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R. Goswami et al.
the composites show higher hardness as compared to the base Al alloys. Furthermore,
it has been realized that the enhancement of mechanical properties of MMCs composites mostly depends on the interfacial characteristics of the metal/ceramic interface,
as these composites mostly fail as a result of interfacial de-bonding [5, 6].
Although significant progress has been made on aluminum-based MMCs, these
composite materials would not be useful for applications at elevated temperatures. In
number of applications, such as, heat sinks and electrical components, a moderate to
a high level of strength is necessary at relatively elevated temperatures. Copper-based
composites would be useful candidates owing to the high thermal, electrical conductivities and good elevated temperature strength. To improve the strength of copper,
solid solution strengthening mechanism can be applied, for example, solid solution
strengthening in copper–tin (Cu–Sn), copper–nickel (Cu–Ni) and copper–zinc (Cu–
Zn) alloys. However, it comes with a number of limitations as alloying of copper
results in a considerable loss in thermal and electrical conductivity. Alternatively,
previous work showed that copper can be strengthened by the incorporation fine
ceramic particulates, which minimizes the loss of thermal and electrical conductivity
[7]. Several powder metallurgical methods have been implemented to embed oxide
nanoparticles into the metallic matrix as opposed to the conventional techniques,
such as melting and casting. The conventional techniques result in segregation in the
melt due to differences in the density between the metal and ceramic particulates.
The typical methods for incorporating the ceramic particulates into copper matrix
are mechanical mixing or ball milling of copper powders with ceramic particulates
and internal oxidation. It has been shown that the internal oxidation process provides
the highest strength levels copper–aluminum-based MMCs [7]. In fact, a dispersion
strengthened copper has been commercially developed, which exhibits high strength
and high electrical and thermal conductivities. The mechanical properties of this
dispersion strengthened copper can be modified by varying the aluminum content.
The motivation of this research work is to investigate the fine scale microstructure
using transmission electron microscopy (TEM) and the strengthening mechanism of
the dispersion strengthened copper. In addition, the dispersion alumina oxide inhibits
recrystallization of the copper at relatively elevated temperatures. Therefore, it would
be of interest to study the microstructure in detail. Here, we demonstrate that the
internal oxidation of copper matrix containing small amount of aluminum produces
a fine dispersion of metastable gamma alumina (γ-Al 2 O 3 ) particles in copper matrix.
Furthermore, using the high-resolution TEM (HRTEM), we showed the formation
of extremely fine nanoparticles of copper oxide formed epitaxially throughout the
matrix. The improvement in modulus and hardness is shown to be associated with
the nanodispersion of copper oxide and alumina in copper matrix.
Experimental
We used commercial cold rolled plate of copper-1.5% alumina composite to investigate the fine scale microstructure. The rolled composite was subject to high strain
rate deformation at relatively high temperatures. The composite was characterized
R. Goswami et al.
the composites show higher hardness as compared to the base Al alloys. Furthermore,
it has been realized that the enhancement of mechanical properties of MMCs composites mostly depends on the interfacial characteristics of the metal/ceramic interface,
as these composites mostly fail as a result of interfacial de-bonding [5, 6].
Although significant progress has been made on aluminum-based MMCs, these
composite materials would not be useful for applications at elevated temperatures. In
number of applications, such as, heat sinks and electrical components, a moderate to
a high level of strength is necessary at relatively elevated temperatures. Copper-based
composites would be useful candidates owing to the high thermal, electrical conductivities and good elevated temperature strength. To improve the strength of copper,
solid solution strengthening mechanism can be applied, for example, solid solution
strengthening in copper–tin (Cu–Sn), copper–nickel (Cu–Ni) and copper–zinc (Cu–
Zn) alloys. However, it comes with a number of limitations as alloying of copper
results in a considerable loss in thermal and electrical conductivity. Alternatively,
previous work showed that copper can be strengthened by the incorporation fine
ceramic particulates, which minimizes the loss of thermal and electrical conductivity
[7]. Several powder metallurgical methods have been implemented to embed oxide
nanoparticles into the metallic matrix as opposed to the conventional techniques,
such as melting and casting. The conventional techniques result in segregation in the
melt due to differences in the density between the metal and ceramic particulates.
The typical methods for incorporating the ceramic particulates into copper matrix
are mechanical mixing or ball milling of copper powders with ceramic particulates
and internal oxidation. It has been shown that the internal oxidation process provides
the highest strength levels copper–aluminum-based MMCs [7]. In fact, a dispersion
strengthened copper has been commercially developed, which exhibits high strength
and high electrical and thermal conductivities. The mechanical properties of this
dispersion strengthened copper can be modified by varying the aluminum content.
The motivation of this research work is to investigate the fine scale microstructure
using transmission electron microscopy (TEM) and the strengthening mechanism of
the dispersion strengthened copper. In addition, the dispersion alumina oxide inhibits
recrystallization of the copper at relatively elevated temperatures. Therefore, it would
be of interest to study the microstructure in detail. Here, we demonstrate that the
internal oxidation of copper matrix containing small amount of aluminum produces
a fine dispersion of metastable gamma alumina (γ-Al 2 O 3 ) particles in copper matrix.
Furthermore, using the high-resolution TEM (HRTEM), we showed the formation
of extremely fine nanoparticles of copper oxide formed epitaxially throughout the
matrix. The improvement in modulus and hardness is shown to be associated with
the nanodispersion of copper oxide and alumina in copper matrix.
Experimental
We used commercial cold rolled plate of copper-1.5% alumina composite to investigate the fine scale microstructure. The rolled composite was subject to high strain
rate deformation at relatively high temperatures. The composite was characterized