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Evolution of texture during hot rolling of aluminum borate whisker-reinforced 6061 aluminum alloy composite
Materials Science and Engineering A 528 (2011) 3243–3248
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Evolution of texture during hot rolling of aluminum borate whisker-reinforced 6061 aluminum alloy composite S.C. Xu a,b , L.D. Wang a , P.T. Zhao a , W.L. Li a , Z.W. Xue a , W.D. Fei a,∗ a b
School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, PR China Institute of Condensed Matter Physics and Materials Science, Jilin Normal University, Siping 136000, PR China
a r t i c l e
i n f o
Article history: Received 20 October 2010 Received in revised form 27 December 2010 Accepted 30 December 2010 Available online 8 January 2011 Keywords: Composites Aluminum borate whisker Hot rolling X-ray diffraction Texture
a b s t r a c t The textures and microstructures of hot-rolled aluminum borate whisker-reinforced 6061 aluminum alloy composites with different reductions have been investigated. Compared with the typical rolling textures for monolithic aluminum alloys, different hot-rolling textures were found in the matrix of the composites. New plate textures are dominant for the specimens with 30% and 50% reductions, and fiber textures for the specimen with 70% reduction. The increasing reduction transforms some plate textures into fiber textures, and develops the original fiber textures. Furthermore, some typical rolling and recrystallization texture components for monolithic aluminum alloys with low orientation distribution densities can be observed in the composites with 50% and 70% reductions. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Discontinuously reinforced metal matrix composites, in particular aluminum matrix composites reinforced by whiskers, have been extensively investigated owing to their low density, low thermal expansion coefficients and high mechanical properties [1–6]. However, these materials exhibit lower ductility compared with monolithic aluminum alloys. Because the hot plastic forming technologies are very important for the fabrication of composite parts, many investigations have been conducted on the hot deformation processing of aluminum matrix composites reinforced by whiskers or particles, including hot forging [7–9], hot compression [10–13], hot extrusion [14–18], and hot rolling [19–22]. The studies on the mechanical properties of the composites after hot forming indicate that the interfacial state plays a very important role in the properties of hot-rolled composites [19,20]. Some studies on the rolling texture of aluminum matrix composites reinforced by whiskers with low volume fraction have been done [21]. However, the rolling texture evolution of aluminum matrix composites reinforced by whiskers with high volume fraction has only been investigated to a limited extent [22].
∗ Corresponding author at: School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, PR China. Tel.: +86 451 86413908; fax: +86 451 86413908. E-mail address: [email protected] (W.D. Fei). 0921-5093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2010.12.103
The rolling textures of aluminum alloys and other face-centred cubic metals have been well studied [23,24]. The typical rolling textures of aluminum alloys can be described by orientation concentrations assembled along the “␣-fiber” and the “-fiber”. The former starts at Goss {0 1 1} 1 0 0 and goes to Brass {1 1 0} 1 1 2, and the latter starts at Brass {1 1 0} 1 1 2 and goes through S {1 2 3} 6 3 4 to Copper {1 1 2} 1 1 1. However, aluminum matrix composites reinforced by whiskers may develop different rolling textures because of the effect resulting from whisker rotation and fracture on the aluminum matrix deformation in the rolling process [22]. Zhang et al. [22] studied the texture of cold-rolled SiCw/Al composites after hot extrusion. The results showed that the dominant rolling texture components {1 1 2} 1 1 1, {1 0 0} 0 1 1 and {1 2 3} 6 3 4 in the composites weakened compared with those in corresponding cold-rolled aluminum because of the restriction effect of whiskers on the aluminum matrix deformation. Although some studies have been done on the rolling processing of whisker-reinforced aluminum matrix composite, the deep rolling processing of as-fabricated composite reinforced with a high volume fraction of whisker is also a challenge. Up to now, the evolution mechanism of rolling texture, which is closely related to the composite deformation mechanism, has not been understood comprehensively yet. In the present study, as-cast aluminum borate (Al18 B4 O33 , denoted by ABO) whisker reinforced 6061 aluminum alloy composite (denoted as ABOw/6061Al) was rolled to different reductions at 500 ◦ C. The texture evolution and microstructures of the hot-rolled specimens were investigated by X-ray diffraction (XRD) and scanning electron microscope (SEM).
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Fig. 1. SEM photograph of as-cast ABOw/6061Al composite.
2. Experimental The 6061 aluminum alloy matrix composite reinforced with 20 vol% of ABO whiskers, fabricated by squeeze casting technique, was used in the present investigation. In order to improve the rolling deformation ability of the composite, SnO2 was coated on the surface of whiskers by a chemical precipitation method [14] before the composite fabrication. Sn resulting from interfacial reaction in the process of squeeze casting prefers existing at the interface between ABO whisker and aluminum matrix, and hardly dissolves into aluminum matrix according to Al–Sn phase diagram. During hot rolling deformation process, Sn exists at the interface in liquid state according to its low melting point of 231.9 ◦ C, which can contribute to the interface sliding and the whisker rotation [14]. The specimens with the dimension of 7 mm × 45 mm × 60 mm were cut from the composite ingot for hot rolling. To avoid cracking at a large rolling ratio and keep temperature uniform during multi-pass hot rolling process, the specimens were packaged by aluminum alloy beforehand. The specimens were heated at 500 ◦ C for 15 min, and then rolled to final reductions of 30%, 50% and 70% with 3, 5 and 7 passes, respectively. No lubrication was used and no interpass annealing was applied during hot rolling process. After taken from the mid-thickness of the rolled sheets, the specimens were ground, mechanically polished, and finally etched using 10% hydrochloric acid water solution to remove the surface defor-
Fig. 3. ODFs of hot-rolled ABOw/6061Al composite with 30% reduction.
mation layer for texture analysis. Texture measurement was carried out on a Philips X’Pert diffractometer with Euler’s rotation sample stage. For each specimen, three incomplete pole figures {1 1 1}, {2 0 0} and {2 2 0} were measured up to a maximum tilt angle of 75◦ by the Schulz back-reflection method using CuK␣ radiation. Orientation distribution functions (ODFs) were calculated from the three pole figures after correction by the method of Bunge, and the results of the calculated ODFs are presented in constant ϕ2 sections in 5-degree steps in Euler space defined by the Euler angles ϕ1 , ˚, ϕ2 . The microstructures of the specimens were examined using S3000 scanning electron microscope. The specimens were ground, mechanically polished, and etched using a 5% hydrofluoric acid water solution for microstructure observation. 3. Results and discussion 3.1. Microstructure and texture of as-cast composite Fig. 1 shows the SEM image of as-cast ABOw/6061Al composite. In the image, no crack or void can be found, and whiskers with a relatively large aspect ratio distribute uniformly and randomly in the as-cast composite. The {1 1 1} pole figure of as-cast ABOw/6061Al composite is shown in Fig. 2. It can be seen that the orientation of the aluminum matrix grains is essentially random. This indicates that no obviously preferred orientation exists in the aluminum matrix. 3.2. Hot-rolling textures of ABOw/6061Al composites
Fig. 2. {1 1 1} pole figure of as-cast ABOw/6061Al composite.
Fig. 3 shows the ODFs of hot-rolled ABOw/6061Al composite with 30% reduction. It can be found that some obvious and strong plate textures are formed in the aluminum matrix of the compos-
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Fig. 4. ODFs of hot-rolled ABOw/6061Al composite with 50% reduction.
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Fig. 5. ODFs of hot-rolled ABOw/6061Al composite with 70% reduction.
Fig. 6. Texture evolution of hot-rolled composites with different reductions: (a) maximum in densities of plate textures in the composites with different reductions, (b) ODFs vs ϕ1 for ˚ = 25◦ and ϕ2 = 70◦ , corresponding to {3 1 6} fiber texture, (c) ODFs vs ϕ1 for ˚ = 15◦ and ϕ2 = 55◦ , corresponding to {3 2 13} fiber texture, and (d) ODFs vs ϕ1 for different fiber textures in the composite with 70% reduction.
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Fig. 7. Three-dimensional ODFs of hot-rolled composites with different reductions of 30% (a), 50% (b) and 70% (c).
ite during hot rolling process. The dominant plate textures consist of {2 1 2} 3 10 2, {3 3 2} 1 1 3 and {1 2 2} 2 1 0, corresponding to the Euler space angles of (15◦ , 45◦ , 65◦ ), (90◦ , 65◦ , 45◦ ) and (0◦ , 45◦ , 25◦ ), whose orientation distribution densities are 33.0, 22.7 and 16.0, respectively. Furthermore, two weak and partial fiber textures, {3 2 13} and {3 1 6} orientations, can be found in the ϕ2 = 55◦ and 70◦ sections. The {h k l} orientation is commonly used to denote that the plane {h k l} is parallel to the rolling plane. Thus, the formation of the two fiber texture components indicates that there is a tendency that the {3 2 13} and {3 1 6} planes of the aluminum matrix grains tend to be parallel to the rolling plane with higher probability. Fig. 4 shows the ODFs of hot-rolled ABOw/6061Al composite with 50% reduction. It can be found that the rolling texture components change a lot, compared with those in the composite with 30% reduction. For the specimen with 50% reduction, the dominant plate texture components found initially at 30% reduction disappear, accompanied with the formation of some new plate textures. The new plate texture components consist of {2 3 13} 2 3 1, {3 2 13} 4 7 2 and {1 3 6} 0 2 1, corresponding to (90◦ , 15◦ , 35◦ ), (65◦ , 15◦ , 55◦ ) and (70◦ , 25◦ , 20◦ ), whose orientation distribution densities are 10.9, 12.5 and 11.3, respectively. The dominant plate texture components alter, and their orientation distribution densities decrease with the reduction. For the specimen with 50% reduction, the dominant plate textures tend to develop along the direction of equiangularity of ˚ in the ϕ2 = 20◦ , 70◦ , 35◦ and 55◦
sections, resulting in the development of {3 1 6} and {3 2 13} orientations. The tendency to form fiber textures is more apparent than that at 30% reduction. The orientation distribution intensities of the two fiber texture components increase with the reduction. As shown in Fig. 4, there are some other plate texture components with low intensities in the ODFs, such as D {4 4 11} 11 11 8 (a typical rolling texture component of fcc metals), Cube {0 0 1} 1 0 0 and R {1 2 4} 2 1 1 (two typical recrystallization texture components of fcc metals). Their orientation distribution densities are relatively low due to the effect of whiskers with a large volume fraction [22]. Fig. 5 shows the ODFs of hot-rolled ABOw/6061Al composite with 70% reduction. With increasing reduction, the dominant plate textures found in the specimen with 50% reduction develop into obvious fiber textures, and the main fiber texture components composed of {3 1 6} and {3 2 13} orientations become stronger and more complete. Simultaneously, a maximum intensity can be found at {3 1 6} orientation in the ϕ2 = 70◦ section corresponding to {3 1 6} 7 9 5. Otherwise, two new but weak fiber texture components, {1 4 1} and {1 2 2} orientations, are found in the specimen. For the composites, the typical rolling texture components and recrystallization texture components of fcc metals vary with the reduction. As the reduction increases from 50% to 70%, the orientation distribution density of D {4 4 11} 11 11 8 increases slightly (from 4.0 to 4.2). For the recrystallization texture, R {1 2 4} 2 1 1 increases its orientation distribution density from 2.3 to 3.9, and
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Cube {0 0 1} 1 0 0 disappears. The observation suggests that the texture evolution of hot-rolled ABOw/6061Al composite may be the result of both deformation and recrystallization. As discussed above, the texture evolution of hot-rolled ABOw/6061Al composite is more complex than that of monolithic aluminum alloys. The variation of texture with the reduction is summarized in Fig. 6. As shown in Fig. 6(a), the densities of the plate textures decrease rapidly with the reduction, accompanied with the variation of texture component. From the curves of ODFs versus ϕ1 shown in Fig. 6(b) and (c), it can be seen that some rolling texture components, in spite of displaying an extension trend, still have plate texture feature at a low reduction, such as {3 2 13} 4 7 2. With increasing reduction, they gradually extend to fiber textures. Fig. 6(d) shows that as the reduction increases to 70%, the fiber textures develop rather completely and only the plate texture of {3 1 6} 7 9 5 can be confirmed with high degree concentration. Fig. 7 shows the three-dimensional ODFs of hot-rolled composites with different reductions. Here the texture evolution behavior from plate textures to fiber textures is indicated more intuitively. From the above analysis, the texture evolution of hot-rolled ABOw/6061Al composite can be summarized as follows. Strong plate textures exist only in the composite with a low reduction and the fiber textures become dominant components at a high reduction. Moreover, the fiber textures formed at a high reduction stem from the extending of some plate textures formed at a low reduction. 3.3. Microstructures of the hot-rolled composites Fig. 8 shows the SEM micrographs of hot-rolled ABOw/6061Al composites with different reductions. The whiskers in the as-cast ABOw/6061Al composite suffer a further rotation and fracture during rolling process. As shown in Fig. 8(a), the whisker fracture is not serious, and a few whiskers are rotated in the composite with 30% reduction. With increasing reduction, the extent of whisker rotation strengthens and the longitudinal direction of whisker tends to be parallel to the rolling plane and direction. Again, the average length of whiskers decreases significantly because of the serious fracture of lots of whiskers, as shown in Fig. 8(b) and (c). 3.4. Discussion Based on the above analysis, one can conclude that the formation of rolling textures in the ABOw/6061Al composite, different from the typical rolling textures for the monolithic aluminum alloys, results from the present of the whiskers with a high volume fraction in the composite. The addition of whiskers changes the microstructure of the composite and induces residual stress in the aluminum matrix, which leads to the formation of different textures in the hot-rolled ABOw/6061Al composites with different reductions. For monolithic metals, the external stress applied to metal grains is constant, and the internal slip systems in a given grain are fixed in the crystallography coordinate system of the grain during rolling deformation process. Therefore, similar textures have been found in the metals with the same crystal structure, e.g. the “-fiber” textures in fcc metals. However, the stress state in the metal matrix grains can be changed greatly due to the addition of whiskers with large aspect ratio. For whisker-reinforced aluminum matrix composite, besides external stress, there is an additional stress resulting from thermal mismatch stress between whisker and aluminum matrix [25]. Because of large aspect ratio of whisker, the thermal mismatch stress in the matrix nearby whisker is highly anisotropic, and the stress concentration in the matrix region around the ends of whisker is very large [5,26]. It is clear that the resultant stress in the matrix grains of the composite is the sum of external stress and
Fig. 8. SEM micrographs of hot-rolled ABOw/6061Al composites with different reductions of 30% (a), 50% (b) and 70% (c).
thermal mismatch stress. During rolling deformation process, the resultant stress in the matrix grains near whiskers changes continuously in that whisker rotate and fracture. As a result, some new plate texture components, different from the typical rolling texture components in monolithic aluminum alloys, are formed in the aluminum matrix of the ABOw/6061Al composite in the initial rolling stage.
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With increasing reduction, more and more whiskers rotate their longitude directions to be parallel to the rolling plane and direction. It leads to a more disordered stress distribution in the rolling plane, but a highly symmetrical stress distribution about the normal direction of the rolling plane. Consequently, some fiber textures are formed in the composites with a high reduction. It can be easily understood that the effect of thermal mismatch stress in the matrix grains far from whisker is weaker, which results in the grain deformation in this region similar to the grain deformation in monolithic alloys. Therefore, some typical rolling and recrystallization texture components of aluminum alloys are observed in the hot-rolled composites with a relatively high reduction, such as D {4 4 11} 11 11 8, Cube {0 0 1} 1 0 0 and R {1 2 4} 2 1 1. However, these textures are not dominant ones in the hotrolled ABOw/6061Al composites. 4. Conclusions A systematic analysis of hot-rolling texture evolution in ABOw/6061Al composite has been carried out at different reductions. The principal conclusions that can be drawn from this research are the following:
tion to an increasing extent. Meanwhile, the whisker fracture becomes more serious. The change of resultant stress of external stress and thermal mismatch stress may be responsible for the formation of new plate textures and fiber textures. Acknowledgements The work was supported by the National High Technology Research and Development Program of China and by a grant from the Specialized Research Fund for the Doctoral Program of Higher Education (No.20070213042). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
1. It is interesting to note that the hot-rolling texture evolution of ABOw/6061Al composite is distinguished from that of monolithic aluminum alloys, which results from the effect of the whiskers in the composite. 2. The dominant plate textures composed of {2 1 2} 3 10 2, {3 3 2} 1 1 3 and {1 2 2} 2 1 0 are formed in the composite with 30% reduction. As the reduction increases from 30% to 50%, some obvious fiber textures, {3 1 6} and {3 2 13} orientations, are formed through a transformation from plate textures to fiber textures. Further rolling deformation leads to the development of the fiber textures. 3. During the hot rolling process, the whisker rotation and fracture take place. With increasing reduction, the whiskers rotate their longitude directions to be parallel to the rolling plane and direc-
[11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26]
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Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Evolution of texture during hot rolling of aluminum borate whisker-reinforced 6061 aluminum alloy composite S.C. Xu a,b , L.D. Wang a , P.T. Zhao a , W.L. Li a , Z.W. Xue a , W.D. Fei a,∗ a b
School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, PR China Institute of Condensed Matter Physics and Materials Science, Jilin Normal University, Siping 136000, PR China
a r t i c l e
i n f o
Article history: Received 20 October 2010 Received in revised form 27 December 2010 Accepted 30 December 2010 Available online 8 January 2011 Keywords: Composites Aluminum borate whisker Hot rolling X-ray diffraction Texture
a b s t r a c t The textures and microstructures of hot-rolled aluminum borate whisker-reinforced 6061 aluminum alloy composites with different reductions have been investigated. Compared with the typical rolling textures for monolithic aluminum alloys, different hot-rolling textures were found in the matrix of the composites. New plate textures are dominant for the specimens with 30% and 50% reductions, and fiber textures for the specimen with 70% reduction. The increasing reduction transforms some plate textures into fiber textures, and develops the original fiber textures. Furthermore, some typical rolling and recrystallization texture components for monolithic aluminum alloys with low orientation distribution densities can be observed in the composites with 50% and 70% reductions. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Discontinuously reinforced metal matrix composites, in particular aluminum matrix composites reinforced by whiskers, have been extensively investigated owing to their low density, low thermal expansion coefficients and high mechanical properties [1–6]. However, these materials exhibit lower ductility compared with monolithic aluminum alloys. Because the hot plastic forming technologies are very important for the fabrication of composite parts, many investigations have been conducted on the hot deformation processing of aluminum matrix composites reinforced by whiskers or particles, including hot forging [7–9], hot compression [10–13], hot extrusion [14–18], and hot rolling [19–22]. The studies on the mechanical properties of the composites after hot forming indicate that the interfacial state plays a very important role in the properties of hot-rolled composites [19,20]. Some studies on the rolling texture of aluminum matrix composites reinforced by whiskers with low volume fraction have been done [21]. However, the rolling texture evolution of aluminum matrix composites reinforced by whiskers with high volume fraction has only been investigated to a limited extent [22].
∗ Corresponding author at: School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, PR China. Tel.: +86 451 86413908; fax: +86 451 86413908. E-mail address: [email protected] (W.D. Fei). 0921-5093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2010.12.103
The rolling textures of aluminum alloys and other face-centred cubic metals have been well studied [23,24]. The typical rolling textures of aluminum alloys can be described by orientation concentrations assembled along the “␣-fiber” and the “-fiber”. The former starts at Goss {0 1 1} 1 0 0 and goes to Brass {1 1 0} 1 1 2, and the latter starts at Brass {1 1 0} 1 1 2 and goes through S {1 2 3} 6 3 4 to Copper {1 1 2} 1 1 1. However, aluminum matrix composites reinforced by whiskers may develop different rolling textures because of the effect resulting from whisker rotation and fracture on the aluminum matrix deformation in the rolling process [22]. Zhang et al. [22] studied the texture of cold-rolled SiCw/Al composites after hot extrusion. The results showed that the dominant rolling texture components {1 1 2} 1 1 1, {1 0 0} 0 1 1 and {1 2 3} 6 3 4 in the composites weakened compared with those in corresponding cold-rolled aluminum because of the restriction effect of whiskers on the aluminum matrix deformation. Although some studies have been done on the rolling processing of whisker-reinforced aluminum matrix composite, the deep rolling processing of as-fabricated composite reinforced with a high volume fraction of whisker is also a challenge. Up to now, the evolution mechanism of rolling texture, which is closely related to the composite deformation mechanism, has not been understood comprehensively yet. In the present study, as-cast aluminum borate (Al18 B4 O33 , denoted by ABO) whisker reinforced 6061 aluminum alloy composite (denoted as ABOw/6061Al) was rolled to different reductions at 500 ◦ C. The texture evolution and microstructures of the hot-rolled specimens were investigated by X-ray diffraction (XRD) and scanning electron microscope (SEM).
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Fig. 1. SEM photograph of as-cast ABOw/6061Al composite.
2. Experimental The 6061 aluminum alloy matrix composite reinforced with 20 vol% of ABO whiskers, fabricated by squeeze casting technique, was used in the present investigation. In order to improve the rolling deformation ability of the composite, SnO2 was coated on the surface of whiskers by a chemical precipitation method [14] before the composite fabrication. Sn resulting from interfacial reaction in the process of squeeze casting prefers existing at the interface between ABO whisker and aluminum matrix, and hardly dissolves into aluminum matrix according to Al–Sn phase diagram. During hot rolling deformation process, Sn exists at the interface in liquid state according to its low melting point of 231.9 ◦ C, which can contribute to the interface sliding and the whisker rotation [14]. The specimens with the dimension of 7 mm × 45 mm × 60 mm were cut from the composite ingot for hot rolling. To avoid cracking at a large rolling ratio and keep temperature uniform during multi-pass hot rolling process, the specimens were packaged by aluminum alloy beforehand. The specimens were heated at 500 ◦ C for 15 min, and then rolled to final reductions of 30%, 50% and 70% with 3, 5 and 7 passes, respectively. No lubrication was used and no interpass annealing was applied during hot rolling process. After taken from the mid-thickness of the rolled sheets, the specimens were ground, mechanically polished, and finally etched using 10% hydrochloric acid water solution to remove the surface defor-
Fig. 3. ODFs of hot-rolled ABOw/6061Al composite with 30% reduction.
mation layer for texture analysis. Texture measurement was carried out on a Philips X’Pert diffractometer with Euler’s rotation sample stage. For each specimen, three incomplete pole figures {1 1 1}, {2 0 0} and {2 2 0} were measured up to a maximum tilt angle of 75◦ by the Schulz back-reflection method using CuK␣ radiation. Orientation distribution functions (ODFs) were calculated from the three pole figures after correction by the method of Bunge, and the results of the calculated ODFs are presented in constant ϕ2 sections in 5-degree steps in Euler space defined by the Euler angles ϕ1 , ˚, ϕ2 . The microstructures of the specimens were examined using S3000 scanning electron microscope. The specimens were ground, mechanically polished, and etched using a 5% hydrofluoric acid water solution for microstructure observation. 3. Results and discussion 3.1. Microstructure and texture of as-cast composite Fig. 1 shows the SEM image of as-cast ABOw/6061Al composite. In the image, no crack or void can be found, and whiskers with a relatively large aspect ratio distribute uniformly and randomly in the as-cast composite. The {1 1 1} pole figure of as-cast ABOw/6061Al composite is shown in Fig. 2. It can be seen that the orientation of the aluminum matrix grains is essentially random. This indicates that no obviously preferred orientation exists in the aluminum matrix. 3.2. Hot-rolling textures of ABOw/6061Al composites
Fig. 2. {1 1 1} pole figure of as-cast ABOw/6061Al composite.
Fig. 3 shows the ODFs of hot-rolled ABOw/6061Al composite with 30% reduction. It can be found that some obvious and strong plate textures are formed in the aluminum matrix of the compos-
S.C. Xu et al. / Materials Science and Engineering A 528 (2011) 3243–3248
Fig. 4. ODFs of hot-rolled ABOw/6061Al composite with 50% reduction.
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Fig. 5. ODFs of hot-rolled ABOw/6061Al composite with 70% reduction.
Fig. 6. Texture evolution of hot-rolled composites with different reductions: (a) maximum in densities of plate textures in the composites with different reductions, (b) ODFs vs ϕ1 for ˚ = 25◦ and ϕ2 = 70◦ , corresponding to {3 1 6} fiber texture, (c) ODFs vs ϕ1 for ˚ = 15◦ and ϕ2 = 55◦ , corresponding to {3 2 13} fiber texture, and (d) ODFs vs ϕ1 for different fiber textures in the composite with 70% reduction.
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Fig. 7. Three-dimensional ODFs of hot-rolled composites with different reductions of 30% (a), 50% (b) and 70% (c).
ite during hot rolling process. The dominant plate textures consist of {2 1 2} 3 10 2, {3 3 2} 1 1 3 and {1 2 2} 2 1 0, corresponding to the Euler space angles of (15◦ , 45◦ , 65◦ ), (90◦ , 65◦ , 45◦ ) and (0◦ , 45◦ , 25◦ ), whose orientation distribution densities are 33.0, 22.7 and 16.0, respectively. Furthermore, two weak and partial fiber textures, {3 2 13} and {3 1 6} orientations, can be found in the ϕ2 = 55◦ and 70◦ sections. The {h k l} orientation is commonly used to denote that the plane {h k l} is parallel to the rolling plane. Thus, the formation of the two fiber texture components indicates that there is a tendency that the {3 2 13} and {3 1 6} planes of the aluminum matrix grains tend to be parallel to the rolling plane with higher probability. Fig. 4 shows the ODFs of hot-rolled ABOw/6061Al composite with 50% reduction. It can be found that the rolling texture components change a lot, compared with those in the composite with 30% reduction. For the specimen with 50% reduction, the dominant plate texture components found initially at 30% reduction disappear, accompanied with the formation of some new plate textures. The new plate texture components consist of {2 3 13} 2 3 1, {3 2 13} 4 7 2 and {1 3 6} 0 2 1, corresponding to (90◦ , 15◦ , 35◦ ), (65◦ , 15◦ , 55◦ ) and (70◦ , 25◦ , 20◦ ), whose orientation distribution densities are 10.9, 12.5 and 11.3, respectively. The dominant plate texture components alter, and their orientation distribution densities decrease with the reduction. For the specimen with 50% reduction, the dominant plate textures tend to develop along the direction of equiangularity of ˚ in the ϕ2 = 20◦ , 70◦ , 35◦ and 55◦
sections, resulting in the development of {3 1 6} and {3 2 13} orientations. The tendency to form fiber textures is more apparent than that at 30% reduction. The orientation distribution intensities of the two fiber texture components increase with the reduction. As shown in Fig. 4, there are some other plate texture components with low intensities in the ODFs, such as D {4 4 11} 11 11 8 (a typical rolling texture component of fcc metals), Cube {0 0 1} 1 0 0 and R {1 2 4} 2 1 1 (two typical recrystallization texture components of fcc metals). Their orientation distribution densities are relatively low due to the effect of whiskers with a large volume fraction [22]. Fig. 5 shows the ODFs of hot-rolled ABOw/6061Al composite with 70% reduction. With increasing reduction, the dominant plate textures found in the specimen with 50% reduction develop into obvious fiber textures, and the main fiber texture components composed of {3 1 6} and {3 2 13} orientations become stronger and more complete. Simultaneously, a maximum intensity can be found at {3 1 6} orientation in the ϕ2 = 70◦ section corresponding to {3 1 6} 7 9 5. Otherwise, two new but weak fiber texture components, {1 4 1} and {1 2 2} orientations, are found in the specimen. For the composites, the typical rolling texture components and recrystallization texture components of fcc metals vary with the reduction. As the reduction increases from 50% to 70%, the orientation distribution density of D {4 4 11} 11 11 8 increases slightly (from 4.0 to 4.2). For the recrystallization texture, R {1 2 4} 2 1 1 increases its orientation distribution density from 2.3 to 3.9, and
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Cube {0 0 1} 1 0 0 disappears. The observation suggests that the texture evolution of hot-rolled ABOw/6061Al composite may be the result of both deformation and recrystallization. As discussed above, the texture evolution of hot-rolled ABOw/6061Al composite is more complex than that of monolithic aluminum alloys. The variation of texture with the reduction is summarized in Fig. 6. As shown in Fig. 6(a), the densities of the plate textures decrease rapidly with the reduction, accompanied with the variation of texture component. From the curves of ODFs versus ϕ1 shown in Fig. 6(b) and (c), it can be seen that some rolling texture components, in spite of displaying an extension trend, still have plate texture feature at a low reduction, such as {3 2 13} 4 7 2. With increasing reduction, they gradually extend to fiber textures. Fig. 6(d) shows that as the reduction increases to 70%, the fiber textures develop rather completely and only the plate texture of {3 1 6} 7 9 5 can be confirmed with high degree concentration. Fig. 7 shows the three-dimensional ODFs of hot-rolled composites with different reductions. Here the texture evolution behavior from plate textures to fiber textures is indicated more intuitively. From the above analysis, the texture evolution of hot-rolled ABOw/6061Al composite can be summarized as follows. Strong plate textures exist only in the composite with a low reduction and the fiber textures become dominant components at a high reduction. Moreover, the fiber textures formed at a high reduction stem from the extending of some plate textures formed at a low reduction. 3.3. Microstructures of the hot-rolled composites Fig. 8 shows the SEM micrographs of hot-rolled ABOw/6061Al composites with different reductions. The whiskers in the as-cast ABOw/6061Al composite suffer a further rotation and fracture during rolling process. As shown in Fig. 8(a), the whisker fracture is not serious, and a few whiskers are rotated in the composite with 30% reduction. With increasing reduction, the extent of whisker rotation strengthens and the longitudinal direction of whisker tends to be parallel to the rolling plane and direction. Again, the average length of whiskers decreases significantly because of the serious fracture of lots of whiskers, as shown in Fig. 8(b) and (c). 3.4. Discussion Based on the above analysis, one can conclude that the formation of rolling textures in the ABOw/6061Al composite, different from the typical rolling textures for the monolithic aluminum alloys, results from the present of the whiskers with a high volume fraction in the composite. The addition of whiskers changes the microstructure of the composite and induces residual stress in the aluminum matrix, which leads to the formation of different textures in the hot-rolled ABOw/6061Al composites with different reductions. For monolithic metals, the external stress applied to metal grains is constant, and the internal slip systems in a given grain are fixed in the crystallography coordinate system of the grain during rolling deformation process. Therefore, similar textures have been found in the metals with the same crystal structure, e.g. the “-fiber” textures in fcc metals. However, the stress state in the metal matrix grains can be changed greatly due to the addition of whiskers with large aspect ratio. For whisker-reinforced aluminum matrix composite, besides external stress, there is an additional stress resulting from thermal mismatch stress between whisker and aluminum matrix [25]. Because of large aspect ratio of whisker, the thermal mismatch stress in the matrix nearby whisker is highly anisotropic, and the stress concentration in the matrix region around the ends of whisker is very large [5,26]. It is clear that the resultant stress in the matrix grains of the composite is the sum of external stress and
Fig. 8. SEM micrographs of hot-rolled ABOw/6061Al composites with different reductions of 30% (a), 50% (b) and 70% (c).
thermal mismatch stress. During rolling deformation process, the resultant stress in the matrix grains near whiskers changes continuously in that whisker rotate and fracture. As a result, some new plate texture components, different from the typical rolling texture components in monolithic aluminum alloys, are formed in the aluminum matrix of the ABOw/6061Al composite in the initial rolling stage.
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With increasing reduction, more and more whiskers rotate their longitude directions to be parallel to the rolling plane and direction. It leads to a more disordered stress distribution in the rolling plane, but a highly symmetrical stress distribution about the normal direction of the rolling plane. Consequently, some fiber textures are formed in the composites with a high reduction. It can be easily understood that the effect of thermal mismatch stress in the matrix grains far from whisker is weaker, which results in the grain deformation in this region similar to the grain deformation in monolithic alloys. Therefore, some typical rolling and recrystallization texture components of aluminum alloys are observed in the hot-rolled composites with a relatively high reduction, such as D {4 4 11} 11 11 8, Cube {0 0 1} 1 0 0 and R {1 2 4} 2 1 1. However, these textures are not dominant ones in the hotrolled ABOw/6061Al composites. 4. Conclusions A systematic analysis of hot-rolling texture evolution in ABOw/6061Al composite has been carried out at different reductions. The principal conclusions that can be drawn from this research are the following:
tion to an increasing extent. Meanwhile, the whisker fracture becomes more serious. The change of resultant stress of external stress and thermal mismatch stress may be responsible for the formation of new plate textures and fiber textures. Acknowledgements The work was supported by the National High Technology Research and Development Program of China and by a grant from the Specialized Research Fund for the Doctoral Program of Higher Education (No.20070213042). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
1. It is interesting to note that the hot-rolling texture evolution of ABOw/6061Al composite is distinguished from that of monolithic aluminum alloys, which results from the effect of the whiskers in the composite. 2. The dominant plate textures composed of {2 1 2} 3 10 2, {3 3 2} 1 1 3 and {1 2 2} 2 1 0 are formed in the composite with 30% reduction. As the reduction increases from 30% to 50%, some obvious fiber textures, {3 1 6} and {3 2 13} orientations, are formed through a transformation from plate textures to fiber textures. Further rolling deformation leads to the development of the fiber textures. 3. During the hot rolling process, the whisker rotation and fracture take place. With increasing reduction, the whiskers rotate their longitude directions to be parallel to the rolling plane and direc-
[11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26]
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