AZ91 alloy nanocomposites reinforced with Mg-coated graphene: Phases distribution, interfacial microstructure, and property analysis

https://doi.org/10.1016/j.jallcom.2021.163484Get rights and content

Highlights

  • The evenly distributed graphene in the matrix played a significant role in grain refinement.

  • A large number of fine β phases were dispersed and precipitated.

  • The orientation relationship (OR) between β-Mg17Al12 and α-Mg was shown as [—3 —1 —1]β-Mg17Al12)∥[1 —1 0 —1]α-Mg.

  • A nano-scale close contact interface improves the bonding strength of the interface.

  • The grain refinement and load transfer were the mechanisms to enhance the mechanical properties.

Abstract

A new organic chemical reduction method was successfully used to synthesize magnesium-coated graphene (GNPs), and xGNPs/AZ91 nanocomposites with different contents were fabricated by vacuum hot-pressing sintering. The microstructure of the composite was mainly composed of the matrix (α-Mg) and the precipitated phase (β-Mg17Al12) with different morphologies such as rods, spindles, and granules. The coarse irregular β phases precipitated along the grain boundaries, while fine rod-like β phases were distributed inside the crystal grains. With the increase of GNPs content, the grain and structure are significantly refined under the action of two mechanisms of increasing the nucleation rate and hindering the growth of grains. The average grain size of the 2.5-wt% GNPs/AZ91 composite dropped from 40.78 µm to 25.39 µm, a reduction of 37.7%. In addition, the orientation relationship (OR) between β-Mg17Al12 and α-Mg was shown as [—3 —1 —1]β-Mg17Al12)∥[1 —1 0 —1]α-Mg. Further, finer β phases were further precipitated in the grain boundaries and matrix. Moreover, the β precipitated phase and the GNP, as well as the GNP and magnesium-matrix formed a nano-scale contact interface and a diffusion bonding interface, thereby greatly enhancing the interface bonding strength between GNP and the matrix. Compared with AZ91 alloy, the grain refinement and load transfer caused by GNPs increased the microhardness of the composite by 17.6% and the friction coefficient was decreased by 37.4%. The significant improvement in the wear resistance of the composites was due to the effect of the lubricating layer formed by GNPs on the wear surface, which changed from severe delamination wear to slight delamination and abrasive wear behavior.

Introduction

AZ is a Mg-Al-Zn series magnesium alloy with low cost and high specific strength. It is currently the most abundant and widely used magnesium alloy series. Among them, the density of AZ91 alloy (Al content ≈ 9 wt%, Zn content<1 wt%) is only 1.82 g/cm3, which is equivalent to 2/3 of aluminum alloy and 1/4 of cast iron. Because of its with good castability and corrosion resistance, it has attracted considerable attention [1], [2], [3]. However, the AZ91 magnesium alloy has limited mechanical strength and poor friction properties. Therefore, there is an urgent need to prepare high-performance reinforced magnesium-matrix composites to meet the rapid development needs of the aerospace and weaponry industries [4], [5], [6], [7], [8], [9]. For example, Wang et al. used a vacuum pressureless infiltration process to manufacture 45 vol% continuous carbon fiber (CF) reinforced magnesium-based composites. The interface reaction produces nanocrystalline particles (MgO, ZrO2, ZrC), which exhibit high tensile strength [10]. Zhang et al. studied the effect of Gd content on the interface microstructure and mechanical behavior of carbon fiber reinforced magnesium composites (Cf/Mg). It was found that the alloying element Gd is enriched in the form of nano-level Gd2O3 interlayers near the interface, and the granular Mg7Gd strengthens the interface bonding [11]. Yang et al. analyzed the morphology and phase composition of SiC-doped Cf/Mg composites, and characterized the interface bonding [12]. Chen et al. prepared AZ91D-GNPs composite material for the first time through a thixotropic process and found that a strong interface bond was formed between GNPs and the alloy matrix. The addition of GNPs has a positive effect on the grain refinement of composite materials, reducing porosity and improving fluidity. This preparation method has broad application prospects for large-scale industrial production [13]. Among the many strengthening and reinforcing phases, graphene (GNP) is a two-dimensional single-layer sheet of sp2 hybridized carbon atoms, in which the carbon atoms are densely packed in a honeycomb lattice [14], [15]. The special structure leads to its excellent Young’s modulus, yield strength, and tensile strength. These advantages make GNP one of the most potential materials for the preparation of the nano-reinforcement of magnesium-matrix composites [14], [16], [17], [18], [19], [20]. However, due to the low solubility of carbon in metal melts such as Cu and Mg, it is difficult for GNPs with poor wettability to be dispersed in the metal matrix, and hence, it cannot play an effective reinforcing effect [21], [22], [23], [24], [25], [26]. Numerous studies have shown that the main obstacle to prepare the GNP/metal matrix composites is still how to uniformly disperse GNPs in the entire metal matrix without destroying their structural integrity [27], [28]. Another important issue is how to effectively increase the interface bonding between the GNPs and the metal, which will cause more load to be transferred to the GNPs [28], [29], [30]. Therefore, on the basis of extensive research on GNPs/metal composites (MMCs), it is believed that good dispersibility of GNPs and the interface bonding between the reinforcement and the matrix are essential for obtaining high-strength composites [13], [31].

In the traditional forming process of GNPs/magnesium-matrix composites, GNPs with high specific surface areas are often pushed to the liquid front before solidification. Further, they gather in the grain boundary or interdendritic region to form obvious clusters, which considerably limits the strengthening and toughening effect of the composite material [7], [32], [33], [34], [35], [36]. In order to solve the problem of the surface wettability of graphene and the matrix, an attempt was made to disperse GNPs uniformly in the alloy matrix through a new process or chemical synthesis route [31], [36]. For example, Ye et al. fabricated a nickel-coated graphene-reinforced Mg-9Al composite. The in situ formed Mg2Ni-reinforced phase improved the interface bonding between the graphene nanosheets and the magnesium matrix, and produced the effect of grain refinement in the matrix. It was considerably higher than that of the unmodified original GNPs [37]. Wu et al. uniformly distributed the GNPs of the Ni-coated layer on the AZ31 magnesium matrix and maintained the structural integrity of the GNPs. The composite material showed excellent mechanical strength and wear resistance [38]. Sun et al. used the in situ exchange reaction (PM) to heterogeneously nucleate magnesium on GNPs and form a GNP@Mg structure, which served as a reinforcing phase to strengthen the magnesium matrix, which in turn ensured the integrity and the dispersibility of GNPs in the Mg matrix, resulting in significantly enhanced and toughened composites [31]. Du et al. ensured that the GNP and the magnesium matrix are closely combined at the nano level through the melt stirring and hot extrusion process and that the mechanical strength of the composite material is significantly increased. At the same time, the enhancement mechanism of grain refinement and load transfer caused by GNPs was discussed [39]. In contrast, studies have shown that the uniform addition of intact graphene can make magnesium-matrix composites exhibit excellent mechanical strength, and the yield resistance, tensile properties, and compressibility can be significantly enhanced without any loss of ductility [40], [41]. However, AZ91 alloy nanocomposites reinforced with graphene have not been reported yet.

In this work, we report a new organic chemical in situ reduction process of Mg nanoparticles, successfully coating the surface of graphene with a Mg nano-metal coating. This method improved the dispersibility of the GNPs and retained the complete structure of graphene. At the same time, a tight interface bonding between the GNPs and the magnesium matrix was realized during the hot-pressing sintering process. This method not only avoided the introduction of foreign elements, but also successfully deposited nano-metal particles on the surface of the graphene, thereby preserving the complete structure of the graphene and significantly improving the dispersibility of GNPs. In addition, under the conditions of appropriate pressure and sintering temperature, the presence of GNPs during the forming process further promoted the dispersion and precipitation of the strengthening phase. As a result, a close interfacial bonding between GNPs and the magnesium-matrix was realized. Furthermore, the effects of the addition of Mg-coated graphene on the microstructure evolution of magnesium-matrix alloys and the bonding relationship between phases and interfaces were analyzed, and the strengthening mechanism of the GNPs in the composite was clarified. This chemical modification method strategy provides a useful reference for the modification of other reinforcing phases. The prepared high-strength magnesium-matrix composite material can have great application prospects in aerospace, medical, catalysis, electronic, and other fields [26], [42], [43], [44], [45].

Section snippets

Materials and reagents

In this study, the graphene (GNP) (Tangshan Jianhua Technology Development Co., Ltd., 3–10 layers, size: 11–15 µm, thickness: 1 ~ 3.5 nm), magnesium ribbon (China Yellow River Smelter,>99%, specification: 3 mm × 15 mm), iodine (I2) (Zhengzhou Chemical Reagent Factory, AR), absolute ethanol (Tianjin Damao Chemical Reagent Factory, AR), and sodium hydride (NaH) (purchased from Aladdin, with a purity of 60%) were used as purchased. The bromoethane C2H5Br (EtBr) (AR, Damao Chemical Reagent Factory)

Morphology and dispersion characteristics of Mg-coated graphene and xGNPs/AZ91 composite powder

According to the same synthesis method of the previously published article [46], the morphology and interfacial microstructure of Mg-coated graphene were shown in Fig. 2. The reaction equations involved in Mg-coated graphene were: n-C2H5Br + Mg → n-C2H5MgBr, C2H5MgBr + NaH → C2H6 + NaBr + Mg. The organic chemical reduction method successfully nucleated 30–50 nm Mg nanoparticles on the GNP surface (Fig. 2a–d), and the nanoparticles were uniformly and densely coated on the GNP (Fig. 2b and c). As

Conclusions

The Mg nano-metal layer coated on the graphene surface by the organic chemical reduction method had excellent interface bonding, considerably improving the interface wettability with the alloy matrix. In particular, with an increase in the GNPs content, the grain refinement of the 2.5-wt% GNPs/AZ91 composite was the most outstanding. The average grain size was reduced from 40.78 µm to 25.39 µm, which was 37.7% less than that of the AZ91 alloy. Furthermore, a large number of long rod-shaped and

CRediT authorship contribution statement

Zhanyong Zhao: Project administration, Resources, Supervision, Investigation, Data curation, Formal analysis, Writing – original draft. Rongxia Zhao: Formal analysis, Writing – review & editing. Peikang Bai: Investigation, Methodology. Wenbo Du: Formal analysis, Writing – review & editing. Renguo Guan: Formal analysis, Writing – original draft. Die Tie: Project administration, Resources, Supervision. Xiaojing Wang: Investigation, Methodology. Nithesh Naik Conceptualization, Writing – review &

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.

Acknowledgments

The authors would like to thank the National Defense Foundation of China (No.61400040208), the China Postdoctoral Science Foundation (2019M661068), the Key Research and Development Project of Shanxi Province (201903D121009), Scientific and Technological Innovation Projects of Shanxi Province, China (2019L0608), the Major Science and Technology Projects of Shanxi Province, China (No. 20181101009).

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