Influence of Tungsten Nanopowders on Enhancing the Aging …
125
Materials and Methods
Copper containing 2 volume percent in situ tungsten (W) nanoparticles was provided
by MetaLi LLC. Alloying was conducted to fabricate Cu-1.2Cr with approximately 2
volume percent tungsten (W) samples. The copper/tungsten [Cu/W] nanocomposite
was heated to 1250 °C in a graphite crucible by induction heating, with the surface of
the melt purged with argon (Ar) gas to prevent oxidation. Chromium (Cr) powders
wrapped in copper (Cu) foil were submerged in the melt, which was then held at
1250 °C for 30 min with manual mixing before cooling to room temperature. The
samples were then subjected to solution heat treatment at 1000 °C for 1 h, followed
by water quenching and aging at 460 °C for different durations ranging from 0.25 to
48 h. Cold rolling with various thickness reductions up to approximately 50% was
conducted on the Cu-1.2Cr/2 vol.% W nanocomposite (aged for 0.75 h) sample and
the Cu-1.2Cr (aged for 3 h) sample.
Specimens cut from the bulk samples were ground, polished, and then ion milled
(Gatan PIPS) at 4° and 4.5 kV for 1 h to clean the surfaces. The microstructure of the
samples was observed by scanning electron microscopy (SEM, Zeiss Supra 40VP)
equipped with energy-dispersive X-ray spectroscopy (EDS). The microhardness of
samples was measured using an LM 800AT microhardness tester with a load of 200 g
and a dwell time of 10 s.
Results and Discussion
The microstructure of the Cu-1.2Cr/2 volume percent tungsten after solution heat
treatment is shown in Fig. 1. As shown in Fig. 1a, the W nanoparticles form
nanoparticle-rich zones in the matrix. The magnified images of a nanoparticle-rich
zone and the alloy matrix are shown in Fig. 1 b, c respectively. The partial sintering
between adjacent tungsten (W) nanoparticles is observed, which likely occurred in
the melt during alloying. The melting point of the Cu–Cr increases significantly with
the increment of the chromium (Cr) content. To mitigate the sintering of tungsten
(W) nanoparticles, a proposed solution is to design a lower chromium content so that
the alloying temperature and time can be decreased. Besides, since 0.4 wt.% Cr can
be solutionized in copper (Cu) at 1000 °C theoretically, undissolved chromium (Cr)
can be observed in the sample indicated by arrows in Fig. 1b, c. As shown in Fig. 1d,
minor spinodal decomposition between chromium (Cr) and tungsten (W) occurred,
which can be attributed to the locally high concentration of chromium at the start of
the alloying process. Chromium and tungsten do not form intermetallic compounds
according to the Cr-W phase diagram, which otherwise would hurt the mechanical
properties of the nanocomposites. Instead, they can only form tungsten-rich and
chromium-rich solid solutions [16].
The microhardness evolution with the aging time of the Cu-1.2Cr/2 vol.% W
sample and the Cu-1.2Cr control sample is shown in Fig. 2. First, the pure copper–
chromium (Cu–Cr) sample requires 3 h of aging to reach the peak microhardness.
125
Materials and Methods
Copper containing 2 volume percent in situ tungsten (W) nanoparticles was provided
by MetaLi LLC. Alloying was conducted to fabricate Cu-1.2Cr with approximately 2
volume percent tungsten (W) samples. The copper/tungsten [Cu/W] nanocomposite
was heated to 1250 °C in a graphite crucible by induction heating, with the surface of
the melt purged with argon (Ar) gas to prevent oxidation. Chromium (Cr) powders
wrapped in copper (Cu) foil were submerged in the melt, which was then held at
1250 °C for 30 min with manual mixing before cooling to room temperature. The
samples were then subjected to solution heat treatment at 1000 °C for 1 h, followed
by water quenching and aging at 460 °C for different durations ranging from 0.25 to
48 h. Cold rolling with various thickness reductions up to approximately 50% was
conducted on the Cu-1.2Cr/2 vol.% W nanocomposite (aged for 0.75 h) sample and
the Cu-1.2Cr (aged for 3 h) sample.
Specimens cut from the bulk samples were ground, polished, and then ion milled
(Gatan PIPS) at 4° and 4.5 kV for 1 h to clean the surfaces. The microstructure of the
samples was observed by scanning electron microscopy (SEM, Zeiss Supra 40VP)
equipped with energy-dispersive X-ray spectroscopy (EDS). The microhardness of
samples was measured using an LM 800AT microhardness tester with a load of 200 g
and a dwell time of 10 s.
Results and Discussion
The microstructure of the Cu-1.2Cr/2 volume percent tungsten after solution heat
treatment is shown in Fig. 1. As shown in Fig. 1a, the W nanoparticles form
nanoparticle-rich zones in the matrix. The magnified images of a nanoparticle-rich
zone and the alloy matrix are shown in Fig. 1 b, c respectively. The partial sintering
between adjacent tungsten (W) nanoparticles is observed, which likely occurred in
the melt during alloying. The melting point of the Cu–Cr increases significantly with
the increment of the chromium (Cr) content. To mitigate the sintering of tungsten
(W) nanoparticles, a proposed solution is to design a lower chromium content so that
the alloying temperature and time can be decreased. Besides, since 0.4 wt.% Cr can
be solutionized in copper (Cu) at 1000 °C theoretically, undissolved chromium (Cr)
can be observed in the sample indicated by arrows in Fig. 1b, c. As shown in Fig. 1d,
minor spinodal decomposition between chromium (Cr) and tungsten (W) occurred,
which can be attributed to the locally high concentration of chromium at the start of
the alloying process. Chromium and tungsten do not form intermetallic compounds
according to the Cr-W phase diagram, which otherwise would hurt the mechanical
properties of the nanocomposites. Instead, they can only form tungsten-rich and
chromium-rich solid solutions [16].
The microhardness evolution with the aging time of the Cu-1.2Cr/2 vol.% W
sample and the Cu-1.2Cr control sample is shown in Fig. 2. First, the pure copper–
chromium (Cu–Cr) sample requires 3 h of aging to reach the peak microhardness.