- Email: [email protected]
Al18B4O33w composite
Materials Science & Engineering A 615 (2014) 313–319
Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Effect of forging on the microstructure and tensile properties of 2024Al/Al18B4O33w composite W.C. Shi, L. Yuan, Z.Z. Zheng, D.B. Shan n School of Materials Science and Engineering, Harbin Institute of Technology, P.O. Box 435, No. 92 West Dazhi Street, Harbin 150001, Heilongjiang, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 8 July 2014 Received in revised form 25 July 2014 Accepted 27 July 2014 Available online 31 July 2014
The hot forging of the 2024Al/Al18B4O33w composite fabricated by the squeeze casting technique was carried out at 450 1C to improve the ductility. The microstructure and tensile properties of the as-cast and forged 2024Al/Al18B4O33w composites are studied. Non-homogeneous distribution of the reinforcement whiskers was revealed in the as-cast composite. The forging led to significant variations in the average length and distribution of the reinforcement whiskers in comparison with the as-cast composite. After forging, the tensile tests showed an increase in the ultimate tensile strength at room temperature and a decrease at elevated temperature. The forging highly increased the ductility both at room and elevated temperature. & 2014 Elsevier B.V. All rights reserved.
Keywords: Metal matrix composite Forging Elevated temperature Microstructure Tensile properties
1. Introduction Metal matrix composites (MMCs) have been widely used for automotive components and aircraft structures as they provide superior properties which cannot be achieved by traditional materials, such as high specific strength and stiffness, high specific modulus and good wear resistance [1–5]. In recent years, the casting methods are widely used for the simple process and can satisfy a number of requirements of the mechanical and physical properties. However, the ductility at room temperature is low, due to the brittle ceramic reinforcement and the non-uniform microstructure (like porosity and clusters of the whiskers) after casting [6–8]. Some studies demonstrate that the hot plastic forming processes, such as extrusion, rolling and forging, can minimize or eliminate porosity, can lead to a more uniform distribution and also improve the matrix–reinforcement interfacial bonding, with some improvements in strength and ductility [9–14]. Most of these papers deal with the extrusion behavior, while relatively few results are reported on the effect of the forging process on the whisker reinforced aluminum matrix composite [15–18]. By comparison, hot forging seems more problematic than hot extrusion. During the open-die forging, the workpiece suffers from not only compressive but tensile stress and strain also, which
n
Corresponding author. Tel./fax: þ 86 451 86418732. E-mail address: [email protected] (D.B. Shan).
http://dx.doi.org/10.1016/j.msea.2014.07.092 0921-5093/& 2014 Elsevier B.V. All rights reserved.
easily cause damage at the matrix–reinforcement interface. And the deformation degree should be reasonable to ensure uniform deformation without crack and satisfactory microstructure and property [19]. However, relatively few studies have been related to the tensile properties at elevated temperatures of the whiskers composites after forging. This study aims to study the effect of the forging process on microstructure and the tensile properties, at room and elevated temperature, of the whiskers reinforced aluminum matrix composite, consisting of 2024 matrix reinforced with 25 vol% of Al18B4O33.
2. Experimental procedures The composite used in this study was 25 vol% of Al18B4O33 with a diameter of 0.5–1 μm and a length of 10–20 μm reinforced 2024 aluminum alloy, fabricated by the squeeze casting technique. The specimens (20 mm in diameter and 14 mm in height) were cut from the as-cast billet and used for the open-die forging. Open-die forging was carried out by using a 250 kN press and a graphite-based lubricant. The forging process was performed at 450 1C, at an average engineering strain rate of 0.1 s 1. The deformation degrees (reduction in height) were 40%, 60% and 70%. Surface morphology of forged composites at different deformation degrees (Fig. 1) show that there are no damage at the surface at the deformation degrees of 40% and 60%, while the
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Fig. 1. Surface morphology of forged composites at the deformation degrees: (a) 40%; (b) 60%; (c) 70% and (d) details in zone “A”.
Fig. 2. Optical micrographs of (a) as-cast composite and forged composites at the deformation degrees of (b) 40%; (c) 60%; (d) 70%.
W.C. Shi et al. / Materials Science & Engineering A 615 (2014) 313–319
forged composite at the deformation degree of 70% suffers from damage at the surface (Fig. 1c and d). Tensile tests were carried out with a strain rate of 1 10 3 s 1 at temperatures of 25, 440, 460, 480, 500, and 520 1C, using an Instron-5500R universal testing machine. The test temperatures in a three-zone split furnace were conducted with three independent temperature controllers. The tensile samples, gage length of 12 mm, gage width of 3 mm and thickness of 1.5 mm, were machined from the as-cast composites and the forged composite at the deformation degree of 60% with the tensile axis perpendicular to the forging direction. The samples for microstructural investigations by optical microscopy (OM), transmission electron microscopy (TEM) and scanning electron microscopy (SEM), with energy dispersive spectroscopy (EDS), were cut from the as-cast composites and the forged composites along the forging direction. The fracture surfaces of the as-cast and forged composite, after the tensile tests, were examined by SEM to gain an understanding of the failure mechanisms.
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composite and significant decrease in the percent of voids was observed in the forged composite, the presence of which seemed limited to clustered regions. These are mainly induced by the plastic deformation of the matrix and the whisker rotation toward the most stable orientation perpendicular to the forging direction during the hot forging process. However, the different rotate velocities of each point along the length direction of the whiskers and the plastic mismatch between the matrix and the whiskers resulted in the stress concentration in the whiskers, which were easy to cause the primary longer whiskers breakage. As the increased deformation, the primary longer whiskers were broken once again, as shown in Fig. 5d. While a new type of voids between the broken whiskers, which was referred to the breakage of some whiskers to smaller ones, was noticeable at the deformation degree of 60%. At the deformation degree of 40%, the localized strain and the degree of matrix hardening were low, and the matrix was easily squeezed into the voids between the broken whiskers during the plastic forging process. When forging at the degree of 60%, due to the increased localized strain and the more matrix hardening, the matrix was
3. Results and discussions 3.1. Microstructure The optical micrographs of the composites in the as-cast condition and after the forging process are shown in Fig. 2. It was found that the distribution of aluminum borate whiskers in the forged composites was random, which was connected to those already existing in the ascast composite, and agglomerations or clusters were also observed, resulting in whisker-rich and whisker-free region (ovals and strips in Fig. 2). These results are probably due to the low deformation degrees, compared with the typical extrusion ratio and rolling reduction of other studies [17,20–22]. The most apparent difference between the as-cast and forged composite was that the average length of the Al18B4O33 whiskers, initially high in some regions in the as-cast composite, had decreased giving a small average length with the increased plastic deformation degree and some longer reinforcing whiskers were prone to break, as shown in Fig. 3. Fig. 4 shows TEM micrographs for the matrix grain structure of the as-cast composite and forged composite at the degree of 60%. The grain size of the matrix became small after forging due to recrystallization during hot forging deformation, where the thermal mismatch and dislocation offered the energy to activate the recrystallization process [20,23]. SEM (Fig. 5) of the composites indicated that the whisker-free regions became strips in shape which were ovals in the at-cast
Fig. 3. Average length of Al18B4O33 whiskers in as-cast composite and forged composites at different deformation degrees.
Fig. 4. TEM micrographs for the matrix grain structure of (a) as-cast composite and (b) forged composite at the deformation degree of 60%.
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Fig. 5. SEM back-scattered electron images of (a) as-cast composite and forged composites at the deformation degrees of (b) 40%; (c) 60%; (d) 70%.
not easily squeezed into the voids to fill them, which resulted in the new type of voids. While no cracking damage and debonding at the interface between matrix and broken whiskers were observed, as shown in Fig. 6. However, forging with a high reduction in height at the degree of 70%, the higher localized strain and matrix hardening resulted in debonding and the cracking damage (Fig. 5d). In the as-cast condition, the matrix microstructure consisted of aluminum solid solution and Al2CuMg (Fig. 7a and b) strengthening phases, as confirmed by the EDS analyses at the grain boundaries or in regions around the whiskers, forming a brittle nearly continuous network. These solute-enriched regions usually indicate the location of the last-solidifying liquid [24,25]. After the application of the forging process, the intermetallic compounds were probably broken by the plastic deformation (Fig. 5b–d).
3.2. Mechanical properties The ultimate tensile strength and elongation of the as-cast composite and forged composites at room temperature are shown in Fig. 8. The forged composites showed ultimate tensile strengths
and elongations higher than the as-cast composite at room temperature. Similar behavior was also reported after forging, hot extrusion and hot rolling of other MMCs [20,22,26]. This improvement in the forging composite can be mainly related to the effect of the previously discussed matrix hardening and microstructure changes, which showed a reduction of the voids, after forging, with smaller size of matrix grains. The ultimate tensile strength increased with the increased deformation degree. However, the elongation decreased at the deformation degree of 70%, which may be related to the cracking damage and debonding existed, as shown in Fig. 5d. However, the application of the forging process gave rise to a decrease in tensile strength compared with the at-cast composite at elevated temperature, as shown in Fig. 9. It is well known that the main factors which control the material strength of MMCs are the volume fraction, size, aspect ratio and distribution of the whisker, as well as the load carrying capability of the interface. A reduction of the voids after forging, without evidence of damaging induced by the forging process, should therefore increase the loadtransfer capability of the interface. So the decrease in tensile strength might be mainly attributed to the effect of the reinforcement distribution at elevated temperature. After forging, the presence of strips of the whisker-free regions, which were parallel
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Fig. 6. TEM micrographs of the broken whisker without cracking damage and debonding in forged composites at the deformation degree of 60%: (a) the broken whisker; (b) the region near the broken whisker; (c) SADP corresponding to Al and (d) SADP corresponding to Al18B4O33.
to the tensile axis, increased the large plastic mismatch between the matrix and the reinforcing whiskers compared with the at-cast composite. And the increased mismatch led to a consequent concentration of stresses near the reinforcing whiskers [27,28]. At elevated temperature, since the matrix flow stresses is low, the local stresses are not large enough to break the whiskers [13,29]. Cracks easily formed near the breakpoint, interface and the tip of whisker. Then, the whisker-free regions flowed easily and were priority to fracture. This caused a decrease in the ultimate tensile strength and an increase in the elongation of the forging composites, as shown in Fig. 10. The increase in ductility might also be attributed to the reduction of the voids induced by forging.
4. Fracture behavior SEM analyses of the fracture surfaces of the as-cast and forged composite at the deformation degree of 60%, tested at room
temperature (Fig. 11a and b), showed that the main failure mechanisms were interface debonding, while a small amount of broken reinforcing whiskers were present. Interface debonding depends on the high local stress concentration at the interfaces. When high load was transferred from the plastically deforming aluminum matrix to whiskers, the increased large plastic mismatch resulted in the high local stress. Then the high local stress can give rise to interfacial debonding and fracture of the reinforcing whiskers. The higher the tensile strength in the forging composite compared with the as-cast composite, the more the fractured whiskers were observed, as shown in Fig. 11a and b. The ductile dimples and “tear ridges”, fine near-featureless non-circular dimples, might be attributed to the constraints in plastic flow of the aluminum matrix, while the large voids and dimples can be associated with the interfacial debonding and fracture of the reinforcing whiskers. The ductile dimples in the strip of whiskerfree regions nearly parallel to the tensile axis after forging, which made a contribution to an increase in elongation, were smaller than the ovals in the at-cast composite.
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Fig. 7. (a) SEM back-scattered electron image of the as-cast composite and (b) EDS spectrum of the intermetallic compounds.
Fig. 8. Effect of forging on the ultimate tensile strength and elongation of the composites with different deformation degrees at room temperature.
Fig. 10. Elongation of the as-cast and forged composite at the deformation degree of 60%.
weakened the interface bond, which allowed the whiskers to pull out of the matrix (Fig. 11d). The main failure mechanisms are depended on the matrix regions and the voids nucleation. The “tear ridges” in whisker-free regions after forging were also more than the as-cast composite, as shown in Fig. 11c and d.
5. Conclusions
Fig. 9. Ultimate tensile strength of the as-cast and forged composite at the deformation degree of 60%.
At elevated temperature, the local stress decreased. The reinforcing whiskers were not easy to fracture. However, the matrix had to accommodate the stress, which was able to promote voids nucleation concentrated in the matrix. The matrix fracture
In the present study, agglomerations or clusters are present before and after forging process. Forging process reduces the voids and the average length of the Al18B4O33 whiskers. The microstructural modifications in the forged composite induce an increase in the ultimate tensile strength of the material at room temperature. At elevated temperature, a decrease in the ultimate tensile strength and an increase in the tensile elongation of the forging composites are mainly related to the shape of the whiskerfree regions, which deform into strips from ovals in the at-cast composite. Fracture surface, which is characterized by whisker reinforcement debonding at room temperature, is depended on the matrix regions at the elevated temperature.
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Fig. 11. Fracture surfaces of (a) as-cast and (b) forged composite at the deformation degree of 60% tested at room temperature; (c) as-cast and (d) forged composite at the deformation degree of 60% tested at 500 1C.
References [1] N. Chawla, Y.L. Shen, Adv. Eng. Mater. 3 (2001) 357–370. [2] B. Torres, M. Lieblich, J. Ibanez, A. Garcia-Escorial, Scr. Mater. 47 (2002) 45–49. [3] A. Sakthivel, R. Palaninathan, R. Velmurugan, P. Raghothama Rao, J. Mater. Sci. 43 (2008) 7047–7056. [4] J. Pan, S.P. Lee, M. Yoshida, G. Sasaki, N. Fuyama, T. Fujii, H. Fukunaga, Adv. Compos. Mater. 10 (2001) 299–307. [5] D.R. Ni, D.L. Chen, D. Wang, B.L. Xiao, Z.Y. Ma, Mater. Sci. Eng. A 608 (2014) 1–10. [6] X.J. Wang, N.Z. Wang, L.Y. Wang, X.S. Hua, K. Wu, Y.Q. Wang, Y.D. Huang, Mater. Des. 57 (2014) 638–645. [7] V. Imayev, R. Gaisin, E. Gaisina, R. Imayev, H.-J. Fecht, F. Pyczak, Mater. Sci. Eng. A 609 (2014) 34–41. [8] S.C. Xu, L.D. Wang, P.T. Zhao, W.L. Li, W.D. Fei, Mater. Sci. Eng. A 550 (2012) 146–151. [9] S. Dikshit, V. Gurjar, R. Dasgupta, S. Chaturvedi, K.K. Pathak, A.K. Jha, J. Mater. Sci. 45 (2010) 4174–4179. [10] S. Spigarelli, E. Cerri, P. Cavaliere, E. Evangelista, Mater. Sci. Eng. A 327 (2002) 144–154. [11] R. Narayanasamy, T. Ramesh, K.S. Pandey, Mater. Des. 29 (2008) 1212–1227. [12] L.M. Tham, M. Gupta, L. Cheng, Mater. Sci. Eng. A 326 (2002) 355–363. [13] L. Ceschini, G. Minak, A. Morri, Compos. Sci. Technol. 66 (2006) 333–342. [14] S. Lim, Y. Nishida, Scr. Metall. Mater. 32 (1995) 1911–1915.
[15] R. Thimmarayan, G. Thanigaiyarasu, Int. J. Adv. Manuf. Technol. 48 (2010) 625–632. [16] T.S. Srivatsan, M. Al-Hajri, C. Smith, M. Petraroli, Mater. Sci. Eng. A 346 (2003) 91–100. [17] C.H.J. Davies, Key Eng. Mater. 104–107 (1995) 447–458. [18] R. Rahmani Fard, F. Akhlaghi, J. Mater. Process. Technol. 187–188 (2007) 433–436. [19] S.K. Guan, L.H. Wu, P. Wang, Mater. Sci. Eng. A 499 (2009) 187–191. [20] A. EI-Sabbagh, M. Soliman, M.A. Taha, H. Palkowski, J. Mater. Process. Technol. 213 (2013) 1669–1681. [21] Y. Nishida, I. Sigematsu, H. Arima, J.C. Kim, T. Ando, J. Mater. Sci. Lett. 21 (2002) 465–468. [22] C. Badini, G.M. La Vecchia, P. Fino, T. Valente, J. Mater. Process. Technol. 116 (2001) 289–297. [23] A. El-Sabbagh, M. Soliman, M. Taha, H. Palkowski, J. Mater. Process. Technol. 212 (2012) 497–508. [24] P.B. Prangnell, S.J. Barnes, S.M. Roberts, P.J. Withers, Mater. Sci. Eng. A 220 (1996) 41–56. [25] S.J. Barnes, P.B. Prangnell, S.M. Roberts, P.J. Withers, Scr. Metall. Mater. 33 (1995) 323–329. [26] Y.H. Seo, C.G. Kang, Compos. Sci. Technol. 59 (1999) 643–654. [27] L. Wang, J. Hu, Z.J. Li, W.D. Fei, Mater. Sci. Eng. A 497 (2008) 358–362. [28] Z.G. Wang, S. Li, L. Sun, Key Eng. Mater. 104–107 (1995) 729–748. [29] H.Z. Ding, H. Biermann, O. Hartmann, Int. J. Fatigue 25 (2003) 209–220.
Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Effect of forging on the microstructure and tensile properties of 2024Al/Al18B4O33w composite W.C. Shi, L. Yuan, Z.Z. Zheng, D.B. Shan n School of Materials Science and Engineering, Harbin Institute of Technology, P.O. Box 435, No. 92 West Dazhi Street, Harbin 150001, Heilongjiang, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 8 July 2014 Received in revised form 25 July 2014 Accepted 27 July 2014 Available online 31 July 2014
The hot forging of the 2024Al/Al18B4O33w composite fabricated by the squeeze casting technique was carried out at 450 1C to improve the ductility. The microstructure and tensile properties of the as-cast and forged 2024Al/Al18B4O33w composites are studied. Non-homogeneous distribution of the reinforcement whiskers was revealed in the as-cast composite. The forging led to significant variations in the average length and distribution of the reinforcement whiskers in comparison with the as-cast composite. After forging, the tensile tests showed an increase in the ultimate tensile strength at room temperature and a decrease at elevated temperature. The forging highly increased the ductility both at room and elevated temperature. & 2014 Elsevier B.V. All rights reserved.
Keywords: Metal matrix composite Forging Elevated temperature Microstructure Tensile properties
1. Introduction Metal matrix composites (MMCs) have been widely used for automotive components and aircraft structures as they provide superior properties which cannot be achieved by traditional materials, such as high specific strength and stiffness, high specific modulus and good wear resistance [1–5]. In recent years, the casting methods are widely used for the simple process and can satisfy a number of requirements of the mechanical and physical properties. However, the ductility at room temperature is low, due to the brittle ceramic reinforcement and the non-uniform microstructure (like porosity and clusters of the whiskers) after casting [6–8]. Some studies demonstrate that the hot plastic forming processes, such as extrusion, rolling and forging, can minimize or eliminate porosity, can lead to a more uniform distribution and also improve the matrix–reinforcement interfacial bonding, with some improvements in strength and ductility [9–14]. Most of these papers deal with the extrusion behavior, while relatively few results are reported on the effect of the forging process on the whisker reinforced aluminum matrix composite [15–18]. By comparison, hot forging seems more problematic than hot extrusion. During the open-die forging, the workpiece suffers from not only compressive but tensile stress and strain also, which
n
Corresponding author. Tel./fax: þ 86 451 86418732. E-mail address: [email protected] (D.B. Shan).
http://dx.doi.org/10.1016/j.msea.2014.07.092 0921-5093/& 2014 Elsevier B.V. All rights reserved.
easily cause damage at the matrix–reinforcement interface. And the deformation degree should be reasonable to ensure uniform deformation without crack and satisfactory microstructure and property [19]. However, relatively few studies have been related to the tensile properties at elevated temperatures of the whiskers composites after forging. This study aims to study the effect of the forging process on microstructure and the tensile properties, at room and elevated temperature, of the whiskers reinforced aluminum matrix composite, consisting of 2024 matrix reinforced with 25 vol% of Al18B4O33.
2. Experimental procedures The composite used in this study was 25 vol% of Al18B4O33 with a diameter of 0.5–1 μm and a length of 10–20 μm reinforced 2024 aluminum alloy, fabricated by the squeeze casting technique. The specimens (20 mm in diameter and 14 mm in height) were cut from the as-cast billet and used for the open-die forging. Open-die forging was carried out by using a 250 kN press and a graphite-based lubricant. The forging process was performed at 450 1C, at an average engineering strain rate of 0.1 s 1. The deformation degrees (reduction in height) were 40%, 60% and 70%. Surface morphology of forged composites at different deformation degrees (Fig. 1) show that there are no damage at the surface at the deformation degrees of 40% and 60%, while the
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Fig. 1. Surface morphology of forged composites at the deformation degrees: (a) 40%; (b) 60%; (c) 70% and (d) details in zone “A”.
Fig. 2. Optical micrographs of (a) as-cast composite and forged composites at the deformation degrees of (b) 40%; (c) 60%; (d) 70%.
W.C. Shi et al. / Materials Science & Engineering A 615 (2014) 313–319
forged composite at the deformation degree of 70% suffers from damage at the surface (Fig. 1c and d). Tensile tests were carried out with a strain rate of 1 10 3 s 1 at temperatures of 25, 440, 460, 480, 500, and 520 1C, using an Instron-5500R universal testing machine. The test temperatures in a three-zone split furnace were conducted with three independent temperature controllers. The tensile samples, gage length of 12 mm, gage width of 3 mm and thickness of 1.5 mm, were machined from the as-cast composites and the forged composite at the deformation degree of 60% with the tensile axis perpendicular to the forging direction. The samples for microstructural investigations by optical microscopy (OM), transmission electron microscopy (TEM) and scanning electron microscopy (SEM), with energy dispersive spectroscopy (EDS), were cut from the as-cast composites and the forged composites along the forging direction. The fracture surfaces of the as-cast and forged composite, after the tensile tests, were examined by SEM to gain an understanding of the failure mechanisms.
315
composite and significant decrease in the percent of voids was observed in the forged composite, the presence of which seemed limited to clustered regions. These are mainly induced by the plastic deformation of the matrix and the whisker rotation toward the most stable orientation perpendicular to the forging direction during the hot forging process. However, the different rotate velocities of each point along the length direction of the whiskers and the plastic mismatch between the matrix and the whiskers resulted in the stress concentration in the whiskers, which were easy to cause the primary longer whiskers breakage. As the increased deformation, the primary longer whiskers were broken once again, as shown in Fig. 5d. While a new type of voids between the broken whiskers, which was referred to the breakage of some whiskers to smaller ones, was noticeable at the deformation degree of 60%. At the deformation degree of 40%, the localized strain and the degree of matrix hardening were low, and the matrix was easily squeezed into the voids between the broken whiskers during the plastic forging process. When forging at the degree of 60%, due to the increased localized strain and the more matrix hardening, the matrix was
3. Results and discussions 3.1. Microstructure The optical micrographs of the composites in the as-cast condition and after the forging process are shown in Fig. 2. It was found that the distribution of aluminum borate whiskers in the forged composites was random, which was connected to those already existing in the ascast composite, and agglomerations or clusters were also observed, resulting in whisker-rich and whisker-free region (ovals and strips in Fig. 2). These results are probably due to the low deformation degrees, compared with the typical extrusion ratio and rolling reduction of other studies [17,20–22]. The most apparent difference between the as-cast and forged composite was that the average length of the Al18B4O33 whiskers, initially high in some regions in the as-cast composite, had decreased giving a small average length with the increased plastic deformation degree and some longer reinforcing whiskers were prone to break, as shown in Fig. 3. Fig. 4 shows TEM micrographs for the matrix grain structure of the as-cast composite and forged composite at the degree of 60%. The grain size of the matrix became small after forging due to recrystallization during hot forging deformation, where the thermal mismatch and dislocation offered the energy to activate the recrystallization process [20,23]. SEM (Fig. 5) of the composites indicated that the whisker-free regions became strips in shape which were ovals in the at-cast
Fig. 3. Average length of Al18B4O33 whiskers in as-cast composite and forged composites at different deformation degrees.
Fig. 4. TEM micrographs for the matrix grain structure of (a) as-cast composite and (b) forged composite at the deformation degree of 60%.
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Fig. 5. SEM back-scattered electron images of (a) as-cast composite and forged composites at the deformation degrees of (b) 40%; (c) 60%; (d) 70%.
not easily squeezed into the voids to fill them, which resulted in the new type of voids. While no cracking damage and debonding at the interface between matrix and broken whiskers were observed, as shown in Fig. 6. However, forging with a high reduction in height at the degree of 70%, the higher localized strain and matrix hardening resulted in debonding and the cracking damage (Fig. 5d). In the as-cast condition, the matrix microstructure consisted of aluminum solid solution and Al2CuMg (Fig. 7a and b) strengthening phases, as confirmed by the EDS analyses at the grain boundaries or in regions around the whiskers, forming a brittle nearly continuous network. These solute-enriched regions usually indicate the location of the last-solidifying liquid [24,25]. After the application of the forging process, the intermetallic compounds were probably broken by the plastic deformation (Fig. 5b–d).
3.2. Mechanical properties The ultimate tensile strength and elongation of the as-cast composite and forged composites at room temperature are shown in Fig. 8. The forged composites showed ultimate tensile strengths
and elongations higher than the as-cast composite at room temperature. Similar behavior was also reported after forging, hot extrusion and hot rolling of other MMCs [20,22,26]. This improvement in the forging composite can be mainly related to the effect of the previously discussed matrix hardening and microstructure changes, which showed a reduction of the voids, after forging, with smaller size of matrix grains. The ultimate tensile strength increased with the increased deformation degree. However, the elongation decreased at the deformation degree of 70%, which may be related to the cracking damage and debonding existed, as shown in Fig. 5d. However, the application of the forging process gave rise to a decrease in tensile strength compared with the at-cast composite at elevated temperature, as shown in Fig. 9. It is well known that the main factors which control the material strength of MMCs are the volume fraction, size, aspect ratio and distribution of the whisker, as well as the load carrying capability of the interface. A reduction of the voids after forging, without evidence of damaging induced by the forging process, should therefore increase the loadtransfer capability of the interface. So the decrease in tensile strength might be mainly attributed to the effect of the reinforcement distribution at elevated temperature. After forging, the presence of strips of the whisker-free regions, which were parallel
W.C. Shi et al. / Materials Science & Engineering A 615 (2014) 313–319
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Fig. 6. TEM micrographs of the broken whisker without cracking damage and debonding in forged composites at the deformation degree of 60%: (a) the broken whisker; (b) the region near the broken whisker; (c) SADP corresponding to Al and (d) SADP corresponding to Al18B4O33.
to the tensile axis, increased the large plastic mismatch between the matrix and the reinforcing whiskers compared with the at-cast composite. And the increased mismatch led to a consequent concentration of stresses near the reinforcing whiskers [27,28]. At elevated temperature, since the matrix flow stresses is low, the local stresses are not large enough to break the whiskers [13,29]. Cracks easily formed near the breakpoint, interface and the tip of whisker. Then, the whisker-free regions flowed easily and were priority to fracture. This caused a decrease in the ultimate tensile strength and an increase in the elongation of the forging composites, as shown in Fig. 10. The increase in ductility might also be attributed to the reduction of the voids induced by forging.
4. Fracture behavior SEM analyses of the fracture surfaces of the as-cast and forged composite at the deformation degree of 60%, tested at room
temperature (Fig. 11a and b), showed that the main failure mechanisms were interface debonding, while a small amount of broken reinforcing whiskers were present. Interface debonding depends on the high local stress concentration at the interfaces. When high load was transferred from the plastically deforming aluminum matrix to whiskers, the increased large plastic mismatch resulted in the high local stress. Then the high local stress can give rise to interfacial debonding and fracture of the reinforcing whiskers. The higher the tensile strength in the forging composite compared with the as-cast composite, the more the fractured whiskers were observed, as shown in Fig. 11a and b. The ductile dimples and “tear ridges”, fine near-featureless non-circular dimples, might be attributed to the constraints in plastic flow of the aluminum matrix, while the large voids and dimples can be associated with the interfacial debonding and fracture of the reinforcing whiskers. The ductile dimples in the strip of whiskerfree regions nearly parallel to the tensile axis after forging, which made a contribution to an increase in elongation, were smaller than the ovals in the at-cast composite.
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Fig. 7. (a) SEM back-scattered electron image of the as-cast composite and (b) EDS spectrum of the intermetallic compounds.
Fig. 8. Effect of forging on the ultimate tensile strength and elongation of the composites with different deformation degrees at room temperature.
Fig. 10. Elongation of the as-cast and forged composite at the deformation degree of 60%.
weakened the interface bond, which allowed the whiskers to pull out of the matrix (Fig. 11d). The main failure mechanisms are depended on the matrix regions and the voids nucleation. The “tear ridges” in whisker-free regions after forging were also more than the as-cast composite, as shown in Fig. 11c and d.
5. Conclusions
Fig. 9. Ultimate tensile strength of the as-cast and forged composite at the deformation degree of 60%.
At elevated temperature, the local stress decreased. The reinforcing whiskers were not easy to fracture. However, the matrix had to accommodate the stress, which was able to promote voids nucleation concentrated in the matrix. The matrix fracture
In the present study, agglomerations or clusters are present before and after forging process. Forging process reduces the voids and the average length of the Al18B4O33 whiskers. The microstructural modifications in the forged composite induce an increase in the ultimate tensile strength of the material at room temperature. At elevated temperature, a decrease in the ultimate tensile strength and an increase in the tensile elongation of the forging composites are mainly related to the shape of the whiskerfree regions, which deform into strips from ovals in the at-cast composite. Fracture surface, which is characterized by whisker reinforcement debonding at room temperature, is depended on the matrix regions at the elevated temperature.
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Fig. 11. Fracture surfaces of (a) as-cast and (b) forged composite at the deformation degree of 60% tested at room temperature; (c) as-cast and (d) forged composite at the deformation degree of 60% tested at 500 1C.
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