Deformation behavior and strengthening mechanisms in a CNT-reinforced bimodal-grained aluminum matrix nanocomposite
Introduction
Ultrafine-grained metal matrix nanocomposites (MMNCs) containing nano-reinforcements exhibit high strengthening effects [[1], [2], [3]]. Carbon nanotube (CNT) is one of the most promising nano-reinforcement candidates in MMNCs owing to its high strength, elastic modulus and nanometer size along with its excellent electrical and thermal conductivities [4,5]. As such, CNT-reinforced aluminum matrix nanocomposites (AMNCs) have received a great deal of attention [6,7]. The strengthening in such nanocomposites is primarily attributed to load transfer, dislocation strengthening, and Orowan strengthening [[7], [8], [9]]. The main strengthening mechanism in CNT-reinforced nanocomposites in the case of high aspect ratio of CNTs is reported to be load transfer [10,11]. However, CNTs are normally shortened due to the shearing effect during high-energy ball milling (HEBM), thereby decreasing the length or aspect ratio of CNTs and increasing the potential significance of Orowan strengthening based on their interaction with dislocations [12,13].
The ultrafine-grained MMNCs often exhibit high strength but low ductility stemming from the insufficient ductility in the matrix, which impedes their structural applications [14]. To solve the strength-ductility trade-off dilemma, tailoring/engineering the microstructure with a distribution of grain sizes via proper processing techniques was proposed as an effective strategy. For example, heterogeneous [[15], [16], [17]], gradient [18], or bimodal [[19], [20], [21]] grain distributions have been observed to demonstrate a synergetic increase in strength and ductility. One of the most effective methods of fabricating such a grain distribution is to introduce coarse grains (CGs) in the ultrafine-grains (UFGs) to achieve a balanced mechanical properties [17,22,23]. Under these circumstances geometrically necessary dislocations (GNDs) would be generated near the boundaries between the CGs and UFGs, and gradually enhance the strength of CGs to be close to that of UFGs. As a result, the deformation behavior of bimodal-grained materials is different from that of pure coarse-grained or ultrafine-grained materials. The addition of CNTs into such a bimodal-grained matrix is anticipated to further enhance the strength. Thus, the underlying strengthening mechanisms in such a CNT-reinforced bimodal-grained nanocomposite need to be explored.
The present study was undertaken with three primary objectives: Firstly, fabricate a unique CNT-reinforced bimodal-grained Al–Cu–Mg nanocomposite and base alloy with high strength and ductility; secondly, characterize microstructure-strength relationship; and finally, identify and quantify the underlying strengthening mechanisms and validate with experimental results. Multiple strengthening mechanisms including load transfer, dislocation strengthening and Orowan looping are taken into consideration to elucidate the enhanced mechanical properties in CNT-reinforced Al–Cu–Mg nanocomposite.
Section snippets
Raw material and fabrication
Atomized 2009Al (Al-4wt.%Cu-1.5 wt%Mg) powders with a diameter of ~10 μm were used as raw materials. The CNTs of ~98% purity with an outer diameter of 10–30 nm, provided by Tsinghua University were prepared via chemical vapor deposition (CVD). The as-received 2009Al powders mixed with 4 vol% CNTs were milled in an attritor at 400 rpm for 6 h with a ball-to-powder mass ratio of 15:1. The ball-milled powders were further mixed with 25 vol% as-received 2009Al powders using a dual axis mixer at
Microstructure
The microstructural features of the extruded B-AA and B-CNT/AA in T4 condition are shown in Fig. 1, Fig. 2, respectively. The microstructures consisted of a mixture of UFG (equiaxed) matrix with ~25% CG (columnar) bands or lamellae that align in the extrusion direction (ED), see supplementary Fig.S2. The reduction in the grain size of both materials was in general attributed to ball milling, and dynamic recrystallization (DRX) that occurred during plastic deformation at higher working
Discussion
Fig. 4 presents typical TEM images showing a remarkable bimodal grain distribution and CNT dispersion, and a HRTEM image showing the matrix-CNT interface structure in the nanocomposite. The microstructure observed via TEM in Fig. 4(a) consisting of distinctive CGs and UFGs in layers is consistent with that via EBSD (Fig. 2) and OM (Fig.S2). Second-phase Al2Cu particles can be clearly seen from the TEM images as indicated by arrows. As well, CNTs were observed to be randomly dispersed in the Al
Conclusions
In this study, a CNT-reinforced bimodal-grained 2009Al nanocomposite along with its base alloy was fabricated by two-step high energy ball milling, hot pressing, extrusion followed by T4 heat treatment. A relationship between the bimodal-grained microstructure and mechanical properties in the base alloy was first established and its strengthening effect was analyzed. The simultaneous presence of UFGs and CGs with a high dislocation density led to an excellent strength-ductility harmony in the
CRediT authorship contribution statement
S.M.A.K. Mohammed: Data curation, Formal analysis, Investigation, Methodology, Validation, Writing – original draft. D.L. Chen: Conceptualization, Funding acquisition, Methodology, Resources, Supervision, Writing – review & editing. Z.Y. Liu: Methodology, Validation, Writing – review & editing. D.R. Ni: Methodology, Writing – review & editing. Q.Z. Wang: Methodology, Writing – review & editing. B.L. Xiao: Methodology, Resources, Writing – review & editing. Z.Y. Ma: Funding acquisition,
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 would like to thank the Natural Sciences and Engineering Research Council of Canada (NSERC) and the National Natural Science Foundation of China (Nos. 51931009, 51871214, 51871215) in the form of international research collaboration. Z.Y. Liu, D.R. Ni, Q.Z. Wang, B.L. Xiao, Z.Y. Ma further thank Key Research Program of Frontier Sciences, CAS (No. QYZDJ-SSW-JSC015) and National Key R&D Program of China (No. 2017YFB0703104). The authors would also like to thank Messrs. Q. Li, A.
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