Influence of Tungsten Nanopowders on Enhancing the Aging …
127
The Cu1.2Cr/2 vol.% W sample reaches peak aging condition after 45–60 min,
suggesting that tungsten (W) nanoparticles can facilitate the aging process. Second,
the nanoparticle-rich zones and metal zones of the Cu1.2Cr/2 vol.% W sample
exhibit distinct aging behaviors. The nanoparticle-rich zones reach peak condition 15 min earlier than the metal zones in Cu1.2Cr/2 volume percent tungsten
(W), further confirming that tungsten nanoparticles accelerate aging. The facilitated
aging possibly results from the enhanced heterogeneous nucleation and growth of
chromium precipitates induced by the nanoparticle/matrix interfaces and the dislocations generated by nanoparticles. Atomic-scale investigation of Al–Zn–Mg–Cu
matrix nanocomposite with uniformly distributed titanium diboride (TiB 2 ) nanoparticles revealed that the semi-coherent TiB 2 /Al interfaces reduced precipitate nucleation energy barrier and acted as short-circuit diffusion paths for transporting solute
atoms and vacancies to accelerate the growth rate of precipitates [17]. Despite the
distinct microhardness evolution, the nanoparticle-rich zones and metal zones of the
nanocomposite sample reach similar hardness at the aging time of 45 min, which will
ensure the homogeneous mechanical properties of the nanocomposite sample. Thus,
the optimal aging time of the copper chromium (Cu–Cr)/W nanocomposite is 45 min.
The average microhardness of the Cu–Cr/W nanocomposite sample aged for 45 min
and the peak-aged Cu–Cr control sample is 160.74 ± 8.22 HV and 150. 46 ± 4.34
HV, respectively. An increase of 10.28 HV in microhardness after aging is achieved
by the addition of W nanoparticles. The enhanced strengthening possibly results from
the modified Cr precipitate formation induced by the tungsten (W) nanoparticles, the
Orowan strengthening by the tungsten (W) nanoparticles, and the Hall–Petch effect
from grain refinement by W nanoparticles.
The microhardness of the metal zones in Cu–Cr/W becomes notably higher than
that of the nanoparticle-rich zones when the sample is over-aged starting from 1 h
despite the lower hardness of the metal zones before aging. More study is still needed
to explain this novel phenomenon. It is proposed that the modified chromium precipitation thermodynamics and kinetics induced by tungsten nanoparticles are responsible. The microhardness of the aged sample is related to the concentration, size,
and type of the precipitates [8], which can be modified by the addition of the tungsten nanoparticles. The shape, size, orientation relationship, and phase transportation
of the chromium precipitates are still under discussion because large lattice distortions caused by nanoscale chromium precipitates make them difficult to observe
under TEM, especially at the early stage of nucleation [3, 4]. The equilibrium crystallographic structure of chromium is body-centered cubic [bcc], but it could have
metastable structures in the early stage of precipitation [4]. The sequence of precipitate formation in Cu-0.71 wt% Cr aged at 450 °C was reported to be super-saturation
solid solution, GP zones (fcc Cr-rich phase), fcc Cr phase, order face-centered cubic
(fcc) chromium (Cr) phase, and bcc chromium (Cr) phase [3]. The addition of other
alloying elements, such as zirconium (Zr), magnesium (Mg), and iron (Fe), can
modify the chromium (Cr) precipitation kinetics. It was reported that additional Mg
in Cu–Cr alloy accelerated the process of precipitate nucleation with enriched magnesium areas as nucleation centers and restrained the precipitate growth by magnesium
segregation along the precipitate surface, which reduced the precipitate size and
127
The Cu1.2Cr/2 vol.% W sample reaches peak aging condition after 45–60 min,
suggesting that tungsten (W) nanoparticles can facilitate the aging process. Second,
the nanoparticle-rich zones and metal zones of the Cu1.2Cr/2 vol.% W sample
exhibit distinct aging behaviors. The nanoparticle-rich zones reach peak condition 15 min earlier than the metal zones in Cu1.2Cr/2 volume percent tungsten
(W), further confirming that tungsten nanoparticles accelerate aging. The facilitated
aging possibly results from the enhanced heterogeneous nucleation and growth of
chromium precipitates induced by the nanoparticle/matrix interfaces and the dislocations generated by nanoparticles. Atomic-scale investigation of Al–Zn–Mg–Cu
matrix nanocomposite with uniformly distributed titanium diboride (TiB 2 ) nanoparticles revealed that the semi-coherent TiB 2 /Al interfaces reduced precipitate nucleation energy barrier and acted as short-circuit diffusion paths for transporting solute
atoms and vacancies to accelerate the growth rate of precipitates [17]. Despite the
distinct microhardness evolution, the nanoparticle-rich zones and metal zones of the
nanocomposite sample reach similar hardness at the aging time of 45 min, which will
ensure the homogeneous mechanical properties of the nanocomposite sample. Thus,
the optimal aging time of the copper chromium (Cu–Cr)/W nanocomposite is 45 min.
The average microhardness of the Cu–Cr/W nanocomposite sample aged for 45 min
and the peak-aged Cu–Cr control sample is 160.74 ± 8.22 HV and 150. 46 ± 4.34
HV, respectively. An increase of 10.28 HV in microhardness after aging is achieved
by the addition of W nanoparticles. The enhanced strengthening possibly results from
the modified Cr precipitate formation induced by the tungsten (W) nanoparticles, the
Orowan strengthening by the tungsten (W) nanoparticles, and the Hall–Petch effect
from grain refinement by W nanoparticles.
The microhardness of the metal zones in Cu–Cr/W becomes notably higher than
that of the nanoparticle-rich zones when the sample is over-aged starting from 1 h
despite the lower hardness of the metal zones before aging. More study is still needed
to explain this novel phenomenon. It is proposed that the modified chromium precipitation thermodynamics and kinetics induced by tungsten nanoparticles are responsible. The microhardness of the aged sample is related to the concentration, size,
and type of the precipitates [8], which can be modified by the addition of the tungsten nanoparticles. The shape, size, orientation relationship, and phase transportation
of the chromium precipitates are still under discussion because large lattice distortions caused by nanoscale chromium precipitates make them difficult to observe
under TEM, especially at the early stage of nucleation [3, 4]. The equilibrium crystallographic structure of chromium is body-centered cubic [bcc], but it could have
metastable structures in the early stage of precipitation [4]. The sequence of precipitate formation in Cu-0.71 wt% Cr aged at 450 °C was reported to be super-saturation
solid solution, GP zones (fcc Cr-rich phase), fcc Cr phase, order face-centered cubic
(fcc) chromium (Cr) phase, and bcc chromium (Cr) phase [3]. The addition of other
alloying elements, such as zirconium (Zr), magnesium (Mg), and iron (Fe), can
modify the chromium (Cr) precipitation kinetics. It was reported that additional Mg
in Cu–Cr alloy accelerated the process of precipitate nucleation with enriched magnesium areas as nucleation centers and restrained the precipitate growth by magnesium
segregation along the precipitate surface, which reduced the precipitate size and