- Email: [email protected]
The influence of temperature and strain rate on microstructural evolution of polycrystalline copper
ELSEVIER
Materials Science and Engineering A234&236 (1997) 810-813
The influence of temperature and strain rate on microstructural evolution of polycrystalline copper Dirk Ostwaldt Freiherg
Unioersity
of Mining
and Technolog>j,
Institute
*, Peter Klimanek
of Physical
Metallurgy,
Gustav-Zeuner-Strasse
5, D-09596
Freiberg,
Germane
Received 10 February 1997: received in revised form 1 April 1997
Abstract A ballistic testing system is introduced which allows compression tests at high strain rates within a wide range of temperature. The paper describes the experimental results obtained for polycrystalline copper at different strain rates i (10V4 sP I I i I lo4 s ~ I), large strains E (up to E - 1 .O) and various temperatures T (22, 400 and 610°C). Stress strain diagrams were calculated and conclusions about strain rate sensitivity were drawn. The total dislocation density is determined by X-ray diffraction. The inhomogeneity of deformation can be made visible by measuring the microhardness along the specimen axis. The data are the basis for modelling the evolution of microstructure at high strain rates and elevated temperatures. 0 1997 Elsevier Science S.A. Ke,word.y:
High
strain
rate deformation;
Compression
test; Copper;
Microstructure
1. Introduction
2. Ballistic testing system
Materials behaviour and evolution of microstructure during plastic deformation strongly depend on temperature as well as strain rate. Dynamic loading of materials may lead to deformation inhomogeneities caused by (a) inertia effects and wave propagation as consequences of the loading pulse [l], (b) adiabatic heating due to the very short deformation time (usually less than 500 ps) and (c) increasing rate sensitivity following a transition of thermal activated to completely damped dislocation movement [2]. For copper, two regions of strain rate sensitivity exist. In the range of low and medium rates, there is only a slight influence of strain rate on stress. Above i - lo3 SC’ the sensitivity increasesrapidly and stress is a linear function of strain rate [3]. These effects usually complicate the acquisition of data about mechanical behaviour and the analysing of microstructure. Therefore, the performance of such experiments requires not only equipment of high technical standard but also a careful monitoring of the analysing processes.
A ballistic testing system was constructed to perform compression tests at high strain rates [4,5]. The schematic layout is shown in Fig. 1. A projectile is accelerated in the barrel by expansion of compressednitrogen.
* Corresponding author. Tel.: + 49 3731 392097; 393657; e-mail: [email protected] 0921-5093/97/$17.00 6 1997 Elsevier PII SO921-5093(97)00305-5
Science
S.A. All
fax:
-
Pressure chamber Projectile retainer
-
Barrel
- Punch - Spacing washer
- Heat container - Water tank - Strain gages - Transmitter bar
+ 49 3731 Fig.
rights
reserved.
1. Schematic
layout
of the ballistic
testing
system.
D. Ostwaldt,
P. Klimanek
/Materials
Science
Its velocity is determined by measurement of flight time using photodiodes installed at the end of the barrel. The specimen is inside a heat container. For compression tests at elevated temperatures, the container (with specimen and punch) is heated up in an external furnace. To freeze the microstructure of the deformed material, it is possible to eject the specimen immediately after the compression test into a water tank. The measurement of displacement is performed by an optical setup. A photo sensitive detector records the displacement of the punch. The compression force is determined by measurement of the elastic strain of the transmitter bar using strain gages. A calibration procedure is required to transform the measured voltages in forces. The upper end of the transmitter bar is cooled by water to protect the strain gages from the high temperatures and to maintain the validity of calibration. Compression tests at strain rates between - 7 x lo2 S -’ and h 1 x lo4 s-l and temperatures from room temperature up to 1000°C can be performed with this device. The insert of space washers between heat container and punch allows the breakoff of the compression test at certain strains. 3. Experimental
details
A copper rod (E-Cu99.96) with a diameter of 12 mm was fabricated by extrusion. Cylindrical specimens with a height-diameter-ratio of 1.5 and diameters between 5 and 10 mm were produced from this rod. The specimens underwent a heat treatment to get a well defined initial state. After recrystallization the average grain size is 40 urn. The specimens were compressed up to a maximum strain E of 1.0 at strain rates i between 1 x 1O-4 s-r and 8 x lo3 SK’ and various temperatures. In addition to room temperature, deformation tests were performed at 400 and 610°C. The compression tests at strain rates in the low and medium range were performed with a servohydraulic testing system MTS-810. The ballistic testing system was used for dynamic testing. Before being tested the specimens were lubricated with MoS,. Consequently the influence of friction between specimen surface and plates was reduced. For analysing the evolution of microstructure, several methods were at disposal. For that reason, the compressed specimens were cut in two manners, some parallel to the cylinder axis which at the same time is the direction of deformation, the others perpendicular to this. Information about deformation inhomogeneities were derived from observations by optical microscopy and the measurement of microhardness along the cylinder axis (parallel cut). Determination of the dislocation density was carried by analysis of X-ray diffraction line broadening (perpendicular cut) [6,7].
and Engineering
A234-236
Fig. 2. Stress strain curves atures and strain rates.
(1997)
811
810-813
for copper
compressed
at various
temper-
4. Results and discussion All results are summarized in Figs. 2-5 and Fig. 6. Fig. 2 shows the calculated stress strain diagrams. Fig. 3 gives information about rate dependence of flow stress. A selection of micrographs is presented in Fig. 4. Fig. 5, showing the microhardness along the cylinder axis, allows conclusions about deformation inhomogeneities. Finally, the dislocation densities are plotted in Fig. 6. The stress strain curves determined at room temperature increase monotonously (Fig. 2). This is typical behaviour of metals deformed at low homologous temperatures. At high strains, the grains are strongly compressed (Fig. 4(a)). The influence of adiabatic heating is important for strain rates above 1 s ~ ‘. In these cases dynamic recovering processes (caused by thermal activation and superposing the strain hardening) are largely
400
F z
200
e---$3’ A’ ’ ’ ’
’ ’ ‘/L’jlE-Cu99.96, E=O.lS
-x-
”
22% 400% 61OOC 22% [2]
hs=o.30 II 22%
b 0
10-4 10-2 100 lo2 10’ i [s-l]
Fig. 3. Rate dependence different temperatures.
of stress at constant
strains
for copper
at
812
D. Ostwaldt,
P. Klimanek
/Materials
Science
and Engineering
Fig. 6. Total ature, strain
Fig. 4. Micrographs and strain rates.
of compressed
copper
for various
temperatures
enhanced. Therefore, increase of stressbecomes smaller at large strains. Under dynamic loading, the heat transfer between specimen, plates and environment is nearly completely suppressed.This leads to adiabatic test conditions. A recalculation of isothermal stressvalues was not carried out because all microstructural data are influenced by this increasing temperature. Consequently, a better comparability between stress values and microstructure is guaranteed. The increase of temperature at the dynamic loading leads to a softening of the material. This effect is reinforced by a rapid reduction of strain rate at high deformations, causing decreasing stressestoo. Nevertheless, there is a distinct 150
I ---
--
100 -m-
3 0.
-fj
100
b
22OC/610%-’
22%/110~9'
E u
I
.-t
=
E-Cu99.96 initial state e=o.3-0.4 ~=0.6-0.7 E=l.O-1.1
50
-0
1.0
0.5
Normalized Fig. 5. Microhardness rate and axial position.
1 1.0
0.5
distance in dependence
from on temperature,
surface strain,
strain
A234-236
(1997)
0
0.5 E
dislocation and strain
density rate.
810-813
1.0 of copper
in dependence
on temper-
increase of stress level at high strain rates in comparison with the values determined at low rates. In all cases, small deformation inhomogeneities caused by friction were noticeable. The deformation is handicapped in the surface zones. This effect influences the microhardness; the hardness in the central region of specimen is higher than in the surface zones (Fig. 5). Also, higher strain rates produce slightly higher values of microhardness. The stress strain diagrams for compression tests at elevated temperatures (Fig. 2) show a complete different behaviour. The curves reach a steady state at high strains. At 610°C all curves pass through a local maximum. This indicates that dynamic recrystallization takes place. At 400°C three types of curves are to be seen. At quasistatic rates (10V4 SK’ lil 10-l SK’), the stressstrain curves are the same as at 610°C (with higher stress level). The local maximum in the curve is missing in the medium range of strain rate (10’ s - i I i I 10’ SK’). However, the steady state occurs. Under dynamic conditions, such a maximum appears, followed by a rapid decrease of stress. The dynamic recrystallization starts in the centre of specimen where the driving force is high (Fig. 5). This is confirmed by micrographs (for tests at 400°C). They show dynamically recrystallized grains at quasistatic rates as well at dynamic loading (Fig. 4(b) and (c)). Becauseof the very short deformation time the grain growth is not so pronounced under dynamic conditions. Fig. 3 gives information about the rate dependence of flow stress at constant strains. However, this is not comparable to a determination of the rate sensitivity in tests with a sudden increase or decrease of strain rate, but the plot seemssuitable here for an estimation of the influence of strain rate on stress. The rate dependence at room temperature is in accordance with the results of other authors (for example [2], seeFig. 3). At low rates the increase is only slight. However, it becomes steep at rates above (lo’... 103) s ~ l. Another effect is visible: The higher the strain, the stronger the rate dependence of stress. However the graph looks different at elevated
D. Ostwaldt,
P. Klimanek/Materials
Science
temperatures. The influence of strain rate in the quasistatic range becomes more noticeable, but the increment of stress decreases for transition to dynamic loading. The dislocation densities (Fig. 6) are in good accordance with the results mentioned above. Their dependence on strain and temperature is consistent with the behaviour and levels of the flow curves. For room temperature deformation, the dislocation density increases with strain, but there is a stagnation at strains around E - 1.0 and under dynamic conditions. This is due to friction which causes still higher stresses but no effects in the level of dislocation densities resp. microstructure. Because of dynamic recrystallization, at high temperatures the dislocation densities are similar for all strain rates. The values reach the level of initial state ( - 1 x 1O’O cm - *). Only at room temperature a higher strain rate leads to higher dislocation densities.
5. Conclusions For room temperature deformation of polycrystalline copper, stress increases strongly at transition from quasistatic to dynamic rates. However, this dependence becomes smaller at higher temperatures, whereas the influence of quasistatic rates rises. Dynamic recrystallization occurs for 610°C at all strain rates and for 400°C at rates lower than 10 ~ ’ s - ’ and greater than lo3 SK’. Deformation inhomogeneities appear. The central part of the specimen is the region of the strongest deformation, i.e. the highest microhardness. Dynamic
and Engineering
A234-236
(1997)
810-813
813
recrystallization starts in this region under appropriate test conditions. At room temperature, dislocation density depends strongly on strain and slightly on strain rate. However, there is no significant influence of strain and strain rate on dislocation density at elevated temperatures due to dynamic recrystallization.
Acknowledgements The authors are grateful for the financial support the Volkswagen-Stiftung.
by
References [I] J.Z. Malinowski, J.R. Klepaczko, Int. J. Mech. Sci. 28 (1986) 381. [2] P.S. Follansbee, G. Regazzoni, U.F. Kocks, in: J. Harding (Ed.), Proceedings 3rd Conference on the Mechanical Properties at High Rates of Strain, Oxford, 9-12 April 1984, Conference Series 70, Institute of Physics, London, 1984, 71. [3] J.R. Klepaczko, in: W.J. Ammann et al. (Eds.), Proceedings 1st International Conference on Effects of Fast Transient Loadings, Lausanne, 26-27 August 1987, A.A. Balkema, Rotterdam/ Brookfield, 1988, 3. [4] K.-E. Hensger, P. Khmanek, U. Trinks, U. Martin, K. Cyrener, Steel Res. 62 (1991) 124. [5] D. Ostwaldt, W. Pantleon, P. Klimanek, Mat.-wiss. u. Werkstofftech. 27 (1996) 417. [6] M.A. Krivoglaz, O.V. Martynenko, K.P. RjaboSapka, Fiz. Metall. Metalloved. 55 (1983) 5. [7] P. Klimanek, in: J. Hasek (Ed.), X-ray and Neutron Structure Analysis in Materials Science, Plenum Press, New York, 1989, 125.
Materials Science and Engineering A234&236 (1997) 810-813
The influence of temperature and strain rate on microstructural evolution of polycrystalline copper Dirk Ostwaldt Freiherg
Unioersity
of Mining
and Technolog>j,
Institute
*, Peter Klimanek
of Physical
Metallurgy,
Gustav-Zeuner-Strasse
5, D-09596
Freiberg,
Germane
Received 10 February 1997: received in revised form 1 April 1997
Abstract A ballistic testing system is introduced which allows compression tests at high strain rates within a wide range of temperature. The paper describes the experimental results obtained for polycrystalline copper at different strain rates i (10V4 sP I I i I lo4 s ~ I), large strains E (up to E - 1 .O) and various temperatures T (22, 400 and 610°C). Stress strain diagrams were calculated and conclusions about strain rate sensitivity were drawn. The total dislocation density is determined by X-ray diffraction. The inhomogeneity of deformation can be made visible by measuring the microhardness along the specimen axis. The data are the basis for modelling the evolution of microstructure at high strain rates and elevated temperatures. 0 1997 Elsevier Science S.A. Ke,word.y:
High
strain
rate deformation;
Compression
test; Copper;
Microstructure
1. Introduction
2. Ballistic testing system
Materials behaviour and evolution of microstructure during plastic deformation strongly depend on temperature as well as strain rate. Dynamic loading of materials may lead to deformation inhomogeneities caused by (a) inertia effects and wave propagation as consequences of the loading pulse [l], (b) adiabatic heating due to the very short deformation time (usually less than 500 ps) and (c) increasing rate sensitivity following a transition of thermal activated to completely damped dislocation movement [2]. For copper, two regions of strain rate sensitivity exist. In the range of low and medium rates, there is only a slight influence of strain rate on stress. Above i - lo3 SC’ the sensitivity increasesrapidly and stress is a linear function of strain rate [3]. These effects usually complicate the acquisition of data about mechanical behaviour and the analysing of microstructure. Therefore, the performance of such experiments requires not only equipment of high technical standard but also a careful monitoring of the analysing processes.
A ballistic testing system was constructed to perform compression tests at high strain rates [4,5]. The schematic layout is shown in Fig. 1. A projectile is accelerated in the barrel by expansion of compressednitrogen.
* Corresponding author. Tel.: + 49 3731 392097; 393657; e-mail: [email protected] 0921-5093/97/$17.00 6 1997 Elsevier PII SO921-5093(97)00305-5
Science
S.A. All
fax:
-
Pressure chamber Projectile retainer
-
Barrel
- Punch - Spacing washer
- Heat container - Water tank - Strain gages - Transmitter bar
+ 49 3731 Fig.
rights
reserved.
1. Schematic
layout
of the ballistic
testing
system.
D. Ostwaldt,
P. Klimanek
/Materials
Science
Its velocity is determined by measurement of flight time using photodiodes installed at the end of the barrel. The specimen is inside a heat container. For compression tests at elevated temperatures, the container (with specimen and punch) is heated up in an external furnace. To freeze the microstructure of the deformed material, it is possible to eject the specimen immediately after the compression test into a water tank. The measurement of displacement is performed by an optical setup. A photo sensitive detector records the displacement of the punch. The compression force is determined by measurement of the elastic strain of the transmitter bar using strain gages. A calibration procedure is required to transform the measured voltages in forces. The upper end of the transmitter bar is cooled by water to protect the strain gages from the high temperatures and to maintain the validity of calibration. Compression tests at strain rates between - 7 x lo2 S -’ and h 1 x lo4 s-l and temperatures from room temperature up to 1000°C can be performed with this device. The insert of space washers between heat container and punch allows the breakoff of the compression test at certain strains. 3. Experimental
details
A copper rod (E-Cu99.96) with a diameter of 12 mm was fabricated by extrusion. Cylindrical specimens with a height-diameter-ratio of 1.5 and diameters between 5 and 10 mm were produced from this rod. The specimens underwent a heat treatment to get a well defined initial state. After recrystallization the average grain size is 40 urn. The specimens were compressed up to a maximum strain E of 1.0 at strain rates i between 1 x 1O-4 s-r and 8 x lo3 SK’ and various temperatures. In addition to room temperature, deformation tests were performed at 400 and 610°C. The compression tests at strain rates in the low and medium range were performed with a servohydraulic testing system MTS-810. The ballistic testing system was used for dynamic testing. Before being tested the specimens were lubricated with MoS,. Consequently the influence of friction between specimen surface and plates was reduced. For analysing the evolution of microstructure, several methods were at disposal. For that reason, the compressed specimens were cut in two manners, some parallel to the cylinder axis which at the same time is the direction of deformation, the others perpendicular to this. Information about deformation inhomogeneities were derived from observations by optical microscopy and the measurement of microhardness along the cylinder axis (parallel cut). Determination of the dislocation density was carried by analysis of X-ray diffraction line broadening (perpendicular cut) [6,7].
and Engineering
A234-236
Fig. 2. Stress strain curves atures and strain rates.
(1997)
811
810-813
for copper
compressed
at various
temper-
4. Results and discussion All results are summarized in Figs. 2-5 and Fig. 6. Fig. 2 shows the calculated stress strain diagrams. Fig. 3 gives information about rate dependence of flow stress. A selection of micrographs is presented in Fig. 4. Fig. 5, showing the microhardness along the cylinder axis, allows conclusions about deformation inhomogeneities. Finally, the dislocation densities are plotted in Fig. 6. The stress strain curves determined at room temperature increase monotonously (Fig. 2). This is typical behaviour of metals deformed at low homologous temperatures. At high strains, the grains are strongly compressed (Fig. 4(a)). The influence of adiabatic heating is important for strain rates above 1 s ~ ‘. In these cases dynamic recovering processes (caused by thermal activation and superposing the strain hardening) are largely
400
F z
200
e---$3’ A’ ’ ’ ’
’ ’ ‘/L’jlE-Cu99.96, E=O.lS
-x-
”
22% 400% 61OOC 22% [2]
hs=o.30 II 22%
b 0
10-4 10-2 100 lo2 10’ i [s-l]
Fig. 3. Rate dependence different temperatures.
of stress at constant
strains
for copper
at
812
D. Ostwaldt,
P. Klimanek
/Materials
Science
and Engineering
Fig. 6. Total ature, strain
Fig. 4. Micrographs and strain rates.
of compressed
copper
for various
temperatures
enhanced. Therefore, increase of stressbecomes smaller at large strains. Under dynamic loading, the heat transfer between specimen, plates and environment is nearly completely suppressed.This leads to adiabatic test conditions. A recalculation of isothermal stressvalues was not carried out because all microstructural data are influenced by this increasing temperature. Consequently, a better comparability between stress values and microstructure is guaranteed. The increase of temperature at the dynamic loading leads to a softening of the material. This effect is reinforced by a rapid reduction of strain rate at high deformations, causing decreasing stressestoo. Nevertheless, there is a distinct 150
I ---
--
100 -m-
3 0.
-fj
100
b
22OC/610%-’
22%/110~9'
E u
I
.-t
=
E-Cu99.96 initial state e=o.3-0.4 ~=0.6-0.7 E=l.O-1.1
50
-0
1.0
0.5
Normalized Fig. 5. Microhardness rate and axial position.
1 1.0
0.5
distance in dependence
from on temperature,
surface strain,
strain
A234-236
(1997)
0
0.5 E
dislocation and strain
density rate.
810-813
1.0 of copper
in dependence
on temper-
increase of stress level at high strain rates in comparison with the values determined at low rates. In all cases, small deformation inhomogeneities caused by friction were noticeable. The deformation is handicapped in the surface zones. This effect influences the microhardness; the hardness in the central region of specimen is higher than in the surface zones (Fig. 5). Also, higher strain rates produce slightly higher values of microhardness. The stress strain diagrams for compression tests at elevated temperatures (Fig. 2) show a complete different behaviour. The curves reach a steady state at high strains. At 610°C all curves pass through a local maximum. This indicates that dynamic recrystallization takes place. At 400°C three types of curves are to be seen. At quasistatic rates (10V4 SK’ lil 10-l SK’), the stressstrain curves are the same as at 610°C (with higher stress level). The local maximum in the curve is missing in the medium range of strain rate (10’ s - i I i I 10’ SK’). However, the steady state occurs. Under dynamic conditions, such a maximum appears, followed by a rapid decrease of stress. The dynamic recrystallization starts in the centre of specimen where the driving force is high (Fig. 5). This is confirmed by micrographs (for tests at 400°C). They show dynamically recrystallized grains at quasistatic rates as well at dynamic loading (Fig. 4(b) and (c)). Becauseof the very short deformation time the grain growth is not so pronounced under dynamic conditions. Fig. 3 gives information about the rate dependence of flow stress at constant strains. However, this is not comparable to a determination of the rate sensitivity in tests with a sudden increase or decrease of strain rate, but the plot seemssuitable here for an estimation of the influence of strain rate on stress. The rate dependence at room temperature is in accordance with the results of other authors (for example [2], seeFig. 3). At low rates the increase is only slight. However, it becomes steep at rates above (lo’... 103) s ~ l. Another effect is visible: The higher the strain, the stronger the rate dependence of stress. However the graph looks different at elevated
D. Ostwaldt,
P. Klimanek/Materials
Science
temperatures. The influence of strain rate in the quasistatic range becomes more noticeable, but the increment of stress decreases for transition to dynamic loading. The dislocation densities (Fig. 6) are in good accordance with the results mentioned above. Their dependence on strain and temperature is consistent with the behaviour and levels of the flow curves. For room temperature deformation, the dislocation density increases with strain, but there is a stagnation at strains around E - 1.0 and under dynamic conditions. This is due to friction which causes still higher stresses but no effects in the level of dislocation densities resp. microstructure. Because of dynamic recrystallization, at high temperatures the dislocation densities are similar for all strain rates. The values reach the level of initial state ( - 1 x 1O’O cm - *). Only at room temperature a higher strain rate leads to higher dislocation densities.
5. Conclusions For room temperature deformation of polycrystalline copper, stress increases strongly at transition from quasistatic to dynamic rates. However, this dependence becomes smaller at higher temperatures, whereas the influence of quasistatic rates rises. Dynamic recrystallization occurs for 610°C at all strain rates and for 400°C at rates lower than 10 ~ ’ s - ’ and greater than lo3 SK’. Deformation inhomogeneities appear. The central part of the specimen is the region of the strongest deformation, i.e. the highest microhardness. Dynamic
and Engineering
A234-236
(1997)
810-813
813
recrystallization starts in this region under appropriate test conditions. At room temperature, dislocation density depends strongly on strain and slightly on strain rate. However, there is no significant influence of strain and strain rate on dislocation density at elevated temperatures due to dynamic recrystallization.
Acknowledgements The authors are grateful for the financial support the Volkswagen-Stiftung.
by
References [I] J.Z. Malinowski, J.R. Klepaczko, Int. J. Mech. Sci. 28 (1986) 381. [2] P.S. Follansbee, G. Regazzoni, U.F. Kocks, in: J. Harding (Ed.), Proceedings 3rd Conference on the Mechanical Properties at High Rates of Strain, Oxford, 9-12 April 1984, Conference Series 70, Institute of Physics, London, 1984, 71. [3] J.R. Klepaczko, in: W.J. Ammann et al. (Eds.), Proceedings 1st International Conference on Effects of Fast Transient Loadings, Lausanne, 26-27 August 1987, A.A. Balkema, Rotterdam/ Brookfield, 1988, 3. [4] K.-E. Hensger, P. Khmanek, U. Trinks, U. Martin, K. Cyrener, Steel Res. 62 (1991) 124. [5] D. Ostwaldt, W. Pantleon, P. Klimanek, Mat.-wiss. u. Werkstofftech. 27 (1996) 417. [6] M.A. Krivoglaz, O.V. Martynenko, K.P. RjaboSapka, Fiz. Metall. Metalloved. 55 (1983) 5. [7] P. Klimanek, in: J. Hasek (Ed.), X-ray and Neutron Structure Analysis in Materials Science, Plenum Press, New York, 1989, 125.