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Fracture mechanism of dispersion-strengthened Cu-Al2O3 nanosystem

Authors:

Abstract

The method of 'in-situ tensile test in SEM' is suitable for investigations of fracture mechanisms because it enables to observe and document deformation processes directly, thank to which the initiation and development of plastic deformation and fracture can be reliably described. The deformation and fracture mechanisms of Cu-Al2O3 nanomaterials with 5 vol. % of Al2O3 phase has been analyzed using technique of the 'in-situ tensile testing in SEM'. It has been shown that the deformation process causes break-up of large Al2O3 particles and decohesion of smaller ones. The final fracture path is influenced also by boundaries of nanograins, through which the principal crack propagates towards the sample exterior surface. Based on the experimental observations a model of damage and/or fracture mechanisms has been proposed.
Acta Metallurgica Slovaca, Vol. 18, 2012, No. 2-3, p. 76-81 76
FRACTURE MECHANISM OF DISPERSION-STRENGTHENED Cu-Al2O3
NANOSYSTEM
M. Besterci1), O. Velgosová2)*, J. Ivan3), T. Kvačkaj4), P. Kulu5)
1) Institute of Materials Research, Slovak Academy of Sciences, Košice, Slovak Republic
2) Department Materials Science, Faculty of Metallurgy, Technical University, Košice, Slovak
Republic
3) Institute of Materials and Machine Mechanics, Slovak Academy of Sciences, Bratislava,
Slovak Republic
4 Department of Metals Forming, Faculty of Metallurgy, Technical University, Košice, Slovak
Republic
5) Tallin University of Technology, Department of Materials Technology, Tallinn, Estonia
Received 21.03.2012
Accepted 14.09.2012
*Corresponding author: e-mail: oksana.velgosova@tuke.sk, tel.: 00421 55 602 2427, fax.:
00421 55 602 2428, Department of Materials Science, Faculty of Metallurgy, Technical
University of Košice, Park Komenského 11, 04200 Košice, Slovak Republic
Abstract
The method of “in-situ tensile test in SEM” is suitable for investigations of fracture mechanisms
because it enables to observe and document deformation processes directly, thank to which the
initiation and development of plastic deformation and fracture can be reliably described. The
deformation and fracture mechanisms of Cu-Al2O3 nanomaterials with 5 vol. % of Al2O3 phase
has been analyzed using technique of the “in-situ tensile testing in SEM”. It has been shown that
the deformation process causes break-up of large Al2O3 particles and decohesion of smaller
ones. The final fracture path is influenced also by boundaries of nanograins, through which the
principal crack propagates towards the sample exterior surface. Based on the experimental
observations a model of damage and/or fracture mechanisms has been proposed.
Keywords: dispersion strengthened Cu-Al2O3 composite; mechanical alloying; fracture
mechanism
1 Introduction
The method of “in-situ tensile testing in SEM” is suitable for investigations of fracture
mechanisms because it enables to observe and document deformation processes directly, by
which the initiation and development of plastic deformation and fracture can be reliably
described.
We have analyzed, as reported in [1-3] the fracture of Cu-Al2O3 and Cu-TiC systems by direct
monitoring of the strain and fracture in a scanning electron microscope (SEM) (in situ tensile
test in SEM). Both systems were prepared by different powder metallurgy technologies. The
dispersed oxides and carbides in the matrix were not coherent. Differences in particle size and
distribution caused differences in fracture mechanism, although both factures were ductile
transcrystalline with dimples. The method of in situ tensile test in SEM is suitable for
observation and evaluation of fracture mechanism directly under the load. This method was used
for analyzing Al and Cu systems (Al-Al4C3 systems with different volume fraction of secondary
Acta Metallurgica Slovaca, Vol. 18, 2012, No. 2-3, p. 76-81 77
phase [4-6] and Cu-Al2O3 system [7, 8]), Al-Si-Fe alloys [9] and Al-Si system [10]. Fracture
mechanism depends on many factors, for instance amount of dispersion particle, their shape and
size, characteristics of matrix material, grain boundary characteristics etc. [11-16].
Owing to the excellent high-temperature properties and sufficiently high values of electrical and
thermal conductivity, the dispersion-strengthened Cu-Al2O3 materials, prepared by the methods
of powder metallurgy, have found use as conductors in electrical machines employed at high
temperatures, in contacts, in electrodes and in vacuum technique parts.
The aim of work is to analyze fracture mechanism in the Cu-Al2O3 nanocomposite system with 5
vol. % of Al2O3 phase and to propose a damage model.
2 Experimental materials and methods
Reaction milling and mechanical alloying was used to prepare the samples. Cu powder with the
calculated addition of Al was homogenized by attrition in oxidizing atmosphere. The
distribution of the obtained CuO was uniform. A subsequent treatment at 750 °C induced the
reaction of CuO with the added Al powder, and led to the formation of Al2O3 particles. The
remaining CuO was reduced by attrition in a mixture of H2 + H2O (rate 1:100). The powder was
compacted by cold pressing and hot extrusion at 750 °C-800 °C.
Microstructured material with 5 vol. % Al2O3 was transformed by the ECAP (Equal Channel
Angular Pressing) method in two passes into a nanocomposite material. The experimental
material was pressed through two right angled (90°) channels of a special die by route “C”. The
ECAP technology allows obtaining the very fine grained microstructure nanostructure by
multiple pressings through the die, Fig. 1 [17].
Fig.1 Schema of ECAP [17] Fig.2 The shape and dimension of a specimen
This material with dimensions of Ø10 x 70 mm was deformed by the ECAP technique in two
passes at room temperature in a hydraulic press in the equipment described in [17].
For the purposes of investigation very small flat tensile test pieces, Fig.2, (7x3 mm, gauge
length is 7 mm) with 0.15 mm thickness were prepared by electroerosive machining, keeping the
loading direction identical to the direction of extrusion. They were ground and polished
mechanically to a thickness of approximately 0.1 mm.
The final operation consisted in double-sided final polishing of specimens with an ion thinning
machine. The specimens were ground and polished down to a thickness of approximately 0.1
Acta Metallurgica Slovaca, Vol. 18, 2012, No. 2-3, p. 76-81 78
mm. Finally, the specimens were finely polished on both sides by ion gunning. The test pieces
were fitted into special deformation grips inside the scanning electron microscope JEM 100 C,
which enables direct observation and measurement of the deformation by ASID-4D equipment.
From each one of the systems five samples were prepared.
3 Results and discussion
The microstructure of the starting material with 5 vol. % Al2O3 was fine-grained (the mean
matrix grain size was 1 m) Fig.3, heterogeneous, with Al2O3 particles distributed in parallel
rows as a consequence of extrusion and ECAP. The experimental materials were deformed at
20 °C at a strain rate of 6.6x10-4 s-1 in the elastic region. The replica is on Fig. 4 and
difractogram is on Fig.5. Particles of size less than 0.25 m were classified as effective
dispersion particles of category A, and particles of size greater than 0.25 m as particles of
category B; the latter are ineffective from the point of view of strengthening; although they
affect the deformation process and plastic properties. In addition to the particles mentioned, the
material also contained impurities, which were introduced during the preparation process of the
material.
Fig.3 The microstructure of the
starting material on thin foil Fig.4 Replica of material with 5 vol. % Al2O3
The material after ECAP is on the border of nanostructured materials. The TEM micrographs,
Fig. 6, showed that the mean grain size was 100-200 nm, small amount of dislocations are
present in nanograins, too, but mostly on the boundaries. The nanostructure formation takes
place most probably by a three-stage mechanism, described in [18-20]. This model has been
experimentally verified only for several specific materials but in our case it seems to be probably
usable. The model includes creation of cell structure, then formation of transitory cell
nanostructure with large angle disorientation, and finally formation of nanostructured grains
with size of approx. 100 nm. However, here one has to consider the retarding effect due to
present dispersoid particles. The simulation and mechanical properties of Cu-Al2O3 system are
described in [21].
Deformation process of the loaded layer causes fracture of large, B-type, particles in the middle
of the specimen (Fig. 7), which initializes fracture path roughly perpendicular to the loading
direction. The fracture path is determined also by decohesion of smaller particles (type A)
(Fig. 8). Since the volume fractions of Al2O3 particles are small, their distribution in lines does
Acta Metallurgica Slovaca, Vol. 18, 2012, No. 2-3, p. 76-81 79
not influence the trajectory of fracture which has low relative deformation ε = 0.1. Unlike the
microstructured Cu based composites, in this case it has been shown that the nanograin
boundaries play an important role. In the final phase (Fig. 9) a crack propagates along the
nanograin boundaries, which has been observed experimentally on the crack line (profile), and it
is documented also by the ductile fracture surface with typical dimples in Fig. 10.
Fig.5 Difractogram of
Al2O3 phase Fig.6 The TEM micrographs
of the material after
ECAP
Fig.7 Fracture of large (B)
Al2O3 particles in the
middle of the layer
Fig.8 Decohesion of
smaller particles
category A
Fig.9 The final phase of
the crack
Fig.10 Surface morphology
(dimples) of the
material with 5 vol. %
Al2O3
A detailed study of the deformation changes showed that the crack initiation was caused by
decohesion, and occasionally also by rupture of the large particles. Decohesion is a result of
different physical properties of different phases of the system. The Cu matrix has significantly
higher thermal expansion coefficient and lower elastic modulus ( = 17.0 x 10-6 K-1,
E = 129.8 GPa) than Al2O3 ( = 8.3 x 10-6 K-1 and E = 393 GPa). Large differences in the
thermal expansion coefficients result in high stress gradients, which arise on the interphase
boundaries during the hot extrusion. Since matrix>particle, high compressive stresses can be
expected. However, because the stress gradients arise due to the temperature changes, during
cooling (which results in increase of the stress peaks) their partial relaxation can occur.
Superposition of the external load and the internal stresses can initiate cracking at interphase
boundaries. This is in accordance also with the dislocation theories which argue that the particles
in composite may cause an increase in the dislocation density as a result of thermal strain
Acta Metallurgica Slovaca, Vol. 18, 2012, No. 2-3, p. 76-81 80
mismatch between the ceramic particles and the matrix during preparation and/or thermal
treatment. In our case, the coefficient of thermal expansion of the matrix is much higher than
that of the secondary particles and the resulting thermal tension may relax around the matrix-
particle interfaces by emitting dislocations, whose density can be calculated according to
procedure described in [22].
a
c
Fig.11 a,b,c,d Model of the fracture mechanism
Based on the microstructure changes observed in the process of deformation, the following
model of fracture mechanism is proposed (Fig. 11a,b,c,d):
a) The microstructure in the initial state is characterized by Al2O3 particles, categorized as B.
b) With increasing tensile load local cracks, predominantly on specimen side surfaces, are
formed by decohesion of smaller A particles.
c,d) In further increasing deformation of nanocomposite materials the nanograin boundaries
start to play an important role. Since the volume fraction of these boundaries is high and the
size of the B and A particles is equal to the matrix grain size, crack propagates preferentially
along the nanograin boundaries in a 45° angle.
4 Conclusion
Based on the microstructure changes obtained in the process of deformation of the dispersion
strengthened Cu-Al2O3 alloys a model of fracture mechanism was proposed. With increasing
tensile load the local cracks, predominantly on specimen’s side surfaces, are formed by rupture
of large B and decohesion of smaller A particles. The orientation of cracks tends to be
perpendicular to the loading direction, depending on the particle volume fraction. The final
rupture, i.e. interconnection of the side cracks along the loading direction, takes place at
nanograin boundaries, depending on the volume fractions of oxide (Al2O3) particles in a 45°
angle.
Acta Metallurgica Slovaca, Vol. 18, No. 2-3, 2012, p. 76-81 81
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Acknowledgement
The work was supported by the Slovak National Grant Agency under the Project VEGA
2/0025/11.
... Both initial materials were then transformed by the ECAP method in order to induce severe plastic deformation and to achieve very fine grained microstructures, and in such way to turn them into nanocomposite systems. The as-received samples were pressed in two passes through two right angled (90°) channels of a special die (Besterci et al., 2012). The notification of the experimental materials is explained inTable 1. ...
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