In-situ observation of interaction between dislocations and carbon nanotubes in aluminum at elevated temperatures
Introduction
In the past decade, increasing attention has been paid to carbon nanotube (CNT)-reinforced aluminum (Al) metal matrix composites (MMCs) because of their promising light weight and excellent mechanical properties [1], [2]. Many studies have shown the possibility to achieve high strength over 300 MPa of CNTs/Al composites by using powder metallurgy methods [2]. However, one critical problem of these composites for future applications is the limited ductility of near or less than 5% in tensile elongation [3], [4]. To improve plasticity, some studies optimized composite microstructures related to reinforcements, such as CNT dispersion [5], interface [6] and Al4C3 amount [7]. However, limit attention has been paid to the matrix microstructure changes induced by CNT additions [1], [8].
In metals and alloys, the misorientation angle of grain boundaries is a significant microstructural feature to influence the deformation behavior and mechanical properties [9], [10]. Regarding Al alloys, some researches showed that low-angle grain boundaries (LAGBs) can improve the plasticity due to the cross slip mechanism between adjacent grains [11], [12]. Recently, Chen et al. [3] reported a large fraction of LAGBs in CNTs/Al composites fabricated at high sintering temperature of 900 K, contributing to the excellent plasticity of tensile elongation larger than 20% as well as tensile strength over 200 MPa. The control of grain features in CNTs/Al composites may break new ground in fabricating high-performance Al MMCs. However, the formation mechanism of the high-fraction LAGBs was still unclear. To this end, in this study we applied in-situ transmission electronic microscopy observations on a low-temperature-sintered CNTs/Al composite to investigate the interaction between dislocations and CNTs at elevated temperatures.
Section snippets
Experimental methods
Pure Al powder and multi-walled CNTs (MWCNTs, ~10 nm in diameter and ~1 μm in length, Baytubes C150P, Bayer Material Science Co., Japan) were used as starting materials. To disperse CNTs in Al matrix, flaky Al powders were prepared by high energy balling and then bathed in isopropyl alcohol (IPA) based solution with 1 wt% CNTs in a plastic bottle on a rocking ball milling machine. To further consolidate the composite powders, spark plasma sintering (SPS) and following hot extrusion were
Results and discussion
Fig. 1 shows the grain boundary (GB) characteristics obtained by EBSD of pure Al and CNTs/Al composites sintered at 800 K and 900 K. Obviously high-angle GBs (defined as GBs with misorientation angles larger than 15°) are dominant in pure Al sintered at 800 K and 900 K (Fig. 1a) and the CNTs/Al composite sintered at 800 K (Fig. 1b). However, with the CNTs/Al composite sintered at 900 K, the fraction of low-angle GBs (LAGBs, smaller than 15°, red and green lines in Fig. 1c) increases remarkably.
Conclusions
This study investigates the formation mechanism of high fraction LAGBs at elevated temperatures in Al MMCs. From in-situ high-temperature TEM observations on CNTs/Al composite sintered at 800 K, the abundant CNTs can be trapped by long CNTs and evolve to LAGBs. The abundant dislocations and long CNTs are internal factors and high temperature is the external one to produce high fraction LAGBs and consequently outstanding tensile properties. Our study provides new insight into designing high
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
This study is supported by the Fundamental Research Funds for the Central Universities (G2018KY0301), from the National Natural Science Foundation of China (No. 51901183), and the Research Fund of the State Key Laboratory of Solidification Processing (2019-TS-13).
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