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A study on the precipitates in T4 treated Al-Mg alloys
Scripta Materiab,
Vol. 34, No. 3, pp. 357-362, 1996 Elsevier Science Ltd Copyright 0 1996 Acta Metallurgica Inc. F’rinted in the USA. All rights reserved 1359-6462/96 $12.00 + .OO
Perganron 0956-716X(95)00487-4
A STUDY ON THE PRECIPITATES IN T4 TREATED Al-Mg ALLOYS W.D. Fei’ and S.B. Kang2 ‘P.O. Box 433, Harbin Institute of Technology, Harbin 150001, P. R. China 2Korea Institute of Machinery and Metals, Changwon 64 1-O10, Korea (Received October 14, 1994) (Revised April 11, 1995) Introduction
Al-Mg alloys have been widely used as auto-body sheet materials because of their high strength to weight ratio. A disadvrmtage of Al-Mg alloys is a lower formability compared with low carbon steels. Most studies on Al-Mg alloys, therefore, have focussed on the investigation of mechanical properties and formability and their relation to microstructures [l-5]. The mechanical properties and formability of these alloys can be modified by heat treatment [6]. T4 heat treatment of Al-Mg alloys was found to improve the corrosion resistance, mechanical properties and formability [6,7]. One problem associated with heat treating auto-body sheets is that the size of sheets is so large that the cooling rate varies greatly from region to region. In the region where the cooling rate is relatively low, the second phase particles can be easily formed especially for Al-Mg alloys with high Mg content. The presence of the second phase particles can affect the mechanical properties of Al-Mg alloys. It is well known that the !supersaturated Al-Mg solid solution is transformed into a-Al and p phase. The p phase is usually given as Al, Mg, (37.3 wt% magnesium), although this composition is outside the limits of existence (34.7 to 37.1 wt%) [8]. The formula Al, Mg, (36 wt”/omagnesium) fits the composition of the solid phase and most of the proposed structure [8,9]. Three different kinds of crystalline structure have been suggested for Al 8 Mg 5 phase [S], but it has not been well understood about crystalline structure of Al, Mg,. In the Al-Mg alloys containing Mg in the range of 5.2 wt% to 7.5 wt%, the strength and ductility were increased linearly with Mg content increment. [lo] As a part of developing new materials of AI-Mg alloys used for auto-body sheet materials, various binary Al-Mg alloys containing 2.46 wt”/ou pto 11.07 WV?? Mg were considered. [ 1I] The purpose of this study is to examine the crystal structure of 8 precipitate in these Al-Mg alloys with different Mg content in more detail. ExDerimental
Procedures
Magnesium contents of two different binary Al-Mg alloys used in this study were 2.46 wt% and 11.07 wt%, respectively. The 25mm thick ingots were homogenized at 480°C for 24h, then hot rolled down to 3.Omm thick she:etswith a reduction ratio of 86% and cold rolled to 0.5mm thick sheets with a reduction
3.57
358
PRECIPITATION
rN AI-Mg ALLOYS
Vol. 34, No. 3
ratio of 83%. The specimens were cut from the sheets with the thickness of 0.5mm, and heat treated at 450°C in air for 30min and quenched into water at room temperature. TEM specimens were thinned by the standard chemical thinning method. TEM observation was carried out using JEOL-2000 type transmission electron microscope. The structures of the precipitates were examined by selected area diffraction (SAD) technique. The accelerating voltage was 200kV and the camera length was about 100cm. Results and Discussion
The microstructure and SAD patterns for an Al-2.46wt% Mg alloy are shown in Fig. 1. Fig. 1(b) and Fig. 1(d) show SAD patterns taken from area “A” and area “A”&“B”, respectively as indicated in Fig. 1(a). From Fig. l(a), (b) and (c), it was found that there exist three phases in this alloy. The first one is the matrix, a-Al, and the second and the third ones are precipitates. It is necessary to note that the materials were homogenized at 480°C for 24h before rolling process. According to the phase diagram of Al-Mg system [8], it can be confirmed that the phases indicated by “A” and “B” in Fig. l(a) are precipitates. In Fig. 1 (b) and (c), the diffraction spots can be divided into two groups; one is the diffraction pattern of aluminum, and the other is that of precipitate. It can be found that the precipitate has FCC structure and the lattice constant is about 1.2Omn. This agrees with the lattice constant (1.24mn) of Al, Mg, phase having FCC structure [8], so this type of Al, Mg, phase is symbolized by p, phase. There are many regular diffraction spots arising from the double diffraction. This can be explained by the fact that the unit cell of Al, Mg, is much larger than that of aluminum [ 12,131.Double di%-action spots are not described in Fig. l(c). From the diffraction pattern shown in Fig. l(b) and (c), the orientation relationship between the matrix, a-Al, and the precipitate phase, p l phase, can be drawn as follows: W0IAI II U331,IY <013>*, II
From Fig. 1 (a), (b) and (c), it was determined that the precipitate in area “A” was p phase having FCC structure (p,) and the one in area “B” was another phase with a different crystal structure. The diffraction pattern taken from area “A” and “B” is shown in Fig. l(d). The double diffraction spots were also observed clearly in the SAD pattern. The index of diffraction spots excluding the double diffraction spots is shown in Fig. l(e). It can be determined that the precipitate in area “B” has a hexagonal structure and the lattice constants are about a=l.O3mn, c=l.68mn. This agrees with the lattice constants (a=1 .OOOnm,c=l.636nm) of p’phase given by reference [ 141. So the precipitate in area “B” shown in Fig. l(a) is considered as p’ phase. A TEM micrograph and SAD patterns of an Al-l l.O7wt% Mg alloy are shown in Fig. 2. From Fig. 2(a), the precipitate in area “A” can be determined as p , phase having FCC structure and the one in area “B” has a hexagonal structure with the lattice constants of a=l.l2nm, c=l.64nm. This agrees with the values (a=1.12-l.l38mn, c=1.60-1.788nm) ofthe l3phase having hexagonal structure [8]. The p phase having hexagonal structure is symbolized by p2phase. From Fig. 2 (c), the orientation relationship among a-Al and two types of p phase in an Al- ll.O7wt% Mg alloy can be determined as follows: {311)r., II 10110~P~,<114’P, II <2116’P, {I3I],, II {I31]~,?<3IO’,, ll
Vol. 34, No. 3
PRECIPITATION IN Al-Mg ALLOYS
359
63
Figure 1. TEM photographs of Al-2.46wt?hMg alloy, (a) morphology, (b) and (c) are the SAD patterns from area “A” indicated in (a), while (d) and (e) are the SAD patterns from area “A” and “B” indicated in (a).
360
PRECIPITATION
IN Al-Mg
ALLOYS
Vol. 34, No. 3
-_
Olllgz 10128,
Figure 2. TEM photographs of Al-l I .07wt%Mg alloy, (a) morphology, (b) and (c) are the SAD patterns from area “A” indicated in (a), while (d) and (e) are the SAD patterns from area “B” indicated in (a).
Vol. 34, No. 3
PRECIPITATION IN Al-Mg ALLOYS
10
20
30
40 so 60 ~1% Mg
70
361
80
90
loo Mg
Figure 3. Phase diagram of AI-Mg system
hexagonal structure. This can be understood from the difference of composition and the transformation temperature between these two alloys. Fig. 3 shows the phase digram of AI-Mg system, and the dashed line illustrates the schematic diagram of pseudo phase diagram at the nonequilibrium condition. From Fig 3, it is obvious that the transformation temperature in the high Mg containing Al-Mg alloy is much higher than that in the low Mg containing Al-Mg alloy. It is reported that the p 1phase is stable one and p’phase is metastable one in Al-Mg alloy [S]. The p, phase can only form at high temperature, while p’phase can form at low temperature [S]. In the high Mg containing Al-Mg alloy, the transformation occurs at high temperature and long elapsed time. Two types of p phases (p, and p,) were observed. While, in the low Mg containing Al-Mg alloy, the transformation occurs relatively at low temperature and short elapsed time. Stable p phase (p ,) and metastable p’phase were observed. The oriental.ion relationship between cl-Al and p, phase in Al-2.46wt% Mg alloy is different from that in Al-l l.O7wt% Mg alloy. It may be due to the different lattice constant of a-Al matrix in two alloys, but the lattice constant of p, phase is constant. In this case, the orientation relationship may change to reduce the elastic energy in the interface between matrix and p, phase. Conclusions
On the basis of the present investigation of the precipitates in T4 treated Al-Mg alloy, the following conclusions can be drawn. (1) In Al-2.46wt?/ Mg alloy, the precipitates observed are p, phase having FCC structure, and p’phase having a hexagonal structure. While, in Al- ll.O7wt?h Mg alloy, the precipitates observed are p, phase having FCC structure and pz phase having a hexagonal structure. (2) In Al-2.46wt% Mg alloy, the orientation relationships are: {2001,, II {I33),,,
I
PRECIPITATION
362
While, in Al- ll.O7wt%
Mg alloy, the orientation
IN AI-Mg ALLOYS
relationships
Vol. 34. No. 3
are:
{3111P, II {Ol10~P~,P, II <2116>,, {131)*1 II {13lIPb {3101*, II <114>,, References 1. 2. 3. 4. 5. 6.
8. 9.
10. 11. 12. 13. 14.
Y.L. Liu and S.B. Kang: Scripta Metall., 30(1994), 487 S.L. Lee, S.T. Wu: Metall. Trans. A, 17A(1986), 1043 T. Sato, Y. Kojima and T. Takahashi: Metall. Trans. A, 13A(1982), 1373 T.R. MeNelley, E.W. Lee, M.E. Mills: Metall. Trans. A, 17A(1986), 1035 T. Daitoh, T. Takai: J. Jpn. Inst. Light Met., 39(1989), 873 S.B. Kang, C.Y. Lim and H.W. Kim: J. Society of Korea Automobile Eng. SAE No 943721,2(1994), 95 H.W. Kim and S.B. Kang: PRICM-2, Kyongju, Korea, June 18-22, 1995 L.F. Mondolfo: Aluminum Alloys—Structure & Properties, London, Butterworth, 1976,312-3 13 J.E. Hatch: Aluminum-Properties and Physical Metallurgy, American Society for Metals, Metals Park, Ohio, 1984,47 M. Yanagawa and S. Oie: Light Metals, 41,2(1991), 119 S.B. Kang et aI: Research Report of Korea Institue of Machinery and Metals, Korea, 1993 P. Hirsch, A. Howie et al: Electron Microscopy of Thin Crystals, Robert E. Krieger, 1977, 149 W.X. Liu, X.Y. Huang, Y.R. Chen: Micro-Analysis of Materials Microstructure, Tianjin Univ., 1989,335 A.K. Sachdev: Metall. Trans. A, 21A(1990), 165
Vol. 34, No. 3, pp. 357-362, 1996 Elsevier Science Ltd Copyright 0 1996 Acta Metallurgica Inc. F’rinted in the USA. All rights reserved 1359-6462/96 $12.00 + .OO
Perganron 0956-716X(95)00487-4
A STUDY ON THE PRECIPITATES IN T4 TREATED Al-Mg ALLOYS W.D. Fei’ and S.B. Kang2 ‘P.O. Box 433, Harbin Institute of Technology, Harbin 150001, P. R. China 2Korea Institute of Machinery and Metals, Changwon 64 1-O10, Korea (Received October 14, 1994) (Revised April 11, 1995) Introduction
Al-Mg alloys have been widely used as auto-body sheet materials because of their high strength to weight ratio. A disadvrmtage of Al-Mg alloys is a lower formability compared with low carbon steels. Most studies on Al-Mg alloys, therefore, have focussed on the investigation of mechanical properties and formability and their relation to microstructures [l-5]. The mechanical properties and formability of these alloys can be modified by heat treatment [6]. T4 heat treatment of Al-Mg alloys was found to improve the corrosion resistance, mechanical properties and formability [6,7]. One problem associated with heat treating auto-body sheets is that the size of sheets is so large that the cooling rate varies greatly from region to region. In the region where the cooling rate is relatively low, the second phase particles can be easily formed especially for Al-Mg alloys with high Mg content. The presence of the second phase particles can affect the mechanical properties of Al-Mg alloys. It is well known that the !supersaturated Al-Mg solid solution is transformed into a-Al and p phase. The p phase is usually given as Al, Mg, (37.3 wt% magnesium), although this composition is outside the limits of existence (34.7 to 37.1 wt%) [8]. The formula Al, Mg, (36 wt”/omagnesium) fits the composition of the solid phase and most of the proposed structure [8,9]. Three different kinds of crystalline structure have been suggested for Al 8 Mg 5 phase [S], but it has not been well understood about crystalline structure of Al, Mg,. In the Al-Mg alloys containing Mg in the range of 5.2 wt% to 7.5 wt%, the strength and ductility were increased linearly with Mg content increment. [lo] As a part of developing new materials of AI-Mg alloys used for auto-body sheet materials, various binary Al-Mg alloys containing 2.46 wt”/ou pto 11.07 WV?? Mg were considered. [ 1I] The purpose of this study is to examine the crystal structure of 8 precipitate in these Al-Mg alloys with different Mg content in more detail. ExDerimental
Procedures
Magnesium contents of two different binary Al-Mg alloys used in this study were 2.46 wt% and 11.07 wt%, respectively. The 25mm thick ingots were homogenized at 480°C for 24h, then hot rolled down to 3.Omm thick she:etswith a reduction ratio of 86% and cold rolled to 0.5mm thick sheets with a reduction
3.57
358
PRECIPITATION
rN AI-Mg ALLOYS
Vol. 34, No. 3
ratio of 83%. The specimens were cut from the sheets with the thickness of 0.5mm, and heat treated at 450°C in air for 30min and quenched into water at room temperature. TEM specimens were thinned by the standard chemical thinning method. TEM observation was carried out using JEOL-2000 type transmission electron microscope. The structures of the precipitates were examined by selected area diffraction (SAD) technique. The accelerating voltage was 200kV and the camera length was about 100cm. Results and Discussion
The microstructure and SAD patterns for an Al-2.46wt% Mg alloy are shown in Fig. 1. Fig. 1(b) and Fig. 1(d) show SAD patterns taken from area “A” and area “A”&“B”, respectively as indicated in Fig. 1(a). From Fig. l(a), (b) and (c), it was found that there exist three phases in this alloy. The first one is the matrix, a-Al, and the second and the third ones are precipitates. It is necessary to note that the materials were homogenized at 480°C for 24h before rolling process. According to the phase diagram of Al-Mg system [8], it can be confirmed that the phases indicated by “A” and “B” in Fig. l(a) are precipitates. In Fig. 1 (b) and (c), the diffraction spots can be divided into two groups; one is the diffraction pattern of aluminum, and the other is that of precipitate. It can be found that the precipitate has FCC structure and the lattice constant is about 1.2Omn. This agrees with the lattice constant (1.24mn) of Al, Mg, phase having FCC structure [8], so this type of Al, Mg, phase is symbolized by p, phase. There are many regular diffraction spots arising from the double diffraction. This can be explained by the fact that the unit cell of Al, Mg, is much larger than that of aluminum [ 12,131.Double di%-action spots are not described in Fig. l(c). From the diffraction pattern shown in Fig. l(b) and (c), the orientation relationship between the matrix, a-Al, and the precipitate phase, p l phase, can be drawn as follows: W0IAI II U331,IY <013>*, II
From Fig. 1 (a), (b) and (c), it was determined that the precipitate in area “A” was p phase having FCC structure (p,) and the one in area “B” was another phase with a different crystal structure. The diffraction pattern taken from area “A” and “B” is shown in Fig. l(d). The double diffraction spots were also observed clearly in the SAD pattern. The index of diffraction spots excluding the double diffraction spots is shown in Fig. l(e). It can be determined that the precipitate in area “B” has a hexagonal structure and the lattice constants are about a=l.O3mn, c=l.68mn. This agrees with the lattice constants (a=1 .OOOnm,c=l.636nm) of p’phase given by reference [ 141. So the precipitate in area “B” shown in Fig. l(a) is considered as p’ phase. A TEM micrograph and SAD patterns of an Al-l l.O7wt% Mg alloy are shown in Fig. 2. From Fig. 2(a), the precipitate in area “A” can be determined as p , phase having FCC structure and the one in area “B” has a hexagonal structure with the lattice constants of a=l.l2nm, c=l.64nm. This agrees with the values (a=1.12-l.l38mn, c=1.60-1.788nm) ofthe l3phase having hexagonal structure [8]. The p phase having hexagonal structure is symbolized by p2phase. From Fig. 2 (c), the orientation relationship among a-Al and two types of p phase in an Al- ll.O7wt% Mg alloy can be determined as follows: {311)r., II 10110~P~,<114’P, II <2116’P, {I3I],, II {I31]~,?<3IO’,, ll
Vol. 34, No. 3
PRECIPITATION IN Al-Mg ALLOYS
359
63
Figure 1. TEM photographs of Al-2.46wt?hMg alloy, (a) morphology, (b) and (c) are the SAD patterns from area “A” indicated in (a), while (d) and (e) are the SAD patterns from area “A” and “B” indicated in (a).
360
PRECIPITATION
IN Al-Mg
ALLOYS
Vol. 34, No. 3
-_
Olllgz 10128,
Figure 2. TEM photographs of Al-l I .07wt%Mg alloy, (a) morphology, (b) and (c) are the SAD patterns from area “A” indicated in (a), while (d) and (e) are the SAD patterns from area “B” indicated in (a).
Vol. 34, No. 3
PRECIPITATION IN Al-Mg ALLOYS
10
20
30
40 so 60 ~1% Mg
70
361
80
90
loo Mg
Figure 3. Phase diagram of AI-Mg system
hexagonal structure. This can be understood from the difference of composition and the transformation temperature between these two alloys. Fig. 3 shows the phase digram of AI-Mg system, and the dashed line illustrates the schematic diagram of pseudo phase diagram at the nonequilibrium condition. From Fig 3, it is obvious that the transformation temperature in the high Mg containing Al-Mg alloy is much higher than that in the low Mg containing Al-Mg alloy. It is reported that the p 1phase is stable one and p’phase is metastable one in Al-Mg alloy [S]. The p, phase can only form at high temperature, while p’phase can form at low temperature [S]. In the high Mg containing Al-Mg alloy, the transformation occurs at high temperature and long elapsed time. Two types of p phases (p, and p,) were observed. While, in the low Mg containing Al-Mg alloy, the transformation occurs relatively at low temperature and short elapsed time. Stable p phase (p ,) and metastable p’phase were observed. The oriental.ion relationship between cl-Al and p, phase in Al-2.46wt% Mg alloy is different from that in Al-l l.O7wt% Mg alloy. It may be due to the different lattice constant of a-Al matrix in two alloys, but the lattice constant of p, phase is constant. In this case, the orientation relationship may change to reduce the elastic energy in the interface between matrix and p, phase. Conclusions
On the basis of the present investigation of the precipitates in T4 treated Al-Mg alloy, the following conclusions can be drawn. (1) In Al-2.46wt?/ Mg alloy, the precipitates observed are p, phase having FCC structure, and p’phase having a hexagonal structure. While, in Al- ll.O7wt?h Mg alloy, the precipitates observed are p, phase having FCC structure and pz phase having a hexagonal structure. (2) In Al-2.46wt% Mg alloy, the orientation relationships are: {2001,, II {I33),,,
I
PRECIPITATION
362
While, in Al- ll.O7wt%
Mg alloy, the orientation
IN AI-Mg ALLOYS
relationships
Vol. 34. No. 3
are:
{3111P, II {Ol10~P~,
8. 9.
10. 11. 12. 13. 14.
Y.L. Liu and S.B. Kang: Scripta Metall., 30(1994), 487 S.L. Lee, S.T. Wu: Metall. Trans. A, 17A(1986), 1043 T. Sato, Y. Kojima and T. Takahashi: Metall. Trans. A, 13A(1982), 1373 T.R. MeNelley, E.W. Lee, M.E. Mills: Metall. Trans. A, 17A(1986), 1035 T. Daitoh, T. Takai: J. Jpn. Inst. Light Met., 39(1989), 873 S.B. Kang, C.Y. Lim and H.W. Kim: J. Society of Korea Automobile Eng. SAE No 943721,2(1994), 95 H.W. Kim and S.B. Kang: PRICM-2, Kyongju, Korea, June 18-22, 1995 L.F. Mondolfo: Aluminum Alloys—Structure & Properties, London, Butterworth, 1976,312-3 13 J.E. Hatch: Aluminum-Properties and Physical Metallurgy, American Society for Metals, Metals Park, Ohio, 1984,47 M. Yanagawa and S. Oie: Light Metals, 41,2(1991), 119 S.B. Kang et aI: Research Report of Korea Institue of Machinery and Metals, Korea, 1993 P. Hirsch, A. Howie et al: Electron Microscopy of Thin Crystals, Robert E. Krieger, 1977, 149 W.X. Liu, X.Y. Huang, Y.R. Chen: Micro-Analysis of Materials Microstructure, Tianjin Univ., 1989,335 A.K. Sachdev: Metall. Trans. A, 21A(1990), 165