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Cleavage and intergranular fracture in Al–Mg alloys
Scripta Materialia 49 (2003) 387–392 www.actamat-journals.com
Cleavage and intergranular fracture in Al–Mg alloys D.M. Jiang
a,*
, C.L. Wang a, J. Yu a, Z.Z. Gao b, Y.T. Shao b, Z.M. Hu a b
b
Harbin Institute of Technology, Harbin 150001, China Northeast Light Alloy Co. Ltd., Harbin 150060, China
Received 12 November 2002; received in revised form 15 May 2003; accepted 27 May 2003
Abstract Intergranular and cleavage fractures have been found in the commercial Al–Mg alloys with high Na content. Dimpled fracture has been found in the Al–Mg alloy with low Na content. Pit etching study indicated the cleavage fracture was {1 0 0} crystal plane. The tramp element Na was the embrittling agent. Ó 2003 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Aluminum alloys; Tension test; Fracture; Transmission electron microscopy; Tramp element
1. Introduction Al–Mg alloys are widely used in industry. The low-magnesium containing Al–Mg alloys have the best formability and corrosion resistance. The high-magnesium containing Al–Mg alloys have reasonable good castability and high strength. The corrosion problems plus the tendency to age-softening have thwarted effect to develop commercial alloys with high-magnesium contents than 5.5% in 5456 [1]. In Al–Mg alloys, stress corrosion cracking and hydrogen embrittlement cracking are almost exclusively intergranular. The intergranular fracture was embrittled without dimples and deformed marks on the surface [2,3]. With the increase of Mg content, the ductility of the Al–Mg alloys becomes worse. The alloys show high fracture tendency during hot or cold defor-
* Corresponding author. Tel.: +86-451-6414-234; fax: +86451-6414-178. E-mail address: [email protected] (D.M. Jiang).
mation [4,5]. However, the fracture behavior and the reason of this fracture are not clear entirely. High temperature embrittlement phenomenon was reported 40 years ago in the high purity Al– Mg alloys [5]. The embrittlement was ascribed to the adsorption of free Na on internal surfaces generated in plastic flow and consequent modification of the growth of grain boundary cavities. Recent study was made in the Al–5%Mg alloys with high purity. The embrittlement was thought to be caused by a trace of sodium, calcium or tritium [6–8]. In those conditions, the cracking was entirely intergranular and very widespread [5–8]. Sodium is a common impurity in primary aluminum and magnesium ingots of commercial grade and high purity, so that a trace amount of sodium in an Al–Mg alloy is not unusual. Cleavage fracture in Al–Mg alloy 5383 was only reported by Deschamps et al. [9]. The alloy showed cleavage facets and intergranular rupture during the secondary tensile test at 490–560 °C. In this paper, several commercial Al–Mg alloys were rolled or tensile tested at different temperature
1359-6462/03/$ - see front matter Ó 2003 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. doi:10.1016/S1359-6462(03)00304-X
388
D.M. Jiang et al. / Scripta Materialia 49 (2003) 387–392
to study the influence of the deformation condition and chemical composition on fracture behavior of the alloys.
2. Experimental The materials used were several Al–Mg alloys with commercial grade. The chemical compositions of the alloys are given in Table 1. The ingot was scalped and homogenized at 530 °C for 24 h, continuously rolled at the temperature ranges of 460–300 °C from the thickness of 280 to 20 mm by eight passes. Finally the plates were stretched 1.5% for flattening at room temperature. The alloy A1 was tensile tested at room temperature. The alloy showed tensile elongation of 20%. One plate from the alloy A2 was cracked during stretching at room temperature after hot rolling. The alloy showed very low tensile elongation (less than 1.5%). The alloy A3 was cracked after the first hot rolling at 450 °C. The fracture surfaces of the alloys were examined by a scanning electron microscope (S-570) equipped with an energy dispersive spectrometer. Microstructures of the alloys were studied by optical microscopy and transmission electron microscopy.
3. Results The tensile fracture of the alloy A1 is shown in Fig. 1. Overall fracture occurred as shear fracture with its surface parallel to the maximum shear stress direction. All the fracture surfaces were covered with dimples. The tensile elongation of the alloy was 22%, which is also the normal value for commercial Al alloys. The alloy A2 exhibited very low tensile elongation with less than 1.5%. Mixed fracture with
Fig. 1. Overall tensile fracture in the alloy A1 (SEM).
intergranular rupture and cleavage facets was found by SEM observation. The overall fracture surface was perpendicular to the tensile axis. Fig. 2 shows the overall morphology of the tensile fracture. A large proportion of the fracture was intergranular. The fracture was embrittled with very smooth appearance. Dimples and secondary phase particles could be seen on the intergranular fracture surface. Those dimples were isolated and large in size. No small dimples and deformed marks were found on the fracture surface (Fig. 3). The cleavage facets were perpendicular to the tensile axis. The cleavage fracture initiated in the grains and propagated outside. It was also observed that the cleavage fracture initiated on the grain boundary and propagated to one side. River patterns could be seen clearly on the fracture surface (Fig. 4). Quantitative analysis gave 8–10% of cleavage fracture in all the fracture. The other part of the fracture was intergranular rupture. The alloy A3 showed similar fracture morphology to the alloy A2. The overall fracture
Table 1 Chemical compositions of the Al–Mg alloys (wt%) Alloy
Mg
Mn
Fe
Si
Cu
Zn
Ti
Ni
Na
Al
A1 A2 A3
5.40 4.70 6.35
0.65 0.65 0.70
0.18 0.18 0.15
0.12 0.10 0.12
0.04 0.05 0.10
0.10 0.15 0.06
0.09 0.07 0.06
0.03 0.05 0.03
0.0004 0.0013 0.0015
Bal. Bal. Bal.
D.M. Jiang et al. / Scripta Materialia 49 (2003) 387–392
389
Fig. 2. Overall tensile fracture in the alloy A2 (SEM).
Fig. 4. Cleavage fracture in the alloy A2 (SEM).
Fig. 3. Intergranular fracture in the alloy A2 (SEM).
Fig. 5. Overall tensile fracture in the alloy A3 (SEM).
surface was perpendicular to the tensile axis, mixed with intergranular rupture and cleavage facets. However, the cleavage fracture was dominant (Fig. 5). Quantitative analysis gave 75% of cleavage fracture in all the fracture. The other part of the fracture was intergranular rupture. Isolated dimples and secondary phase particles could be seen on the intergranular fracture surface. The size of the cleavage facets are much larger in the A3 than that in the A2 alloy.
EDS analysis on the fracture surface of all the alloys revealed that the secondary phase particles on the fracture surface have high Fe or Si content, which indicates that those particles are common inclusions in Al alloys. Both the cleavage facet and the smooth area on the surface of intergranular fracture of the alloys A2 and A3 showed almost the similar composition as the bulk composition given by the chemical method in Table 1. Sodium was not detected on the surfaces of the
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D.M. Jiang et al. / Scripta Materialia 49 (2003) 387–392
Table 2 Chemical compositions of the alloy A3 on the fracture surface (wt%) Alloy
Mg
Mn
Fe
Si
Cu
Zn
Ti
Ni
Na
Al
A B C
6.80 6.30 6.35
0.65 0.72 0.70
0.17 0.07 0.15
0.12 0.04 0.12
0.04 0.02 0.10
0.08 0.03 0.06
0.02 0.00 0.06
0.00 0.01 0.03
0.0000 0.0000 0.0015
Bal. Bal. Bal.
A: intergranular fracture surface; B: cleavage fracture surface; C: bulk composition given by chemical method.
intergranular fracture and cleavage fracture. The result of the EDS analysis for alloy A3 has been given in Table 2. Pit etching study found square etched pits on the cleavage fracture surface, which indicates that the cleavage fracture surface is {1 0 0} plane. Fig. 6 shows the etched pits on the cleavage fracture surface of the A3 alloy. The specimens for microstructure observation were machined before tensile tests for the alloy A1, after stretching 1.5% for the alloy A2 and after the first hot rolling at 450 °C for the alloy A3. The alloys A1 and A2 exhibited unrecrystallized microstructure with elongated grains. While the alloy A3 exhibited equiaxed grains. Fig. 7 shows the optical micrographs of the alloys A2 and A3. Transmission electron microscopic observation found that the A1 and A2 alloys exhibited subgrains of 2–4 lm in the grains. Subgrains were
Fig. 7. Optical micrographs of the different alloys: (a) alloy A2; (b) alloy A3.
much coarse and did not developed well in the alloy A3. 4. Discussion Fig. 6. Etched pits on the cleavage fracture surface of the alloy A3.
Fracture that is associated with only small amount of plastic deformation and characterized
D.M. Jiang et al. / Scripta Materialia 49 (2003) 387–392
by fracture planes parallel to low-index crystallographic planes is known as cleavage. In an inert environment, cleavage is common for bcc and hcp metals and alloys at low temperature. But the cleavage fracture is also observed in two fcc metals, namely iridium and rhodium [10–12]. For other fcc metals and alloys, cleavage can only occur in certain embrittling environment. In the literature, few observations of cleavage in aluminum and aluminum alloys have been reported. Aluminum alloys usually showed cleavage fracture in some specific environment, which made the alloys brittle by stress corrosion cracking or hydrogen embrittlement mechanics [13–15]. For commercial aluminum alloys, cleavage in laboratory environment was only reported in some commercial Al–Li alloys [15,16]. The size of the facets in the alloy was 10–30 lm, much smaller than the grain size of the alloy, which is different to the common cleavage fracture since cleavage cracks normally propagate until they reach the grain boundary. The results also showed the proportion and size of the cleavage facets decreased as strain rate increased. It looks as though the cleavage is controlled by a diffusion mechanism. Sodium was detected on the surface of the cleavage fracture and the effect of sodium might be similar to that of hydrogen in hydrogen embrittlement. The result also showed that the tendency to intergranular fracture increased with the Na content in the Al–Li alloy. In order to study the effect of Na on fracture mode, Na was added to the high purity Al–Zn– Mg–Cu alloy 7075. Cleavage facets were also found in the alloy with high Na content (35 ppm) when tested in the room temperature [17]. The alloy showed similar feature to the Al–Li alloys. In this research, Na was also detected on the cleavage fracture surface. The percentage of the cleavage facets was very low and the alloys exhibited macro-ductile rupture. In commercial aluminum alloys, ductile fracture with the dimples is the usual fracture mode. Intergranular fracture in commercial Al–Mg alloy was only found after SCC and fatigue tests [2–4]. In the high purity Al–5.5at%Mg alloy, Na, Ca and Sr could induce high temperature embrittlement based on intergranular fracture at around 300 °C
391
depending on strain rate [6–8]. The EDS and AES studies revealed that the embrittlement caused by Na, Ca and Sr, for those elements were detected on the fracture surface. The alloy exhibited macrobrittle rupture with very low reduction in area in this condition. Recently, cleavage in Al–Mg alloy was reported with striking similarities [9]. The possible reason of this cleavage fracture was proposed by liquid phase appears at the grain boundaries. If a sufficient number of grain boundaries were locally embrittled, the load transferred to the other grains became large enough to induce cleavage. However, this paper did not give the content of Na as well as its effect. There was also a big difference in cleavage and intergranular fracture in the different Al alloys induced by Na. The cleavage fracture in the Al–Li and Al–Zn–Mg–Cu alloys occurred at the room temperature. The percentage of the cleavage facets was less than 10% and the alloys showed macroductile fracture. Whereas the intergranular fracture occurred at high temperature in Al–Mg alloys. The percentage of the intergranular fracture could be more than 90%, so the alloy showed macro-brittle fracture with very low tensile elongation compared with the same alloy without intergranular fracture. From the present result, it is clear that Na made the alloys A2 and A3 brittle and induced intergranular and cleavage fracture. The main difference for the alloys A1, A2 and A3 is their Na content. The alloys A2 and A3 have higher Na content (13–15 ppm) and show mixed fractures with intergranular rupture and cleavage facets. Whereas the alloy A1 has lower Na content and shows dimpled fracture. However, Na was not detected on the surfaces of the cleavage and intergranular fracture. The alloy A2 showed tensile elongation of less than 1.5%. This value is much lower than the normal ductility of the Al–Mg alloys. The alloy A3 cracked at high temperature and the rolling reduction was only 10%. So the alloy also exhibited very low ductility. The Al–Mg alloys could exhibit macro-brittle fractures with intergranular rupture and cleavage facets. This result is unique and never reported before.
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D.M. Jiang et al. / Scripta Materialia 49 (2003) 387–392
For commercial aluminum alloys, macro-brittle fracture scarcely occurred at room temperature in laboratory environment. So it is possible that the cracks in the alloy A2 initiate during hot rolling at high temperature. Then the cracks developed and fracture occurred during stretching. In order to examine this presumption, 12 specimens were machined from the cracked alloy A3 plate and tensile tested at the temperature range 30–520 °C. The alloy showed more than 70% of cleavage fracture when it was tensile tested at 520 °C. At the room temperature, the alloy showed ductile fracture with dimples. Present results also show that Na elements could induce brittle fracture with intergranular rupture or cleavage facets in Al–Mg alloys. Comparatively, the alloys have higher tendency to intergranular fracture than cleavage fracture. Only 2 ppm Na elements could induce intergranular fracture in high purity Al–Mg alloys [8]. Si, Bi and Sb [5,8] could form the compounds with Na so as to decrease the deleterious effect. In those condition, cleavage fracture did not occur. For the alloys tested were with commercial grade, other element and impurities might also decrease the tendency to intergranular fracture. If the Na content was high enough, it could induce cleavage fracture in those alloys. However, this is just the preliminary result, and systematic research is required.
5. Conclusion The Al–Mg alloys with high Na content exhibit mixed fracture with intergranular rupture and cleavage facets. The alloy with low Na content exhibits ductile fracture with dimples. The cleav-
age fracture is first found in commercial Al alloys except in Al–Li alloy. The tramp element Na is the embrittling agent.
Acknowledgement The financial support of the Natural Science Fund of Heilongjiang Province is greatly acknowledged.
References [1] Staley JT. In: Vasudevan AK, Doherty RD, editors. Aluminum––contemporary research and applications. New York: Academic Press; 1989. p. 14–7. [2] Higashi K, Ohnishi T, Nagatani Y, Okabayashi K. J Jpn Inst Light Met 1981;31:386–412. [3] Higashi K, Ohnishi T, Nagatani Y, Okabayashi K. J Jpn Inst Light Met 1981;31:644. [4] Jiang D, Kang SB, Kim HW, Ko HS. Sci Technol Weld Joining 2000;5:183. [5] Ransley CE, Talbot DEJ. J Inst Met 1959–60;88:150. [6] Horikava K, Kuramoto S, Kanno M. In: Proceedings of 6th International Conference on Aluminum Alloys, ICAA6, Toyohashi, Japan; 1998. p. 1021. [7] Okada H, Kanno M. Scripta Mater 1997;37:781. [8] Ueda K, Horikawa K, Kanno M. Scripta Mater 1997;37:1105. [9] Deschamps A, Peron S, Brechet Y, Ehestrom J-C, Poizat L. Mater Sci Eng A 2001;319–321:583. [10] Gandhi C, Ashby MF. Scripta Metall 1979;13:371. [11] Hecker S, Rour DL, Stein DF. Metall Trans 1978;9A:481. [12] Westwood ARC, Preece CM, Kamdar MH. ASM Trans 1976;60:211. [13] Wanhill RJH. Corrosion 1974;30:371. [14] Koch GH. Corrosion 1979;35:73. [15] Nelson JL, Pugh EN. Metall Trans 1975;6A:1495. [16] Jiang D, Wang Y, Hong B, Lei TC, Sun FL. J Mater Sci Lett 1996;15:1597. [17] Miller WS, Thomas MP, White J. Scripta Mater 1987;21:663.
Cleavage and intergranular fracture in Al–Mg alloys D.M. Jiang
a,*
, C.L. Wang a, J. Yu a, Z.Z. Gao b, Y.T. Shao b, Z.M. Hu a b
b
Harbin Institute of Technology, Harbin 150001, China Northeast Light Alloy Co. Ltd., Harbin 150060, China
Received 12 November 2002; received in revised form 15 May 2003; accepted 27 May 2003
Abstract Intergranular and cleavage fractures have been found in the commercial Al–Mg alloys with high Na content. Dimpled fracture has been found in the Al–Mg alloy with low Na content. Pit etching study indicated the cleavage fracture was {1 0 0} crystal plane. The tramp element Na was the embrittling agent. Ó 2003 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Aluminum alloys; Tension test; Fracture; Transmission electron microscopy; Tramp element
1. Introduction Al–Mg alloys are widely used in industry. The low-magnesium containing Al–Mg alloys have the best formability and corrosion resistance. The high-magnesium containing Al–Mg alloys have reasonable good castability and high strength. The corrosion problems plus the tendency to age-softening have thwarted effect to develop commercial alloys with high-magnesium contents than 5.5% in 5456 [1]. In Al–Mg alloys, stress corrosion cracking and hydrogen embrittlement cracking are almost exclusively intergranular. The intergranular fracture was embrittled without dimples and deformed marks on the surface [2,3]. With the increase of Mg content, the ductility of the Al–Mg alloys becomes worse. The alloys show high fracture tendency during hot or cold defor-
* Corresponding author. Tel.: +86-451-6414-234; fax: +86451-6414-178. E-mail address: [email protected] (D.M. Jiang).
mation [4,5]. However, the fracture behavior and the reason of this fracture are not clear entirely. High temperature embrittlement phenomenon was reported 40 years ago in the high purity Al– Mg alloys [5]. The embrittlement was ascribed to the adsorption of free Na on internal surfaces generated in plastic flow and consequent modification of the growth of grain boundary cavities. Recent study was made in the Al–5%Mg alloys with high purity. The embrittlement was thought to be caused by a trace of sodium, calcium or tritium [6–8]. In those conditions, the cracking was entirely intergranular and very widespread [5–8]. Sodium is a common impurity in primary aluminum and magnesium ingots of commercial grade and high purity, so that a trace amount of sodium in an Al–Mg alloy is not unusual. Cleavage fracture in Al–Mg alloy 5383 was only reported by Deschamps et al. [9]. The alloy showed cleavage facets and intergranular rupture during the secondary tensile test at 490–560 °C. In this paper, several commercial Al–Mg alloys were rolled or tensile tested at different temperature
1359-6462/03/$ - see front matter Ó 2003 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. doi:10.1016/S1359-6462(03)00304-X
388
D.M. Jiang et al. / Scripta Materialia 49 (2003) 387–392
to study the influence of the deformation condition and chemical composition on fracture behavior of the alloys.
2. Experimental The materials used were several Al–Mg alloys with commercial grade. The chemical compositions of the alloys are given in Table 1. The ingot was scalped and homogenized at 530 °C for 24 h, continuously rolled at the temperature ranges of 460–300 °C from the thickness of 280 to 20 mm by eight passes. Finally the plates were stretched 1.5% for flattening at room temperature. The alloy A1 was tensile tested at room temperature. The alloy showed tensile elongation of 20%. One plate from the alloy A2 was cracked during stretching at room temperature after hot rolling. The alloy showed very low tensile elongation (less than 1.5%). The alloy A3 was cracked after the first hot rolling at 450 °C. The fracture surfaces of the alloys were examined by a scanning electron microscope (S-570) equipped with an energy dispersive spectrometer. Microstructures of the alloys were studied by optical microscopy and transmission electron microscopy.
3. Results The tensile fracture of the alloy A1 is shown in Fig. 1. Overall fracture occurred as shear fracture with its surface parallel to the maximum shear stress direction. All the fracture surfaces were covered with dimples. The tensile elongation of the alloy was 22%, which is also the normal value for commercial Al alloys. The alloy A2 exhibited very low tensile elongation with less than 1.5%. Mixed fracture with
Fig. 1. Overall tensile fracture in the alloy A1 (SEM).
intergranular rupture and cleavage facets was found by SEM observation. The overall fracture surface was perpendicular to the tensile axis. Fig. 2 shows the overall morphology of the tensile fracture. A large proportion of the fracture was intergranular. The fracture was embrittled with very smooth appearance. Dimples and secondary phase particles could be seen on the intergranular fracture surface. Those dimples were isolated and large in size. No small dimples and deformed marks were found on the fracture surface (Fig. 3). The cleavage facets were perpendicular to the tensile axis. The cleavage fracture initiated in the grains and propagated outside. It was also observed that the cleavage fracture initiated on the grain boundary and propagated to one side. River patterns could be seen clearly on the fracture surface (Fig. 4). Quantitative analysis gave 8–10% of cleavage fracture in all the fracture. The other part of the fracture was intergranular rupture. The alloy A3 showed similar fracture morphology to the alloy A2. The overall fracture
Table 1 Chemical compositions of the Al–Mg alloys (wt%) Alloy
Mg
Mn
Fe
Si
Cu
Zn
Ti
Ni
Na
Al
A1 A2 A3
5.40 4.70 6.35
0.65 0.65 0.70
0.18 0.18 0.15
0.12 0.10 0.12
0.04 0.05 0.10
0.10 0.15 0.06
0.09 0.07 0.06
0.03 0.05 0.03
0.0004 0.0013 0.0015
Bal. Bal. Bal.
D.M. Jiang et al. / Scripta Materialia 49 (2003) 387–392
389
Fig. 2. Overall tensile fracture in the alloy A2 (SEM).
Fig. 4. Cleavage fracture in the alloy A2 (SEM).
Fig. 3. Intergranular fracture in the alloy A2 (SEM).
Fig. 5. Overall tensile fracture in the alloy A3 (SEM).
surface was perpendicular to the tensile axis, mixed with intergranular rupture and cleavage facets. However, the cleavage fracture was dominant (Fig. 5). Quantitative analysis gave 75% of cleavage fracture in all the fracture. The other part of the fracture was intergranular rupture. Isolated dimples and secondary phase particles could be seen on the intergranular fracture surface. The size of the cleavage facets are much larger in the A3 than that in the A2 alloy.
EDS analysis on the fracture surface of all the alloys revealed that the secondary phase particles on the fracture surface have high Fe or Si content, which indicates that those particles are common inclusions in Al alloys. Both the cleavage facet and the smooth area on the surface of intergranular fracture of the alloys A2 and A3 showed almost the similar composition as the bulk composition given by the chemical method in Table 1. Sodium was not detected on the surfaces of the
390
D.M. Jiang et al. / Scripta Materialia 49 (2003) 387–392
Table 2 Chemical compositions of the alloy A3 on the fracture surface (wt%) Alloy
Mg
Mn
Fe
Si
Cu
Zn
Ti
Ni
Na
Al
A B C
6.80 6.30 6.35
0.65 0.72 0.70
0.17 0.07 0.15
0.12 0.04 0.12
0.04 0.02 0.10
0.08 0.03 0.06
0.02 0.00 0.06
0.00 0.01 0.03
0.0000 0.0000 0.0015
Bal. Bal. Bal.
A: intergranular fracture surface; B: cleavage fracture surface; C: bulk composition given by chemical method.
intergranular fracture and cleavage fracture. The result of the EDS analysis for alloy A3 has been given in Table 2. Pit etching study found square etched pits on the cleavage fracture surface, which indicates that the cleavage fracture surface is {1 0 0} plane. Fig. 6 shows the etched pits on the cleavage fracture surface of the A3 alloy. The specimens for microstructure observation were machined before tensile tests for the alloy A1, after stretching 1.5% for the alloy A2 and after the first hot rolling at 450 °C for the alloy A3. The alloys A1 and A2 exhibited unrecrystallized microstructure with elongated grains. While the alloy A3 exhibited equiaxed grains. Fig. 7 shows the optical micrographs of the alloys A2 and A3. Transmission electron microscopic observation found that the A1 and A2 alloys exhibited subgrains of 2–4 lm in the grains. Subgrains were
Fig. 7. Optical micrographs of the different alloys: (a) alloy A2; (b) alloy A3.
much coarse and did not developed well in the alloy A3. 4. Discussion Fig. 6. Etched pits on the cleavage fracture surface of the alloy A3.
Fracture that is associated with only small amount of plastic deformation and characterized
D.M. Jiang et al. / Scripta Materialia 49 (2003) 387–392
by fracture planes parallel to low-index crystallographic planes is known as cleavage. In an inert environment, cleavage is common for bcc and hcp metals and alloys at low temperature. But the cleavage fracture is also observed in two fcc metals, namely iridium and rhodium [10–12]. For other fcc metals and alloys, cleavage can only occur in certain embrittling environment. In the literature, few observations of cleavage in aluminum and aluminum alloys have been reported. Aluminum alloys usually showed cleavage fracture in some specific environment, which made the alloys brittle by stress corrosion cracking or hydrogen embrittlement mechanics [13–15]. For commercial aluminum alloys, cleavage in laboratory environment was only reported in some commercial Al–Li alloys [15,16]. The size of the facets in the alloy was 10–30 lm, much smaller than the grain size of the alloy, which is different to the common cleavage fracture since cleavage cracks normally propagate until they reach the grain boundary. The results also showed the proportion and size of the cleavage facets decreased as strain rate increased. It looks as though the cleavage is controlled by a diffusion mechanism. Sodium was detected on the surface of the cleavage fracture and the effect of sodium might be similar to that of hydrogen in hydrogen embrittlement. The result also showed that the tendency to intergranular fracture increased with the Na content in the Al–Li alloy. In order to study the effect of Na on fracture mode, Na was added to the high purity Al–Zn– Mg–Cu alloy 7075. Cleavage facets were also found in the alloy with high Na content (35 ppm) when tested in the room temperature [17]. The alloy showed similar feature to the Al–Li alloys. In this research, Na was also detected on the cleavage fracture surface. The percentage of the cleavage facets was very low and the alloys exhibited macro-ductile rupture. In commercial aluminum alloys, ductile fracture with the dimples is the usual fracture mode. Intergranular fracture in commercial Al–Mg alloy was only found after SCC and fatigue tests [2–4]. In the high purity Al–5.5at%Mg alloy, Na, Ca and Sr could induce high temperature embrittlement based on intergranular fracture at around 300 °C
391
depending on strain rate [6–8]. The EDS and AES studies revealed that the embrittlement caused by Na, Ca and Sr, for those elements were detected on the fracture surface. The alloy exhibited macrobrittle rupture with very low reduction in area in this condition. Recently, cleavage in Al–Mg alloy was reported with striking similarities [9]. The possible reason of this cleavage fracture was proposed by liquid phase appears at the grain boundaries. If a sufficient number of grain boundaries were locally embrittled, the load transferred to the other grains became large enough to induce cleavage. However, this paper did not give the content of Na as well as its effect. There was also a big difference in cleavage and intergranular fracture in the different Al alloys induced by Na. The cleavage fracture in the Al–Li and Al–Zn–Mg–Cu alloys occurred at the room temperature. The percentage of the cleavage facets was less than 10% and the alloys showed macroductile fracture. Whereas the intergranular fracture occurred at high temperature in Al–Mg alloys. The percentage of the intergranular fracture could be more than 90%, so the alloy showed macro-brittle fracture with very low tensile elongation compared with the same alloy without intergranular fracture. From the present result, it is clear that Na made the alloys A2 and A3 brittle and induced intergranular and cleavage fracture. The main difference for the alloys A1, A2 and A3 is their Na content. The alloys A2 and A3 have higher Na content (13–15 ppm) and show mixed fractures with intergranular rupture and cleavage facets. Whereas the alloy A1 has lower Na content and shows dimpled fracture. However, Na was not detected on the surfaces of the cleavage and intergranular fracture. The alloy A2 showed tensile elongation of less than 1.5%. This value is much lower than the normal ductility of the Al–Mg alloys. The alloy A3 cracked at high temperature and the rolling reduction was only 10%. So the alloy also exhibited very low ductility. The Al–Mg alloys could exhibit macro-brittle fractures with intergranular rupture and cleavage facets. This result is unique and never reported before.
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D.M. Jiang et al. / Scripta Materialia 49 (2003) 387–392
For commercial aluminum alloys, macro-brittle fracture scarcely occurred at room temperature in laboratory environment. So it is possible that the cracks in the alloy A2 initiate during hot rolling at high temperature. Then the cracks developed and fracture occurred during stretching. In order to examine this presumption, 12 specimens were machined from the cracked alloy A3 plate and tensile tested at the temperature range 30–520 °C. The alloy showed more than 70% of cleavage fracture when it was tensile tested at 520 °C. At the room temperature, the alloy showed ductile fracture with dimples. Present results also show that Na elements could induce brittle fracture with intergranular rupture or cleavage facets in Al–Mg alloys. Comparatively, the alloys have higher tendency to intergranular fracture than cleavage fracture. Only 2 ppm Na elements could induce intergranular fracture in high purity Al–Mg alloys [8]. Si, Bi and Sb [5,8] could form the compounds with Na so as to decrease the deleterious effect. In those condition, cleavage fracture did not occur. For the alloys tested were with commercial grade, other element and impurities might also decrease the tendency to intergranular fracture. If the Na content was high enough, it could induce cleavage fracture in those alloys. However, this is just the preliminary result, and systematic research is required.
5. Conclusion The Al–Mg alloys with high Na content exhibit mixed fracture with intergranular rupture and cleavage facets. The alloy with low Na content exhibits ductile fracture with dimples. The cleav-
age fracture is first found in commercial Al alloys except in Al–Li alloy. The tramp element Na is the embrittling agent.
Acknowledgement The financial support of the Natural Science Fund of Heilongjiang Province is greatly acknowledged.
References [1] Staley JT. In: Vasudevan AK, Doherty RD, editors. Aluminum––contemporary research and applications. New York: Academic Press; 1989. p. 14–7. [2] Higashi K, Ohnishi T, Nagatani Y, Okabayashi K. J Jpn Inst Light Met 1981;31:386–412. [3] Higashi K, Ohnishi T, Nagatani Y, Okabayashi K. J Jpn Inst Light Met 1981;31:644. [4] Jiang D, Kang SB, Kim HW, Ko HS. Sci Technol Weld Joining 2000;5:183. [5] Ransley CE, Talbot DEJ. J Inst Met 1959–60;88:150. [6] Horikava K, Kuramoto S, Kanno M. In: Proceedings of 6th International Conference on Aluminum Alloys, ICAA6, Toyohashi, Japan; 1998. p. 1021. [7] Okada H, Kanno M. Scripta Mater 1997;37:781. [8] Ueda K, Horikawa K, Kanno M. Scripta Mater 1997;37:1105. [9] Deschamps A, Peron S, Brechet Y, Ehestrom J-C, Poizat L. Mater Sci Eng A 2001;319–321:583. [10] Gandhi C, Ashby MF. Scripta Metall 1979;13:371. [11] Hecker S, Rour DL, Stein DF. Metall Trans 1978;9A:481. [12] Westwood ARC, Preece CM, Kamdar MH. ASM Trans 1976;60:211. [13] Wanhill RJH. Corrosion 1974;30:371. [14] Koch GH. Corrosion 1979;35:73. [15] Nelson JL, Pugh EN. Metall Trans 1975;6A:1495. [16] Jiang D, Wang Y, Hong B, Lei TC, Sun FL. J Mater Sci Lett 1996;15:1597. [17] Miller WS, Thomas MP, White J. Scripta Mater 1987;21:663.