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(Ti,W)C composites prepared by microwave sintering
Journal of Alloys and Compounds 590 (2014) 168–175
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Microstructure and mechanical properties of Al/(Ti,W)C composites prepared by microwave sintering R.R. Zheng a, Y. Wu a, S.L. Liao a, W.Y. Wang b, W.B Wang b, A.H. Wang a,⇑ a State Key Laboratory of Material Processing and Die & Mould Technology, School of Materials Science & Engineering, Huazhong University of Science & Technology, Wuhan 430074, PR China b Henan Key Laboratory of Non-ferrous Metals Materials Science & Processing Technology, School of Materials Science & Engineering, Henan University of Science & Technology, Luoyang 471023, PR China
a r t i c l e
i n f o
Article history: Received 19 August 2013 Received in revised form 29 November 2013 Accepted 3 December 2013 Available online 8 December 2013 Keywords: Aluminum matrix composites Microwave sintering Microstructure Mechanical properties
a b s t r a c t (Ti,W)C particulate reinforced 6061 Al alloy composites were prepared by microwave sintering method in this investigation. Green compacts containing different content of reinforcements (i.e., 10, 20 and 30 wt.% (Ti,W)C) were prepared and then sintered by microwave heating at four temperatures (i.e., 500 °C, 520 °C, 540 °C, 560 °C) with a constant soaking time of 45 min. The porosity was increased with an increase in the additive amount of (Ti,W)C particulates, firstly increased and then decreased with an increase in sintering temperature. The microstructure was characterized by uniform distribution of (Ti,W)C particulates in the Al alloy matrix. Grain size growth trend was readily controlled and small grain size was obtained even at higher sintering temperature owing to the addition of (Ti,W)C phase. Mechanical tests indicated that the average hardness, compressive yield strength as well as compression strength were enhanced with an increase in both (Ti,W)C particulate amount and sintering temperature (range from 520 °C to 560 °C). The interfacial structures between Al/(Ti,W)C, Al/Al and (Ti,W)C/(Ti,W)C were revealed by TEM to investigate bounding mechanism during microwave sintering process. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Aluminum metal matrix composites (AMC) have been widely used in aerospace, car industry and military due to their excellent properties, such as high specific modulus, strength and toughness. Conventionally, extrusion forging, squeeze casting and powder metallurgy (PM) are used to produce AMC [1,2]. Compared with traditional methods, PM method is believed to be a suitable process to fabricated near net shape products [3]. In addition, the low processing temperature of the PM method can avoid interaction between matrix and reinforcements [4]. Moreover, it has better control on the microstructure where better distribution of the reinforcements is possible, and this great influence on the end properties [5]. However, conventional PM processing of composites often need long sintering time, which results in grain coarsening and poor performance of mechanical properties [6]. Compared with traditional heating methods, microwave sintering presents distinct advantages, including energy efficient, environmental friendly, enhanced densification, and especially, smaller grain size due to the faster heating rate and the lower sintering temperature [7,8]. Additionally, attributed to uniform volumetric heating and smaller pores in the sintered green compacts, ⇑ Corresponding author. Tel.: +86 27 87180507. E-mail address: [email protected] (A.H. Wang). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.12.016
the microstructure and mechanical properties can be significantly improved [8–10]. Therefore, since the success of microwave sintering metallic materials in 1999 [11], many researchers pay attention to the processing of metal based materials using microwave heating. Up to now, the research has been carried out on ferrous, copper, tungsten and magnesium with limited studies on aluminum and its composites [6]. The present studies on AMC mainly focused on the system of Al/SiC. Leparoux et al. [1] described SiCreinforced Al-matrix composites with high SiC content can be effectively sintered using microwave. They also pointed out that the SiC particulate size has a predominant influence on the heating rate. Thakur et al. [12] reported that two-directional microwave assisted rapid sintering could be used for fabricating Al/(Ti + SiC) and ascribed the improvement of mechanical properties to fine microstructure. It is well known that pure aluminum does not have enough strength compared with aluminum alloys. So far, 6061 Al alloy has received lots of attention for its good ductility, excellent corrosion resistance, good strength and low price [13]. (Ti,W)C may be the most widely known tungsten carbide because of its exceptional hardness, and superior wear resistance [14]. But there is no research on (Ti,W)C particulate reinforced Al matrix. In addition, little effort has been made to investigate the effect of the amount of reinforcement and different levels of sintering temperature have on the microstructure and the
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properties of AMC using microwave heating. Moreover, there has been no research to study the interfacial characteristic between Al matrix and the ceramic reinforcements fabricated by microwave heating. Therefore, 6061 Al alloy matrix and (Ti,W)C reinforcement were selected to synthesize AMC by microwave sintering, and the influence of microwave sintering parameters and (Ti,W)C reinforcement amount on microstructure mechanical properties was investigated in present study.
2. Material and methods The 6061 Al alloy powder with a size range of 5–10 lm (acquired from Zhejiang Bainianyin Company, China) and the angular (Ti,W)C powder with a size range of 0.1–10 lm (procured from Ningxiang Shuangdun Metal Co., Ltd., China) were used as raw materials. The corresponding morphologies and chemical compositions are listed in Fig. 1 and Table 1, respectively. Powders were mixed by mechanical milling using a planetary ball mill for 1 h at 200RPM with a ball to powder ratio of 5:1 by a solvent of ethanol and a dispersing agent of polyethylene glycol. Cuboid green compacts (23 7 7 mm) with different (Ti,W)C particulate content, i.e., 0 wt.%, 10 wt.% (4.04 vol.%), 20 wt.% (8.66 vol.%) and 30 wt.% (13.98 vol.%), were prepared by cold compression moulding forming at a pressure of 400 MPa for 2 min. A microwave oven with frequency 2.45 GHz and power 1.4 kW (Changsha SYNO-THERM Corporation, China) was applied for microwave sintering. Silicon carbide and alumina mixture was buried around the samples to realize rapid and uniform heating from both inside and outside of the green compacts [12]. Microwave heating process was carried out at four different temperatures (500 °C, 520 °C, 540 °C and 560 °C) in high pure argon atmosphere (99.99%) with a heating rate of 20 °C/min. Accurate temperature of samples was monitored using an infrared thermometer and the temperature was controlled by changing microwave power. The green compacts were firstly heated up to 400 °C for half an hour to eliminate residual additives and then heated to the sintering temperature for a soaking time of 45 min, finally, the sintered samples were cooled down to room temperature in air. The absolute density of sintered AMC in polished condition was measured using Archimedes principle. An electronic balance (Yueping, FA2104J) with an accuracy of 0.0001 g was used for recording. The general porosity of the sintered samples was calculated by the following formulae:
e ¼ ð1
q Þ 100 q0
ð1Þ
where q and q0 are the density of the specimen and its corresponding theoretical density, respectively. Three samples were tested for all conditions and the mean value was calculated to make the results more accurate. A JEOL-7600F scanning electron microscopy (SEM) was utilized to evaluate grain morphology and distribution of (Ti,W)C particulates. A high resolution transmission electron microscope (HRTEM, JEOL-2010) coupled with energy dispersive spectrometer (EDS) were utilized to evaluate interfacial characteristics and chemical composition. The corresponding phase structure was determined using X-ray diffraction (XRD, X’Pert PRO, PANalytical, The Netherlands) with Cu Ka radiation. Microhardness measurements were carried out by a Buehler Micromet II Microhardness Tester under a load of 100 gf and 15 s dwell time according to the ASTM standard E384–99. Compressive tests were carried out according to ASTM standard E9 at room temperature with a compressive rate of 0.5 mm/min by a universal material testing machine (AG-100KN). Three samples were tested for all conditions and the mean value was calculated.
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3. Results 3.1. Macro-scale morphology and relative density of the sintered samples The pure 6061 Al alloy samples and the Al/(Ti,W)C composite samples were successfully accomplished by microwave heating and microwave-coupled SiC heating. Fig. 2 shows the heating curve of Al and (Ti,W)C green compacts under microwave sintering, indicating that both materials have good ability to absorb microwave. The sintered samples with good macro-scale morphology (i.e., no porosity, no cracks nor shape change) were successfully produced at the sintering temperature ranging from 500 °C to 560 °C, as shown in Fig. 3. Bubbling and distortion occurred at the temperature of 580 °C (Fig. 3), which might be caused by the melting of 6061 aluminum alloy. This result is similar to the result reported by Leparoux [1], in which microwave sintered Al/SiC system at a sintered temperature below and near the melting point of Al (660 °C) but in the sintering process, the melting temperature of Al was reached and liquid Al was expelled to the sample surface. So in this paper, we studied the effect of the temperature ranging from 500 °C to 560 °C to avoid distortion. The absolute density and the porosity for each case are shown in Fig. 4. It is obvious that an increase in the amount of reinforcements leads to an increase in absolute density. On the contrary, the addition of (Ti,W)C particulates results in an increase in porosity. The effect of sintering temperature on the porosity is a little bit complex. For the samples with the same (Ti,W)C contents, the porosity increases firstly and then decreases. 3.2. Microstructure feature of the sintered samples The influence of both reinforcement content and sintering temperature on matrix grain size is summarized in Fig. 5. Compared with pure 6061Al, the grain size of the composites with 30 wt.% (Ti,W)C decreases up to 11%, 13%, 15%, and 17% at four temperatures of 500 °C, 520 °C, 540 °C, 560 °C, respectively. Raising the temperature leads to an increase of the grain size. The ratio of grain size growth caused by sintering temperature increasing from 500 °C to 560 °C is 32%, 28%, 25% and 23% at four (Ti,W)C contents of 0 wt.%, 10 wt.% (4.04 vol.%), 20 wt.% (8.66 vol.%) and 30 wt.% (13.98 vol.%), respectively. Therefore, the grain growth was significantly inhibited and the inhibition extent contributed by introducing (Ti,W)C particulates increased with an increase in (Ti,W)C content. The edge of the grain turns to be smoother with an increase in sintering temperature. A small degree of isolated porosity with small pores can still be found even at the highest sintering temperature of 560 °C. Typical SEM micrographs with various mass fractions of reinforcements are displayed in Fig. 6. The (Ti,W)C particulates
Fig. 1. SEM micrographs of (a) 6061 Al alloy powder and (b) (Ti,W)C powder.
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Table 1 The chemical composition (wt.%) of 6061 alloy powder. Others
Cu
Si
Fe
Mn
Mg
Zn
Cr
Ti
Al
<0.10
0.15– 0.40
0.40– 0.80
0.28
<0.15
0.80– 1.20
<0.05
0.04– 0.35
<0.15
Bal.
Fig. 5. The effect of sintering temperature and (Ti,W)C amount on grain size of the composites.
Fig. 2. The heating curves of Al and (Ti,W)C compacts under microwave sintering.
Fig. 3. Macro-morphology of the samples sintered at 500 °C, 520 °C, 540 °C, 560 °C and 580 °C (from left to right).
are uniformly distributed throughout the matrix, and few agglomerations can be found even at the highest reinforcement content (30 wt.%). Quantitative metallographic analysis reveals the average area percentages of the samples with 10 wt.%, 10 wt.% (4.04 vol.%), 20 wt.% (8.66 vol.%) and 30 wt.% (13.98 vol.%) (Ti,W)C are 4.13%, 8.76% and 14.02%, respectively. Most of the (Ti,W)C particulates distributed along grain boundaries and a small proportion of smaller particulates distributed inside the grains at every temperature and content. The results of X-ray diffraction analysis conducted on 6061Al alloy powders, (Ti,W)C powders and composites are shown in Fig. 7. Individual matching peaks of Al phase and C5Ti4W phase can be observed, which indicates that there was no significant interfacial reaction between 6061Al alloy and (Ti,W)C particulate. Table 2. HRTEM analysis indicated that sound and gap-free interfaces between Al alloy grains and Al/(Ti,W)C in the composites was observed, as shown in Fig. 8. Fig. 8(a) and (b) show the interface between (Ti,W)C phase and Al alloy grain sintered at 560 °C, the characteristic features are some white phase was formed in the junction region and the corners of (Ti,W)C are not as acute as the raw powder, suggesting that liquid phase appeared around the (Ti,W)C during the sintering process. Fig. 8(b) shows the interface between the liquid Al phase and the (Ti,W)C observed at high magnification. The image consists of three distinctive regions inwards: white region, black region and a distinctive layer between them. The corresponding electron diffraction patterns inserted in
Fig. 4. The effect of sintering temperature and (Ti,W)C amount on: (a) absolute density; (b) porosity.
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Fig. 6. SEM micrographs showing the distribution of the reinforcement sintered at 560 °C for: (a) 6061 + 10 wt.% (Ti,W)C; (b) 6061 + 20 wt.% (Ti,W)C; (c) 6061 + 30 wt.% (Ti,W)C.
presence of voids and debonding area can also be found in Fig. 8(d), the inserted electron diffraction indicates both of the two phases aside the interface are (Ti,W)C particulates. 3.3. Mechanical behavior
Fig. 7. XRD patterns of 6061 Al alloy powder, (Ti,W)C powder and samples sintered at 560 °C.
Table 2 The chemical composition (wt.%) of (Ti,W)C particle. Others
Total carbon
Free carbon
W
Ti
O
N
Fe
<0.10
12.64
0.21
47.05
39.60
0.18
0.14
0.11
Fig. 8(b) indicate that the white and black region are Al and (Ti,W)C, respectively. The ragged edge of (Ti,W)C particulate (Fig. 8(a)) and the distinctive layer show that there has been a considerable diffusion between the two phases. Table 3 summarized the chemical compositions of different regions in Fig. 8, which proves the liquid phase is Al and shows clearly the distinctive layer contains Al, Ti and W. According to the EDS analysis, diffusion occurred at between 6061Al matrix and (Ti,W)C particulates, which leads to the formation of diffusion layer. Typical high resolution images revealed stable, strongly connected and defect-free interfaces in Fig. 8(c), which was identified to be Al. Additionally, some parallel lines in the Al grains may indicate the presence of stacking fault (Fig. 8(a) and (c)). Consistent and tight interface with no
The variation in microhardness of samples with different (Ti,W)C reinforcement contents sintered at different temperatures is plotted in Fig. 9. Obviously, the addition of the (Ti,W)C particulates could effectively upgrade the microhardness. For all Al/ (Ti,W)C samples, the corresponding microhardness change slightly at the sintering temperature from 500 °C to 520 °C, and increases significantly at the sintering temperature from 520 °C to 560 °C. However, the microhardness of the pure 6061 Al sample reduces with an increase in the sintering temperature from 500 °C to 560 °C. The effect of the amount of (Ti,W)C phase on compressive yield stress, compression strength and elongation at the sintering temperature of 560 °C is plotted in Fig. 10. The properties were enhanced by the addition of (Ti,W)C, and the increasing extent of the compressive yield strength was 19, 52 and 137 MPa achieved by various (Ti,W)C addition contents (i.e., 10 wt.%, 20 wt.% and 30 wt.%) respectively, in comparison with the pure 6061 Al alloy samples. The compressive strength increases from 346 MPa to 474 MPa when the (Ti,W)C content increases from 10 wt.% to 30 wt.% (Fig. 10(a)). However, increasing the amount of reinforcements leads to a reduction in elongation, and the pure 6061 Al alloy sample displayed best plasticity (Fig. 10(b)). Fig. 11(a) illustrates the compressive yield strength and compressive strength of the 30 wt.% (Ti,W)C reinforced 6061Al alloy samples as a function of sintering temperature. The trend of the compressive yield strength is similar to the compression strength. There is no obvious difference in compressive yield strength and compression strength between the samples sintered at 520 °C and 500 °C. However, the sintering temperature ranging from 520 °C to 560 °C results in an increase of compression strength, the growth ratio of which is 39%. At the same time, the increase in sintering temperature from 520 °C to 560 °C also leads the compressive yield stress increased by 36%. The effect of temperature on
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Fig. 8. HRTEM images of the interfaces: (a) the interface between (Ti,W)C and liquid Al phase; (b) micrograph at high magnification; (c) the Al/Al interface; (d) the (Ti,W)C/ (Ti,W)C interface.
Table 3 Chemical composition (wt.%) of the phases in Fig. 11. Elements
Al
Ti
W
C
Cr
Cu
Region A B C D
69.40 31.80 98.46 99.12
11.45 24.60 0.59 0.14
17.12 41.05 – –
– – 0.01 –
0.28 – 0.11 0.06
1.75 2.55 0.83 0.68
elongation is illustrated in Fig. 11(b). With the increasing of the temperature, only a slightly but not significantly increase can be observed in elongation. Fig. 12 illustrates the effect of (Ti,W)C content and temperature on stress–strain behavior of the composites. The strengthening effect of the reinforcement and the decrease in elongation can be clearly observed with the increase of (Ti,W)C content (Fig. 12(a)). The effect of temperature on stress–strain behavior of the 6061 Al/(Ti,W)C composite was shown in Fig. 12(b), which is in agreement with Fig. 11.
4. Discussion 4.1. Densification mechanism of the Al/(Ti,W)C composite by microwave sintering
Fig. 9. The effect of sintering temperature and (Ti,W)C amount on hardness of composites.
The pure 6061 Al alloy and the pure Al/(Ti,W)C green compacts were successfully sintered using microwave sintering method with different sintering parameters. Low porosity was achieved in present study. This would attribute to the volumetric heating during microwave sintering, and sintering process was initiated throughout the sample, which has been proven to contribute to high dense samples [15–17]. What is more, atoms may accelerate diffusion when samples are exposed in the microwave fields and the densification can be enhanced in a short holding time [18]. Results suggest that the increase of the mass fraction of (Ti,W)C causes an increase in density, which is contributed by the higher specific gravity of (Ti,W)C than that of 6061 Al alloy. However, the porosity was increased by the addition of (Ti,W)C. This effect on densification of sintered green compacts is similar to the result reported by Rahimian [19]. The random distribution of (Ti,W)C particulates acts as a barrier to the diffusion of Al atom, which
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Fig. 10. The effect of (Ti,W)C amount on: (a) compression yield strength and compression strength; (b) elongation.
Fig. 11. The effect of sintering temperature on: (a) compression yield strength and compression strength; (b) elongation.
Fig. 12. The effect of (a) (Ti,W)C content and (b) sintering temperature on stress–strain behavior of the composites.
hinders the rate of densification by reducing the mass transport processes. This hindering becomes more prominent if these second phase particulates stay at the grain boundary. Therefore, the increase of such second phase is expected to reduce the rate of densification. The temperature has a twofold effect on the densification of composites. With the increase of the temperature, the porosity increased firstly and decreased later. The reason why porosity was increased as the temperature raising is not clear. Raised the temperature from 520 °C to 560 °C, the porosity decreased due to high
diffusion rates. Increasing the sintering temperature causes the easier diffusion of atoms which help the better sinter ability and resulting to a high density of the composites. Besides, the appearance of a small amount of liquid aluminum phase around (Ti,W)C sintered at high temperature would be expected to provide a faster transport part for diffusion, thus enhancing the densification process [20]. The studies conducted on the samples fabricated by PM technology using microwave sintering revealed narrow grain distribution, uniform distribution of the reinforcements, and well-bonded
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interfaces of Al/Al, (Ti,W)C /(Ti,W)C Al/(Ti,W)C. This can be attributed to the appropriate selection of green compaction, blending, and sintering parameters. Moreover, it can also be correlated with the advantage of microwave sintering which have been proved to produce the composites with small grain size, homogeneous distribution of reinforcements and good interfacial characteristic [15,21,22]. As compared with pure 6061 Al, the addition of (Ti,W)C have a significant effect on the reduction of the grain size. There is a noticeable decrease in average grain size as larger amounts of reinforcements were added. This can be explained by the well-known Zener formula [15,23]:
ds ¼
4adp 3V p
ð2Þ
where a is a constant, dp is radius of the particulates, Vp is fractional volume of particulates. As it can be seen from Eq. (2), the grain size is determined by the reinforcing particulate size and volume fraction. At the present study, a large amount of reinforcements distribute at the grain boundary and the limitation of reinforcements to contribute as grain refiner in the composites. For both pure 6061 Al and the composite samples, large grain size was observed in the samples sintered at high temperature. The growth rate of the grain depends on the migration of grain boundaries, the rising temperature facilitate the easier diffusion and hence results in larger grain size. However, the microstructure becomes uniform with the addition of the reinforcements. Smaller grains and narrower grain distribution were obtained in the composite with 30 wt.% (Ti,W)C particulates due to the inhabitation of the reinforcements on microstructural coarsening. For all of the composites, fairly well-distributed (Ti,W)C particulates with few clusters can be observed. The development of homogeneous microstructure in relation to second phase distribution can be attributed to: (a) suitability of parameters during blending, (b) well relative particulate size (RPS) ratio between the matrix and reinforced particulates. Bhanu Prasad et al. [24] reported that the degree of homogeneity of the particulate dispersion is determined by the RPS ratio. Particulate clustering is most likely to occur in higher RPS ratios (e.g. 40:1; 25:1; 18:1) while the lower RPS ratios (e.g. 5:1 and 3:1) promote a more uniform distribution of the reinforcement in the matrix. In this study, 6061 powders with a size range of 5–10 lm and (Ti,W)C with a Fisher particulate size of 2.98 lm, leads to a homogeneous microstructure in the composites. The observation of the interfacial characteristics conducted on the interfaces of Al/(Ti,W)C, (Ti,W)C/(Ti,W)C and Al/Al were found to be very good. In the case of Al/(Ti,W)C interface, the presence of liquid phase between Al and (Ti,W)C may be due to: (a) (Ti,W)C exhibits better absorbency of microwave than 6061 Al alloy, that is, the temperature of (Ti,W)C will surpass the temperature of Al alloy during the microwave sintering process, in this way, heat will transfer from (Ti,W)C to Al alloy, resulting in the melting of Al phase adjacent to (Ti,W)C, (b) initial powders are very small and the (Ti,W)C particulates are angular, in the microwave sintering, smaller particulate have a better absorbility [1], so the small bodies especially sharp corners of (Ti,W)C turn to hotspot during microwave sintering. Additionally, due to acute angle effect, heat concentration occurred at the sharp corners lead to the melting of small Al particulates. The liquid phase would be expected to provide a faster transport path for diffusion, thus interdiffusion occurred between Al and (Ti,W)C, resulting in the formation of diffusion layer. Besides, the appearance of interdiffusion might be due to the microwave effect, which is considered to promote the atom diffusion [6,18,25]. Even though the interdiffusion affects the Al/(Ti,W)C interfacial microstructure, no appearance of interfa-
cial reaction can be observed between the matrix and the second phase, which is in accordance with the results of X-ray diffraction studies conducted on 6061 aluminum alloy powder, (Ti,W)C powder and the sintered composites. It may be attributed to the low sintering temperature, rapid heating, the considerable advantages of microwave sintering, which make it possible to minimize the chances of interfacial reaction and hence maintaining a cleaner interface [15]. The stacking faults in the Al grain might be caused by the large difference in thermal expansion coefficients values of the matrix and the reinforcement which caused thermal stress induced during cooling in the air. Consistent interface at the boundary of (Ti,W)C particulates suggests that solid diffusion took place between (Ti,W)C phases even at a low sintering temperature of 560 °C. This may attributed to the advantage of microwave sintering which have been proved to enhance diffusion process and the (Ti,W)C particulates could be heated effectively at a low power during the microwave sintering (Fig. 2) [18]. 4.2. Improvement mechanism of mechanical properties For all the tested samples, the average microhardness of the microwave sintered samples is higher than the corresponding value for the reference 6061 Al alloy and the hardness increases with the increasing amount of (Ti,W)C. This can be explained by: (a) the law of mixtures, which shown in the following equation [19]:
Hc ¼ Hm fm þ Hr fr
ð3Þ
where Hc, Hm, and Hr are the hardness of the composite, matrix and reinforcement, respectively, fm and fr are the fraction of matrix and reinforcement, respectively. The microhardness is increased since the (Ti,W)C is inherently harder than 6061 Al alloy, (b) grain refinement strengthening, the addition of (Ti,W)C with smaller grains and narrower grain distribution is responsible for the increase of hardness of Al/(Ti,W)C composites, (c) dispersion strengthening, the fairly well distribution of the reinforcements improves the ability of the soft matrix to resist deformation. Sintering temperature is another parameter which influences the microhardness of the composite. In the Al/(Ti,W)C composites, raising the temperature from 500 °C to 520 °C, the microhardness changed little due to the decrease of the densification. However, further increase of the temperature enhances the microhardness due to reduced porosity and the enhanced interfacial bonding. In the pure 6061 Al alloy samples, the microhardness decreased with the temperature rising, this negative effect is caused by the grain coarsening. The superior values of compressive yield strength and compressive strength were obtained in present study compared with the previous studies on aluminum matrix composite (Figs. 10 and 11) [19,26]. This is possibly attributed to the suitable combination of 6061Al alloy powders and (Ti,W)C particulates and the optimal microwave sintering parameters [12]. The results of the ambient temperature compressive tests on the composite samples revealed that an increase in the amount of (Ti,W)C leads to an increase in the compressive yield strength and compressive strength of 6061Al alloy. This can primarily be attributed as follows: (a) Hall–Petch law, the strength of the polycrystalline increases with grain refinement which can be expressed by the following equation:
ry ¼ r0 þ K y d1=2
ð4Þ
where ry is the yield strength, r0 and Ky are constants for given material, and d is the average grain diameter. According to Eq. (4), smaller grain size will lead to more grain boundaries which can act as strong obstacles to the dislocation motion, thus resulting to
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higher yield stress and compressive strength. Moreover, the uniformity of microstructure of the grain size is also contributed to higher strength, (b) the stable interface structure may ensure the strengthening effect of the second phase by loading transfer on the Al matrix [27,28], (c) the presence of high dislocation density in the matrix due to the large difference in thermal expansion coefficients values between the matrix and the reinforcements [12,15,29], and (d) dispersion hardening. A decrease of elongation with the increasing of the reinforcement can be found from Fig. 10(b), this can be attributed to the fact that the more reinforcements are added, the shorter of the interparticulate spacing will be. In this way, the movement of dislocations will be harder because there are more barriers, and then the dislocations pile up happens, causing the decrease in elongation [26]. The trend of the effect of temperature on compressive yield strength and compressive strength is similar to that of the microhardness. The compressive yield strength increases with the sintering temperature increases. Increasing the temperature causes a decrease in miro-porosities and this is equal to lower stress concentration regions and therefore, has a beneficial effect on mechanical properties. Furthermore, good matrix-reinforcement interfacial integrity, higher interfacial bond strength could effectively transfer load from the Al matrix to the second phase, thus strengthening the mechanical property. No obvious change on the compressive yield strength and compressive strength when the temperature increased from 500 °C to 520 °C due to grain coarsening and the decline in density. The effect of the temperature on elongation is not obvious owing to the small temperature ranges. But raised the temperature to 560 °C leads to the rise in elongation, this can be due to the bonding strength between the (Ti,W)C and 6061Al become higher in such circumstances. 5. Conclusions 1. Low porosity of 6061 Al alloy and Al/(Ti,W)C composites with good macro-scale morphology were successfully fabricated using microwave sintering method at diverse temperatures and amounts of (Ti,W)C because of the higher microwave absorptivity of both 6061 Al alloy and Al/(Ti,W)C. 2. The grain growth trend has been significantly restrained by introducing (Ti,W)C phase while increasing temperature facilitates grain growth. (Ti,W)C particulates are uniformly distributed and predominantly distributed along grain boundaries. Sound grain boundaries and phase interfaces were generated by microwave sintering the low sintering temperature. 3. The average microhardness, compressive yield strength as well as compression strength were enhanced with an increase of both (Ti,W)C phase amount and the temperature (ranging from 520 °C to 560 °C). The microhardness increased to a maximum value of 96HV, the yield stress and compressive strength increased to a maximum value of 236 MPa and 473 MPa, respectively. 4. Microwave heating mainly induced diffusion between Al alloy and/or (Ti,W)C particles and generated well-bonded interfaces. Meanwhile, a small amount of liquid Al phase appeared around the (Ti,W)C due to the better absorbing ability of (Ti,W)C to microwave and heat concentration due to angular initial powder. The appearance of liquid might promoted interfacial diffusion.
Acknowledgement The author would like to thank the financial support provided by the National Basic Research Program of China ‘‘973’’ (No.
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Microstructure and mechanical properties of Al/(Ti,W)C composites prepared by microwave sintering R.R. Zheng a, Y. Wu a, S.L. Liao a, W.Y. Wang b, W.B Wang b, A.H. Wang a,⇑ a State Key Laboratory of Material Processing and Die & Mould Technology, School of Materials Science & Engineering, Huazhong University of Science & Technology, Wuhan 430074, PR China b Henan Key Laboratory of Non-ferrous Metals Materials Science & Processing Technology, School of Materials Science & Engineering, Henan University of Science & Technology, Luoyang 471023, PR China
a r t i c l e
i n f o
Article history: Received 19 August 2013 Received in revised form 29 November 2013 Accepted 3 December 2013 Available online 8 December 2013 Keywords: Aluminum matrix composites Microwave sintering Microstructure Mechanical properties
a b s t r a c t (Ti,W)C particulate reinforced 6061 Al alloy composites were prepared by microwave sintering method in this investigation. Green compacts containing different content of reinforcements (i.e., 10, 20 and 30 wt.% (Ti,W)C) were prepared and then sintered by microwave heating at four temperatures (i.e., 500 °C, 520 °C, 540 °C, 560 °C) with a constant soaking time of 45 min. The porosity was increased with an increase in the additive amount of (Ti,W)C particulates, firstly increased and then decreased with an increase in sintering temperature. The microstructure was characterized by uniform distribution of (Ti,W)C particulates in the Al alloy matrix. Grain size growth trend was readily controlled and small grain size was obtained even at higher sintering temperature owing to the addition of (Ti,W)C phase. Mechanical tests indicated that the average hardness, compressive yield strength as well as compression strength were enhanced with an increase in both (Ti,W)C particulate amount and sintering temperature (range from 520 °C to 560 °C). The interfacial structures between Al/(Ti,W)C, Al/Al and (Ti,W)C/(Ti,W)C were revealed by TEM to investigate bounding mechanism during microwave sintering process. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Aluminum metal matrix composites (AMC) have been widely used in aerospace, car industry and military due to their excellent properties, such as high specific modulus, strength and toughness. Conventionally, extrusion forging, squeeze casting and powder metallurgy (PM) are used to produce AMC [1,2]. Compared with traditional methods, PM method is believed to be a suitable process to fabricated near net shape products [3]. In addition, the low processing temperature of the PM method can avoid interaction between matrix and reinforcements [4]. Moreover, it has better control on the microstructure where better distribution of the reinforcements is possible, and this great influence on the end properties [5]. However, conventional PM processing of composites often need long sintering time, which results in grain coarsening and poor performance of mechanical properties [6]. Compared with traditional heating methods, microwave sintering presents distinct advantages, including energy efficient, environmental friendly, enhanced densification, and especially, smaller grain size due to the faster heating rate and the lower sintering temperature [7,8]. Additionally, attributed to uniform volumetric heating and smaller pores in the sintered green compacts, ⇑ Corresponding author. Tel.: +86 27 87180507. E-mail address: [email protected] (A.H. Wang). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.12.016
the microstructure and mechanical properties can be significantly improved [8–10]. Therefore, since the success of microwave sintering metallic materials in 1999 [11], many researchers pay attention to the processing of metal based materials using microwave heating. Up to now, the research has been carried out on ferrous, copper, tungsten and magnesium with limited studies on aluminum and its composites [6]. The present studies on AMC mainly focused on the system of Al/SiC. Leparoux et al. [1] described SiCreinforced Al-matrix composites with high SiC content can be effectively sintered using microwave. They also pointed out that the SiC particulate size has a predominant influence on the heating rate. Thakur et al. [12] reported that two-directional microwave assisted rapid sintering could be used for fabricating Al/(Ti + SiC) and ascribed the improvement of mechanical properties to fine microstructure. It is well known that pure aluminum does not have enough strength compared with aluminum alloys. So far, 6061 Al alloy has received lots of attention for its good ductility, excellent corrosion resistance, good strength and low price [13]. (Ti,W)C may be the most widely known tungsten carbide because of its exceptional hardness, and superior wear resistance [14]. But there is no research on (Ti,W)C particulate reinforced Al matrix. In addition, little effort has been made to investigate the effect of the amount of reinforcement and different levels of sintering temperature have on the microstructure and the
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properties of AMC using microwave heating. Moreover, there has been no research to study the interfacial characteristic between Al matrix and the ceramic reinforcements fabricated by microwave heating. Therefore, 6061 Al alloy matrix and (Ti,W)C reinforcement were selected to synthesize AMC by microwave sintering, and the influence of microwave sintering parameters and (Ti,W)C reinforcement amount on microstructure mechanical properties was investigated in present study.
2. Material and methods The 6061 Al alloy powder with a size range of 5–10 lm (acquired from Zhejiang Bainianyin Company, China) and the angular (Ti,W)C powder with a size range of 0.1–10 lm (procured from Ningxiang Shuangdun Metal Co., Ltd., China) were used as raw materials. The corresponding morphologies and chemical compositions are listed in Fig. 1 and Table 1, respectively. Powders were mixed by mechanical milling using a planetary ball mill for 1 h at 200RPM with a ball to powder ratio of 5:1 by a solvent of ethanol and a dispersing agent of polyethylene glycol. Cuboid green compacts (23 7 7 mm) with different (Ti,W)C particulate content, i.e., 0 wt.%, 10 wt.% (4.04 vol.%), 20 wt.% (8.66 vol.%) and 30 wt.% (13.98 vol.%), were prepared by cold compression moulding forming at a pressure of 400 MPa for 2 min. A microwave oven with frequency 2.45 GHz and power 1.4 kW (Changsha SYNO-THERM Corporation, China) was applied for microwave sintering. Silicon carbide and alumina mixture was buried around the samples to realize rapid and uniform heating from both inside and outside of the green compacts [12]. Microwave heating process was carried out at four different temperatures (500 °C, 520 °C, 540 °C and 560 °C) in high pure argon atmosphere (99.99%) with a heating rate of 20 °C/min. Accurate temperature of samples was monitored using an infrared thermometer and the temperature was controlled by changing microwave power. The green compacts were firstly heated up to 400 °C for half an hour to eliminate residual additives and then heated to the sintering temperature for a soaking time of 45 min, finally, the sintered samples were cooled down to room temperature in air. The absolute density of sintered AMC in polished condition was measured using Archimedes principle. An electronic balance (Yueping, FA2104J) with an accuracy of 0.0001 g was used for recording. The general porosity of the sintered samples was calculated by the following formulae:
e ¼ ð1
q Þ 100 q0
ð1Þ
where q and q0 are the density of the specimen and its corresponding theoretical density, respectively. Three samples were tested for all conditions and the mean value was calculated to make the results more accurate. A JEOL-7600F scanning electron microscopy (SEM) was utilized to evaluate grain morphology and distribution of (Ti,W)C particulates. A high resolution transmission electron microscope (HRTEM, JEOL-2010) coupled with energy dispersive spectrometer (EDS) were utilized to evaluate interfacial characteristics and chemical composition. The corresponding phase structure was determined using X-ray diffraction (XRD, X’Pert PRO, PANalytical, The Netherlands) with Cu Ka radiation. Microhardness measurements were carried out by a Buehler Micromet II Microhardness Tester under a load of 100 gf and 15 s dwell time according to the ASTM standard E384–99. Compressive tests were carried out according to ASTM standard E9 at room temperature with a compressive rate of 0.5 mm/min by a universal material testing machine (AG-100KN). Three samples were tested for all conditions and the mean value was calculated.
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3. Results 3.1. Macro-scale morphology and relative density of the sintered samples The pure 6061 Al alloy samples and the Al/(Ti,W)C composite samples were successfully accomplished by microwave heating and microwave-coupled SiC heating. Fig. 2 shows the heating curve of Al and (Ti,W)C green compacts under microwave sintering, indicating that both materials have good ability to absorb microwave. The sintered samples with good macro-scale morphology (i.e., no porosity, no cracks nor shape change) were successfully produced at the sintering temperature ranging from 500 °C to 560 °C, as shown in Fig. 3. Bubbling and distortion occurred at the temperature of 580 °C (Fig. 3), which might be caused by the melting of 6061 aluminum alloy. This result is similar to the result reported by Leparoux [1], in which microwave sintered Al/SiC system at a sintered temperature below and near the melting point of Al (660 °C) but in the sintering process, the melting temperature of Al was reached and liquid Al was expelled to the sample surface. So in this paper, we studied the effect of the temperature ranging from 500 °C to 560 °C to avoid distortion. The absolute density and the porosity for each case are shown in Fig. 4. It is obvious that an increase in the amount of reinforcements leads to an increase in absolute density. On the contrary, the addition of (Ti,W)C particulates results in an increase in porosity. The effect of sintering temperature on the porosity is a little bit complex. For the samples with the same (Ti,W)C contents, the porosity increases firstly and then decreases. 3.2. Microstructure feature of the sintered samples The influence of both reinforcement content and sintering temperature on matrix grain size is summarized in Fig. 5. Compared with pure 6061Al, the grain size of the composites with 30 wt.% (Ti,W)C decreases up to 11%, 13%, 15%, and 17% at four temperatures of 500 °C, 520 °C, 540 °C, 560 °C, respectively. Raising the temperature leads to an increase of the grain size. The ratio of grain size growth caused by sintering temperature increasing from 500 °C to 560 °C is 32%, 28%, 25% and 23% at four (Ti,W)C contents of 0 wt.%, 10 wt.% (4.04 vol.%), 20 wt.% (8.66 vol.%) and 30 wt.% (13.98 vol.%), respectively. Therefore, the grain growth was significantly inhibited and the inhibition extent contributed by introducing (Ti,W)C particulates increased with an increase in (Ti,W)C content. The edge of the grain turns to be smoother with an increase in sintering temperature. A small degree of isolated porosity with small pores can still be found even at the highest sintering temperature of 560 °C. Typical SEM micrographs with various mass fractions of reinforcements are displayed in Fig. 6. The (Ti,W)C particulates
Fig. 1. SEM micrographs of (a) 6061 Al alloy powder and (b) (Ti,W)C powder.
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Table 1 The chemical composition (wt.%) of 6061 alloy powder. Others
Cu
Si
Fe
Mn
Mg
Zn
Cr
Ti
Al
<0.10
0.15– 0.40
0.40– 0.80
0.28
<0.15
0.80– 1.20
<0.05
0.04– 0.35
<0.15
Bal.
Fig. 5. The effect of sintering temperature and (Ti,W)C amount on grain size of the composites.
Fig. 2. The heating curves of Al and (Ti,W)C compacts under microwave sintering.
Fig. 3. Macro-morphology of the samples sintered at 500 °C, 520 °C, 540 °C, 560 °C and 580 °C (from left to right).
are uniformly distributed throughout the matrix, and few agglomerations can be found even at the highest reinforcement content (30 wt.%). Quantitative metallographic analysis reveals the average area percentages of the samples with 10 wt.%, 10 wt.% (4.04 vol.%), 20 wt.% (8.66 vol.%) and 30 wt.% (13.98 vol.%) (Ti,W)C are 4.13%, 8.76% and 14.02%, respectively. Most of the (Ti,W)C particulates distributed along grain boundaries and a small proportion of smaller particulates distributed inside the grains at every temperature and content. The results of X-ray diffraction analysis conducted on 6061Al alloy powders, (Ti,W)C powders and composites are shown in Fig. 7. Individual matching peaks of Al phase and C5Ti4W phase can be observed, which indicates that there was no significant interfacial reaction between 6061Al alloy and (Ti,W)C particulate. Table 2. HRTEM analysis indicated that sound and gap-free interfaces between Al alloy grains and Al/(Ti,W)C in the composites was observed, as shown in Fig. 8. Fig. 8(a) and (b) show the interface between (Ti,W)C phase and Al alloy grain sintered at 560 °C, the characteristic features are some white phase was formed in the junction region and the corners of (Ti,W)C are not as acute as the raw powder, suggesting that liquid phase appeared around the (Ti,W)C during the sintering process. Fig. 8(b) shows the interface between the liquid Al phase and the (Ti,W)C observed at high magnification. The image consists of three distinctive regions inwards: white region, black region and a distinctive layer between them. The corresponding electron diffraction patterns inserted in
Fig. 4. The effect of sintering temperature and (Ti,W)C amount on: (a) absolute density; (b) porosity.
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Fig. 6. SEM micrographs showing the distribution of the reinforcement sintered at 560 °C for: (a) 6061 + 10 wt.% (Ti,W)C; (b) 6061 + 20 wt.% (Ti,W)C; (c) 6061 + 30 wt.% (Ti,W)C.
presence of voids and debonding area can also be found in Fig. 8(d), the inserted electron diffraction indicates both of the two phases aside the interface are (Ti,W)C particulates. 3.3. Mechanical behavior
Fig. 7. XRD patterns of 6061 Al alloy powder, (Ti,W)C powder and samples sintered at 560 °C.
Table 2 The chemical composition (wt.%) of (Ti,W)C particle. Others
Total carbon
Free carbon
W
Ti
O
N
Fe
<0.10
12.64
0.21
47.05
39.60
0.18
0.14
0.11
Fig. 8(b) indicate that the white and black region are Al and (Ti,W)C, respectively. The ragged edge of (Ti,W)C particulate (Fig. 8(a)) and the distinctive layer show that there has been a considerable diffusion between the two phases. Table 3 summarized the chemical compositions of different regions in Fig. 8, which proves the liquid phase is Al and shows clearly the distinctive layer contains Al, Ti and W. According to the EDS analysis, diffusion occurred at between 6061Al matrix and (Ti,W)C particulates, which leads to the formation of diffusion layer. Typical high resolution images revealed stable, strongly connected and defect-free interfaces in Fig. 8(c), which was identified to be Al. Additionally, some parallel lines in the Al grains may indicate the presence of stacking fault (Fig. 8(a) and (c)). Consistent and tight interface with no
The variation in microhardness of samples with different (Ti,W)C reinforcement contents sintered at different temperatures is plotted in Fig. 9. Obviously, the addition of the (Ti,W)C particulates could effectively upgrade the microhardness. For all Al/ (Ti,W)C samples, the corresponding microhardness change slightly at the sintering temperature from 500 °C to 520 °C, and increases significantly at the sintering temperature from 520 °C to 560 °C. However, the microhardness of the pure 6061 Al sample reduces with an increase in the sintering temperature from 500 °C to 560 °C. The effect of the amount of (Ti,W)C phase on compressive yield stress, compression strength and elongation at the sintering temperature of 560 °C is plotted in Fig. 10. The properties were enhanced by the addition of (Ti,W)C, and the increasing extent of the compressive yield strength was 19, 52 and 137 MPa achieved by various (Ti,W)C addition contents (i.e., 10 wt.%, 20 wt.% and 30 wt.%) respectively, in comparison with the pure 6061 Al alloy samples. The compressive strength increases from 346 MPa to 474 MPa when the (Ti,W)C content increases from 10 wt.% to 30 wt.% (Fig. 10(a)). However, increasing the amount of reinforcements leads to a reduction in elongation, and the pure 6061 Al alloy sample displayed best plasticity (Fig. 10(b)). Fig. 11(a) illustrates the compressive yield strength and compressive strength of the 30 wt.% (Ti,W)C reinforced 6061Al alloy samples as a function of sintering temperature. The trend of the compressive yield strength is similar to the compression strength. There is no obvious difference in compressive yield strength and compression strength between the samples sintered at 520 °C and 500 °C. However, the sintering temperature ranging from 520 °C to 560 °C results in an increase of compression strength, the growth ratio of which is 39%. At the same time, the increase in sintering temperature from 520 °C to 560 °C also leads the compressive yield stress increased by 36%. The effect of temperature on
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Fig. 8. HRTEM images of the interfaces: (a) the interface between (Ti,W)C and liquid Al phase; (b) micrograph at high magnification; (c) the Al/Al interface; (d) the (Ti,W)C/ (Ti,W)C interface.
Table 3 Chemical composition (wt.%) of the phases in Fig. 11. Elements
Al
Ti
W
C
Cr
Cu
Region A B C D
69.40 31.80 98.46 99.12
11.45 24.60 0.59 0.14
17.12 41.05 – –
– – 0.01 –
0.28 – 0.11 0.06
1.75 2.55 0.83 0.68
elongation is illustrated in Fig. 11(b). With the increasing of the temperature, only a slightly but not significantly increase can be observed in elongation. Fig. 12 illustrates the effect of (Ti,W)C content and temperature on stress–strain behavior of the composites. The strengthening effect of the reinforcement and the decrease in elongation can be clearly observed with the increase of (Ti,W)C content (Fig. 12(a)). The effect of temperature on stress–strain behavior of the 6061 Al/(Ti,W)C composite was shown in Fig. 12(b), which is in agreement with Fig. 11.
4. Discussion 4.1. Densification mechanism of the Al/(Ti,W)C composite by microwave sintering
Fig. 9. The effect of sintering temperature and (Ti,W)C amount on hardness of composites.
The pure 6061 Al alloy and the pure Al/(Ti,W)C green compacts were successfully sintered using microwave sintering method with different sintering parameters. Low porosity was achieved in present study. This would attribute to the volumetric heating during microwave sintering, and sintering process was initiated throughout the sample, which has been proven to contribute to high dense samples [15–17]. What is more, atoms may accelerate diffusion when samples are exposed in the microwave fields and the densification can be enhanced in a short holding time [18]. Results suggest that the increase of the mass fraction of (Ti,W)C causes an increase in density, which is contributed by the higher specific gravity of (Ti,W)C than that of 6061 Al alloy. However, the porosity was increased by the addition of (Ti,W)C. This effect on densification of sintered green compacts is similar to the result reported by Rahimian [19]. The random distribution of (Ti,W)C particulates acts as a barrier to the diffusion of Al atom, which
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Fig. 10. The effect of (Ti,W)C amount on: (a) compression yield strength and compression strength; (b) elongation.
Fig. 11. The effect of sintering temperature on: (a) compression yield strength and compression strength; (b) elongation.
Fig. 12. The effect of (a) (Ti,W)C content and (b) sintering temperature on stress–strain behavior of the composites.
hinders the rate of densification by reducing the mass transport processes. This hindering becomes more prominent if these second phase particulates stay at the grain boundary. Therefore, the increase of such second phase is expected to reduce the rate of densification. The temperature has a twofold effect on the densification of composites. With the increase of the temperature, the porosity increased firstly and decreased later. The reason why porosity was increased as the temperature raising is not clear. Raised the temperature from 520 °C to 560 °C, the porosity decreased due to high
diffusion rates. Increasing the sintering temperature causes the easier diffusion of atoms which help the better sinter ability and resulting to a high density of the composites. Besides, the appearance of a small amount of liquid aluminum phase around (Ti,W)C sintered at high temperature would be expected to provide a faster transport part for diffusion, thus enhancing the densification process [20]. The studies conducted on the samples fabricated by PM technology using microwave sintering revealed narrow grain distribution, uniform distribution of the reinforcements, and well-bonded
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interfaces of Al/Al, (Ti,W)C /(Ti,W)C Al/(Ti,W)C. This can be attributed to the appropriate selection of green compaction, blending, and sintering parameters. Moreover, it can also be correlated with the advantage of microwave sintering which have been proved to produce the composites with small grain size, homogeneous distribution of reinforcements and good interfacial characteristic [15,21,22]. As compared with pure 6061 Al, the addition of (Ti,W)C have a significant effect on the reduction of the grain size. There is a noticeable decrease in average grain size as larger amounts of reinforcements were added. This can be explained by the well-known Zener formula [15,23]:
ds ¼
4adp 3V p
ð2Þ
where a is a constant, dp is radius of the particulates, Vp is fractional volume of particulates. As it can be seen from Eq. (2), the grain size is determined by the reinforcing particulate size and volume fraction. At the present study, a large amount of reinforcements distribute at the grain boundary and the limitation of reinforcements to contribute as grain refiner in the composites. For both pure 6061 Al and the composite samples, large grain size was observed in the samples sintered at high temperature. The growth rate of the grain depends on the migration of grain boundaries, the rising temperature facilitate the easier diffusion and hence results in larger grain size. However, the microstructure becomes uniform with the addition of the reinforcements. Smaller grains and narrower grain distribution were obtained in the composite with 30 wt.% (Ti,W)C particulates due to the inhabitation of the reinforcements on microstructural coarsening. For all of the composites, fairly well-distributed (Ti,W)C particulates with few clusters can be observed. The development of homogeneous microstructure in relation to second phase distribution can be attributed to: (a) suitability of parameters during blending, (b) well relative particulate size (RPS) ratio between the matrix and reinforced particulates. Bhanu Prasad et al. [24] reported that the degree of homogeneity of the particulate dispersion is determined by the RPS ratio. Particulate clustering is most likely to occur in higher RPS ratios (e.g. 40:1; 25:1; 18:1) while the lower RPS ratios (e.g. 5:1 and 3:1) promote a more uniform distribution of the reinforcement in the matrix. In this study, 6061 powders with a size range of 5–10 lm and (Ti,W)C with a Fisher particulate size of 2.98 lm, leads to a homogeneous microstructure in the composites. The observation of the interfacial characteristics conducted on the interfaces of Al/(Ti,W)C, (Ti,W)C/(Ti,W)C and Al/Al were found to be very good. In the case of Al/(Ti,W)C interface, the presence of liquid phase between Al and (Ti,W)C may be due to: (a) (Ti,W)C exhibits better absorbency of microwave than 6061 Al alloy, that is, the temperature of (Ti,W)C will surpass the temperature of Al alloy during the microwave sintering process, in this way, heat will transfer from (Ti,W)C to Al alloy, resulting in the melting of Al phase adjacent to (Ti,W)C, (b) initial powders are very small and the (Ti,W)C particulates are angular, in the microwave sintering, smaller particulate have a better absorbility [1], so the small bodies especially sharp corners of (Ti,W)C turn to hotspot during microwave sintering. Additionally, due to acute angle effect, heat concentration occurred at the sharp corners lead to the melting of small Al particulates. The liquid phase would be expected to provide a faster transport path for diffusion, thus interdiffusion occurred between Al and (Ti,W)C, resulting in the formation of diffusion layer. Besides, the appearance of interdiffusion might be due to the microwave effect, which is considered to promote the atom diffusion [6,18,25]. Even though the interdiffusion affects the Al/(Ti,W)C interfacial microstructure, no appearance of interfa-
cial reaction can be observed between the matrix and the second phase, which is in accordance with the results of X-ray diffraction studies conducted on 6061 aluminum alloy powder, (Ti,W)C powder and the sintered composites. It may be attributed to the low sintering temperature, rapid heating, the considerable advantages of microwave sintering, which make it possible to minimize the chances of interfacial reaction and hence maintaining a cleaner interface [15]. The stacking faults in the Al grain might be caused by the large difference in thermal expansion coefficients values of the matrix and the reinforcement which caused thermal stress induced during cooling in the air. Consistent interface at the boundary of (Ti,W)C particulates suggests that solid diffusion took place between (Ti,W)C phases even at a low sintering temperature of 560 °C. This may attributed to the advantage of microwave sintering which have been proved to enhance diffusion process and the (Ti,W)C particulates could be heated effectively at a low power during the microwave sintering (Fig. 2) [18]. 4.2. Improvement mechanism of mechanical properties For all the tested samples, the average microhardness of the microwave sintered samples is higher than the corresponding value for the reference 6061 Al alloy and the hardness increases with the increasing amount of (Ti,W)C. This can be explained by: (a) the law of mixtures, which shown in the following equation [19]:
Hc ¼ Hm fm þ Hr fr
ð3Þ
where Hc, Hm, and Hr are the hardness of the composite, matrix and reinforcement, respectively, fm and fr are the fraction of matrix and reinforcement, respectively. The microhardness is increased since the (Ti,W)C is inherently harder than 6061 Al alloy, (b) grain refinement strengthening, the addition of (Ti,W)C with smaller grains and narrower grain distribution is responsible for the increase of hardness of Al/(Ti,W)C composites, (c) dispersion strengthening, the fairly well distribution of the reinforcements improves the ability of the soft matrix to resist deformation. Sintering temperature is another parameter which influences the microhardness of the composite. In the Al/(Ti,W)C composites, raising the temperature from 500 °C to 520 °C, the microhardness changed little due to the decrease of the densification. However, further increase of the temperature enhances the microhardness due to reduced porosity and the enhanced interfacial bonding. In the pure 6061 Al alloy samples, the microhardness decreased with the temperature rising, this negative effect is caused by the grain coarsening. The superior values of compressive yield strength and compressive strength were obtained in present study compared with the previous studies on aluminum matrix composite (Figs. 10 and 11) [19,26]. This is possibly attributed to the suitable combination of 6061Al alloy powders and (Ti,W)C particulates and the optimal microwave sintering parameters [12]. The results of the ambient temperature compressive tests on the composite samples revealed that an increase in the amount of (Ti,W)C leads to an increase in the compressive yield strength and compressive strength of 6061Al alloy. This can primarily be attributed as follows: (a) Hall–Petch law, the strength of the polycrystalline increases with grain refinement which can be expressed by the following equation:
ry ¼ r0 þ K y d1=2
ð4Þ
where ry is the yield strength, r0 and Ky are constants for given material, and d is the average grain diameter. According to Eq. (4), smaller grain size will lead to more grain boundaries which can act as strong obstacles to the dislocation motion, thus resulting to
R.R. Zheng et al. / Journal of Alloys and Compounds 590 (2014) 168–175
higher yield stress and compressive strength. Moreover, the uniformity of microstructure of the grain size is also contributed to higher strength, (b) the stable interface structure may ensure the strengthening effect of the second phase by loading transfer on the Al matrix [27,28], (c) the presence of high dislocation density in the matrix due to the large difference in thermal expansion coefficients values between the matrix and the reinforcements [12,15,29], and (d) dispersion hardening. A decrease of elongation with the increasing of the reinforcement can be found from Fig. 10(b), this can be attributed to the fact that the more reinforcements are added, the shorter of the interparticulate spacing will be. In this way, the movement of dislocations will be harder because there are more barriers, and then the dislocations pile up happens, causing the decrease in elongation [26]. The trend of the effect of temperature on compressive yield strength and compressive strength is similar to that of the microhardness. The compressive yield strength increases with the sintering temperature increases. Increasing the temperature causes a decrease in miro-porosities and this is equal to lower stress concentration regions and therefore, has a beneficial effect on mechanical properties. Furthermore, good matrix-reinforcement interfacial integrity, higher interfacial bond strength could effectively transfer load from the Al matrix to the second phase, thus strengthening the mechanical property. No obvious change on the compressive yield strength and compressive strength when the temperature increased from 500 °C to 520 °C due to grain coarsening and the decline in density. The effect of the temperature on elongation is not obvious owing to the small temperature ranges. But raised the temperature to 560 °C leads to the rise in elongation, this can be due to the bonding strength between the (Ti,W)C and 6061Al become higher in such circumstances. 5. Conclusions 1. Low porosity of 6061 Al alloy and Al/(Ti,W)C composites with good macro-scale morphology were successfully fabricated using microwave sintering method at diverse temperatures and amounts of (Ti,W)C because of the higher microwave absorptivity of both 6061 Al alloy and Al/(Ti,W)C. 2. The grain growth trend has been significantly restrained by introducing (Ti,W)C phase while increasing temperature facilitates grain growth. (Ti,W)C particulates are uniformly distributed and predominantly distributed along grain boundaries. Sound grain boundaries and phase interfaces were generated by microwave sintering the low sintering temperature. 3. The average microhardness, compressive yield strength as well as compression strength were enhanced with an increase of both (Ti,W)C phase amount and the temperature (ranging from 520 °C to 560 °C). The microhardness increased to a maximum value of 96HV, the yield stress and compressive strength increased to a maximum value of 236 MPa and 473 MPa, respectively. 4. Microwave heating mainly induced diffusion between Al alloy and/or (Ti,W)C particles and generated well-bonded interfaces. Meanwhile, a small amount of liquid Al phase appeared around the (Ti,W)C due to the better absorbing ability of (Ti,W)C to microwave and heat concentration due to angular initial powder. The appearance of liquid might promoted interfacial diffusion.
Acknowledgement The author would like to thank the financial support provided by the National Basic Research Program of China ‘‘973’’ (No.
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