propagation (FCP)
mechanism. Wrought magnesium alloy such as the
extruded AZ61 have good mechanical properties and
widely utilized especially for chassis in car and other
strategic applications. Until now, there has been
growing interest in studies on fatigue behaviour of
magnesium alloy. These include crack growth behaviour
in die cast AZ91D [6], low cycle fatigue behaviour of
die cast AZ91E-T6 [7], fatigue crack growth of rolled
plate AZ31 [8], fatigue of calibre rolled AZ91D [9],
fatigue of extruded AZ91D [10], fatigue behaviour of
die cast AZ91, AM60B and AZ91E-T4 in very high
cycle regime [11]-[13]. Hilpert and Wagner examined
the fatigue performance of extruded AZ80 in ambient
air and NaCl [14]. It was found that there was no
pronounced effect of NaCl solution on fatigue life at
higher stress but fatigue life was considerably reduced
at stress below 125 MPa. Shih et al. studied the fatigue
life of AZ61A and reported that cracks initiate from
subsurface or surface inclusions [15]. There is no clear
information or finding as to whether heat treatment
(aging) accelerates the FCP rate of magnesium alloy or
does not. Bag and Zhou pointed out that the aging
treatment reduces the FCG rate of as-cast AZ91 alloys
mostly, due to the larger deviation and branching of the
crack from the plane of maximum stress caused by the
inhomogeneous microstructure [16]. However, in
contrast, Kobayashi et al. proposed that the precipitates
have a negative effect on the FCP rate of AZ91 [17].
This study aims to investigate the influence of
solution treatment on the FCP rate of extruded AZ61
magnesium alloy, and the emphasis is on understanding
the influence of solution treatment on the FCP rate of
AZ61.
AZ80 magnesium alloys have been performed by
Uematsu et al. [18].
II. Experimental Procedure
To investigate the effect of heat treatment on fatigue
crack propagation rate of the AZ61 magnesium alloy;
all samples were firstly heated in furnace to 400oC and
held for one hour before quenching them in water.
Time and temperature range used in this study were
based on the ASTM Standard [19]. Figure 2 shows the
microstructure of the material used. The average grain
size was about 20 μm.
The material used in this study was AZ61
magnesium alloy. The chemical composition of the
material used is shown in Table 1. AZ61 magnesium
alloy has about 6% of aluminium and 1% of zinc as its
major alloying elements.
Prior to the investigation on effect of solution
treatment on fatigue crack propagation behaviour of
AZ61 magnesium alloy, series of heat treatment
processes were performed to identify the effect of each
process on the mechanical property, i.e. hardness of the
alloy after treatment.
Solution treatment was performed at 400oC for 60
minutes followed by quenching in water. To investigate
the effect of aging temperature, samples were heated to
100, 150, 200, 250oC for 30 minutes before quenched
in water. The hardness of these samples were then
measured and compared to the hardness of solution
treated sample. It was found that the aging treated
samples showed significantly lower hardness compared
to that of solution treated sample, which exhibited
hardness value of Hv 71 as shown in Fig.1 (a). The
increase in aging temperature improved the hardness
but it was then saturated at the temperature above
200oC at Hv 60. Based on these results, further detail
aging treatment was performed to investigate the effect
of aging time on the hardness of AZ61 magnesium
alloy. Aging treatment was applied on solution heattreated samples at different time intervals of 30, 60, 90,
120 and 150 minutes at aging temperature of 200 oC.
Figure 1(b) shows the result of effect of aging time on
hardness. From the result, it shows that AZ61
magnesium reached the peak aging after only 60
minute of aging time. The hardness at the peak aged
was Hv 61.
From the above initial test results, solution treated
sample demonstrated the best mechanical property in
hardness compared to that of aged samples. Based on
these results, it is worth to investigate the effect of
solution heat treatment on fatigue behaviour of AZ61
magnesium alloy before proceeding detail investigation
on the effect of aging treatment of the investigated
(a) Effect of aging temperature at aging time of 30 minutes
(b) Effect of aging time at 200oC
Fig. 1. Vickers hardness as a function of aging treatment
TABLE 1
THE CHEMICAL COMPOSITIONS OF AZ61
MAGNESIUM ALLOY (WT.%)
Al
Zn
Fe
Si
Mn
Mg
6.53
0.96
0.002
0.024
0.164
Bal.
alloy. Moreover, similar study on the effect of aging
treatment on fatigue behaviour in extruded AZ61 and
(a) As-extruded
(b) Solution treated
Fig. 2. The optical micrograph of (a) as-extruded and (b) solution
treated samples
(a)
(2)
Here, F(α) is a boundary correction factor which
depends on the ratio of the crack length a to the width
of the specimen W. For the CCT test specimen used in
this study, the boundary correction factor is given as
[19],
N BW (1Pmax
ys
2a / W )
F ( ) 1 0.025 2 0.06 4
(1)
K
a F
sec
2
(3)
2a
W
(4)
Here, σN is the nominal stress, σys is the yield stress,
Pmax is the maximum load, B is the specimen thickness,
W is the width of gauge position and a is the crack
length. A screw type fixture was used in the CCT
specimen. To avoid the excessive lateral deflection or
buckling of the CCT specimen during the test, the
gauge length and thickness of gage position was
limited to 12 mm and 2 mm, respectively. The gage
position was then polished with 500 to 1500 grit emery
papers to obtain a smooth surface.
where
The fatigue crack propagation rate test was
conducted by using a pneumatic fatigue testing
machine (14 kN maximum capacity) and to investigate
the effect of heat treatment on fatigue crack
propagation behaviour. The tests were performed at
frequency of 10 Hz by using sinusoidal loading form.
A stress ratio R = 0.1 was applied in the tests. The
loading direction was in the extrusion direction of the
material and the testing was carried out at room
temperature.
The procedure for introducing a pre-crack was
followed the ASTM standard [20]. The specimen was
aligned so that the load distribution is symmetrical. The
load ratio R during pre-cracking is the same as the load
ratio used in the fatigue crack propagation test. The
pre-cracking was interrupted after a pre-crack length
equal to 0.1 of specimen thickness was attained at
maintained pre-cracking propagation rates of about 10-8
m/cycle.
The crack propagation curve (crack propagation
rate da/dN versus stress intensity factor range ΔK) was
obtained by using K-decreasing and K-increasing test
procedures. The decreasing and increasing load steps
are 5 - 7% of the previous loading value. The stress
intensity factor value for the CCT specimen was
calculated using the following equation:
The crack length was measured using travelling
microscope. The threshold stress intensity factor ΔKth
was determined when a crack growth is not observed
for 106 cycles. A hole with a 1 mm diameter was drilled
in the centre of the specimen before introducing a 1.35
mm notch by EDM (electrical-discharge machining) to
facilitate fatigue pre-cracking.
III. Result and Discussion
The comparison of fatigue strength of solution
treated and extruded AZ61 magnesium alloy is shown
in Figure 4. The figure shows that fatigue strength of
the solution treated samples increase as that compared
to the fatigue strength of the as-extruded AZ61
samples. The fatigue limit for solution treated and asextruded AZ61 were 180 MPa and 150 MPa,
respectively. The higher fatigue strength observed for
300
Maximum stress (MPa)
(a)
The specimen used for fatigue crack propagation
rate test was centre cracked-plate tension (CCT)
specimen. Figure 3 shows the geometry of the
specimen according to ASTM E647-08 standard [20].
The dimension of the specimen was determined by
following equation according to the test standard:
250
Solution treated
As-extruded
200
150
100 3
10
104
105
106
107
108
Number of cycles to failure, Nf (cycles)
Fig. 3. Centre cracked-plate tension (CCT) specimen used in
the FCP tests
Fig. 4. Fatigue strengths of solution treated and as-extruded samples
TABLE 2
MECHANICAL PROPERTIES OF AZ61 MAGNESIUM ALLOY
Yield
Stress,
σy (MPa)
Material
type
Asextruded
Ave.
Solution
treated
Vickers
Hardness
(Hv)
244
265
270
309
329
308
67
(Ave. of
10 points)
268
315
308
292
288
381
324
322
296
342
As-received
(Low hardness matrix)
Ave.
Ultimate
Tensile
Strength,
σuts (MPa)
71
(Ave. of
10 points)
Surface
Fatigue crack initiation
the solution treated sample is believed due to higher
tensile strength and also higher hardness properties
compared to that of the as-extruded sample as shown in
Table 2.
After the solution treatment the increment in
hardness from Hv 67 to Hv 71 is believed due to the
solid solution strengthening. In the heat treatment
process, the solution treated samples were heated into
the solid solution zone where atoms of alloying
elements dissolved into the matrix. In this condition,
the samples were quenched in water, which limit the
time for precipitation to takes place.
Optical micrographs revealed that there is no
precipitation of second phase observed in the solution
treated sample. Further, the aging processes performed
Fatigue crack
propagation
Cyclic load
Matrix
Matrix
Inclusion
Crack
P.S.B
Δai
Δai+1
Δai< Δai+1
Cyclic load
Surface
Solution treated
(High hardness matrix)
Crack
Matrix
Inclusion
Crack
Crack
High stress
Concentration site
Matrix
Δai
Δai+1
Δai< Δai+1
Fig. 5. Mechanisms of crack initiation and propagation for as-extruded and solution treated AZ61 magnesium alloy
at different aging times and temperatures were unable
to achieve higher hardness compared to that of solution
treated sample due to limitation of second phase
precipitation. This result was in aligned with the results
obtained by Uematsu et al. who reported that
precipitation of Mg17Al12 in AZ61 magnesium alloy is
very limited due to low percentage of Al content as
compared to other magnesium alloy with higher Al
content such as AZ80 [22].
figure, the threshold value for solution treated AZ61
magnesium alloy is at 0.91 MPa√m. This value is
almost at par to that of the extruded magnesium alloy
where the threshold value is at 0.92 MPa√m. From the
result, it can be concluded that heat treatment does not
affect the threshold value of AZ61 magnesium alloy.
However, a slight difference in the fatigue crack
propagation resistance is been demonstrated at higher
ΔK region. Similar FCP behaviour of the AZ91D
magnesium alloy was also reported by Kobayashi et al.
[17].
The increased in hardness of solution treated
sample resulted in difficulty for the extrusion and
intrusion of persistence slip bands to occur and
consequently the fatigue crack tends to initiate at
higher stress concentration sites such as inclusions or
foreign particles at sub-surface. For the as-extruded
samples, the extrusion and intrusion play a prominent
role in initiating fatigue crack. Therefore, the increased
in hardness and yield strength in solution treated
samples delay the fatigue crack initiation and result in a
higher fatigue life. The initiation and propagation
mechanisms of fatigue crack in both samples are
illustrated in Fig. 5.
The FCP curve of solution treated AZ61
magnesium alloy as a function of stress intensity factor
range (ΔK) at room temperature is shown in Fig. 7. The
FCP curve for as-extruded AZ61 magnesium alloy is
also plotted in the same figure for comparison [8].
From the figure, it can be noted that there is not much
difference in the FCP resistance for both curves at a
low ΔK region. However the difference of fatigue crack
propagation resistance can be seen at a higher ΔK
region above 2.0 MPa√m. Fatigue crack propagation
resistance for solution treated samples is found lower
as compared to the as-extruded samples. It can be
considered that the crack in as-extruded AZ61 has
more frequent chances of encounter with grain
boundaries due to the smaller grain size, resulting in a
slower propagation rate. This argument is similar to the
FCP behaviour of AZ31B-L as mentioned by Uematsu
et al. [22].
The arrows in the figure indicate that the threshold
value of stress intensity factor range at ΔKth. From the
(5)
da
C K m
dN
where C is a constant and m is the slope of the curve on
the log-log plot. Values of constants C and m were
calculated using the least square method and the results
are shown in Table 3.
b
a
Fig. 6. Observations were done using SEM equipped EDX. (a)
Foreign particle (b) Flat surface
10-6
Fatigue crack propagation rate, da/dN (m/cycle)
Detailed fracture surface observations showed that
foreign particle was found at the fatigue fracture origin
of the solution treated samples especially for the
samples which exhibited higher fatigue life more than
105 cycles as shown in Fig. 6(a). The foreign particle
size observed was about 20 to 30µm. Pile-up of slips
deformation at near the foreign particle during fatigue
cycles contributed to high stress concentration at
around the foreign particle and resulted in fatigue crack
initiation. In contrast, SEM observation results on
fracture surface of as-extruded samples showed that
there was no evidence of foreign particle at the fatigue
fracture origin. The fatigue crack initiation site was
relatively flat as shown in Fig. 6(b).
The da/dN-ΔK curves obtained at the Paris regime
can be expressed as:
Solution treated
As-extruded
(Sajuri et al. [21])
10-7
10-8
10-9
10-10
10-11
10-12
0.1
0.5
1
5
10
Stress intensity factor range, Δ K (MPa.m 1/2)
Fig. 7. Fatigue crack propagation behavior of solution treated and asextruded AZ61 magnesium alloy
TABLE 3
CRACK PROPAGATION PARAMETER AT THE PARIS
REGIME FOR AZ61 MAGNESIUM ALLOY
ΔKth
(MPa)
m
C
(m/cycle)
As-extruded
0.92
2.1
2.6x10-9
Solution treated
0.91
2.4
3.7x10-7
Material type
slip fracture. The directions of the slips patterns are
almost the same as the crack propagation directions.
Some faceted surface of cleavage fracture was appears
together with the featureless slip fracture as shown in
Fig. 8 (a). The striations-like slip markings can be seen
from the fracture surface as shown in Fig. 8 (b).
V. Conclusion
Experimental investigation on the effect of solution
treatment to the fatigue crack propagation behaviour of
AZ61 magnesium alloy was carried out. Based on the
results the following conclusions are made:
1. Solution treated sample demonstrated superior
mechanical properties and higher fatigue limit as
compared to as-extruded AZ61 magnesium alloy.
(a)
Overview
2. Foreign particle in sub-surface served as the fatigue
fracture origin for solution treated samples
especially at high fatigue life region.
3. The solution treatment shifted the FCP curve at high
ΔK region to the left and demonstrated lower
fatigue crack propagation resistance as compared to
as-extruded sample which has more frequent
chances of encounter with grain boundaries due to
the smaller grain size, resulting in a slower
propagation rate.
(b)
Micrograph of fracture surface at ‘a’
in (a) at low ΔK region
4. The threshold stress intensity factor range ΔKth for
solution treated and as-extruded are almost identical
at the value of 0.91 MPa√m.
Acknowledgement
The author would like to thank Prof. Dr. Y. Mutoh of
Nagaoka University of Technology, Japan for providing
the extruded AZ61 magnesium alloy and the Ministry
of Science, Technology and Innovation (MOSTI),
Malaysia for sponsoring this project under the Science
Fund Research Grant (03-01-02-SF0048).
(c)
Micrograph of fracture surface at ‘b’
in (a) at high ΔK region
Fig. 8. Fracture surface observations of FCP test for solution treated
specimen.
IV. Observation of Fracture Surface
Figure 8 shows the fracture surface of fatigue crack
propagation test specimens tested at R = 0.1 for
solution treated AZ61 magnesium alloy. Figures 8(a),
(b) and (c) show the macroscopic and microscopic
fractograph at the low ΔK and at high ΔK regions,
respectively. The fracture surface appearance of the
entire fracture surface was predominantly a featureless
References
[1]
[2]
[3]
[4]
[5]
B.L. Mordika and T. Ebert, Magnesium. Propertiesapplications-potential, Material Science and Engineering.
A302, pp. 37-45, 2001.
L. Duffy, Magnesium Alloy: The light choice for aerospace,
Materials World, pp. 127-130, 1996.
J. F. King, G. A. Fowler and P. Lyon, Corrosion resistant
magnesium alloy for aerospace casting, Light Weight Alloys for
Aerospace application II, The Mineral, Metals & Materials
Society, pp. 423-438, 1991.
A. A. Luo, Recent magnesium alloy development for
automotive powertrain application, Mat. Science Forum, pp.
419-422, 2003.
S. Schumann and H. Friedrich, Current and Future Use of
Magnesium in the Automobile Industry, Material Science
Forum. Vol. 419-422, pp. 51-55, 2003.
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
[21]
[22]
T. Shibusawa, Y. Kobayashi and K. Ishikawa, Fatigue crack
propagation in Die cast AZ91D magnesium alloy, J. Japan Inst.
Metal, 61(4), pp. 423-438, 1997.
D. L. Goodenberger and J.R.I. Stephens, Fatigue of AZ91E-T6
cast magnesium alloy, Engineering Mat. Technology. 115, pp.
391-397, 1993.
K. Tokaji, M. Kamakura, Y. Ishizumi, and N. Hasegawa,
Fatigue behaviour and fracture mechanism of a rolled AZ31
magnesium alloy, Int. J. Fatigue, 26, pp1217-1224, 2004.
T. Fujii, N. Fuyama, and C. Masuda, Fatigue fracture
mechanisms for calibre rolled AZ91D magnesium alloys,
Materials Science Forum, 419-422, pp109-114, 2003.
Z. Sajuri, Y. Miyashita and Y. Mutoh, Effect of stress ratio on
fatigue crack groeth behaviour of magnesium alloy, Materials
Science Forum, 419-422, pp81-89, 2003.
H.R. Mayer, H. J. Lipowsky, M. Rosch, R. Stich, and S.
Stanzl-Tschegg, Application of ultrasound for fatigue testing of
light weight alloy, Fatigue Frac Engineering Mat. Struct., 22,
pp591-599, 2004.
K. Gall, G. Biallas, H.J. Maier, P. Gullett, M.F. Horstemeyer,
D.L. McDowell and J. Fan, In-situ observations of high cycle
fatigue mechanisms in cast AM60B magnesium in vacuum and
water vapor environments, Int. J. Fatigue, 26, pp59-70, 2004.
M. F. Horstemeyer, N. Yang, K. Gall, D. L. McDowell, J. Fan,
P. M. Gullett. High cycle fatigue of a die cast AZ91E-T4
magnesium alloy, Acta Mater, 52, pp. 1327-1336, 2004.
M. Hilpert and L. Wagner, Corrosion behaviour of highstrength magnesium alloy AZ80, J. Mat. Engineering and
Performance, 9(4), pp402-407, 2000.
T. S. Shih, W. S. Liu, Y. J. Chen. Fatigue of as-extruded AZ61A
magnesium alloy, Mater. Sci. Eng.,325, pp. 152-162, 2002.
A. Bag and W. Zhou. Tensile and fatigue behaviour of AZ91D
magnesium alloy, Journal of Materials Science Letters, 20,
pp. 457-459, 2001.
Y. Kobayashi, T. Shibusawa, K. Ishikawa. Environmental effect
of fatigue crack propagation of magnesium alloy, Mater. Sci.
Eng. A, 234-236, pp. 220-222, 1997.
Y.Uematsu, K. Tokaji, M. Matsumoto. Effects of aging
treatment on fatigue behaviour in extruded AZ61 and AZ80
magnesium alloy, Mater. Sci. and Eng. A, 517, pp. 138-145,
2009.
Standard practice for heat treatment of magnesium alloy
[B661]. Annual book of ASTM Standards, Vol. 02.02. 1999,
Philadelphia.
ASTM Standard E647-08, Annual Book of ASTM Standards,
Vol. 03.01, 2008, 685-686.
Z. Sajuri, T. Umehara, Y. Miyashita, Y. Mutoh. Fatigue-Life
Prediction of Magnesium Alloys for Structural Applications,
Advanced Engineering Materials, Vol. 5(12), pp. 910-916,
2003.
Y. Uematsu, K. Tokaji, M. Kamakura, K. Uchida, H. Shibata, T.
Bekku, Effects of extrusion conditions on grain refinement and
fatigue behavior in magnesium alloy, Mater. Sci. and Eng. A,
434, pp. 131-140, 2006.
Authors’ information
1,2,3,4
Dept. of Mechanical and Materials Engineering, Faculty of
Engineering & Built Environment, Universiti Kebangsaan Malaysia,
43600 UKM Bangi, Selangor, Malaysia.
1
Mohd Ahadlin Mohd Daud is a PhD student in Department of
Mechanical & Materials Engineering,
Universiti Kebangsaan Malaysia. He
received his Master Degree (Mechanical
Eng.) from Universiti Putra Malaysia in
2002. His research interest includes
fatigue and fracture mechanics, and
manufacturing process.
2
Dr Zainuddin
Sajuri is a senior
lecturer at the
Department
of
Mechanical &
Materials Engineering,
Universiti
Kebangsaan Malaysia.
He received his
PhD (Material Science)
from Nagaoka
University, Japan in
2005. He has
taught several courses
throughout the
undergraduate
and
postgraduate
curriculum.
His
research
interest includes fatigue
and
fracture
mechanics, advanced materials and powder metallurgy
3
Dr Syarif Junaidi is a senior lecturer at the Department of
Mechanical &
Materials Engineering,
Universiti Kebangsaan Malaysia. He
received his PhD (Material Science) from
Kyushu University, Japan in 2003. He has
taught several courses throughout the
undergraduate
and
postgraduate
curriculum. His research interest includes
materials processing, advanced materials
and powder metallurgy.
4
Dr Zaidi Omar is an Associate Professor
at the Department of Mechanical &
Materials
Engineering,
Universiti
Kebangsaan Malaysia. He received his
PhD (Mechanical Eng.) from Sheffield
University, UK in 2005. He has taught
several
courses
throughout
the
undergraduate
and
postgraduate
curriculum. His research interest includes
semisolid processing, advanced materials
and metal matrix composite.