VOLUME 31

ISSUE 2
of Achievements in Materials December
and Manufacturing Engineering 2008

Analysis of the Portevin - Le Chatelier

effect in tin bronzes at elevated
temperatures
W. Ozgowicz*, B. Grzegorczyk
Division of Constructional and Special Materials,
Institute of Engineering Materials and Biomaterials, Silesian University of Technology,
ul. Konarskiego 18a, 44-100 Gliwice, Poland
* Corresponding author: E-mail address: wojciech.ozgowicz@polsl.pl
Received 02.09.2008; published in revised form 01.12.2008

Materials

ABSTRACT
Purpose: The aim of the present paper is the determination of the effect of the chemical composition and
temperature of deformation of standardized tin bronzes and bronze modified with zirconium on the Portevin
– Le Chatelier (PLC) phenomenon, mainly basing on the shape of stress-strain curves within the temperature
range of 100-300oC and observations of their structure. The reasons of the occurrence of such en effect are so
far no fully known and explained and the opinions concerning is physical basis vary.
Design/methodology/approach: Of essential design in this research is determination of the dependence of PLC
effect on the chemical composition, temperature and strain rate and the preliminary heat treatment and grain
size. The main method used in this investigation is tensile test at elevated temperature.
Findings: The main conclusions are following: the PLC effect on tin bronzes with a micro-addition of zirconium
in an amount of 0.01-0.05% depends the temperature of deformation in the tensile test in the range of 100-300oC
and the chemical composition of the alloys; the type of serration revealed on the σ–ε curve depend mainly on the
temperature of deformation and can be differ during the respective stages of the analyzed curves.
Practical implications: In this paper implications for practice are not taken into consideration.
Originality/value: In this paper an additive type of serration observed on the curves σ–ε, denoted by the symbol
D, is new.
Keywords: Metallic alloys; Plastic instability; Portevin - Le Chatelier effect; Tensile test at elevated temperature

[1-5, 10, 11, 13-15] concerning this phenomenon, but there is still
1. Introduction
1. Introduction no explicit explanation of the reasons of its occurrence in some
definite conditions of plastic deformation. This phenomenon is
2In solid solutions of many metallic alloys, both interstitial still the topic of numerous investigations, verifying the actual
and substitutional ones, deformed at elevated temperature the theories of the phenomenon PLC, and concerning the aspect of
phenomenon of heterogenic deformation occurs, generally called modeling it and its computer simulation [6-9, 12].
Portevin – Le Chatelier effect (PLC). The most characteristic Up-to-date theories concerning the PLC effect assume a prior
feature of this phenomenon is the presence of irregularities on the the influence of alloy atoms with dislocation which hampers the
tensile curves (the so-called teeth or steps - Fig. 1) which occur in dislocation movement, thus preventing the plastic deformation of
a definite range of temperature and strain rate (ȑ) depending of the alloys. In the case of a defined strain rate (ȑ), therefore, such a
kind of the investigated alloys. There are several publications stress is required which is higher than the resistance of the

© Copyright by International OCSCO World Press. All rights reserved. 2008 Research paper 281
 Journal of Achievements in Materials and Manufacturing Engineering Volume 31 Issue 2 December 2008

dislocation movement and the effect of the alloy atoms with dislocations from the atmosphere of alloy atoms, but also of the
dislocation. The break away of the dislocation from the alloy renewed flow of atoms towards the dislocation, which is a
atom results in a drop of the stress indispensable for the further diffusive process. The occurrence of the PLC effect is, in the case
dislocation movement by the magnitude of this effect. The model of the assumed model, conditioned by the equalization of the
suggested by Cottrell [1, 3-5, 10, 13, 15] motivates qualitatively dislocation rate and the velocity of diffusion of the alloy atoms
the cyclic changes of stresses recorded during the tensile test. The [2, 3, 10]. As the average rates of diffusion are small if compared
moment of dislodging of the dislocation from the blocking atoms with normal dislocation rates, the effect of interdependence of
is displayed by a violent drop of the stresses determining the alloy atoms and dislocation becomes more evident when the strain
deformation. Experimentally confirmed oscillations of stresses rate is small [1, 15]. A detailed analysis of this problem requires,
(serrations of stresses) in the course of the heterogeneous therefore, the determination of the dependence of the dislocation
deformation usually depend on the kind of the applied strength rate and the rate of the migration of atom alloys on the stress,
machine [9-11] in most cases, irregularities along the curve are to temperature, strain rate and structure of the alloys. Studies of
be observed when a “soft” machine is use, the construction of references [1, 2, 4, 5, 10-13, 15] concerning the PLC effect
which does not permit to measure sudden changes of the load. indicate that in many cases the fundamental model of this
The most reliable image of those changes of the load is warranted phenomenon is not satisfactory and does not explain the
by “rigid” machines; deprived of the inertion of the system of experimental results. There are also only few publications
measurements, displaying only minimum elastic deformation in verifying the assumed model from the viewpoint of the structure.
the machine. It has been found that the magnitude and character, The aim of the investigations was to analyze preliminarily the
particularly the repeatability of the stress serrations on the ı–İ PLC effect, revealed in the course of the static stretching of tin
curve, depends also on the degree of dislodging of the dislocation bronzes containing about 6-7% Sn from industrial and laboratory
from the presence of blocking atoms. smelting with a comparable concentration of alloy elements and
modified with a micro-addition of zirconium in an amount of up
to 0.1% in the temperature range from 100 to 300oC and
a constant strain rate of about 10-3s-1.

2. Experimental
2. Experimental procedure
procedure
Experiments were carried out on tin bronze samples, mainly
of the type CuSn7 resulting from industrial and laboratory
casting shaped as bars with a diameter of about 30 mm and plates
about 20 mm thick, with a chemical composition as shown in Table 1.
In this investigations also model bronzes were used, modified
with microddition of zirconium within the range of 0.01-0.1%.
The tensile tests were carried out within the temperature range of
Fig. 1 Types of “teeth” characteristic for the heterogeneous 100y300oC at strain rate of 1.19•10-3 s-1 on a strength machine
deformation of the Portevin – Le Chatelier phenomenon [10] INSTRON (4405) with a loading range up to 100kN with
computer control of the tensile test and numeral recording of the
An analysis on of the PLC effect requires, therefore, the results. The dimensions of the stretched sample are to be seen in
consideration of not only the conditions of dislodging of Fig. 2.

Table 1.
Chemical composition of the studied alloys
Chemical composition, wt %
Kind of the alloy
Sn P Zr Bi Pb Si Zn Cu

1 CuSn7 6.07 0.0037 - 0.0003 0.0052 0.001 0.0130 res.

2 CuSn7Zr0.01 6.86 0.0006 0.020 0.00004 0.0049 0.002 0.0130 res.
3 CuSn7Zr0.05 6.8 - 0.001 - - - - res.
4 CuSn7Zr0.1 7.16 0.0001 0.0005 0.0008 0.004 0.012 0.0220 res.
5 CuZr0.01 0.0001 0.0008 0.015 0.0001 0.0004 - 0.0001 res.
6 CuZr0.05 0.0006 0.0001 0.0665 0.0001 0.0003 - 0.0013 res.

282 Research paper W. Ozgowicz, B. Grzegorczyk

 Materials

Table 2.
Parameters analyzed on tensile curve of the investigated alloys displaying the PLC effect
Deform. Type of ǻF + ǻF- ǻx IJǻx ǻL IJǻL
Alloy temp.
o serration [N] [N] [mm] [s] [mm] [s]
[ C]
150 A 168 184 0.144 4.33 0.732 21.97
CuSn7 250 B 298 255 0.033 1.01 - -
(1) C 163 243 0.040 1.27 0.084 1.50
300
D 131 016 0.055 1.65 - -
A 448 216 0.160 4.87 0.770 23.11
150
B 200 199 0.084 2.54 - -
CuSn7Zr0.01 B 230 205 0.039 1.18 - -
200
(2) D 326 189 0.080 2.39 - -
B 283 568 0.033 0.99 - -
280
C 384 384 0.077 2.04 0.612 18.36
A+B 506 383 0.126 3.79 0.424 12.71
200
CuSn7Zr0.05 (3) B 288 256 0.044 1.32 - -
300 C 171 177 0.045 1.36 0.122 3.67
A 410 502 0.105 3.17 0.692 20.78
CuSn7Zr0.1 150 A+B 325 351 0.074 2.23 0.692 20.78
(4) B 241 201 0.044 1.30 - -
250 B 371 330 0.051 1.52 - -
CuZr0.01 A 420 29 0.091 2.75 0.133 3.99
200
(5) B 299 275 0.041 1.23 - -
CuZr0.05
250 B 355 335 0.051 1.54 - -
(6)

'x – quantity of deformation during a single cycle;

'L - quantity of deformation in repeated cycles;
W'L – duration of single cycle;
W'L – duration of repeated cycles;

(Hs) and the characteristic periods of oscillation of stress ('x, 'L)

during the unstable plastic flow of material in the range of stable
deformations ('H). Stress were measured with on accuracy of
0.001kN, the deformation with an accuracy of 1μm.

3. Experimental
3. Experimental results
results

The results of the analysis of tensile curves of the investigated

model copper alloys within the temperature range of the PLC
Fig. 2. Sample of tensile test at elevated temperatures effect have been gathered in Table 2 and in the diagrams
presented in Figs. 3, 5, 6 and 8-14. Mainly two-compositional tin
Metallographic investigations on a light microscope were bronzes (Cu-Sn) were analyzed with a concentration of Sn about
carried out longitudinal polished sections of samples stretched at 7% (alloy 1) and three-elemental alloys (Cu-Sn-Zr) modified by a
elevated temperature, prepared in the conventional way and microaddition of zirconium with a concentration of about
etched in an agent based on K2Cr2O7 diluted with water. The 0.01-0.1% (alloy 2-4), as well as for the sake of comparison alloys
phenomenon of heterogeneous plastic deformation was analyzed of pure copper (0FHC) with a microaddition of Zr with a
basing on ı–İ curves, classifying the kind of serration, the concentration of 0.01% (alloy 5) and 0.05% (alloy 6). The
amplitude of the stress ('Fr), the range of homogeneous strain characteristics of typical parameter of tensile curves displaying

Analysis of the Portevin - Le Chatelier effect in tin bronzes at elevated temperatures 283
 Journal of Achievements in Materials and Manufacturing Engineering Volume 31 Issue 2 December 2008

Fig. 3. Tensile stress-strain curve for CuSn7 alloy at 150°C

the PLC effect comprised the increase of forces 'F+ and their revealing regular teeth of type B (Fig. 5) or at a temperature of
violent drop 'F- concerning the subsequent oscillations ('x, W'x) 350oC a complex character of serration of type C and D (Fig. 6).
or periods of repeated oscillations ('L, W'L). It has been found that
the shape of the curves ı–İ in the tested range of temperatures
(100-300oC) at a constant strain rate ȑ=1.19•10-3 s-1 depends
essentially on the temperature of deformation and chemical
composition of the tested alloys. The heterogeneous stage of
deformation PLC, occurring in the form of serration on the tensile
curves in usually preceded by an preliminary range of
homogeneous plastic deformation İs.
The model alloy CuSn7 (alloy 1) displays the first signs of
heterogeneous deformation in steady-state of he curve į–İ at a
temperature of 100oC. The PLC effect is however insignificant
and the periodical character of the teeth approximates the shape of
the “teeth” type A suggested in the literature [10]. A distinct PLC
effect has been recorded only at a temperature of 150oC (Fig. 3),
with a characteristic arrangement of the teeth corresponding
exactly to the standard A (Fig. 1). It has been found that in the
case of this type serration the repeatability of the periods of the
cycle of stress changes ('L, W'L § 22s) is considerable and the
increase of the force and violent drops are comparable (Table 2). Fig. 4. Single – phase structure of bronze CuSn7 with sliding lines
The observed shape of the analyzed curve ı–İ at a and deformation bands; tensile temp.150oC, deformation zone of
temperature of 150oC confirms, among others, the theorem of A. the sample; 500x
Korbel [10] saying that at lower temperatures of deformation after
the homogeneous deformation (İs) teeth of type A are to be Serration type B and C is characterized by the fact that the
observed. After deformation at such a temperature the structure rising part of the curve of strengthening displays a characteristic
displays a large number of slip lines and deformation bands slope due to the elastic behaviour of the system. It is to be supposed
(Fig. 4) confirming the hypothesis that teeth of type A are mainly that in the case of maximum stresses glide bands or shear bands are
connected with the nucleation of deformation bands, which nucleated (Fig. 7) accompanied by a violent drop of forces. Plastic
analogically to LĦder’s bands in the case of the yield points of deformation changes, therefore , from the elastic to the plastic one.
This is connected with a large number of local deformation band
soft carbon steel are shifted across the entire length of the sample.
which do not necessarily are dislocated across the whole sample.
It has been found that every periodical drop of forces along the
On the ı–İ there are also local oscillations of the forces, frequently
curve ı–İ corresponds to the activation of any single deformation not homogeneous in their character (Fig. 5), sometimes divided by
band. An increase of the temperature of deformation to about ranges of homogeneous deformation (Fig. 6). The PLC effect in this
250oC involves a modification of the shape of the curve ı–İ alloy disappears above the temperature of deformation 310oC.

284 Research paper W. Ozgowicz, B. Grzegorczyk

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Fig. 5. Tensile stress-strain curve for CuSn7 alloy at 250°C

Fig. 6. Tensile stress-strain curve for CuSn7 alloy at 300°C

The modification of the basic bronze with concentration 0.01% The PLC effect of the investigated pure copper modified by Zr
Zr (alloy 2) does not effect any essential changes in the occurrence with a concentration of 0.01% (alloy 5) was detected within a
of the PLC effect, if compared with alloy 1. It has been found, narrow tensile temperature range of about 200oC. It has been found
however, that at the tested temperatures a complex character of that the curve ı–İ displays periodically ('L) teeth of the general
serration is to be observed, viz. at 150oC the teeth type A and B type A locally modulated by B teeth, occurring cyclically ('x) in
(Fig. 8) and at 200oC teeth of type A, B, D and at 280oC teeth of the periods W'L (Fig. 13). Moreover, the cyclically occurring
type B and C (Fig. 9). Moreover, it has been found that at lower increase of the force exceeding the level of the approximated tensile
temperatures of deformation and at lower values of İ usually teeth curve, followed by violent drop below this level. The PLC effect of
of type A do occur, whereas at higher temperatures and higher
pure copper containing 0.05% Zr (alloy 6) could be observed at
value of İ teeth of type B and C.
higher tensile temperature of about 250-300oC. This denotes a
An increased concentration of the addition of Zr in this basic
higher energy of activation for the diffusion of Zr atoms in
bronze (up to 0.05% - alloy 3) and to about 0.1% (alloy 4) does not
result is essential changes in the temperature range of the PLC D -solution. The stress-strain curves recorded at 300oC display
effect, and the type of characteristic serration along the curve ı–İ characteristic teeth of type B, localized merely in the initial stage of
(Figs. 10-12). It has been found, however, that in alloy 3 no the strengthening curve (Fig. 14). The revealed complex character
serration occurred which would correspond the standard A of serration type A+B in pure copper modified with Zr is similar to
(Figs. 10, 11), and in alloy 4 no serration was detected classified as the serration of tensile curves of the alloy 3 and 4 within the
type C (Fig. 12). temperature range of 150-200 oC (Figs. 10, 12).

Analysis of the Portevin - Le Chatelier effect in tin bronzes at elevated temperatures 285
 Journal of Achievements in Materials and Manufacturing Engineering Volume 31 Issue 2 December 2008

Fig. 7. Single – phase structure of bronze CuSn7 with the shear bands; tensile temp. 250°C, break–off zone of the sample; 500x

Fig. 8. Tensile stress-strain curve for CuSn7Zr0.01% alloy at 150°C

Fig. 9. Tensile stress-stain curve for CuSn7Zr0.01% alloy at 280°C

286 Research paper W. Ozgowicz, B. Grzegorczyk

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Fig. 10. Tensile stress-strain curve for CuSn7Zr0.05% alloy at 200°C

Fig. 11. Tensile stress-strain for CuSn7Zr0.05% alloy at 300°C

Fig. 12. Tensile stress-strain curve forCuSn7Zr0.1% alloy at 150°C

Analysis of the Portevin - Le Chatelier effect in tin bronzes at elevated temperatures 287
 Journal of Achievements in Materials and Manufacturing Engineering Volume 31 Issue 2 December 2008

Fig. 13. Tensile stress-strain curve for CuZr0.01% alloy at 200°C

Fig. 14. Tensile stress-strain curve for CuZr0.05% alloy at 250°C

An analysis of the parameters of the curves ı–İ of the those of type B. The analysis of the parameters of the curves ı–İ
investigated alloys with a PLC effect has shown that in the case of displayed also the occurrence of an additive type of serration A+B
teeth type A the repeatability of the cycle a heterogeneous (Figs. 12, 13) and the presence of teeth described as type D
deformation ('L, W'L) at a constant strain rate and test temperature (Fig. 6). The quantities of the individual cycles 'x in teeth type D
of 150oC is considerable, independent of the chemical approach the value determined for teeth type B. The shape of the
composition of the alloy (Table 2). The amplitude of the drop of strengthening curve is also similar within the range of elastic
forces ('F-) increases, however, with the growing content of strain in a single cycle of stress changes. Different, however, is
zirconium in the alloy Nos.1, 2 and 4. In the case of teeth type B it the shape of the curve of decreasing forces, suggesting changes of
has been noticed that the magnitude of the respective cycles ('x) the kinetics of the progress of deformation after the break–away
decreases with the temperature of deformation and does not of dislocation from the zirconium atoms blocking, the dislocation
depend on the chemical composition of the investigated alloys. in the D – solution.
The repeatability of the cycle 'x is also considerable and
indicates a dependance on the temperature of the PLC effect.
Teeth of type C at a comparible tensile temperature (about 300oC)
are characterized by a shorter cycle of heterogeneous deformation 4.4. Conclusions
Conclusions
(W'L) than teeth of type A and a symmetrical distribution of the
quantities 'F+ and 'F-. The influence of the content of zirconium The investigations permit to draw the following conclusions:
on the shape of the strengthening curve within the range of elastic Polycrystalline tin bronzes type CuSn7 display a
strain in the respective cycles of the teeth type C is similar to heterogeneous plastic deformation, the so-called PLC effect, in

288 Research paper W. Ozgowicz, B. Grzegorczyk

 Materials

the temperature range of stretching amounting to about 100-300oC [4] K. Chihab, K. Bouabdalah, Influence of the structural
and strain rate 1.2˜10-3s-1. anisotropy on the nonuniform plasticity of the PLC effect,
The heterogeneity appearing on the curves ı–İ depend mainly Annales de Chimie Science des Materiaux 3 (2000) 171-178.
on the temperature of plastic deformation and chemical [5] P. Hähner, On the critical conditions of the Portevin – Le
composition of the tested alloys. Chatelier effect, Acta Materiala 9 (1997) 3695-3706.
The type and parameters of the “teeth” on the curves ı–İ may [6] Z. Jiang, Spatial characteristics of the Portevin – Le
differ in the respective stages of the analyzed diagrams. Chatelier deformation bands in Al-4 at %Cu polycrystals,
The investigated alloys display on the curves ı–İ a distinct Materials Science and Engineering 21 (2005) 154-164.
range of homogeneous deformation (Hs) preceding the [7] J. Kang, Effect of type-B Portevin–Le Chatelier bands in
characteristic PLC effect. uniaxial tension of strip cast AA5754 sheets, Scripta
Materialia 53 (2005) 499-503.
The degree of deformation Hs decreases distinctly with the
[8] F.B. Klose, A. Ziegenbein, F.Hagemann, H. Neuhäuser,
rising temperature of plastic deformation.
P. Hähner, M. Abbadi, A. Zeghloul, Analysis of Portevin–
The amplitude of cyclic load changes ('F+-) on the curves ı–İ
of the investigated alloys grows distinctly with the increasing Le Chatelier serration of type B in Al-Mg, Materials Science
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investigated bronzes leads to an intensive PLC effect within a P. Hähner, Portevin–Le Chatelier effect in strain and stress
wider range of the temperature of plastic deformation. controlled tensile tests, Computational Materials Science 26
Model bronzes with a microaddition of zirconium is (2003) 80-86.
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type A+B, as well as of the “teeth” denoted by the symbol D. the substitutional solid solutions, Metallurgy and Foundry
The investigated bronzes deformed at elevated temperature Practice Bulletin 474 (1974) 9-70.
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Univeritet, Budapest, 2002.
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