Microstructure and Mechanical Properties of Aluminum: Graphene Composites Produced by Powder Metallurgical Method

Abstract

Aluminum-based composites containing 0.1-0.7 wt.% graphene were produced by the powder metallurgical process. The microstructure of the nanocomposites was studied using LM, SEM, EDS, TEM, and XRD. A relatively uniform distribution of graphene nano-platelets and compacted agglomerates in composites with different graphene contents was found. A mechanical bond between the nano-platelets and the aluminum matrix was established. Under the conditions used, neither particles nor nanoparticles of Al₄C₃ are formed. The microhardness and mechanical properties were studied. It was found out they are the highest in the composite containing 0.1% graphene. The change of these properties with the graphene content increasing has the same character. Agglomeration of GNPs occurs in all composites containing graphene, but a part of GNPs remain in a dispersed, non-agglomerated state. The microhardness and mechanical properties depend on this part of individual GNPs. It was proven that the observed increase in the yield strength of the studied composites is due to Orowan's strengthening mechanism. The fracture mechanism of GNPs de-bonding and pull-out from the aluminum matrix of the composite was experimentally proved.

Appendix

From the theory is known that the moving dislocations interact with the differently sized reinforcement particles differently. Recall that the strengthening effect of the reinforcement particles manifests itself in two different ways: (1) when below $\left\langle {r_{p} } \right\rangle_{{{\text{crit}}}}$ it is due to the friction that the dislocation experiences when cutting through the reinforcement particle, (2) above this size it is due to looping of the dislocation around the reinforcement, i.e., the Orowan’s mechanism. In the theory of dislocations is shown that the critical size of the reinforcing particle, which distinguishes these two mechanisms, is related to the mechanical properties of the matrix and the reinforcement as follows:

$$\left\langle {r_{p} } \right\rangle_{{{\text{crit}}}} \,\approx\, \frac{{Eb^{2} }}{{2\left( {1 + \nu } \right)\Gamma }}.$$

(A1)

Here, E is the Young modulus of the reinforcer and Γ is the antiphase boundary energy (Jm⁻²), which varies depending on the degree of the coherence between the lattices of the reinforcer and the matrix (according to Ref 37) for coherent lattices Γ = 0.1 Jm⁻², while for the non-coherent lattices it is Γ = 1 Jm⁻²). Replacing in, E_Al = 398 GPa, ν_Al = 0.23 and assuming a semi-coherence between the lattices of the reinforcer and the Al-matrix, Γ =0.5 Jm⁻², for the critical radius is obtained, $\left\langle {r_{p} } \right\rangle_{{{\text{crit}}}}$~14 nm, or ~2 nm for non-coherent lattices. The mismatch between the experimental and the evaluated relative yield strength, therefore, suggests that a certain coherency exists between the lattices of the GNP and the Al.