AA6061 Composites Prepared by Spark Plasma Sintering

Accepted Manuscript Title: Hot Deformation Behaviour of SiC/AA6061 Composites Prepared by Spark Plasma Sintering Author: Xiaopu Li, Chongyu Liu, Kun Luo, Mingzhen Ma, Riping Liu PII: DOI: Reference:

S1005-0302(15)00221-2 http://dx.doi.org/doi: 10.1016/j.jmst.2015.12.006 JMST 615

To appear in:

Journal of Materials Science & Technology

Received date: Revised date: Accepted date:

22-9-2015 19-11-2015 28-11-2015

Please cite this article as: Xiaopu Li, Chongyu Liu, Kun Luo, Mingzhen Ma, Riping Liu, Hot Deformation Behaviour of SiC/AA6061 Composites Prepared by Spark Plasma Sintering, Journal of Materials Science & Technology (2015), http://dx.doi.org/doi: 10.1016/j.jmst.2015.12.006. 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 proof before it is published in its final 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.

Hot Deformation Behaviour of SiC/AA6061 Composites Prepared by Spark Plasma Sintering Xiaopu Li 1, Chongyu Liu 2,3,*, Kun Luo 2,3, Mingzhen Ma 1, Riping Liu 1 1

State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, China 2 Key Laboratory of New Processing Technology for Nonferrous Metal & Materials, Ministry of Education, Guilin University of Technology, Guilin 541004, China 3 Guangxi Key Laboratory of Universities for Clean Metallurgy and Comprehensive Utilization of Non-ferrous Metal Resources, Guilin University of Technology, Guilin 541004, China [Manuscript received 22 September 2015; received in revised form 19 November 2015; accepted 28 November 2015] * Corresponding author. Ph.D.; Tel: +86 335 8057047; Fax: +86 335 8074545. E-mail address: [email protected] (Chongyu Liu); [email protected] (Xiaopu Li).

In this study, SiC/AA6061 composites with different SiC volume fractions (5%, 10%, 15% and 20%) were fabricated by spark plasma sintering. The deformation behaviour of the composites was studied by uniaxial compression test at temperatures from 573 K to 773 K and strain rates between 0.001 s−1 to 1 s−1. Results indicate that the flow stress of SiC/AA6061 composites increases with the increase of SiC volume fraction, with the decrease of deformation temperature and the decrease of strain rate. The main deformation mechanism of the composites is dynamic recrystallisation (DRX), and the DRX degree depends on the processing parameters of deformation. Higher SiC volume fraction, higher deformation temperature and lower deformation strain rate promote the occurrence of DRX. The strain rate sensitivity and deformation activation energy of SiC/AA6061 composites are calculated. Results show that with the increase in deformation temperature and the decrease in SiC volume fraction, the strain rate sensitivity of the composites increases. From 573 K to 773 K, the average deformation activation energy of 5vol.%SiC/AA6061, 10vol.%SiC/AA6061, 15vol.%SiC/AA6061 and 20vol.%SiC/AA6061 are 207.91, 230.88, 237.7 and 249.87 kJ/mol, respectively. The optimum hot working zone of the SiC/AA6061 composites is in the temperature range of 723 K to 773 K at strain rates from 0.1 s−1 to 1 s−1. Key words: Spark plasma sintering; Hot deformation; Al alloy matrix composites;

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1. Introduction Aluminum matrix composites (AMCs) have attracted considerable attention because of their light weight, high strength, high stiffness and high wear resistance[1,2]. In particular, particlereinforced AMCs have strong application potential in aerospace and automotive industry because they not only have good mechanical and wear properties, but are also economically viable[3,4]. Several methods are used to fabricate particle-reinforced AMCs, including casting[5,6], pressureless infiltration[7], spray forming[8] and powder metallurgy (PM)[9]. In these methods, PM has some notable advantages. For example, PM can produce AMCs with uniform distribution of reinforcements because the fabrication process is not affected by the wettability of the reinforcements and Al. In addition, PM can prevent undesirable reaction because of low processing temperature and can accurately design the volume fraction of reinforcement particles[10]. As a rapid sintering technology, spark plasma sintering (SPS) has received considerable attention over the past few years. SPS shows faster heating rate, lower sintering temperature and shorter sintering time than conventional PM techniques. Furthermore, the discharge process during SPS can clean the surface and enhance the surface activation energy of powder. Thus, SPS can maintain the nano- and submicron- structures of powder-based materials after consolidation, obtain strong adhesion between the reinforcement and the matrix, and fabricate fully dense materials[11]. To date, SPS has been used to fabricate several AMCs, including SiC/Al[12,13], CNT/Al[14], Al2O3/Al[15], diamond/Al[16], ZrB2/Al[17], metallic glass/Al[18], B4C/Al [19]and intermetallic composites/Al[20]. Plastic deformation through forging, extrusion, or rolling is usually employed to improved mechanical properties of metals[21]. However, such AMCs are characterized by high work hardening rate, poor forming ability and low elongation at room temperature, and controlling the deformation workability of AMCs remains a challenge[22]. Although valuable investigations concentrating on the deformation of AMCs have been performed, limited works can be found dealing with deformation behaviour of particle-reinforced AMCs which were prepared by SPS. In this study, SiC/AA6061 composites with different contents of SiC were prepared through milling and SPS. Uniaxial compression tests were performed at temperatures ranging from 573 K to 773 K and strain rates from 0.001 s−1 to 1 s−1. The hot deformation parameters, including activation energy and strain rate sensitivity at different temperatures, as well as the flow localization parameter, were investigated. Furthermore, deformation process parameters of SiC/AA6061 composites were optimized. 2. Experimental 2.1. Sample preparation AA6061 alloy (Al–0.8Mg, 0.4Si, 0.1Mn, 0.1Cu, 0.2Zn, 0.06Cr, 0.2Ti, 0.7Fe) particles which are 5 μm to 60 μm in size and SiC particles which are 100 nm to 2 μm in size were used as raw materials. Fig. 1 shows an SEM image of the AA6061 and SiC particles used in this study.

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Matrix powders containing various SiC volume fractions (5%, 10%, 15% and 20%) were mixed with a planetary mill in a WC vial. Milling was performed at 300 rpm for 2 h by using WC balls. The weight ratio of balls to powder was fixed to 5:1, and the vial was filled with argon during milling. SPS was performed in an SPS apparatus (SPS-3.20MK-IV). The compaction pressure was 50 MPa, and the heating rate was 323 K/min. The sintering temperature and time were set at 833 K and 3 min, respectively. After SPS, SiC/AA6061 composites of 30 mm in diameter and 15 mm in height were obtained. 2.2. Thermomechanical simulation Compression tests on the cylindrical samples with the diameter of 10 mm and the length of 15 mm were performed at Gleeble 3500-type thermomechanical simulation test machine. Five temperatures, namely, 573, 623, 673, 723, and 773 K, and four strain rates, namely, 1, 0.1, 0.01 and 0.001s−1, were used in the deformation process. Tantalum chip and lubricant was laid between the punch and the specimen to minimize the friction. Samples were heated to the test temperature with the heating rate of 283 K/s. After heating, the samples were held at the test temperature for 1 min, and then hot compressed up to a true strain of 0.7. After hot deformation, the samples were cooled down to room temperature by immersing them into water. 2.3. Characterisation The microstructures of the samples were observed via scanning electron microscopy (SEM) and transmission electron microscopy (TEM). TEM specimens were prepared via ion milling. 3. Results and discussion Fig. 2 shows SEM images of the 10vol.%SiC/AA6061 powder mixture after 2 h of highenergy ball milling. The low-magnification image shows that the AA6061 powder was slightly finer than the unmilled AA6061 powder (Fig. 2(a)). Fig. 2(b) shows the mixtures with uniform distributions of nano-size SiC powder on the surface of submicron-size AA6061 powder. Fig. 3 shows back-scattered SEM (BSE) images of the as-prepared (SPS) 10vol.%SiC/AA6061 composites before and after hot deformation. Grey and black regions corresponded to Al and SiC, respectively. Fig. 3(a) shows the as-prepared 10vol.%SiC/AA6061 appeared as heterogeneous materials consisting of SiC agglomerates separating Al grains. Several microvoids appeared in the Al matrix. Numerous SiC particles were drawn out during sample polishing; hence, some cavities were retained on the sample surface. Fig. 3(b) shows BSE images of the as-deformed 10vol.%SiC/AA6061. Al particles were elongated perpendicular to the deformation direction, and the size of Al particles was refined obviously. The density of voids was decreased, which indicates the relative density of 10vol.%SiC/AA6061 composite increased after hot deformation.

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Fig. 4 shows the true stress-true strain curves of 5vol.%SiC/AA6061 composite at different temperatures with strain rates 1 s−1. The flow behaviour of 5vol.%SiC/AA6061 composite was significantly affected by deformation temperature as shown by the curves of the composites. When the deformation temperature is 573 K, with increasing strain, the flow stress rapidly increases until the strain is 0.1. Then, it slowly decreases. Fig. 5(a) shows TEM image of as-prepared 5vol.%SiC/AA6061 composite. Some dislocations were generated at Al/SiC interface due to the large difference in thermal expansion between Al matrix and SiC. Fig. 5(b) shows 5vol.%SiC/AA6061 composite after 0.1 strain at 573 K. more dislocations were accumulated around SiC particles. At the initial deformation stage, the dislocation rapidly increases and accumulates. Then, strength of the composite increases[23]. Fig. 5(c) shows TEM image of 5vol.%SiC/AA6061 composite after 0.5 strain at 573 K. The well-defined grain boundaries were presented in this sample, which means dynamic recrystallisation (DRX) has occurred, and the size of some recrystallized grain is as fine as 200 nm, as shown in Fig. 5(c). The density of dislocation in the Al matrix near the matrixreinforcement interfaces of 5vol.%SiC/AA6061 composite after 0.5 strain is obviously lower than that of 5vol.%SiC/AA6061 composite after 0.1 strain. Thus, dynamic recovery (DRV) also occurred in the 5vol.%SiC/AA6061 composite during hot compression[24]. When compression strain is higher than 0.1, the softening effect by DRX and DRV overrides the hardening effect by work hardening. Thus, the flow stress of 5vol.%SiC/AA6061 composite decreases. The flow stress value nearly did not change after the strain of 0.65. This phenomenon means that the softening and hardening effects in 5vol.%SiC/AA6061 composite reach to dynamic equilibrium[25]. Fig. 5(d) shows TEM image of 5vol.%SiC/AA6061 composite after 0.7 strain at 573 K. The recrystallized grain was coarsened and the density of dislocation was further decreased in the composite. The flow stress and the strain value which correspond to peak stress (peak stress–strain value) of the SiC/AA6061 composite both decrease with the increase of deformation temperature. For example, when the deformation temperature is 773 K, the flow stress of 5vol.%SiC/AA6061 composite reach peak value (49 MPa) at the strain of 0.03. At a constant deformation strain rate, the higher deformation temperature provides more energy to promote the annihilation and rearrangement of dislocations and grain rotation. Therefore, it promotes the occurrence of DRX and DRV[26]. Fig. 6 shows the true stress–true strain curves of 5vol.%SiC/AA6061 composite with different strain rates at 573 K. The flow stress and peak stress–strain value both increase with the increase of strain rates. The dislocations have no enough time to annihilation and rearrangement during high strain rates deformation[27]. Thus, low strain rate promotes the occurrence of DRX and DRV at a constant deformation temperature. Fig. 7 shows the flow true stress–true strain curves of the SiC/AA6061 composites with different contents of SiC. The flow stress of SiC/AA6061 composites increases with the contents

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of SiC, whereas the peak stress–strain value shows an opposite trend. During compression deformation, the hard SiC particles impede the motion of the soft Al matrix, thereby increasing the dislocation density in the Al matrix near the matrix–reinforcement interfaces (Fig. 5(a)). Thus, the flow stress of SiC/AA6061 composites increases with the contents of SiC. The SiC/AA6061 composites with high SiC volume fraction show lower peak stress–strain value. This phenomenon indicated that the SiC particles can promote the occurrence of DRX[22]. More dislocations which were introduced by SiC provide more energy to promote DRX. Thus, more recrystallized grains emerged near the SiC particles, as shown in Fig. 5(b). Strain rate sensitivity parameter at a given strain can be computed using the following equation: ln

(1)

ln

where m is the strain rate sensitivity of the flow stress at a given strain of and ε and are the applied strain rate and the flow stress, respectively. Table 1 displays the stress values of 0.3 strains ( 0.3) of SiC/AA6061 composite with different deformation strain rates. Linear fitting was performed to ln 0.3‒ln ε , as displayed in Fig. 8. At the next stage, the m for 0.3 strain (m0.3) was defined by calculating the slope of the line in ln 0.3−ln ε as indicated in Fig. 9. The m of SiC/AA6061 composites ranges from 0.083 to 0.014 when the contents of SiC particles range from 5% to 20% and deformation temperature ranges from 573 K to 773 K. The m of the composites increases with the increase of deformation temperature and decrease of SiC volume fraction. The value of the strain rate sensitivity, m, is an important parameter to characterize the deformation capacity of composite materials, and higher m value indicates better deformation capacity. When the m is negative, stress concentration often leads to the plastic instability of composites[28]. The effect of temperature and strain rate on material deformation behaviour is expressed by Zener–Hollomon parameter, with Z as given by Eq. (2)[29]:  Q  Z  ε ex p    RT 

(2)

The correlation between Z and the flow stress of metal is given by Eq. (3): Z = A[sinh( ασ )]

n

(3)

where σ is flow stress, ε is strain rate, Q is deformation activation energy, R is gas constant (8.314 J mol‒1 K‒1), T is temperature, α and β are stress adjustment factors, A is the material constant, and n and n´ are the stress exponents, α=β/ n  .

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Table 2 shows the peak stress value of SiC/AA6061 composites. The deformation activation energy of SiC/AA6061 composites is computed based on Table 2, Eq. (2) and Eq. (3). The values of n  and β were obtained from the slope of the lnσp–ln ε and σp–ln ε plots, respectively, by linear fit method, based on Eq. (4) and Eq. (5). These two plots for the SiC/AA6061 composites are shown in Fig. 10 and Fig. 11, respectively. n =

β =

 [ln ε ]  [ln σ ]

 [ln ε ] σ

(4)

(5)

Eq. (6) was obtained from Eq. (2) and Eq. (3): Q   n ε = A[sin h ( α σ )] ex p     RT 

(6)

Taking logarithm and differential on both sides of Eq. (6) gives:     ln[sinh( α σ )]   ln ε Q = R     (1 / T )   ln[sinh( α σ )]  T  ε

(7)

The Q value was obtained from the slope of the linear fitting of ln[sinh(ασp)]−ln ε and ln[sinh(ασp)]−1/T plot, respectively, based on Eq. (7). In addition, these two plots for the SiC/AA6061 composites are shown in Fig. 12 and Fig. 13, respectively. The average Q values of 5vol.%SiC/AA6061, 10vol.%SiC/AA6061, 15vol.%SiC/AA6061 and 20vol.%SiC/AA6061 composites are 207.91, 230.88, 237.7 and 249.87 kJ/mol, respectively, when deformed at 573 K to 773 K. When the deformation activation energy is greater than that of lattice self-diffusion, the DRX is the main controlling deformation mechanism[30]. In the present study, the Q values are greater than the self-diffusion activation energy of 144 kJ/mol for the Al matrix[25]. Therefore, It is can be deduced that the deformation mechanism for the composites should be dominated by DRX. It is worth mentioning that the deformation activation energy of the 5vol.%SiC/AA6061 composite in this study is higher than that of solid solution treatment 20vol.%B4C/AA6061 composite[25]. The AMCs prepared by SPS were characterized by fine matrix grains due to the faster heating rate, lower sintering temperature and shorter sintering time during SPS process. The fine grains AMCs often show higher deformation activation energy due to the boundary strengthening[31]. Higher deformation temperature and lower deformation strain rate promote the occurrence of DRX. The reduction of flow stress which is caused by DRX is beneficial to the deformation

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process of composites[22]. The results showed that the temperature of 573 K with strain rate of 1 s−1 is sufficient for DRX. However, considering the cooling encounter through contact with the processing tools, the optimum hot processing range of the SiC/AA6061 composites should be at the temperature range of 723 K to 773 K with strain rate of 0.1 s−1 to 1 s−1. 4. Conclusions (1) The SiC/AA6061 composites prepared by SPS were heterogeneous materials consisting of SiC agglomerates that separate the Al grains. (2) The main deformation mechanism of the SiC/AA6061 composites is DRX when the deformation was performed at 573 K to 773 K and 0.001 s−1 to 1 s−1. The flow stress of SiC/AA6061 composites increases with the increase of SiC volume fraction, the decrease of deformation temperature and the decrease of strain rate. The peak stress–strain value of SiC/AA6061 composites decreases with the increase of SiC volume fraction, the decrease of deformation temperature and the decrease of strain rate. (3) The m of SiC/AA6061 composites ranges from 0.083 to 0.014 when the content of SiC particles ranges from 5% to 20%, and deformation temperature ranges from 573 K to 773 K. The m of the composites increases with the increase in deformation temperature and decrease of SiC volume fraction. (4) The average Q values of 5vol.%SiC/AA6061, 10vol.%SiC/AA6061, 15vol.%SiC/AA6061 and 20vol.%SiC/AA6061 composites are 207.91, 230.88, 237.7 and 249.87 kJ/mol, respectively, when deformed at 573 K to 773 K. Acknowledgements This work was funded by the National Basic Research Program of China (No. 2013CB733000), Guangxi Natural Science Foundation (No. 2015GXNSFBA139238), and the Guangxi ‘Bagui’ Teams for Innovation and Research.

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Figure and table captions Fig. 1 SEM image of the as-received (a) AA6061 and (b) SiC particles. Fig. 2 SEM image of 10vol.%SiC/AA6061 powers after 2 h of high energy ball milling: (a) low magnification and (b) high magnification. Fig. 3 SEM images of (a) as-prepared 10vol.%SiC/AA6061 and (b) as-deformed 10vol.%SiC/AA6061 (deformation performed at 673 K and 1 s−1 strain rates, true strain is 0.7). Fig. 4 Flow true stress–true strain curves of the 5vol.%SiC/AA6061 composite at different temperatures with strain rates 1 s−1. Fig. 5 TEM images of (a) as-prepared 5vol.%SiC/AA6061 and as-deformed 5vol.%SiC/AA6061 after strain of (b) 0.1, (c) 0.5, and (d) 0.7 (deformation performed at 573 K and 1 s−1 strain rates). Fig. 6 Flow true stress‒true strain curves of the 5vol.%SiC/AA6061 composite with different strain rates at 573 K. Fig. 7 Flow true stress–true strain curves of the SiC/AA6061 composites with different contents of SiC (deformation performed at 573 K and 1 s−1 strain rates). Fig. 8 Linear fitting plots of ln

0.3‒ln

ε in: (a) 5vol.%SiC/AA6061; (b) 10vol.%SiC/AA6061, (c)

15vol.%SiC/AA6061 and (d) 20vol.%SiC/AA6061. Fig. 9 m0.3 of SiC/AA6061 composites. Fig. 10 Linear fitting plots of ln(σp)−ln( ε ) in: (a) 5vol.%SiC/AA6061, (b) 10vol.%SiC/AA6061, (c) 15vol.%SiC/AA6061 and (d) 20vol.%SiC/AA6061. Fig. 11 Linear fitting plots of σp−ln ε in: (a) 5vol.%SiC/AA6061, (b) 10vol.%SiC/AA6061, (c) 15vol.%SiC/AA6061 and (d) 20vol.%SiC/AA6061. Fig. 12 Linear fitting plots of ln[sinh(ασp)]−ln ε in: (a) 5vol.%SiC/AA6061, (b) 10vol.%SiC/AA6061, (c) 15vol.%SiC/AA6061 and (d) 20vol.%SiC/AA6061. Fig. 13 Linear fitting plots of 1000/T−ln[sinh(ασp)] in: (a) 5vol.%SiC/AA6061, (b) 10vol.%SiC/AA6061, (c) 15vol.%SiC/AA6061 and (d) 20vol.%SiC/AA6061.

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Table list: Table 1 Stress values of 0.3 strain of SiC/AA6061 composites

SiC fraction

5%

10%

15%

20%

0.3 (MPa)

T (°C)

at different strain rates (s‒1)

1 s‒1

0.1 s‒1

0.01 s‒1

0.001 s‒1

300

124.27

96.11

84.54

63.86

350

87.09

71.1

58.2

44.9

400

69.4

52.6

44.05

34.4

450

55.23

38.36

32.09

26.9

500

41.38

32.38

26.53

19.87

300

133.91

110.83

88.11

72.58

350

102.33

81.18

64.88

54.71

400

80.15

65.11

51.42

41.67

450

58.5

47.29

38.14

29.54

500

43.97

32.49

27.13

21.26

300

142.89

123.03

97.23

79.87

350

112.89

88.66

73.19

60.77

400

88.18

67.67

56.74

46.52

450

63.29

48.89

40.51

32.55

500

46.91

36.1

28.54

23.57

300

168.43

140.12

111.3

95.9

350

123.25

97.63

80.93

67.55

400

99.11

74.1

60.02

54.04

450

68.25

49.57

41.47

36.15

500

49.03

40.39

30.12

26.09

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Table 2 Peak stress value of SiC/AA6061 composites SiC fraction

5%

10%

15%

20%

σp (MPa) at different strain rates T (°C)

1 s‒1

0.1 s‒1

0.01 s‒1

0.001 s‒1

300

125.95

100.25

88.22

67.92

350

89.31

77.19

64.49

52.01

400

75.88

59.44

50.79

40.19

450

60.13

45.17

40.32

30.79

500

48.11

38.84

30.75

24.15

300

153.08

127.69

94.38

79.2

350

112.12

96.85

71.86

63.95

400

85.39

71.43

55.88

52.02

450

64.5

56.39

47.52

35.68

500

52.36

40.3

32.0

26.56

300

154.21

130.98

104.15

88.5

350

122.22

101.12

83.7

71.95

400

98.17

78.42

66.23

55.79

450

71.73

57.38

47.92

39.58

500

54.41

43.99

35.78

30.03

300

175.94

143.96

114.47

98.64

350

128.97

102.22

85.63

72.53

400

105.02

80.21

67.17

58.56

450

73.1

58.36

49.46

42.38

500

55.51

45.93

37.01

30.8

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Fig. 8

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Fig. 9

Fig. 10

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