# Enhancing the Strengthening Effect of Graphene-Nanoplates in Al Matrix Composites by Heterogeneous Matrix Design

^{1}

^{2}

^{*}

## Abstract

**:**

## 1. Introduction

^{2}/g), which is very easy to agglomerate [8]. In this case, GNPs are not beneficial for the improvement of composite’s properties, while they will become a source of cracks, leading to crack propagation of the composite under load. Rashad et al. [9] studied the microstructure and mechanical properties of GNPs/Al composites with different contents. It was found that when the content of GNPs was high, it would stack into graphite particles due to the effect of the π–π bond between layers, which led to the decrease in material plasticity. Shin et al. [10] compared and analyzed the strengthening behavior of GNPs and carbon nanotubes in Al matrix composites, and found that they can be described by the modified shear lag model (Equation (1)),

_{c}and σ

_{m}are the yield strength of the composite and matrix, V

_{r}and V

_{m}are the volume fraction of carbon nanomaterials and matrix, respectively, S is the contact interface area, A is the cross-sectional area, and τ

_{m}is the interfacial shear strength of the matrix. It can be seen that the larger diameter and thinner thickness of GNPs will make a more obvious contribution to the strengthening effect under the condition of uniform dispersion and the same content.

_{2}into the amorphous Mg

_{69}Cu

_{11}Y

_{20}shell and obtained 3.3 GPa ultra-high strength, which is close to the theory. Huang et al. [30] proposed the idea of “micro inhomogeneous”, and prepared TiBw/Ti composites with a quasi-continuous network distribution of reinforcements via the in situ method, which greatly improved the strength and plasticity of traditional titanium matrix composites and caused researchers to think about the non-uniform design of composites and other materials [31].

## 2. Materials and Methods

#### 2.1. Finite Element Method (FEM) Simulation

^{2}. The size of the GNPs sheet is 10 × 0.5 µm

^{2}.

#### 2.2. Materials and Preparations

#### 2.3. Characterizations and Mechanical Tests

## 3. Results and Discussion

#### 3.1. FEM Simulation and Microstructure Investigation of As-Cast Composites

#### 3.2. Microstructure Evolution of Composites after Deformation Treatment

_{D}/I

_{G}is positively correlated with defect density, which can be used to evaluate the defect degree of GNPs; therefore, this study mainly uses I

_{D}/I

_{G}and 2D peak frequency to qualitatively analyze the state of GNPs in composites. As shown in Figure 9, there is a blue shift of the G peak after ball milling. After ball milling, the morphology of GNPs changed significantly compared with before, especially the thickness of GNPs decreased significantly, which is consistent with many reports in the literature [4,7,9,10]. By comparing the Raman spectra of the composites, it can be found that the position of the G peak of heterogeneous composites is higher than that of homogeneous materials, while the position of the 2D peak is lower than that of homogeneous materials. This means that the heterogeneous matrix has a better effect on eliminating GNPs agglomeration and reducing the number of GNPs layers, which is beneficial to the mechanical properties of the composites. In addition, comparing the I

_{D}/I

_{G}of the two composites, it is found that the change of defect degree is not obvious, which may be due to the production of new FLG during hot extrusion. In order to evaluate the effect of the heterogeneous matrix, it is necessary to combine the tests of properties, especially the elastic modulus.

_{4}C

_{3}can be observed in Figure 13c. In comparison, it can be observed from Figure 13d that after solution aging, the Al

_{2}CuMg phase appears near the Al grain in the composite. It is a type of nano-scaled precipitation phase in 2024Al, which is conducive to the strengthening of the material; however, it is worth mentioning that most of the interfaces are well bonded, as shown in Figure 13e. It can be seen that this piece of GNP is thinner than the raw material, which can play a better effect in transmitting load, which is beneficial to obtaining composites with high mechanical properties.

#### 3.3. Mechanical Properties of Composites

#### 3.4. Strengthening Mechanism of GNPs/Al Composites

_{L}); grain refinement results from the pinning effect of GNPs (∆σ

_{G}

_{(R)}); the thermal mismatch mechanism is caused by the generation of dislocations due to the different coefficient of thermal expansion (CTE) between Al matrix and the reinforcements (∆σ

_{T}); the Orowan strengthening mechanism is related to the Orowan looping system (∆σ

_{Oro}); therefore, the multiple strengthening mechanisms operating in the composite can be expressed as Equation (2). The following calculation will take (GNPs/6061Al)/2024Al composite as the research object.

^{1/2}[45]; d is the average grain size of the composite after hot extrusion, as shown in Figure 11, and it can be measured as 2.01 μm. So, the results show that the contribution of the grain refinement effect is about 9.1 MPa. This effect is mainly related to the inhibition of reinforcement on grain grown during the preparation process and the hot extrusion.

^{−6}/K and 23.6 × 10

^{−6}/K for the matrix Al. The contribution of the thermal mismatch mechanism to tensile strength can be estimated by the following formula [46]:

_{m}is the shear modulus of the matrix and can be calculated using the basic parameters of Al. According to the literature, the G

_{m}is about 27.5 GPa [46]; ∆α is the difference in CTE between the two parts, matrix, and reinforcement; ∆T is the gradient in the temperature from hot extrusion (500 °C) to the ambient temperature (25 °C). In addition, the V

_{R}and d

_{R}are the volume fraction and approximate diameter of reinforcements, respectively; d

_{R}can be obtained from microstructure observation. According to Equation (4), the contribution of the thermal mismatch mechanism is about 9.7 MPa.

_{m}and b are the shear modulus and the Burgers vector of matrix Al. V

_{R}is the volume fraction of reinforcements in the composite. The result indicates that the contribution from the Orowan looping system is about 24.2 MPa.

## 4. Conclusions

## Author Contributions

## Funding

## Data Availability Statement

## Acknowledgments

## Conflicts of Interest

## References

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**Figure 1.**FEM simulation of the effect of heterogeneous matrix on the deformation behavior of GNPs: (

**a**) model diagram; (

**b**) strength difference–strain curve.

**Figure 2.**Microstructure of the raw materials used in this work: (

**a**) SEM image of the 6061Al powder; (

**b**) SEM image of GNPs; (

**c**) mixed powder after ball milling. Some GNPs have been marked.

**Figure 4.**Schematic of the deformation treatment process of the heterogeneous matrix design composite.

**Figure 5.**Morphology of the (GNPs/6061Al)/2024Al and GNPs/6061Al composites before the extrusion treatment: (

**a**) (GNPs/6061Al)/2024Al; (

**b**) GNPs/6061Al.

**Figure 6.**XRD analysis results of the as-cast composites and alloys: (

**a**) (GNPs/6061Al)/2024Al and GNPs/6061Al composites; (

**b**) 6061Al/2024Al alloys and 6061Al alloys.

**Figure 7.**EDS analysis result of as-cast 0.6 wt.% (GNPs/6061Al)/2024Al composite: (

**a**) microstructure, (

**b**) C element, (

**c**) Al element, (

**d**) Cu element.

**Figure 8.**EDS analysis result of 0.6 wt.% (GNPs/6061Al)/2024Al composite after hot extrusion: (

**a**) microstructure, (

**b**) C element, (

**c**) Al element, (

**d**) Cu element.

**Figure 10.**(

**a**) Raman results of composites GNPs/6061Al and (GNPs/6061Al)/2024Al composite after extrusion; (

**b**,

**c**) partial diagram of Raman results.

**Figure 11.**EBSD analysis results of extruded composites: (

**a**) the grain map and (

**b**) the grain boundary (GB) map of GNPs/6061Al composite; (

**c**) the grain map and (

**d**) the grain boundary (GB) map of (GNPs/6061Al)/2024Al; (

**e**) the distribution of grain size; (

**f**) fraction of GB characteristics; (

**g**) number fraction of kernel average misorientation (KAM) distribution.

**Figure 12.**The (111) pole figures of extruded composites (

**a**) GNPs/6061Al (

**d**) (GNPs/6061Al)/2024Al composite. (

**b**) and (

**e**) inverse pole figures, and inverse pole figure (IPF) maps of (

**c**) GNPs/6061Al (

**f**) (GNPs/6061Al)/2024Al.

**Figure 13.**TEM analysis results of (GNPs/6061Al)/2024Al composite after extrusion: (

**a**) Bright-field image; (

**b**) HADDF image; (

**c**–

**e**) high magnification images, the inset of (

**c**,

**d**) FFT—the result of the selected area.

**Figure 14.**Representative mechanical properties of the composites and matrixes after extrusion: (

**a**) tensile stress–strain curves; (

**b**) comparison of the tensile strength, yield strength, elongation, and Young’s modulus; (

**c**) strain hardening curves calculated from stress–strain curves.

**Figure 15.**Fracture surfaces of the extruded composites: (

**a**,

**b**) GNPs/6061Al composite; (

**c**,

**d**) (GNPs/6061Al)/2024Al composite; (

**e**,

**f**) 6061Al matrix alloy; (

**g**,

**h**) 6061Al/2024Al matrix alloy.

Stress (MPa) | ${\mathit{\sigma}}_{\mathit{y}}$ | ${\mathit{\sigma}}_{\mathit{y}}+50$ | ${\mathit{\sigma}}_{\mathit{y}}+100$ | ${\mathit{\sigma}}_{\mathit{y}}+120$ |
---|---|---|---|---|

Plastic strain | 0 | 0.5 | 1.5 | 2 |

Young’s Modulus | Poisson’s Ratio | Density | |
---|---|---|---|

GNPs | 1000 GPa | 0.1 | 2.25 g/cm^{3} |

Al alloy | 70 GPa | 0.3 | 2.7 g/cm^{3} |

Element | Mg | Si | Cu | Fe | Zn | Al |

6061Al powder | 1.12 | 0.75 | 0.32 | 0.65 | 0.22 | Bal. |

Element | Cu | Mg | Mn | Zn | Cr | Al |

2024Al | 4.05 | 1.65 | 0.75 | 0.22 | 0.07 | Bal. |

Process | Ball Milling | Pressure Infiltration |
---|---|---|

(GNPs/6061Al)/2024Al | 6061Al powder + GNPs | 2024Al |

GNPs/6061Al | 6061Al powder + GNPs | 6061Al |

Specimen | Condition | YS (MPa) | UTS (MPa) | EI. (%) | E (GPa) |
---|---|---|---|---|---|

0.6 wt.%(GNPs/6061Al)/2024Al | Extruded | 194.2 ± 7.3 | 338.1 ± 9.2 | 10.0 ± 1.1 | 87.2 ± 0.2 |

0.6 wt.%GNPs/6061Al | Extruded | 161.1 ± 5.2 | 310.5 ± 4.5 | 7.4 ± 1.4 | 86.7 ± 0.3 |

0.6 wt.%(GNPs/6061Al)/2024Al | As-cast | 125.8 ± 2.6 | 249.5 ± 6.1 | 5.7 ± 0.8 | 83.2 ± 0.2 |

0.6 wt.%GNPs/6061Al | As-cast | 119.3 ± 3.8 | 227.3 ± 3.2 | 5.0 ± 1.1 | 82.8 ± 0.1 |

6061Al/2024Al | Extruded | 102.8 ± 6.1 | 207.4 ± 5.7 | 24.8 ± 1.6 | 82.5 ± 0.3 |

6061Al alloy | Extruded | 71.3 ± 5.8 | 161.5 ± 7.1 | 21.9 ± 2.7 | 79.6 ± 0.1 |

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**MDPI and ACS Style**

Shao, P.; Sun, K.; Zhu, P.; Liu, K.; Zhang, Q.; Yang, W.; Wang, Z.; Sun, M.; Zhang, D.; Kidalov, S.; Xiao, H.; Wu, G. Enhancing the Strengthening Effect of Graphene-Nanoplates in Al Matrix Composites by Heterogeneous Matrix Design. *Nanomaterials* **2022**, *12*, 1833.
https://doi.org/10.3390/nano12111833

**AMA Style**

Shao P, Sun K, Zhu P, Liu K, Zhang Q, Yang W, Wang Z, Sun M, Zhang D, Kidalov S, Xiao H, Wu G. Enhancing the Strengthening Effect of Graphene-Nanoplates in Al Matrix Composites by Heterogeneous Matrix Design. *Nanomaterials*. 2022; 12(11):1833.
https://doi.org/10.3390/nano12111833

**Chicago/Turabian Style**

Shao, Puzhen, Kai Sun, Ping Zhu, Kai Liu, Qiang Zhang, Wenshu Yang, Zhijun Wang, Ming Sun, Dingyue Zhang, Sergey Kidalov, Haiying Xiao, and Gaohui Wu. 2022. "Enhancing the Strengthening Effect of Graphene-Nanoplates in Al Matrix Composites by Heterogeneous Matrix Design" *Nanomaterials* 12, no. 11: 1833.
https://doi.org/10.3390/nano12111833