Saturation Dislocation Microstructures In A Copper

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Saturation Dislocation Microstructures In A Copper Single Crystal
During Fatigue In HClO4 Aqueous Solution
1
J H Yang1, X P Zhang1, Y -W Mai1, W P Jia2 and W Ke 2
Centre for Advanced Materials Technology (CAMT), School of Aerospace, Mechanical and
Mechatronic Engineering, The University of Sydney, NSW 2006, Australia
2
Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
ABSTRACT: A copper single crystal was tested at room temperature in air and in a 0.1M HClO4 solution
under the symmetric tension-compression load mode, with loading axis parallel to the [013] direction. The
dislocation structures were characterised using the electron channeling contrast (ECC) technique of scanning
electron microscopy (SEM) and using transmission electron microscopy (TEM). The results show that the
saturation dislocation structures in samples subjected to corrosion fatigue in the 0.1M HClO4 aqueous
solution manly had the form of cells, dislocation wall-like and veins, which differ from the dislocation
structures of dislocation wall-like and veins in the air environment.
1 INTRODUCTION
The dislocation structure of a material is directly associated with its mechanical behaviour. The
dislocation structures of cyclically deformed specimens [Ackermann et al., 1984; Basinski and
Basinski, 1992; Suresh, 1998], especially for single crystals of copper [Laird et al., 1986; Yang et al.,
2003; Mughrabi, 1983], have been extensively studied. Cyclic deformation and persistent slip bands
(PSBs) are both important to crack initiation and crack propagation [Zhang et al., 2001]. Many
investigators have found that the service properties of components subject to corrosion fatigue
conditions are considerably degraded compared with the fatigue properties in air environments
materials [Ortner et al., 1987; Yan et al., 1985]. Moreover, the corrosion fatigue properties of the
materials have been found to be even worse than the simple sum of the separate effects of cyclic
loading and corrosion [Yan et al., 1985]. Therefore, a study of environmental effects on cyclic
deformation and PSB behavior can be expected to improve our understanding of the mechanism of
corrosion fatigue, crack nucleation and life behavior. Unfortunately, this subject has rarely been
studied in recent years because the phenomena are complicated. Most attention has been devoted to
crack propagation. Contrary to the abundant availability of dislocation structures in specimens
cyclically tested in air, TEM and SEM observations of specimens fatigue-tested in corrosive
environments are still very limited [Hahn and Duquette, 1978; Yan et al., 1985]. The present
investigation examines dislocation structures in a copper single crystal that has been subjected to
corrosion fatigue testing, so as to provide a better understanding of environment-cyclic deformation
interactions without the complicating factor of the crack path sensitivity of grain boundaries.
Figure 1 shows the location of the orientations of Cu single crystal in the stereographic standard
triangle and the classification of different types of slip orientations. In Fig.1, the three pinnacles
correspond to [001], [111] and [011] multiple-slip-oriented orientations, respectively, and the three
sides correspond to quite different double-slip orientations, i.e., coplanar slip (011/ 111 ), conjugate
slip (001/ 111 ) and critical slip (001/011) orientations, respectively. The [013] orientation locates at
the (001/011) side, and corresponds to a critical double slip. In this work, fatigue tests were
performed on a copper single crystal with loading axis parallel to [013] double-slip-oriented at room
temperature in air and 0.1 M HClO4 solution respectively, under the symmetric tension-compression
load mode. Dislocation structures were observed and characterized using TEM and the ECC
technique of SEM.
SIF2004 Structural Integrity and Fracture. http://eprint.uq.edu.au/archive/00000836
111
Single slip
001
lip
uble s
nar do
Copla
p
sli
e
bl
ou
d
ate
ug
j
n
Co
Multiple slip
011
013
Multiple slip
Critical double slip
Multiple slip
Fig. 1. Stereographic triangle showing the orientations of Cu single crystal
and the classification of orientations with different slip systems.
2 EXPERIMENT MATERIAL AND PROCEDURE
The material used in the present work was a copper single crystal grown from OFHC copper with a
purity of 99.999% by the Bridgman technique. Specimens used for corrosion fatigue tests were cut
by electric discharge machining, which had the final dimension of 7 × 10 × 75 mm3 and a gauge
section of 7 × 5 × 15 mm3, as shown in Fig.2. The tensile axis of the specimens is parallel to the
[013] orientation. The orientation of the specimens was determined by the Laue back-reflection
technique with accuracy within ± 2°. To eliminate any residual stress effect, all specimens were
electro-polished before testing.
[013]
7
[013]
10
20
5
15
20
75
Fig. 2. The size and shape of specimens used in this study.
A special apparatus was designed to keep the specimen under well-controlled corrosion fatigue
conditions. Figure 3 shows schematically the configuration of the testing system, which consisted of
four parts: mechanical testing machine (Parts 5 and 6), solution deaeration system (Part 1), argon
purification train (Parts 2 and 3), and potential control electronics (Part 4). Deaerated 0.1 M HClO4
solution was applied as the aqueous environment. Fatigue tests were conducted in symmetric
tension-compression load mode at room temperature under the plastic strain control and potential
control conditions using an Instron servohydraulic testing machine (8501, 100 kN). A triangular
waveform signal with frequencies of 0.05-0.5Hz was used for the constant plastic strain amplitude
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control. The applied plastic shear strain amplitudes were 1.6 × 10-3 and 6.8 × 10-3, respectively.
Stress-strain hysteresis loops were on-time recorded and monitored on an X-Y recorder.
3
2
Argonin
Solutionin
1
5
4
Argon out
6
Solution out
1. Solution reservoir
2. Argon purification furnace
3. Argon cylinder
4. Potentiostat
5. Instron fatigue machine
6. X-Y recorder
Fig. 3. Schematic diagram of testing system.
Specimens were deformed cyclically up to the occurrence of the saturation stage of cyclic
deformation. Thin foils for TEM studies were first sliced from the gauge part of the corrosion
fatigued specimen parallel to some specific crystallographic planes (111), ( 111) and (121) , by
electric discharge machining; then mechanically thinned down to dozens of micron thick and finally
polished by a conventional twin-jet method. TEM observations were performed using JEOL2000FX II electron microscope operated at 200kV. The experiment parameters used in corrosion
fatigue are shown in Table 1.
Table 1 Experiment parameters in corrosion fatigue tests
Specimen
designation
S1
S2
S3
S4
Plastic strain
amplitude
6.8 × 10-3
1.6 × 10-3
6.8 × 10-3
1.6 × 10-3
Environment
solution
Air
0.1N HClO4
0.1N HClO4
0.1N HClO4
Potential (VSCE)
N/A
Free Corrosion
0.005
0.005
3 RESULTS AND DISCUSSIONS
3.1 Cyclic hardening curve of specimens tested in air
Figure 4 shows cyclic hardening curves of the [013] double-slip-oriented copper single crystal
which was tested at room temperature in air at plastic shear strain amplitude of 1.6 × 10-3 and 6.8 ×
10-3. It can be seen from Fig. 4 that the shear stress increases with increasing number of cycles and
finally reaches constant and enters a saturation state. Moreover, the hardening curves for the
different environmental conditions (see Table 1) showed no significant differences. This implies that
the environment has no significant influence on the hardening process.
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70
-3
6 .8 x1 0
-3
1 .6 x1 0
Axial stress MPa
60
50
40
30
20
10
0
1000
2000
3000
4000
C yc lic num b e r
Fig. 4. Cyclic hardening curves of [013] double-slip-oriented copper single crystals
under two different plastic strain amplitudes.
3.2 Surface morphology and saturation dislocation structures in specimens tested in air
Figure 5 shows observations of the surface deformation morphology and dislocation structures of
the specimen S1 cyclically loaded in air for 3500 cycles (by this cycle number the specimen was in
the saturation stage of cyclic deformation). In Fig.5 (a), the parallel macro deformation bands (DBs),
at an angle of 45° with respect to the loading direction, can be seen clearly. These macro DBs can
spread over the whole surface. In Fig. 5 (b), the dislocation structure mainly consists of dislocation
wall-like and vein structures. The observed plane is (111). Regarding the critical double-slip crystals,
Gong et al. [Gong et al., 1996] reported that PSB ladders, veins and labyrinths constitute the main
dislocation features in the critical double-slip crystal cycled at intermediate strain amplitudes and
cell structures developed at higher strain amplitudes. Jin and Winter (Jin and Winter, 1984) also
found PSB ladders and veins in the [012] critical double-slip crystal cycled at an intermediate strain
amplitude of 3.0× 10−3. Recently Li et al. [Li et al., 2002] reported that the labyrinth structure
becomes the main dislocation configuration in cyclically deformed [017] critical double-slip
crystals (γpl = 1.2 × 10-4 ) and some narrow PSB ladder-like structures are embedded in the labyrinth
structures (γpl = 9.4 × 10-4 ). The present work revealed that the dislocation wall-like and vein
structures become the main dislocation configuration in the [013] critical double-slip copper single
crystal.
1)
(1 1
(b)
wa
ll -l
ik
e
PS
Bs
(a)
Di
sl
oc
at
io
n
DB
25 µ m
Fig. 5. SEM images of specimen S1: (a) surface morphology; and
(b) SEM-ECC image of dislocation wall-like and vein microstructures
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3.3 Surface morphology and saturation dislocation structures in specimen S2
Figure 6 shows observations of the surface deformation morphology and dislocation structures of
the specimen S2 which was cyclically tested in a 0.1M HClO4 aqueous solution at free corrosion
potential for 4000 cycles (also being in the saturation stage of cyclic deformation). In the 0.1 M
HClO4 aqueous solution, without any corrosion product that does not dissolve in water was formed
on the specimen surface, a copper atom is easy to lose electron to become a copper ion and dissolve
in medium solution, is typical active system. Observation shows that the surface has slip traces and
partial dissolution of copper atoms, without any apparent oxide films formed (see Fig.6a). SEMECC and TEM observations show that the saturation dislocation structures, after corrosion fatigue
in 0.1M HClO4 aqueous solution, are manly consisted of dislocation walls-like, veins (see Fig.6b)
and cells (see Fig.6c).
(a)
(b)
Veins
Di
ik e
ll - l
wa
ip
sl
n
tio
ca
y
ar
im
s lo
Pr
10 µm
(c)
Cell
400 nm
Fig. 6. SEM and TEM images of saturation dislocation structures of fatigued copper single crystal
in 0.1M HClO4 solution at free corrosion potential: (a) surface morphologies; (b) SEM-ECC
images of dislocation walls-like and vein microstructures; (c) TEM image of dislocation cells.
Yan et al. [Yan et al., 1985] have studied the dislocation structure of copper single crystals
oriented for single slip cycled in 0.1M HClO4 and under different polarization potentials. TEM
samples were observed both after saturation and after fracture. They reported that although much
higher strain localization was observed in the crystals cycled at anodic potentials, the dislocation
structures observed were very similar to those of specimens tested at cathodic potentials and in air.
For a plastic shear strain amplitude of 2.0× 10−3, regular loop patches (veins) and dipolar walls
(dislocation wall) were observed. For a higher strain amplitude of 4.0 × 10−3, dipolar walls
associated with secondary slip were found in addition to regular primary walls. We have studied the
dislocation structure of copper single crystals oriented for double slip cycled in 0.1M HClO4 and
under free corrosion potential for 4000 cycles (also being in the saturation stage of cyclic
SIF2004 Structural Integrity and Fracture. http://eprint.uq.edu.au/archive/00000836
deformation). The present work revealed that the saturation dislocation microstructures, after
corrosion fatigue in the aqueous solution of 0.1M HClO4, are manly consisted of dislocation walllike, veins and cells, which differ from the dislocation structures in the air.
4 CONCLUSIONS
1) The dislocation walls-like and veins constitute the main dislocation feature in the critical doubleslip copper single crystals cycled deformation in open-air condition.
2) For double-slip-oriented copper single crystal cyclically stressed to the saturation stage in a 0.1
M HClO4 aqueous solution, the specimen surface has slip traces and partial dissolution of copper
atoms, but without oxide film formed.
3) The saturation dislocation microstructures, after corrosion fatigue in the aqueous solution of
0.1M HClO4, are manly consisted of dislocation walls-like, veins and cells, which differ from
dislocation structures of dislocation wall-like and veins in open-air condition.
ACKNOWLEDGMENTS
Support of this work by the Australian Research Council (ARC) through a Large Grant (A10009166)
is gratefully acknowledged. YWM also acknowledges the award of an Australian Federation
Fellowship tenable at the University of Sydney.
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