Fracture and Fatigue Behavior of Cemented Carbides: 3D

BEST PAPER
AWARD
FRACTURE AND
FATIGUE BEHAVIOR OF
CEMENTED CARBIDES:
3D FOCUSED ION
BEAM TOMOGRAPHY
OF CRACK–
MICROSTRUCTURE
INTERACTIONS
Jose María Tarragó,* Emilio Jimenez-Piqué,**
Miquel Turón-Viñas,*** Lorenzo Rivero,**** Ihsan Al-Dawery,*****
Ludwig Schneider***** and Luis Llanes******
INTRODUCTION
Cemented carbides, also called hardmetals, are a group of powder metallurgy (PM) liquid-phase-sintered materials consisting of brittle refractory carbides of the transition metals (e.g., WC, TiC, TaC) embedded in a metallic
matrix.1 The success of this ceramic–metal composite resides in the combination of the hardness and wear resistance of the ceramic particles with the
toughness of the metallic phase. It makes cemented carbides the materials of
choice in several engineering and tooling applications, such as metal cutting,
mining, rock drilling, metal forming, structural components, and wear-resistant parts.2
The exceptional toughness level exhibited by cemented carbides is related to
highly effective toughening by constrained deformation of the ductile phase.3–6
Ductile-phase (cobalt ligaments) reinforcement of a brittle matrix (tungsten
carbide network) is a prime example of toughening mechanisms that act in the
crack wake to screen the crack tip from the far-field driving force.7 From a
fracture viewpoint, such toughening has proven to be a successful microstructural design strategy in brittle-like materials because it implies the existence
of a crack-growth-resistance-curve (R-curve) behavior that imparts damage
tolerance, and thus improved in-service reliability to the corresponding structural components.8 Thus, the first step of crack propagation in cemented carbides is the formation of a crack from a microstructural heterogeneity that
subsequently advances continuously in the brittle phase and circumvents the
ductile regions leaving bridging ligaments across the crack faces.5 In this way,
The fracture and fatigue
phenomena in WC-cobalt
cemented carbides (hardmetals) have been subjects
extensively investigated in
the last 30 years. From
these studies, it is well
established that the metallic
binder phase plays a key
role as the toughening and
fatigue-susceptible agent in
these materials, as its effective ductility is critical for
defining crack-growth resistance and cyclic-induced
degradation. However,
experimental proof of the
role of toughening and
fatigue micromechanisms
has usually been presented
on the basis of post-failure
fractographic examination.
In this work, the fracture
and fatigue behavior of
WC-cobalt is investigated
and a 3D characterization of
crack–microstructure interaction during stable crack
growth in hardmetals is carried out in order to gain a
better understanding of the
failure processes in cemented carbides under monotonic and cyclic loads. In doing
so, focused ion beam/field
emission scanning electron
microscopy (FIB/FESEM),
3D tomography, and imaging reconstruction are
combined with systematic
mechanical and indentation
protocols for assessing
crack-extension behavior
of cemented carbides.
Experimental findings
clearly highlight existing
differences regarding failure
mechanisms operative
under monotonic and cyclic
loads, and provide new and
interesting insights for
understanding them.
*PhD Fellow, **Tenured Assistant Professor, ***PhD Fellow, ****MSc Engineer, ******Full Professor, CIEFMA, Universitat Politècnica de
Catalunya, Barcelona, Spain, 08028; E-mail: [email protected], *****Senior Engineer in R&D/Technology, Sandvik Hyperion,
Coventry, United Kingdom CV40XG
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International Journal of Powder Metallurgy
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FRACTURE AND FATIGUE BEHAVIOR OF CEMENTED CARBIDES: 3D FOCUSED ION BEAM TOMOGRAPHY OF CRACK–MICROSTRUCTURE INTERACTIONS
a multiligament zone (~2–4 ligaments) is developed in
the direction of crack propagation.4,5 As the crack propagates, the binder ligaments elongate and microcavities
are formed inside the ligament to maintain volume constancy. Using finite element calculations it was demonstrated that binder sites exposed to a high local plastic
strain and stress triaxiality are preferred zones for
microvoid nucleation.9 First, a void will be followed by
others along the plane linking the adjacent cracks in
the carbide phase and finally the ligament fractures by
void growth and coalescence. Fracture along the carbide–binder interface proceeds in a similar manner to
that in the binder, i.e., by the nucleation, growth, and
coalescence of microcavities. However, shallow and
closely spaced microvoids are evidenced when the crack
runs parallel and close to the binder–carbide interface.
Fischmeister et al.9 attributed the formation of such a
fine dimple structure to the fulfillment of high-stress
triaxiality conditions in the binder zone close to the carbide interface.
Cemented carbides, despite the plasticity developed
by the binder ligaments, exhibit brittle-like behavior.
This fracture mode, such as in other brittle materials,
is governed by unstable propagation of flaws that may
be inherent to processing, or induced in the forming
process or under service conditions.10,11 Thus,
strength and reliability of hardmetals are dependent on
the size, geometry, and distribution of existing flaws.
Following this idea, linear elastic fracture mechanics
(LEFM) is recognized as an acceptable theoretical basis
to describe and rationalize the fracture behavior of
cemented carbides.12,13 On the other hand, a statistical tool is required to rationalize the rupture strength
of this brittle material, as it is dependent on the size
and geometry of the flaws. To this end, Weibull statistics are commonly employed to evaluate not only rupture strength, but also the reliability of the material.
Meanwhile, as is the case for other brittle-like composite systems (e.g., ceramic and intermetallic-base
materials) where crack-tip shielding mechanisms prevail,14 the susceptibility of hardmetals to mechanical
degradation under cyclic loading is well established.15–19 Schleinkofer et al. documented a strong
strength degradation of cemented carbides under
cyclic loads that predominantly occur in the ductile
binder phase.17,18,20 They established that subcritical
crack growth is the controlling stage for fatigue failure
in cemented carbides, not crack nucleation.17,20,21 On
the other hand, taking into account this second
assumption and that fracture rupture behavior of
cemented carbides has been extensively rationalized
within the LEFM framework, Torres et al. proposed the
fatigue-crack-growth threshold as the effective toughness under cyclic loading.19 Experimental validation
2
for such an approach was presented for a series of WCcobalt hardmetal grades.22 Moreover, the results cited
a strong influence of microstructure on the fatigue sensitivity of hardmetals, depending on the compromising
role of the metallic binder, as both a toughening and
fatigue-susceptible agent.23 These investigations also
point out a similarity to the Paris-Erdogan law for
fatigue-crack-growth kinetics reported for structural
ceramics and intermetallics. Binder fatigue degradation, also observed in the experimental applied
stress–fatigue life (S–N) data published by Sailer et
al.,24 has been rationalized on the basis of fatigueinduced accumulation of the FCC to HCP phase transformation within the cobalt binder.17 However, most of
the experimental support validating the above-referred
failure scenarios are either indirect, through clear evidences of different fractographic features associated
with fracture and fatigue,4,17,23 or rather limited (but
direct), by means of transmission electron microscopy
(TEM) of slices (local and small areas) in regions
around crack tips.17,21,25
Over the past few years, new advanced characterization techniques have been successfully implemented
for microstructure, surface modification and deformation/damage characterization in various areas of materials research. Among them, the focused ion beam
(FIB) technique has shown to be extremely versatile for
overcoming many of the experimental limitations/difficulties ascribed to more conventional approaches,
such as scanning electron microscopy (SEM) and/or
TEM, atomic force microscopy, and X-ray diffraction.
Fruitful examples of its implementation for quantifying
microstructure, as well as evaluating tribological and
mechanical phenomena in cemented carbides, are
cited by Cairney et al.,26 Beste et al.,27 Mingard et
al.,28 and Gant et al.29
It is the purpose of this study to assess the fracture
and fatigue behavior of a WC-cobalt cemented carbide
and to use the FIB technique for bringing new insights
into the toughening and fatigue micromechanisms
operative in hardmetals when subjected to monotonic
(toughness) and cyclic (fatigue) loading conditions. In
doing so, the main focus will be to document crack–
microstructure interactions at regions close to the tip
of cracks arrested after stable growth, and to discuss
results on the basis of existing knowledge on fracture
and fatigue phenomena for these materials.
EXPERIMENTAL ASPECTS
The material studied is a medium-grained WCcobalt cemented carbide supplied by Sandvik
Hyperion. The key microstructural parameters (binder
content (w/o), mean grain size (dWC), carbide contiguity (CWC), and binder mean free path (λbinder)) are listed
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FRACTURE AND FATIGUE BEHAVIOR OF CEMENTED CARBIDES: 3D FOCUSED ION BEAM TOMOGRAPHY OF CRACK–MICROSTRUCTURE INTERACTIONS
TABLE I. MICROSTRUCTURAL PARAMETERS
Binder
(w/o)
dWC
(μm)
CWC
λbinder
(μm)
11% Co
1.12 ±0.71
0.38 ±0.07
0.42 ±0.28
in Table I. Mean grain size was measured following the
linear intercept method by means of FESEM, using a
JEOL-7001F unit. Carbide contiguity and binder mean
free path were deduced from best-fit equations,
attained after compilation and analysis of data published in the literature,3,5 on the basis of empirical
relationships given by Roebuck and Almond,30 but
extending them to include the influence of carbide
size.12,13
Mechanical characterization includes hardness
(HV30), flexural strength (σr), fracture toughness (KIc)
and fatigue-crack-growth (FCG) parameters. Hardness
was measured using a Vickers diamond pyramidal
indenter under a load of 294N. In all the other cases,
testing was conducted using a four-point bending fully
articulated test jig with inner and outer spans of 20
mm and 40 mm, respectively. Flexural strength tests
were performed on an Instron 8511 servohydraulic
machine at room temperature. At least 15 specimens
(45 mm × 4 mm × 3 mm) were tested per grade. The
surface (which was later subjected to the maximum
tensile loads) was polished to a mirror-like finish and
the edges were chamfered to reduce their effect as
stress raisers. Results were analyzed using Weibull
statistics.
Fracture toughness and FCG parameters were determined using 45 mm × 10 mm × 5 mm single-edge precracked-notch beam (SEPNB) specimens with a notch
length-to-specimen width ratio of 0.3. Compressive
cyclic loads were induced in the notched beams to
nucleate a sharp crack; details are given elsewhere.31
The sides of the SEPNB specimens were polished to follow stable crack growth using a high-resolution confocal microscope. Fracture toughness was determined by
testing the SEPNB specimens to failure at stressintensity-factor load rates ~2 MPa√m/s. FCG behavior
was assessed for two different load-ratio (R) values, 0.1
and 0.5, using a Rumul Testronic resonant testing
machine at load frequencies ~150 Hz.
Effective evaluation of crack–microstructure interactions requires a procedure for introducing sharp precracks into specimens free of residual stresses. To this
end, a sharp indentation is suggested as a simple and
practical precracking technique, since hardmetals are
brittle enough.32 Tomography characterization implemented in this study involves automated serial sectioning using FIB and imaging of each milled surface with
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International Journal of Powder Metallurgy
FESEM. However, precise and detailed imaging of
regions close to the tips of cracks, just as they develop
after unloading the indenter, may become a difficult
task. The main reason behind it is the intrinsic indentation residual stress field. Thus, as material is
removed through FIB milling, the effective stress state
at the crack tip changes and the latter gets displaced.
This experimental inconvenience may be overcome by
the relief of indentation residual stresses before FIB.
This was approached by controlled crack extension
under cyclic loads. In this way much longer cracks can
be induced without any indentation residual stresses
acting at their tips.
One of the purposes of this investigation is to document crack–microstructure interaction during stable
crack growth under both monotonic and cyclic loads. It
then requires the arresting of cracks after stable
growth under each of the loading conditions. Indeed,
the residual stress-relief procedure could be taken as
the final step for assessing fatigue micromechanisms.
However, to attain similar cracks under monotonic
loading demands one further step: testing to failure of
specimens containing multiple indentations.33
Strength-indentation tests were conducted on specimens (4 mm × 3 mm × 45 mm) under four-point bending (outer and inner spans of 40 and 20 mm,
respectively). Four controlled flaw patterns were produced, 2 mm apart, in the center of the prospective
tensile surface of each flexure specimen by applying a
load of 490N using a Vickers diamond pyramid. Care
was taken to orient one set of the corresponding
Palmqvist cracks of each indentation flaw parallel to
the cross section of the specimen where the prospective rupture would occur. Indentation residual stresses for all the controlled flaws were relieved by
subjecting the specimens to tensile cyclic bending (load
ratio of 0.1 and frequency 10 Hz) in order to induce
stable crack growth. Finally, fracture of the cracked
specimens was induced in flexure, under either monotonic or cyclic loads. All the specimens ruptured at one
of the controlled flaws. Stably grown and arrested
cracks remained for the other three indentations. FIB
serial sectioning was then carried out in regions close
to the tips of these surviving cracks. A schematic of the
process is shown in Figure 1.
In order to document crack–microstructure interaction during stable crack extension, combined
FIB/FESEM (Zeiss Neon 40) was used. Before ion
milling, a thin protective platinum layer was deposited
on small areas of interest, corresponding to regions
close to tips of arrested cracks, along surface crack
paths. U-shaped trenches with one cross-sectional
surface (perpendicular to the crack path and to the
specimen surface) were produced by FIB (Figure 2(a)).
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FRACTURE AND FATIGUE BEHAVIOR OF CEMENTED CARBIDES: 3D FOCUSED ION BEAM TOMOGRAPHY OF CRACK–MICROSTRUCTURE INTERACTIONS
Figure 1. Schematic representation of process to generate stable cracks for subsequent sectioning by FIB/FESEM
(a)
(b)
Figure 2. FESEM images of trenches generated by FIB: (a) previous to sequential milling and (b) after completion of sequential milling, around the regions of interest
4
Subsequently a series of crack–microstructure interactions was obtained by periodic removal of the material
by FIB, within the U-shaped crater parallel to the
cross-sectional surface, using automated software
(Figure 2(b)). A 15 µm × 15 µm × 10 µm3 volume was
ion milled and ~600 images were obtained.
The 3D reconstruction of the images was carried out
using commercial Avizo software. The images were
aligned by applying image processing techniques in
order to correct, equalize, and differentiate the grey
level of the three phases (WC, cobalt, and cracks). The
next step consisted of segmenting the three phases to
reconstruct the 3D volume.
TABLE II: HARDNESS, STRENGTH, AND FRACTURE MECHANICS
PARAMETERS FOR WC-COBALT HARDMETAL
RESULTS AND DISCUSSION
Hardness, Strength, and Toughness
Hardness, flexural strength, Weibull modulus, and
fracture toughness of the material are listed in Table II.
The dispersion in flexural strength is relatively small
and, accordingly, the corresponding Weibull analysis
yields high values, indicative of high reliability from a
structural viewpoint (Figure 3(a)). Fractographic examination reveals that critical defects are abnormal
coarse grains and carbide agglomerates (Figure 3(b))
with an equivalent diameter (2acr) ~20–30 μm. It is in
Fatigue Behavior
FCG rates are plotted against the range and the
maximum applied stress intensity factor, ∆K (Figure
4(a)) and Kmax (Figure 4(b)), respectively, for the two
load ratios studied. As previously reported for
WC-cobalt cemented carbides,19,23 the hardmetal
grade under consideration exhibits: (i) a large-power
dependence of FCG rates on ∆K, and (ii) subcritical
crack growth at ∆K values much lower than the fracture toughness. Also, a pronounced load-ratio effect is
observed in the dependence of FCG on ∆K. However, as
HV30
(GPa)
12.8 ±0.2
Flexural Strength Weibull
(MPa)
Modulus
3,101 ±102
36
Estimated 2acr
KIc
(MPa √m)
(μm)
13.9 ±0.3
31 ±2
agreement with values estimated from a direct implementation of the basic LEFM equation relating
strength, toughness, and critical flaw size (Table II), by
considering defects as surface semicircular flaws. This
supports the use of LEFM for rationalizing the fracture
behavior of the hardmetal grade studied.
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FRACTURE AND FATIGUE BEHAVIOR OF CEMENTED CARBIDES: 3D FOCUSED ION BEAM TOMOGRAPHY OF CRACK–MICROSTRUCTURE INTERACTIONS
(a)
(b)
Figure 3. (a) Fracture Weibull
distribution and (b) agglomerate
of carbides acting as a critical
flaw for fracture
(a)
(b)
Figure 4. da/dN behavior vs. (a) ∆K, and (b) Kmax, for the two load ratios studied
observed for other brittle materials, R effects are largely reduced when plotting FCG against Kmax. This is an
indication of the predominance of static failure over
cyclic failure modes.16,23
The FCG–fatigue life relationship proposed and validated by Torres et al. for WC-cobalt cemented carbides19 has been extended for the hardmetal studied.
In doing so, a classical approach on the basis of fatigue
limit, within an infinite-life framework, and FCG
threshold (Kth) is implemented by defining the latter as
the effective toughness under fatigue for a given critical flaw size. Thus, the fatigue limit (σf ) is deduced
from the stress-intensity factor threshold of a small
non-propagating crack emanating from a defect of critical size (2acr), according to a relationship of the form:
Kth
σf = Y ____
√acr
(1)
Hence, the fundamental LEFM correlation between
strength, stress-intensity factor, and defect size also
applies to natural defects in cemented carbides. This
assertion may be made considering that: (i) the size of
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International Journal of Powder Metallurgy
the critical flaws is larger than both dWC and √binder; (ii)
plasticity is confined to process zone ahead of the crack
tip; and (iii) process zones governing fracture (multiligament zones behind the crack tip) extends over a relatively short distance (~five ligaments).19,34 Thus,
fatigue-limit values can be estimated from the relation
given by equation (2), under the assumption that flaws
controlling strength have the same size, geometry, and
distribution under monotonic and cyclic loading.
Kth
σf = σr ____
KIc
(2)
In an attempt to validate the estimated fatigue limit,
an experimental study was conducted using 15 samples, following an up-and-down (staircase) loading
regime for fatigue testing (Figure 5(a)), and defining
“infinite fatigue life” at 106 cycles. Predicted and experimentally determined fatigue limits are listed in Table
III. The FCG threshold–fatigue limit correlation is validated by the excellent agreement attained between
them. Furthermore, the fractographic examination
conducted on failed specimens reveals that size, geom-
5
FRACTURE AND FATIGUE BEHAVIOR OF CEMENTED CARBIDES: 3D FOCUSED ION BEAM TOMOGRAPHY OF CRACK–MICROSTRUCTURE INTERACTIONS
TABLE III: FATIGUE-CRACK-GROWTH THRESHOLD AND
PREDICTED AND EXPERIMENTALLY DETERMINED FATIGUE-LIMIT
VALUES IN TERMS OF MAXIMUM APPLIED STRESS
FCG Threshold
(MPa m1/2)
Predicted Fatigue Limit
(MPa)
Experimental Fatigue
Limit (MPa)
7.6 ±0.2
1,696 ±80
1,632 ±130
etry, and the nature of the initial critical defects are
similar under both monotonic- and cyclic-loading conditions (Figure 5(b)). However, under the application of
cyclic loads, the defects grow until they reach a critical
size and fracture is unstable.
Crack–Microstructure Interactions
After failure, the fracture surfaces were examined
using FESEM. Clear differences are evidenced when
comparing fractographic features corresponding to stable crack growth (Figure 6(a)) and unstable crack growth
(a)
(Figure 6(b)). While in the former, “step-like” fatigue
damage features are discerned within the metal binder,
in the latter well-defined dimples are evident, suggesting
a ductile fracture mechanism. Fracture under cyclic
loading in the cobalt binder appears to follow a faceted,
crystallographic fracture mode, as can be appreciated by
the sharp angular facets localized within broken binder
regions. With the purpose of documenting and understanding the crack-growth processes under monotonic
and cyclic loads, a tomographic reconstruction of the
process zone at the crack tip has been performed by
means of the FIB/FESEM technique.
Stable Crack Growth Under Monotonic Loading
Figure 7 shows crack-microstructure details along
the path of an arrested crack, after stable growth
under monotonic loading. From these preliminary findings, many of the conclusions drawn by Sigl, Exner
and Fischmeister4,5 are validated, namely, (1) continu(b)
Figure 5. (a) Up-and-down (staircase) fatigue test to determine the mean fatigue limit for the WC-nickel cemented carbide, and (b) example of critical defect
growth under cyclic loading
(a)
(b)
Figure 6. FESEM images corresponding to (a) stable crack growth under cyclic loads (R = 0.1) and (b) unstable crack growth under monotonic loading for
WC-cobalt hardmetal. Fatigue facets in (a) and ductile dimples in (b) are evident in the metallic phase
6
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FRACTURE AND FATIGUE BEHAVIOR OF CEMENTED CARBIDES: 3D FOCUSED ION BEAM TOMOGRAPHY OF CRACK–MICROSTRUCTURE INTERACTIONS
ous cracks enclose cobalt regions (ligaments) that elongate during crack opening by void formation with localized plastic deformation at the bridges between the
voids, (2) ligaments finally fail by void coalescence, (3)
the mean depth of the plastic zone is always smaller
than the mean-free path in the binder, and (4) partition
of crack paths between binder and carbide does not
correspond to the volume fraction but significantly
favors the ductile binder. Additionally, local blunting
effects as crack fronts end within the binder (as
invoked by McVeigh and Liu,35 although at a higherlength scale) and the role of carbide corners as stress
risers are clearly discerned.
In conclusion, the multiligament zone may be unambiguously established as the primary foundation for
rationalizing R-curve behavior and exceptional fracture
toughness of cemented carbides. Within this framework, a description and understanding of microstructural effects on fracture toughness, R-curve
characteristics, strength variability, and damage tolerance for hardmetals are also validated. In Figure 8 the
3D reconstruction of the microstructure containing the
crack propagated under monotonic loading is shown.
The light blue corresponds to the WC carbides, the
dark blue corresponds to the cobalt binder, and the red
area corresponds to the crack path. An examination of
the reconstruction permits a clear visualization and
understanding of the fracture process (microvoid
nucleation and growth). A clear example of the process
is presented in Figure 9.
Figure 7. Stable crack growth under monotonic loading: FESEM micrographs showing crack–microstructure interactions. Binder ligaments are clearly
discerned in these images, corresponding to serial sections obtained by FIB/FESEM tomography
Figure 8. 3D reconstruction of process zone for stable crack growth under monotonic loading. Light blue and dark blue correspond to the WC skeleton and the
cobalt phase, respectively; red represents the cracked region. A volume of 6 μm (X axis) x 10 μm (Y axis) x 7.5 μm (Z axis) was reconstructed
Figure 9. Microvoid formation and growth within a cobalt ligament
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FRACTURE AND FATIGUE BEHAVIOR OF CEMENTED CARBIDES: 3D FOCUSED ION BEAM TOMOGRAPHY OF CRACK–MICROSTRUCTURE INTERACTIONS
Unstable Crack Growth Under Cyclic Loading
Crack–microstructure interaction during stable
crack growth under cyclic loading is shown in Figure
10. Different from the failure scenario discerned under
monotonic loading, fatigue micromechanisms in
cemented carbides are less defined and understood.
Taking this into consideration, images attained
through FIB “slice and view” shed further light for proposing and/or validating specific failure micromechanisms: (1) subcritical fatigue-crack growth is more
predominantly located in the ductile binder phase than
under monotonic loading;9 (2) fatigue-crack extension
follows crystallographic-like paths (steps); and (3)
crack-arrest phenomena within binder ligaments
appear to be normal, particularly in regions far from
microstructure-related stress risers. Although the
crystallographic nature of the steps is evident, their
intrinsic origin as related to shear bands, stacking
faults and/or twins resulting from FCC-to-HCP phase
transformation is not clear. The fact that step-like
markings on fatigue-fracture surfaces have been
observed not only in cobalt-base hardmetals but also
in nickel- and cobalt/nickel-base grades,36,37 makes
clear that fatigue susceptibility of cemented carbides
goes beyond changes in deformation mode induced by
the FCC-to-HCP transformation, well established in
WC-cobalt grades.38 As in monotonic loading, the 3D
reconstructed images of the whole volume and the
individual phases is presented in Figure 11. It is interesting to note that under cyclic loading the crack prop-
Figure 10. Stable crack growth under cyclic loading: FESEM micrographs showing crack–microstructure interactions. A crystallographic-like fracture path within the binder is evidenced in these images, corresponding to serial sections obtained by FIB/FESEM tomography
Figure 11. 3D reconstruction of process zone for stable crack growth under monotonic loading. Light and dark blue correspond to the WC skeleton and the
cobalt phase, respectively; red represents the cracked region. A volume of 10 μm (X axis) x 10 μm (Y axis) x 8 μm (Z axis) was reconstructed
Figure 12. Crack propagation within a binder pool under the application of cyclic loads
8
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FRACTURE AND FATIGUE BEHAVIOR OF CEMENTED CARBIDES: 3D FOCUSED ION BEAM TOMOGRAPHY OF CRACK–MICROSTRUCTURE INTERACTIONS
agates through the ductile metallic phase and bridging
ligaments are not formed. Figure 12 shows the fatigue
failure process in a binder pool.
CONCLUSIONS
The fracture and fatigue behavior of a WC-cobalt
cemented carbide has been investigated. A detailed
study of crack–microstructure interactions during stable crack growth under monotonic and cyclic loads has
also been conducted by the complementary use of
mechanical and indentation testing protocols and
FIB/FESEM tomography. From the results the following conclusions may be drawn:
1. The fracture behavior of the WC-cobalt cemented
carbide has been successfully rationalized using
LEFM, on the basis of a reliable assessment of fracture toughness as well as the size and geometry of
critical flaws.
2. As previously reported for cemented carbides, the
WC-cobalt grade studied exhibited a large-power
dependence of FCG rates on ∆K, subcritical crack
growth at ∆K values lower than the fracture toughness, and pronounced load-ratio effects in FCG–∆K
curves. R effects are largely reduced when plotting
FCG against Kmax. This indicates the predominance
of static over dynamic failure modes.
3. Fatigue behavior (“infinite fatigue life”) can be
rationalized by a fatigue mechanics approach considering the initiation of subcritical crack growth as
the control parameter in fatigue failure (Kth is the
effective toughness under cyclic loads).
4. The key role played by the metallic binder phase as
the toughening and fatigue susceptible constituent
in cemented carbides is validated. Unequivocal
proof of the multiligament zone as the foundation
for understanding toughness and R-curve behavior
in hardmetals is provided. Fatigue susceptibility of
these materials is associated with inhibition of
such toughening mechanism under cyclic loading.
In this case, stable crack growth follows a crystallographic path through the metallic binder, distinct
from the typical ductile failure mode found under
monotonic loading.
ACKNOWLEDGEMENTS
This work was supported financially by the Spanish
Ministerio de Economía y Competitividad (Grant
MAT2012-34602). The authors are grateful to T.
Trifonov (CRnE) for his technical assistance during
FIB/FESEM research. Additionally, J.M. Tarragó
acknowledges a PhD scholarship received from the collaborative Industry–University program between
Sandvik Hyperion and the Universitat Politècnica de
Catalunya.
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International Journal of Powder Metallurgy
REFERENCES
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International Journal of Powder Metallurgy

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