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Crack Initiation Analysis in AZ31 Magnesium Alloy Based on Electron Backscatter Diffraction (EBSD)
Available online at www.sciencedirect.com
ScienceDirect Procedia Materials Science 3 (2014) 790 – 792
20th European Conference on Fracture (ECF20)
Crack Initiation Analysis in AZ31 Magnesium Alloy based on Electron Backscatter Diffraction (EBSD) Yoshihiko Uematsua, *, Toshifumi Kakiuchia, Yoshifumi Kamiyab b
a Gifu University, 1-1 Yanagido, Gifu 501-1193, Japan Murata Machinery Ltd., 163 Takeda-Mukaishiro-cho, Fushimi-ku, Kyoto 612-8686, Japan
Abstract Plane bending fatigue test had been conducted to figure out fatigue crack initiation mechanism in magnesium alloy, AZ31, with HCP crystallographic structure. The initial crystallographic structure was analyzed by an electron backscatter diffraction (EBSD) method. Subsequently, the fatigue test was periodically terminated and time-series EBSD analysis was performed. Basal slip and primary twin operated predominantly. In a twin band, secondary twin operated, and resulted in the fatigue crack initiation. The crack initiation was strongly affected by Schmid factor of grain and twin band. Published byaccess Elsevier Ltd. © 2014 Elsevier The Authors. © 2014 Ltd. This is an open article under the CC BY-NC-ND license Selection and peer-review under responsibility of the Norwegian University of Science and Technology (NTNU), Department of (http://creativecommons.org/licenses/by-nc-nd/3.0/). Structural Engineering. Selection and peer-review under responsibility of the Norwegian University of Science and Technology (NTNU), Department of Structural Engineering Keywords: Type your keywords here, separated by semicolons ;
1. Introduction Magnesium (Mg) alloy has hcp structure, where plastic deformation at room temperature is induced only by basal slip and twining. It is well known that fatigue crack initiation behavior is strongly affected by crystallographic orientations. But the effect of crystallographic orientation on fatigue crack initiation in Mg alloy is not understood. In the present study, fatigue test was performed using Mg alloy, where crystallographic orientations were analyzed in advance, and subsequently fatigue crack initiation behavior was investigated based on time-series EBSD analysis.
* Corresponding author. Tel.: +81-58-293-2501; fax: +81-58-293-2491 E-mail address: [email protected]
2211-8128 © 2014 Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and peer-review under responsibility of the Norwegian University of Science and Technology (NTNU), Department of Structural Engineering doi:10.1016/j.mspro.2014.06.129
791
Yoshihiko Uematsu et al. / Procedia Materials Science 3 (2014) 790 – 792
2. Experimental procedure The material used is AZ31 magnesium roll plate with the average grain size of 15ȝm. The as-received material was friction stir processed to give sever plastic deformation, and subsequently annealed at 500ºC. The severe plastic deformation followed by annealing resulted in the abnormal grain growth, where the average grain size of 510ȝm. The grain coarsening was applied to observe transgranular fatigue crack initiation behavior. The mechanical properties are; ı0.2: 116MPa, ıB: 295MPa for the as-received material, and ı0.2: 46MPa, ıB: 157MPa for the graincoarsened material. The plate fatigue test specimen with the thickness of 4mm and the gauge length of 8mm was used for the plane bending fatigue test. Shallow notch was introduced at the center of the gauge section to limit the crack initiation site. Fatigue test was conducted at the load ratio R=-1 and frequency f=10Hz. 3. Results and discussion 3.1. Macroscopic EBSD analysis S-N diagram of the as-received and grain-coarsened materials is shown in Fig.1, where fatigue strength was decreased by the grain coarsening. Based on the S-N diagram, a fatigue test was conducted at the stress amplitude ıa=130MPa, where the number of cycles to failure was Nf§4.4×104cycles according to the S-N diagram. The fatigue test was periodically terminated and the specimen surface was observed by EBSD. The IPF map of the specimen surface before fatigue test (N/Nf=0%) is shown in Fig.2(a). The maximum grain size is about 680ȝm, and the grain orientation is almost random. It is considered that the texture induced during rolling process had disappeared due to the abnormal grain growth during annealing. The IPF map of the fatigued specimen is shown in Fig.2(b), where N/Nf=8%. Transgranular lines are densely recognized in coarse grains. It is considered that these lines were formed due to basal slip or twining deformation. EBSD analysis could reveal Euler angles of each grain, and based on those angles and angle analysis (Sugeta et al. (2007)), it was identified that the lines in Fig.2(b) were formed by the operation of primary twining of { 1012 } system.
Stress amplitude Va (MPa)
160
Laboratory air plane bending R=í1
140 120 100 AZ31 asíreceived coarse grains
80 10
4
10
5
10
6
10
7
Number of cycles to failure Nf
10
8
700ȝm
700ȝm
Fig. 1. S-N diagram.
Fig.2. EBSD image: (a) N/Nf = 0%, (b) N/Nf =8%.
792
Yoshihiko Uematsu et al. / Procedia Materials Science 3 (2014) 790 – 792
3.2. Fatigue crack initiation When the fatigued specimen (N/Nf=8%) was observed in detail, two small fatigue cracks were found on the specimen surface. Fig.3 shows SEM image and IPF map of one small fatigue crack with the length of 15ȝm. From the IPF map (Fig.3(b)), it was found that small crack initiated within the twin band with the width of 5ȝm. Based on the angle analysis, this twin was identified as primary ( 1012 ) twin. Three twin systems in this grain have Schmid factor (SF) of 0.159, 0.072 and 0.067, where ( 1012 ) twin has the smallest SF. But it was found that all three twin systems operated in this grain. It should be noted that the maximum SF of basal slip was 0.466, indicating that the basal slip system must have operated in this grain. EBSD analysis was also performed in the twin band shown in Fig.3(b), resulting in the maximum SF of 0.496 in basal slip system. In this twin band, fatigue crack with the length of 15ȝm was found but the crack was not parallel to the primary twin boundary. The primary twin band has the maximum SF of 0.209 for the secondary twin ( 1011 ), and the angle between secondary twin and horizontal line should be 77º based on the angle analysis (Sugeta et al. (2007)). This angle coincide with the angle of the initiated fatigue crack as shown in Fig.3(b). Based on the TEM observation, Koike et al. (2010) had revealed that secondary twin within primary twin band could be stress concentration site. Consequently, it could be assumed that primary twin band was firstly formed in the grain with high SF of basal slip system by cyclic loading, and subsequently secondary twin operated within the primary twin band with high SF of basal slip. Stress concentration induced by secondary twin within primary twin band resulted in small fatigue initiation. 4. Conclusion Plane bending fatigue test was conducted, and twining operation during fatigue test was observed by EBSD. It was found that primary twin operated and subsequently secondary twin operated within the primary twin band. The stress concentration at the secondary twin resulted in the fatigue crack initiation. References Sugeta, A., Uematsu, Y., Jono, M., 2007. A Study on the Mechanism of Small Fatigue Crack Deflection Behavior in Alpha-Brass by Means of In-Situ Atomic Force Microscopy and Crystallo-Graphic Orientation Analysis. Key Engineering 353-358, 1225-1228. Koike, J., Fujiyama, N., Ando, D., Sutou, Y., 2010. Roles of deformation twinning and dislocation slip in the fatigue failure mechanism of AZ31 Mg alloys. Scripta Materialia 63, 747-750.
loading direction Fig.3. Micrographs showing fatigue crack initiated from twin at N/Nf =8% : (a) SEM image and (b) EBSD image.
ScienceDirect Procedia Materials Science 3 (2014) 790 – 792
20th European Conference on Fracture (ECF20)
Crack Initiation Analysis in AZ31 Magnesium Alloy based on Electron Backscatter Diffraction (EBSD) Yoshihiko Uematsua, *, Toshifumi Kakiuchia, Yoshifumi Kamiyab b
a Gifu University, 1-1 Yanagido, Gifu 501-1193, Japan Murata Machinery Ltd., 163 Takeda-Mukaishiro-cho, Fushimi-ku, Kyoto 612-8686, Japan
Abstract Plane bending fatigue test had been conducted to figure out fatigue crack initiation mechanism in magnesium alloy, AZ31, with HCP crystallographic structure. The initial crystallographic structure was analyzed by an electron backscatter diffraction (EBSD) method. Subsequently, the fatigue test was periodically terminated and time-series EBSD analysis was performed. Basal slip and primary twin operated predominantly. In a twin band, secondary twin operated, and resulted in the fatigue crack initiation. The crack initiation was strongly affected by Schmid factor of grain and twin band. Published byaccess Elsevier Ltd. © 2014 Elsevier The Authors. © 2014 Ltd. This is an open article under the CC BY-NC-ND license Selection and peer-review under responsibility of the Norwegian University of Science and Technology (NTNU), Department of (http://creativecommons.org/licenses/by-nc-nd/3.0/). Structural Engineering. Selection and peer-review under responsibility of the Norwegian University of Science and Technology (NTNU), Department of Structural Engineering Keywords: Type your keywords here, separated by semicolons ;
1. Introduction Magnesium (Mg) alloy has hcp structure, where plastic deformation at room temperature is induced only by basal slip and twining. It is well known that fatigue crack initiation behavior is strongly affected by crystallographic orientations. But the effect of crystallographic orientation on fatigue crack initiation in Mg alloy is not understood. In the present study, fatigue test was performed using Mg alloy, where crystallographic orientations were analyzed in advance, and subsequently fatigue crack initiation behavior was investigated based on time-series EBSD analysis.
* Corresponding author. Tel.: +81-58-293-2501; fax: +81-58-293-2491 E-mail address: [email protected]
2211-8128 © 2014 Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and peer-review under responsibility of the Norwegian University of Science and Technology (NTNU), Department of Structural Engineering doi:10.1016/j.mspro.2014.06.129
791
Yoshihiko Uematsu et al. / Procedia Materials Science 3 (2014) 790 – 792
2. Experimental procedure The material used is AZ31 magnesium roll plate with the average grain size of 15ȝm. The as-received material was friction stir processed to give sever plastic deformation, and subsequently annealed at 500ºC. The severe plastic deformation followed by annealing resulted in the abnormal grain growth, where the average grain size of 510ȝm. The grain coarsening was applied to observe transgranular fatigue crack initiation behavior. The mechanical properties are; ı0.2: 116MPa, ıB: 295MPa for the as-received material, and ı0.2: 46MPa, ıB: 157MPa for the graincoarsened material. The plate fatigue test specimen with the thickness of 4mm and the gauge length of 8mm was used for the plane bending fatigue test. Shallow notch was introduced at the center of the gauge section to limit the crack initiation site. Fatigue test was conducted at the load ratio R=-1 and frequency f=10Hz. 3. Results and discussion 3.1. Macroscopic EBSD analysis S-N diagram of the as-received and grain-coarsened materials is shown in Fig.1, where fatigue strength was decreased by the grain coarsening. Based on the S-N diagram, a fatigue test was conducted at the stress amplitude ıa=130MPa, where the number of cycles to failure was Nf§4.4×104cycles according to the S-N diagram. The fatigue test was periodically terminated and the specimen surface was observed by EBSD. The IPF map of the specimen surface before fatigue test (N/Nf=0%) is shown in Fig.2(a). The maximum grain size is about 680ȝm, and the grain orientation is almost random. It is considered that the texture induced during rolling process had disappeared due to the abnormal grain growth during annealing. The IPF map of the fatigued specimen is shown in Fig.2(b), where N/Nf=8%. Transgranular lines are densely recognized in coarse grains. It is considered that these lines were formed due to basal slip or twining deformation. EBSD analysis could reveal Euler angles of each grain, and based on those angles and angle analysis (Sugeta et al. (2007)), it was identified that the lines in Fig.2(b) were formed by the operation of primary twining of { 1012 } system.
Stress amplitude Va (MPa)
160
Laboratory air plane bending R=í1
140 120 100 AZ31 asíreceived coarse grains
80 10
4
10
5
10
6
10
7
Number of cycles to failure Nf
10
8
700ȝm
700ȝm
Fig. 1. S-N diagram.
Fig.2. EBSD image: (a) N/Nf = 0%, (b) N/Nf =8%.
792
Yoshihiko Uematsu et al. / Procedia Materials Science 3 (2014) 790 – 792
3.2. Fatigue crack initiation When the fatigued specimen (N/Nf=8%) was observed in detail, two small fatigue cracks were found on the specimen surface. Fig.3 shows SEM image and IPF map of one small fatigue crack with the length of 15ȝm. From the IPF map (Fig.3(b)), it was found that small crack initiated within the twin band with the width of 5ȝm. Based on the angle analysis, this twin was identified as primary ( 1012 ) twin. Three twin systems in this grain have Schmid factor (SF) of 0.159, 0.072 and 0.067, where ( 1012 ) twin has the smallest SF. But it was found that all three twin systems operated in this grain. It should be noted that the maximum SF of basal slip was 0.466, indicating that the basal slip system must have operated in this grain. EBSD analysis was also performed in the twin band shown in Fig.3(b), resulting in the maximum SF of 0.496 in basal slip system. In this twin band, fatigue crack with the length of 15ȝm was found but the crack was not parallel to the primary twin boundary. The primary twin band has the maximum SF of 0.209 for the secondary twin ( 1011 ), and the angle between secondary twin and horizontal line should be 77º based on the angle analysis (Sugeta et al. (2007)). This angle coincide with the angle of the initiated fatigue crack as shown in Fig.3(b). Based on the TEM observation, Koike et al. (2010) had revealed that secondary twin within primary twin band could be stress concentration site. Consequently, it could be assumed that primary twin band was firstly formed in the grain with high SF of basal slip system by cyclic loading, and subsequently secondary twin operated within the primary twin band with high SF of basal slip. Stress concentration induced by secondary twin within primary twin band resulted in small fatigue initiation. 4. Conclusion Plane bending fatigue test was conducted, and twining operation during fatigue test was observed by EBSD. It was found that primary twin operated and subsequently secondary twin operated within the primary twin band. The stress concentration at the secondary twin resulted in the fatigue crack initiation. References Sugeta, A., Uematsu, Y., Jono, M., 2007. A Study on the Mechanism of Small Fatigue Crack Deflection Behavior in Alpha-Brass by Means of In-Situ Atomic Force Microscopy and Crystallo-Graphic Orientation Analysis. Key Engineering 353-358, 1225-1228. Koike, J., Fujiyama, N., Ando, D., Sutou, Y., 2010. Roles of deformation twinning and dislocation slip in the fatigue failure mechanism of AZ31 Mg alloys. Scripta Materialia 63, 747-750.
loading direction Fig.3. Micrographs showing fatigue crack initiated from twin at N/Nf =8% : (a) SEM image and (b) EBSD image.