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Environmental effect of fatigue crack propagation of magnesium alloy
ELSEVIER
Materials
Environmental
Science
A234-236
(1997)
220-222
effect of fatigue crack propagation alloy
Yasuo Kobayashi Department
and Engineering
of Mechanical
Engineering,
Faculty
Received
*, Toshinori
Shibusawa,
of Engineering,
University,
2100 Kujirai-Nakanodai,
in revised
form
28 Jaimary
Toyo
1997; received
of magnesium
Keisuke Ishikawa 11 March
Kawagoe,
Saitama
350, Japan
1997
Abstract Fatigue crack propagation tests were carried out for a magnesium alloy in various kinds of environment. The presence of oxygen causes the production of oxide film on the fresh fracture surfaces made during the cyclic loading. Therefore, the fatigue crack behavior depends strongly upon the environment. The fatigue tests have been conducted in dry and wet argon gas as well as in air. The wet atmosphere, in particular, accelerated the fatigue crack propagation rate. The presence of the oxide film would restrict the deformation of the matrix beneath the hard film and promote hydrogen embrittlement in the wet condition. 0 1997 Elsevier Science S.A. Keywords:
Environmental
effect;
Fatigue
crack
propagation;
Magnesium
alloy
1. Introduction
2. Experiments
Magnesium alloys are extremely useful structural materials for transportation, because of their higher specific strength compared with other metals and alloys [ 11. Reduction of vehicle weight is essential to save energy. Magnesium, however, is very reactive to the environment, above all to the presence of oxygen and water. Hence, we have investigated the effect of oxygen and humidity on the mechanical properties of magnesiumaluminum-zinc alloy, AZ91D. The alloy is currently one of strongest magnesium alloys. The most important mechanical property is fatigue, since information on fatigue allows the lifetime and safety of the structure to be determined.
2.1. Material and heat treatment
Table 1 Chemical
composition
of AZ91D
The magnesium alloy for this experiment is a modified pore-free alloy. This alloy is therefore heat-treatable. Its chemical composition is shown in Table 1 and the applied heat treatments are as-cast (F), solution treatment (T4), and aging (T6a, T6b). The most common microstructures are dual phases composed of lamellar nodules and a-matrix except for the solid-solution one. 2.2. Fatigue tests Whole fatigue tests were carried out in dry and wet
(mass %)
Al
Zn
Mn
Si
Fe
Be
Ni
CU
Mg
9.0
0.68
0.17
0.051
0.002
0.0015
O.OOl>
O.OOl>
Bal.
* Corresponding 0921-5093/97/$17.00
author.
Tel.:
+ 81 492 391340;
0 1997 Elsevier
PII SO921-5093(97)00202-S
Science
fax:
+ 81 492 339779;
S.A. All rights
reserved.
e-mail:
[email protected]
Y. Kobayashi
et al. /Materials
Science
and Engineering
A2346236
(1997)
220-222
221
a, z % 0
i . 1x10-4 z u .
0
z
Fig. 1. Dimension
of specimen
for FCPR
3. Results and discussion 3.1. EjJect of heat treatment we have investigated
Ar,0.5Hz,R=0.2
1xlo-5
test (mm).
5xlo-6
atmosphere at room temperature. The specimen dimension used for the fatigue crack propagation test is shown in Fig. 1. The thickness of the specimen is 8 mm. The humidity of the test chamber was checked by a hygrometer.
First.
dry
the effect of heat treat-
3.5
10
AU Fig. 3. Effect
of oxygen
/
14
MPa&
on FCPR
for AZ91D
(T6b).
ment (microstructures) on the relationship between the fatigue crack propagation rate (FCPR), da/dN, and the stress intensity factor range, AK, for the magnesium alloy. Fig. 2 shows the effect of the heat treatment on the fatigue crack propagation behavior. The T4 alloy yields the lowest FCPR among them, since the microstructure is homogeneous solid solution. The presence of the nodule accelerates the FCPR (T6). Besides,
2x1 o-3 2x1 o-2
AV nv
1x10-3
IC
n
1x10-’
a 0 2
a, u s-3 0 ’ 1x10-3
2 ,
T
E
1 xl o-4 .
z u .
0
F (air,
:
0
T4 (air,
A
T6a (air,
5Hz,
R=O.Z)
V
T6b (air,
5Hz,
R=0.2)
5Hz,
FP0.2)
5Hz,
Z u . 4
R=O.2)
1x10-4
1 xl o-5
5x1 o-6 + 3.5
-n AK
/
10
MPa&
15
1x10-5 7x1 o-6
of heat
treatment
on FCPR
for
AZ9lD
dry
Ar,0.2Hz,R=0.2
wet
air,lOHz,R=O.P
V
dry air,lOHz,R=0.2 I
3.5 ,
18
AK Fig. 2. Effect temperature.
0 A
/
MPaE
at room Fig. 4. Effect
of moisture
on FCPR
for AZYlD
(T6b).
222
Y. Kohayashi
b Fig.
5. Scanning
electron
et al. j Materials
crack micrographs
Science
propagation of the fatigue
fracture
the precipitates in the matrix further promote the FCPR. Both the nodules and the precipitates have little effect on the S-N behavior, but have a negative effect on the FCPR [2]. In spite of the increase in the yield and the tensile strength, they do not improve the fatigue properties of the alloy.
and Engineering
A.?34-236
(1997)
2206222
di rection surfa ce of AZ91 D for AK & 4.5 MPa
20pm ,/&
in air: (a) wet; (b) dry.
surfaces for AZ91D are shown in Fig. number of striations were observed on the the steady propagation region. We did not big difference between the wet and the spheres.
5. A large surfaces in recognize a dry atmo-
3.2. Effect of environment 4. Conclusions The effect of oxygen on the FCPR is shown in Fig. 3. For the higher frequency, FCPR shows a small difference between exposure in air and in argon. As the frequency decreases, FCPR is higher in air than in argon. The oxide film which is formed at lower frequency is thicker than at high frequency. Since it depends upon the reaction time, the effect would be larger for a lower frequency. The oxide film is so brittle that the FCPR remarkably depends upon frequency as well as the crack closure [3]. The effect of moisture (water vapor) on the FCPR is shown in Fig. 4. FCPR in the wet atmosphere is higher than in either dry air or dry argon. The FCPR of the magnesium alloy depends upon the presence of water, which induces hydrogen embrittlement, contrary to the account given in Emley [41. 3.3. Fructography Scanning
electron
micrographs
of fatigue
fracture
Aging, which stimulates production of nodules and precipitates, accelerates the FCPR of the magnesium alloy in every environment. Oxidation produces a negative effect on the FCPR of AZ91D. The wet atmosphere would contribute more to the higher FCPR through hydrogen embrittlement.
References [I] W.E. Smith, Structure and Properties of Engineering Alloys. McGraw-Hill, New York, 1993, p. 537. [2] K. Ishikawa, Y. Kobayashi, T. Ito, Characteristics of fatigue crack propagation in heat treatable die cast magnesium alloy, in: E.W. Lee, NJ. Kim, K.V. Jata and W.E. Frazier (Eds.), Light Weight Alloys for Aerospace Applications III, TMS, New York. 1995, p. 449. [3] P.K. Liaw, W.A. Logsdon, Eng. Fract. Mech. 22 (1985) 115. [4] E.F. Emley, Principles of Magnesium Technology, Pergamon Press. Oxford, 1966, p. 194.
Materials
Environmental
Science
A234-236
(1997)
220-222
effect of fatigue crack propagation alloy
Yasuo Kobayashi Department
and Engineering
of Mechanical
Engineering,
Faculty
Received
*, Toshinori
Shibusawa,
of Engineering,
University,
2100 Kujirai-Nakanodai,
in revised
form
28 Jaimary
Toyo
1997; received
of magnesium
Keisuke Ishikawa 11 March
Kawagoe,
Saitama
350, Japan
1997
Abstract Fatigue crack propagation tests were carried out for a magnesium alloy in various kinds of environment. The presence of oxygen causes the production of oxide film on the fresh fracture surfaces made during the cyclic loading. Therefore, the fatigue crack behavior depends strongly upon the environment. The fatigue tests have been conducted in dry and wet argon gas as well as in air. The wet atmosphere, in particular, accelerated the fatigue crack propagation rate. The presence of the oxide film would restrict the deformation of the matrix beneath the hard film and promote hydrogen embrittlement in the wet condition. 0 1997 Elsevier Science S.A. Keywords:
Environmental
effect;
Fatigue
crack
propagation;
Magnesium
alloy
1. Introduction
2. Experiments
Magnesium alloys are extremely useful structural materials for transportation, because of their higher specific strength compared with other metals and alloys [ 11. Reduction of vehicle weight is essential to save energy. Magnesium, however, is very reactive to the environment, above all to the presence of oxygen and water. Hence, we have investigated the effect of oxygen and humidity on the mechanical properties of magnesiumaluminum-zinc alloy, AZ91D. The alloy is currently one of strongest magnesium alloys. The most important mechanical property is fatigue, since information on fatigue allows the lifetime and safety of the structure to be determined.
2.1. Material and heat treatment
Table 1 Chemical
composition
of AZ91D
The magnesium alloy for this experiment is a modified pore-free alloy. This alloy is therefore heat-treatable. Its chemical composition is shown in Table 1 and the applied heat treatments are as-cast (F), solution treatment (T4), and aging (T6a, T6b). The most common microstructures are dual phases composed of lamellar nodules and a-matrix except for the solid-solution one. 2.2. Fatigue tests Whole fatigue tests were carried out in dry and wet
(mass %)
Al
Zn
Mn
Si
Fe
Be
Ni
CU
Mg
9.0
0.68
0.17
0.051
0.002
0.0015
O.OOl>
O.OOl>
Bal.
* Corresponding 0921-5093/97/$17.00
author.
Tel.:
+ 81 492 391340;
0 1997 Elsevier
PII SO921-5093(97)00202-S
Science
fax:
+ 81 492 339779;
S.A. All rights
reserved.
e-mail:
[email protected]
Y. Kobayashi
et al. /Materials
Science
and Engineering
A2346236
(1997)
220-222
221
a, z % 0
i . 1x10-4 z u .
0
z
Fig. 1. Dimension
of specimen
for FCPR
3. Results and discussion 3.1. EjJect of heat treatment we have investigated
Ar,0.5Hz,R=0.2
1xlo-5
test (mm).
5xlo-6
atmosphere at room temperature. The specimen dimension used for the fatigue crack propagation test is shown in Fig. 1. The thickness of the specimen is 8 mm. The humidity of the test chamber was checked by a hygrometer.
First.
dry
the effect of heat treat-
3.5
10
AU Fig. 3. Effect
of oxygen
/
14
MPa&
on FCPR
for AZ91D
(T6b).
ment (microstructures) on the relationship between the fatigue crack propagation rate (FCPR), da/dN, and the stress intensity factor range, AK, for the magnesium alloy. Fig. 2 shows the effect of the heat treatment on the fatigue crack propagation behavior. The T4 alloy yields the lowest FCPR among them, since the microstructure is homogeneous solid solution. The presence of the nodule accelerates the FCPR (T6). Besides,
2x1 o-3 2x1 o-2
AV nv
1x10-3
IC
n
1x10-’
a 0 2
a, u s-3 0 ’ 1x10-3
2 ,
T
E
1 xl o-4 .
z u .
0
F (air,
:
0
T4 (air,
A
T6a (air,
5Hz,
R=O.Z)
V
T6b (air,
5Hz,
R=0.2)
5Hz,
FP0.2)
5Hz,
Z u . 4
R=O.2)
1x10-4
1 xl o-5
5x1 o-6 + 3.5
-n AK
/
10
MPa&
15
1x10-5 7x1 o-6
of heat
treatment
on FCPR
for
AZ9lD
dry
Ar,0.2Hz,R=0.2
wet
air,lOHz,R=O.P
V
dry air,lOHz,R=0.2 I
3.5 ,
18
AK Fig. 2. Effect temperature.
0 A
/
MPaE
at room Fig. 4. Effect
of moisture
on FCPR
for AZYlD
(T6b).
222
Y. Kohayashi
b Fig.
5. Scanning
electron
et al. j Materials
crack micrographs
Science
propagation of the fatigue
fracture
the precipitates in the matrix further promote the FCPR. Both the nodules and the precipitates have little effect on the S-N behavior, but have a negative effect on the FCPR [2]. In spite of the increase in the yield and the tensile strength, they do not improve the fatigue properties of the alloy.
and Engineering
A.?34-236
(1997)
2206222
di rection surfa ce of AZ91 D for AK & 4.5 MPa
20pm ,/&
in air: (a) wet; (b) dry.
surfaces for AZ91D are shown in Fig. number of striations were observed on the the steady propagation region. We did not big difference between the wet and the spheres.
5. A large surfaces in recognize a dry atmo-
3.2. Effect of environment 4. Conclusions The effect of oxygen on the FCPR is shown in Fig. 3. For the higher frequency, FCPR shows a small difference between exposure in air and in argon. As the frequency decreases, FCPR is higher in air than in argon. The oxide film which is formed at lower frequency is thicker than at high frequency. Since it depends upon the reaction time, the effect would be larger for a lower frequency. The oxide film is so brittle that the FCPR remarkably depends upon frequency as well as the crack closure [3]. The effect of moisture (water vapor) on the FCPR is shown in Fig. 4. FCPR in the wet atmosphere is higher than in either dry air or dry argon. The FCPR of the magnesium alloy depends upon the presence of water, which induces hydrogen embrittlement, contrary to the account given in Emley [41. 3.3. Fructography Scanning
electron
micrographs
of fatigue
fracture
Aging, which stimulates production of nodules and precipitates, accelerates the FCPR of the magnesium alloy in every environment. Oxidation produces a negative effect on the FCPR of AZ91D. The wet atmosphere would contribute more to the higher FCPR through hydrogen embrittlement.
References [I] W.E. Smith, Structure and Properties of Engineering Alloys. McGraw-Hill, New York, 1993, p. 537. [2] K. Ishikawa, Y. Kobayashi, T. Ito, Characteristics of fatigue crack propagation in heat treatable die cast magnesium alloy, in: E.W. Lee, NJ. Kim, K.V. Jata and W.E. Frazier (Eds.), Light Weight Alloys for Aerospace Applications III, TMS, New York. 1995, p. 449. [3] P.K. Liaw, W.A. Logsdon, Eng. Fract. Mech. 22 (1985) 115. [4] E.F. Emley, Principles of Magnesium Technology, Pergamon Press. Oxford, 1966, p. 194.