Designing heterogeneous structures is a promising pathway for overcoming the trade-off between strength and toughness in metal matrix composites (MMCs). Herein, we report an innovative strategy for fabricating graphene nanosheet-Cu reinforced Al matrix (GNS-Cu/Al) composites with heterogeneous structure. This strategy involves the consolidation of unique composite powders with core-shell grain structure, which are synthesized with the aid of in-situ GNS-Cu hybrids. Results reveal that the fabricated GNS-Cu/Al composite exhibits multiple microstructural heterogeneities, including both heterogeneous grain structure and reinforcement spatial distribution, which endow the composite with a prominent combination of tensile strength of ∼437 MPa, fracture elongation of ∼12.5% and toughness of ∼48.7 MJ m−3. It is confirmed that such microstructural heterogeneities in GNS-Cu/Al composite contribute significant hetero-deformation induced (HDI) stress strengthening and sustained strain hardening, making the key mechanical properties of GNS-Cu/Al considerably outperform the counterpart of Cu/Al composite. Moreover, the coordinated deformation and crack bridging/blunting behaviors are demonstrated to be responsible for the exceptional toughness of GNS-Cu/Al composite. This work offers a promising bottom-up tactic to fabricate Al matrix composites with heterogeneous structures and superior mechanical performances for structural applications. 

Exceptional mechanical properties of aluminum matrix composites with heterogeneous structure induced by in-situ graphene nanosheet-Cu hybrids

Boron (B) and graphene oxide nanoplates (GOs) reinforced Ti6Al4V (Ti64) titanium matrix composite ((B + GOs)/Ti64) with a heterogeneous network structure was prepared via spark plasma sintering (SPS). The results show that the heterogeneous network structure composites consist of fine grain zones with a network structure and coarse grain zones with good ductility. Compared with Ti64 alloy, heterogeneous network structure (B + GOs)/Ti64 composites has higher strength and ductility, both of which are superior to homogeneous network structure (B + GOs)/Ti64composite. The increase in strength is attributed to the grain refinement effect within the fine grain zone and the load transfer effect of the reinforcement at the interface. The ductility is mainly dependent on the existence of large size matrix and the improvement of deformation coordination and cracks propagation by the heterogeneous network structure. Finally, the conflict between the strength and plasticity of the composites is effectively reconciled. This provides a new idea for the design of high-performance titanium matrix composites. 

A novel heterogeneous network structure titanium matrix composite with a combination of strength and ductility

A Novel Heterogeneous Network Structure Titanium Matrix Composite with a Combination of Strength and Ductility

Laminate carbon nanotube (CNT)/Al–Cu–Mg composites consisting of alternate ductile layers (coarse or ultrafine grain Al) free of CNTs and brittle layers (ultrafine grain) rich in CNTs, were prepared in the powder metallurgy route. It was found that the strength-ductility of the composites was improved remarkably, as compared with those of uniform composites. Mechanical incompatibility between different layers during tensile deformation produced a large number of geometrically necessary dislocations (GNDs) between the ductile layers and the brittle layers, which inhibited the strain localization, thereby enhancing the strength-ductility. Compared with the laminate composite using the coarse grain Al as the ductile layers, the strength of the laminate composite using ultrafine grain Al as ductile layers further increased by 14%, while the elongation remained unchanged. This was because the ultrafine grain rather than coarse grain of the ductile layers could lead to higher strength and had better coordination with the brittle layers. 

Enhanced strength–ductility synergy of carbon nanotube/Al–Cu–Mg composites via introducing laminate structure and grain modification

Silicon carbide particles reinforced 6xxxAl (SiCp/6xxxAl) composites have high strength but low elongation after artificial aging (AA), leading to poor formability. To obtain components with precise dimensions, complex shapes and great load-bearing abilities, it is demanded for SiCp/6xxxAl composites to be cold plastically formed during natural aging (NA) and served after AA. However, the process is limited due to the unclear effects of NA on AA hardening. In this study, the influence of NA on precipitation behaviors and hardening abilities during AA of a SiCp/Al-Mg-Si composite and unreinforced Al-Mg-Si alloy was investigated using hardness, tension, differential scanning calorimetry, electron probe and transmission electron microscopy tests. It is shown that in both the composite and Al-Mg-Si alloy, clusters formed during NA inhibit the formation of β'' phases and promote the formation of coarse β′ phases, so NA exhibits negative effects on AA hardening. However, what different from the Al-Mg-Si alloy is that the quenching dislocations in the composite promote the formation of precipitates and thus alleviate the inhibition of NA on precipitation. In addition, the composite has a lower proportion of coarse β′ phases due to the depletion of Mg by chemical reactions at or near SiCp-Al interfaces. Consequently, the negative NA effect in the composite is much weaker than that in the Al-Mg-Si alloy. This study clarifies how SiCp affect the negative NA effect on AA hardening and is expected to deepen the understanding of aging hardening behaviors of SiCp/Al composites. 

Effects of natural aging on precipitation behavior and hardening ability of peak artificially aged SiCp/Al-Mg-Si composites

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.

X. N. Li

Papers

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