Metallic glasses have been the subject of intense scientific study since the 1960s, owing to their unique properties such as high strength, large elastic limit, high hardness, and amorphous microstructure. However, bulk metallic glasses have not been used in the high strength structural applications for which they have so much potential, owing to a highly localized failure mechanism that results in catastrophic failure during unconfined loading. In this thesis, bulk metallic glass matrix composites are designed with the combined benefits of high yield strengths and tensile ductility. This milestone is achieved by first investigating the length scale of the highly localized deformation, known as shear bands, that governs fracture in all metallic glasses. Under unconfined loading, a shear band grows to a certain length that is dependent on the fracture toughness of the glass before a crack nucleates and fracture occurs. Increasing the fracture toughness and ductility involves adding microstructural stabilization techniques that prevent shear bands from lengthening and promotes formation of multiple shear bands. To accomplish this, we develop in-situ formed bulk metallic glass matrix-composites with soft crystalline dendrites whose size and distribution are controlled through a novel semi-solid processing technique. The new alloys have a dramatically increased room-temperature ductility and a fracture toughness that appears to be similar to the toughest steels. Owing to their low modulus, the composites are therefore among the toughest known materials, a claim that has recently been confirmed independently by a fracture mechanics group. We extend our toughening strategy to a titanium-vanadium-based glass-dendrite composite system with density as low as 4.97 g/cm3. The new low-density composites rival the mechanical properties of the best structural crystalline Ti alloys. We demonstrate new processing techniques available in the highly toughened composites: room temperature cold rolling, work hardening, and thermoplastic forming. This thesis is a proven road map for developing metallic glass composites into real structural engineering materials.

Designing metallic glass matrix composites with high toughness and tensile ductility

One-dimensional carbon nanotubes and two-dimensional graphene nanosheets with unique electrical, mechanical and thermal properties are attractive reinforcements for fabricating light weight, high strength and high performance metal-matrix composites. Rapid advances of nanotechnology in recent years enable the development of advanced metal matrix nanocomposites for structural engineering and functional device applications. This review focuses on the recent development in the synthesis, property characterization and application of aluminum, magnesium, and transition metal-based composites reinforced with carbon nanotubes and graphene nanosheets. These include processing strategies of carbonaceous nanomaterials and their composites, mechanical and tribological responses, corrosion, electrical and thermal properties as well as hydrogen storage and electrocatalytic behaviors. The effects of nanomaterial dispersion in the metal matrix and the formation of interfacial precipitates on these properties are also addressed. Particular attention is paid to the fundamentals and the structure–property relationships of such novel nanocomposites.

Recent progress in the development and properties of novel metal matrix nanocomposites reinforced with carbon nanotubes and graphene nanosheets

After almost three decades of intensive fundamental research and development activities, intermetallic titanium aluminides based on the ordered γ-TiAl phase have found applications in automotive and aircraft engine industry. The advantages of this class of innovative high-temperature materials are their low density and their good strength and creep properties up to 750 °C as well as their good oxidation and burn resistance. Advanced TiAl alloys are complex multi-phase alloys which can be processed by ingot or powder metallurgy as well as precision casting methods. Each process leads to specific microstructures which can be altered and optimized by thermo-mechanical processing and/or subsequent heat treatments. The background of these heat treatments is at least twofold, i.e., concurrent increase of ductility at room temperature and creep strength at elevated temperature. This review gives a general survey of engineering γ-TiAl based alloys, but concentrates on β-solidifying γ-TiAl based alloys which show excellent hot-workability and balanced mechanical properties when subjected to adapted heat treatments. The content of this paper comprises alloy design strategies, progress in processing, evolution of microstructure, mechanical properties as well as application-oriented aspects, but also shows how sophisticated ex situ and in situ methods can be employed to establish phase diagrams and to investigate the evolution of the micro- and nanostructure during hot-working and subsequent heat treatments.

Design, Processing, Microstructure, Properties, and Applications of Advanced Intermetallic TiAl Alloys†

Rapid innovation in nanotechnology in recent years enabled development of advanced metal matrix nanocomposites for structural engineering and functional devices. Carbonous materials, such as graphite, carbon nanotubes (CNT's), and graphene possess unique electrical, mechanical, and thermal properties. Owe to their lubricious nature, these carbonous materials have attracted researchers to synthesize lightweight self-lubricating metal matrix nanocomposites with superior mechanical and tribological properties for several applications in automotive and aerospace industries. This review focuses on the recent development in mechanical and tribological behavior of self-lubricating metallic nanocomposites reinforced by carbonous nanomaterials such as CNT and graphene. The review includes development of self-lubricating nanocomposites, related issues in their processing, their characterization, and investigation of their tribological behavior. The results reveal that adding CNT and graphene to metals decreases both coefficient of friction and wear rate as well as increases the tensile strength. The mechanisms involved for the improved mechanical and tribological behavior is discussed.

Mechanical and tribological properties of self-lubricating metal matrix nanocomposites reinforced by carbon nanotubes (CNTs) and graphene – A review

RGO flakes are homogeneously dispersed in a Cu matrix through a molecular-level mixing process. This novel fabrication process prevents the agglomeration of the RGO and enhances adhesion between the RGO and the Cu. The yield strength of the 2.5 vol% RGO/Cu nanocomposite is 1.8 times higher than that of pure Cu. The strengthening mechanism of the RGO is investigated by a double cantilever beam test using the graphene/Cu model structure.

Enhanced Mechanical Properties of Graphene/Copper Nanocomposites Using a Molecular-Level Mixing Process

https://ro.uow.edu.au/cgi/viewcontent.cgi?article=1069&context=eispapers

Reinforcement with graphene nanosheets in aluminum matrix composites

Microstructure refinement of cast TiAl alloys by β solidification

The invention discloses a preparation method of a graphene reinforced metal-matrix composite, which comprises the following steps of: firstly, dispersing the graphene oxide on the surface of the flaky metal powder; and then obtaining the graphene/metal composite powder through the reducing treatment; and at last, carrying out densification treatment by adopting a powder metallurgic technology to obtain the compact graphene reinforced metal-matrix composite. The flaky metal powder has the plane two-dimensional form, is inclined to the directional piling to form a laminated structure, and is helpful for inducing the graphene orientation distribution and giving play to the reinforcing effect. The preparation method disclosed by the invention is simple and feasible, is capable of regulating the graphene content and is suitable for preparing the massive composite.

Preparation method of graphene reinforced metal-matrix composite

Refining of coarse lamellar microstructure of TiAl alloys by rapid heat treatment

The invention discloses a graphene and carbon nanotube mixed enhanced metal-matrix composite material and a preparation method thereof. The graphene and carbon nanotube mixed enhanced metal-matrix composite material is characterized in that graphene and a carbon nanotube are mutually connected to constitute an enhanced network in a metal matrix, wherein the graphene is few-layer graphene with 10 layers or less, the radius-thickness ratio of the graphene is larger than 200, and the volume fraction of the graphene is 0.1-1%; and the carbon nanotube is a single-wall, double-wall or multi-wall carbon nanotube, the length-diameter ratio of the carbon nanotube is larger than 20, and the volume fraction of the carbon nanotube is 0.5-5%. Compared with the composite material enhanced only by the carbon nanotube, the graphene and carbon nanotube mixed enhanced metal-matrix composite material disclosed by the invention not only has greatly improved mechanical properties, but also has more excellent electric conduction and heat conduction properties, and is a multi-purpose structure and function integrated material. In addition, the preparation method provided by the invention based on slurry blending and graphene oxide reduction is simple and efficient and is easy for large-scale production.

J.N. Wang

Papers

Reinforcement with graphene nanosheets in aluminum matrix composites

Microstructure refinement of cast TiAl alloys by β solidification

Preparation method of graphene reinforced metal-matrix composite

Refining of coarse lamellar microstructure of TiAl alloys by rapid heat treatment

Graphene and carbon nanotube mixed enhanced metal-matrix composite material and preparation method thereof