Improving the high-cycle fatigue strength of heterogeneous carbon nanotube/Al-Cu-Mg composites through grain size design in ductile-zones
Graphical abstract
Introduction
With the continuous upgrading of high-tech equipment in aerospace, electronics, nuclear power and other fields, the demand for metal matrix composites (MMCs) with strong structural and performance designability, excellent physical and mechanical properties is growing rapidly [[1], [2], [3], [4], [5], [6], [7]]. Among them, carbon nanotube (CNT) reinforced Al matrix (CNT/Al) composites have attracted great attention due to their high specific strength, high specific modulus and good machinability [[8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18]]. However, the CNT/Al composites have a significant drawback of low ductility, which limits their industrial application. This is mainly due to the lower dislocation storability of the fine grains, and the strong pinning effect of CNTs on dislocation gliding [[19], [20], [21]].
The heterogeneous structures consisting of ductile-zones (DZs) and brittle-zones (BZs) by varied distribution of reinforcements or grain sizes, have been demonstrated as a promising approach to have a better trade off of the strength-ductility in ultrafine grained (UFG) metals and composites [[22], [23], [24], [25], [26], [27], [28]]. Recently, some scholars studied the heterogeneous CNT/Al composites, and found that they had better strength-ductility than the uniform CNT/Al composites [[29], [30], [31], [32], [33], [34]]. For example, Liu et al. [31] fabricated the heterogeneous CNT/Al-Cu-Mg composite by powder metallurgy method combined with subsequent hot extrusion, and reported that it achieved more than 100% elongation increase with nearly no loss of the tensile strength as compared to the uniform CNT/Al-Cu-Mg composite. The enhanced elongation was attributed to the greatly suppressed strain localization and effectively blunted micro-cracks due to the inhomogeneous structure. Meanwhile, geometrically necessary dislocations were induced between the DZs and BZs, leading to extra-strengthening beyond the rule-of-mixtures. On the basis of the toughening strategy with heterogeneous structures, Tan et al. [34] fabricated the heterogeneous CNT/Al-Cu-Mg composite with trimodal grain structure, and found that both the elongation and tensile strength of the heterogeneous composite were higher than the uniform composite. This achievement in heterogeneous CNT/Al composites further confirmed the beneficial role of tailoring grain structure in improving the strength-ductility of CNT/Al composites.
For many industrial applications, the fatigue performance is a key criterion of structural materials. Therefore, it is of great importance to investigate the fatigue behavior. So far, the investigations of the heterogeneous materials were mainly focused on their tensile properties, investigations on the fatigue behaviors were quite rare. The fatigue behaviors of the uniform CNT/Al composites were investigated in recent years [35,36]. Shin et al. [35] found that the addition of CNTs was helpful to improve the fatigue strength, which was mainly due to that the prevailing bridging behavior of CNTs suppressed the formation of catastrophic cracks. However, for the heterogeneous CNT/Al composites, there was no related study on their fatigue behaviors.
According to the traditional view, the high-cycle fatigue (HCF) strength of the uniform materials was closely related to their static tensile strength, and the high static tensile strength usually corresponded to the high value of the fatigue strength [35,37,38]. On the other hand, the fatigue cracks preferentially nucleated in the local deformation area, which deteriorated the fatigue properties. For example, Nelson et al. [39] found that the HCF cracks of the heterogeneous Al alloys nucleated in the low-strength coarse grained (CG) zones. Liu et al. [40] found that the HCF strength of the Cu-Al alloys mainly depended on the most vulnerable area within the inhomogeneous grain structure, and had less relation on the overall static mechanical properties. Therefore, it was generally believed that the inhomogeneous microstructure might not be good for the improvement of the fatigue properties. This poses a challenge for the application of heterogeneous composites under the fatigue conditions.
In recent years, some researchers found that the gradient materials with the grain transition from the nano-size at the sample surface to the micro-size at the sample center had the excellent fatigue properties. For example, Lu et al. [41] and Zhang et al. [42] fabricated Cu and TWIP steels with gradient grain structure respectively, and found that their HCF strengths were higher than those of CG and UFG counterparts. Qian et al. [43] studied the fatigue behavior of heterogeneous nickel with different grain sizes in the DZs, and found that the HCF strength was significantly improved as the grain size in the DZs was lower than 1 μm, which was even higher than that of the uniform UFG nickel. However, the mechanism of improving fatigue performance has not been well understood. It is not clear yet whether adjusting the grain size in the DZs could improve the fatigue performance of heterogeneous CNT/Al composites.
In this study, the heterogeneous CNT/Al composites with two different grain sizes in the DZs as well as the uniform CNT/Al composite were fabricated by powder metallurgy route. The fatigue performance and cyclic lives at different stress amplitudes were tested and the microstructures were analyzed. The aim is to (a) clarify the effect of heterogeneous structure on the fatigue properties, and (b) develop heterogeneous CNT/Al composites with high fatigue strength without reducing the strength-ductility.
Section snippets
Experimental
In the present work, CNT/Al composites with heterogeneous structures were fabricated through powder metallurgy routes, as shown in Fig. 1. Atomized 2009Al (Al-4 wt.% Cu-1.5 wt% Mg) powders with approximately 10 μm diameters were used as raw metal materials. CNTs (~98% purity) fabricated by chemical vapor deposition had an outer diameter of 10–30 nm and a length of ~5 μm. No extra pre-treatment was conducted on CNTs. 3 vol% CNTs were high energy ball milled with 10 μm as-received 2009Al powders
Initial microstructure
Fig. 3 shows the OM images of the two kinds of heterogeneous CNT/2009Al composites. It can be seen that the bright zones aligned along the extrusion direction were embedded in the dark zones. According to our previous investigations [[30], [31], [32]], the bright zones were the DZs originating from the additional matrix powders, and the dark zones were the BZs originating from the milled CNT/Al composite powders. The DZ bands for the Hetero-CG DZ were slightly thinner (Fig. 3(a)) than those of
Conclusions
In this study, the uniform and two kinds of heterogeneous CNT/2009Al composites with different grain sizes in the DZs were fabricated by powder metallurgy routes. The high cycle fatigue tests with the stress ratio of 0.1 and frequency of 20 Hz were carried out, and the microstructures before and after fatigue test were analyzed. The following conclusions can be drawn:
- (1)
All the three CNT/2009Al composites, including the uniform composite, the heterogeneous composite with coarse grain ductile-zones
Author statement
All authors contribute substantially to the paper. K. Ma (first author) carried out the data collection, data analysis and manuscript writing; X.N. Li fabricated the composites and tested the tensile properties; K. Liu, X.G. Chen helped to revise the manuscript; Z.Y. Liu designed the experiment and revised the manuscript; B.L. Xiao participated in the design of the experiment and analysis of the experimental data; Z.Y. Ma revised the manuscript and provided the funding. All authors read and
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 work was supported by: (a) Key R&D Program of China under grant (No. 2017YFB0703104); (b) Key Research Program of Frontier Sciences, CAS (NO. QYZDJ-SSW-JSC015); (c) National Natural Science Foundation of China (No. 51871215, No. 51931009, No.51871214); (d) the Youth Innovation Promotion Association CAS (2020197). K. Ma would like to acknowledge the support from China Scholarship Council.
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