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G. Yao et al.
improved the mechanical properties [5]. For this study, the hypothesis is as follows.
Chromium precipitates in nanoparticle-rich zones grew to larger sizes than those
outside the nanoparticle-rich zones when overaged, due to the higher growth rate
induced by the high diffusivity paths such as copper/tungsten interfaces, leading to
the lower hardness in the nanoparticle-rich zones. Meanwhile, it is hypothesized that
precipitates with a higher number density formed in the metal zones arising from a
higher Cr solute concentration, due to possibly enhanced Cr dissolution in solution
heat treatment or changed thermodynamics by W nanoparticles, leading to the higher
hardness.
The Cu–Cr/W nanocomposite exhibits higher thermal stability than pure Cu–Cr.
Poor softening resistance of Cu–Cr alloys at elevated temperatures has been reported
[5]. Extended aging of the samples leads to over-aging and decreased microhardness.
In this study, the microhardness of the nano-treated Cu–Cr during prolonged aging
almost plateaus from 2 up to 48 h. Meanwhile, the microhardness of the pure Cu–Cr
shows a continuously decreasing trend when aging time extends from 3 to 48 h.
After aging at 460 °C for 48 h, the overall hardness of nano-treated Cu–Cr is still
higher than the control sample. The high thermal stability of nanocomposites has
been reported [18]. Unlike Cr precipitates, W nanoparticles are thermally stable and
continuously induce strengthening in the sample even at high temperatures, which
leads to the high thermal stability of the nano-treated sample.
To indicate the evolution of Cr distribution during aging, detailed microstructural
characterizations of the nano-treated Cu–Cr are shown in Fig. 3. EDS line scan was
conducted in the solutionized sample across a W nanoparticle, as shown in Fig. 3a.
The corresponding counts of Cr and W signals along the line are shown in Fig. 3b. The
peak of the W signal coincides with the W nanoparticle with only background noise in
the matrix, which verifies the immiscibility of tungsten in copper. Chromium signal
is detected in the alloy matrix, indicating that chromium is solutionized in the matrix.
Besides, relatively high counts of Cr are detected inside the tungsten nanoparticles,
revealing that some chromium is also dissolved in the tungsten nanoparticles. On
the other hand, different chromium distribution is observed in the over-aged sample.
The EDS line scan results across a tungsten nanoparticle in Fig. 3c in the 48-haged nanocomposite sample are shown in Fig. 3d. A Cr signal peak appears at the
Cu/W interface, as indicated by the square in Fig. 3 c, d. Although more evidence is
needed especially TEM analysis, it indicates that copper/tungsten interfaces can act
as heterogeneous nucleation sites for chromium.
Cold rolling was applied to the optimally aged samples to further increase the
microhardness. The microhardness of the Cu–Cr/W (aged for 45 min) and the pure
Cu–Cr (aged for 3 h) with different thickness reductions by cold rolling is shown in
Fig. 4. Although effective hardness increase is achieved for both samples due to strain
hardening, the nano-treated Cu–Cr consistently shows a higher hardness than the pure
Cu–Cr. The microhardness of both samples saturates after a thickness reduction of
approximately 40%. After an approximately 40% thickness reduction, the microhardness of the nano-treated sample reaches around 194 HV. For future work, TEM
study will be conducted for a fundamental understanding of the nanoparticle-induced
precipitation formation and enhanced strengthening in the Cu–Cr alloy. Besides, the
G. Yao et al.
improved the mechanical properties [5]. For this study, the hypothesis is as follows.
Chromium precipitates in nanoparticle-rich zones grew to larger sizes than those
outside the nanoparticle-rich zones when overaged, due to the higher growth rate
induced by the high diffusivity paths such as copper/tungsten interfaces, leading to
the lower hardness in the nanoparticle-rich zones. Meanwhile, it is hypothesized that
precipitates with a higher number density formed in the metal zones arising from a
higher Cr solute concentration, due to possibly enhanced Cr dissolution in solution
heat treatment or changed thermodynamics by W nanoparticles, leading to the higher
hardness.
The Cu–Cr/W nanocomposite exhibits higher thermal stability than pure Cu–Cr.
Poor softening resistance of Cu–Cr alloys at elevated temperatures has been reported
[5]. Extended aging of the samples leads to over-aging and decreased microhardness.
In this study, the microhardness of the nano-treated Cu–Cr during prolonged aging
almost plateaus from 2 up to 48 h. Meanwhile, the microhardness of the pure Cu–Cr
shows a continuously decreasing trend when aging time extends from 3 to 48 h.
After aging at 460 °C for 48 h, the overall hardness of nano-treated Cu–Cr is still
higher than the control sample. The high thermal stability of nanocomposites has
been reported [18]. Unlike Cr precipitates, W nanoparticles are thermally stable and
continuously induce strengthening in the sample even at high temperatures, which
leads to the high thermal stability of the nano-treated sample.
To indicate the evolution of Cr distribution during aging, detailed microstructural
characterizations of the nano-treated Cu–Cr are shown in Fig. 3. EDS line scan was
conducted in the solutionized sample across a W nanoparticle, as shown in Fig. 3a.
The corresponding counts of Cr and W signals along the line are shown in Fig. 3b. The
peak of the W signal coincides with the W nanoparticle with only background noise in
the matrix, which verifies the immiscibility of tungsten in copper. Chromium signal
is detected in the alloy matrix, indicating that chromium is solutionized in the matrix.
Besides, relatively high counts of Cr are detected inside the tungsten nanoparticles,
revealing that some chromium is also dissolved in the tungsten nanoparticles. On
the other hand, different chromium distribution is observed in the over-aged sample.
The EDS line scan results across a tungsten nanoparticle in Fig. 3c in the 48-haged nanocomposite sample are shown in Fig. 3d. A Cr signal peak appears at the
Cu/W interface, as indicated by the square in Fig. 3 c, d. Although more evidence is
needed especially TEM analysis, it indicates that copper/tungsten interfaces can act
as heterogeneous nucleation sites for chromium.
Cold rolling was applied to the optimally aged samples to further increase the
microhardness. The microhardness of the Cu–Cr/W (aged for 45 min) and the pure
Cu–Cr (aged for 3 h) with different thickness reductions by cold rolling is shown in
Fig. 4. Although effective hardness increase is achieved for both samples due to strain
hardening, the nano-treated Cu–Cr consistently shows a higher hardness than the pure
Cu–Cr. The microhardness of both samples saturates after a thickness reduction of
approximately 40%. After an approximately 40% thickness reduction, the microhardness of the nano-treated sample reaches around 194 HV. For future work, TEM
study will be conducted for a fundamental understanding of the nanoparticle-induced
precipitation formation and enhanced strengthening in the Cu–Cr alloy. Besides, the