The design, microstructure and tensile properties of B4C particulate reinforced 6061Al neutron absorber composites

Journal of Alloys and Compounds 632 (2015) 23–29

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The design, microstructure and tensile properties of B4C particulate reinforced 6061Al neutron absorber composites H.S. Chen a, W.X. Wang a,⇑, Y.L. Li a, P. Zhang a, H.H. Nie b, Q.C. Wu a a b

School of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, China Shanxi Coal-Mining Administrators College, Taiyuan 030024, China

a r t i c l e

i n f o

Article history: Received 24 October 2014 Received in revised form 8 January 2015 Accepted 11 January 2015 Available online 17 January 2015 Keywords: Monte Carlo B4C/6061Al Vacuum hot pressing Hot rolling

a b s t r a c t Based on the Monte Carlo Particle transport program MCNP, a novel boron carbide particulate reinforced 6061Al composite for neutron shielding (B4C/6061Al NACs) with high strength and low density was designed. The NACs with four volume fractions (10%, 20%, 30% and 40%) were successfully fabricated by vacuum hot pressing followed by hot rolling (VPHR) in atmospheric environments. The calculation results indicated that the neutron transmission ratio decreased with the increasing of B4C content and the thickness of plates. B4C particle is uniformly distributed in the matrix, exhibiting the good bonding in interface. The phases of neutron absorbers were mainly B4C and Al, and a spot of AlB2 and Al3BC. The grain of the matrix was refined and the dislocation was formed around the particles. With increasing the B4C content, the particles gathered, breakage appeared, and the tensile strength of composite first increased and then decreased. The failure mode of B4C/6061Al NACs included: the interfacial debonding and the cleavage fracture of particles. Ó 2015 Published by Elsevier B.V.

1. Introduction Boron carbide (B4C) has captured the attention of the ceramic materials community to a large extent. B4C possesses high melting point, super-hardness and the good absorbing neutron property, so it has become a major focus in nuclear shielding field [1–3]. The boron includes 10B and 11B isotopes, while the nuclear reaction only happened between the 10B and neutron, and the reaction products form as lithium and helium [4,5]. The average natural abundance of 10B in boron is 19.9% and the thermal neutron absorption cross-section is 3837b [5]. Thus, B4C is a perfect neutron shielding material. The neutron shielding materials mainly include boron-containing stainless steel [6,7], cadmium [8], boron-containing-PE [9], Al/ B, Fe/Al/B [10,11] and glass fiber/B4C epoxy resin composite [12]. However, B4C/Al composites, a high performance neutron shielding materials [13], have been developed recently. A number of manufacturing techniques, such as casting [14], infiltration [15], hot extrusion [16] and in-situ compositing [17] have been used to produce B4C/Al neutron absorbers. Barbara et al. [18] investigated the production of 7.5%B4C/Al composites adopting solid–liquid ⇑ Corresponding author. Tel./fax: +86 351 6010076. E-mail address: [email protected] (W.X. Wang). http://dx.doi.org/10.1016/j.jallcom.2015.01.048 0925-8388/Ó 2015 Published by Elsevier B.V.

composite casting. Abenojar et al. [19] produced 10% carbide–aluminum composites using mechanical alloying. Mohanty et al. [20] studied the fabrication process and mechanical properties of 0– 25% B4C/Al composites. Morteza [21] analyzed the strengthening mechanisms in particulate 5 and 10 vol.% B4C/1100Al alloy composites produced by repeated roll bonding process. Kalaiselvan et al. [22] applied modified stir casting technique to production of AA6061-B4C composites. The microstructure and mechanical properties of the composites are analyzed. Dai et al. [23] calculated the neutron transmission coefficient of 3–9 cm-thick B4C/Al composite with 5–15% B4C content in air, water, 200–1400 ppm H3BO3 solution, irradiated by 0.5–20 MeV neutrons and 235U thermal neutron fission source. Thus, the numerical analysis study on the relationship among the neutron transmission coefficient, B4C volume fraction and thickness will help the B4C/6061Al NACs design and application in different field. In this paper, based on the Monte-Carlo simulation, the relationships among the B4C content, the plate thickness and the neutron transmission ratio were obtained. The B4C/6061Al NACs with four volume fractions were successfully fabricated by VSHR method. Compared with other methods, it is a feasible method to fabricate B4C/6061Al NACs with high B4C content, good mechanical properties and uniform distribution.

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0.20

2.1. Material design

0.18

The Monte Carlo Particle transport program was invented at The Los Alamos National Laboratories, USA in the 1940s. MCNP is one of Monte Carlo simulations, which is to track a large number of particles lose operation steps and statistics, record the particle transport information in the process of moving, and given related physical parameters to calculate the neutron, photon and neutron–photon coupling transport problems. It can be applied to calculate the performance of the shielding material. Based on the Monte Carlo Particle transport program MCNP, the isotropic point source with energy of 0.025 MeV was used and the distance between the point source and input plane was 13 cm in vacuum. The specimens with dimensions of 5 cm  5 cm were used in the simulation, and the specimens were supposed to contain no pore, crack, compression and inflation, and the B4C particles symmetrically distributed in the 6061Al matrix. The number of seeds implanted in material was 8  107, and physical model was presented in Fig. 1. The neutron transmission ratio (y = I0/I) of material can be calculated by the following equation [5]: Rx

I0 =I ¼ e

ð1Þ

where I0 is the neutron transmission intensity, I is the incident intensity of the neutron, R is the macroscopic transmission cross section, x is the thickness of the neutron absorbing material (mm). Fig. 2 presents the simulated relationship among the neutron transmission ratio, B4C content and thickness. For the neutron absorbers material with different B4C contents, the relationship between the neutron transmission ratio and thickness was determined as follows: y(10%) = e0.98763x, y(20%) = e1.96735x, y(30%) = e2.91003x, y(40%) = e3.81738x. It can be found that the neutron transmission ratio decreased with increasing B4C particles content and the thickness of the plates. 2.2. Material preparation In the present study, water atomized 6061Al alloy particles with diameter of 14 lm as the matrix, and the chemical composition was shown in Table 1, supplied from Beijing Xingrongyuan technology, Co., Ltd, China. The B4C powders after preoxidation treatment at 420 °C for 1 h were selected as the reinforced particulars with an average size of 23 lm and the chemical composition was presented in Table 2, supplied from Dalian Boentan technology, Co., Ltd, China. The purpose of pre-oxidation behavior of B4C powders in air was formed a layer of B2O3 oxide film on the surface of particles. The B2O3 (melting point: 450 °C) oxide film reacts with Al2O3 and Al at the interface (Fig. 6), it is beneficial to improve the interface bonding strength. According to the MCNP simulation results, the mixed powders with 10%, 20%, 30% and 40% B4C content were ball milled using a QM-3B high energy ball milling machine. Ball milling was carried out in air at a rotating speed of 1500 r/min for 40 min with ball-to-powder weight ratio of 10:1. ZrO2 balls, with diameters of 10 mm and 6 mm, were used as grinding media. Fig. 3 shows BSE image of the milled powders of 30% B4C mixed with 6061Al. The specimens were fabricated by vacuum hot pressing followed by hot rolling in atmospheric environments. The mixed powers were put into the high quality graphite mold, and then the vacuum hot pressing was performed at an elevated temperature of 640 °C under 120 MPa for about 2.5 h, both heating rate and cooling rate were kept at 10 °C/min. The dimensions of vacuum hot pressing composites were 90 mm  90 mm  20–22 mm. Vacuum hot pressing composites were cross rolled in atmospheric environments, applying the predetermined allowable strain per pass and rolling at temperatures of 500 °C, followed by an annealing treatment at 500 °C for 40 min after per pass. The thickness of plate was 4 mm after rolling. At last, the different B4C content plates were solution treated at 530 °C for 2 h with subsequent water quenching and over aged at 185 °C for 2 h to stabilize the precipitates. The fabricated specimens were examined with an optical microscope (OM, CMM-20) and a scanning electron microscope (SEM, JSM-6700F) equipped with energy dispersive spectrometer (EDS). The phases were identified by X-ray diffrac-

I0

Didtance（L）

10% B4C/6061Al 20% B4C/6061Al 30% B4C/6061Al 40% B4C/6061Al

0.16 0.14 0.12 0.10 0.08 0.06 0.04 0.02 0.00

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5

Thickness (mm) Fig. 2. The relationship between the neutron transmission ratio and thickness with different B4C contents.

Table 1 Chemical composition of 6061Al alloy (wt.%). Si

Fe

Cu

Mn

Mg

Cr

Zn

Ti

Al

0.6

0.7

0.25

0.15

0.8

0.1

0.25

0.15

Rest

Table 2 Chemical composition of B4C (wt.%). B

C

Ca

Fe

Si

F

Cl

80.0

18.1

0.3

1.0

0.5

0.025

0.075

Fig. 3. BSE images of 30% B4C/6061Al milled powders.

tion (XRD, D/MAX2400, Cu Ka). A transmission electron microscopy (TEM, JEM-2100F) was used to identify the phases. The tensile tests were conducted in DNS200 tension machine at room temperature (RT) with an initial strain of 1.0  103 s1. In our study, three specimens were tested and the average value was employed for the yield strength of the alloy.

The exit surface Z axis

I

Neutron transmission ratio ( I/I0 )

2. Experimental procedures

Y axis O

Source

Detector X axis

The incident plane

Neutron absorber

Fig. 1. The sketch of physical model used in the simulation.

3. Results and discussion 3.1. Characteristics of B4C/6061Al NACs Fig. 4 illustrates the OM micrographs of B4C/6061Al NACs with different contents of B4C. It can be observed from Fig. 4 that B4C

H.S. Chen et al. / Journal of Alloys and Compounds 632 (2015) 23–29

particles distributed homogeneously in the matrix. Small B4C particles distributed among big ones, with aluminum particles surrounding them. Mixed radius of B4C particles are helpful to improve the density of the B4C/6061Al neutron absorbers, and most of B4C particles remained intact and a clear profile. Rolling results in improving particle distribution and less agglomeration, improving particles/matrix wetting and decreasing voids%. The amounts of the B4C particles breakage increase with increasing content of B4C due to the compressive forces. Fig. 5 shows SEM images of the 30% B4C/6061Al NACs. The EDS analysis taken from the composites showed that the composite consists of 6061Al matrix (Fig. 5b) and B4C particles (Fig. 5c). The crushing of big B4C particle was caused by load transfer during the rolling process, as shown in Fig. 5a. Moreover, neither pore nor crack was observed at the interface as shown in Fig. 5d. This indicates proper bonding of B4C particles with the 6061Al matrix. The thickness of the interface diffusion layer is in the range of 200–250 nm. On the one hand, the oxygen element is implanted onto the 6061Al particles surface during the fabrication of B4C/ 6061Al NACs and a thin layer of oxide film is formed on surfaces of matrix particles. Mg belongs to the higher activity elements and also can facilitate bonding, which breaks the oxide film. According to the Gibbs theory, magnesium enriched on the surface of aluminum and reducing interface tension is beneficial to the diffusion and wetting. On the other hand, a layer of B2O3 film is formed on surfaces of B4C after pre-oxidized. The B2O3 film is helpful for reduce the reaction temperature. So the combining strength of the interface can be improved. The EDS linear scanning showed the gradient distribution of aluminum, boron, carbon and magnesium exist at the interface (Fig. 5f). Fig. 6 depicts the X-ray diffraction patterns of neutron absorber materials. It indicates that all the composites are consisted of Al and B4C phases together with small amounts of AlB2 and Al3BC phases. The products of AlB2 and Al3BC were formation by the following reaction of B4C + Al = AlB2 + Al3BC at the 627–868 °C [24]. In general, the Al4C3 phase formed due to the reaction between B4C and Al. Arslan et al. [25] have reported that B4C–Al composites

25

are composed of various combination of Al3BC, AlB2, AlB12C2 and Al4C3 phases. In addition, the research suggests that interfacial reactions at the interface of B4C/6061Al system creates MgO, B and Al2O3 etc. Variation of Gibbs free energy versus temperature and the chemical reactions at the interface are shown in Fig. 7. However, we cannot find Al4C3, MgO, B and Al2O3 diffraction peaks in the XRD pattern is ascribed to the quantity is less. The schematic of two stages of interface between B4C and Al are shown in Fig. 5e. Stage I: the initial stage of vacuum hot pressing. Stage II: the stage of after rolling. Fig. 8 shows the bright fields of TEM of different kinds of microstructures in the B4C/6061Al neutron absorbers. Nano-grains were observed near a B4C particle, which was caused by the misfit strain (as shown in square box in Fig. 8a) at the interface between matrix and particles during deformation [26,27]. This misfit strain results in the enforced strain gradient in the matrix near a particle. Therefore, a region with high dislocation density and large orientation gradient is formed near a particle, which is called particle deformation zones (PDZs) [26]. Additionally, the rolling leads to more uniformity in particles’ distribution and hence more particles act as nucleation sites and aluminum grains solidify on the particles [28]. This has a final effect on the refinement of the matrix grains. 3.2. Tensile properties Fig. 9 shows yield strength (rYS, 0.2% proof stress), ultimate tensile strength (rUTS, the ultimate tensile strength) and elongation to fracture of B4C/6061Al NACs. It can be seen that the tensile properties of the 6061Al with B4C particles is significantly improved while the elongation to fracture exhibits obvious declined compared with the 6061Al alloy. The tensile strength first increased and then decreased on increasing the B4C particles content. The values of UTS lie within the range of 132–256 MPa depending on Vf of B4C. Kalaiselvan (2014) reported that UTS of 12%B4C/6061Al is 215 MPa, since he prepared the composite by casting. For a successful development of B4C/6061Al NACs, it is essential to understand the relevant strengthening mechanisms of these

Fig. 4. OM micrographs of the B4C/6061Al with different contents of B4C: (a) 10%, (b) 20%, (c) 30% and (d) 40%.

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H.S. Chen et al. / Journal of Alloys and Compounds 632 (2015) 23–29

Stage ĉ

Stage Ċ

(e) 6061Al B4C B2O3 Al2O3

B

B Al4C3 AlB2 Al3BC MgO

Fig. 5. SEM micrographs of B4C/6061Al NACs: (a) under lower magnification, (b and c) point scanning, (d) under higher magnification, (e) schematics of interface and (f) line scanning.

composites. In general, failure procedures are simply divided into three stages: elastic stage, yield stage and fracture. However, the increase in the yield strength of B4C/6061Al NACs can be attributed to the following reasons: (1) the mismatch of coefficient of thermal expansion between B4C particles and 6061Al matrix leads to geometrically necessary dislocation generation around B4C particles during preparation; (2) The existence of B4C particles in NACs can transfer load from matrix to the hard reinforcements; (3) Orowan strengthening mechanism exists in the NACs. Dislocation strengthening can play an important role. The preparation of neutron absorber including two stages: vacuum hot pressing and hot rolling, the different coefficient of thermal expansion between B4C (4.4  106 K1) and 6061Al (23.6  106 K1) lead to the dislocation density increase around the B4C particles in the 6061Al matrix of neutron absorbers, but the material

strengthen and begin to brittle simultaneously. The elongation rate was only 2.5% of 30% B4C/6061Al neutron absorbers. Dislocation strengthening can be calculated by [29,30]:

rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 12f DaDT bd

Dr1 ¼ KGb

ð2Þ

where K = 1.25 is a constant, Da = 19.2  106 K1 is value of difference in the coefficient of thermal expansion between B4C and 6061Al, DT is the difference between the processing (hot rolling:773 K) and room temperature (293 K), the value DT = 480 K was calculated, G is shear modulus and can be calculated by equation: G = E/2(1 + vm), G = 52.23 GPa (E and vm is elastic modulus and Poisson’s ratio of matrix, respectively), b = 0.286 is Burgers vector, d = 23 lm and f is the average diameter and volume fraction of

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H.S. Chen et al. / Journal of Alloys and Compounds 632 (2015) 23–29

Strength/MPa

200

6061Al 10% B4 C/6061Al 20% B4 C/6061Al 30% B4 C/6061Al 40% B4 C/6061Al

8 7 6 5

150

4 100

3 2

50 0

1 UTS

YS

Elongation

0

Fig. 9. Tensile properties of 6061Al with and without B4C particles. Fig. 6. X-ray diffraction patterns of B4C/6061Al NACs.

4Al+3O2=2Al2O 3 (1)

ΔG/(kJ/mol)

ΔG/(kJ/mol)

-1400 -1450 -1500 -1550 -1600 0

-2350 -2400 -2450 -2500 -2550 -2600 -2650 -2700 -2750

100 200 300 400 500 600 700

-460 3Mg+Al2 O3=3MgO+2Al (2)

3B4C+4Al=Al4 C3+12B (3)

-470

Δ G/(kJ/mol)

-1350

-480 -490 -500 -510 -520

0

T/䉝

100 200 300 400 500 600 700

0

T/䉝

100 200 300 400 500 600 700

T/䉝

10

0 -5 -10 -15 -20

0

100 200 300 400 500 600 700

T/䉝

-112 -114 -116 -118 -120 -122 -124 -126 -128

-82

B2O3+3Mg=3MgO+2B (5)

B2 O 3+2Al=Al2O3+2B (6)

-84

ΔG/(kJ/mol)

B4C+4O2=2B2O3+CO2 (4)

ΔG/(kJ/mol)

ΔG/(kJ/mol)

5

-86 -88 -90 -92 -94

0

100 200 300 400 500 600 700

0

T/䉝 Fig. 7. The temperature dependence of Standard Gibbs energies for different reactions.

Fig. 8. TEM images of B4C/6061Al NACs: (a) near B4C particle and (b) the matrix.

100 200 300 400 500 600 700

T/䉝

Elongation/%

250

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H.S. Chen et al. / Journal of Alloys and Compounds 632 (2015) 23–29

where rm = 103 MPa is the yield stress of the 6061Al matrix. The yield stress of neutron absorbers is improved by load transfer is strongly depend on the volume fraction of B4C particles. The Dr2 for 10%, 20%, 30% and 40% neutron absorbers are 5.15 MPa, 10.30 MPa, 15.45 MPa and 20.60 MPa, respectively. Particles shearing stress plays an important role in the grain refinement of 6061Al matrix during the processing, the grain size of matrix is up to nanometer level. Meanwhile, dislocation pile-up and tangling around B4C particles were also observed after deformation, the microstructure as shown in Fig. 4. The B4C particles breakage under the rolling force and becomes smaller, the B4C particles became more homogeneous and it has a positive effect on strength of neutron absorber, the strengthening mechanism is equal to Orowan strengthening, it can be calculated by [32,33]:

Dr3 ¼ Fig. 10. SEM micrograph of 40% B4C/6061Al NACs.

B4C particles. According to the calculation, the Dr1 for 10%, 20%, 30% and 40% neutron absorbers are 24.21 MPa, 34.23 MPa, 41.93 MPa and 48.42 MPa, respectively. The B4C particles have improved the load transfer ability for the neutron absorbers. For the load transfer, an increase in the yield stress caused can be calculated by [31]:

Dr2 ¼ 0:5rm f

ð3Þ

0:13Gb d ln k 2b

ð4Þ

where k is inter-particle spacing, the inter-particle spacing has also been decrease on increasing the B4C particle content. According to calculation, the inter-particle spacing for 10%, 20%, 30% and 40% neutron absorbers are 11 lm, 8 lm, 4.5 lm, and 2 lm. The values of Dr3 are 1.86 MPa, 2.55 MPa, 4.54 MPa and 10.21 MPa, respectively. By the above analysis, the ultimate strength of the neutron absorber increases with increasing particle content and decreasing of particle size. But the strength decreases if the contents of B4C exceeds 30%, due to the agglomeration of B4C, pore and the fracture

Table 3 Comparison of the yield strength of theoretic and experiment values (MPa). B4C contents (%)

Dr 1

Dr 2

Dr 3

Theoretic value

Experiment value

10 20 30 40

24.21 34.23 41.83 48.42

5.15 10.30 15.45 20.60

1.86 2.55 4.54 10.21

31.22 47.08 61.52 79.23

41 ± 1 56 ± 1 81 ± 1 60 ± 1

Fig. 11. The tensile fracture surfaces of the 30% B4C/6061Al composite: (a) lower magnification of fracture surface; (b) the B4C particles fracture; (c) the tensile dimples in 6061Al; (d) particle/matrix interface debonding.

H.S. Chen et al. / Journal of Alloys and Compounds 632 (2015) 23–29

of B4C are increases within the materials, as shown in Fig. 10. Comparison of the yield strength of theoretic and experiment values are presented in Table 3. Fig. 11 shows that the tensile fracture surfaces of 30% B4C/ 6061Al NACs. It can be seen by SEM that the B4C particles and the salient of 6061Al matrix (Fig. 11a). The salient of 6061Al matrix similar to dimple due to the toughness of 6061Al. The clear contour exist at the matrix due to the particles pullout (Fig. 11c), cracks appearing at the particle/matrix interface (Fig. 11d). The fresh section of particle was observed (Fig. 11b), the particles breakage was formed during hot rolling, the initiation of cracks at this place. Therefore, the failure mode of B4C/6061Al neutron absorber including: the interfacial debonding and the cleavage fracture of particle.

References

4. Conclusions

[13]

Simulation illustrates the neutron transmission ratio decreased on increasing the B4C particles content and the thickness of the plates. The macroscopic transmission cross section for 10%, 20%, 30% and 40% are 0.987, 1.967, 2.910 and 3.817, respectively. B4C/ 6061Al neutron absorbers were successfully produced by vacuum hot pressing followed by hot rolling. Rolling results in improving particle distribution and less agglomeration, improving particles/ matrix wetting and decreasing voids%. The particle distributions were fairly uniform in composites. The phases of B4C/6061Al NACs include B4C, Al, AlB2 and Al3BC. Compared with the 6061Al the yield strength and ultimate tensile strength of the B4C/6061Al neutron absorbers are enhanced while ductility to fracture is slightly decreased. The strengthening mechanism of B4C/6061Al NACs can be attributed to the following factors: (a) grain refinement and dislocation strengthening mechanism, (b) load transfer effect and (c) Orowan strengthening. Acknowledgment This work was supported by ‘‘The Key Science and Technology Program of Shanxi Province, China’’ (Grant No. 20130321024).

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