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Mechanical strength of the binary compound Ni3Al
Materials Science and Engineering A239 – 240 (1997) 169 – 173
Mechanical strength of the binary compound Ni3Al B. Matterstock *, J.L. Martin, J. Bonneville, T. Kruml, P. Spa¨tig De´partement de Physique, Ecole Polytechnique Fe´de´rale de Lausanne, CH-1015 Lausanne, Switzerland
Abstract The mechanical properties of binary Ni76.6Al23.4 single crystalline specimens have been studied in compression tests over a wide range of temperatures (293–1100 K). The resolved proof stress (t0.2%) and the corresponding work-hardening rate (u0.2%) have been measured as a function of temperature. A technique of repeated stress relaxations has been used to investigate the variation in the density of mobile dislocations that occurs during such transient tests. These experiments have been complemented with strain-rate changes for characterising the strain-rate sensitivity of the flow stress. The high hardening rates and their variation with temperature seem to correlate well with the values of the mobile dislocation exhaustion rates. The strain rate sensitivity of the stress exhibits negative values between 293 and 600 K, below the stress anomaly domain. These mechanical parameters are discussed in terms of the available dislocation mechanisms. © 1997 Elsevier Science S.A. Keywords: Ordered intermetallics; Strain rate sensitivity; Portevin – Le Chatelier effect; Strength anomaly
1. Introduction
2. Experimental details
During the last 10 years, a considerable effort has been devoted to the characterisation and understanding of the strength anomaly of Ni3Al compounds [1]. One of our contributions to this effort has been the measurement and interpretation of the mechanical properties, using conventional compression tests, but also developing appropriate transient tests (successive relaxations [2] and repeated creep tests [3]) in an attempt to separate the respective contributions to the strain-rate of the mobile dislocation density on the one hand and their velocities on the other hand. These methods were successfully applied to single crystals of Ni3 (Al, 1 at.% Ta), Ni3 (Al, 3 at.% Hf) [4,5]. The results suggested that alloying effects are quite complex, i.e. not only influence the stress level at a given temperature, but also alter the deformation mechanisms as indicated by the variation of the activation volume with temperature. Therefore, the investigation of the mechanical properties of the binary compound has been undertaken with special emphasis on off stoichiometry effects. The present report deals with single crystals of composition Ni76.6Al23.4, i.e. on the Nickel rich side of the stoichiometric compound.
Two sets of binary Ni3Al specimens of two different origins but having similar compositions (76.6% Ni and 23.4% Al) have been used. One set of specimens was prepared from a single crystalline bar that has been kindly supplied by Professor D.P. Pope at the University of Pennsylvania. In this case, the specimens were electro-discharge machined with a [1( 23] compression axis, yielding a Schmid factor of 0.46 for the primary [1( 01](111) octahedral slip system. Shear stresses t and shear strains g are resolved on the primary octahedral glide system. The second set of specimens was prepared from a columnar grained rod kindly provided by Professor S. Hanada at the Tohoku University, Sendai. The growth direction of the columnar grains corresponds to a 011 crystallographic orientation and the grains are slightly misoriented, the misorientations are less than 5° from each other about this direction, so that the rod can be considered as a highly polygonised single crystal. The deformation axes were chosen perpendicular to the 011 average growth direction and were approximately parallel to a [2( 33] orientation which, in this case, corresponds to a maximum Schmid factor of about 0.37 for octahedral slip. For all the specimens, the gauge length was 6.5 mm with a gauge cross section of 3.2 mm× 3.2 mm.
* Corresponding author. 0921-5093/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S 0 9 2 1 - 5 0 9 3 ( 9 7 ) 0 0 5 7 7 - 7
170
B. Matterstock et al. / Materials Science and Engineering A239–240 (1997) 169–173
The mechanical tests were performed in compression under helium atmosphere in a Schenck RMC 100 testing machine, at an engineering strain-rate of 5× 10 − 5 s − 1. The specimens were deformed at various temperatures that range from 295 to 1100 K. Each deformation test has been performed on a virgin specimen. Indeed, it was shown that the well accepted procedure which consists in performing successive deformation tests at different temperatures on the same sample, yields significant errors [4]. The strain-rate sensitivity of the flow stress (S) has been measured by using both stress relaxation tests and strain-rate changes. In the latter experiment, the nominal strain-rate was abruptly increased, respectively decreased, by a factor of 11. A technique of repeated stress relaxations has been used for measuring the variation in the density of mobile dislocations [6] that takes place during relaxation. The point of the latter technique is to measure the true activation area of the dislocation velocity which is considered as thermally activated. The structural changes which are included in the apparent activation area, which is measured in single relaxation tests or strain rate jump experiments, are therefore isolated. They can be characterised by the plastic hardening rate and the change of mobile dislocation density during the transient. These two parameters can be determined, with a satisfactory accuracy, by comparing the values of the true and apparent activation areas, respectively. Detailed information about this experimental procedure has already been given elsewhere [2].
3. Experimental results
3.1. [1( 23] oriented specimens The resolved proof stress measured at a 0.2% offset plastic strain (t0.2%) is shown in Fig. 1 as a function of
Fig. 1. t0.2% as a function of temperature for the [1( 23] single crystals.
Fig. 2. Work-hardening coefficient as a function of temperature for the [1( 23] single crystals.
the deformation temperature. t0.2% is approximately constant from room temperature up to 425 K and then sharply increases up to a temperature of about 1000 K, referred to as the peak temperature (Tp,t ) in the following, above which it finally decreases. The general aspect of t0.2% with temperature is in agreement with previously reported results on Ni3Al single crystals having both identical orientation and composition [7]. The related work-hardening rate (u0.2%) reported in Fig. 2 exhibits a temperature dependence which is similar to that of t0.2%. However, u0.2% peaks at a temperature of about 850 K (Tp,u ) which is below Tp,t. This anomalous dependence of u with temperature has already been reported by various authors for the binary compound as well as for alloyed compounds. It has also been shown that the variation of u with temperature is dependent on the deformation procedure, that is: u exhibits two peaks as a function of temperature when measured on a specimen that is successively deformed at several increasing temperatures, while one peak only is present when virgin specimens are used for each temperature [4]. The repeated stress relaxation technique allows for the measurement of a structural parameter (V) which accounts for the microstructural changes that take place during relaxations. The latter include both the variation in the density of mobile dislocations (rm) and of the internal stress (ti) [6]. Since V results from the variations in both ti and rm, it is not possible to determine each of these parameters separately without further hypothesis. However, the plastic deformation being always very small during a stress relaxation test, it seems reasonable to assume that ti does not vary too much during relaxation. Then, the value of V allows us to determine the variation of rm during relaxation. Fig. 3 shows, in the anomaly domain, the values of u0.2% together with that of d0.2% = Drm/rmo(rmo is the mobile
B. Matterstock et al. / Materials Science and Engineering A239–240 (1997) 169–173
171
Table 1 Comparison of the mechanical parameters of three different Ni3Al compounds [4,9]
Tp,t t0.2% max Tp,u u0.2% max
Fig. 3. Work-hardening coefficient and dislocation exhaustion rate at gp =0.2%. [1( 23] single crystals.
dislocation density at the beginning of the relaxation) as a function of t0.2%. The d0.2% values are calculated for a decrease in strain-rate of one order of magnitude during relaxation. A fair correlation is found in the variation of d0.2% and u0.2% with t0.2%. They both rapidly increase in the first part of the anomaly domain, indicating that an increase in the exhaustion rate results in an increase of the work-hardening rate and viceversa. They peak at approximately the same stress (temperature) level, before the stress peak, above which d0.2% drastically decreases while the decrease of u0.2% is much more moderate.
3.2. [2( 33] oriented specimens As a whole, the plots of t0.2% and u0.2%, respectively, as a function of temperature, exhibit the same trends as above (see Figs. 2 and 3). Strain-rate jump experiments
Ni3Al
Ni3 (Al, Hf)
Ni3 (Al, Ta)
1000 230 850 5400
650 400 600 7800
800 190 750 5300
K MPa K MPa
K MPa K MPa
K MPa K MPa
were also performed on the samples. According to the observed transient, two temperature domains could be defined below the peak temperature (see Fig. 4): for 300B TB 600 K (domain I), the stress jump, following a strain rate increment, includes a positive transient Dttr followed by a stress decrease, measured through Dtss which exhibits negative values. Stress instabilities are also visible along the deformation curves in this temperature range. In domains II (700 K 5 T5 900 K) and III (900 K5T5 1100 K), below and above the peak temperature respectively, the transients are ‘normal’ ones, with a larger amplitude in domain III. A detailed description of these transients can be found in [8].
4. Discussion A comparison of the mechanical properties of single crystals of the binary compound (present study) and single crystals containing Ta and Hf additions respectively [4,5], yields the following remarks: For each of the three compounds, a peak temperature for t0.2% can be defined which is higher than the peak temperature for u0.2%. The respective values for the peak temperatures and maximum values of t0.2% and u0.2% are listed in Table 1 for three different compounds.
Fig. 4. The temperature domains of the proof stress and the associated transients in strain-rate jump experiments. [2( 33] single crystals.
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B. Matterstock et al. / Materials Science and Engineering A239–240 (1997) 169–173
Table 1 shows that the effects of alloying are quite complex. It affects the various peak temperatures, as well as the maximum stress and the maximum workhardening rate. This is corroborated by activation volume measurements. The plots of the latter parameter as a function of temperature exhibit different trends, depending on the presence of a given alloying element [4,5]. This means that different deformation mechanisms operate in the various types of compounds. The most significant results of stress relaxation experiments reported here, concern the values of the exhaustion rate of mobile dislocations. The same experiments have also yielded values of the true activation areas [9]. Their variations with temperature are as follows. As the temperature increases in domain I, this parameter increases from 1200b 2 to 1800b 2, it decreases in regime II from 1900b 2 to 900b 2, then exhibits low values ( 5100b 2) in regime III. The relatively large values of the activation area, in domains I and II, clearly indicate that thermal activation is not too effective for dislocation motion in the corresponding temperature ranges. However, the same experiments provide values for the exhaustion rate of mobile dislocations at least during the relaxations. Looking at the plots of Fig. 3, it is remarkable that the stress at which the work-hardening rate u0.2% is maximum, coincides with a maximum dislocation exhaustion rate. This can be interpreted by considering that dislocation exhaustion takes place in the crystal by storage without (or with restricted) annihilation. This type of correlation also supports the method that we use to determine the dislocation exhaustion rate, and the related assumptions for interpretation. It also indicates that the exhaustion phenomena, which take place during plastic deformation at the onset of the transient experiment, are quite comparable with those of the stress–strain curve just before the relaxation. In addition, the values of exhaustion rates which are measured here between 50 and 70% are quite high, corresponding to the high work-hardening rates that these compounds exhibit, as compared with pure metals. This suggests that the present Ni3Al compounds exhibit specific dislocation storage mechanisms which affect both the flow stress and the work-hardening coefficient. From the data of strain rate jump experiments, the rate sensitivity can be measured as a function of temperature. The latter parameter is defined as: S=(Dt/D ln g; )
the present data show that Sss (though of small magnitude) is negative in the present experiments in temperature domain I, where t0.2% is constant or slightly increases with temperature. This behaviour which is different from the previously reported ones, may be attributed to different alloying elements (Ti in [10] and Hf+ B in [11]). It is well known that the two effects observed here, in temperature domain I, i.e. serrations on the stress– strain curve and a negative strain-rate sensitivity, can be attributed to the Portevin–Le Chatelier effect. This is clearly evidenced in copper solid solutions as an example [12]. In addition, for the latter alloys, an anomalous increase of the flow stress is also observed, due to the Portevin–Le Chatelier effect. However, we do not think that this effect is responsible for the strength anomaly of the present Ni3Al samples, for the following reasons: the anomaly is observed in the transition domain (I+II) and domain II in Fig. 4 in which S is positive and no serrations are observed; the present increase in flow stress is large as compared with that of the Cu solid solutions. The Portevin–Le Chatelier effect is connected with the mechanism of dynamic strain ageing, in which moving dislocations interact with diffusing point defects, impurities or solute atoms. In the present state of our investigations, the diffusing species is not identified yet. However, these results emphasise the role of point defects or impurities with regard to the flow stress of Ni3Al compounds. Finally, a comparison of the mechanical properties of the two sets of specimens investigated in this study shows that at a given temperature in the strength anomaly domain t0.2% is lower in the [1( 23] than in the [2( 33] orientation (see Figs. 1 and 4) but peaks at a higher temperature in the former orientation. These features about the stress orientation dependence of t0.2% are in fair agreement with results previously reported for Ni3(Al,Ta) single crystals [13].
(1)
From early investigations of the mechanical properties of Ni3Al compounds, this parameter has been measured by some authors [10]. The stress transient following an increase of strain rate is usually reported to look like the schematics of Fig. 5. Dttr is positive (or negligible). Consequently, Str, defined by relation Eq. (1) above, is positive (or negligible) [10,11]. However,
Fig. 5. Schematic stress transient associated with a strain-rate jump.
B. Matterstock et al. / Materials Science and Engineering A239–240 (1997) 169–173
5. Conclusion The mechanical parameters of the binary compound Ni76.6Al23.4 have been determined and a fair agreement is found between the temperature variations of the work-hardening coefficient and the mobile dislocation exhaustion rate. In the low temperature domain, which corresponds to the onset of the strength anomaly, the Portevin–Le Chatelier effect is evidenced. This effect alone is not likely to account for the large amplitude of the strength anomaly of this compound.
Acknowledgements The financial support of Fonds National Suisse is gratefully acknowledged.
References [1] P. Veyssie`re and G. Saada, in: F.R.N. Nabarro, M.S. Duesberry (Eds.), Dislocations in Solids, Vol. 10, North Holland, Amsterdam, 1996, ch. 53, p. 253.
.
173
[2] P. Spa¨tig, J. Bonneville, J.L. Martin, Mater. Sci. Eng. A167 (1993) 73. [3] A. Orlova, J. Bonneville, P. Spa¨tig, Mater. Sci. Eng. A191 (1995) 85. [4] P. Spa¨tig, J. Bonneville, J.L. Martin, Proc. ‘Euromat 95’, vol. G and H, Padua, 1995, p. 97. [5] P. Spa¨tig, J. Bonneville, J.L. Martin, in: Oikawa et al. (Eds.), Strength of Materials, The Japan Institute of Metals, 1994, p. 353. [6] P. Spa¨tig, J. Bonneville, J.L. Martin, in: J.A. Horton, I. Baker, S. Hanada, R.D. Noebe, D.S. Schwartz (Eds.), High-Temperature Ordered Intermetallic Alloys VI, Mater. Res. Soc. Symp. Proc., 364, Part 2, Pittsburgh, PA, 1995, p. 713. [7] F. Heredia, Ph.D. Dissertation, University of Pennsylvania, 1990. [8] B. Matterstock, J. Bonneville, J.L. Martin, Proc. 5th European Conf. on Advanced Materials and Processes and Applications, vol. 1, Fed. Eur. Mater. Soc., 1997, p. 163. [9] J. Bonneville, J.L. Martin, P. Spa¨tig, B. Viguier, B. Matterstock, in: C.C Koch, N.S. Stoloff, C.T. Liu, A. Wanner (Eds.), High-Temperature Ordered Intermetallic Alloys VII, Mater. Res. Soc. Symp. Proc., vol. 460, Pittsburgh, PA, 1997, p. 419. [10] P.H. Thornton, R.G. Davies, J.L. Johnston, Metall. Trans. 1 (1970) 207. [11] S.S. Ezz, P.B. Hirsch, Philos. Mag. A69 (1994) 105. [12] H. Traub, H. Neuha¨user, Ch. Schwink, Acta Met. 25 (1977) 437. [13] Y. Umakoshi, D.P. Pope, V. Vitek, Acta Met. 32 (1984) 449.
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Mechanical strength of the binary compound Ni3Al B. Matterstock *, J.L. Martin, J. Bonneville, T. Kruml, P. Spa¨tig De´partement de Physique, Ecole Polytechnique Fe´de´rale de Lausanne, CH-1015 Lausanne, Switzerland
Abstract The mechanical properties of binary Ni76.6Al23.4 single crystalline specimens have been studied in compression tests over a wide range of temperatures (293–1100 K). The resolved proof stress (t0.2%) and the corresponding work-hardening rate (u0.2%) have been measured as a function of temperature. A technique of repeated stress relaxations has been used to investigate the variation in the density of mobile dislocations that occurs during such transient tests. These experiments have been complemented with strain-rate changes for characterising the strain-rate sensitivity of the flow stress. The high hardening rates and their variation with temperature seem to correlate well with the values of the mobile dislocation exhaustion rates. The strain rate sensitivity of the stress exhibits negative values between 293 and 600 K, below the stress anomaly domain. These mechanical parameters are discussed in terms of the available dislocation mechanisms. © 1997 Elsevier Science S.A. Keywords: Ordered intermetallics; Strain rate sensitivity; Portevin – Le Chatelier effect; Strength anomaly
1. Introduction
2. Experimental details
During the last 10 years, a considerable effort has been devoted to the characterisation and understanding of the strength anomaly of Ni3Al compounds [1]. One of our contributions to this effort has been the measurement and interpretation of the mechanical properties, using conventional compression tests, but also developing appropriate transient tests (successive relaxations [2] and repeated creep tests [3]) in an attempt to separate the respective contributions to the strain-rate of the mobile dislocation density on the one hand and their velocities on the other hand. These methods were successfully applied to single crystals of Ni3 (Al, 1 at.% Ta), Ni3 (Al, 3 at.% Hf) [4,5]. The results suggested that alloying effects are quite complex, i.e. not only influence the stress level at a given temperature, but also alter the deformation mechanisms as indicated by the variation of the activation volume with temperature. Therefore, the investigation of the mechanical properties of the binary compound has been undertaken with special emphasis on off stoichiometry effects. The present report deals with single crystals of composition Ni76.6Al23.4, i.e. on the Nickel rich side of the stoichiometric compound.
Two sets of binary Ni3Al specimens of two different origins but having similar compositions (76.6% Ni and 23.4% Al) have been used. One set of specimens was prepared from a single crystalline bar that has been kindly supplied by Professor D.P. Pope at the University of Pennsylvania. In this case, the specimens were electro-discharge machined with a [1( 23] compression axis, yielding a Schmid factor of 0.46 for the primary [1( 01](111) octahedral slip system. Shear stresses t and shear strains g are resolved on the primary octahedral glide system. The second set of specimens was prepared from a columnar grained rod kindly provided by Professor S. Hanada at the Tohoku University, Sendai. The growth direction of the columnar grains corresponds to a 011 crystallographic orientation and the grains are slightly misoriented, the misorientations are less than 5° from each other about this direction, so that the rod can be considered as a highly polygonised single crystal. The deformation axes were chosen perpendicular to the 011 average growth direction and were approximately parallel to a [2( 33] orientation which, in this case, corresponds to a maximum Schmid factor of about 0.37 for octahedral slip. For all the specimens, the gauge length was 6.5 mm with a gauge cross section of 3.2 mm× 3.2 mm.
* Corresponding author. 0921-5093/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S 0 9 2 1 - 5 0 9 3 ( 9 7 ) 0 0 5 7 7 - 7
170
B. Matterstock et al. / Materials Science and Engineering A239–240 (1997) 169–173
The mechanical tests were performed in compression under helium atmosphere in a Schenck RMC 100 testing machine, at an engineering strain-rate of 5× 10 − 5 s − 1. The specimens were deformed at various temperatures that range from 295 to 1100 K. Each deformation test has been performed on a virgin specimen. Indeed, it was shown that the well accepted procedure which consists in performing successive deformation tests at different temperatures on the same sample, yields significant errors [4]. The strain-rate sensitivity of the flow stress (S) has been measured by using both stress relaxation tests and strain-rate changes. In the latter experiment, the nominal strain-rate was abruptly increased, respectively decreased, by a factor of 11. A technique of repeated stress relaxations has been used for measuring the variation in the density of mobile dislocations [6] that takes place during relaxation. The point of the latter technique is to measure the true activation area of the dislocation velocity which is considered as thermally activated. The structural changes which are included in the apparent activation area, which is measured in single relaxation tests or strain rate jump experiments, are therefore isolated. They can be characterised by the plastic hardening rate and the change of mobile dislocation density during the transient. These two parameters can be determined, with a satisfactory accuracy, by comparing the values of the true and apparent activation areas, respectively. Detailed information about this experimental procedure has already been given elsewhere [2].
3. Experimental results
3.1. [1( 23] oriented specimens The resolved proof stress measured at a 0.2% offset plastic strain (t0.2%) is shown in Fig. 1 as a function of
Fig. 1. t0.2% as a function of temperature for the [1( 23] single crystals.
Fig. 2. Work-hardening coefficient as a function of temperature for the [1( 23] single crystals.
the deformation temperature. t0.2% is approximately constant from room temperature up to 425 K and then sharply increases up to a temperature of about 1000 K, referred to as the peak temperature (Tp,t ) in the following, above which it finally decreases. The general aspect of t0.2% with temperature is in agreement with previously reported results on Ni3Al single crystals having both identical orientation and composition [7]. The related work-hardening rate (u0.2%) reported in Fig. 2 exhibits a temperature dependence which is similar to that of t0.2%. However, u0.2% peaks at a temperature of about 850 K (Tp,u ) which is below Tp,t. This anomalous dependence of u with temperature has already been reported by various authors for the binary compound as well as for alloyed compounds. It has also been shown that the variation of u with temperature is dependent on the deformation procedure, that is: u exhibits two peaks as a function of temperature when measured on a specimen that is successively deformed at several increasing temperatures, while one peak only is present when virgin specimens are used for each temperature [4]. The repeated stress relaxation technique allows for the measurement of a structural parameter (V) which accounts for the microstructural changes that take place during relaxations. The latter include both the variation in the density of mobile dislocations (rm) and of the internal stress (ti) [6]. Since V results from the variations in both ti and rm, it is not possible to determine each of these parameters separately without further hypothesis. However, the plastic deformation being always very small during a stress relaxation test, it seems reasonable to assume that ti does not vary too much during relaxation. Then, the value of V allows us to determine the variation of rm during relaxation. Fig. 3 shows, in the anomaly domain, the values of u0.2% together with that of d0.2% = Drm/rmo(rmo is the mobile
B. Matterstock et al. / Materials Science and Engineering A239–240 (1997) 169–173
171
Table 1 Comparison of the mechanical parameters of three different Ni3Al compounds [4,9]
Tp,t t0.2% max Tp,u u0.2% max
Fig. 3. Work-hardening coefficient and dislocation exhaustion rate at gp =0.2%. [1( 23] single crystals.
dislocation density at the beginning of the relaxation) as a function of t0.2%. The d0.2% values are calculated for a decrease in strain-rate of one order of magnitude during relaxation. A fair correlation is found in the variation of d0.2% and u0.2% with t0.2%. They both rapidly increase in the first part of the anomaly domain, indicating that an increase in the exhaustion rate results in an increase of the work-hardening rate and viceversa. They peak at approximately the same stress (temperature) level, before the stress peak, above which d0.2% drastically decreases while the decrease of u0.2% is much more moderate.
3.2. [2( 33] oriented specimens As a whole, the plots of t0.2% and u0.2%, respectively, as a function of temperature, exhibit the same trends as above (see Figs. 2 and 3). Strain-rate jump experiments
Ni3Al
Ni3 (Al, Hf)
Ni3 (Al, Ta)
1000 230 850 5400
650 400 600 7800
800 190 750 5300
K MPa K MPa
K MPa K MPa
K MPa K MPa
were also performed on the samples. According to the observed transient, two temperature domains could be defined below the peak temperature (see Fig. 4): for 300B TB 600 K (domain I), the stress jump, following a strain rate increment, includes a positive transient Dttr followed by a stress decrease, measured through Dtss which exhibits negative values. Stress instabilities are also visible along the deformation curves in this temperature range. In domains II (700 K 5 T5 900 K) and III (900 K5T5 1100 K), below and above the peak temperature respectively, the transients are ‘normal’ ones, with a larger amplitude in domain III. A detailed description of these transients can be found in [8].
4. Discussion A comparison of the mechanical properties of single crystals of the binary compound (present study) and single crystals containing Ta and Hf additions respectively [4,5], yields the following remarks: For each of the three compounds, a peak temperature for t0.2% can be defined which is higher than the peak temperature for u0.2%. The respective values for the peak temperatures and maximum values of t0.2% and u0.2% are listed in Table 1 for three different compounds.
Fig. 4. The temperature domains of the proof stress and the associated transients in strain-rate jump experiments. [2( 33] single crystals.
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B. Matterstock et al. / Materials Science and Engineering A239–240 (1997) 169–173
Table 1 shows that the effects of alloying are quite complex. It affects the various peak temperatures, as well as the maximum stress and the maximum workhardening rate. This is corroborated by activation volume measurements. The plots of the latter parameter as a function of temperature exhibit different trends, depending on the presence of a given alloying element [4,5]. This means that different deformation mechanisms operate in the various types of compounds. The most significant results of stress relaxation experiments reported here, concern the values of the exhaustion rate of mobile dislocations. The same experiments have also yielded values of the true activation areas [9]. Their variations with temperature are as follows. As the temperature increases in domain I, this parameter increases from 1200b 2 to 1800b 2, it decreases in regime II from 1900b 2 to 900b 2, then exhibits low values ( 5100b 2) in regime III. The relatively large values of the activation area, in domains I and II, clearly indicate that thermal activation is not too effective for dislocation motion in the corresponding temperature ranges. However, the same experiments provide values for the exhaustion rate of mobile dislocations at least during the relaxations. Looking at the plots of Fig. 3, it is remarkable that the stress at which the work-hardening rate u0.2% is maximum, coincides with a maximum dislocation exhaustion rate. This can be interpreted by considering that dislocation exhaustion takes place in the crystal by storage without (or with restricted) annihilation. This type of correlation also supports the method that we use to determine the dislocation exhaustion rate, and the related assumptions for interpretation. It also indicates that the exhaustion phenomena, which take place during plastic deformation at the onset of the transient experiment, are quite comparable with those of the stress–strain curve just before the relaxation. In addition, the values of exhaustion rates which are measured here between 50 and 70% are quite high, corresponding to the high work-hardening rates that these compounds exhibit, as compared with pure metals. This suggests that the present Ni3Al compounds exhibit specific dislocation storage mechanisms which affect both the flow stress and the work-hardening coefficient. From the data of strain rate jump experiments, the rate sensitivity can be measured as a function of temperature. The latter parameter is defined as: S=(Dt/D ln g; )
the present data show that Sss (though of small magnitude) is negative in the present experiments in temperature domain I, where t0.2% is constant or slightly increases with temperature. This behaviour which is different from the previously reported ones, may be attributed to different alloying elements (Ti in [10] and Hf+ B in [11]). It is well known that the two effects observed here, in temperature domain I, i.e. serrations on the stress– strain curve and a negative strain-rate sensitivity, can be attributed to the Portevin–Le Chatelier effect. This is clearly evidenced in copper solid solutions as an example [12]. In addition, for the latter alloys, an anomalous increase of the flow stress is also observed, due to the Portevin–Le Chatelier effect. However, we do not think that this effect is responsible for the strength anomaly of the present Ni3Al samples, for the following reasons: the anomaly is observed in the transition domain (I+II) and domain II in Fig. 4 in which S is positive and no serrations are observed; the present increase in flow stress is large as compared with that of the Cu solid solutions. The Portevin–Le Chatelier effect is connected with the mechanism of dynamic strain ageing, in which moving dislocations interact with diffusing point defects, impurities or solute atoms. In the present state of our investigations, the diffusing species is not identified yet. However, these results emphasise the role of point defects or impurities with regard to the flow stress of Ni3Al compounds. Finally, a comparison of the mechanical properties of the two sets of specimens investigated in this study shows that at a given temperature in the strength anomaly domain t0.2% is lower in the [1( 23] than in the [2( 33] orientation (see Figs. 1 and 4) but peaks at a higher temperature in the former orientation. These features about the stress orientation dependence of t0.2% are in fair agreement with results previously reported for Ni3(Al,Ta) single crystals [13].
(1)
From early investigations of the mechanical properties of Ni3Al compounds, this parameter has been measured by some authors [10]. The stress transient following an increase of strain rate is usually reported to look like the schematics of Fig. 5. Dttr is positive (or negligible). Consequently, Str, defined by relation Eq. (1) above, is positive (or negligible) [10,11]. However,
Fig. 5. Schematic stress transient associated with a strain-rate jump.
B. Matterstock et al. / Materials Science and Engineering A239–240 (1997) 169–173
5. Conclusion The mechanical parameters of the binary compound Ni76.6Al23.4 have been determined and a fair agreement is found between the temperature variations of the work-hardening coefficient and the mobile dislocation exhaustion rate. In the low temperature domain, which corresponds to the onset of the strength anomaly, the Portevin–Le Chatelier effect is evidenced. This effect alone is not likely to account for the large amplitude of the strength anomaly of this compound.
Acknowledgements The financial support of Fonds National Suisse is gratefully acknowledged.
References [1] P. Veyssie`re and G. Saada, in: F.R.N. Nabarro, M.S. Duesberry (Eds.), Dislocations in Solids, Vol. 10, North Holland, Amsterdam, 1996, ch. 53, p. 253.
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