Z.Y. Liu

Bio: Z.Y. Liu is an academic researcher from Center for Advanced Materials. The author has contributed to research in topics: Grain boundary & Composite number. The author has an hindex of 3, co-authored 6 publications receiving 16 citations.

Enhancing strength-ductility synergy of carbon nanotube/7055Al composite via a texture design by hot-rolling

Abstract: The limited ductility of carbon nanotubes reinforced Al matrix (CNT/Al) composites hinders their engineering applications, which is an urgent problem to be solved. Herein, texture optimization through hot-rolling was employed to improve the strength-ductility of the CNT/7055Al composites. Microstructural examinations indicate that the composite subjected to extrusion (CNT/7055Al-E) had a bimodal structure with the coarse and ultrafine grains. The composite subjected to extrusion followed by hot-rolling (CNT/7055Al-ER) exhibited a uniform ultrafine grain structure. No further structure damage or aggregation was detected for the CNTs after hot-rolling. The texture analysis demonstrates that the CNT/7055Al-E had a fiber texture, while the CNT/7055Al-ER had a plate texture including {011} , {113} and {215} components with higher texture intensity. Compared with the CNT/7055Al-E, the CNT/7055Al-ER increased twofold in elongation with a slight decrease in strength, which was attributed to the favorable grain orientations, the increased proportion of low angle grain boundaries, as well as the fine and densely distributed precipitates. Further, the mechanical property anisotropy of the composite was weakened after hot-rolling due to the elimination of coarse grain bands.

Deformation behavior and strengthening mechanisms in a CNT-reinforced bimodal-grained aluminum matrix nanocomposite

Abstract: The aim of this study was to identify deformation behavior and strengthening mechanisms of a carbon nanotube (CNT)-reinforced bimodal-grained Al–Cu–Mg nanocomposite and its base alloy fabricated by two-step ball milling, powder metallurgy and extrusion. A superior strength-ductility synergy stemming from the concurrent presence of ultrafine grains (UFGs) and coarse grains (CGs) was achieved. Singly-dispersed CNTs in UFGs and sound CNT/Al interfacial bond contributed to a significant improvement in the strength of the nanocomposite. The predominant strengthening mechanism in the CNT-reinforced nanocomposite was identified to be Orowan looping due to severe shearing of CNTs into nano-sized fragments during ball milling, along with load-transfer and thermal mismatch-induced dislocation strengthening mechanisms. The predicted yield strength of the nanocomposite was in agreement with the experimental value obtained. The findings in this study help pave the way for developing high-performance lightweight materials with a superior strength-ductility synergy via incorporating CNTs with novel bimodal grain structures.

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.

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

Abstract: 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.