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Influence of Strain Rate and Temperature on Mechanical Properties and Fracture Mechanism of Dispersion Strengthened Al12Al4C3 System

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In the presented work the change in fracture for the Al-12Al4C3 system was investigated and analysed at temperatures from 20 to 400 °C and strain rates from 2.5·10 -5 to 10-1 s-1. At room temperature 20 °C, during tensile testing at strain rates in the tested region, the strain is first controlled by work hardening, expressed by the exponent n. In the second generally smaller part the deformation is limited by local straining and forming the neck. There is a marked decrease of plastic properties for the strain rate ε& = 2.5·10 -5 s-1 with the growth of temperature in the investigated region. It is explained by changes in the micromechanism of deformation and fracture. Fracture surface shows the transition from ductile fracture with dimples at 20 °C, to intercrystalline fractures, with the growth of the temperature, an indication of exhausted grain boundary plasticity. The intercrystalline fracture initiation tends, it is supposed, to be localized in triple points. At temperatures 400 °C, and ε& = 10 -1 s-1 there is a marked growth of plastic properties. The first part of the strain characterized by work hardening is very short. After the short growth of the stress to maximum, a deformation mechanism, showing presence of thermally and mechanically activated dynamic recovery processes takes place. The strain is this way uniform all over the body of the test piece. The fracture process ends with the increase of cavities and transcrystalline fracture with deep pimples.
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Effect of Strain Rate and Temperature on Mechanical
Properties and Fracture Mechanism of the Dispersion
Strengthened A1-12A14C3 System
Oksana Velgosova'*, Michal Besterci2, Pritt Kulu1
'Technical University Faculty of Metallurgy, Department of Non-ferrous Materials and Waste Treatment,
Letnä 9/A, Kosice 04200, Slovakia
2 Institute of Materials Research, Slovak Academy of Sciences, Watsonova 47, Kosice 043 53, Slovakia
3Tallin University of Technology, Etritajate tee 5, 19086 Tallinn, Estonia
(Received January 21, 2005: final form February 22, 2005.)
ABSTRACT
The change in fracture for the AI-I2AI4C3 system
has been investigated at temperatures from 293 to
673 K. Under strain rates from έ = 2.5· 10'5 to έ = 10'1
s'1. During tensile testing at room temperature, the
strain was controlled by dislocation movement and
reactions. The first part of the strain was characterized
by work hardening expressed by the exponent "n" and
the second part was expressed by the local strain in the
neck. At high temperatures presently investigated the
principal mechanism for originating the strain is
attributed to the dynamic recovery process. The effect
of strain rate on facture was also analysed.
Keywords: Dispersion strengthened materials;
Fracture mechanism; SEM analysis
1. INTRODUCTION
The dispersion strengthened alloys AI-AI4C3
prepared by mechanical alloying using powder
metallurgy are promising structural materials enabling
significant weight reduction for use of aircraft,
automobile and materials at elevated temperatures /1-3/.
We have described /4/ the optimised condition for
producing composite materials by mechanical alloying
and their mechanical properties including plasticity at
elevated temperatures were presented in 151. In /6/, the
high temperature stability of this alloy was described for
long time exposition. In /7-8/, the microstructure changc
was examined after deformation at elevated
temperatures and the fracture mechanism in creep was
also studied. The model of "in-situ" fracture mechanism
for these materials was described /9,10/. It should be
mentioned that the kinetics and mechanism of
superplastic deformation of various Al alloys are given
in many works /11-16/.
The purpose of this work is to present the fracture at
different strain rates and temperature for the A1-12A14C3
system. Temperatures and strain rates are from 293 to
673 Κ and from έ=2.5·10"5 to έ =10'' s'1, respectively.
Considering the results reported in /17/ and the strain
induced coefficient m ranging from 0.03 to 0.06, this
work covers the strain rates from creep to the activation
of superplastic behavior.
2. EXPERIMENTAL MATERIAL AND
METHODS
The AI-AL1C3 composite dispersion strengthened by
12 vol.% of AI4C3, was prepared by mechanical
"Corresponding author. E-mail: oksana.velgosova@tuke.sk. Fax number: (+42195) 633 70 48
183
Vol.
24, No. 3. 2005 Effect of Strain Rate and Temperature on Mechanical Properties
And Fracture Mechanism of the Dispersion
alloying. The Al powder, grain size under 100 μηι, with
1% of KS 2.5 graphite was milling for 90 minutes.
Granulate was compacted under the pressure of 600
MPa and annealed at 823 Κ with a 94% reduction.
The dispersed particles are oriented in layers in the
direction of extrusion. The effective size of the particles
was found to be 25 χ 85 nm, as shown in Fig. 1. There
Fig. 1: Effective size of AI4C3 particles on the foil.
were also larger dispersed particles and their
distribution was estimated to be from 85 nm to 1 μπι in
size, making up about 30% of dispersoid amount,
observed by Scanning Electron Microscopy (SEM) and
optical metallography. The A14C3 and A1203 particles
are located in the grain boundaries as well as inside the
Al grains. The observed microstructure is fine, even,
with grains less than 1 μπι, elongated in the direction of
extrusion as shown in Fig. 2.
Test pieces of 3 mm in diameter and 15 mm gauge
length were machined for tensile test. They were
positioned in longitudinal direction, in the direction of
extrusion. For the evaluation of strain and fracture
mechanisms, SEM observation was made for the
fracture surfaces.
3. RESULT AND DISCUSSION
Figure 3 shows the yield strength, Rpo.2, and the
[
200 nm
Fig. 2: The grain size of Al matrix on the foil.
600
500
OJ
§•300
200
100
90
80
70
60
50 _
£
40 N
30
20
10
250 450 t 650
Fig. 3: Influence of temperature and strain rate on
yield strength Rpo.2 and reduction of area Z.
reduction of area, Z, as a function of temperature. The
results with strain rates applied are also included in this
figure. The results with strain rate of έ=10"' s"' at 673
Κ indicate a rapid increase in the value of Z. However,
184
Oksana Velgosova et al. High Temperature Materials and Processes
no substantial change is detected in the value of yield
strength. It may be worthy of note in Fig. 4 that this is
supported by the results obtained for tensile strength,
Rm, and elongation, A5.
Figure 5 shows the fracture surface of the sample
loaded with high strain rate of έ =10"' s"' at 293 K. On
the other hand, the result for the sample with low strain
rate of e=2.510"5 s"' is given in Fig. 6. There are no
significant differences in these two results. It is also
noted that these two samples are ductile and
transcrystalline fracture with dimples. The dimples are
found shallow with their typical dimension of 0.45 μπι.
Fracture surfaces fractured under strain rate of
Έ=2.5Ί0"5 S"' at 573 Κ show the underdeveloped
intercrystalline facets. They are underdeveloped because
the so-called ductile fracture takes place at the end of
fracture. Under strain rate of έ=10"' s"1, on the other
hand, the fracture is considered to be transcrystalline
with dimples. The dimples are deeper and larger than is
the case at 293 K. The characteristic dimple diameter is
around 0.6 μπι.
Fig. 5: Transcrystalline fracture surface obtained with
strain rate of έ = 10"1 s'1 at 293 K.
Fig. 6: Transcrystalline fracture surface obtained with
strain rate of έ = 2.5· 10"5 s"1 at 293 K.
Under low strain rate of έ =2.5-10'5 s"1 at 673 K, the
fracture is found to take place in the reduction of area
Z=8%. Typical micro facets of the fracture are given in
Fig. 7. As discussed for metallic materials, the
developed intercrystalline facets are present, with
dimensions corresponding to the fine grain size, and
great angle disorientation. There are small parts of
185
έ = 10Λ_1
Fig. 4: Influence of temperature and strain rate on
tensile strength Rm and elongation A5.
Vol. 24, No. 3. 2005 Effect of Strain Rate and Temperature on Mechanical Properties
And Fracture Mechanism of the Dispersion
Fig. 7: Intercrystalline fracture surface obtained with
strain rate of έ = 2.5105 s'1 at 673 K.
fracture showing crests of ductile facets. Under strain
rate of έ =10"' s"' the fracture is ended at the reduction
of area Z=64%, where the fracture is considered to be
ductile transcrystalline with developed deep dimples as
shown in Fig.8. The characteristic dimple dimension is
estimated to be 0.65 μπι.
Fig. 8: Transcrystalline fracture surface obtained with
strain rate of έ = 10"' s'1 at 673 K.
The temperature dependence of the middle dimples
diameter is shown in Fig. 9 using the case under strain
rate έ=10"' s"'. The fracture under strain rate έ =
2.5·
10"5
s"' at 673 Κ has been considered intercrystalline
fracture, by finding the result of damage to grain
boundaries formed by interactions of dislocations with
dispersed particles on grain boundaries. However, the
fine particles on grain boundaries are important for
diffusion creep and strength properties of dispersion
strengthened system at high temperatures.
Transcrystalline fractures under high strain rates
(£=10"' s"1) show a remarkable recovery in the plateau
part of the stress-strain curve and this is attributed to the
dynamic polygonisation, resulting in the equilibrium of
strengthening and weakening. The fracture is caused by
voids occurring at the interfaces dispersoid-matrix first
on larger particles, after a great amount of strain. The
voids are growing and coalesce into dimples of the
transcrystalline fracture.
200 300 400 500 600 700
temperature [K]
Fig. 9: The middle dimple diameter as a function of
temperature using the case with strain rate
of έ = 10"' s'1
4. CONCLUSION
The change in fracture for the A1-12A14C3 system
has been investigated at temperatures from 293 to 673 Κ
and with strain rates from έ= 2.5· 10"5 to έ = 10"' s"'.
The results are summarized as follows.
1. At 293 K, during tensile testing under strain rates
186
Oksana Velgosova et al. High Temperature Materials and Processes
presently tested, the strain is at first controlled by
work hardening, expressed by the exponent n. In the
next stage, the deformation is, more or less, affected
by local straining and forming the neck.
2. There is a marked decrease of plastic properties
under strain rate of έ= 2.5·ΙΟ"5 s'1 with increasing
temperature. The results are considered relevant to
changes in micromechanism of deformation and
fracture. At 293 K, fracture surface shows the
transition from ductile fracture with dimples to
intercrystalline fracture, suggesting the exhausted
grain boundary plasticity with increasing
temperature.
3. In the results under strain rate of £=10~'s"1 at 673
K, the first part of the strain characterized by work
hardening was found to be very short. Then, the
stress quickly reached the maximum. In this stage,
the thermally and mechanically activated dynamic
recovery is quite likely to take place where the strain
is uniform all over the body of the test piece. The
fracture process indicates an increase of cavities and
it is described by transcrystalline fracture with deep
dimples.
ACKNOWLEDGEMENT
This work has been supported by grant 2/5142/25.
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