2024Al nanocomposite with a quasi-network architecture

Accepted Manuscript Enhanced strength and ductility in ZrB2/2024Al nanocomposite with a quasi-network architecture Z.Y. Zhang, G. Chen, S.L. Zhang, Y.T. Zhao, R. Yang, M.P. Liu PII:

S0925-8388(18)34346-9

DOI:

https://doi.org/10.1016/j.jallcom.2018.11.214

Reference:

JALCOM 48440

To appear in:

Journal of Alloys and Compounds

Received Date: 6 June 2018 Revised Date:

14 November 2018

Accepted Date: 16 November 2018

Please cite this article as: Z.Y. Zhang, G. Chen, S.L. Zhang, Y.T. Zhao, R. Yang, M.P. Liu, Enhanced strength and ductility in ZrB2/2024Al nanocomposite with a quasi-network architecture, Journal of Alloys and Compounds (2018), doi: https://doi.org/10.1016/j.jallcom.2018.11.214. 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.

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Graphical abstract

ACCEPTED MANUSCRIPT Enhanced strength and ductility in ZrB2/2024Al nanocomposite with a quasi-network architecture Z.Y. Zhanga, G. Chena, S.L. Zhanga, Y.T. Zhaoa∗, R. Yangb, M.P. Liua a School of Material Science and Engineering, Jiangsu University, Zhenjiang 212013, China

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b School of Transportation, Zhenjiang College, Zhenjiang 212028, China Abstract: In order to achieve high strength and high ductility, 2vol.% ZrB2/2024Al nanocomposite with a quasi-network reinforcement distribution was designed and prepared by a combination of direct melt reaction and T6 heat treatment. The microstructural evolution

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shows that the in situ ZrB2 nanoparticles were self-assembled into clusters that were pushed towards α-Al boundaries and formed a quasi-network architecture; T6 heat treatment

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effectively eliminated interference of the coexisting eutectics. As expected, the unique heterogeneous structure results in a remarkable increase in the tensile modulus, strength and ductility of as-fabricated composite as compared with the matrix alloy. The superior strengthening effect can be attributed to the novel quasi-network architecture, matrix grain refinement and excellent load-bearing capacity, while the ductility enhancement can be

clusters.

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ascribed to the matrix grain refinement and high damage tolerance of ZrB2 nanoparticle

Keywords: Aluminum matrix composites; In situ synthesis; Quasi-network microstructure;

1. Introduction

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Mechanical properties

Discontinuously reinforced metal matrix composites, especially particulate reinforced

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aluminum matrix composites (AMCs), are of great interest due to their low cost and high specific strength as well as modulus [1, 2]. Over the past decades, to the construction of composite structures, the traditional practice is to pursue a homogenous dispersed reinforcements within the metallic matrix. However, despite some encouraging results, AMCs with a uniform microstructure usually exhibit a “trade-off” state between strength and ductility, which largely limits their engineering applications [3-5]. To achieve balanced mechanical properties, regulating the spatial distribution of reinforcing particles in the ∗

Corresponding author. Tel./fax.: +86 511 88797783 E-mail address: [email protected] 1

ACCEPTED MANUSCRIPT multiple length scale has emerged as a novel strategy [6]. A series of composites with unique architectures, including isolated cluster [7, 8], layered structures [9, 10] and network framework [11, 12], etc., were successfully designed and prepared by various methods. Among them, Huang et al. [13-15] reported a tailored network microstructure in TiBw/Ti in

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situ composites that were fabricated by reaction hot pressing. They concluded that the continuous TiBw boundary phase effectively strengthened the composites while the relatively softer

Ti

matrix

contributed

to

the

ductility.

A similar

network

skeleton

in

(Al3Zrp+Al2O3np)/2024Al composite were found in the work of Kaveendran et al. [16], in

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which the hybrid particles were discontinuously arranged around the 2024Al particles. The results showed that simultaneous increase in tensile strength and ductility, as compared to

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those of the uniformly distributed counterparts, can be ascribed to various toughening mechanisms offered by nanosized particles and quasi-network architecture. To authors' knowledge, direct melt reaction (DMR) during casting is of advantages in the fabrication of complex compositional alloys and parts. In the current work, the network-like architectures in AMCs by DMR with T6 heat treatment was employed to establish a

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quasi-network architecture for in situ ZrB2/2024Al nanocomposite. The scenario originates from one fact that nanoparticles have a great tendency to aggregate to the interfaces of primary α-Al dendrites and T6 heat treatment can eliminate interference of second phase particles or solute segregation. Therefore, the heterogeneous microstructural evolution in

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ZrB2/2024Al nanocomposite was studied and the corresponding strengthening and ductility-enhancing mechanisms were discussed in detail.

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2. Experimental procedure Based

on

the

reaction

equation

[17-19]:

3K2ZrF6+6KBF4+10Al

=

3ZrB2+9KAlF4+K3AlF6, in situ 2vol.% ZrB2/2024Al nanocomposite was synthesized by DMR in a laboratory scale. The experimental process was as follows. Commercial 2024Al alloy ingots were heated upto 850~870 °C in a graphite crucible using a resistance furnace, and then a mixture salts, i.e., potassium hexafluorozirconate (K2ZrF6, purity >98.5%) and potassium tetrafluoroborate (KBF4, purity >98.5%), were added into the melt. After sufficient stirring for 30 min, the reacted slags were removed from the crucible and some appropriate Mg was replenished. The composite slurry of 750 °C was degassed with 0.5 wt.% 2

ACCEPTED MANUSCRIPT hexachloroethane (C2Cl6) and finally poured into a preheated steel mold at about 200 °C. For a comparative purpose, the as-cast 2024Al ingots were adopted as the reference. The chemical compositions of both materials determined by inductively coupled plasma emission spectrometer (XSeries II ICP-MS) are listed in Table 1. The stoichiometric calculation from

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the mass of Zr element (Table 1) revealed that the actual content of ZrB2 reinforcement (1.94 vol.%) was very close to the nominal value.

In order to optimize microstructure and properties, T6 heat treatment was further carried out on these castings, including solution treatment (505 °C × 2 h), 20 °C water quenching and

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artificial aging (180 °C × 8 h). The microhardness (HV) of both materials was measured by using a Vickers hardness tester with a load of 30 kg. Tensile specimens with a gauge section

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of 10×2×1 mm3 were machined and tested with a constant crosshead speed of 1 mm/min at room temperature. At least three samples were tested for each material. X-ray diffractometer (XRD, Rigaku D/MAX 2500) was performed to identify the phases of the specimens. Optical microscope (OM, Zeiss Axio Observer Z1m), scanning electron microscopy (SEM, JSM-7800F) equipped with energy dispersive X-ray spectroscopy (EDS) and transmission

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electron microscopy (TEM, FEI Tecnai-G2) were used to perform microstructural examinations. The average grain size was assessed by the mean linear intercept method described in ASTM standard E 112-88.

Material

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Table 1 Chemical compositions of the as-cast alloy and nanocomposite

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Si

Element (wt.%)

Fe

Cu

Mn

Mg

Zn

Zr

B

Al

2024Al

0.50

0.50

4.51

0.50

1.40

0.25

-

-

Bal.

ZrB2/2024Al

0.45

0.43

4.46

0.42

1.48

0.14

2.68

0.71

Bal.

3. Results

3.1 Microstructural evolution Fig. 1 displays the XRD pattern of the as-cast composite, where the indexed peaks correspond to α-Al, ZrB2, Al2Cu and Al2CuMg phases. The occurrence of ZrB2 peaks indicates that the in situ reinforcement was successful synthesized in the matrix by DRM method. By the way, the presence of secondary phases (Al2Cu and Al2CuMg) is primarily 3

ACCEPTED MANUSCRIPT owing to the non-equilibrium solidification process. 800

∗ Al2CuMg 400

200

○ 0 20

○

∗

○

∗

30

○

40

∗

○

50

2θ (degree)

60

70

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Intensity (cps)

600

ZrB2/2024Al

Al ZrB2 ○ Al2Cu

80

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Fig. 1 XRD patterns of the as-cast ZrB2/2024Al nanocomposite.

Fig. 2 shows the typical as-cast microstructures of the unreinforced and reinforced

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samples. The 2024Al alloy exhibits a two-phase dendrite as being composed of α-Al and eutectics (Fig. 2a). Combined with XRD result and inserted SEM image, the coarse α-Al grains with uneven sizes are mainly delineated by Al2Cu+Al2CuMg phases. As in situ ZrB2 nanoparticles were introduced into the matrix alloy (Fig. 2b), the grain morphology transforms from cellular to equiaxed dendritic and their average size decreases from 220±10

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µm to 160±8 µm, by approximately 27 % reduction. It is also seen from Fig. 2b that almost of ZrB2 nanoparticles prefer to aggregate to the dendritic interfaces and assemble into clusters (~30 µm), thus leading to significant grain refinement by the growth restriction mechanism

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[20, 21]. TEM and SEM were used to explore the cluster nature. As shown in Fig. 2c, these clusters mainly consist of a large number of nano-sized particles embedded in the Al matrix.

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Two common shapes, i.e., rectangle and nearly hexagon, are observed and the corresponding electron diffraction patterns confirm these nanodispersoids to be ZrB2. Figs. 2d and e give the SEM-EDS element mappings of the clusters located at boundaries. Note that the distribution of ZrB2 can be discerned from Zr map, given the EDS detector fail to probe the B light elements. Interestingly, Mg and Cu elements are found to coexist with copious ZrB2 nanoparticles, which is in accordance with the previous reports of TiB2/Al-Cu composites [22, 23]. Such behavior may be understood from the solidification sequence of the melt [22-25]. Specifically, the primary α-Al began to nucleate and growth once the melt temperature fell below the liquidus, during which ZrB2 nanoparticles in the melt were pushed by the growing 4

ACCEPTED MANUSCRIPT dendrites. As the temperature continuously went down, the non-equilibrium eutectic reactions, e.g., L→α-Al+Al2Cu (about 548 °C [26]) occurred near the last solidified regions. Therefore, in situ ZrB2 nanoparticles always tangle up with Al2Cu+Al2CuMg phases in the present composite and T6 heat treatment is required to eliminate solute segregation or dissolve

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eutectics.

Fig. 2 Typical grain structures of (a) the as-cast 2024Al alloy and (b) the in situ ZrB2/2024Al

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nanocomposites; (c-e) TEM and SEM-EDS element mappings revealing the cluster constitution.

Figs. 3a and b represent the SEM images of the T6 state unreinforced and reinforced samples, respectively. For the 2024Al alloy (Fig. 3a), the white phases left correspond to the high-melting point compounds. EDS confirms that these remnants are the complex intermetallics containing Al, Cu, Fe, Mn and Si elements (e.g., AlFeSi [27], AlCuFeMn [28]). While in the ZrB2/2024Al nanocomposites (Fig. 3b), a quasi-network architecture surrounded by ZrB2 nanoparticle clusters and a few intermetallics is clearly visible. Although a large amount of ZrB2 nanoparticles (average size of 160 nm) concentrate on interdendritic and grain 5

ACCEPTED MANUSCRIPT boundaries, they are actually uniform and free of Cu as well as Mg elements if seen within the clusters. These results indicate that Al2Cu+Al2CuMg phases, even inside the clusters, almost dissolved into the matrix after solution treatment, which is similar as reported of Geng et al. [24]. Figs. 3c and d give the morphology of aging precipitates in both materials. The

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needle-shaped precipitates with length of ~120 nm and width of ~5 nm are observed lying on {100}Al planes, which can be identified as S’, one of S (Al2CuMg) metastable phase, by the [100]Al SAED pattern [29, 30]. In contrast, there are no significant difference in the size and density of the S’ phase between the 2024Al alloy and in situ ZrB2/2024Al nanocomposites. It

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seems that the presence of quasi-network structure has little influence on the aging

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precipitation process.

Fig. 3 Microstructure of (a, c) the 2024Al-T6 alloy and (b, d) the ZrB2/2024Al-T6 nanocomposites. 3.2 Mechanical properties Fig. 4 exhibits the tensile engineering stress-strain curves of the T6 treated nanocomposite and the matrix alloy. The related property parameters derived from uniaxial tensile tests are also listed in Table 2. As expected, the elastic modulus (E), 0.2% yield 6

ACCEPTED MANUSCRIPT strength (σ0.2), work-hardening rate ( Θ=dσture / dεtrue ), ultimate tensile strength (σb) and elongation to failure (δ) of the ZrB2 nanoparticle reinforced specimens are remarkably improved. For example, the quasi-network structured composite displays a 16.0 and 29.8% increase in σ0.2 and σb as compared with the matrix alloy. Meanwhile, its δ reaches 7.0%,

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improving approximate 119% over the matrix (3.2%). On the other hand, the ZrB2/2024Al nanocomposite exhibits higher hardness than the unreinforced matrix, as listed in Table 2. This can be primarily attributed to: (i) an efficient grain refinement; (ii) high constraint to matrix deformation as a result of the presence of quasi-network architecture; (iii) strong

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load-bearing capacity between the matrix and ZrB2 clusters. 400

ZrB2/2024Al

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300

2024Al

250

10000

200

Work-hardening rate (MPa)

Engineering Stress (MPa)

350

150 100 50

2024Al ZrB2/2024Al

8000

6000

4000

2000

0

0

2

4

6

8

10

True strain

0 2

4

6

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0

8

10

12

14

Engineering Strain (%)

Fig. 4 Tensile stress-strain curves of the 2024Al-T6 alloy and the ZrB2/2024Al-T6 nanocomposites.

Material

E (GPa)

σ0.2 (MPa)

σb (MPa)

δ (%)

Hardness (HV)

69.8±0.3

200±6

285±7

3.2±0.4

142±11

76.4±0.4

232±10

370±8

7.0±0.5

118±14

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2024Al-T6

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Table 2 Mechanical properties of the matrix alloy and composite measured from various tests.

ZrB2/2024Al-T6

Fig. 5 depicts the tensile fractography of the matrix alloy and nanocomposites taken from different planes. The vertical fracture surface of the 2024Al alloy is dominated by cleavage facets, tear ridges and cavities (Fig. 5a). Some crushing particles, i.e., Al(CuFeMnSi) are usually detected around the cavities. The typical brittle transgranular fracture explains well why low elongation was attained in the matrix alloy. For the ZrB2/2024Al nanocomposite (Fig. 5b), amounts of dimples with shallow depth and small size are evident at the grain boundaries. Observation at a high magnification reveals that ZrB2 nanoparticles are distributed at the 7

ACCEPTED MANUSCRIPT bottom of the dimples. This means that toughening clusters underwent microvoid nucleation, growth and coalescence. Furthermore, it can be seen from Figs. 5c and d that crack propagates along the quasi-network boundaries. The interrupted crack branching or microcrack near the main path indicate that ZrB2 clusters offer a large resistance to crack propagation and thus

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absorb more energy.

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Fig. 5 Fracture morphologies of (a) the 2024Al-T6 alloy and (b-d) the ZrB2/2024Al-T6 nanocomposites taken from different planes.

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4. Discussion

4.1 Strengthening mechanism The strengthening mechanism associated with this heterogeneous composite can be mainly attributed to the high contiguity of the cluster containing ZrB2 nanoparticles. Fig. 6a shows a schematic illustration of the quasi-network architecture for the ZrB2/2024Al-T6 nanocomposite. Considering that the vast majority of ZrB2 nanoparticle clusters locate at the network boundary and ignoring the presence of a few intermetallics, this region with variable thicknesses can be treated as one new phase (named phase I), whilst the matrix occupied by S’ precipitates can be seen as another phase (termed phase II). Based on the H-S bound theory 8

ACCEPTED MANUSCRIPT [31, 32], such network skeleton is close to the upper bound structure, i.e., a strong and toughening phase-I covers a softer phase-II. In the present case, the phase I plays multiple important roles in strength enhancement, including (i) effective obstacle against dislocation glide, similar to “grain boundary strengthening” [33], (ii) the reduced grain size due to the

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growth restriction, (iii) strong load-bearing capacity achieved by the excellent bonding between the two phases. It is worthy mentioned that a certain proportion of clusters retained at the interdendritic boundaries can further increase the strengthening effect by load transferring. All the above factors are responsible for the higher hardness, elastic modulus and tensile

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strength of the composite.

Fig. 6 Schematic illustrations of (a) a quasi-network architecture and (b) crack propagation path in in situ ZrB2/2024Al nanocomposites.

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4.2 Ductility-enhancing mechanism

The ductility enhancement is a result of an integrated effect of grain refinement and

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phase I. Firstly, fine equiaxed dendrites make the load scatter to more α-Al grains and thus reduces the nucleating flaws during plastic deformation. In addition, more tortuous boundaries induced by fine equiaxed dendrite can impede the mobile dislocations, which is beneficial to the work-hardening rate and increase strain-bearing ability to some extent [34]. Secondly, in term of the ductility “intergranular failure”, it is reasonable to infer that the phase I effectively toughens the composite by improving damage tolerance. The analyses of Figs. 5b-d indicate that fracture failure in the composite is a progressive process that involves crack nucleation and propagation. Microcrack prefers to initiate at local regions under a higher tensile stress, e.g., larger cluster or intermetallics, which is confirmed by the observations 9

ACCEPTED MANUSCRIPT marked by circles in Fig. 5d. Subsequently, the stress/strain gradient compels these microcracks to coalesce along the phase I, where ZrB2 nanoparticles effectively alter the path of microcrack propagation and result in crack bridging, branching and deflection (Fig. 6b). The process can slow the crack propagation rate and inevitably consume more fracture energy,

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giving rise to the improvement of fracture toughness. 5. Conclusions

In this paper, a simple method following direct melt reaction and T6 heat treatment was proposed to fabricate ZrB2/2024Al nanocomposite with a quasi-network reinforcement

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distribution. Microstructural observations indicate that in situ nanoparticle clusters were always tangled up with Al2Cu+Al2CuMg eutectics during solidification and subsequently

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formed into a ZrB2 quasi-network architecture after T6 heat treatment. Compared with that of the matrix alloy, the as-received composite exhibits superior tensile strength mainly due to the unique heterogeneous structure, matrix grain refinement and excellent load-bearing capacity. Moreover, the simultaneously achieved high ductility can be attributed to matrix grain refinement and high damage tolerance of ZrB2 nanoparticle clusters.

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Acknowledgements

This work was supported by Natural Science Foundation of China (U1664254, U1710124, 51801074) and Key Research and Development Program of Jiangsu Province (BE2015148). References

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ACCEPTED MANUSCRIPT A quasi-network architecture is established in ZrB2/2024Al nanocomposite




The preparation process combines direct melt reaction method and T6 heat treatment




Such heterogeneous microstructure improves strength and ductility over the matrix




Enhanced ductility is mainly attributed to high damage tolerance of ZrB2 clusters

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