The objective of this study is to provide insights and guidance for the rational design of high-performance metal matrix nanocomposites (MMNCs) with tunable functional properties for widespread applications. Microstructure-property relationship is a long-term focus for MMNCs, and incorporation of different nanoparticles would change these properties differently. Since the metals/alloys are unique with their high electron concentration and strongly coupled interaction between electrons and other configurational (micro-)structures, introducing nanoparticles into them will significantly influence electrical, thermal, chemical, and electrochemical properties. Given the various demands from industrial fields, MMNCs with predictable and reliable functional properties are urgently needed. However, there still exist significant challenges in utilizing optimal functional properties with a suitable combination of metals/alloys and nanoparticles. Several reasons contribute to the deadlock situations: First, the supreme functional properties would limit the selection of nanoparticles, but specific desired nanoparticles may be hard to fabricate, incorporate, and uniformly disperse in metals/alloys. Second, due to the lack of studies into nanoparticles-affected functional properties, the relationships between microstructures, processing routes, and final performance are too complicated to be determined. Therefore, though mechanical properties have been studied for decades, the lack of functional performance study hinders the use of metal matrix nanocomposites.
In this dissertation, a wide variety of MMNCs with a rational selection of nanoparticles were fabricated both in situ and ex situ to experimentally study the nanoparticle effects on their functional properties. Electrical, thermal, chemical (mainly anti-oxidation), electrochemical, and other selective functional properties (mainly tribological performance) were studied. The underlying mechanisms and (semi-)quantitative models have been investigated and developed to reveal nanoparticles’ role in tuning these properties, providing insightful guidelines for the rational design of high-performance MMNCs.
The metal matrices in MMNCs are full of free electrons, and the high concentration of electrons is the most crucial factor to determine the electrical performance and other functional properties in MMNCs. In this study, the electrical performance of MMNCs fabricated by both in situ and ex situ methods have been investigated. Both Cu and Al alloys as the highly conductive matrices have been studied, and the metal-like ceramic nanoparticles such as WC, TiC, TiB2, and ZrB2 were used. First, the electron performance was measured with large temperature scanning on physical property measurement system (PPMS), and the role of the electron concentration and the electron diffusivity have been decoupled in the MMNC system. The experiments demonstrated that the reduced electrical conductivity is associated with a reduced electron concentration by the interfacial electron localization. Second, with the understanding of the reduced apparent free electron concentration in MMNCs, a quantitative model was developed to depict the electrical conductivity change after the nanoparticle incorporation, and the feasibility of this model has been confirmed in Cu- and Al-based MMNC systems. This part of the study illustrates the fundamental role of the matrix-nanoparticle interface and explain the quantitative influences of the interfaces on the electronic parameters related to electrical conductivity. The developed model to predict electrical conductivity is necessary for the MMNC applications in electronic sectors.
With this understanding, since electrons are the dual carrier for both electricity and heat, the thermal performance of the MMNCs has been systematically investigated. In this study, ex situ nanocomposites of Cu alloy (i.e., Cu-Ag/WC) and in situ nanocomposites of Al (i.e., Al-TiC, Al-TiB2, and Al-ZrB2) were the primary focus. With the detailed microstructure investigation, matrix-nanoparticle interfacial characterization, and thermal parameter analyses, the influence of nanoparticles on the thermal performance of metal matrix nanocomposites has been clarified. The changes in heat capacity (by differential scanning calorimetry), thermal diffusivity (by laser flash method), and thermal conductivity have been decoupled. The contribution from electronic and phonic thermal transport has been compared. This part of the study confirms the electron behavior change by nanoparticles. It illustrates a semi-quantitative relationship and close links between the investigated electronic and thermal properties in MMNCs. The analyzed systems (i.e., Cu and Al nanocomposites) would be critical to rational design and applications of MMNCs in thermal management fields.
Similarly, due to the high concentration of electrons of metal matrices and their relatively high activity, even with the conductive ceramic nanoparticles, it is of interest to investigate how metal matrix nanocomposites respond to environments. Oxidation and corrosion are the two primary degradation forms of metals in the environment, potentially compromising their service life and significantly limiting their applications in various conditions. This part of the study would be divided into two sections accordingly: Firstly, how the high-temperature oxidation process is influenced and how the temperature-dependent stability is tuned by nanoparticles have been investigated. Quantitative information about the thermal oxidation in Cu-based (i.e., Cu-40 wt.% Zn/WC) and Al-based (i.e., Al/ZrB2) nanocomposites have been obtained via ex situ and in situ (mainly in situ XRD) measurement methods. Two distinctive oxidation layer growth modes (e.g., continuous growth in Cu-40 wt.% Zn/WC and self-limiting growth in Al/ZrB2 nanocomposites) have been identified, respectively. The thermal oxidation kinetics and dynamics in MMNCs have also been clarified. The interactions among nanoparticles, microstructures, and oxidation driving force have been studied, and the potential applications and effective prevention measures have also been proposed. Second, metal corrosion is a process associated with electron transfer and ion transport. During the process, corroded by-products (mainly oxides and hydro-oxides) will appear on the metal surface as a protection layer. Therefore, to understand the corrosion performance in MMNCs, the electron behavior and oxide growth were integrated. In this part of the study, aluminum alloy 7075 and A206 nanocomposites (i.e., wrought AA7075-TiB2 and AA7075-TiC as well as cast A206-TiC) were the main focus. During the experiments, the corrosion processes on the freshly exposed surface, passivated surface (with oxide layer), and passivated and then immersed surface (after being pitted) were compared, and their corrosion dynamical characteristics have been depicted with polarization potential scanning and electrochemical impedance scanning (EIS). Different corrosion performances, including pitting, intergranular corrosion, and stress crack corrosion, have been investigated under ASTM standards. The interplay between microstructures, oxidation, and corrosion has been quantitatively studied. In short, the corrosion study of Al nanocomposites has advanced the understanding of the corrosive degradation process. It would also fundamentally shed light on possible measures of promoting the overall anti-corrosion performance in MMNCs.
Moreover, given other essential applications of MMNCs, other functional properties linked with tribological and manufacturing fields have been investigated.
In summary, this dissertation’s extensive experimental studies have provided a useful fundamental understanding of the promising and tunable functional properties in various MMNCs. The starting point of electron behavior has created a unique angle to look into these functional properties by linking microstructures, electrons, and incorporated nanoparticles. Then, thermal properties, anti-oxidation performance, and anti-corrosion performance have been systematically studied, and the (semi-)quantitative models have been developed. Finally, tribological properties have been studied. This study advances the knowledge for rational design and manufacturing of high-performance MMNCs with desirable predictable functional properties for numerous applications
