Improving the high-cycle fatigue strength of heterogeneous carbon nanotube/Al-Cu-Mg composites through grain size design in ductile-zones

Abstract

Heterogeneous structure consisting of brittle-zones (BZs) rich of carbon nanotubes (CNTs) and ductile-zones (DZs) free of CNTs, was an effective way to improve the strength-ductility of CNT reinforced Al (CNT/Al) composites. Two heterogeneous CNT/2009Al composites with coarse grain (CG, ~2 μm) DZs or ultra-fine grain (UFG, ~500 nm) DZs were fabricated and achieved enhanced strength-ductility. However, the heterogeneous composite with CG DZs had a lower high-cycle fatigue strength as well as fatigue strength/tensile strength ratio than the uniform composite, while the heterogeneous composite with UFG DZs exhibited the increased fatigue strength and the same level of fatigue strength/tensile strength ratio compared to the uniform composite. It was found that the improved fatigue properties for the heterogeneous composite with the UFG DZs could attribute to two reasons. Firstly, the UFG for the DZs significantly increased the strength of DZs, which effectively reduced the strain localization in the DZs. Secondly, the dislocations piling up at the grain boundaries of the BZs, as well as the stress concentration at the boundaries between the DZs and BZs were relieved due to the coordinated micro-strain for the heterogeneous structure. This provided a simple strategy for the structural design of heterogeneous composites with high fatigue strength.

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.