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Fatigue crack propagation and fracture mechanisms of wrought magnesium alloys in different environments
International Journal of Fatigue 31 (2009) 1137–1143
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International Journal of Fatigue journal homepage: www.elsevier.com/locate/ijfatigue
Fatigue crack propagation and fracture mechanisms of wrought magnesium alloys in different environments Keiro Tokaji a,*, Masaki Nakajima b, Yoshihiko Uematsu a a b
Department of Mechanical and Systems Engineering, Gifu University, 1-1 Yanagido, Gifu 501-1193, Japan Department of Mechanical Engineering, Toyota National College of Technology, 2-1 Eisei-cho, Toyota 471-8525, Japan
a r t i c l e
i n f o
Article history: Received 24 September 2008 Received in revised form 26 December 2008 Accepted 28 December 2008 Available online 1 January 2009 Keywords: Fatigue crack propagation Wrought magnesium alloy Environment Frequency Fracture mechanism
a b s t r a c t Fatigue crack propagation (FCP) was studied on wrought magnesium alloys, AZ31 and AZ61, in laboratory air, dry air and distilled water. In laboratory air, the FCP rate versus stress intensity factor plots consisted of two sections with different slopes, which was clearly recognized after allowing for crack closure. This was attributed to the transition in fracture mechanisms operated. In distilled water, FCP rates were nearly the same as in laboratory air, while in dry air, an order of magnitude slower than in laboratory air and distilled water. After allowing for crack closure, the environmental effects still existed and FCP rates were the fastest in laboratory air, then in distilled water, in dry air in decreasing order. Fractography revealed that the fracture mechanisms operated in laboratory air and in distilled water were different, possibly hydrogen embrittlement and anodic dissolution, respectively. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Magnesium (Mg) alloys are very attractive as structural materials in order to achieve high performance and energy saving of machines and structures, because of their advantages such as light weight, high specific strength and stiffness, good machinability and recyclability. Mg alloys are generally classified into two categories; wrought alloys and cast alloys. Cast alloys have advantages such as cost saving and flexibility in fabrication compared with wrought alloys. Therefore, there have recently been increasing applications of cast alloys in transportation industries, particularly in automobile industry. On the other hand, since wrought alloys posses much better mechanical properties than cast alloys, they are expected to be applied to load-bearing components for which fatigue is critical. Therefore, it is necessary to evaluate various fatigue properties of wrought Mg alloys. Studies on the fatigue properties of Mg alloys have been conducted for a long time. Ogarevic and Stephens [1] have reviewed the results reported between 1923 and 1990. In this review, many S–N data and fatigue crack propagation (FCP) are presented for various alloys, but most of them are rather old. Considering this situation of fatigue research and a further development of Mg alloys, most recent data on various fatigue properties have to be accumulated. With increasing interest of Mg alloys in recent years, many studies on the fatigue properties have been reported. Most of them * Corresponding author. Tel.: +81 58 293 2500; fax: +81 58 293 2491. E-mail address: [email protected] (K. Tokaji). 0142-1123/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijfatigue.2008.12.012
are focused on fatigue strength, fatigue life, cyclic deformation and corrosion fatigue, but there have been very limited works on FCP behaviour [2–4]. In this study, the FCP behaviour of two wrought Mg alloys, rolled AZ31 and extruded AZ61, was investigated. FCP experiments were first performed in laboratory air that is the ordinary environment in which materials are used. Fracture mechanisms were discussed based on crack closure measurement and fractographic observation. Based on the FCP response in laboratory air, further experiments were then conducted in dry air and in distilled water, and the environmental effects and associated fracture mechanisms were discussed.
2. Experimental details 2.1. Materials The materials used are rolled AZ31 plate with a thickness of 6 mm and extruded AZ61 plate with a thickness of 5 mm, which are the same materials as employed in previous reports [5–7]. Their chemical compositions (wt.%) are given in Table 1. Both alloys have an equiaxed grain structure in all planes of the plates. As an example, Fig. 1 shows the microstructures on a crosssection perpendicular to the rolling or extrusion direction. The average grain sizes are approximately 60 lm and 14 lm in AZ31 and AZ61, respectively, but in AZ61 the grain sizes near the plate surfaces were larger than those in the core [7].
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Table 1 Chemical compositions of materials (wt.%). Alloy
Al
Zn
Mn
Si
Cu
Fe
Ni
AZ31 AZ61
3 6.5
1 1.0
0.1
0.1
0.05
0.005
0.005
a travelling microscope and crack closure was measured by an unloading elastic compliance method using a strain gauge mounted on the back face of the specimens. After experiment, fracture surfaces were analyzed in detail using a scanning electron microscope (SEM). 3. Results and discussion 3.1. FCP characteristics in laboratory air
Fig. 1. Microstructures on cross-section perpendicular to rolling or extrusion direction: (a) AZ31 and (b) AZ61.
Table 2 Mechanical properties of materials. Alloy AZ31 AZ61
r0.2 (MPa)
Tensile strength rB (MPa)
Elongation / (%)
Vickers hardness HV
110 –
224 278
30 25
53 72
0.2% Proof stress
Based on XRD analysis, it was found that both alloys have a crystallographic texture, where (0 0 0 2) basal planes are parallel to the plate surface in AZ31 [5] and are inclined approximately 30° against the extrusion direction in AZ61 [8]. The mechanical properties in the rolling or extrusion direction are listed in Table 2. As can be seen in the table, AZ61 exhibits higher tensile strength and hardness than AZ31.
3.1.1. Relationships between FCP rate and stress intensity factor range The relationship between FCP rate, da/dN, and stress intensity factor range, DK, DKeff, is shown in Fig. 2, where DKeff is the effective stress intensity factor range. When FCP rate is characterized in terms of DK, AZ61 exhibits slightly faster FCP rates than AZ31 in the entire DK region studied. After allowing for crack closure (da/ dN–DKeff plots), the FCP rates for both alloys become identical, indicating that the small difference in FCP rates seen in the da/ dN–DK plots is attributed to difference of the crack closure levels induced by fracture surface roughness due to grain size. It should be noted that the da/dN–DK plots for both alloys consist of two sections with slightly different slopes and the slope change occurs at the DK value of 3.5–4 MPa m1/2 that is called hereafter the transition stress intensity factor, DKT. The slope change is clearly recognized after allowing for crack closure and the DKeff value at which the slopes changed is 2.5–3 MPa m1/2 regardless of alloy system. 3.1.2. Fractographic observation of fracture surfaces Figs. 3 and 4 reveal SEM micrographs of fracture surfaces at different DK levels in AZ31 and AZ61, respectively. Fracture surface morphology is similar in both alloys, indicating that the fracture mechanisms operated do not depend on alloy system. It should be noted that the fracture surfaces are essentially brittle in character regardless of DK level, but differences in appearance are evident above and below DKT. At DK levels above DKT (Figs. 3a and 4a), the
Compact type (CT) specimens with a 50.8 mm width were used in FCP experiments. The thicknesses are 6 mm in AZ31 and 5 mm in AZ61. The specimens were machined in L–T orientation, where L is the longitudinal direction, i.e. the rolling or extrusion direction, and T is the transverse direction. Before experiment, the specimen surface was mechanically polished using progressively finer grades of emery paper. 2.3. Procedures FCP experiments were performed at a stress ratio, R, of 0.05 using a 19 kN capacity electro-hydraulic fatigue testing machine. Test environments evaluated were uncontrolled laboratory air, dry air and distilled water. The humidity and temperature in laboratory air were 50–70% and 20–25 °C, respectively, and the dew point of dry air was 60 °C. The distilled water was kept at 30 °C and circulated by a pump between a corrosion cell attached to the specimen surface and a reserved tank. Sinusoidal test frequencies used were 10 Hz in laboratory air and in dry air and 1 Hz in distilled water, but additional experiments were conducted at the frequencies of 0.01, 0.1, 1 and 10 Hz in order to examine the effect of frequency on FCP rate. A precrack of 2 mm long from the starter notch root was introduced under a decreasing stress intensity factor range, DK, condition, and then increasing and decreasing DK tests were started according to ASTM standards [9]. Crack length was monitored with
Crack propagation rate da/dN (mm/cycle)
2.2. Specimen
10
–3
10
–4
10
–5
10
–6
Magnesium alloy Laboratory air L–T orientation R=0.05 AZ31 AZ61
Open: ΔK
Solid: ΔK eff
10
–7
1 5 10 0.3 0.5 Stress intensity factor range 1/2 ΔK, ΔK eff (MPam )
Fig. 2. Relationship between FCP rate and stress intensity factor range in laboratory air.
K. Tokaji et al. / International Journal of Fatigue 31 (2009) 1137–1143
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Fig. 3. SEM micrographs of fracture surfaces for AZ31 in laboratory air: (a) DK = 5.0 MPa m1/2, (b) DK = 3.0 MPa m1/2 and (c) DK = 2.5 MPa m1/2.
Fig. 4. SEM micrographs of fracture surfaces for AZ61 in laboratory air: (a) DK = 5.0 MPa m1/2, (b) DK = 3.0 MPa m1/2 and (c) DK = 2.5 MPa m1/2.
fracture surfaces are covered with packets with fine steps whose sizes appear to coincide with the grain size. This appearance is very similar to transgranular, quasi-cleavage fracture surface morphology observed in pure Mg under environmentally-induced cracking conditions or in liquid nitrogen [10]. On the other hand, at DK levels below DKT (Figs. 3c and 4c), transgranular, cleavage-like facets with river-patterns (indicated by arrow) can be seen extensively. At DK levels near DKT (Figs. 3b and 4b), the mixed morphology of packets with fine steps and cleavage-like facets is observed. Based on these fractographic observations, it is suggested that the fracture mechanism transition takes place at DKT, which would lead to the sharp slope changes in the da/dN–DKeff plots. As an example, Fig. 5 shows a magnified view of the fracture surface at a DK level above DKT in AZ31. Many fine steps nearly parallel to the FCP direction can be seen, but striations are not observed. Based on detailed examination of fracture surfaces in both alloys, no striations were present in the entire DK region studied. 3.1.3. Transition in fracture mechanisms As shown in Fig. 2, the da/dN–DKeff plots in laboratory air consisted of two sections with different slopes. Similar behaviour has been reported in aluminium alloy [11] and pure titanium [12], which is attributed to the fracture mechanism transition. Also in
Fig. 5. Magnified view of fracture surface at DK = 5.0 MPa m1/2 for AZ31 in laboratory air.
the present Mg alloys, the slope changes were related to the transition in fracture mechanisms operated, because fractographic observations clearly revealed different fracture surface appearances below and above DKT at which the slopes changed (Figs. 3 and 4). It has been considered that the fracture mechanism transition correlates with the development of a reversed plastic zone size equal to the grain size. The reversed plastic zone size, rpc.eff, for plane strain condition is given by the following equation
1 DK eff 2 rpc;eff ¼ pffiffiffiffiffiffi 2 2p 2ry
ð1Þ
where ry is the yield stress of material. In AZ31, substituting ry = 110 MPa and DKeff = 3 MPa m1/2 to the above equation, the rpc.eff value at the transition is obtained to be approximately 21 lm, which is not consistent with the average grain size of 60 lm. In addition, the transition stress intensity factor ranges were the same in both alloys with different strengths and grain sizes. These results indicate that the fracture mechanism transition is not due to the correlation between the reversed plastic zone size and the grain size. As described previously, fracture surfaces were brittle and the appearances were clearly different above and below DKT. This suggests that the fracture mechanism transition would occur due to an environmental effect where FCP rate is the controlling parameter. 3.1.4. Comparison in FCP behaviour between Mg alloys and other light metals Fig. 6 represents the comparison in FCP behaviour between the present Mg alloys and other light metals such as aluminium alloys [13,14] and pure titanium [12]. When FCP rate is characterized in term of DK (open mark in Fig. 6a), the Mg alloys exhibit faster FCP rates than the other metals in the entire DK region. This indicates that the FCP resistance of the Mg alloys is considerably lower than aluminium alloys and pure titanium. After allowing for crack closure (solid mark in Fig. 6a), the FCP resistance of the Mg alloys is still significantly lower than the other light metals. The modulus of elasticity, E, is one of the most important material variables controlling the FCP response. Hence, FCP rate is
K. Tokaji et al. / International Journal of Fatigue 31 (2009) 1137–1143
10
–2
10
–3
10
–4
10
10
Magnesium alloy AZ31 AZ61 Aluminium alloy 6063–T5 [14] 7075–T6 [13]
Crack propagation rate d a/dN (mm/cycle)
Crack propagation rate d a/dN (mm/cycle)
1140
Open: ΔK Solid: ΔK eff
–5
–6
Titanium Pure Ti [12] –7
10 0.5
1 5 10 Stress intensity factor range 1/2 ΔK, ΔK eff (MPam )
10
10
–3
10
–4
10
–5
10
10
50
–2
Magnesium alloy AZ31 AZ61 Aluminium alloy 6063–T5 [14] 7075–T6 [13]
–6
Titanium Pure Ti [12] –7
10
–5
–4
10 1/2 ΔK eff /E (m )
10
–3
Fig. 6. Comparison of FCP responses between magnesium alloys and other light metals. FCP rate is characterized in terms of (a) stress intensity factor range and effective stress intensity factor range and (b) effective stress intensity factor range normalized with respect to modulus of elasticity.
10
10
10
10
10
10
is believed that the FCP response of the Mg alloys is affected by moisture in the entire DK region. In particular, below DKT, FCP rates are enhanced by extensive cleavage-like fracture resulting from slower FCP rate, i.e. more time allowed for environmental attack. 3.2. FCP characteristics in dry air and distilled water 3.2.1. Relationships between FCP rate and stress intensity factor range Based on the FCP response and fracture surface observation in laboratory air, it was strongly suggested that moisture or humidity exerts a significant influence on FCP in Mg alloys. Thus further FCP experiments were performed in a moisture-free environment, dry air, and in an aqueous environment, distilled water. The relation-
–2
–3
–4
10 Magnesium alloy AZ31 R=0.05 Laboratory air Distilled water Dry air
Crack propagation rate d a/dN (mm/cycle)
Crack propagation rate d a/dN (mm/cycle)
shown in Fig. 6b as a function of DKeff normalized with respect to E, i.e. DKeff/E. The differences in FCP rates between 6063Al alloy and pure titanium seen in Fig. 6a are almost eliminated, but the Mg alloys and 7075Al alloy exhibit still faster FCP rates than those metals. It is worth noting that Mg alloys and 7075Al alloy possess much greater environmental susceptibility than 6063Al alloy and pure titanium. It is suggested, therefore, that the lower FCP resistance of the Mg alloys and 7075Al alloy may be attributed to the effect of environment, probably the presence of moisture or humidity in laboratory air. It has been indicated that laboratory air is a corrosive environment for Mg alloys [15] and corrosion fatigue occurs above a relative humidity of 80% [16]. In FCP, the effect of moisture in laboratory air would become much severer at the crack tip due to the capillary condensation [17]. Therefore, it
Open: ΔK Solid: ΔK eff
–5
–6
–7
0.3 0.5
1
5
10
Stress intensity factor range 1/2 ΔK, ΔK eff (MPam )
50
10
10
10
10
10
–2
–3
–4
Magnesium alloy AZ61 R=0.05 Laboratory air Distilled water Dry air Open: ΔK Solid: ΔK eff
–5
–6
–7
0.3 0.5
1
5
10
50
Stress intensity factor range 1/2 ΔK, ΔK eff (MPam )
Fig. 7. Relationships between FCP rate and stress intensity factor range in dry air and distilled water: (a) AZ31 and (b) AZ61.
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ships between FCP rate and stress intensity factor range for AZ31 and AZ61 in those environments are demonstrated in Fig. 7a and b, respectively. Also included in the figure are the FCP characteristics in laboratory air for comparison. It can be seen that both alloys show the same dependence of FCP response on environment. When FCP rates are characterized in terms of DK, they are nearly the same in laboratory air and in distilled water, whereas an order of magnitude slower in dry air than in those environments in the entire DK region. After allowing for crack closure (da/dN–DKeff plots), the effect of environment is still clearly recognized, indicating that FCP behaviour is purely influenced by environment. As can be seen in the figures, FCP rates are the fastest in laboratory air, then in distilled water, in dry air in decreasing order. Note that the enhancement of FCP rate is much more remarkable in labora-
Crack length a (mm)
6
Magnesium alloy AZ31 (a) Laboratory air Dry air
Laboratory
5 air
Distilled water
4 3
ΔK=3.5MPam
0
1/2
10 20 0 Number of cycles
Crack length a (mm)
6 5
ΔK=3.5MPam
1/2
20
Magnesium alloy AZ61 (b) Laboratory air Dry air
Laboratory
4 air
10 4 N (x10 )
Distilled water
3 2 1 0
ΔK=3.5MPam
1/2
ΔK=3.5MPam
10 20 0 10 4 Number of cycles N (x10 )
1/2
20
Fig. 8. FCP responses at constant DK of 3.5 MPa m1/2 under changing-environment conditions: (a) AZ31 and (b) AZ61.
10
10
–3
10
Magnesium alloy AZ31 Laboratory air Distilled water Dry air –4
–5
ΔK=3.5MPam
10
3.2.4. Fractographic observation of fracture surfaces SEM micrographs of fracture surfaces in dry air for AZ31 and AZ61 are revealed in Figs. 10 and 11, respectively. It can be seen
Crack propagation rate da/dN (mm/cycle)
Crack propagation rate da/dN (mm/cycle)
10
3.2.2. Effect of environment under changing-environment test In order to further understand the effect of environment, additional FCP experiments were performed at a constant DK level of 3.5 MPa m1/2 under changing-environment conditions. The obtained results are shown in Fig. 8, where the environment was changed from laboratory air to distilled water or from laboratory air to dry air in a single specimen. As can be seen in the figure, again both alloys exhibit a similar FCP response. After changing the environment, the FCP rates in the following environment, i.e. distilled water or dry air, are achieved immediately and they are consistent with the rates at the DK value of 3.5 MPa m1/2 obtained in increasing and decreasing DK tests shown in Fig. 7. Note that no transitional effect is seen just after the change of environment. This implies that a clean, bare material at the crack tip would react with the environment immediately, indicating very high susceptibility to environment in Mg alloys. 3.2.3. Effect of test frequency on FCP rate Corrosion is a time-dependent phenomenon, thus there have been many studies on the effect of test frequency on FCP behaviour. It is generally known that FCP rate is enhanced with decreasing frequency. The dependence of FCP rate on frequency is represented in Fig. 9, where experiments were performed under a constant DK condition of 3.5 MPa m1/2. In dry air, FCP rates are nearly constant over a wide range of the frequencies from 0.01 Hz to 10 Hz in both alloys. Thus the frequency effect is negligible, which seems reasonable. In laboratory air, although FCP rates tend to be slightly faster at low frequencies, the effect of frequency is rather small. In distilled water, a distinct dependence on frequency can be seen and FCP rates become faster with decreasing frequency. When the frequency is larger than 1 Hz, the FCP rates in laboratory air and in distilled water are the same, but below that frequency, the FCP rates in distilled water are faster than those in laboratory air. From these results, it is believed that the fracture mechanisms operated in laboratory air and in distilled water would be different.
2 1
tory air than in distilled water. It should also be noted that there exists a large difference in FCP rate between laboratory air and dry air, suggesting that moisture or humidity in laboratory air affects significantly the FCP behaviour. Hence, laboratory air is no longer a non-aggressive, reference environment for Mg alloys.
10
Magnesium alloy AZ61 Laboratory air Distilled water Dry air –4
–5
1/2
–6
0.01
10
–3
0.1 1 10 Test fre quency f (Hz)
ΔK=3.5MPam
10
1/2
–6
0.01
0.1 1 10 Test fre quency f (Hz)
Fig. 9. Effect of test frequency on FCP rate: (a) AZ31 and (b) AZ61.
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Fig. 10. SEM micrographs of fracture surfaces for AZ31 in dry air: (a) DK = 6.5 MPa m1/2, (b) DK = 5.0 MPa m1/2 and (c) DK = 3.5 MPa m1/2.
Fig. 11. SEM micrographs of fracture surfaces for AZ61 in dry air: (a) DK = 6.5 MPa m1/2, (b) DK = 5.0 MPa m1/2 and (c) DK = 3.5 MPa m1/2.
in the figures that the fracture surface appearance is similar in both alloys. As described previously, in laboratory air, the fracture surface appearances were different above and below DKT, but fracture surfaces were essentially brittle. On the other hand, it should be noted that in dry air, the fracture surfaces are ductile regardless of DK level. Fig. 12 reveals a magnified view of the fracture surface at a high DK level for AZ31 where striations are not observed. No
Fig. 12. Magnified view of fracture surface at DK = 6.5 MPa m1/2 for AZ31 in dry air.
striations were seen for AZ61 either in the entire DK region studied. The change in fracture surface appearance from brittle to ductile suggests that there exists a significant effect of environment on FCP response in laboratory air. In distilled water, fracture surfaces could not be observed because they were totally covered with corrosion products, indicating that anodic dissolution severely occurred in this environment. Fig. 13a shows the fracture surface appearance before and after the change of the environments from laboratory air to dry air. It can be seen that the fracture surface morphology changes from brittle to ductile immediately, which corresponds to the fracture surface appearance in Figs. 3b and 10b, respectively. No transitional region is recognized and thus the fracture mechanism in dry air operates immediately after changing the environments. This again indicates the very high susceptibility to environment in Mg alloys. The fracture surface appearance in distilled water under the changing-environment condition from laboratory air to distilled water is revealed in Fig. 13b. As can be seen in the figure, the fracture surface is covered with corrosion products, suggesting that anodic dissolution occurs remarkably. It should be noted that there was no evidence of any anodic dissolution or corrosion products in laboratory air, even under long-term FCP experiment near the threshold regime.
Fig. 13. SEM micrographs of fracture surfaces at constant DK of 3.5 MPa m1/2 under changing-environment conditions in AZ61: (a) appearance before and after change of environments from laboratory air to dry air and (b) appearance in distilled water after change of environment from laboratory air to distilled water.
K. Tokaji et al. / International Journal of Fatigue 31 (2009) 1137–1143
3.2.5. Operative fracture mechanisms in corrosive environments In both Mg alloys, FCP rates in laboratory air and in distilled water were the same when they are characterized in terms of DK, but after allowing for crack closure, FCP rates became faster in laboratory air than in distilled water (Fig. 7). This difference in FCP rate is due to pure environmental effect, because there were no differences in crack shielding other than crack closure such as crack deflection and bifurcation between both environments. Fractographic observations clearly revealed differences in fracture surface appearance between laboratory air and distilled water. In laboratory air, fracture surfaces were brittle in character such as step-like pattern, cleavage-like facets and river-pattern, while in distilled water, they were totally covered with corrosion products. This strongly suggests that the fracture mechanisms operated are different between both environments, thereby intrinsic FCP rates become faster in laboratory air than in distilled water. As possible mechanisms operated in laboratory air and in distilled water, hydrogen embrittlement (HE) and anodic dissolution, will be now considered. There have been some studies on HE [18], stress corrosion cracking (SCC) [19] and environment-assisted cracking (EAC) [10,20] in pure Mg and Mg alloys and it has been indicated that SCC results from HE [19] and EAC occurs even in laboratory air, which is promoted by HE [20]. In laboratory air, local environment in the vicinity of the crack tip would become a wet condition because of the capillary condensation of moisture [17], but it is not severe to cause anodic dissolution. Therefore, it is believed that FCP is enhanced by embrittlement at the crack tip due to the diffusion of hydrogen evolved by the corrosion reaction between the exposed bare surface and moisture contained in laboratory air. The amount of produced hydrogen is not so much, but it has been indicated that a small amount of hydrogen is sufficient to cause embrittlement [10]. In addition, it has also been reported that FCP is enhanced by HE in cast AZ91D alloy [4]. In distilled water, as the results of the reaction with a bare metal at the crack tip, anodic dissolution occurs severely, resulting in corrosion products. Larger amounts of hydrogen than in laboratory air would be evolved, but hydrogen could not reach the bare metal surface because corrosion products such as Mg(OH)2 act as a protective barrier to block the hydrogen diffusion [10]. Therefore, localized crack tip anodic dissolution is believed to be responsible for the enhanced FCP in distilled water. 4. Conclusions The fatigue crack propagation (FCP) behaviour of two wrought Mg alloys, rolled AZ31 and extruded AZ61, was investigated in laboratory air, dry air and distilled water. The environmental effect and associated fracture mechanisms were discussed. Based on the results obtained in this study, the following conclusions can be made: 1. In laboratory air, the da/dN–DK plots of both alloys consisted of two sections with slightly different slopes, and after allowing for crack closure, the slope change became much more remarkable. This was attributed to the transition in fracture mechanisms operated because the fracture surface morphology was different above and below the transition stress intensity factor range at which the slopes changed. 2. After allowing for both crack closure and the modulus of elasticity, the Mg alloys still exhibited lower FCP resistance than aluminium alloys such as 6063 and 7075 and pure titanium, from which the effect of moisture contained in laboratory air was suggested.
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3. In both alloys, FCP rates were the same in laboratory air and in distilled water, while an order of magnitude slower in dry air in the entire DK region. After allowing for crack closure, the environmental effect still existed and FCP rates were the fastest in laboratory air, then in distilled water and in dry air in decreasing order. 4. When environment was changed from laboratory air to dry air or to distilled water, FCP rates and fracture surface morphology in the following environment were attained immediately, i.e. no transitional effect was seen, indicating very high susceptibility to environment. 5. Over a wide range of test frequencies from 0.01 Hz to 10 Hz, the dependence of FCP rate on frequency was not seen in dry air and a little effect of frequency was observed in laboratory air, while FCP rates were enhanced with decreasing frequency in distilled water. 6. Fracture surface morphology was essentially brittle with stepped pattern, cleavage-like facets and river-pattern in laboratory air, while ductile in dry air regardless of DK level. In distilled water, the fracture surfaces were totally covered with corrosion products. Since there was no evidence of corrosion products in laboratory air, the fracture mechanisms operated in laboratory air and in distilled water were different, possibly hydrogen embrittlement and anodic dissolution, respectively. References [1] Ogarevic VV, Stephens RI. Fatigue of magnesium alloys. Annu Rev Mater Sci 1990;20:147–77. [2] Pook LR, Greenan AF. Fatigue crack-growth characteristics of two magnesium alloys. Eng Fract Mech 1973;5(4):821–6. [3] Liaw PK, Ho TL, Donald JK. Near-threshold fatigue crack growth behavior in a magnesium alloy. Scripta Metal 1984;18(8):821–4. [4] Kobayashi Y, Shibusawa T, Ishikawa K. Environmental effect of fatigue crack propagation of magnesium alloy. Mater Sci Eng 1997;A234–236:220–2. [5] Tokaji K, Kamakura M, Hasegawa N, Tsuboi Y. Fatigue crack propagation in magnesium alloy AZ31 rolled plate. J Soc Mater Sci, Jpn 2003;52(7):821–6. [6] Tokaji K, Kamakura M, Ishiizumi Y, Hasegawa N. Fatigue behaviour and fracture mechanism of a rolled AZ31 magnesium alloy. Int J Fatigue 2004;26(11):1217–24. [7] Kamakura M, Tokaji K, Ishiizumi Y, Hasegawa N. Fatigue behaviour and fracture mechanism of an extruded AZ61 magnesium alloy. J Soc Mater Sci, Jpn 2004;53(12):1371–7. [8] Uematsu Y, Tokaji K, Kamakura M, Uchida K, Shibata H, Bekku N. Effect of extrusion conditions on grain refinement and fatigue behaviour in magnesium alloys. Mater Sci Eng 2006;A434:131–40. [9] Annual book of ASTM Standards, Part 10: metals-mechanical, fracture, corrosion testing; fatigue: erosion and wear; effect of temperature, E647–81; 1982. [10] Stampella RS, Procter RPM, Ashworth V. Environmentally-induced cracking of magnesium. Corros Sci 1984;24(4):325–41. [11] Jono M, Song J, Mikami S, Ohgaki M. Fatigue crack growth and crack closure behavior of structural materials. J Soc Mater Sci, Jpn 1981;33(367):468–74. [12] Ogawa T, Tokaji K, Kameyama Y. Fatigue crack growth characteristics of pure titanium. J Soc Mater Sci, Jpn 1989;38(432):1026–32. [13] Tokaji K, Ogawa T, Kameyama Y. The effects of stress ratio on the growth behaviour of small fatigue cracks in an aluminum alloy 7075–T6 (with special interest in stage I crack growth). Fatigue Fract Eng Mater Struct 1990;13(4):411–21. [14] Tokaji K, Goshima Y. Fatigue behaviour of 6063 aluminium alloy in corrosive environments. J Soc Mater Sci, Jpn 2002;51(12):1411–6. [15] Hipert M, Wagner L. Environmental effects on the HCF behavior of the magnesium alloys AZ31 and AZ80. In: Kaplan HI, Hryn J, Clow B, editors. Magnesium Tech 2000. The Minerals, Metals & Materials Society; 2000. p. 375–81. [16] Sajuri ZB, Miyashita Y, Mutoh Y. Fatigue characteristics of an extruded AZ61 magnesium alloy. J Jpn Inst Light Metals 2002;52(4):161–6. [17] Komai K. Kozozairyo-no-kankyokyodosekkei. Tokyo: Yokendo Ltd.; 1993. [18] Chakrapani DG, Pugh EN. Hydrogen embrittlement in a Mg–Al alloy. Metal Trans 1975;7A:173–8. [19] Makar GL, Kruger J, Sieradzki K. Stress corrosion cracking of rapidly solidified magnesium–aluminum alloys. Corros Sci 1993;34(8):1311–42. [20] Marrow TJ, Ahmad AB, Khan IN, Sim SMA, Torkamani S. Environment-assisted cracking of cast WE43–T6 magnesium. Mater Sci Eng 2004;A387–389: 419–23.
Contents lists available at ScienceDirect
International Journal of Fatigue journal homepage: www.elsevier.com/locate/ijfatigue
Fatigue crack propagation and fracture mechanisms of wrought magnesium alloys in different environments Keiro Tokaji a,*, Masaki Nakajima b, Yoshihiko Uematsu a a b
Department of Mechanical and Systems Engineering, Gifu University, 1-1 Yanagido, Gifu 501-1193, Japan Department of Mechanical Engineering, Toyota National College of Technology, 2-1 Eisei-cho, Toyota 471-8525, Japan
a r t i c l e
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Article history: Received 24 September 2008 Received in revised form 26 December 2008 Accepted 28 December 2008 Available online 1 January 2009 Keywords: Fatigue crack propagation Wrought magnesium alloy Environment Frequency Fracture mechanism
a b s t r a c t Fatigue crack propagation (FCP) was studied on wrought magnesium alloys, AZ31 and AZ61, in laboratory air, dry air and distilled water. In laboratory air, the FCP rate versus stress intensity factor plots consisted of two sections with different slopes, which was clearly recognized after allowing for crack closure. This was attributed to the transition in fracture mechanisms operated. In distilled water, FCP rates were nearly the same as in laboratory air, while in dry air, an order of magnitude slower than in laboratory air and distilled water. After allowing for crack closure, the environmental effects still existed and FCP rates were the fastest in laboratory air, then in distilled water, in dry air in decreasing order. Fractography revealed that the fracture mechanisms operated in laboratory air and in distilled water were different, possibly hydrogen embrittlement and anodic dissolution, respectively. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Magnesium (Mg) alloys are very attractive as structural materials in order to achieve high performance and energy saving of machines and structures, because of their advantages such as light weight, high specific strength and stiffness, good machinability and recyclability. Mg alloys are generally classified into two categories; wrought alloys and cast alloys. Cast alloys have advantages such as cost saving and flexibility in fabrication compared with wrought alloys. Therefore, there have recently been increasing applications of cast alloys in transportation industries, particularly in automobile industry. On the other hand, since wrought alloys posses much better mechanical properties than cast alloys, they are expected to be applied to load-bearing components for which fatigue is critical. Therefore, it is necessary to evaluate various fatigue properties of wrought Mg alloys. Studies on the fatigue properties of Mg alloys have been conducted for a long time. Ogarevic and Stephens [1] have reviewed the results reported between 1923 and 1990. In this review, many S–N data and fatigue crack propagation (FCP) are presented for various alloys, but most of them are rather old. Considering this situation of fatigue research and a further development of Mg alloys, most recent data on various fatigue properties have to be accumulated. With increasing interest of Mg alloys in recent years, many studies on the fatigue properties have been reported. Most of them * Corresponding author. Tel.: +81 58 293 2500; fax: +81 58 293 2491. E-mail address: [email protected] (K. Tokaji). 0142-1123/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijfatigue.2008.12.012
are focused on fatigue strength, fatigue life, cyclic deformation and corrosion fatigue, but there have been very limited works on FCP behaviour [2–4]. In this study, the FCP behaviour of two wrought Mg alloys, rolled AZ31 and extruded AZ61, was investigated. FCP experiments were first performed in laboratory air that is the ordinary environment in which materials are used. Fracture mechanisms were discussed based on crack closure measurement and fractographic observation. Based on the FCP response in laboratory air, further experiments were then conducted in dry air and in distilled water, and the environmental effects and associated fracture mechanisms were discussed.
2. Experimental details 2.1. Materials The materials used are rolled AZ31 plate with a thickness of 6 mm and extruded AZ61 plate with a thickness of 5 mm, which are the same materials as employed in previous reports [5–7]. Their chemical compositions (wt.%) are given in Table 1. Both alloys have an equiaxed grain structure in all planes of the plates. As an example, Fig. 1 shows the microstructures on a crosssection perpendicular to the rolling or extrusion direction. The average grain sizes are approximately 60 lm and 14 lm in AZ31 and AZ61, respectively, but in AZ61 the grain sizes near the plate surfaces were larger than those in the core [7].
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Table 1 Chemical compositions of materials (wt.%). Alloy
Al
Zn
Mn
Si
Cu
Fe
Ni
AZ31 AZ61
3 6.5
1 1.0
0.1
0.1
0.05
0.005
0.005
a travelling microscope and crack closure was measured by an unloading elastic compliance method using a strain gauge mounted on the back face of the specimens. After experiment, fracture surfaces were analyzed in detail using a scanning electron microscope (SEM). 3. Results and discussion 3.1. FCP characteristics in laboratory air
Fig. 1. Microstructures on cross-section perpendicular to rolling or extrusion direction: (a) AZ31 and (b) AZ61.
Table 2 Mechanical properties of materials. Alloy AZ31 AZ61
r0.2 (MPa)
Tensile strength rB (MPa)
Elongation / (%)
Vickers hardness HV
110 –
224 278
30 25
53 72
0.2% Proof stress
Based on XRD analysis, it was found that both alloys have a crystallographic texture, where (0 0 0 2) basal planes are parallel to the plate surface in AZ31 [5] and are inclined approximately 30° against the extrusion direction in AZ61 [8]. The mechanical properties in the rolling or extrusion direction are listed in Table 2. As can be seen in the table, AZ61 exhibits higher tensile strength and hardness than AZ31.
3.1.1. Relationships between FCP rate and stress intensity factor range The relationship between FCP rate, da/dN, and stress intensity factor range, DK, DKeff, is shown in Fig. 2, where DKeff is the effective stress intensity factor range. When FCP rate is characterized in terms of DK, AZ61 exhibits slightly faster FCP rates than AZ31 in the entire DK region studied. After allowing for crack closure (da/ dN–DKeff plots), the FCP rates for both alloys become identical, indicating that the small difference in FCP rates seen in the da/ dN–DK plots is attributed to difference of the crack closure levels induced by fracture surface roughness due to grain size. It should be noted that the da/dN–DK plots for both alloys consist of two sections with slightly different slopes and the slope change occurs at the DK value of 3.5–4 MPa m1/2 that is called hereafter the transition stress intensity factor, DKT. The slope change is clearly recognized after allowing for crack closure and the DKeff value at which the slopes changed is 2.5–3 MPa m1/2 regardless of alloy system. 3.1.2. Fractographic observation of fracture surfaces Figs. 3 and 4 reveal SEM micrographs of fracture surfaces at different DK levels in AZ31 and AZ61, respectively. Fracture surface morphology is similar in both alloys, indicating that the fracture mechanisms operated do not depend on alloy system. It should be noted that the fracture surfaces are essentially brittle in character regardless of DK level, but differences in appearance are evident above and below DKT. At DK levels above DKT (Figs. 3a and 4a), the
Compact type (CT) specimens with a 50.8 mm width were used in FCP experiments. The thicknesses are 6 mm in AZ31 and 5 mm in AZ61. The specimens were machined in L–T orientation, where L is the longitudinal direction, i.e. the rolling or extrusion direction, and T is the transverse direction. Before experiment, the specimen surface was mechanically polished using progressively finer grades of emery paper. 2.3. Procedures FCP experiments were performed at a stress ratio, R, of 0.05 using a 19 kN capacity electro-hydraulic fatigue testing machine. Test environments evaluated were uncontrolled laboratory air, dry air and distilled water. The humidity and temperature in laboratory air were 50–70% and 20–25 °C, respectively, and the dew point of dry air was 60 °C. The distilled water was kept at 30 °C and circulated by a pump between a corrosion cell attached to the specimen surface and a reserved tank. Sinusoidal test frequencies used were 10 Hz in laboratory air and in dry air and 1 Hz in distilled water, but additional experiments were conducted at the frequencies of 0.01, 0.1, 1 and 10 Hz in order to examine the effect of frequency on FCP rate. A precrack of 2 mm long from the starter notch root was introduced under a decreasing stress intensity factor range, DK, condition, and then increasing and decreasing DK tests were started according to ASTM standards [9]. Crack length was monitored with
Crack propagation rate da/dN (mm/cycle)
2.2. Specimen
10
–3
10
–4
10
–5
10
–6
Magnesium alloy Laboratory air L–T orientation R=0.05 AZ31 AZ61
Open: ΔK
Solid: ΔK eff
10
–7
1 5 10 0.3 0.5 Stress intensity factor range 1/2 ΔK, ΔK eff (MPam )
Fig. 2. Relationship between FCP rate and stress intensity factor range in laboratory air.
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Fig. 3. SEM micrographs of fracture surfaces for AZ31 in laboratory air: (a) DK = 5.0 MPa m1/2, (b) DK = 3.0 MPa m1/2 and (c) DK = 2.5 MPa m1/2.
Fig. 4. SEM micrographs of fracture surfaces for AZ61 in laboratory air: (a) DK = 5.0 MPa m1/2, (b) DK = 3.0 MPa m1/2 and (c) DK = 2.5 MPa m1/2.
fracture surfaces are covered with packets with fine steps whose sizes appear to coincide with the grain size. This appearance is very similar to transgranular, quasi-cleavage fracture surface morphology observed in pure Mg under environmentally-induced cracking conditions or in liquid nitrogen [10]. On the other hand, at DK levels below DKT (Figs. 3c and 4c), transgranular, cleavage-like facets with river-patterns (indicated by arrow) can be seen extensively. At DK levels near DKT (Figs. 3b and 4b), the mixed morphology of packets with fine steps and cleavage-like facets is observed. Based on these fractographic observations, it is suggested that the fracture mechanism transition takes place at DKT, which would lead to the sharp slope changes in the da/dN–DKeff plots. As an example, Fig. 5 shows a magnified view of the fracture surface at a DK level above DKT in AZ31. Many fine steps nearly parallel to the FCP direction can be seen, but striations are not observed. Based on detailed examination of fracture surfaces in both alloys, no striations were present in the entire DK region studied. 3.1.3. Transition in fracture mechanisms As shown in Fig. 2, the da/dN–DKeff plots in laboratory air consisted of two sections with different slopes. Similar behaviour has been reported in aluminium alloy [11] and pure titanium [12], which is attributed to the fracture mechanism transition. Also in
Fig. 5. Magnified view of fracture surface at DK = 5.0 MPa m1/2 for AZ31 in laboratory air.
the present Mg alloys, the slope changes were related to the transition in fracture mechanisms operated, because fractographic observations clearly revealed different fracture surface appearances below and above DKT at which the slopes changed (Figs. 3 and 4). It has been considered that the fracture mechanism transition correlates with the development of a reversed plastic zone size equal to the grain size. The reversed plastic zone size, rpc.eff, for plane strain condition is given by the following equation
1 DK eff 2 rpc;eff ¼ pffiffiffiffiffiffi 2 2p 2ry
ð1Þ
where ry is the yield stress of material. In AZ31, substituting ry = 110 MPa and DKeff = 3 MPa m1/2 to the above equation, the rpc.eff value at the transition is obtained to be approximately 21 lm, which is not consistent with the average grain size of 60 lm. In addition, the transition stress intensity factor ranges were the same in both alloys with different strengths and grain sizes. These results indicate that the fracture mechanism transition is not due to the correlation between the reversed plastic zone size and the grain size. As described previously, fracture surfaces were brittle and the appearances were clearly different above and below DKT. This suggests that the fracture mechanism transition would occur due to an environmental effect where FCP rate is the controlling parameter. 3.1.4. Comparison in FCP behaviour between Mg alloys and other light metals Fig. 6 represents the comparison in FCP behaviour between the present Mg alloys and other light metals such as aluminium alloys [13,14] and pure titanium [12]. When FCP rate is characterized in term of DK (open mark in Fig. 6a), the Mg alloys exhibit faster FCP rates than the other metals in the entire DK region. This indicates that the FCP resistance of the Mg alloys is considerably lower than aluminium alloys and pure titanium. After allowing for crack closure (solid mark in Fig. 6a), the FCP resistance of the Mg alloys is still significantly lower than the other light metals. The modulus of elasticity, E, is one of the most important material variables controlling the FCP response. Hence, FCP rate is
K. Tokaji et al. / International Journal of Fatigue 31 (2009) 1137–1143
10
–2
10
–3
10
–4
10
10
Magnesium alloy AZ31 AZ61 Aluminium alloy 6063–T5 [14] 7075–T6 [13]
Crack propagation rate d a/dN (mm/cycle)
Crack propagation rate d a/dN (mm/cycle)
1140
Open: ΔK Solid: ΔK eff
–5
–6
Titanium Pure Ti [12] –7
10 0.5
1 5 10 Stress intensity factor range 1/2 ΔK, ΔK eff (MPam )
10
10
–3
10
–4
10
–5
10
10
50
–2
Magnesium alloy AZ31 AZ61 Aluminium alloy 6063–T5 [14] 7075–T6 [13]
–6
Titanium Pure Ti [12] –7
10
–5
–4
10 1/2 ΔK eff /E (m )
10
–3
Fig. 6. Comparison of FCP responses between magnesium alloys and other light metals. FCP rate is characterized in terms of (a) stress intensity factor range and effective stress intensity factor range and (b) effective stress intensity factor range normalized with respect to modulus of elasticity.
10
10
10
10
10
10
is believed that the FCP response of the Mg alloys is affected by moisture in the entire DK region. In particular, below DKT, FCP rates are enhanced by extensive cleavage-like fracture resulting from slower FCP rate, i.e. more time allowed for environmental attack. 3.2. FCP characteristics in dry air and distilled water 3.2.1. Relationships between FCP rate and stress intensity factor range Based on the FCP response and fracture surface observation in laboratory air, it was strongly suggested that moisture or humidity exerts a significant influence on FCP in Mg alloys. Thus further FCP experiments were performed in a moisture-free environment, dry air, and in an aqueous environment, distilled water. The relation-
–2
–3
–4
10 Magnesium alloy AZ31 R=0.05 Laboratory air Distilled water Dry air
Crack propagation rate d a/dN (mm/cycle)
Crack propagation rate d a/dN (mm/cycle)
shown in Fig. 6b as a function of DKeff normalized with respect to E, i.e. DKeff/E. The differences in FCP rates between 6063Al alloy and pure titanium seen in Fig. 6a are almost eliminated, but the Mg alloys and 7075Al alloy exhibit still faster FCP rates than those metals. It is worth noting that Mg alloys and 7075Al alloy possess much greater environmental susceptibility than 6063Al alloy and pure titanium. It is suggested, therefore, that the lower FCP resistance of the Mg alloys and 7075Al alloy may be attributed to the effect of environment, probably the presence of moisture or humidity in laboratory air. It has been indicated that laboratory air is a corrosive environment for Mg alloys [15] and corrosion fatigue occurs above a relative humidity of 80% [16]. In FCP, the effect of moisture in laboratory air would become much severer at the crack tip due to the capillary condensation [17]. Therefore, it
Open: ΔK Solid: ΔK eff
–5
–6
–7
0.3 0.5
1
5
10
Stress intensity factor range 1/2 ΔK, ΔK eff (MPam )
50
10
10
10
10
10
–2
–3
–4
Magnesium alloy AZ61 R=0.05 Laboratory air Distilled water Dry air Open: ΔK Solid: ΔK eff
–5
–6
–7
0.3 0.5
1
5
10
50
Stress intensity factor range 1/2 ΔK, ΔK eff (MPam )
Fig. 7. Relationships between FCP rate and stress intensity factor range in dry air and distilled water: (a) AZ31 and (b) AZ61.
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ships between FCP rate and stress intensity factor range for AZ31 and AZ61 in those environments are demonstrated in Fig. 7a and b, respectively. Also included in the figure are the FCP characteristics in laboratory air for comparison. It can be seen that both alloys show the same dependence of FCP response on environment. When FCP rates are characterized in terms of DK, they are nearly the same in laboratory air and in distilled water, whereas an order of magnitude slower in dry air than in those environments in the entire DK region. After allowing for crack closure (da/dN–DKeff plots), the effect of environment is still clearly recognized, indicating that FCP behaviour is purely influenced by environment. As can be seen in the figures, FCP rates are the fastest in laboratory air, then in distilled water, in dry air in decreasing order. Note that the enhancement of FCP rate is much more remarkable in labora-
Crack length a (mm)
6
Magnesium alloy AZ31 (a) Laboratory air Dry air
Laboratory
5 air
Distilled water
4 3
ΔK=3.5MPam
0
1/2
10 20 0 Number of cycles
Crack length a (mm)
6 5
ΔK=3.5MPam
1/2
20
Magnesium alloy AZ61 (b) Laboratory air Dry air
Laboratory
4 air
10 4 N (x10 )
Distilled water
3 2 1 0
ΔK=3.5MPam
1/2
ΔK=3.5MPam
10 20 0 10 4 Number of cycles N (x10 )
1/2
20
Fig. 8. FCP responses at constant DK of 3.5 MPa m1/2 under changing-environment conditions: (a) AZ31 and (b) AZ61.
10
10
–3
10
Magnesium alloy AZ31 Laboratory air Distilled water Dry air –4
–5
ΔK=3.5MPam
10
3.2.4. Fractographic observation of fracture surfaces SEM micrographs of fracture surfaces in dry air for AZ31 and AZ61 are revealed in Figs. 10 and 11, respectively. It can be seen
Crack propagation rate da/dN (mm/cycle)
Crack propagation rate da/dN (mm/cycle)
10
3.2.2. Effect of environment under changing-environment test In order to further understand the effect of environment, additional FCP experiments were performed at a constant DK level of 3.5 MPa m1/2 under changing-environment conditions. The obtained results are shown in Fig. 8, where the environment was changed from laboratory air to distilled water or from laboratory air to dry air in a single specimen. As can be seen in the figure, again both alloys exhibit a similar FCP response. After changing the environment, the FCP rates in the following environment, i.e. distilled water or dry air, are achieved immediately and they are consistent with the rates at the DK value of 3.5 MPa m1/2 obtained in increasing and decreasing DK tests shown in Fig. 7. Note that no transitional effect is seen just after the change of environment. This implies that a clean, bare material at the crack tip would react with the environment immediately, indicating very high susceptibility to environment in Mg alloys. 3.2.3. Effect of test frequency on FCP rate Corrosion is a time-dependent phenomenon, thus there have been many studies on the effect of test frequency on FCP behaviour. It is generally known that FCP rate is enhanced with decreasing frequency. The dependence of FCP rate on frequency is represented in Fig. 9, where experiments were performed under a constant DK condition of 3.5 MPa m1/2. In dry air, FCP rates are nearly constant over a wide range of the frequencies from 0.01 Hz to 10 Hz in both alloys. Thus the frequency effect is negligible, which seems reasonable. In laboratory air, although FCP rates tend to be slightly faster at low frequencies, the effect of frequency is rather small. In distilled water, a distinct dependence on frequency can be seen and FCP rates become faster with decreasing frequency. When the frequency is larger than 1 Hz, the FCP rates in laboratory air and in distilled water are the same, but below that frequency, the FCP rates in distilled water are faster than those in laboratory air. From these results, it is believed that the fracture mechanisms operated in laboratory air and in distilled water would be different.
2 1
tory air than in distilled water. It should also be noted that there exists a large difference in FCP rate between laboratory air and dry air, suggesting that moisture or humidity in laboratory air affects significantly the FCP behaviour. Hence, laboratory air is no longer a non-aggressive, reference environment for Mg alloys.
10
Magnesium alloy AZ61 Laboratory air Distilled water Dry air –4
–5
1/2
–6
0.01
10
–3
0.1 1 10 Test fre quency f (Hz)
ΔK=3.5MPam
10
1/2
–6
0.01
0.1 1 10 Test fre quency f (Hz)
Fig. 9. Effect of test frequency on FCP rate: (a) AZ31 and (b) AZ61.
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Fig. 10. SEM micrographs of fracture surfaces for AZ31 in dry air: (a) DK = 6.5 MPa m1/2, (b) DK = 5.0 MPa m1/2 and (c) DK = 3.5 MPa m1/2.
Fig. 11. SEM micrographs of fracture surfaces for AZ61 in dry air: (a) DK = 6.5 MPa m1/2, (b) DK = 5.0 MPa m1/2 and (c) DK = 3.5 MPa m1/2.
in the figures that the fracture surface appearance is similar in both alloys. As described previously, in laboratory air, the fracture surface appearances were different above and below DKT, but fracture surfaces were essentially brittle. On the other hand, it should be noted that in dry air, the fracture surfaces are ductile regardless of DK level. Fig. 12 reveals a magnified view of the fracture surface at a high DK level for AZ31 where striations are not observed. No
Fig. 12. Magnified view of fracture surface at DK = 6.5 MPa m1/2 for AZ31 in dry air.
striations were seen for AZ61 either in the entire DK region studied. The change in fracture surface appearance from brittle to ductile suggests that there exists a significant effect of environment on FCP response in laboratory air. In distilled water, fracture surfaces could not be observed because they were totally covered with corrosion products, indicating that anodic dissolution severely occurred in this environment. Fig. 13a shows the fracture surface appearance before and after the change of the environments from laboratory air to dry air. It can be seen that the fracture surface morphology changes from brittle to ductile immediately, which corresponds to the fracture surface appearance in Figs. 3b and 10b, respectively. No transitional region is recognized and thus the fracture mechanism in dry air operates immediately after changing the environments. This again indicates the very high susceptibility to environment in Mg alloys. The fracture surface appearance in distilled water under the changing-environment condition from laboratory air to distilled water is revealed in Fig. 13b. As can be seen in the figure, the fracture surface is covered with corrosion products, suggesting that anodic dissolution occurs remarkably. It should be noted that there was no evidence of any anodic dissolution or corrosion products in laboratory air, even under long-term FCP experiment near the threshold regime.
Fig. 13. SEM micrographs of fracture surfaces at constant DK of 3.5 MPa m1/2 under changing-environment conditions in AZ61: (a) appearance before and after change of environments from laboratory air to dry air and (b) appearance in distilled water after change of environment from laboratory air to distilled water.
K. Tokaji et al. / International Journal of Fatigue 31 (2009) 1137–1143
3.2.5. Operative fracture mechanisms in corrosive environments In both Mg alloys, FCP rates in laboratory air and in distilled water were the same when they are characterized in terms of DK, but after allowing for crack closure, FCP rates became faster in laboratory air than in distilled water (Fig. 7). This difference in FCP rate is due to pure environmental effect, because there were no differences in crack shielding other than crack closure such as crack deflection and bifurcation between both environments. Fractographic observations clearly revealed differences in fracture surface appearance between laboratory air and distilled water. In laboratory air, fracture surfaces were brittle in character such as step-like pattern, cleavage-like facets and river-pattern, while in distilled water, they were totally covered with corrosion products. This strongly suggests that the fracture mechanisms operated are different between both environments, thereby intrinsic FCP rates become faster in laboratory air than in distilled water. As possible mechanisms operated in laboratory air and in distilled water, hydrogen embrittlement (HE) and anodic dissolution, will be now considered. There have been some studies on HE [18], stress corrosion cracking (SCC) [19] and environment-assisted cracking (EAC) [10,20] in pure Mg and Mg alloys and it has been indicated that SCC results from HE [19] and EAC occurs even in laboratory air, which is promoted by HE [20]. In laboratory air, local environment in the vicinity of the crack tip would become a wet condition because of the capillary condensation of moisture [17], but it is not severe to cause anodic dissolution. Therefore, it is believed that FCP is enhanced by embrittlement at the crack tip due to the diffusion of hydrogen evolved by the corrosion reaction between the exposed bare surface and moisture contained in laboratory air. The amount of produced hydrogen is not so much, but it has been indicated that a small amount of hydrogen is sufficient to cause embrittlement [10]. In addition, it has also been reported that FCP is enhanced by HE in cast AZ91D alloy [4]. In distilled water, as the results of the reaction with a bare metal at the crack tip, anodic dissolution occurs severely, resulting in corrosion products. Larger amounts of hydrogen than in laboratory air would be evolved, but hydrogen could not reach the bare metal surface because corrosion products such as Mg(OH)2 act as a protective barrier to block the hydrogen diffusion [10]. Therefore, localized crack tip anodic dissolution is believed to be responsible for the enhanced FCP in distilled water. 4. Conclusions The fatigue crack propagation (FCP) behaviour of two wrought Mg alloys, rolled AZ31 and extruded AZ61, was investigated in laboratory air, dry air and distilled water. The environmental effect and associated fracture mechanisms were discussed. Based on the results obtained in this study, the following conclusions can be made: 1. In laboratory air, the da/dN–DK plots of both alloys consisted of two sections with slightly different slopes, and after allowing for crack closure, the slope change became much more remarkable. This was attributed to the transition in fracture mechanisms operated because the fracture surface morphology was different above and below the transition stress intensity factor range at which the slopes changed. 2. After allowing for both crack closure and the modulus of elasticity, the Mg alloys still exhibited lower FCP resistance than aluminium alloys such as 6063 and 7075 and pure titanium, from which the effect of moisture contained in laboratory air was suggested.
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3. In both alloys, FCP rates were the same in laboratory air and in distilled water, while an order of magnitude slower in dry air in the entire DK region. After allowing for crack closure, the environmental effect still existed and FCP rates were the fastest in laboratory air, then in distilled water and in dry air in decreasing order. 4. When environment was changed from laboratory air to dry air or to distilled water, FCP rates and fracture surface morphology in the following environment were attained immediately, i.e. no transitional effect was seen, indicating very high susceptibility to environment. 5. Over a wide range of test frequencies from 0.01 Hz to 10 Hz, the dependence of FCP rate on frequency was not seen in dry air and a little effect of frequency was observed in laboratory air, while FCP rates were enhanced with decreasing frequency in distilled water. 6. Fracture surface morphology was essentially brittle with stepped pattern, cleavage-like facets and river-pattern in laboratory air, while ductile in dry air regardless of DK level. In distilled water, the fracture surfaces were totally covered with corrosion products. Since there was no evidence of corrosion products in laboratory air, the fracture mechanisms operated in laboratory air and in distilled water were different, possibly hydrogen embrittlement and anodic dissolution, respectively. References [1] Ogarevic VV, Stephens RI. Fatigue of magnesium alloys. Annu Rev Mater Sci 1990;20:147–77. [2] Pook LR, Greenan AF. Fatigue crack-growth characteristics of two magnesium alloys. Eng Fract Mech 1973;5(4):821–6. [3] Liaw PK, Ho TL, Donald JK. Near-threshold fatigue crack growth behavior in a magnesium alloy. Scripta Metal 1984;18(8):821–4. [4] Kobayashi Y, Shibusawa T, Ishikawa K. Environmental effect of fatigue crack propagation of magnesium alloy. Mater Sci Eng 1997;A234–236:220–2. [5] Tokaji K, Kamakura M, Hasegawa N, Tsuboi Y. Fatigue crack propagation in magnesium alloy AZ31 rolled plate. J Soc Mater Sci, Jpn 2003;52(7):821–6. [6] Tokaji K, Kamakura M, Ishiizumi Y, Hasegawa N. Fatigue behaviour and fracture mechanism of a rolled AZ31 magnesium alloy. Int J Fatigue 2004;26(11):1217–24. [7] Kamakura M, Tokaji K, Ishiizumi Y, Hasegawa N. Fatigue behaviour and fracture mechanism of an extruded AZ61 magnesium alloy. J Soc Mater Sci, Jpn 2004;53(12):1371–7. [8] Uematsu Y, Tokaji K, Kamakura M, Uchida K, Shibata H, Bekku N. Effect of extrusion conditions on grain refinement and fatigue behaviour in magnesium alloys. Mater Sci Eng 2006;A434:131–40. [9] Annual book of ASTM Standards, Part 10: metals-mechanical, fracture, corrosion testing; fatigue: erosion and wear; effect of temperature, E647–81; 1982. [10] Stampella RS, Procter RPM, Ashworth V. Environmentally-induced cracking of magnesium. Corros Sci 1984;24(4):325–41. [11] Jono M, Song J, Mikami S, Ohgaki M. Fatigue crack growth and crack closure behavior of structural materials. J Soc Mater Sci, Jpn 1981;33(367):468–74. [12] Ogawa T, Tokaji K, Kameyama Y. Fatigue crack growth characteristics of pure titanium. J Soc Mater Sci, Jpn 1989;38(432):1026–32. [13] Tokaji K, Ogawa T, Kameyama Y. The effects of stress ratio on the growth behaviour of small fatigue cracks in an aluminum alloy 7075–T6 (with special interest in stage I crack growth). Fatigue Fract Eng Mater Struct 1990;13(4):411–21. [14] Tokaji K, Goshima Y. Fatigue behaviour of 6063 aluminium alloy in corrosive environments. J Soc Mater Sci, Jpn 2002;51(12):1411–6. [15] Hipert M, Wagner L. Environmental effects on the HCF behavior of the magnesium alloys AZ31 and AZ80. In: Kaplan HI, Hryn J, Clow B, editors. Magnesium Tech 2000. The Minerals, Metals & Materials Society; 2000. p. 375–81. [16] Sajuri ZB, Miyashita Y, Mutoh Y. Fatigue characteristics of an extruded AZ61 magnesium alloy. J Jpn Inst Light Metals 2002;52(4):161–6. [17] Komai K. Kozozairyo-no-kankyokyodosekkei. Tokyo: Yokendo Ltd.; 1993. [18] Chakrapani DG, Pugh EN. Hydrogen embrittlement in a Mg–Al alloy. Metal Trans 1975;7A:173–8. [19] Makar GL, Kruger J, Sieradzki K. Stress corrosion cracking of rapidly solidified magnesium–aluminum alloys. Corros Sci 1993;34(8):1311–42. [20] Marrow TJ, Ahmad AB, Khan IN, Sim SMA, Torkamani S. Environment-assisted cracking of cast WE43–T6 magnesium. Mater Sci Eng 2004;A387–389: 419–23.