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Tensile properties of Al–12Si matrix composites reinforced with Ti–Al-based particles
Journal of Alloys and Compounds 630 (2015) 256–259
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Tensile properties of Al–12Si matrix composites reinforced with Ti–Al-based particles Z. Wang a,b,c,⇑, K.G. Prashanth a, A.K. Chaubey d, L. Löber a, F.P. Schimansky e, F. Pyczak e, W.W. Zhang b, S. Scudino a, J. Eckert a,f a
IFW Dresden, Institut für Komplexe Materialien, Postfach 270116, D-01171 Dresden, Germany School of Mechanical and Automotive Engineering, South China University of Technology, Guangzhou 510640, China WPI Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan d Institute of Minerals and Materials Technology, Bhubaneswar 751013, India e Helmholtz-Zentrum Geesthacht, D-21502 Geesthacht, Germany f TU Dresden, Institut für Werkstoffwissenschaft, D-01062 Dresden, Germany b c
a r t i c l e
i n f o
Article history: Received 25 November 2014 Received in revised form 25 December 2014 Accepted 26 December 2014 Available online 20 January 2015 Keywords: Metal matrix composites TiAl based alloy Powder metallurgy Mechanical properties
a b s t r a c t Lightweight composites consisting of an Al–12Si matrix reinforced with Ti–Al–Nb–Mo–B (TNM) crystalline particles have been fabricated by powder metallurgy through hot pressing and hot extrusion. A detailed microstructural characterization was carried out along with the analysis of their mechanical properties. The results reveal that the tensile properties of the composites are effectively improved by the addition of the reinforcing TNM particles. The high strength combined with the low density gives rise to composites with good specific strength. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction Al–Si alloys, widely used foundry alloys, have significant technological importance due to their good castability and high specific properties, e.g. high specific strength, finding a wide variety of applications in the transportation sector [1]. However, the relatively low strength of Al–Si alloys restricts their application. The development of aluminum matrix composites (AMCs), through the combination of the low density and good ductility of the aluminum matrices and the high hardness and strength of the reinforcements, is one of the promising ways to strengthen Al alloys [2–10]. Ceramic particles are conventionally used as reinforcements; however, their use generally implies wettability and incompatibility problems with the metallic matrix. For example, the poor wettability of the ceramic particles may lead to weak interfaces between metallic matrix and reinforcements [2]. Also the thermal expansion differences between the ceramic and metal may induce crack formation of the ceramic particles during the producing upon cooling [3]. The poor interface and the cracks may then act as weak ⇑ Corresponding author at: WPI Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan. Tel.: +81 22 217 5956. E-mail addresses: [email protected], [email protected] (Z. Wang). http://dx.doi.org/10.1016/j.jallcom.2014.12.254 0925-8388/Ó 2015 Elsevier B.V. All rights reserved.
areas and severely deteriorate the mechanical properties of ceramic-reinforced composites. This work aims to develop AMCs reinforced with lightweight metallic particles having good mechanical and thermal compatibility with the Al matrix, being simultaneously inexpensive compared with other non-ceramic reinforcements, such as quasicrystals and metallic glasses [4–10]. Ti–Al–Nb–Mo–B (TNM) alloys are lightweight materials with good room and elevated temperature mechanical properties [11]. These properties make these alloys the ideal candidate as reinforcement in the AMCs. In this study, gas atomized Ti52.4Al42.2Nb4.4Mo0.9B0.06 TNM powder is used as reinforcing agent in Al–12Si matrix composites and their structure and mechanical behavior have been characterized in detail. 2. Experimental In order to ensure the homogeneous distribution of the reinforcing particles, powder mixtures consisting of gas atomized Al–12Si (wt.%) with 20 and 40 vol.% of gas atomized Ti52.4Al42.2Nb4.4Mo0.9B0.06 (at.%) were prepared by room temperature milling using a Retsch PM400 planetary ball mill with hardened steel balls and vials (milling time 1.5 h; rotational speed 100 rpm). Bulk samples were prepared by uniaxial hot pressing followed by hot extrusion under Ar atmosphere. During hot pressing, the powders were heated up to 673 K at a heating rate of 40 K/min and then a pressure of 600 MPa was applied for 10 min; after that, the pressure was removed and the sample was cooled down to room temperature using air cooling. The hot pressed sample was extruded (extrusion ration 6:1) at 673 K using a
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pressure of 700 MPa. The extrusion time is in the range of 30–60 min, depending on the volume fraction of the TNM particles. The microstructure was studied by scanning electron microscopy (SEM), using a Gemini 1530 microscope equipped with an energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD) using a D3290 PANalytical X’pert PRO diffractometer (Co Ka radiation, k = 0.17889 nm). Tensile tests were carried out with an INSTRON 8562 testing facility under quasistatic loading (strain rate of 1 104 s1) at room temperature. Vickers microhardness measurements were carried out using a Shimadzu HMV2000 hardness testing machine (applied load 0.025 kgf (0.245 N), dwell time 10 s).
3. Results and discussion Fig. 1 shows the morphology of the Al–12Si and TNM powders. The particles display spherical shape with diameter in the range of 3–50 lm and average value of 11 lm for the Al–12Si powders and with diameter in the range of 3–55 lm and average value of 17 lm for the TNM particles. The XRD patterns of the composites reinforced with 20 and 40 vol.% of TNM (f = 20 and f = 40), Al–12Si matrix and TNM reinforcement are presented in Fig. 2. The diffraction signals of a-Al solid solution and Si (from Al–12Si matrix) along with the signals of the TNM phase, such as AlTi3 and MoNb are observed. No diffraction peaks belonging to additional phases can be detected, implying that no reaction occurs between matrix and reinforcement during consolidation. Fig. 3a–c shows the SEM micrographs of the cross-section of the composites with f = 20 and 40. No pores are observed, indicating high densification of the materials. The high densification of the composites is further confirmed by the high density achieved for the extruded Al–12Si alloy and composites f = 20 and 40, respectively, 2.628 ± 0.006 g/cm3, 2.891 ± 0.098 g/cm3 and 3.186 ± 0.030 g/cm3. The density of the TNM particles is 3.972 g/cm3. Because there is no additional phase formed during consolidation (see Fig. 2), the porosity can be approximately calculated by the difference between the measured and theoretical density based on the assumption that no reaction between the reinforcements and matrix occurred. As a result, the calculated porosity for the extruded Al–12Si alloy and composites with f = 20 and 40 are 0.8%, 0.8% and 0%, respectively. The SEM images in Fig. 3 display a microstructure consisting of bright areas (the TNM reinforcement) homogeneously distributed in the Al–12Si matrix (the dark area). However, the presence of clusters of TNM particles can be also observed, particularly for the sample with f = 40 (Fig. 3b). EDX analysis of the composite with f = 40 (Fig. 3d–f), corresponding to the area in Fig. 3c, shows that the bright areas are rich in Ti and the dark matrix is rich in Al and Si. The small dark-gray particles present in the Al–12Si matrix (Fig. 3c) are rich in Si, indicating the precipitation of Si from the Al–12Si matrix. The Si precipitation is corroborated by the XRD results (Fig. 2), where the intensity of the Si peaks increases with the increasing volume fraction of TNM. These results indicate that the addition of TNM particles can accelerate Si precipitation from the Al–12Si matrix. The diffusion of Ti from the TNM particles to the Al–12Si matrix (see Fig. 3f) enhances the Si precipitation, which in turn reduces the Si solubility in Al. The diffusion of Ti into the Al–12Si matrix may happen during of these powder metallurgy processes like mechanical milling, hot pressing and hot extrusion.
Fig. 2. XRD patterns obtained from composites (f = 20 and f = 40), Al–12Si alloy and TNM alloy.
Typical room temperature uni-axial tensile stress–strain curves for the composites are shown in Fig. 4 together with the curve for the unreinforced Al–12Si matrix. The yield strength ry (0.2% offset) increases from 104 MPa for Al–12Si to about 125 MPa and 150 MPa for the composites reinforced with 20 vol.% and 40 vol.% TNM, respectively. Similarly, the ultimate tensile strength (UTS) increases from 174 MPa for Al–12Si to 200 MPa and 240 MPa respectively for the composites with f = 20 and 40. At the same time, the specific strength increases with increasing TNM volume fraction, from 66 103 N m kg1 of Al–12Si alloy to 69 N m kg1 (f = 20) and 75 N m kg1 (f = 40). The increase in strength of the composites occurs at the expense of the plastic strain, which decreases from 16% for the unreinforced matrix to about 10% and 4% for the composites with 20% and 40% reinforcement. The fracture morphology of the unreinforced Al–12Si matrix and composites after the tensile tests are shown in Fig. 5. Ductile and brittle failure modes are found in the Al–12Si matrix and TNM particles, respectively: micro-scaled dimples are observed in the matrix, whereas flowery cleavages are visible in the TNM particles. In addition TNM particle cracking and TNM particle decohesion from the matrix were also observed after tensile test. This might be attributed to the interfacial strength, which is close to the strength of the TNM particle. This suggests that good bonding is established between the TNM particle and Al–12Si matrix by the hot consolidation process. It is interesting to note that the dimple size in the Al–12Si matrix increases with increasing volume fraction of TNM particle, which is clearly seen from the high magnification SEM images shown in Fig. 5d–f. This is quantitatively summarized in Fig. 6, which shows that the dimple diameter increases from 1.36 lm for the unreinforced Al–12Si alloy to 1.66 lm and 2.16 lm for the composites with f = 20 and 40. Fractured Si precipitates are also observed at the ends of the
Fig. 1. SEM micrographs for the gas-atomized powders: (a) Al–12Si powder, (b) TNM powder.
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Fig. 3. SEM micrographs and EDX of the composites: (a) f = 20, (b and c) f = 40 and (d–f) the corresponding EDX of the microstructure as shown in (c).
Fig. 4. Tensile stress–strain curves for the composites and Al–12Si alloy, the inset image is the experimental yield strength of the composites as a function of the reinforcement content and yield strength calculated by the iso-stress model. The strength of TNM alloy is taken from the research [12].
dimples (Fig. 5c and f), indicating that the nucleation of the cracks occur preferentially near the small brittle Si precipitates, which is closely analogous with the annealing of Al–12Si produced by selective laser melting [13]. This is supported by the microstructure of
Fig. 6. The fractography dimple size as a function of the TNM volume fraction.
the composites, where the dimple size is close to the size of the Si precipitates, as seen in the composites with f = 40 (Figs. 3c and 5f). The hardness of the TNM particle in the composites is 562 ± 8 Hv, which is much higher than the matrix (64 ± 3 Hv). The strengthening
Fig. 5. SEM images showing the fracture surfaces of the composites: (a and d) f = 0, (b and e) f = 20 and (c and f) f = 40, where (f) is the higher magnification image from the rectangle area in (c).
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be noted that the strength of the Al–12Si alloy from the reference [18] is higher while the ductility is lower than the present work, which may be due to the presence of different Si morphology (needle-like Si phase). Although the present Al–Si composites reinforced with TNM particles display a significantly lower strength than the composites reinforced with flyash, they are stronger than the composites reinforced with ceramic particles or graphite. In addition, they exhibit good plastic deformation and thus represent a positive combination of strength and ductility, as observed for the composite reinforced with 20 vol.%, which shows ultimate strength of 200 MPa along with elongation of 10%. 4. Conclusion
Fig. 7. A comparison of Al–Si matrix composites reinforced with different phases. (See above-mentioned references for further information.)
effect observed in Fig. 4 is in good agreement with the value calculated by one of the Rule of Mixtures, the iso-stress model rcy ¼ ðV p =rpy þ V m =rmy Þ1 ; where V is the volume fraction, ry is the yield strength and the subscripts c, p and m indicate the composite, the reinforcement and the matrix), which assumes that the composite exhibits equal stress in the two components matrix and reinforcement [14] (see inset in Fig. 4). On the other hand, the strength calculated by the iso-strain (rcy = Vp rpy + Vm rmy), which assumes that the two components experience the same strain during deformation, is 157 MPa and 347 MPa for the composites with f = 20 and 40, respectively. This is much higher than the real experimental value; the reason is probably that the distance between the TNM particles is too large to transfer the load efficiently from the soft matrix to the hard particles, resulting into a larger strain in the matrix. This is in accord with the observation that the composites with low volume fraction of hard particles in the soft matrix obey the iso-stress model rather than iso-strain model [7,14]. Beside the strengthening effect of the TNM particles, the Si precipitates may also contribute to the increased strength. However, the strengthening contribution of the Si particles is expected to be low; for example, the yield strength increases only marginally from 75 MPa to 81 MPa with increasing the Si content of the Al–Si alloys from 10 to 19 wt.% [15]. However, the presence of the Si particles has a significant effect on the fracture of the composites. The cracks that nucleate at the Si precipitates, along with the de-bonding of TNM particles, act as the potential nucleation sites for the macroscopic cracks and finally deteriorate the plasticity of the composites. The effect of the TNM particles as strengthening agent for the Al–Si matrix is compared in Fig. 7 with different kinds of reinforcements. Most of the composites, including the composites reinforced with the typical ceramics, such as SiC and Al2O3, are located at the left-bottom area, where both the UTS and elongation are relatively low. A significant enhancement of the strength of Al–Si matrix composites has been achieved by using flyash particulates [18]; the UTS of these composites can reach 305 MPa by adding 15 wt.% flyash particles. However, the elongation for these composites is rather limited, ranging between 2% and 7%. It has to
Highly dense Al–12Si matrix composite reinforced with TNM particles have been synthesized by powder metallurgy through hot pressing followed by hot extrusion. The TNM reinforcements have good bonding with the matrix and accelerate the precipitation of Si in the matrix. Compared with the unreinforced Al–12Si, the composites exhibit improved tensile yield and ultimate strength, and specific strength, which are caused by the reinforcement effect of the TNM particles. However, the composites also display reduced plasticity owing to the addition of TNM particles and Si precipitation in the matrix. Acknowledgements This work is supported by the Guangdong – Natural Science Foundation of China (GD-NSFC) Foundation (U1034001), Natural Science Foundation of China (51374110). Z. Wang would like to acknowledge the fellowship support from the China Scholarship Council. References [1] V. Raghavan, Physical Metallurgy: Principles and Practice, Prentice Hall of India Private Limited, New Delhi, 1998. [2] N. Chawla, K.K. Chawla, Metal Matrix Composites, Springer, New York, 2006. [3] S. Elomari, R. Boukhili, D.J. Lloyd, Acta Mater. 44 (1996) 1873–1882. [4] Z. Wang, J. Tan, B. Sun, S. Scudino, K.G. Prashanth, W. Zhang, Y. Li, J. Eckert, Mater. Sci. Eng. A 600 (2014) 53–58. [5] D. Dudina, K. Georgarakis, M. Aljerf, Y. Li, M. Braccini, A. Yavari, A. Inoue, Composites Part A 41 (2010) 1551–1557. [6] F. Ali, S. Scudino, M.S. Anwar, R.N. Shahid, V.C. Srivastava, V. Uhlenwinkel, M. Stoica, G. Vaughan, J. Eckert, J. Alloys Comp. 607 (2014) 274–279. [7] S. Scudino, G. Liu, K.G. Prashanth, B. Bartusch, K.B. Surreddi, B.S. Murty, J. Eckert, Acta Mater. 57 (2009) 2029–2039. [8] S. Scudino, K.B. Surreddi, S. Sager, M. Sakaliyska, J.S. Kim, W. Loser, J. Eckert, J. Mater. Sci. 43 (2008) 4518–4526. [9] Z. Wang, J. Tan, S. Scudino, B. Sun, R. Qu, J. He, K.G. Prashanth, W. Zhang, Y. Li, J. Eckert, Adv. Powder Technol. 25 (2014) 635–639. [10] M. Aljerf, K. Georgarakis, D. Louzguine-Luzgin, A.L. Moulec, A. Inoue, A. Yavari, Mater. Sci. Eng. A 532 (2012) 325–330. [11] H. Clemens, W. Wallgram, S. Kremmer, V. Güther, A. Otto, A. Bartels, Adv. Eng. Mater. 10 (2008) 707–713. [12] D. Hu, Intermetallics 10 (2002) 851–858. [13] K.G. Prashanth, S. Scudino, H. Klauss, K.B. Surreddi, L. Loeber, Z. Wang, A. Chaubey, U. Kühn, J. Eckert, Mater. Sci. Eng. A 590 (2014) 153–160. [14] H.S. Kim, Mater. Sci. Eng. A 289 (2000) 30–33. [15] M. Gupta, S. Ling, J. Alloys Comp. 287 (1999) 284–294. [16] J. Ejiofor, R. Reddy, JOM 49 (1997) 31–37. [17] I. EI-Mahallawi, H. Abdelkader, L. Yousef, A. Amer, J. Mayer, A. Schwedt, Mater. Sci. Eng. A 556 (2012) 76–87. [18] M. Ramachandra, K. Radhakrishna, J. Mater. Sci. 40 (2005) 5989–5997.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Tensile properties of Al–12Si matrix composites reinforced with Ti–Al-based particles Z. Wang a,b,c,⇑, K.G. Prashanth a, A.K. Chaubey d, L. Löber a, F.P. Schimansky e, F. Pyczak e, W.W. Zhang b, S. Scudino a, J. Eckert a,f a
IFW Dresden, Institut für Komplexe Materialien, Postfach 270116, D-01171 Dresden, Germany School of Mechanical and Automotive Engineering, South China University of Technology, Guangzhou 510640, China WPI Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan d Institute of Minerals and Materials Technology, Bhubaneswar 751013, India e Helmholtz-Zentrum Geesthacht, D-21502 Geesthacht, Germany f TU Dresden, Institut für Werkstoffwissenschaft, D-01062 Dresden, Germany b c
a r t i c l e
i n f o
Article history: Received 25 November 2014 Received in revised form 25 December 2014 Accepted 26 December 2014 Available online 20 January 2015 Keywords: Metal matrix composites TiAl based alloy Powder metallurgy Mechanical properties
a b s t r a c t Lightweight composites consisting of an Al–12Si matrix reinforced with Ti–Al–Nb–Mo–B (TNM) crystalline particles have been fabricated by powder metallurgy through hot pressing and hot extrusion. A detailed microstructural characterization was carried out along with the analysis of their mechanical properties. The results reveal that the tensile properties of the composites are effectively improved by the addition of the reinforcing TNM particles. The high strength combined with the low density gives rise to composites with good specific strength. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction Al–Si alloys, widely used foundry alloys, have significant technological importance due to their good castability and high specific properties, e.g. high specific strength, finding a wide variety of applications in the transportation sector [1]. However, the relatively low strength of Al–Si alloys restricts their application. The development of aluminum matrix composites (AMCs), through the combination of the low density and good ductility of the aluminum matrices and the high hardness and strength of the reinforcements, is one of the promising ways to strengthen Al alloys [2–10]. Ceramic particles are conventionally used as reinforcements; however, their use generally implies wettability and incompatibility problems with the metallic matrix. For example, the poor wettability of the ceramic particles may lead to weak interfaces between metallic matrix and reinforcements [2]. Also the thermal expansion differences between the ceramic and metal may induce crack formation of the ceramic particles during the producing upon cooling [3]. The poor interface and the cracks may then act as weak ⇑ Corresponding author at: WPI Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan. Tel.: +81 22 217 5956. E-mail addresses: [email protected], [email protected] (Z. Wang). http://dx.doi.org/10.1016/j.jallcom.2014.12.254 0925-8388/Ó 2015 Elsevier B.V. All rights reserved.
areas and severely deteriorate the mechanical properties of ceramic-reinforced composites. This work aims to develop AMCs reinforced with lightweight metallic particles having good mechanical and thermal compatibility with the Al matrix, being simultaneously inexpensive compared with other non-ceramic reinforcements, such as quasicrystals and metallic glasses [4–10]. Ti–Al–Nb–Mo–B (TNM) alloys are lightweight materials with good room and elevated temperature mechanical properties [11]. These properties make these alloys the ideal candidate as reinforcement in the AMCs. In this study, gas atomized Ti52.4Al42.2Nb4.4Mo0.9B0.06 TNM powder is used as reinforcing agent in Al–12Si matrix composites and their structure and mechanical behavior have been characterized in detail. 2. Experimental In order to ensure the homogeneous distribution of the reinforcing particles, powder mixtures consisting of gas atomized Al–12Si (wt.%) with 20 and 40 vol.% of gas atomized Ti52.4Al42.2Nb4.4Mo0.9B0.06 (at.%) were prepared by room temperature milling using a Retsch PM400 planetary ball mill with hardened steel balls and vials (milling time 1.5 h; rotational speed 100 rpm). Bulk samples were prepared by uniaxial hot pressing followed by hot extrusion under Ar atmosphere. During hot pressing, the powders were heated up to 673 K at a heating rate of 40 K/min and then a pressure of 600 MPa was applied for 10 min; after that, the pressure was removed and the sample was cooled down to room temperature using air cooling. The hot pressed sample was extruded (extrusion ration 6:1) at 673 K using a
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pressure of 700 MPa. The extrusion time is in the range of 30–60 min, depending on the volume fraction of the TNM particles. The microstructure was studied by scanning electron microscopy (SEM), using a Gemini 1530 microscope equipped with an energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD) using a D3290 PANalytical X’pert PRO diffractometer (Co Ka radiation, k = 0.17889 nm). Tensile tests were carried out with an INSTRON 8562 testing facility under quasistatic loading (strain rate of 1 104 s1) at room temperature. Vickers microhardness measurements were carried out using a Shimadzu HMV2000 hardness testing machine (applied load 0.025 kgf (0.245 N), dwell time 10 s).
3. Results and discussion Fig. 1 shows the morphology of the Al–12Si and TNM powders. The particles display spherical shape with diameter in the range of 3–50 lm and average value of 11 lm for the Al–12Si powders and with diameter in the range of 3–55 lm and average value of 17 lm for the TNM particles. The XRD patterns of the composites reinforced with 20 and 40 vol.% of TNM (f = 20 and f = 40), Al–12Si matrix and TNM reinforcement are presented in Fig. 2. The diffraction signals of a-Al solid solution and Si (from Al–12Si matrix) along with the signals of the TNM phase, such as AlTi3 and MoNb are observed. No diffraction peaks belonging to additional phases can be detected, implying that no reaction occurs between matrix and reinforcement during consolidation. Fig. 3a–c shows the SEM micrographs of the cross-section of the composites with f = 20 and 40. No pores are observed, indicating high densification of the materials. The high densification of the composites is further confirmed by the high density achieved for the extruded Al–12Si alloy and composites f = 20 and 40, respectively, 2.628 ± 0.006 g/cm3, 2.891 ± 0.098 g/cm3 and 3.186 ± 0.030 g/cm3. The density of the TNM particles is 3.972 g/cm3. Because there is no additional phase formed during consolidation (see Fig. 2), the porosity can be approximately calculated by the difference between the measured and theoretical density based on the assumption that no reaction between the reinforcements and matrix occurred. As a result, the calculated porosity for the extruded Al–12Si alloy and composites with f = 20 and 40 are 0.8%, 0.8% and 0%, respectively. The SEM images in Fig. 3 display a microstructure consisting of bright areas (the TNM reinforcement) homogeneously distributed in the Al–12Si matrix (the dark area). However, the presence of clusters of TNM particles can be also observed, particularly for the sample with f = 40 (Fig. 3b). EDX analysis of the composite with f = 40 (Fig. 3d–f), corresponding to the area in Fig. 3c, shows that the bright areas are rich in Ti and the dark matrix is rich in Al and Si. The small dark-gray particles present in the Al–12Si matrix (Fig. 3c) are rich in Si, indicating the precipitation of Si from the Al–12Si matrix. The Si precipitation is corroborated by the XRD results (Fig. 2), where the intensity of the Si peaks increases with the increasing volume fraction of TNM. These results indicate that the addition of TNM particles can accelerate Si precipitation from the Al–12Si matrix. The diffusion of Ti from the TNM particles to the Al–12Si matrix (see Fig. 3f) enhances the Si precipitation, which in turn reduces the Si solubility in Al. The diffusion of Ti into the Al–12Si matrix may happen during of these powder metallurgy processes like mechanical milling, hot pressing and hot extrusion.
Fig. 2. XRD patterns obtained from composites (f = 20 and f = 40), Al–12Si alloy and TNM alloy.
Typical room temperature uni-axial tensile stress–strain curves for the composites are shown in Fig. 4 together with the curve for the unreinforced Al–12Si matrix. The yield strength ry (0.2% offset) increases from 104 MPa for Al–12Si to about 125 MPa and 150 MPa for the composites reinforced with 20 vol.% and 40 vol.% TNM, respectively. Similarly, the ultimate tensile strength (UTS) increases from 174 MPa for Al–12Si to 200 MPa and 240 MPa respectively for the composites with f = 20 and 40. At the same time, the specific strength increases with increasing TNM volume fraction, from 66 103 N m kg1 of Al–12Si alloy to 69 N m kg1 (f = 20) and 75 N m kg1 (f = 40). The increase in strength of the composites occurs at the expense of the plastic strain, which decreases from 16% for the unreinforced matrix to about 10% and 4% for the composites with 20% and 40% reinforcement. The fracture morphology of the unreinforced Al–12Si matrix and composites after the tensile tests are shown in Fig. 5. Ductile and brittle failure modes are found in the Al–12Si matrix and TNM particles, respectively: micro-scaled dimples are observed in the matrix, whereas flowery cleavages are visible in the TNM particles. In addition TNM particle cracking and TNM particle decohesion from the matrix were also observed after tensile test. This might be attributed to the interfacial strength, which is close to the strength of the TNM particle. This suggests that good bonding is established between the TNM particle and Al–12Si matrix by the hot consolidation process. It is interesting to note that the dimple size in the Al–12Si matrix increases with increasing volume fraction of TNM particle, which is clearly seen from the high magnification SEM images shown in Fig. 5d–f. This is quantitatively summarized in Fig. 6, which shows that the dimple diameter increases from 1.36 lm for the unreinforced Al–12Si alloy to 1.66 lm and 2.16 lm for the composites with f = 20 and 40. Fractured Si precipitates are also observed at the ends of the
Fig. 1. SEM micrographs for the gas-atomized powders: (a) Al–12Si powder, (b) TNM powder.
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Fig. 3. SEM micrographs and EDX of the composites: (a) f = 20, (b and c) f = 40 and (d–f) the corresponding EDX of the microstructure as shown in (c).
Fig. 4. Tensile stress–strain curves for the composites and Al–12Si alloy, the inset image is the experimental yield strength of the composites as a function of the reinforcement content and yield strength calculated by the iso-stress model. The strength of TNM alloy is taken from the research [12].
dimples (Fig. 5c and f), indicating that the nucleation of the cracks occur preferentially near the small brittle Si precipitates, which is closely analogous with the annealing of Al–12Si produced by selective laser melting [13]. This is supported by the microstructure of
Fig. 6. The fractography dimple size as a function of the TNM volume fraction.
the composites, where the dimple size is close to the size of the Si precipitates, as seen in the composites with f = 40 (Figs. 3c and 5f). The hardness of the TNM particle in the composites is 562 ± 8 Hv, which is much higher than the matrix (64 ± 3 Hv). The strengthening
Fig. 5. SEM images showing the fracture surfaces of the composites: (a and d) f = 0, (b and e) f = 20 and (c and f) f = 40, where (f) is the higher magnification image from the rectangle area in (c).
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be noted that the strength of the Al–12Si alloy from the reference [18] is higher while the ductility is lower than the present work, which may be due to the presence of different Si morphology (needle-like Si phase). Although the present Al–Si composites reinforced with TNM particles display a significantly lower strength than the composites reinforced with flyash, they are stronger than the composites reinforced with ceramic particles or graphite. In addition, they exhibit good plastic deformation and thus represent a positive combination of strength and ductility, as observed for the composite reinforced with 20 vol.%, which shows ultimate strength of 200 MPa along with elongation of 10%. 4. Conclusion
Fig. 7. A comparison of Al–Si matrix composites reinforced with different phases. (See above-mentioned references for further information.)
effect observed in Fig. 4 is in good agreement with the value calculated by one of the Rule of Mixtures, the iso-stress model rcy ¼ ðV p =rpy þ V m =rmy Þ1 ; where V is the volume fraction, ry is the yield strength and the subscripts c, p and m indicate the composite, the reinforcement and the matrix), which assumes that the composite exhibits equal stress in the two components matrix and reinforcement [14] (see inset in Fig. 4). On the other hand, the strength calculated by the iso-strain (rcy = Vp rpy + Vm rmy), which assumes that the two components experience the same strain during deformation, is 157 MPa and 347 MPa for the composites with f = 20 and 40, respectively. This is much higher than the real experimental value; the reason is probably that the distance between the TNM particles is too large to transfer the load efficiently from the soft matrix to the hard particles, resulting into a larger strain in the matrix. This is in accord with the observation that the composites with low volume fraction of hard particles in the soft matrix obey the iso-stress model rather than iso-strain model [7,14]. Beside the strengthening effect of the TNM particles, the Si precipitates may also contribute to the increased strength. However, the strengthening contribution of the Si particles is expected to be low; for example, the yield strength increases only marginally from 75 MPa to 81 MPa with increasing the Si content of the Al–Si alloys from 10 to 19 wt.% [15]. However, the presence of the Si particles has a significant effect on the fracture of the composites. The cracks that nucleate at the Si precipitates, along with the de-bonding of TNM particles, act as the potential nucleation sites for the macroscopic cracks and finally deteriorate the plasticity of the composites. The effect of the TNM particles as strengthening agent for the Al–Si matrix is compared in Fig. 7 with different kinds of reinforcements. Most of the composites, including the composites reinforced with the typical ceramics, such as SiC and Al2O3, are located at the left-bottom area, where both the UTS and elongation are relatively low. A significant enhancement of the strength of Al–Si matrix composites has been achieved by using flyash particulates [18]; the UTS of these composites can reach 305 MPa by adding 15 wt.% flyash particles. However, the elongation for these composites is rather limited, ranging between 2% and 7%. It has to
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