Materials Science & Engineering A 724 (2018) 164–170
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Materials Science & Engineering A
journal homepage: www.elsevier.com/locate/msea
Tensile behaviors of pure copper with different fraction of nonequilibrium
grain boundaries
T
⁎
Yunpeng Wanga,b, Ruidong Fua,b, , Lei Jinga,b, Deli Sanga,b, Yijun Lia,b
a
b
State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao, Hebei 066004, PR China
College of Materials Science and Engineering, Yanshan University, Qinhuangdao, Hebei 066004, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords:
Nanocrystalline materials
Mechanical properties
Microstructure
Grain boundaries
Copper
Pure copper with different fraction of nonequilibrium grain boundaries were achieved by friction stir processing
(FSP) under air, water and liquid nitrogen cooling conditions. Tensile behaviors at room temperature exhibited
significant difference for above three cases involving different fraction of nonequilibrium grain boundaries. The
case with nitrogen cooling showed better combination of strength and elongation for the largest fraction of high
energy nonequilibrium boundaries, which contribute to emit dislocations from grain boundaries and suppress
grain boundary sliding. Fully relaxed grain boundaries in air cooling samples can suppress the grain boundary
sliding and dislocation emission causing high stress and very low elongation. However, appropriate relaxed grain
boundaries in the water cooling samples will promote grain boundary sliding and the increase of elongation. The
grain coarsening during tensile deformation was observed in those samples with nonequilibrium grain boundaries and will increase elongation and cause work softening behavior of these samples.
1. Introduction
It is well known that the strength or hardness of metals increases
with decreasing grain sizes following the classical Hall-Petch relationship [1,2]. When grain size reduces into the submicrometer or nanometer scale, however, the experimental observations are mixed due to
the higher density of grain boundaries (GBs). In these cases, continuous
hardening of some materials was detected [3,4], and softening attributed to grain coarsening was also reported for fine-grained samples
during deformation [5,6]. The different observed behaviors mean that
an ambiguity remains as to the governing plastic deformation mechanism in the fine-grained materials with high density GBs. Recently,
Lu et al. found that hardness of electrodeposited nano-grained (NGed)
Ni–Mo alloy were adjustable with GB stabilization through relaxation
and Mo segregation, and the reduced GB energy stabilizes the NGed
structures as the thermodynamic driving force for grain coarsening is
lowered [7]. It demonstrates that the characteristic of the GBs strongly
influences the strength and hardness of fine-grained materials.
For the achievement of fine-grained materials, severe plastic deformation is a promising route. Along with the modification of the grain
structure towards finer grains, the higher number of lattice defects that
are created lead to the modifications of the GB structure during continued strain [8]. High number of densities of extrinsic dislocations
accumulated in the GBs increase the specific energy state and free
⁎
volume of the GBs to form the so-called nonequilibrium GBs [9,10]. The
nonequilibrium GBs with increased specific excess energy states
markedly affect the transport properties of ultrafine grained (UFGed)
materials [11,12], and the results indicate strongly that a distribution of
GBs with a rather distinct distribution of their nonequilibrium character
forms result in a nonuniform distribution of specific mobilities of the
GBs. It can be speculated that the distinct distributions of nonequilibrium GBs should also have a certain impact on the mechanical
properties of fine-grained materials, but at present, there are few specific studies on the respect. Thus, it is necessary to take sight into the
deformation mechanism of fine-grained materials with different fraction and energy state nonequilibrium GBs.
In this work, we prepared three fractions of nonequilibrium GBs in
pure copper by using friction stir processing (FSP) with a shoulderless
conical tool and different cooling conditions. Microstructures in the
processed zones (PZs) in term of grain size and nonequilibrium GBs
were characterized. The tensile property and deformation mechanisms
of the PZs were investigated in detail.
2. Experimental
The commercially pure copper (99.9%) with dimensions
120 mm × 40 mm × 3 mm were chosen as experimental materials. The
FSP was performed using a shoulderless conical tool. Three cooling
Corresponding author at: State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao, Hebei 066004, PR China.
E-mail address: rdfu@ysu.edu.cn (R. Fu).
https://doi.org/10.1016/j.msea.2018.03.086
Received 11 November 2017; Received in revised form 20 March 2018; Accepted 21 March 2018
Available online 22 March 2018
0921-5093/ © 2018 Elsevier B.V. All rights reserved.
Materials Science & Engineering A 724 (2018) 164–170
Y. Wang et al.
the PZs under three cooling conditions. It shows that the grains in the
PZs are equiaxed and significantly refined for the three cases. As the
shoulderless tool is used during FSP under the three conditions, the
microstructure in each area of the PZs is basically the same through the
observation of TEM. Moreover, there are few dislocations in the interior
of grains. The characteristic of spreading of boundary thickness extinction contours is observed in some grains in NC (Fig. 2a) and WC
(Fig. 2b) samples. This grain boundary characteristic indicates the existence of nonequilibrium GBs with high-level stress concentrations
[10], which are generally observed in the UFGed or NGed materials
refined by severe plastic deformation. The extinction contours are relatively most obvious in the NC samples and the nonequilibrium GB
characteristic has been verified through the high angle GBs are tightly
surrounded by many low angle GBs by ASTAR technique in Ref. [13]. In
comparison, the straight and sharp GBs in the AC sample indicate stabilized equilibrium GBs with few extrinsic dislocation accumulations.
Meanwhile, the mixture feature with the grain boundaries in the NC
and AC samples can be found in the WC samples. It implies the fraction
of nonequilibrium GBs in the WC samples should be between that of the
NC and AC samples. The grain size distribution histograms (Fig. 2d–f)
were calculated from a large number of TEM images for the three cases.
It shows that the grain size of the NC samples ranges from 25 to 228 nm
with 45.3% of NGs, while there is a broad grain size distribution (from
90 to 405 nm) and only 1.7% of NGs in the WC samples. In the AC
samples, the grain size has increased into the range from 110 to 560 nm
and no NGs are found. Such microstructure of the AC samples is relaxed
structures with equilibrium GBs because of the higher peak temperature
and lower strain during FSP. The statistic of average grain size is 109,
220 and 259 nm for the NC, WC and AC samples, respectively. As
mentioned above, the smaller grain size is, the larger the strain undergone by the deformed metal is. In another word, the fraction of the
nonequilibrium GBs or the internal energy should be the highest in the
samples with the smallest grain size.
Fig. 3 shows the orientation maps and statistic GB misorientation
distributions of the three samples. Considering the difference of the
grain size of the three samples, ASTAR™ system is used for the NC
samples to obtain more accurate grain structure information and EBSD
is used for the WC and AC samples to obtain grain structure information
of more amount of grains. The TEM microstructures are further confirmed by the ASTAR and EBSD. The equiaxed grains are surrounded by
the high angle GBs (> 15°) with grain sizes of 118 nm, 216 nm and
265 nm for NC, WC and AC samples, respectively. The grain sizes are
similar to the statistical results of TEM. There are few low angle GBs in
the interior of grains. It is noted that the distributions of grain boundary
misorientation angles of the three samples are similar. And the large
fraction of low angle GBs in NC samples is because of the small step size
can obtain more micro information in the interior of grains as Fig. 3a
shown.
It has been known that the internal energy of deformed metal can
indirectly reflected the fraction of nonequilibrium GBs [15]. Thus, DSC
was employed to find the variation of the internal energy in the PZs
with different cooling conditions (Fig. 4). As shown in Fig. 4, no visible
thermal (exothermic or endothermic) effect is detected in the BM and
AC samples during the heating process. When the WC samples is heated,
a rather weak exothermic signal is observed in the DSC curve at about
215 °C (Tp) from 175 °C (Ton) to 250 °C (Tend), of which the integrated
enthalpy (or enthalpy release) is about 0.32 J/g. In the NC samples, a
strong exothermic reaction is detected at a temperature range from
about 100 °C (Ton) to 215 °C (Tend). And Tp is about 150 °C. The exothermic peak during which a total integrated enthalpy change of about
1.47 J/g was detected. It is much higher than that in the WC samples
and is about twice that in the UFGed Cu deformed by equal-channel
angular pressing (0.8 J/g) [16].
The exothermic reaction in these samples is attribution to the static
recrystallization process during DSC heating [17]. The micro-strain
release process of the deformed materials causes the onset of
conditions, named air cooling (AC), water cooling (WC) and liquid nitrogen cooling (NC), were employed during FSP. For the WC and NC
samples, the plates were immerged in cooling medium during whole
process of FSP. The shoulderless tool, setup and processing steps can be
found in Ref. [13]. The tool traveled at 20 mm/min with rotational
speed of 1200 rpm for the three cases.
Microstructural examination was completed with transmission
electron microscopy (TEM). TEM observation was carried out on a
JEOL-2010 microscope operating at 200 kV. Thin foils for TEM cut from
the PZs were twin-jet electropolished by a solution of 30% nitric acid
and 70% methanol at 263 K. The GB character and distribution were
confirmed by ASTAR™ system installed in the NanoMEGAS Precession
Electron Diffraction platform for the NC sample and high-resolution
electron backscatter diffraction (EBSD) for the WC and AC samples. In
the ASTAR™ system, diffraction patterns of the TEM samples were recorded with a 0.3° procession angle and a scanning step size of 3 nm.
EBSD scans were performed on a SU5000 scanning electron microscope
with a step size of 20 nm. The results were all analyzed using an TSL
OIM system. Differential scanning calorimetry (DSC; Discovery DSC
250 instrument) was used to study the thermal characteristics of the
samples. Aluminum pans were used for both the sample and the reference. The three samples were sealed in aluminum pans and heated in
a flowing argon atmosphere at a constant heating rate of 5 °C/min from
25 °C to 300 °C.
For the tensile test, the dog-bone-shaped specimens with a gauge
length of 5 mm and a width of 2 mm were machined from the same
location, on the top surface and paralleled to the processing direction of
the PZs, and then polished to a thickness of 0.35 mm. Uniaxial tensile
tests were conducted at room temperature at an initial strain rate of
1 × 10−3 s−1 using an Instron-5948 MicroTester with a video noncontact extensometer.
3. Results
Fig. 1 shows the cross-sectional macrographs of the defect free PZs
formed under three cooling conditions. The boundary (arrowed in
Fig. 1a) between the PZ and base metal (BM) looks very clear on both
advanced (AS) and retreated side (RS) for the three cases. The area of
the PZs decreases with enhancing the cooling rate, i.e., the PZ area of in
AC samples is largest, followed by WC samples, and NC samples. The
effect of “hard shell” [14], resulting from the restrain effect of the cold
BM to the deformed metal in the PZs, can account for the formation of
the PZs. Accordingly, the difference in PZ areas under three conditions
can also be easily understood.
Fig. 2 shows the microstructures and the grain size distributions of
Fig. 1. Cross-sectional macrostructures of the PZs for samples: (a) NC; (b)WC and (c) AC.
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Fig. 2. TEM bright images and grain size distributions of (a, d) NC, (b, e) WC and (c, f) AC samples, respectively.
Fig. 3. Orientation maps and statistic GB misorientation distributions of (a, d) NC, (b, e) WC and (c, f) AC samples, respectively. High angle boundaries (> 15°) and low angle boundaries
(< 15°) are sketched by black and white lines, respectively.
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Fig. 5. Engineering stress–strain curves of NC, WC, AC samples and fully annealed
copper.
Fig. 4. Typical DSC curves for the samples of BM, NC, WC and AC at a heating rate of
5 °C/min.
samples, and is evidently higher than AC samples. It is should be noted
that the dislocation density calculated by XRD contains the dislocations
in the interior of grains and on the GBs and the distribution of nonequilibrium GBs in the samples is nonuniform, so the calculated γGB of
the samples is lower than the actual results. It evident that the GBs in
the NC samples is in the high energy states. The results proved that the
fraction of nonequilibrium GBs with high energy state is largest in the
NC samples. In the WC samples, the fraction of nonequilibrium GBs is
small. And there is basically no nonequilibrium GBs in the Ac samples.
The comparisons of engineering tensile stress–strain curves among
the three samples and fully annealed copper are shown in Fig. 5. It can
be found that the four samples exhibit completely distinct tensile
properties. Although the NC and AC samples exhibit different yield
strength (σy) of 381 MPa and 457 MPa, respectively, they show the
nearly equal ultimate tensile strengths (σUTS) of ~ 550 MPa, which was
approximately three times that of fully annealed copper. However, the
elongation (δf) of the AC samples is very low, only ~ 2.5%, compared
with the elongation of about 25% for the NC samples. By contrast, the
WC sample shows relatively lower yield strength of ~ 360 MPa, ultimate strength of ~ 420 MPa and moderated elongation of ~ 8%. The
above variations in tensile properties can not be explained if only
considering the difference in grain size. Moreover, the tensile softening
behaviors after peak stress also indicate that the tensile deformation
mechanisms must relate to a dynamic evolution of the microstructures
or GB characteristics in three samples during tensile deformation.
recrystallization. For the BM and AC samples, no thermal effect is detected in the measured temperature range is the evidence that stored
internal energy corresponding to the micro-strain is much lower. In
contrast, the micro-strain in the WC samples is relatively larger and the
NC samples contain the largest micro-strain. Besides, the onset temperature of recrystallization reflects how much internal energy is stored
inside materials. The lowest onset temperature of NC samples also
implies the largest stored energy in the samples. In the UFG/NGed
materials, micro-strain is closely related to the GB structure, which
determine the GB energy [15]. Therefore, it is reasonable to consider
the GB energy by the DSC results, especially the micro-strain in the
FSPed samples as a signature of few dislocation in the interior of grains.
The stored internal energy is mainly concentrated in the GBs under such
case involving nonequilibrium GBs. It also should be noted that the
exothermic peak may be observed in the temperature above 300 °C due
to the large fraction of equilibrium GBs in the AC samples than that of
coarse grained BM.
Based on the stored internal energy measured by the DSC, the
specific GB energy (γGB) in the NC and WC samples can be roughly
estimated. In the UFG and NG materials, the majority of stored energy
released during recrystallization is attributed to the disappearance of
high density of GB and lattice dislocations. The energy released from
the other defects such as vacancies and the associated elastic strain
energy are minor and not significantly affecting the analysis present
here [18]. Consequently, the stored energy (ΔH) and specific GB energy
(γGB) can be described as [19]:
4. Discussion
1
∆H = ⋅Ed⋅ρ g + EGB
ρ
(1)
4.1. Effects of nonequilibrium grain boundary on the tensile behavior
γGB = ρ⋅EGB / S
(2)
As shown in Fig. 5, the tensile behaviors of the three PZs show no
monotonic variation with the cooling conditions, which indirectly reflect the grain size in the PZs. Thus, the plotting of various tensile
property involving yield strength (σs), ultimate strength (σb), uniform
elongation (δun) and fracture elongation (δf) versus grain size are illustrated in Fig. 6. If only considering the relation between the strength
and grain size, the AC samples shows obvious deviation to the classical
Hall-Petch relationship. For example, the grain size in the AC samples is
the largest among three cases, but the strength is higher than that of the
WC samples with smaller grain size.
Indeed, the issues on the deviation to the classical Hall-Petch relationship has been reported for the microstructure with NGs [22] or
UFGs [23]. The inverse Hall-Petch effect has been frequently found in
the nanocrystalline materials because the GB sliding or other proposed
models is the main mechanism during tensile deformation [24,25]. In
addition, Du et al. reported that the sub-grain boundary in UFGs had
also contribution to the strength in FSPed metal of high nitrogen
where ρ is the density of Cu samples, Ed is the energy per unit length of
a dislocation, ρg is the dislocation density in the interiors of grain or cell
of the samples, EGB is the GB energy, and S is the GB area in the unit
volume (S ~ 3/D, D is the average grain size of the sample). Here, an
average value of 5 × 10−9 J/m is taken for Ed in Eq. (1) without
making any distinction between edge/screw dislocations, complete/
partial dislocations [18]. And in this work, the dislocation density
within the grains is calculated by X-ray diffraction [20]. The calculated
result is ~ 3.45 × 1014 m−2 for NC samples and 1.87 × 1014 m−2 for
WC samples, respectively, which is much lower than that of the reported results from other severe plastic deformation of Cu (~
1.0 × 1015 m−2) [16,21]. The low dislocation density of the samples is
consistent with the observation of the TEM images in Fig. 2. The calculated γGB is 0.41 J/m2 and 0.14 J/m2 for NC and WC samples, respectively. The γGB of NC samples is much higher than that of WC
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samples in this work is much smaller. Other mechanism may play a role
in the mechanical properties of the Cu samples in the three cases. The
deformed microstructures near the fractured positions of the tensile
samples are shown in Fig. 7. In the deformed regions of the NC and WC
samples, the grains still maintain equiaxed state and there exist the
features of dislocation pile-up in some UFGs. The magnified TEM
images (inset in Fig. 7a and b) show that the dislocations are emitted
from the wavy grain boundaries with high-level stress concentrations,
namely the nonequilibrium GBs [26]. In comparison, the deformed
microstructures in the AC samples are still equiaxed grains without high
density dislocations.
Valiev RZ reported that the nonequilibrium GBs can affect the GB
sliding to facilitate the recovery process leading to a softening effect on
the mechanical properties in ECAP Cu [27]. The fact that grains remained equiaxed after deformation is an indirect evidence of the GB
sliding mechanism, which is the possible deformation mode in UFG/NG
materials [4]. In the NC samples, the deformation mechanism has been
revealed in detail elsewhere [13]. The dislocations emitted from the
high energy stated nonequilibrium GBs pile up in UFGs, and the interaction between the GBs of NG and dislocations originated from
nonequilibrium GBs, combined with the GB sliding are responsible for
the outstanding mechanical properties. In this deformation mechanism,
the effect of GB sliding is suppressed by the large fraction high energy
state nonequilibrium GBs. Then more applied stress is needed to make
the sample occur plastic deformation causing the high yield stress of the
NC samples. And the dislocation pile-up in the UFGs and interaction of
the dislocation and GBs in the NGs increase the strain hardening rate to
a certain extent reduce the softening of tensile samples leading to the
relative high ultimate strength and elongation of the samples. In addition, Valiev RZ et al. found that the GB sliding is sensitive to the GB
energy states and low temperature annealing of UFGed Ti could change
the strength and ductility by changing its GB energy states [28]. In the
WC samples, the relative low proportion low-energy stated nonequilibrium GBs, like the GBs of the NC samples after appropriate annealing process, may promote the GB sliding process, resulting the
ductility in a certain degree. While the GB energy state still contributes
to dislocation emission from nonequilibrium GBs. Premature GB sliding
makes the WC samples occur plastic deformation early with low yield
strength, while the dislocation pile-up also has a certain inhibitory effect on the rapid softening of tensile deformation attributes to the ultimate tensile strengths. However, in the AC samples, it will be more
difficult for the fully relaxed stabilized GBs to emit dislocations and
undergo GB sliding because of the reduction of GB defects in GBs. The
suppressed GB sliding will make the AC samples reach a higher strength
with very low ductility under applied stress. The large number of dislocation involved in the early stage of deformation of NC samples
Fig. 6. Strength and elongation versus grain size for the three sets of samples.
austenitic steel [23]. They modified the classical Hall-Petch equation by
introducing the effective grain size based on the statistic of the high
angle GBs and low angle GBs. However, besides the effect of grain size,
the fraction of sub-grain boundaries in the AC sample (as Fig. 3 shown)
is too low for the relatively high deformation temperature, thus it can
not provide enough strengthening effect in this case. Moreover, the
similar ratio and distribution of GBs in the three samples indicate that
the GB ratio and distribution has negligible impact on the mechanical
properties in the three cases.
Recently, the effects of nonequilibrium GBs on the low temperature
superplasticity in deformed metals have been reported [26]. The AZ91
alloy with nonequilibrium GBs exhibits lower superplastic elongation
than the alloy with equilibrium GBs because dislocation movement is
hampered and GB sliding is less accommodated by the long-range
stresses associated with the nonequilibrium GBs. The result implies that
the energy state of the GBs can also play an important role in the
strengthening of metals. According to the calculated results of the γGBs,
it can be inferred that the existence of nonequilibrium GBs in the NC
and WC samples is an important factor affecting the mechanical properties.
4.2. The deformation mechanism and grain growth during tensile
deformation
For the UFG materials produced by severe plastic deformation with
high dislocation density the dislocation bow-out model was utilized to
interpret the flow stress [27], however, the dislocation density of
Fig. 7. TEM images of the fractured positions taken from samples of: (a) NC; (b) WC and (c) AC. The TEM images inset in (a) and (b) are the magnified images of the position red box
marked, respectively.
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Fig. 8. Area-weighted grain size distributions of different samples before and after tensile deformation: (a), (b): NC; (c), (d) WC and (e), (f): AC, as indicated.
caused by grain coarsening during tensile deformation. Grain coarsening combined with the effect of nonequilibrium GBs acting on NGs
and UFGs make the NC samples exhibit optimum mechanical properties
in the three sets of samples. The enhanced effect of grain size and
nonequilibrium GBs may be higher than the softening effect of grain
coarsening in NC samples.
Based on above discussion, it is noted that the different energy state
nonequilibrium GBs in the grains have different effects on the room
temperature tensile properties. For GB sliding, high energy state
nonequilibrium GBs and stabilized equilibrium GBs can suppress the
process. When the energy state of GBs reaches an appropriate degree,
GB sliding process will be easy to happen in the room temperature
during tensile deformation. The high energy nonequilibrium GBs promote dislocation emission from the GBs. And when the GBs maintain
high stored energy, mechanical induced grain coarsening will occur.
The grain coarsening will cause a high elongation in the deformation
and smaller effect of work softening. The present results suggest that all
the factors related to the internal stored energy, nonequilibrium state of
GBs in the refined materials will have a direct impact on the mechanical
properties and microstructure stability during subsequent deformation.
allows smaller applied stress to make the samples plastically deformed
leading to the yield stress of NC samples is lower than that of AC
samples.
The other noteworthy phenomenon occurred during tensile deformation is the grain growth in the samples. The grain coarsening is
described more quantitatively in the form of grain size distribution
plots of the grain area fraction versus grain size of different samples
before and after tensile deformation, as shown in Fig. 8. The width of
grain size distribution of the NC samples has varied from being narrow
to broader after tensile deformation (Fig. 8a and b). And the small
grains in the NC samples have disappeared. It confirms the occurrence
of obvious grain coarsening in the NC samples. In the WC samples, a
less significant grain coarsening has happened. While the grain size
distribution profiles show no obvious shifts toward the larger grain size
in the AC samples, which indicates there is no grain coarsening.
Observation of the TEM images in the Fig. 7 also gives the evidence
of the extent of the grain coarsening in the NC and WC samples. The
grain coarsening processes modify the GB morphology appreciably.
Accompanying grain coarsening, the GBs become indistinct and more
and more dislocations accumulate in the GBs (as shown in Fig. 7a and
b), which means the dislocation are interacting with GBs and the energy
of GBs have been released, trend to an equilibrium state. The process
will cause the coalescence of grains, which is consistent with the in-situ
observation of grain growth in nanocrystalline Al thin films by TEM at
room temperature [29]. It indicates that the presence of high proportion of nonequilibrium GBs in the NC samples can account for the
mechanically induced grain coarsening [30]. It can be understood as
the result of an energy release due to substantial defects annihilation in
the nonequilibrium GBs. Grain coarsening trend is weak in the WC
samples and is not observed in the AC samples indicate that the
nonequilibrium GBs only reach a certain energy state can promote grain
growth. It also suggests that the observed mechanically-induced grain
coarsening will induce certain degree of “strain softening”, because the
grain coarsening has consumed the accumulated dislocations nearby,
leading the GBs to a low energy state [31,32]. The high elongation in
the NC and WC samples are also attribution to the softening effect
5. Conclusions
In this work, defect free PZs of pure copper are achieved by FSP
under different cooling conditions. The effects of nonequilibrium GBs
on the room temperature tensile properties were discussed in detail.
The main conclusions drawn from this work are as follows:
1. The microstructures of PZs produced by different cooling conditions
have nonequilibrium GBs with different fractions and energy states.
The NC samples contain the highest proportion of high-energy
nonequilibrium GBs. The WC samples have the lower proportion of
nonequilibrium GBs. And there is a feature of stabilized equilibrium
GBs with few extrinsic dislocations in the AC samples. The three
samples exhibit different room temperature mechanical properties
because of the nonequilibrium GBs.
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2. By comparing the deformation mechanisms, large fraction of highenergy nonequilibrium boundaries contributes to emit dislocations
from GB and suppress GB sliding, the appropriate relaxed GB in WC
samples will promote GB sliding and the fully relaxed GB can suppress the GB sliding and dislocation emission.
3. The grain coarsening is observed during deformation in the NC and
WC samples. It is attributed to the high GB energy states and cause
the increased elongation and decreased work hardening during deformation.
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Acknowledgments
This study was supported by the State Key Laboratory of Metastable
Materials
Science
and
Technology,
Yanshan
University
(Grant No. MM2016010).
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