Enhancing strength-ductility synergy of carbon nanotube/7055Al composite via a texture design by hot-rolling

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Abstract

The limited ductility of carbon nanotubes reinforced Al matrix (CNT/Al) composites hinders their engineering applications, which is an urgent problem to be solved. Herein, texture optimization through hot-rolling was employed to improve the strength-ductility of the CNT/7055Al composites. Microstructural examinations indicate that the composite subjected to extrusion (CNT/7055Al-E) had a bimodal structure with the coarse and ultrafine grains. The composite subjected to extrusion followed by hot-rolling (CNT/7055Al-ER) exhibited a uniform ultrafine grain structure. No further structure damage or aggregation was detected for the CNTs after hot-rolling. The texture analysis demonstrates that the CNT/7055Al-E had a <111> fiber texture, while the CNT/7055Al-ER had a plate texture including {011}<111>, {113}<332> and {215}<342> components with higher texture intensity. Compared with the CNT/7055Al-E, the CNT/7055Al-ER increased twofold in elongation with a slight decrease in strength, which was attributed to the favorable grain orientations, the increased proportion of low angle grain boundaries, as well as the fine and densely distributed precipitates. Further, the mechanical property anisotropy of the composite was weakened after hot-rolling due to the elimination of coarse grain bands.

Introduction

Aluminium matrix composites (AMCs) are widely used as lightweight structural materials in engineering [[1], [2], [3], [4], [5]]. With the rapid development of aerospace and transportation fields, the demands for AMCs with high performance are increasingly growing. Owing to the high strength, high modulus and excellent machinability [[6], [7], [8], [9], [10], [11]], carbon nanotubes (CNTs) reinforced aluminium matrix (CNT/Al) composites are considered as one of the most promising structural AMCs.

However, strength increase is usually accompanied by ductility loss for CNT/Al composites [12]. Choi et al. [13] fabricated a 3 vol% CNT/2024Al composite through ball-milling process, achieving a yield strength up to 780 MPa but a poor elongation of only 2%. Liu et al. [14] increased the yield strength of CNT/2009Al composites by 84% with 4.5 vol% CNTs addition through friction stir processing, but the elongation was decreased from 15% to 5%. He et al. [15] fabricated a 3.5 wt% CNT/Al composite by in situ chemical vapour deposition, obtaining a strength increase 180% over the alloy counterpart but showing a low elongation of 3.6%. Such strength-ductility trade-off was a significant bottleneck for the engineering application of CNT/Al composites.

The remarkable ductility loss in CNT/Al composites is mainly from the ultrafine grain (UFG) structure and the strong pinning effect of CNTs. First, the preparation process of the composites usually led to refined grains. Choi et al. [13] reported that the grain sizes of CNT/2024Al composites reduced to 100 nm after 12 h milling. These UFGs were difficult to accommodate dislocations [16]. In addition, Xu et al. [17] reported that the mean space between two closest CNTs was only hundreds of nanometers, even if only a very small fraction (0.1 vol%) of CNTs were uniformly dispersed in Al matrix. These densely distributed CNTs would significantly restrict the deformation of Al matrix [18].

One strategy to overcome this dilemma is construction of a heterogeneous structure [[19], [20], [21]]. Liu et al. [21] constructed a bimodal structure containing the soft CNT-free coarse grain (CG) bands and the hard CNT-rich UFG zones in a CNT/2009Al composite to enhance the strength-ductility. It was found that the CG bands effectively relaxed the stress concentration resulting from CNTs, suppressed the strain localization in the UFG zones and retarded the crack propagation. However, the strength-ductility could not be improved in the direction perpendicular to the CG bands, and the CG bands would cause severe anisotropy [22].

Other research efforts were focused on optimizing fabrication and dispersion processes to improve the ductility [17,[23], [24], [25]]. These research work included improving CNT-Al interface bonding using elevated sintering temperatures [25], reducing CNT damage using shift-speed ball milling [23] and obtaining relatively coarse grains using elemental alloying or short-time ball-milling [17,26]. Although these strategies could enhance the strength-ductility of CNT/Al composites, the grains of CNT/Al composites were difficult to further coarsen, and the severe interface reaction of Al-CNT resulting from increased sintering temperatures was adverse to corrosion resistance.

In general, dislocation slip is the main contributor to ductility at room temperature for aluminium based materials. Crystallographic texture, that is grain orientation distribution, has significant influence on the dislocation start and slip, and inevitably affects the ductility of aluminium based materials [27]. Some researchers have obtained enhanced ductility in alloys or composites by changing crystallographic texture. For instance, Agnew et al. [28] increased the ductility of AZ31B Mg alloys more than twofold by changing the deformation texture into the orientations conducive to basal slip. Jiang et al. [29] pointed out that grains with favorable orientations might account for the high ductility of the B4C/Al nanocomposites. However, so far, there was no investigation on improving the ductility of CNT/Al composites directly through the texture control.

Hot-rolling, a simple and common deformation process for AMCs, can generate various texture components, such as Copper {112}<111>, Brass {011}<211>, S {123}<634>, and Goss {011}<100> [[30], [31], [32]]. Among them, Brass and S components with high Schmid factors of 0.42 are favorable for the deformation of fcc metals. Therefore, it is worthwhile to toughen the CNT/Al composites via a texture optimization by hot-rolling process. To the best of our knowledge, there is also a lack of investigations on texture evolution of the CNT/Al composites during hot-rolling.

Once texture was designed to modify the strength-ductility of the CNT/Al composites, the composites would inevitably produce the mechanical property anisotropy. Previous studies on the CNT/Al composites mainly reported the properties along the deformation direction, only a few studies reported the anisotropy of the CNT/Al composites. Liu et al. [33] investigated the effect of CNT orientations on the properties of CNT/Al composites. They found that with the increase of off-axis angle from 0° to 90°, the properties of the composites decreased. However, they did not consider the effect of matrix texture on the anisotropy. It has been reported that textures and reinforcement orientation in AMCs have an influence on the anisotropy [[34], [35], [36]]. Therefore, it is necessary to analyze the mechanical property anisotropy in the CNT/Al composites in depth considering both of the matrix textures and the reinforcement orientations.

In this study, the CNT/7055Al composite fabricated by high-energy ball milling and powder metallurgy route was first subjected to a hot-extrusion process, and then subjected to a hot-rolling process to form plate textures. The grain size, texture component, CNT distribution and tensile properties of the hot-extruded as well as subsequent hot-rolled CNT/7055Al composites were respectively investigated. The aim of this study is to (a) achieve enhanced strength-ductility of the composites by texture design, (b) clarify the toughening mechanisms of the composites, and (c) explore the mechanical property anisotropy of the CNT/Al composites.

Section snippets

Experimental

The as-received CNTs provided by Cnano Technology Ltd. (Jiangsu, China) had an outer diameter of 10–15 nm and a length of 2–5 μm (Fig. 1 (a)). The 7055Al alloy (Al-8.1 wt% Zn-2.2 wt% Mg-2.2 wt% Cu) powders (Fig. 1 (b)), with an average diameter of 10 μm, were ball-milled with 1 vol% CNTs using an attritor with a rotation rate of 400 rpm for 6 h with a ball-to-powder ratio of 15:1 in a purified argon atmosphere. 1.6 wt% stearic acid (CH3(CH2)16COOH) was added as the process control agent to

Surface morphologies and microstructure

Fig. 2(b) and (c) show the surface morphologies of the CNT/7055Al-E after hot forging and the CNT/7055Al-ER, respectively. The surfaces of both the composites were in good conditions. No macro holes or cracks were observed, indicating that CNT/7055Al composites had excellent hot-forming ability.

Fig. 3 shows the grain structures of the CNT/7055Al-E and CNT/7055Al-ER. The TEM images show that the CNT/7055Al-E had a bimodal structure containing coarse grain (CG) bands ~1.2 μm in width, and UFG

Decreased strength in the CNT/7055Al-ER

The yield strength difference between the CNT/7055Al-E and CNT/7055Al-ER (46 MPa) could be explained by the following equation [45]:τc=σscosϕcosδwhere cosϕcosδ is the Schmid factor; τc is critical shear stress for activating dislocation slip in a single crystal, which is a constant independent of applied force and dependent on crystal structure; σs is the yield stress, δ is the angle between applied load direction and slip direction, and ϕ is the angle between applied load direction and the

Conclusions

The CNT/7055Al composites were fabricated by high-energy ball milling and powder metallurgy. The mechanical behaviors, texture components of the composites subjected to hot-extrusion process (CNT/7055Al-E) and subsequent hot-rolling process (CNT/7055Al-ER) were investigated. The conclusions are as follows:

  • 1)

    The CNT/7055Al-E displayed a bimodal structure containing coarse grain (CG) bands ~1.2 μm in width and ultrafine grains (UFGs) with a grain size of ~330 nm. After hot-rolling, the CG bands

CRediT authorship contribution statement

S. Bi: (first author) carried out the data collection, data analysis and, Writing - original draft, All authors contribute substantially to the paper. Z.Y. Liu: All authors contribute substantially to the paper, revised the manuscript and provided the fund. B.L. Xiao: designed the experiment, All authors contribute substantially to the paper. Y.N. Zan: helped to analyze the microstructure, All authors contribute substantially to the paper. Q.Z. Wang: helped to analyze the microstructure, All

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors gratefully acknowledge the support of (a) the National Natural Science Foundation of China (No. 51871215, No. 51931009, No. 51871214), (b) Key Research Program of Frontier Sciences, CAS (No. QYZDJ-SSW-JSC015), (c) National Key R&D Program of China (No. 2017YFB0703104), (d) the Youth Innovation Promotion Association CAS (2020197).

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