V Congreso Nacional de Pulvimetalurgia Girona 2015. 1, 2 y 3 de julio
XXX-XXX
MICROSTRUCTURAL INFLUENCE ON THE FATIGUE
BEHAVIOUR OF CEMENTED CARBIDES
J. M. Tarragó1,2, L. Schneider3, I. Al-Dawery3, L. Llanes1,2
CIEFMA, Departament de Ciència dels Materials i Enginyeria Metal∙lúrgica, Universitat
Politècnica de Catalunya, 08028, Barcelona
2
CRnE, Centre de Recerca en Nanoenginyeria, Universitat Politècnica de Catalunya, 08028,
Barcelona
3
SANDVIK HYPERION, Coventry CV4 0XG, UK
e-mail del autor: jose.maria.tarrago@upc.edu
1
RESUMEN
En esta investigación se ha evaluado la influencia de la microestructura en el comportamiento a
fatiga de los carburos cementados. Con este propósito, se han seleccionado tres variables
microestructurales (tamaño de grano y la naturaleza química y contenido de fase ligante) y se ha
estudiado su efecto en el comportamiento de crecimiento de grieta por fatiga (FCG). Los
resultados indican una dependencia lineal entre el umbral de fatiga (Kth) y el tamaño de grano
debido a la acción de la deflexión de grietas como un mecanismo de aumento de tenacidad
inmune a la fatiga. Por otra parte, no se ha observado una clara influencia de la naturaleza
química del ligante en el comportamiento a fatiga. Asimismo, se puede establecer el umbral de
fatiga como la tenacidad efectiva de los carburos cementados bajo cargas cíclicas debido a la
excelente correlación encontrada entre el límite a fatiga y Kth. Se ha realizado un análisis
fractográfico detallado de los micromecanismos de daño dentro del ligante y se ha observado
que, independientemente de la naturaleza química del ligante, éste tiene un aspecto
marcadamente cristalográfico donde se aprecian claramente escalones. La presencia de estos
escalones puede ser racionalizada en base a una región de deformación plástica en la punta de la
grieta de tamaño similar a la unidad microestructural.
ABSTRACT
In this study, the microstructural influence on the fatigue behaviour of cemented carbides is
assessed. With this purpose, three microstructural variables (carbides grain size and binder
chemical nature and content) are selected and its effect on the fatigue crack growth (FCG)
behaviour investigated. Results indicate a linear dependence between the FCG threshold (Kth)
and the grain size due to the action of crack deflection as a fatigue immune toughening
mechanisms. On the other hand, binder chemical nature is not found to have a significant
influence on FCG rates. In addition, FCG threshold is stated as the effective toughness due to
the excellent correlation found between determined fatigue limit strength and Kth. A detailed
fractographic study of damage micromechanisms has been conducted and, regardless of the
binder chemical nature, well-defined faceted crystallographic features have been observed
within the binder. Such features are rationalized on the basis of a cyclic plastic zone in front of
the crack tip of similar size to the microstructural unity.
Palabras clave (Keywords): cemented carbides, fatigue behaviour, fractography, fatigue crack
growth threshold
1
Microstructural influence on the fatigue behaviour of cemented carbides
1. INTRODUCTION
A better understanding of service degradation phenomena in hardmetals is required for
industrial manufacturers if material performance and lifetime of tools and components are to be
improved. Among them, fatigue is a relevant one since cemented carbides are commonly used
in applications involving high cyclic stresses, and is associated with premature and unexpected
failure [1]. The more relevant scientific and technical advances on fatigue of cemented carbides
are concentrated in the last two decades. Among them it is worth underlining the findings
reported by Sockel's [2–5] and Llanes's [6–8] groups, developed following different testing
approaches: applied stress–fatigue life (S–N curves) and fatigue crack growth (FCG) tests,
respectively. Schleinkofer and co-workers documented a strong strength degradation of
cemented carbides under cyclic loads mainly related to fatigue degradation localized in the
ductile binder phase [2–4]. Furthermore, through their systematic work, they were able to
identify the subcritical crack growth of preexisting flaws as the controlling stage for fatigue
failure in these materials [2–5]. On the other hand, Torres and co-workers proposed the FCG
threshold as the effective toughness under cyclic loading [6]. Moreover, they pointed out that
fatigue sensitivity of hardmetals is significantly dependent on microstructure, according to the
compromising role played by the binder as the main toughening and fatigue susceptible agent in
cemented carbides [7]. This fatigue degradation ascribed to the metallic phase, also observed
from the experimental S–N data published by Sailer and co-workers [9], has been rationalized –
at least partly – on the basis of the cyclic strain-induced fcc to hcp phase transformation within
the Co binder [2,3]. On the other hand, one of the major trends in hardmetal industry is focused
on finding new binder phases to replace cobalt [10]. Among them nickel has attracted
considerable attention as substitutes for cobalt because of its similarity in structure and
properties [11], besides its good corrosion resistance [11,12]. However, information on the
fatigue behaviour of Ni-base cemented carbides is rather scarce (e.g. Refs. [5,13,14]). In this
regard, nickel binder accumulates deformation in the form of slip plus twinning damage
mechanisms [5,13,14], but without evidence of phase transformation, as it is the case of Co
binder. Thus, it is not clear whether above relationships proposed by Llanes an co-workers
[6,7], regarding either fatigue mechanics perspective or microstructural influence on the basis of
binder mean free path, may be directly extrapolated to other than Co-base hardmetals. Within
this context, one of the main goals of this investigation is to evaluate the influence of the partial
or total replacement of Co binder by Nickel on the fatigue behavior of cemented carbides by
extrapolating the work performed by Llanes and co-workers on WC-Co hardmetals [6,7] to Nicontaining ones. On the other hand, in a recent work Torres et al. [8] pointed out the effective
action of crack deflection as an additional toughening mechanism (in addition to the bridging
mechanism at the crack wake) immune to fatigue loads. Therefore, it is also the purpose of this
investigation to study the influence of microstructural coarsening on the fatigue crack growth
behavior of hardmetals.
2. MATERIALS END EXPERIMENTAL ASPECTS
Six hardmetal grades corresponding to different combinations of binder chemical nature, binder
content and carbide grain size were studied. All materials were supplied by Sandvik Hyperion.
The designations and key microstructural parameters: binder content (wt.% binder), mean grain
size (dWC), carbide contiguity (CWC), and binder mean free path (λbinder) are listed in Table 1.
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 (e.g.
Refs. [15,16]), on the basis of empirical relationships given by Roebuck and Almond [17] but
extending them to include the influence of carbide size [18].
2
Microstructural influence on the fatigue behaviour of cemented carbides
Table 1. Microstructural parameters for the studied hardmetal grades.
Grade
wt.% binder
10CoUF
11CoM
10CoC
15CoM
9NiF
10CoNiM
10%Co
11%Co
10%Co
15%Co
9%Ni
8%Co-2%Ni
dWC
(µm)
0.39 ± 0.19
1.12 ± 0.71
2.33 ± 1.38
1.15 ± 0.92
0.77 ± 0.49
1.04 ± 0.83
CWC
0.46 ± 0.06
0.38 ± 0.07
0.31 ± 0.11
0.30 ± 0.07
0.45 ± 0.08
0.41 ± 0.08
λbinder
(µm)
0.16 ± 0.06
0.42 ± 0.28
0.68 ± 0.48
0.55 ± 0.46
0.27 ± 0.18
0.36 ± 0.29
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 and the reported value is the average of five
measurements. 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 pre-cracked 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 [19]. 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 stress intensity-factor load rates of
~2 MPa√m/s. FCG behaviour 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. Experimental
fatigue limit (‘‘infinite fatigue life’’ defined at 106 cycles) was assessed following the stair-case
method for the 11CoM and 10CoC hardmetal grades. Tests were performed using the same
resonant testing machine, at load frequencies around 150 Hz and under a load ratio (R) of 0.1.
3. RESULTS AND DISCUSSION
Hardness, flexural strength, Weibull modulus, and fracture toughness of the studied materials
are listed in Table 2. The dispersion in flexural strength is relatively small and, accordingly, the
corresponding Weibull analysis yields relatively high values for all investigated hardmetals,
indicative of high reliability from a structural viewpoint.
Table 2. Hardness, strength parameters and fracture toughness for investigated hardmetals.
Grade
10CoUF
11CoM
10CoC
15CoM
9NiF
10CoNiM
HV30
(GPa)
15.7 ± 0.6
12.8 ± 0.2
11.4 ± 0.2
11.2 ± 0.1
13.4 ± 0.1
12.3 ± 0.1
σr
(MPa)
3422 ± 512
3101 ± 102
2489 ± 85
2912 ± 88
3085 ± 182
2720 ± 198
Weibull
modulus
11
36
35
39
20
23
KIc
(MPam)
10.4 ± 0.3
13.9 ± 0.3
15.8 ± 0.3
15.2 ± 0.4
11.5 ± 0.2
14.2 ± 0.4
FCG rate versus the stress intensity factor (Kmax) plots, including data for the two studied load
ratios (0.1 and 0.5), are shown in Figure 1 for the studied cemented carbides. FCG thresholds
3
Microstructural influence on the fatigue behaviour of cemented carbides
(Kth), defined at crack growth rates of 10−6 mm/cycle, were attained following a decremental
loading sequence. As previously reported, studied hardmetals exhibit FCG at Kmax values much
lower than corresponding fracture toughness levels, and large-power dependences between FCG
rates (da/dN) and Kmax.
Figure 1. Determined crack growth rates (da/dN) plotted against Kmax for the investigated cemented
carbides determined at two different load ratios of 0.1 and 0.5
Table 3 shows the determined fatigue threshold and fatigue sensitivity [1−(Kth/KIc)] levels for
the studied hardmetal grades. FCG parameters are quite similar for grades with alike
microstructures (i.e. carbide size and binder content), but different binder chemical nature. On
the other hand, coarse-grained materials exhibit higher fatigue threshold values than those
determined for medium-grained ones. This improvement is related to a more effective crackdeflection [8] mechanism, as microstructure gets coarser, which remains operative under cyclic
loading (i.e. toughening discerned under monotonic loading is not inhibited or degraded by
fatigue). In Figure 2 the FCG threshold is plotted as a function of the carbide mean grain size
for the investigated hardmetals and for the two studied load ratios. It is found that the fatigue
threshold (Kth) correlates linearly with the carbide mean grain size (dWC).
Table 3. Hardness, strength parameters and fracture toughness for investigated hardmetals.
Grade
10CoUF
11CoM
10CoC
15CoM
9NiF
10CoNiM
Kth (R=0.1)
(MPa*m1/2)
6.2
7.6
8.3
7.6
7.2
7.8
Kth (R=0.5)
(MPa*m1/2)
7.2
8.6
10.4
9.1
8.2
9
1- Kth/KIc
(R=0.1)
0.40
0.45
0.47
0.50
0.37
0.45
1- Kth/KIc
(R=0.1)
0.31
0.38
0.34
0.40
0.29
0.37
4
Microstructural influence on the fatigue behaviour of cemented carbides
Figure 2. Fatigue threshold as a function of the mean grain size for the cemented grades studied here.
Llanes et al. [7] investigated the dependence of fatigue sensitivity of WC–Co hardmetals as a
function of the binder mean free path and applied load ratio. A similar study has been attempted
and in Figure 3 the fatigue sensitivity exhibited by the studied cemented carbides is plotted
against the binder mean free path for load ratios of 0.1 and 0.5. It is interesting to note that the
fatigue sensitivity linearly increases with the binder mean free path, indicating that as the binder
ligaments become larger the material is more susceptible to fatigue. Main reason for that is that
the effective toughening given by the metallic phase becomes degraded under the application of
cyclic loads [7]. Meanwhile, fatigue sensitivity of Co, Ni and CoNi based cemented carbides
follow similar trends. On the other hand, experimental results indicate smaller fatigue sensitivity
values for coarse grained grades than that expected from the direct extrapolation of the fatigue
sensitivity data obtained for fine and medium grained materials. Once again, the higher
relevance of crack deflection mechanism as microstructure gets coarser, should here be recalled
for rationalizing such finding [8].
Figure 3. Fatigue sensitivity as a function of binder mean free path for the hardmetals studied here.
Based on the assumption that subcritical crack growth is the controlling stage for fatigue failure
in cemented carbides [2–5], Torres and co-workers proposed and successfully validated a FCG
threshold–fatigue limit correlation for WC–Co hardmetals within the Linear Elastic Fracture
Mechanics (LEFM) framework [6,20]. In doing so, they defined the FCG threshold as the
effective toughness under the application of cyclic loads for an infinite life approach. Thus, the
fatigue limit (σf) can be deduced from the stress intensity factor threshold of a small nonpropagating crack emanating from a defect of critical size, 2acr, through a relationship of type:
𝜎𝑓 = 𝑌 ∙
𝐾𝑡ℎ
√𝑎𝑐𝑟
(1)
5
Microstructural influence on the fatigue behaviour of cemented carbides
where Y is a crack geometry factor. Accordingly, fatigue limit (σf) values can be estimated from
the relation given by expression (1) under the assumption that failure-controlling flaws under
the application of monotonic and cyclic loadings have similar size, geometry and distribution.
𝜎𝑓
𝜎𝑟
=
𝐾𝑡ℎ
𝐾𝐼𝑐
(2)
Aiming to validate the above relationship, fatigue limit values were experimentally determined,
following an up-and-down load (staircase) methodology, for two of the hardmetal grades under
consideration: 11CoM and 10CoC (Figure 4). An excellent agreement is obtained between
predicted – using relationship (2) – and experimentally determined fatigue limits (Table 4),
validating then the referred FCG threshold–fatigue limit correlation. A detailed FE-SEM
fractographic examination of samples broken under cyclic loads pointed out critical defects of
similar nature and geometry to those observed in failed specimens tested under monotonic
loading. However, under the application of cyclic loads these defects act as starting locations for
subcritical crack growth until they reach a critical size where unstable fracture takes place
(Figure 5). As previously reported, stable crack growth region within the binder is characterized
by crystallographic step-like features [7,8,21]. These step-like features have been observed in all
hardmetals studied here, indicative that they are independent on the binder chemical nature and
pointing out the step-like crack morphology to be rather a microstructure size scale effect. In
this regard, it is well-known that the transition from the near-threshold regime to the
intermediate stage: (i) is accompanied by a noticeable change from a microstructure-sensitive to
a microstructure-insensitive fracture behavior; and (ii) occurs when the size of the cyclic plastic
zone (rc) becomes comparable to the characteristic microstructural dimension of the material
under consideration [22]. When plane stress conditions are satisfied, the size of the cyclic
plastic region can be approximated as
1 ∆𝐾𝐼
)
𝜋 2𝜎𝑦
𝑟𝑐 ≈ (
(3)
where ΔKI is the applied stress intensity factor range and σy is the yield strength of the material.
In cemented carbides, the high effective yield stress exhibited by the constrained binder
(between 2 and 4 GPa [16]), together with the relatively low ΔKI values at which stable crack
growth takes place (between 4 and 10 MPa·m1/2, Figure 1), yields a submicrometric plastic
region ahead the crack tip, whose size is then comparable to the binder mean free path. Under
these conditions, microscopic failure modes characterized by localized shear and zig-zag crack
paths may be expected, as it is evidenced in this investigation too. Therefore, such features are
speculated to be related to a cyclic plastic zone in front of the crack tip of similar size to the
microstructural unity [23].
Table 4. Estimated and experimentally determined fatigue limit, in terms of maximum applied stress, for
11CoM and 10CoC hardmetal grades studied.
Grade
11CoM
10CoC
Estimated fatigue limit
(MPa)
1696 ± 70
1308 ± 53
Experimentally determined fatigue limit
(MPa)
1603 ± 97
1250 ± 84
6
Microstructural influence on the fatigue behaviour of cemented carbides
(a)
(b)
Figure 4. Example of a microstructure heterogeneity acting as an initial point for fatigue crack growth.
Fatigue crystallographic features are observed in the vicinity of the starting critical flaw.
Figure 5. Example of a microstructure heterogeneity acting as an initial point for fatigue crack growth.
Fatigue crystallographic features are observed in the vicinity of the starting critical flaw.
4. CONCLUSIONS
The influence of the microstructure on the fatigue behaviour of cemented carbides has been
investigated. Based on obtained results the following conclusions may be drawn:
1. FCG threshold of cemented carbides linearly rises when increasing carbide mean grain
size, due to a more effective action of the crack deflection mechanism. Therefore,
coarse-grained hardmetals are found to be less fatigue sensitive than finer-graded with
similar binder mean free path.
2. FCG threshold and fatigue sensitivity are quite similar for grades with alike
microstructures (i.e. carbide size and binder content), but different binder chemical
7
Microstructural influence on the fatigue behaviour of cemented carbides
nature. This negligible influence of binder chemical nature points out then that ductile
Co, Ni and CoNi binders exhibit similar degradation susceptibility under fatigue of
toughening mechanisms operative under monotonic loading.
3. A fatigue mechanics analysis allows estimating the fatigue limit of hardmetals on the
basis that Kth is the effective toughness under cyclic loads.
4. Stable FCG for the hardmetal studied is characterized by faceted, crystallographic
features within the binder, different from the ductile dimples evidenced in the region of
unstable propagation. Such fatigue failure mode is postulated to be a direct consequence
of the comparable size length scales of microstructure and cyclic plastic zone in front of
the crack tip.
5. ACKWNOLEDGMENTS
This work was financially supported by the Spanish Ministerio de Economía y Competitividad
(GrantMAT2012-34602). In addition, J.M. Tarragó acknowledges the Ph.D. scholarship
received fromthe collaborative Industry-University program between Sandvik Hyperion and
Universitat Politècnica de Catalunya.
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