A nano-indentation study on the mechanical behaviour of the
matrix material in an AA6061-Al2O3 MMC
Citation for published version (APA):
Mussert, K. M., Vellinga, W. P., Bakker, A., & Zwaag, van der, S. (2002). A nano-indentation study on the
mechanical behaviour of the matrix material in an AA6061-Al2O3 MMC.
Journal of Materials Science
,
37
(4),
789-794. https://doi.org/10.1023/A:1013896032331
DOI:
10.1023/A:1013896032331
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Published: 01/01/2002
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JOURNAL OF MATERIALS SCIENCE 37 (2002) 789– 794
A nano-indentation study on the mechanical
behaviour of the matrix material in an
AA6061 - Al
2
O
3
MMC
K. M. MUSSERT
Department of Materials Science, Delft University of Technology,
Rotterdamseweg 137, 2628 AL Delft, The Netherlands
W. P. VELLINGA
Department of Mechanical Engineering, Eindhoven University of Technology,
P.O. Box 513, 5600 MB Eindhoven, The Netherlands
A. BAKKER, S. VAN DER ZWAAG
Department of Materials Science, Delft University of Technology,
Rotterdamseweg 137, 2628 AL Delft, The Netherlands
E-mail: S.VanderZwaag@stm.tudelft.nl
The nano-indentation technique is a suitable technique to measure hardness and elastic
moduli profiles of AA6061 reinforced with Al
2
O
3
particles, since it allows measurements of
mechanical properties on a micrometer range. To investigate possible local variations in
mechanical behaviour of the matrix material due to precipitation reactions being affected
by the presence of ceramic reinforcements, nano-indentation tests were done on both
metal matrix composite (MMC) as well as unreinforced reference material, in three different
heat treatment conditions. Matrix response depends on heat treatment condition, but is
approximately equal for the MMC and the base reference alloy. Due to the various imposed
heat treatments, magnesium enrichment around the ceramic particles was observed, but
hardness and elastic modulus of this interfacial layer could not be measured. To confirm
the preferential segregation of Mg near the particle/matrix interface, linescans were made
with a Scanning Electron Microscope (SEM) equipped with EDS (Energy Dispersive
Spectrum) facilities. The limited width of the Mg rich zone explains the absence of typical
’interphase’ indentations in this investigation. Hardly any differences in calculated elastic
moduli and hardness values were found for the three heat treatment conditions
investigated, when comparing results of AA6061 reference material with results of an
AA6061 matrix in an MMC. This result is of great importance when modelling the
mechanical behaviour of MMCs using the finite element method, since it permits the
assumption that the MMC matrix material behaves similar to the same aluminium alloy
without ceramic reinforcements.
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2002 Kluwer Academic Publishers
1. Introduction
Macroscopic mechanical properties of MMCs are not
only determined by the mechanical properties of the
various constituents, but a considerable contribution
is due to the behaviour of the particle/matrix inter-
face. Stiffening and strengthening rely on load transfer
across this interface, toughness is influenced by crack
deflection and ductility is affected by relaxation of peak
stresses near the interface [1]. Some investigators have
found that, as result of a heat treatment, the matrix re-
gions adjacent to the reinforcement may exhibit a higher
dislocation density when compared with matrix mate-
rial further away from the interface due to mismatch of
the coefficients of thermal expansion (CTE) between
the matrix and the ceramic reinforcements [2]. These
CTE-dislocation effects influence the kinetics of pre-
cipitation. Matrix dislocations may act as nucleation
sites for precipitates and a higher density facilitates pre-
cipitation formation. Furthermore, the dislocations may
act as preferential paths for solute diffusion which can
accelerate the ageing process [2–4].
Since the width of particle/matrix interfaces and the
interparticle distances are of the order of micrometers, it
is practically not possible to determine gradients in me-
chanical properties around the particles using conven-
tional techniques. However, nano-indentation which
measures the resistance to plastic deformation in very
small volumes, may offer some indication of gradients
in mechanical behaviour on the micrometer range. Be-
cause of this capacity, nano-indentation is often used to
0022–2461
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2002 Kluwer Academic Publishers 789
measure thin hard films [5]. However, Leggoe et al. [6]
and Das et al. [7] used the method to study aluminium-
based particle reinforced MMCs. In the latter investiga-
tion, a distinction was made between indentations in a
particle, interface or matrix. Electron probe microanal-
ysis (EPMA), indicated that magnesium enrichment oc-
curred in the interfacial region between the particle and
the matrix and the width of this phase increased with
ageing time. This magnesium enrichment has been at-
tributed to the presence of a MgAl
2
O
4
spinel which
formed during fabrication and which gave rise to a layer
of matrix material around the particles with high hard-
ness. It is of great importance to investigate the occur-
rence of such an interfacial layer and its influence on
the mechanical behaviour of the matrix material in an
MMC compared to the matrix response to heat treating
in the base reference alloy.
In this investigation, nano-indentation testing is used
to measure the hardness and elastic moduli profiles
of AA6061 reinforced with Al
2
O
3
in three differ-
ent heat treatment conditions. Results are compared
with nano-indentation measurements on unreinforced
AA6061 in the same three heat treatment conditions,
to investigate possible changes in the mechanical be-
haviour of the matrix material due to the presence of the
ceramic reinforcement. When hardly any differences
are to be found in the mechanical behaviour of the
MMC matrix material in comparison with the AA6061
reference alloy, finite element calculations on MMCs
are simplified since the matrix behaviour can then be
assumed to behave the same as that of the aluminium
alloy without ceramic reinforcements.
2. Materials used
The MMC used was produced via the stir casting pro-
cess by Duralcan USA and consists of an AA6061-
based alloy and 20 vol% Al
2
O
3
particles. AA6061, a
widely used typical extrusion alloy containing Mg and
Si as the principal alloying elements, is a very suitable
alloy as matrix for MMCs since its properties can be
adjusted by a suitable heat treatment. An unreinforced
AA6061 alloy of almost identical composition was used
as the reference system. The compositions of both the
MMC matrix alloy and the reference AA6061 are listed
in Table I.
To produce known and reproducible precipitation
structures in the AA6061 material as well as the MMC
matrix material, all specimens were solutionized for
2 hours at 530
◦
C followed by water quenching. In or-
der to generate distinctly different precipitates in the
matrix material, three different heat treatments were
imposed making use of the results of a recent study on
the precipitation kinetics in AA6061-based MMCs [8].
In that study, DSC curves were obtained for both ho-
TABLE I Chemical composition of the materials tested in weight
percentage
Material Mg Si Cu Ni Mn Cr Fe Ti
AA6061 0.890 0.628 0.338 0.011 0.104 0.104 0.364 0.030
MMC 1.107 0.446 0.229 0.006 0.001 0.090 0.045 0.012
TABLE II Water quench temperatures for the AA6061 alloy and the
MMC
Material Temperature [
◦
C]
AA6061-B 256
AA6061-C 309
MMC-B 254
MMC-C 300
Figure 1 DSC traces of AA6061 and the Duralcan AA6061 with
20 vol% Al
2
O
3
particles, heating rate 5
◦
C/min [8].
mogenised AA6061 MMC and the reference alloy at a
fixed heating rate of 5
◦
C/min (see Fig. 1). In this fig-
ure, the peaks at around 60
◦
C (labelled A) are related
to Si clustering, those at around 230
◦
C (labelled B) to
β
formation, those at around 280
◦
C (labelled C) to β
formation and those at around 480
◦
C (labelled D) to
β-Mg
2
Si formation.
In the present work, the condition of a matrix contain-
ing a maximum amount of β
precipitates was created
by linear heating the samples to a temperature of 255
◦
C
(condition B), using a heating rate of 5
◦
C/min. A ma-
trix containing a maximum amount of β
precipitates
was created by linear heating the samples to a tem-
perature of 305
◦
C (condition C), using a heating rate
of 5
◦
C/min. To account for small differences in pre-
cipitation kinetics in the MMC and the reference alloy,
the maximum annealing temperatures were adjusted ac-
cording to Table II. The third heat treatment condition
was an as-received T6-condition for the MMC, which
consisted of solutionizing followed by 8 hours holding
time at 175
◦
C and an as-received T651-condition for
the AA6061 alloy, which had additional stretching of
∼2% before artificially ageing.
3. Nano-indentation procedure
The mechanical properties of both matrix material and
MMC were investigated with a nano-indenter devel-
oped at Eindhoven University of Technology which
continuously measures force and displacement as an
indentation is made. The indenter in this instrument
is a Berkovich diamond three-sided pyramid with a
nominal angle of 65.3
◦
and an area-to-depth func-
tion which is the same as that of a Vickers indenter.
For each sample at least ten indentations were made
with a maximum load of 30 mN and the indentations
790
Figure 2 Schematic representation of a loading-unloading curve for a
nano-indentation measurement; P
max
is the load at maximum indenta-
tion, h
max
is the indenter displacement at peak load, h
f
is the final depth
of the contact impression after unloading and S is the initial unloading
stiffness.
were separated by 20 µm. On the MMC sample in
T6-condition, the maximum applied load for the inden-
tations was 100 mN. The samples had to be polished
to 0.25 µm for the indenter to trace the surface, since
any roughness affects the indentation curve. From the
data obtained during unloading of the indentation, elas-
tic displacements can be determined and from these
measurements the elastic modulus, E, can be calcu-
lated. Furthermore, by removing the elastic contribu-
tion from the total displacement, the hardness, H, can
be calculated. A schematic diagram of a typical loading-
unloading curve is shown in Fig. 2.
The displacement plotted in this figure represents the
total displacement of the indenter relative to the ini-
tial position. This value is composed of both elastic
and plastic displacements. An important observation
concerns the shape of the hardness impression after
the indenter is unloaded and the material elastically re-
covers [9]. In metals it turned out, that the impression
formed by a conical indenter is still conical, but with
a larger included tip angle. Doerner and Nix [10] ob-
served that during the initial stages of unloading, the
area of contact remains constant as the indenter is un-
loaded. The unloading stiffness is then related to the
elastic modulus and the projected area of the elastic
contact, A, through the following relationship:
S =
dP
dh
unloading
P=P
max
=
2
√
π
E
r
√
A (1)
where E
r
is the reduced elastic modulus as defined in
the following equation:
1
E
r
=
(1 − ν
2
)
E
+
1 − ν
2
i
E
i
(2)
where E and ν are the Young’s modulus and Poisson’s
ratio for the specimen and E
i
and ν
i
are the same pa-
rameters for the nano-indenter material. To determine
the contact area at peak load, Oliver and Pharr [9] pro-
posed a new method of analysis, since the behaviour
of materials when indented by a Berkovich indenter
can not be described using the flat punch approxima-
tion [10]. Sneddon [11] has derived analytical solutions
for punches of several geometries, including conical
indenters. Like for the Berkovich indenter, the cross-
sectional area of a conical indenter varies as the square
of the depth of contact and its geometry is singular at
the tip. Therefore, Oliver et al. [9] assume that the be-
haviour of a conical indenter gives a better description
of the elastic unloading of an indentation made with a
Berkovich indenter. The area of contact at peak load is
then determined by the geometry of the indenter and
the depth of contact, h
c
. The indenter geometry can
be described by an area function F(h) which relates
the cross-sectional area of the indenter to the distance
from its tip, h. Given that the indenter itself does not
deform significantly, the projected contact area at peak
load can then be calculated from the relation:
A = F(h
c
) (3)
In addition to the elastic modulus, the hardness, H,
defined as the applied load divided by the projected
area of contact between the indenter and the sample
can now be calculated:
H =
P
max
A
(4)
4. Results and discussion
Typical loading-unloading indentation curves acquired
for the MMC in T6 condition are shown in Fig. 3.
In this figure, shallow penetration curves are at-
tributed to indentations in particles and deep penetra-
tions to those in the matrix. In contrast to Das et al. [7]
this investigation did not reveal a third distinct group of
indentations representing an existing ‘interphase’ be-
tween particle and matrix, for any of the heat treated
MMCs. Little variation is observed for the particle in-
dentations, more so for the matrix indentations. Inden-
tation curves with deviant shapes were excluded from
further interpretation. These curves were either a result
of indentations in matrix material with underlying par-
ticles near the surface, indentations on particle-edges
Figure 3 Indentation loading-unloading curves for an Al
2
O
3
particle
reinforced AA6061 MMC in T6 condition.
791
Figure 4 Indentation loading-unloading curve for an indent partially in the matrix and partially in a particle; the indentation is accentuated with a
white line.
whereby the indenter slided off, or they were due to
surface roughness of the sample. The latter is hard to
overcome when investigating MMCs, since it turned
out to be extremely difficult to level an MMC sample
surface. A more detailed analysis of the shape of the
indentation curves in relation to the position of neigh-
bouring particles, showed a perfect example of a devi-
ating curve which can be related to the indenter sliding
off the ceramic particle, see Fig. 4.
Using dark field optical microscopy, most indenta-
tions in matrix material could be tracked down, whereas
indentations in ceramic particles could not be found.
After locating the position of the indentations in the ma-
trix material with dark field, it was possible to switch to
bright field and make photographs for further interpre-
tation. Since the indentations in the MMC in T6 condi-
tion were made with a maximum load of 100 mN, the
identification of the indentations was the easiest com-
pared to the other MMCs and Fig. 5 shows such an
indentation in the matrix.
Since Mg enrichment of an interfacial region would
lead to magnesium depletion in the matrix further away
from the particles and thus reduce the age hardening
effects in these regions [7], this would lead to a correla-
tion between the measured hardness in the matrix and
the distance to the nearest reinforcing particle. For the
successful matrix indentations, this distance was mea-
sured using the photographs made with bright field op-
tical microscopy, but no correlation between hardness
and distance to the nearest Al
2
O
3
particle was found.
From Fig. 5, it can be seen that an indentation with
a maximum load of 100 mN takes up approximately
10 µm. Although measurements by Das et al. [7] were
done with a maximum load of 30 mN, it is hard to un-
derstand that they were able to measure exactly in this
interfacial layer without encountering problems simi-
lar to those found in this investigation as shown in
Fig. 4.
To confirm the preferential segregation of Mg near
the particle/matrix interface, linescans were made with
a SEM equipped with EDS (Energy Dispersive Spec-
trum) facilities. Fig. 6 shows such a linescan for the
MMC material in the T6 condition. The box diagram
at the top of the figure shows the translation of the
chemical composition into the spatial distribution of
the constituent phases; for example, the particles can be
recognised by the increase of oxygen and the decrease
of aluminium. Clear indications of Mg enrichment near
particles were obtained. The average thickness of this
Mg rich zone could not be established exactly, but was
estimated to be approximately 4 µm. Such a value is
in good agreement with data by Das et al. [7], who re-
ported values ranging from 2–5 µm depending on an-
nealing time. The limited width of the Mg rich zone ex-
plains the absence of typical ‘interphase’ indentations
in our work, but also raises serious questions about the
correctness of the ‘interphase’ indentations as reported
by Das et al. This thought is also supported by the fact
that the Mg content in the MMC investigated here is
even higher than that in both MMCs investigated by
Das et al. They investigated two MMCs, one consist-
ing of AA6061 reinforced with 15 vol% angular Al
2
O
3
particles and the other was reinforced with 20 vol%
spherical Al
2
O
3
particles. The first MMC contained
792
![Figure 1 DSC traces of AA6061 and the Duralcan AA6061 with 20 vol% Al2O3 particles, heating rate 5◦C/min [8].](/figures/figure-1-dsc-traces-of-aa6061-and-the-duralcan-aa6061-with-17v3133n.webp)
