- Email: [email protected]
The nano-sized TiC particle reinforced Al–Cu matrix composite with superior tensile ductility
Author's Accepted Manuscript
The nono-sized TiC particle reinforced Al-Cu matrix composite with superior tensile ductility Dongshuai Zhou, Feng Qiu, Qichuan Jiang
www.elsevier.com/locate/msea
PII: DOI: Reference:
S0921-5093(14)01345-8 http://dx.doi.org/10.1016/j.msea.2014.11.006 MSA31724
To appear in:
Materials Science & Engineering A
Received date: 14 May 2014 Revised date: 31 October 2014 Accepted date: 3 November 2014 Cite this article as: Dongshuai Zhou, Feng Qiu, Qichuan Jiang, The nono-sized TiC particle reinforced Al-Cu matrix composite with superior tensile ductility, Materials Science & Engineering A, http://dx.doi.org/10.1016/j.msea.2014.11.006 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
The nono-sized TiC particle reinforced Al-Cu matrix composite with superior tensile ductility Dongshuai Zhou, Feng Qiu*, Qichuan Jiang* Key Laboratory of Automobile Materials, Ministry of Education, and Department of Materials Science and Engineering, Jilin University, No. 5988 Renmin Street, Changchun 130025, P. R. China * Corresponding author. Tex/Fax: +86 431 8509 4699. E-mail: [email protected] Abstract: The high mechanical properties of the TiC particle reinforced Al-Cu matrix composites are highly desirable for a wide range of critical application. However, a long-standing problem for these composites is that they suffer from low ductility and limited formability. Here we fabricated the nano-sized TiC particle reinforced Al-Cu matrix composites by disperse the nano-sized TiC particles into molten Al-Cu alloy. The tensile strength and ductility were significantly improved with the addition of the nano-sized TiC particles. The tensile strength and elongation of the 0.5 wt.% nano-sized TiC particle reinforce Al-Cu matrix composite can reach to 540 MPa and 19.0%, increased by 11.08% and 187.9% respectively, than those of the Al-Cu matrix alloy (485 MPa and 6.6%). Key words: nano-sized TiC particle; composite; ductility; casting.
1
1. Introduction Al alloys have been attracted considerable attentions over the past decades, particularly due to their great weigh reduction potential in the realms of aviation, aerospace and automotive [1-3]. Moreover, the strength of the Al alloy was improved through being reinforced with the ceramic particles. However, the particle reinforced Al matrix composites often process high tensile strength with low ductility which limited their widespread engineering applications [4-8]. So, it is important to simultaneously improve the strength and ductility of the Al matrix composites. Simultaneously improve the strength and ductility has been a major goal over recent decades for the particle reinforced metal matrix composites. One approach is decrease the size of the reinforcing particles. Some researches indicated that when the size of the reinforcing particles decreases below 100 nm, the strength and ductility of the composites could be improved simultaneously [9-11]. But this approach causes non-homogeneous particle dispersion and poor interface bonding. In our previous research, with using the carbon nano-tube (CNT) of high chemical activity, nano-sized TiC particles were synthesized by self-propagating high temperature synthesis (SHS) in the Al-Ti-CNT systems [12]. On the other hand, micro-sized TiC particle (1-5 m) reinforced Al matrix composites were fabricated by adding TiC-Al master alloy, which was made by reaction of Al, Ti and C powders, into the molten Al matrix. It was indicated that the micro-sized TiC particles individually dispersed in the Al matrix. The strength improvement (from 280 MPa to 328 MPa) is attributed to the good wetting and hence strong interfacial bonding between Al and TiC [13]. They
2
provide thoughts and guidance for manufacture the nano-sized TiC particle reinforced metal matrix composites via SHS and stir casting. In this process, the nano-sized TiC reinforcing phase is formed in situ in through SHS reaction. Unlike the conventional metal matrix composites produced by ex situ methods, the in situ composites exhibit the following advantages: (a) the in situ formed reinforcements are thermodynamically stable in the matrix, leading to less degradation in elevated temperature service; (b) the in situ formed reinforcements tend to be fine and well distributed; (c) the interface between the reinforcing phase and the matrix is clean, resulting in a strong interfacial bonding [5,14,15]. Therefore, to investigate the in situ nano-sized particle reinforced metal matrix composites is very important. In the past decade, much of the research has been focused on in situ micro-sized particle reinforced aluminum metal matrix composites due to their potentially low fabrication cost, while less work has been carried out on in situ nano-sized particle reinforced aluminum metal matrix composites. On the other hand, In the Al-Cu alloy, ' (Al2Cu) is one of the primary strengthening precipitates. In the Al-Cu alloy, the precipitation sequence was previously accepted as supersaturated solid solutionG.P.(I)G.P.(II)(or '')' [16]. The '-plates are distributed over all three {100}-plane variants in aged Al-Cu alloys. During plastic deformation, the semi-coherent ' precipitates would restrict by-passed by dislocations [17]. Therefore, the size and spacing of the ' precipitates have a strong influence on the plastic behavior of Al-Cu alloys. It is very important to study the ' precipitates in the Al-Cu matrix composites. However, the investigation on the
3
effect of the nano-sized TiC particles is absent. In this paper, an alternative technique has been employed where nano-sized TiC particles are formed in-situ, from elemental powder mixtures, to make a master alloy which is subsequently added to molten Al-Cu. The structure and mechanical properties of the resulting composite are reported. 2. Experimental The composites were fabricated by dispersion the nano-sized TiC-Al master alloy into molten Al-Cu alloy. The master alloy was prepared by the following steps. The powders of Al, Ti and carbon nano-tube (CNT) were mixed sufficiently by ball milling at a low speed (approximately 70 rpm) for 24h and then pressed into the cylindrical compacts of approximately 28mm in diameter and approximately 25mm in height with green densities of approximately 65% of theoretical. The ball to powder mass ratio is 20:1. The SHS reactions of the compacts were conducted in a vacuum at 1173K for 10 min. The phase compositions in the master alloys were indentified by X-ray diffraction (XRD, Rigaku D/Max 2500PC) with CuK radiation using a scanning speed of 4°/min. The chemical composition in mass% of the casting Al-Cu alloy used in the present experiment was 5.00 Cu, 0.45 Mn, 0.30 Ti, 0.20 Cd, 0.20 V, 0.15 Zr, 0.04 B, and balanced Al. The nano-sized TiC-Al master alloy was then added to the molten Al-Cu alloy which was held at 1073K. The contents were controlled to produce the composites containing 0.1, 0.3, 0.5, 0.7 and 1.0 wt.% nano-sized TiC particles, respectively. Then, the molten melt was poured into pre-heated steel die of
4
200×600×12 mm. Before the tensile test, all the samples underwent the T6 heat treatment (solution at 808K for 12h and aging at 438K for 10h). After T6 heat treatment, all the samples were machined to the tensile dog-bone shaped samples with a gauge cross section of 5.0×2.5 mm and a gauge length of 30.0 mm. Tensile tests were conducted at room temperatures using a servohydraulic materials testing system (MTS, MTS 810) at a constant strain rate of 1×10-4s-1. The microstructure was examined by Olypus opical microscope (OM, Olympus PMG3,) and high resolution transmission electron microscope (HRTEM, JEM-2100F). 3. Results Fig. 1 shows the XRD patterns of the master alloys. As indicated, the master alloy contains only Al and TiC phases without intermediate phase Al3Ti was found. Fig. 2 shows the typical as-cast microstructures of the Al-Cu alloy and the composites. The dendritic microstructure resulting from casting solidification process is clearly revealed. Fig. 3 shows that the nano-sized TiC particles predominantly dispersed in the dendritic interior. It is assumed that the uniform dispersion of the nano-sized particles provides some heterogeneous nucleation sties of the -Al crystal during solidification, resulting in a more refined microstructure. As indicated, the microstructure in the Al-Cu alloy is constituted by the coarse -Al grains with uneven sizes about 100-180 m and the Al2Cu phase at the grain boundaries (Fig. 2a); while in the composites, the average size of the -Al dendrites decrease with the addition of the nano-sized TiC paritlces increasing. That means the TiC particles successfully refined the dendrite of the Al-Cu alloy. Furthermore, compared to the Al-Cu alloy, the
5
morphologies of the -Al grains in the composites changed from cystiform to dendrite with finer sizes. Fig. 4 (a) and (b) show the microstructure of the ' precipitates in the Al-Cu alloy and the composite after the T6 heat treatment, respectively. The plate-shaped ' precipitates form on the {001} planes of the -Al matrix, and the dislocation loops are found around the precipitates. It can be seen that in the Al-Cu alloy, there are only a few amount of the nano-sized ' precipitates with a width of 10-15 nm and length of 180 nm. However, there are large numbers of nano-sized ' precipitates with a width of 2-5 nm and length of 80 nm in the composite. At same time, the distribution of the ' precipitates were more homogeneous. Fig. 5 shows the typical engineering/true stress-strain curves of all the samples and Table 1 lists the detail data of the engineering tensile strength and elongation. As indicated, compared with the Al-Cu alloy, the ultimate tensile strength and elongation of the composites were significantly improved by the addition of the nano-sized TiC particles. The tensile strength of the composites increases with the increase of the TiC, while the elongation increases firstly and then decreases. The 0.5 wt.% nano-sized TiC particle reinforced Al-Cu matrix composite possesses the highest ductility. The tensile strength and the elongation of the 0.5 wt.% TiCp/Al-Cu composite can reach to 540 MPa and 19.0%, respectively. 4. Discussion The mechanisms contributing to the good tensile property of the composites are now analyzed and discussed. Considering all the microstructural features, the
6
improvement of the tensile strength and ductility should be attributed to the finer -Al dendrites and ' participates introduced by the addition of the nano-sized TiC particles. As above mentioned, the nano-sized TiC particles acted as heterogeneous nucleation sties of the -Al crystal during solidification, resulting in a more refined microstructure. As known, the refined -Al dendrite reduces the size of the nucleating flaws and increases the resistance to crack propagation, leading to a higher fracture stress and ductility. More boundaries are formed because of the refined dendrites in the composite. The high boundary concentration and the rosebush-like dendrites play a important role as barriers to the enablement and transmission of the dislocation, which is helpful to improve the tensile strength and ductility. On the other hand, the refined dendrite can also offer a higher resistance to shear localization and shear fracture, and thus stabilize the hydrostatic triaxial stress. Then, the ductile fracture through microvoid nucleation and coalescence can be promoted by this [18,19]. The rosebush-like dendrites microstructure makes it harder for the transgranular fractures which need more energy than the intergranular fracture. Even when the fracture mode is intergranular, the rosebush-like dendrites microstructure can also improve the strength and ductility by extending the crack propagation path. As mentioned above, the size and spacing of the ' precipitates have a strong influence on the plastic behavior of Al-Cu alloys. It was suggested that Cu is likely to segregate to relieve the stress field around the edge components of the dislocation and hence that dislocations can act as nuclei for ' formation [20]. For example, the
7
acceleration of ' formation in fast quenched crystals is due to dislocations formed on quenching [21]. In our experiment, the addition of the nano-sized TiC particles would increase the dislocations in the composites. Then, more ' precipitates were obtained in the composites. Furthermore, because the -Al grains in the composites are finer than those in the Al-Cu alloy, the diffusion distance of the Cu atoms becomes shorter during the solution process [22]. Therefore, the finer and more uniformly distributed ' precipitates were formed in the -Al grains during aging process. It was suggests that the precipitation strengthening, which results from the ability of the nano-scale second-phase precipitates to restrict and impede the dislocation actuation and movement by forcing dislocations to circumvent the nano-scale precipitates, which makes a significant contribution to the strength enhancement. As known, due to the presence of highly-dispersed nano-sized reinforcement (smaller than ~100nm) in a metal matrix, Orowan strengthening becomes more favourable in the materials [23]. Orowan strengthening results from the interaction between dislocations and the dispersed reinforcemnt. In this paper, the dispersed nano-sized TiC particles act as obstacles to hinder the motion of dislocations and then enhanced tensile strength of the composites. In particular, dispersing nano-sized TiC particles and ' precipitates in the grain interior is an effective approach to increase the strength and simultaneously improve the ductility of the composites, because they can generate, pin down and thus accumulate dislocations within the grains. During tensile deformation the retention of an increasing number of dislocations in the grain interior is helpful to improve the
8
tensile strength and elongation. On the other hand, the composite failure is associated with particle cracking and void formation in the matrix within clusters of the particles. Particles cracking by catastrophic propagation of an internal defect is given by the Giffith equation [24]: 1/ 2
Vf
§ 2 EJ · ¸ ¨ © SC ¹
(1)
where f is the stress on the particle, the fracture surface energy, E Young’s modules of the particle, and C the internal crack length. In our study, because the nano-sized TiC particle is fine and distributed uniformly at first, so the internal crack length C is short an the stress on the particle is high. The ductility would be improved by the decrease of the particles cracking. With the increase of the nano-sized TiC particles, some agglomerations would be exited in the composites. The cracks tend to propagate in the agglomeration zone, which would reduce the tensile elongation. 5. Conclusions In this work we have designed, and successfully fabricated the nano-sized TiC particle reinforced Al-Cu matrix composites via the dispersion the nano-sized TiC-Al master alloy into the molten Al-Cu alloy. A good tensile ductility together with the high strength has been derived from the resulting nano-sized TiC particle reinforced Al-Cu matrix composites. The tensile strength and elongation of the 0.5 wt.% nano-sized TiC particle reinforce Al-Cu matrix composite can reach to 540 MPa and 19.0%, increased by 11.08% and 187.9% respectively, than those of the Al-Cu matrix alloy (485 MPa and 6.6%). The significant improvement of the ductility could be attributed to the finer -Al dendrite and ' participates introduced by the addition of 9
the nano-sized TiC particles. Our finding will help guide endeavors to architecture other nano-sized TiC particle reinforced metal matrix composites to simultaneously elevate strength and ductility. Acknowledgment This work is supported by the National Natural Science Foundation of China (No. 51171071, 50971065 and 50531030), National Basic Research Program of China (973 Program) (No.2012CB619600), the Research Fund for the Doctoral Program of High Education of China (No. 20130061110037) and the Project 985-High Performance Materials of Jilin University. References [1] M.Nakai, T.Eto, Mater. Sci. Eng. A 285(2000) 62-68. [2] W.S. Miller, L.Zhang, J.Bottema, A.J. Wittebrood, P.De Smet, A. Haszler, A. Vieregge, Mater. Sci. Eng. A 280(2003) 37-49. [3] A. Heinz, A. Haszler, C. Keidel, S. Moldenhauer, R. Benedictus, W.S. Miller, Mater. Sci. Eng. A 280(2000) 102-107. [4]X.W. Zeng, W.G. Zhang, N. Wei, R.P. Liu, M.Z. Ma, Mater. Sci. Eng. A 443(2007) 224-228. [5]Q.C. Jiang, X.L. Li, H.Y. Wang, Scripta Mater. 48(2003) 713-717. [6]M.K. Premkumar, M.G. Chu, Mater. Sci. Eng. A 202(1995) 172-178. [7]X.C. Tong, A.K. Ghosh, J. Mater. Sci. 36(2001) 4059-4069. [8]F. Tang, M. Hagiwara, J.M. Schoenung, Mater. Sci. Eng. A 407(2005) 306-314. [9]M. Bahrami, K. Dehghani, M.K.B. Givi, Mater. Des. 53(2014) 173-178
10
[10]D.S. Zhou, F. Qiu, Q.C. Jiang, Mater. Sci. Eng. A 596(2014) 98-102. [11]G. Cao, H. Honishi, X. Li, Mater. Sci. Eng. A 486(2008) 357-362. [12]S.B. Jin, P. Shen, D.S. Zhou, Q.C. Jiang. Nanoscale Res. Lett. 6(2011) 1-7. [13]C. Selcuk, A.R. Kennedy, Mater. Lett. 60(2006) 3364-3366. [14] S.C. Tjong, Z.Y. Ma, Mater. Sci. Eng. (R) 29(2000) 49-113. [15] M.A. Matin, L. Lu, M. Gupta, Scripta Mater. 45(2001) 479-486. [16] S.K. Son, M. Takeda, M. Mitome, Y. Bando, T.E. Endo, Mater. Lett. 59(2005) 629. [17] S.Y. Hu, M.I. Baskes, M. Stan, L.Q. Chen, Acta Mater. 47(1999) 1713. [18] Z.M. Xu, Q.C. Jiang, Q.F. Guan, Z.M. He, J. mater. Sci. Lett. 17(1998) 5-9. [19] W.B. Bouaueshi, D.Y. Li, Tribol. Int. 40(2007) 188-189. [20] G. Thomas, M.J. Whelan, Philos. Mag. 4(1959) 511-527. [21] J.M. Silcock, Philos. Mag. 4(1959) 1187-1194. [22] D.S. Zhou, F. Qiu, Q.C. Jiang, Mater. Sci. Eng. A 596(2014) 98-102. [23] Z. Zhang, D.L. Chen, Scripta Mater 54(2006) 1321-1326. [24] X.C. Tong, A.K. Ghosh, J. Mater. Sci. 36(2001) 4059.
11
Table 1 Data of the tensile strength and elongation of the Al-Cu alloy and the nano-sized TiCp/Al-Cu composites reinforced by different contents of TiC particles TiC(wt.%) 0 0.1 0.3 0.5 0.7 1.0
b (MPa) 485 509 522 540 546 552
(%) 6.6 10.7 10.9 19 16.2 12.84
12
Figure Captions Figure 1 The XRD patterns of the master alloy. Figure 2 Typical microstructures of the samples: (a) Al-Cu alloy and (b-f) the TiCp/Al-Cu composites with different mass fraction of TiC: (b) 0.1%; (c) 0.3%; (d) 0.5%; (e) 0.7% and (f) 1.0%. Figure 3 (a) TEM micrographs of the nano-sized TiC particles in the TiCp/Al-Cu composite; (b) HRFEM images of the interface between the nano-sized TiC and the -Al matrix; (c) corresponding SAED pattern. Figure 4 TEM micrographs of the ' precipitates in (a) Al-Cu matrix alloy sample and (b) nano-size TiCp/Al-Cu composite sample and (c) and (d) are the corresponding SAED patterns. Figure 5 Typical (a) engineering and (b) true stress-strain curves of the Al-Cu alloy and the nano-sized TiCp/Al-Cu composites reinforced by different contents of nano-sized TiC particles.
13
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
The nono-sized TiC particle reinforced Al-Cu matrix composite with superior tensile ductility Dongshuai Zhou, Feng Qiu, Qichuan Jiang
www.elsevier.com/locate/msea
PII: DOI: Reference:
S0921-5093(14)01345-8 http://dx.doi.org/10.1016/j.msea.2014.11.006 MSA31724
To appear in:
Materials Science & Engineering A
Received date: 14 May 2014 Revised date: 31 October 2014 Accepted date: 3 November 2014 Cite this article as: Dongshuai Zhou, Feng Qiu, Qichuan Jiang, The nono-sized TiC particle reinforced Al-Cu matrix composite with superior tensile ductility, Materials Science & Engineering A, http://dx.doi.org/10.1016/j.msea.2014.11.006 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
The nono-sized TiC particle reinforced Al-Cu matrix composite with superior tensile ductility Dongshuai Zhou, Feng Qiu*, Qichuan Jiang* Key Laboratory of Automobile Materials, Ministry of Education, and Department of Materials Science and Engineering, Jilin University, No. 5988 Renmin Street, Changchun 130025, P. R. China * Corresponding author. Tex/Fax: +86 431 8509 4699. E-mail: [email protected] Abstract: The high mechanical properties of the TiC particle reinforced Al-Cu matrix composites are highly desirable for a wide range of critical application. However, a long-standing problem for these composites is that they suffer from low ductility and limited formability. Here we fabricated the nano-sized TiC particle reinforced Al-Cu matrix composites by disperse the nano-sized TiC particles into molten Al-Cu alloy. The tensile strength and ductility were significantly improved with the addition of the nano-sized TiC particles. The tensile strength and elongation of the 0.5 wt.% nano-sized TiC particle reinforce Al-Cu matrix composite can reach to 540 MPa and 19.0%, increased by 11.08% and 187.9% respectively, than those of the Al-Cu matrix alloy (485 MPa and 6.6%). Key words: nano-sized TiC particle; composite; ductility; casting.
1
1. Introduction Al alloys have been attracted considerable attentions over the past decades, particularly due to their great weigh reduction potential in the realms of aviation, aerospace and automotive [1-3]. Moreover, the strength of the Al alloy was improved through being reinforced with the ceramic particles. However, the particle reinforced Al matrix composites often process high tensile strength with low ductility which limited their widespread engineering applications [4-8]. So, it is important to simultaneously improve the strength and ductility of the Al matrix composites. Simultaneously improve the strength and ductility has been a major goal over recent decades for the particle reinforced metal matrix composites. One approach is decrease the size of the reinforcing particles. Some researches indicated that when the size of the reinforcing particles decreases below 100 nm, the strength and ductility of the composites could be improved simultaneously [9-11]. But this approach causes non-homogeneous particle dispersion and poor interface bonding. In our previous research, with using the carbon nano-tube (CNT) of high chemical activity, nano-sized TiC particles were synthesized by self-propagating high temperature synthesis (SHS) in the Al-Ti-CNT systems [12]. On the other hand, micro-sized TiC particle (1-5 m) reinforced Al matrix composites were fabricated by adding TiC-Al master alloy, which was made by reaction of Al, Ti and C powders, into the molten Al matrix. It was indicated that the micro-sized TiC particles individually dispersed in the Al matrix. The strength improvement (from 280 MPa to 328 MPa) is attributed to the good wetting and hence strong interfacial bonding between Al and TiC [13]. They
2
provide thoughts and guidance for manufacture the nano-sized TiC particle reinforced metal matrix composites via SHS and stir casting. In this process, the nano-sized TiC reinforcing phase is formed in situ in through SHS reaction. Unlike the conventional metal matrix composites produced by ex situ methods, the in situ composites exhibit the following advantages: (a) the in situ formed reinforcements are thermodynamically stable in the matrix, leading to less degradation in elevated temperature service; (b) the in situ formed reinforcements tend to be fine and well distributed; (c) the interface between the reinforcing phase and the matrix is clean, resulting in a strong interfacial bonding [5,14,15]. Therefore, to investigate the in situ nano-sized particle reinforced metal matrix composites is very important. In the past decade, much of the research has been focused on in situ micro-sized particle reinforced aluminum metal matrix composites due to their potentially low fabrication cost, while less work has been carried out on in situ nano-sized particle reinforced aluminum metal matrix composites. On the other hand, In the Al-Cu alloy, ' (Al2Cu) is one of the primary strengthening precipitates. In the Al-Cu alloy, the precipitation sequence was previously accepted as supersaturated solid solutionG.P.(I)G.P.(II)(or '')' [16]. The '-plates are distributed over all three {100}-plane variants in aged Al-Cu alloys. During plastic deformation, the semi-coherent ' precipitates would restrict by-passed by dislocations [17]. Therefore, the size and spacing of the ' precipitates have a strong influence on the plastic behavior of Al-Cu alloys. It is very important to study the ' precipitates in the Al-Cu matrix composites. However, the investigation on the
3
effect of the nano-sized TiC particles is absent. In this paper, an alternative technique has been employed where nano-sized TiC particles are formed in-situ, from elemental powder mixtures, to make a master alloy which is subsequently added to molten Al-Cu. The structure and mechanical properties of the resulting composite are reported. 2. Experimental The composites were fabricated by dispersion the nano-sized TiC-Al master alloy into molten Al-Cu alloy. The master alloy was prepared by the following steps. The powders of Al, Ti and carbon nano-tube (CNT) were mixed sufficiently by ball milling at a low speed (approximately 70 rpm) for 24h and then pressed into the cylindrical compacts of approximately 28mm in diameter and approximately 25mm in height with green densities of approximately 65% of theoretical. The ball to powder mass ratio is 20:1. The SHS reactions of the compacts were conducted in a vacuum at 1173K for 10 min. The phase compositions in the master alloys were indentified by X-ray diffraction (XRD, Rigaku D/Max 2500PC) with CuK radiation using a scanning speed of 4°/min. The chemical composition in mass% of the casting Al-Cu alloy used in the present experiment was 5.00 Cu, 0.45 Mn, 0.30 Ti, 0.20 Cd, 0.20 V, 0.15 Zr, 0.04 B, and balanced Al. The nano-sized TiC-Al master alloy was then added to the molten Al-Cu alloy which was held at 1073K. The contents were controlled to produce the composites containing 0.1, 0.3, 0.5, 0.7 and 1.0 wt.% nano-sized TiC particles, respectively. Then, the molten melt was poured into pre-heated steel die of
4
200×600×12 mm. Before the tensile test, all the samples underwent the T6 heat treatment (solution at 808K for 12h and aging at 438K for 10h). After T6 heat treatment, all the samples were machined to the tensile dog-bone shaped samples with a gauge cross section of 5.0×2.5 mm and a gauge length of 30.0 mm. Tensile tests were conducted at room temperatures using a servohydraulic materials testing system (MTS, MTS 810) at a constant strain rate of 1×10-4s-1. The microstructure was examined by Olypus opical microscope (OM, Olympus PMG3,) and high resolution transmission electron microscope (HRTEM, JEM-2100F). 3. Results Fig. 1 shows the XRD patterns of the master alloys. As indicated, the master alloy contains only Al and TiC phases without intermediate phase Al3Ti was found. Fig. 2 shows the typical as-cast microstructures of the Al-Cu alloy and the composites. The dendritic microstructure resulting from casting solidification process is clearly revealed. Fig. 3 shows that the nano-sized TiC particles predominantly dispersed in the dendritic interior. It is assumed that the uniform dispersion of the nano-sized particles provides some heterogeneous nucleation sties of the -Al crystal during solidification, resulting in a more refined microstructure. As indicated, the microstructure in the Al-Cu alloy is constituted by the coarse -Al grains with uneven sizes about 100-180 m and the Al2Cu phase at the grain boundaries (Fig. 2a); while in the composites, the average size of the -Al dendrites decrease with the addition of the nano-sized TiC paritlces increasing. That means the TiC particles successfully refined the dendrite of the Al-Cu alloy. Furthermore, compared to the Al-Cu alloy, the
5
morphologies of the -Al grains in the composites changed from cystiform to dendrite with finer sizes. Fig. 4 (a) and (b) show the microstructure of the ' precipitates in the Al-Cu alloy and the composite after the T6 heat treatment, respectively. The plate-shaped ' precipitates form on the {001} planes of the -Al matrix, and the dislocation loops are found around the precipitates. It can be seen that in the Al-Cu alloy, there are only a few amount of the nano-sized ' precipitates with a width of 10-15 nm and length of 180 nm. However, there are large numbers of nano-sized ' precipitates with a width of 2-5 nm and length of 80 nm in the composite. At same time, the distribution of the ' precipitates were more homogeneous. Fig. 5 shows the typical engineering/true stress-strain curves of all the samples and Table 1 lists the detail data of the engineering tensile strength and elongation. As indicated, compared with the Al-Cu alloy, the ultimate tensile strength and elongation of the composites were significantly improved by the addition of the nano-sized TiC particles. The tensile strength of the composites increases with the increase of the TiC, while the elongation increases firstly and then decreases. The 0.5 wt.% nano-sized TiC particle reinforced Al-Cu matrix composite possesses the highest ductility. The tensile strength and the elongation of the 0.5 wt.% TiCp/Al-Cu composite can reach to 540 MPa and 19.0%, respectively. 4. Discussion The mechanisms contributing to the good tensile property of the composites are now analyzed and discussed. Considering all the microstructural features, the
6
improvement of the tensile strength and ductility should be attributed to the finer -Al dendrites and ' participates introduced by the addition of the nano-sized TiC particles. As above mentioned, the nano-sized TiC particles acted as heterogeneous nucleation sties of the -Al crystal during solidification, resulting in a more refined microstructure. As known, the refined -Al dendrite reduces the size of the nucleating flaws and increases the resistance to crack propagation, leading to a higher fracture stress and ductility. More boundaries are formed because of the refined dendrites in the composite. The high boundary concentration and the rosebush-like dendrites play a important role as barriers to the enablement and transmission of the dislocation, which is helpful to improve the tensile strength and ductility. On the other hand, the refined dendrite can also offer a higher resistance to shear localization and shear fracture, and thus stabilize the hydrostatic triaxial stress. Then, the ductile fracture through microvoid nucleation and coalescence can be promoted by this [18,19]. The rosebush-like dendrites microstructure makes it harder for the transgranular fractures which need more energy than the intergranular fracture. Even when the fracture mode is intergranular, the rosebush-like dendrites microstructure can also improve the strength and ductility by extending the crack propagation path. As mentioned above, the size and spacing of the ' precipitates have a strong influence on the plastic behavior of Al-Cu alloys. It was suggested that Cu is likely to segregate to relieve the stress field around the edge components of the dislocation and hence that dislocations can act as nuclei for ' formation [20]. For example, the
7
acceleration of ' formation in fast quenched crystals is due to dislocations formed on quenching [21]. In our experiment, the addition of the nano-sized TiC particles would increase the dislocations in the composites. Then, more ' precipitates were obtained in the composites. Furthermore, because the -Al grains in the composites are finer than those in the Al-Cu alloy, the diffusion distance of the Cu atoms becomes shorter during the solution process [22]. Therefore, the finer and more uniformly distributed ' precipitates were formed in the -Al grains during aging process. It was suggests that the precipitation strengthening, which results from the ability of the nano-scale second-phase precipitates to restrict and impede the dislocation actuation and movement by forcing dislocations to circumvent the nano-scale precipitates, which makes a significant contribution to the strength enhancement. As known, due to the presence of highly-dispersed nano-sized reinforcement (smaller than ~100nm) in a metal matrix, Orowan strengthening becomes more favourable in the materials [23]. Orowan strengthening results from the interaction between dislocations and the dispersed reinforcemnt. In this paper, the dispersed nano-sized TiC particles act as obstacles to hinder the motion of dislocations and then enhanced tensile strength of the composites. In particular, dispersing nano-sized TiC particles and ' precipitates in the grain interior is an effective approach to increase the strength and simultaneously improve the ductility of the composites, because they can generate, pin down and thus accumulate dislocations within the grains. During tensile deformation the retention of an increasing number of dislocations in the grain interior is helpful to improve the
8
tensile strength and elongation. On the other hand, the composite failure is associated with particle cracking and void formation in the matrix within clusters of the particles. Particles cracking by catastrophic propagation of an internal defect is given by the Giffith equation [24]: 1/ 2
Vf
§ 2 EJ · ¸ ¨ © SC ¹
(1)
where f is the stress on the particle, the fracture surface energy, E Young’s modules of the particle, and C the internal crack length. In our study, because the nano-sized TiC particle is fine and distributed uniformly at first, so the internal crack length C is short an the stress on the particle is high. The ductility would be improved by the decrease of the particles cracking. With the increase of the nano-sized TiC particles, some agglomerations would be exited in the composites. The cracks tend to propagate in the agglomeration zone, which would reduce the tensile elongation. 5. Conclusions In this work we have designed, and successfully fabricated the nano-sized TiC particle reinforced Al-Cu matrix composites via the dispersion the nano-sized TiC-Al master alloy into the molten Al-Cu alloy. A good tensile ductility together with the high strength has been derived from the resulting nano-sized TiC particle reinforced Al-Cu matrix composites. The tensile strength and elongation of the 0.5 wt.% nano-sized TiC particle reinforce Al-Cu matrix composite can reach to 540 MPa and 19.0%, increased by 11.08% and 187.9% respectively, than those of the Al-Cu matrix alloy (485 MPa and 6.6%). The significant improvement of the ductility could be attributed to the finer -Al dendrite and ' participates introduced by the addition of 9
the nano-sized TiC particles. Our finding will help guide endeavors to architecture other nano-sized TiC particle reinforced metal matrix composites to simultaneously elevate strength and ductility. Acknowledgment This work is supported by the National Natural Science Foundation of China (No. 51171071, 50971065 and 50531030), National Basic Research Program of China (973 Program) (No.2012CB619600), the Research Fund for the Doctoral Program of High Education of China (No. 20130061110037) and the Project 985-High Performance Materials of Jilin University. References [1] M.Nakai, T.Eto, Mater. Sci. Eng. A 285(2000) 62-68. [2] W.S. Miller, L.Zhang, J.Bottema, A.J. Wittebrood, P.De Smet, A. Haszler, A. Vieregge, Mater. Sci. Eng. A 280(2003) 37-49. [3] A. Heinz, A. Haszler, C. Keidel, S. Moldenhauer, R. Benedictus, W.S. Miller, Mater. Sci. Eng. A 280(2000) 102-107. [4]X.W. Zeng, W.G. Zhang, N. Wei, R.P. Liu, M.Z. Ma, Mater. Sci. Eng. A 443(2007) 224-228. [5]Q.C. Jiang, X.L. Li, H.Y. Wang, Scripta Mater. 48(2003) 713-717. [6]M.K. Premkumar, M.G. Chu, Mater. Sci. Eng. A 202(1995) 172-178. [7]X.C. Tong, A.K. Ghosh, J. Mater. Sci. 36(2001) 4059-4069. [8]F. Tang, M. Hagiwara, J.M. Schoenung, Mater. Sci. Eng. A 407(2005) 306-314. [9]M. Bahrami, K. Dehghani, M.K.B. Givi, Mater. Des. 53(2014) 173-178
10
[10]D.S. Zhou, F. Qiu, Q.C. Jiang, Mater. Sci. Eng. A 596(2014) 98-102. [11]G. Cao, H. Honishi, X. Li, Mater. Sci. Eng. A 486(2008) 357-362. [12]S.B. Jin, P. Shen, D.S. Zhou, Q.C. Jiang. Nanoscale Res. Lett. 6(2011) 1-7. [13]C. Selcuk, A.R. Kennedy, Mater. Lett. 60(2006) 3364-3366. [14] S.C. Tjong, Z.Y. Ma, Mater. Sci. Eng. (R) 29(2000) 49-113. [15] M.A. Matin, L. Lu, M. Gupta, Scripta Mater. 45(2001) 479-486. [16] S.K. Son, M. Takeda, M. Mitome, Y. Bando, T.E. Endo, Mater. Lett. 59(2005) 629. [17] S.Y. Hu, M.I. Baskes, M. Stan, L.Q. Chen, Acta Mater. 47(1999) 1713. [18] Z.M. Xu, Q.C. Jiang, Q.F. Guan, Z.M. He, J. mater. Sci. Lett. 17(1998) 5-9. [19] W.B. Bouaueshi, D.Y. Li, Tribol. Int. 40(2007) 188-189. [20] G. Thomas, M.J. Whelan, Philos. Mag. 4(1959) 511-527. [21] J.M. Silcock, Philos. Mag. 4(1959) 1187-1194. [22] D.S. Zhou, F. Qiu, Q.C. Jiang, Mater. Sci. Eng. A 596(2014) 98-102. [23] Z. Zhang, D.L. Chen, Scripta Mater 54(2006) 1321-1326. [24] X.C. Tong, A.K. Ghosh, J. Mater. Sci. 36(2001) 4059.
11
Table 1 Data of the tensile strength and elongation of the Al-Cu alloy and the nano-sized TiCp/Al-Cu composites reinforced by different contents of TiC particles TiC(wt.%) 0 0.1 0.3 0.5 0.7 1.0
b (MPa) 485 509 522 540 546 552
(%) 6.6 10.7 10.9 19 16.2 12.84
12
Figure Captions Figure 1 The XRD patterns of the master alloy. Figure 2 Typical microstructures of the samples: (a) Al-Cu alloy and (b-f) the TiCp/Al-Cu composites with different mass fraction of TiC: (b) 0.1%; (c) 0.3%; (d) 0.5%; (e) 0.7% and (f) 1.0%. Figure 3 (a) TEM micrographs of the nano-sized TiC particles in the TiCp/Al-Cu composite; (b) HRFEM images of the interface between the nano-sized TiC and the -Al matrix; (c) corresponding SAED pattern. Figure 4 TEM micrographs of the ' precipitates in (a) Al-Cu matrix alloy sample and (b) nano-size TiCp/Al-Cu composite sample and (c) and (d) are the corresponding SAED patterns. Figure 5 Typical (a) engineering and (b) true stress-strain curves of the Al-Cu alloy and the nano-sized TiCp/Al-Cu composites reinforced by different contents of nano-sized TiC particles.
13
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5