Date of Award

August 2016

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Engineering

First Advisor

Pradeep K. Rohatgi

Committee Members

Chang-Soo Kim, Hugo Lopez, Rani Elhajjar, Ryoichi Amano

Keywords

Graphene, Mechanical Properties of Materials, Metal Matrix Composites, Nanocrystalline Materials, Powder Metallurgy, Strengthening Mechanisms

Abstract

Metal matrix composites (MMCs) and Metal Matrix Nano-composites (MMNCs) are promising materials for a number of aerospace, defense, and automobile applications. Among all MMCs and MMNCs, aluminum is the most widely used matrix due to its low density coupled with high stiffness, high specific strength, high specific modulus and low thermal expansion coefficient. While high strengths have been shown in MMCs, they are known to have very limited ductility. However, there are indications that reducing reinforcement size to the nanoscale may improve strain to failure in addition to increase strength. Reducing grain size to the nanoscale has been found to improve material properties specially strength until grain size of around 10 nm. From this preliminary evidence, it would appear that nanocrystalline (NC) metallic materials and NC MMNCs may result in the best properties. Yet, in these materials, the effect of processing conditions and reinforcement type, size, and concentration on material performance is not well understood.

Currently, mainly Powder Metallurgy (PM) techniques appear to be capable of generating NC metallic materials. Milling is used to mix matrix and reinforcement particles as well as reduce the size of particles. The majority of work in synthesizing NC metallic materials has employed cryomilling, which is milling of the metallic powders in a medium of liquid nitrogen (LN2) using stearic acid as a process control agent (PCA). After cryomilling, the milled powder is nanosize, but requires a high temperature/high vacuum degassing step to remove the PCA. However, there are several medium/PCA combinations that could be used that may produce the same results as the relatively complex and expensive LN2/Stearic acid combination. To date, these alternatives remain unexplored.

To consolidate the milled powders techniques such as HIP, CIP, single action compaction, extrusion, and forging can be used. However, to achieve close to theoretical densities it is necessary to perform the consolidation at high temperature. The high temperature causes coarsening of the powder grains and results in a larger grain size. Since it appears that the grain size is one of the main factors in determining the strength and ductility of the material, it is important to understand how reinforcement additions affect grain growth. Furthermore, plastic deformation seems to be required to achieve maximum density. Plastic deformation results in work hardening, which strengthens the material at the expense of ductility. The effect of reinforcement additions on work hardening has also received limited attention.

In order to better understand the effect of reinforcement type, size, and concentration on the processing and mechanical behavior of NC metals and MMNCs, pure Al was mixed with varying concentrations of graphene nanoplatelets (GNPs) and 47 nm alumina nanoparticles (Al2O3np). Instead of cryomilling, milling was conducted at room temperature in ethanol, where ethanol acted as both the medium and PCA. Degassing was accomplished by heating to only 135oC rather than to several hundred degrees. This processing method is considerably less complex and therefore less expensive and results in milled powders of the same size as those achieved by cryomilling followed by high temperature/high vacuum degassing.

The consolidation of the powders was conducted by single action cold compaction and single action hot compaction. This method should minimize textural effects that are produced by other consolidation techniques such as extrusion. Relative density (i.e. the density of the sample divided by the theoretical density) of the final consolidated samples reached nearly 100% in all graphene-reinforced samples regardless of graphene concentration; whereas in Al2O3np-reinforced samples the achievable densities were in the range of 85-95% of theoretical density depending on Al2O3np concentration. These are similar to the relative densities achieved by LN2 cryomilling/HIP/extrusion processing and shows that the room temperature ethanol/cold compaction/hot compaction is a viable alternative synthesis method.

As stated above, mechanical properties of the material are primarily governed by grain size and work-hardening and that it is likely that reinforcement additions have a significant effect on these properties. To understand the strengthening mechanisms of MMNCs, pure NC Al was reinforced with varying concentrations of Al2O3np and GNPs. The results show that i) room temperature milling in ethanol followed by a relatively low temperature drying treatment can produce NC Al and NC Al MMNCs with grain sizes comparable to materials produced by cryomilling, ii) grain boundary strengthening as described by the Hall-Petch relation accounts for the strength of Al- Al2O3np MMNCs, and iii) grain boundary strengthening, solute strengthening, and CTE mismatch strengthening also appear to contribute to the strength in Al-GNP MMNCs. To investigate the tribological behavior of aluminum matrix composites reinforced by GNPs and pure aluminum, pin-on-disk experiments were conducted on samples synthesized in the study. In the experiments, the influence of reinforcement, volume fraction, normal load, and sliding velocity on the tribological performance was investigated. Results showed that the wear rate of Al-1wt.% GNP is increased with increasing normal loads. However, the coefficient of friction (COF) of the Al-1wt.% GNP decreased with increasing normal loads. Formation of carbon film on the worn surface of Al-1wt.% GNP sample and morphology of the worn surfaces of aluminum and composite samples were analyzed by Optical Microscope (OM) and Scanning Electron Microscope (SEM). It was found that the graphene nanoplatelets reinforced nano-composites showed superior tribological properties and demonstrated the ability of the self-lubricating nature of the composite during tribological conditions.

Moreover, diametrical compression of small disk specimens was used to understand the mechanical properties of metal matrix nano composites. Analysis was performed using an inverse method that couples digital image correlation and the analytical closed form formulation. This technique was capable of extracting the tension and compression modulus values in the metal matrix nanocomposite disk specimens. This method for characterization of metal matrix composites have the potential to accelerate the development and study of advanced composite materials.

Available for download on Thursday, August 23, 2018

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