Y. S. KAKHKI et al.: IMPRESSION RELAXATION AND CREEP BEHAVIOR OF Al/SiC NANOCOMPOSITE
611–615
IMPRESSION RELAXATION AND CREEP BEHAVIOR OF Al/SiC
NANOCOMPOSITE
SPROSTITEV VTISA IN OBNA[ANJE Al/SiC NANOKOMPOZITA
PRI LEZENJU
Yasaman Saberi Kakhki, Said Nategh, Tehrani Shamseddin Mirdamadi
Islamic Azad University, Tehran Science and Research Branch, Department of Materials Engineering, Tehran, Iran
s.nategh@srbiau.ac.ir
Prejem rokopisa – received: 2015-06-04; sprejem za objavo – accepted for publication: 2015-08-25
doi:10.17222/mit.2015.113
In the present study, Al-4 % of volume fractions of SiC composites were produced by mechanical alloying and the powders
were gradually compacted at a pressure of 620 MPa. The post-compaction samples were sintered under an argon atmosphere at
873 K and the corresponding creep results were obtained from impression-relaxation. Additionally, compression techniques
were investigated at high temperature (723 K). Enforcement rates of 8.3 × 10–3 and 0.83 × 10–3 mm s–1 and indenter depths of
0.5 mm and 0.8 mm were selected. Results showed a constant relation between stress relaxation and compression creep rate.
Under different conditions of impression-relaxation this constant was 1000 for compression stress of 30 MPa and 32.5 MPa, and
400 for compression stress of 35 MPa, subsequently. This coefficient was affected by the porosity and was stable for different
indenter depths and enforcement rates. The variations in the steady state relaxation rate were reasonable because of different
nano-SiC distributions and porosities in the form of drops. Moreover, in spite of the enforcement rate decrease, the nanosized
reinforcing particles caused a decrease of the relaxation rate. It should be mentioned that the constant coefficient calculated will
be useful to estimate the fracture time which uses the strain rate calculated from impression-relaxation in industrial applications.
Keywords: Al-SiC, creep, impression relaxation, nanocomposite, powder processing
V {tudiji so bili izdelani kompoziti Al-4 % volumenskega dele`a SiC z mehanskim legiranjem, prahovi pa so bili postopoma
stisnjeni do tlaka 620 MPa. Stisnjeni vzorci so bili nato sintrani v atmosferi argona na 873 K. Iz sprostitve pri vtiskovanju je bila
dobljena povezava z rezultati lezenja. Dodatno so bile preiskovane tehnike stiskanja pri visoki temperaturi (723 K). Izbrani sta
bili hitrosti vtiskovanja 8,3 × 10–3 mm s–1 kot tudi 0,83 × 10–3 mm s–1 pri globini vtiska 0,5 mm in 0,8 mm. Rezultati so pokazali
konstanten koeficient med sprostitvijo napetosti in hitrostjo lezenja pri tla~enju. Vrednost konstante je bila pri razli~nih pogojih
sprostitve vtiska 1000 pri napetostih stiskanja 30 MPa, 32,5 MPa in 400 pri napetosti stiskanja 35 MPa. Na koeficient vpliva
tudi poroznost, stabilen pa je pri razli~nih globinah vtisa in hitrostih vtiskovanja. Tudi rezultati sprostitve so bili ob~utljivi
zaradi razli~ne razporeditve SiC nanodelcev in poroznosti v obliki kapljic. Poleg tega, kljub zmanj{evanju hitrosti vtiskovanja,
nanoutrjevalci zmanj{ajo hitrost sprostitve. Pomembno je omeniti, da bo iz hitrosti obremenjevanja in spro{~anja pri vtiskovanju
izra~unani konstantni koeficient uporaben za napovedovanje ~asa poru{itve pri industrijski uporabi.
Klju~ne besede: Al-SiC, lezenje, sprostitev pri vtiskovanju, nanokompozit, obdelava prahu
1 INTRODUCTION
Powder-processed aluminum alloys reinforced with
SiC particles provide significantly enhanced properties
over conventional monolithic materials, such as higher
specific modulus, strength and thermal stability. They are
widely utilized in the aerospace and automobile industry
as ground vehicle brake rotors, or combustion engine
components.1–3 For the goal of investigating the creep
properties, methods such as uniaxial (tension and com-
pression), impression and relaxation are often used.4–8
The stress relaxation test is the ideal method to inves-
tigate the creep behavior of soft materials. In such tests,
specimens are subjected to impressions at a predeter-
mined impression depth level, the cross head is arrested
and the decrement in magnitude (supposed depth) as a
function of time is recorded.5,6,8 The stress–relaxation
test has the advantages of simplicity and speed over the
conventional creep test.5,6 There have been many studies
evaluating the stress exponent and activation energy of
creep mechanisms in different alloys employing various
techniques.4–8 Furthermore, in different studies, the
uniaxial creep properties of Al-SiC with micron sized
reinforcements have been examined,9–13 but the effect of
nanosized reinforcement and the evaluation of the steady
state creep rate with compression and impression rela-
xation methods have not been investigated. Therefore,
this paper aims at investigating the steady state creep rate
of Al with SiC reinforcement (nanoparticles) by com-
pression and impression relaxation tests. This approach
is used to determine the correspondence of the creep
results obtained through these different approaches. It is
worth highlighting that the novelty of the present
research is the evidence of the relaxation creep theory in
these materials. The creep behavior of these composites
with different methods has provided an understanding of
the actual characteristics of nanocomposite. Addition-
ally, this research aims at exploring the creep rate to
evaluate the fracture time, especially in industrial cases,
with porosity, with non-destructive methods.
Materiali in tehnologije / Materials and technology 50 (2016) 4, 611–615 611
UDK 621.763:669.715:67.017 ISSN 1580-2949
Professional article/Strokovni ~lanek MTAEC9, 50(4)611(2016)
2 MATERIALS AND METHODS
The base material used in the present experimental
investigation are synthesized from: atomized Al powders
with a purity of 99.5 % and a particle size of less than 45
micron; reinforcing SiC powders with a purity of 99 %
and a particle size from 45 nm to 65 nm (Figure 1): and
stearic acid with purity of 97.5 % as a process control
agent. Al-4 % of volume fractions SiC composites were
made using a laboratory scale high-energy planetary ball
mill at 260 min–1 with a 2 % stearic acid mass fraction as
a surface active agent and a ball to powder weight ratio
of 15:1. Milling was carried out under an Ar atmosphere
(99.999 % purity). In order to avoid a significant
temperature rise for the 4 h required to complete mixing,
the ball milling process were stopped periodically for 20
min, then resumed for 45 min. The composite powders
obtained by mechanical alloying, were gradually com-
pacted uniaxially at room temperature using a cylindrical
die-punch assembly (double end compaction type) to
620 MPa with a Zwick 1496-2d. The compacted samples
were sintered under Ar atmosphere (99.999 %) for 1 h at
a temperature of 873 K. The density of the composite
was determined by the Archimedes’ principle. Com-
pression creep measurements were made on each sample
(diameter and length of 10 mm) using stresses in the
range of 30–35 MPa with a Santam (STM 150) universal
tensile testing machine.
Table 1 shows the sample specification (diameter of
10 mm and length of 5 mm) for the impression tests. To
measure the impression relaxation creep, a primary
impression of 0.5 mm depth with a speed of 8.3 × 10–4
mm s–1 was used for all specimens. Having reached this
indenter depth, the crosshead was stopped and the
decrease in force with time was recorded. In another test
the speed was increased to 8.3 × 10–3 mm s–1 and inden-
ter depths of 0.5 mm and 0.8 mm were selected. The
stress relaxation results were also obtained.
Table 1: Porosity of samples used in impression tests
Tabela 1: Zna~ilnosti uporabljenih vzorcev pri preizkusu vtiskovanja
Sample Porosity (%)
1 8.5
2 13.6
3 8.1
4 10.5
5 12.2
6 11.4
7 8.9
3 RESULTS AND DISCUSSION
Figure 2 and Table 2 show the compression creep
data. Figure 3 and Table 3 show the impression stress
relaxation tests in different conditions. In Tables 4 and 5,
the relationship between stress reduction in the im-
pression-relaxation tests and creep rate in the uniaxial
creep test is shown.
Y. S. KAKHKI et al.: IMPRESSION RELAXATION AND CREEP BEHAVIOR OF Al/SiC NANOCOMPOSITE
612 Materiali in tehnologije / Materials and technology 50 (2016) 4, 611–615
Figure 2: Compression strain as a function of time and compression
stress
Slika 2: Skr~ek v odvisnosti od ~asa in napetosti pri stiskanju
Figure 1: a) Morphology of as-received aluminum powders, b) TEM-micrograph of as-received nano-SiC powders
Slika 1: a) Morfologija dobavljenih prahov Al, b) TEM-posnetek dobljenega SiC nanoprahu
Table 2: Compression steady state creep rate as a function of stress
and porosity
Tabela 2: Hitrost lezenja pri stiskanju v odvisnosti od napetosti in po-
roznosti
Sample Porosity (%) Compression(MPa)
Compression
creep rate (1/s)
1 12 30 3 × 10–6
2 12 32.5 3 × 10–6
3 16 35 1 × 10–5
Table 3: Stress relaxation rate of composite samples as a function of
indenter depth and enforcement rate
Tabela 3: Hitrost spro{~anja kompozitnih vzorcev v odvisnosti od
globine vtiska in hitrosti vtiskovanja
Sample
Indenter depth
(mm), speed
(× 103 mm s–1)
d/dt (MPa/s)
× 103
1 0.5, 8.3 7
2 0.5, 8.3 0.9
3 0.5, 8.3 3
4 0.8, 8.3 4
5 0.5, 0.83 6
6 0.5, 0.83 3
7 0.5, 0.83 3
Table 4: Coefficient of stress relaxation and compression creep rates
(C) (Compression = 30, 32.5 MPa)
Tabela 4: Koeficient spro{~anja napetosti in hitrost lezenja pri
stiskanju (C) (Compression = 30, 32,5 MPa)
Sample
(a)
Indenter depth
(mm), speed
(×103 mm s–1)
C(MPa)
{
o = (1/C)(d/dt)}
Average (C)
1 0.5, 8.3 2333 C=1211
(samples
(1, 2, 3))
2 0.5, 8.3 300
3 0.5, 8.3 1000
4 0.8, 8.3 1333 C = 1333
5 0.5, 0.83 2000 C = 1333
(samples
(5, 6, 7))
6 0.5, 0.83 1000
7 0.5, 0.83 1000
Table 5: Coefficient of stress relaxation and compression creep rates
(C) (Compression = 35 MPa)
Tabela 5: Koeficient relaksacije napetosti in hitrost lezenja pri
stiskanju (C) (Compression = 35 MPa)
Sample
(b)
Indenter depth
(mm), speed
(×103 mm s–1)
C(MPa)
{
o = (1/C)(d/dt)}
Average (C)
1 0.5, 8.3 700 C = 363
(samples
(1, 2, 3))
2 0.5, 8.3 90
3 0.5, 8.3 300
4 0.8, 8.3 400 C = 400
5 0.5, 0.83 600 C = 400
(samples
(5, 6, 7))
6 0.5, 0.83 300
7 0.5, 0.83 300
It is clear that the stress relaxation is a suitable
criterion of the strain rate because of the constant strain
(indenter depth) during the test.5,14 Figure 3 shows the
positive effect of the enforcement rate of 0.83 × 10–3
mm s–1 and impression depth of 0.8 mm on recovery rate
or viscoelastic coefficient decrease, relaxation (d/dt) or
creep rate. In addition, the porosity and SiC distribution
variations affect the movement of dislocations and
threshold stress or internal friction stress. Experimental
results on the creep behavior of Al composite with mi-
cron sized SiC by A. B. Pandey9 have emphasized the
threshold stress increase with SiC content, interparticle
spacing or particle size decrease. Thus, the creep
resistance has been improved. Similar creep behavior has
been reported in Al reinforced with nanoparticles which
is the Orowan strengthening mechanism under the
nano-reinforcement effect.15,16 In these studies the effect
of oxide nanoparticles which leads to the reduction of
dislocation movement has been clearly shown. Thus, in
agreement with previous studies9,15–17, the reinforcing
phase leads to creep resistance in the composites. Also
the reinforcement distribution and particle size have been
shown to affect the creep behavior of composites.9,18 The
novelty of this research in comparison with earlier work
is in observation of the creep behavior of a composite
using the impression-relaxation method. Initially, the
speed of relaxation is high, as shown in Figure 3. Then
the immobilized dislocations generate resistance and
threshold stress. Consequently, the stress relaxation
decreases until equilibrium between recovery and work
hardening or the steady state condition (constant slope)
is achieved. The stress relaxation behavior of materials
leading to a steady state condition has been observed in
Y. S. KAKHKI et al.: IMPRESSION RELAXATION AND CREEP BEHAVIOR OF Al/SiC NANOCOMPOSITE
Materiali in tehnologije / Materials and technology 50 (2016) 4, 611–615 613
Figure 3: Stress relaxation tests as a function of impression depth and
strain rate: a) samples 1, 2, 3, 4, b) samples 5, 6, 7
Slika 3: Preizkus sprostitve v odvisnosti od globine vtiskovanja in
hitrosti obremenjevanja: a) vzorci 1, 2, 3, 4, b) vzorci 5, 6,7
other materials.19–21 Thus, in composites with 4 % of
volume fractions SiC (nano) produced by mechanical
alloying, compacting with a compression force of 620
MPa, and sintering at 873 K, the primary stress rela-
xation rate decreases due to the creation of immobilized
dislocations (pinning at the nano-reinforcement).
Correspondingly, the existence of voids (depending on
the indenter size) in the form of drops, or more
relaxation, are observable in graphs. Considering Tables
4 and 5, the similarity of the coefficient of compression
creep as well as the impression relaxation rates reveal
minute differences of microstructure or SiC particles
clustering. This issue has been proved in Tables 1 and 2
in which the samples’ densities are shown. These minute
differences are illustrated in the steady state rate
fluctuations of the different samples’ relaxation graphs.
The impression relaxation rate increases with the
increase of porosity and SiC clustering. This result
shows that the impression relaxation test with this
indenter size can be useful for obtaining the actual
composite characteristics within a short test time.
In line with earlier studies,5,14 correlation of the rela-
xation and uniaxial creep data, and the strain rate (
°)
can be estimated using the stress relaxation rate (d/dt)
and the elastic modulus (E) according to the following
Equation (1):


0 1= −
E t
d
d
(1)
According to Tables 4 and 5, the compression and
impression methods are related to each other with the
approximate constant (C), (1000 MPa), even with chang-
ing indenter depth and enforcement speed, taking into
account the role of porosity, Table 1. The stability of the
coefficient is decreased with the enforcement speed or
strain rate (recovery increase) and the higher strength in
samples 5, 6, and 7 is justified by the effect of the nano-
SiC in these composites. It means that the compression
creep rate and impression stress relaxation in these
composites are related by a constant of 1000 (MPa), as
given in Equation (1). It is useful for the determination
of the composite fracture time using equations relating
the strain rate and fracture time. The determination of the
compression strain rate using the impression and im-
pression stress relaxation methods, which rapidly yields
the actual composite characteristics, helps in estimating
the fracture time with the safety factor. In this research,
on the basis of possible applications of this composite at
high temperatures and compression forces (such as in a
piston), the impression relaxation test was carried out at
a temperature of 723 K and with high impression depths,
e.g. 0.5 mm and 0.8 mm, selected for industrial appli-
cations, and the relationship between compression rate
and stress relaxation rate (coefficient) was calculated.
Moreover, the stability of the coefficient (C) over diffe-
rent experiments has been proved. In impression rela-
xation tests the value of the coefficient does not change
with different indenter depth or enforcement speed,
which is related to the SiC content. As Tables 4 and 5
show, the coefficient is affected only by the porosity and
nano SiC distribution. This constant coefficient can be
taken to be a function of the SiC content. These findings
are suitable for the evaluation of fracture times and
compression creep rate for research and industrial appli-
cations.
4 CONCLUSIONS
In the present study, a constant coefficient between
impression relaxation rate and compression creep rate
has been shown. The results indicate that in
impression-relaxation tests, the impression depth and
enforcement speed have no effect on the coefficient. The
SiC reinforcement nanoparticles act as work-hardening
particles. The porosity and the non-symmetrical
distribution of SiC led to variations of the steady state
relaxation rate or creep rate. The constant coefficient was
a function of SiC content, porosity and the composite’s
elastic modulus.
Acknowledgements
The authors would sincerely like to acknowledge the
Materials Engineering Department of Ferdowsi
University of Mashhad for the experimental support of
the research work.
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Materiali in tehnologije / Materials and technology 50 (2016) 4, 611–615 615