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Influence of quenching conditions on the mechanical and structural properties of Al–30 wt% Zn alloy
Materials Science & Engineering A 602 (2014) 105–109
Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Influence of quenching conditions on the mechanical and structural properties of Al–30 wt% Zn alloy A.F. Abd El-Rehim n, M.M. El-Sayed, M.R. Nagy, M. Abd El-Hafez Physics Department, Faculty of Education, Ain Shams University, P.O. Box 5101, Heliopolis 11771, Roxy, Cairo, Egypt
art ic l e i nf o
a b s t r a c t
Article history: Received 7 January 2014 Received in revised form 10 February 2014 Accepted 12 February 2014 Available online 20 February 2014
The present study aims at determining the effect of quenching condition on the mechanical and microstructure properties of Al–30 wt% Zn alloy. Creep curves of slowly cooled and quenched Al–30 wt% Zn alloys were studied at the temperature ranging from 508 to 578 K under constant applied stress. Results of both transient and steady state creep indicated a transition point at 548 K. Transient and steady state creep parameters of the quenched samples were found to be smaller than those of the slowly cooled samples. The activation energy of the transient and steady state creep amounted to 27 and 29 kJ/mol, respectively, before the transition deformation temperature (548 K) due to dislocation mechanism. The energy activating the transient and steady state creep steady state creep in the high temperature region (above the transformation temperature) characterizing as dislocation climb. & 2014 Elsevier B.V. All rights reserved.
Keywords: Al-alloys Creep curves Activation energy
1. Introduction Aluminum and its alloys play an important role in engineering applications, on account of their excellent mechanical and electrical properties as well as corrosion resistance. Research and development work in the field of aluminum alloys occupied therefore, a position of importance. The Aluminum (Al) based alloys mixture with Zinc (Zn) is one of the leading candidates for many end-use applications, due to their attractive physical, mechanical, and superplastic properties in conjunction with good abrasion and wear resistances [1]. The effect of dissolution of β-phase (Zn-rich phase) on the transient creep characteristics of Al–14 wt% Zn alloy has been studied [2]. The energy activating the transient creep characterized the dislocation mechanism. The effect of grain size as well as aging temperature on the creep characteristics of A1–10 wt% Zn alloy have been studied [3]. It has been found that the steady state creep rate decreases by increasing grain size, whereas it increases by increasing aging temperature. The values of the activation energies controlling the creep processes corresponded to that required for dislocation glide. The hardness of Al–5 wt% Zn and Al–5 wt% Zn–0.25 wt% In was measured at room temperature for samples heat treated in the range 300–453 K and dwell times in the range 30–300 s under 50 g load [4]. Softening was observed for all the samples and the hardness decreased with increasing temperature and/or dwell n
Corresponding author. E-mail address: [email protected] (A.F. Abd El-Rehim).
http://dx.doi.org/10.1016/j.msea.2014.02.040 0921-5093 & 2014 Elsevier B.V. All rights reserved.
time. Addition of In to the binary Al–5 wt% Zn alloy led to a significant increase in the hardness. The system Al–Zn is very suitable and particularly attractive for studying microstructure and phase transitions, especially in the supersaturated state. The purpose of the present work is to investigate the influence of quenching conditions and phase transformation on the mechanical properties of the Al–30 wt% Zn alloy.
2. Experimental procedures The Al–30 wt% Zn alloy that is used in the present study was prepared from pure raw materials of high purity; Al (99.99%) and Zn (99.99%). Small pieces of these raw materials were melted together in a high purity graphite crucible and then poured into a cast iron mold of dimensions 150 mm 10 mm 10 mm and allowed to solidify. After annealing for homogenization at 813 K for 4 days, the ingots were swaged and cold drawn into (i) wire samples having 0.8 mm diameter and 50 mm length for creep measurements, and (ii) sheets of 0.6 mm thick for microstructural examinations. The chemical composition of the alloy was verified by means of energy dispersive X-ray analysis (EDX). The samples were divided into two groups. Each group contained some wires suitable for the tensile test and some sheets suitable for structure examination. Two groups were annealed at 813 K for 2 h. One group was rapidly quenched in water (W.Q) at room temperature (300 K). The second group was slowly cooled (S.C) to room temperature with a cooling rate of 11 10 3 K/s. Tensile creep tests were then performed under constant stress of
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21.20 MPa, using a conventional tensile creep testing machine described earlier [5], and at temperatures ranging from 508 to 578 K in 10 K increments with an accuracy of 71 K. The sensitivity of strain measurements was approximately 7 1.0 10 2 mm. Microstructural examinations were carried out using a light optical microscope (VERSAMET-2). The treated sheet specimens for metallographic examination were etched with Keller's reagent. The microstructure was also investigated using XRD measurements. A Shimadzu X-ray diffractometer (Dx-30) was used to trace the structural variations in the samples by analyzing the obtained patterns. The X-ray tube giving copper (Kα) radiation operated at 30 kV and 30 mA emits X-ray beam of wavelength λ ¼0.15406 nm.
3. Results To identify the initial phases, both slowly cooled and quenched sheet samples of Al–30 wt% Zn alloy were examined by XRD at room temperature (300 K), as shown in Fig. 1. It can be seen that the slowly cooled alloy (Fig. 1a) is primarily composed of two phases identified as the α-phase (Al-rich phase of fcc structure) and β-phase (Zn-rich phase of hcp structure). While the quenched alloy (Fig. 1b) consists of one phase α-phase (Al-rich phase of fcc structure). Isothermal creep curves for both slowly cooled and quenched Al–30 wt% Zn samples obtained under different deformation temperatures ranging from 508 to 578 K in steps of 10 K under constant stress of 21.20 MPa are illustrated in Fig. 2. The creep
Fig. 1. X-ray diffraction pattern of (a) the slowly cooled sheet and (b) the quenched sheet samples of Al–30 wt% Zn alloy.
Fig. 2. Creep curves of (a) the slowly cooled and (b) the quenched samples under constant stress of 21.20 MPa at different deformation temperatures as indicated.
Fig. 3. Relation between ln εtr and ln t for (a) the slowly cooled and (b) the quenched samples at different deformation temperatures as indicated.
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curves consist of three distinct stages: transient creep where the strain rate decreases with time, steady state creep with a constant strain rate, and lastly, tertiary creep where the strain rate accelerates to fracture [6]. It was found that the transient creep fits a power law expression of the form [7,8]:
εtr ¼ κ t n
ð1Þ
where εtr is the transient strain, n and κ are the transient creep parameters and ttr is the transient creep time in seconds. The parameters n and κ are correlated from the relation between ln εtr and ln t at different deformation temperatures. n and κ are determined from the slopes and the intercepts of the straight lines in Fig. 3, respectively. Fig. 4 demonstrates the effect of deformation temperature on the values of parameter n of both slowly cooled and quenched samples while Fig. 5 shows the same effect concerning the parameter κ. The dependence of the steady state creep rate, έst, (evaluated from the slopes of the linear parts of the creep curves shown in Fig. 2) for both slowly cooled and quenched samples on the deformation temperature is depicted in Fig. 6. It is clear from Figs. 4–6 that
Fig. 4. Dependence of the transient creep parameter n on the deformation temperature for the slowly cooled and quenched samples.
the slowly cooled samples have higher values of creep parameters than quenched samples,
the creep parameters (n, κ and έst) show two deformation
temperature regions, the low temperature region (below 548 K) and the high temperature region (after 548 K). The values of n, κ and έst increase with increasing deformation temperature and show an anomalous behavior in the vicinity of the transition temperature (548 K) for both slowly cooled and quenched samples.
The activation energy controlling the transient creep behavior in the present work could be calculated assuming that the transient creep parameter κ varies with the deformation temperature, T, according to an Arrhenius-type relation of the form [9]:
κ ¼ constant expð Etr =RTÞ
ð2Þ
while έst satisfied the following equation: έst ¼ constant expð Est =RTÞ
ð3Þ
where R is the universal gas constant and T is the deformation temperature in Kelvin. The activation energies of the transient creep, Etr, were calculated from Fig. 7(a) and (b) relating ln κ and 1000/T (K 1) and found to be 27 and 66 kJ/mol in the low temperature range (508–538 K) and the high temperature range (558–578 K), respectively. The values of the activation energy of steady state creep, Est, were calculated from Fig. 8(a) and (b) relating ln έst and 1000/T (K 1). The mean value of activation energy in the low temperature range was found to be 29 kJ/mol while a value of 73 kJ/mol was obtained in the high temperature range.
Fig. 5. Dependence of the transient creep parameter κ on the deformation temperature for the slowly cooled and quenched samples.
4. Discussion In the present study, the nature of creep behavior is found to be markedly dependent on the microstructural changes and the deformation temperature. The deformation temperature dependence of the calculated creep parameters (n, β and έst) of both slowly cooled and quenched samples shows a sole transition temperature at 548 K which is in good agreement with a previously reported conclusion obtained for the binary Al–Zn alloy [10]. The creep parameters (n, κ and έst) for the binary Al–30 wt% Zn alloy cooled by the two different cooling techniques are found to increase with increasing the deformation temperature and show
Fig. 6. Dependence of the steady state creep rate, έst, on the deformation temperature for the slowly cooled and quenched samples.
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Fig. 7. Relation between ln κ and 103/T in (a) the low temperature range and (b) the high temperature range for slowly cooled and quenched samples.
Fig. 8. Relation between ln έst and 103/T in (a) the low temperature range, (b) the high temperature range for slowly cooled and quenched samples.
two different deformation stages with respect to the temperature (Figs. 4–6). The increase in the creep parameters with deformation temperature has different mechanisms depending on the heat treatment technique, which affect greatly the microstructure of the samples. For slowly-cooled samples, the saturated solid solution decomposes on cooling by preferential precipitation on grain boundaries [11–13]. The cell boundaries move discontinuously forming the equilibrium α- and β-phases [14]. The vacancies and other lattice concentration of defects are, therefore, low for slowly-cooled samples compared with that of the quenched samples. The slower the quench, the greater is the possible vacancy migration distance to the grain boundary sink and the less will be the density of retained vacancies. The relative increase of the creep parameters of the slowly-cooled samples in comparison to that of quenched samples (Figs. 4–6) might be due to the high vacancy concentration existing in the quenched samples (α-phase) [15] which enhances the migration of Zn-vacancy pairs towards dislocations, and the formation of Zn atmospheres around them with the result of a decrease in the density of the mobile lattice defects. It is clear from Figs. 4–6 that by increasing deformation temperature, from 508 to 538 K, creep parameters were found to increase. This behavior could be explained as due to the fact that increasing deformation temperature causes a coarsening process for the β-phase. This process lead to a precipitation process at one of the inter-phase boundary and to a dissolution process at the other inter-phase boundary. These transformation processes are associated with movements of dislocations at the inter-phase boundaries [10]. The anomalous behavior in the creep parameters was found at the temperature 548 K which might be attributed to
the dissolution of the β-phase, thus causing free Zn atoms which consequently move towards the generated mobile dislocations and pinning them. During the entire second deformation temperature (558– 578 K), the increase in the creep parameters has been observed. This behavior is in accordance with what can be expected of thermodynamic grounds. Raising the deformation temperature enhances the dissolution of the incoherent β-phase in the αphase matrix [16]. The increased recovery of the vacancy-type defects enhances the rate of diffusion of solute atoms. Accordingly, β-phase dissolves completely and disappears [17]. Concerning our optical microscope observations, we have confirmed the effect of deformation temperature on mode of precipitation on slowly cooled Al–30 wt% Zn samples. We have observed that precipitation of β-phase on the grain boundaries at the temperature ranging from 508 to 538 K (see Fig. 9a). The heating at the temperature ranging from 548 to 578 K causes dissolution of the formed Zn-rich phase which completely disappears (see Fig. 9b) leading to the enhancement of the creep parameters. The above interpretation is strongly supported by XRD measurements. Fig. 10 shows the dependence of the half line width at maximum intensity, Δ2θ and the integral intensity, I, on the deformation temperature for quenched and slowly cooled crept samples. It was found that I and Δ2θ increased with increasing deformation temperature and exhibited minima at 548 K for all tested samples. The average activation energies of the transient and steady state creep stages were found to be 28 kJ/mol in the low temperature range. The former value indicates that the rate controlling
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Fig. 10. The deformation temperature dependence of (a) X-ray full width at half maximum intensity, Δ2θ, and (b) integral intensity, I, for slowly cooled and quenched crept samples.
Fig. 9. Optical micrographs of slowly cooled Al–30 wt% Zn samples heated at (a) 528 K and (b) 558 K.
mechanism is the dislocation mechanism involving grain sliding or grain migration [14]. The average energies activating both transient and steady state creep stages above the transition temperature was found to be 73 kJ/mol which is consistent with the activation energy for the dislocation climb [18].
5. Conclusions Creep characteristics of Al–30 wt% Zn alloy have been studied in the temperature range from 508 to 578 K for both quenched and slowly cooled specimens. The creep parameters of the quenched specimens were found to be smaller than those of the slowly cooled specimens. These parameters of both types of specimens were generally found to increase with increasing deformation temperature, showing two stages around the transformation temperature (548 K).
References [1] S. Valdez, M.I. Pech-Canul, J.A. Ascencio-Gutiérrez, S.R. Casolco, Int. J. Electrochem. Sci. 8 (2013) 9298. [2] M.S. Saker, M.R. Nagy, M.M. Mostafa, R. Kamel, Egypt. J. Solids 8 (1986) 95. [3] S.B. Youssef, M.H. Beshai, A. Fawzy, M. Sobhy, G. Saad, Cryst. Res. Technol. 30 (1995) 1017. [4] F. Abd El-Salam, L.A. Wahab, R.H. Nada, H.Y. Zahran, J. Mater. Sci. 42 (2007) 3661. [5] M.A. Mahmoud, A.F. Abd El-Rehim, M. Sobhy, R.M. Abdel Rahman, Mater. Sci. Eng. A 528 (2011) 6026. [6] A.F. Abd El-Rehim, H.Y. Zahran, Mater. Sci. Technol. 30 (2014) 434. [7] A.F. Abd El-Rehim, J. Mater. Sci. 43 (2008) 1444. [8] J. Friedel, “Dislocations”, Pergamon Press, London, 1964(p. 304). [9] A.F. Abd El-Rehim, M.A. Mahmoud, J. Mater. Sci. 48 (2013) 2659. [10] F. Abd El-Salam, R.H. Nada, A.M. Abd El-Khalek, Physica B 292 (2000) 71. [11] H. Loffler, Structure and Structure Development in Al–Zn Alloys, Akademie Verlage, Berlin, 1995. [12] M. Hinai, S. Sawaya, H. Masumoto, Trans. JIM 33 (1992) 856. [13] A.Z. Mohamed, M.M. Mostafa, M.S. Sakr, A.A. El-Daly, Phys. Status Solidi A 110 (1988) K13. [14] F. Abd El-Salam, M.M. Mostafa, M.M. El-Sayed, R.H. Nada, Phys. Status Solidi A 144 (1994) 111. [15] M.M. Mostafa, R.H. Nada, F. Abd El-Salam, Phys. Status Solidi A 143 (1994) 297. [16] Z. Boumerzoug, M. Fatmi, Mater. Charact. 60 (2009) 768. [17] M.S. Sakr, Phys. Status Solidi A 126 (1991) K37. [18] J. Negrete, L. Valdes, G.T. Villasenor, Metall. Trans. 14A (1931) 1983.
Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Influence of quenching conditions on the mechanical and structural properties of Al–30 wt% Zn alloy A.F. Abd El-Rehim n, M.M. El-Sayed, M.R. Nagy, M. Abd El-Hafez Physics Department, Faculty of Education, Ain Shams University, P.O. Box 5101, Heliopolis 11771, Roxy, Cairo, Egypt
art ic l e i nf o
a b s t r a c t
Article history: Received 7 January 2014 Received in revised form 10 February 2014 Accepted 12 February 2014 Available online 20 February 2014
The present study aims at determining the effect of quenching condition on the mechanical and microstructure properties of Al–30 wt% Zn alloy. Creep curves of slowly cooled and quenched Al–30 wt% Zn alloys were studied at the temperature ranging from 508 to 578 K under constant applied stress. Results of both transient and steady state creep indicated a transition point at 548 K. Transient and steady state creep parameters of the quenched samples were found to be smaller than those of the slowly cooled samples. The activation energy of the transient and steady state creep amounted to 27 and 29 kJ/mol, respectively, before the transition deformation temperature (548 K) due to dislocation mechanism. The energy activating the transient and steady state creep steady state creep in the high temperature region (above the transformation temperature) characterizing as dislocation climb. & 2014 Elsevier B.V. All rights reserved.
Keywords: Al-alloys Creep curves Activation energy
1. Introduction Aluminum and its alloys play an important role in engineering applications, on account of their excellent mechanical and electrical properties as well as corrosion resistance. Research and development work in the field of aluminum alloys occupied therefore, a position of importance. The Aluminum (Al) based alloys mixture with Zinc (Zn) is one of the leading candidates for many end-use applications, due to their attractive physical, mechanical, and superplastic properties in conjunction with good abrasion and wear resistances [1]. The effect of dissolution of β-phase (Zn-rich phase) on the transient creep characteristics of Al–14 wt% Zn alloy has been studied [2]. The energy activating the transient creep characterized the dislocation mechanism. The effect of grain size as well as aging temperature on the creep characteristics of A1–10 wt% Zn alloy have been studied [3]. It has been found that the steady state creep rate decreases by increasing grain size, whereas it increases by increasing aging temperature. The values of the activation energies controlling the creep processes corresponded to that required for dislocation glide. The hardness of Al–5 wt% Zn and Al–5 wt% Zn–0.25 wt% In was measured at room temperature for samples heat treated in the range 300–453 K and dwell times in the range 30–300 s under 50 g load [4]. Softening was observed for all the samples and the hardness decreased with increasing temperature and/or dwell n
Corresponding author. E-mail address: [email protected] (A.F. Abd El-Rehim).
http://dx.doi.org/10.1016/j.msea.2014.02.040 0921-5093 & 2014 Elsevier B.V. All rights reserved.
time. Addition of In to the binary Al–5 wt% Zn alloy led to a significant increase in the hardness. The system Al–Zn is very suitable and particularly attractive for studying microstructure and phase transitions, especially in the supersaturated state. The purpose of the present work is to investigate the influence of quenching conditions and phase transformation on the mechanical properties of the Al–30 wt% Zn alloy.
2. Experimental procedures The Al–30 wt% Zn alloy that is used in the present study was prepared from pure raw materials of high purity; Al (99.99%) and Zn (99.99%). Small pieces of these raw materials were melted together in a high purity graphite crucible and then poured into a cast iron mold of dimensions 150 mm 10 mm 10 mm and allowed to solidify. After annealing for homogenization at 813 K for 4 days, the ingots were swaged and cold drawn into (i) wire samples having 0.8 mm diameter and 50 mm length for creep measurements, and (ii) sheets of 0.6 mm thick for microstructural examinations. The chemical composition of the alloy was verified by means of energy dispersive X-ray analysis (EDX). The samples were divided into two groups. Each group contained some wires suitable for the tensile test and some sheets suitable for structure examination. Two groups were annealed at 813 K for 2 h. One group was rapidly quenched in water (W.Q) at room temperature (300 K). The second group was slowly cooled (S.C) to room temperature with a cooling rate of 11 10 3 K/s. Tensile creep tests were then performed under constant stress of
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21.20 MPa, using a conventional tensile creep testing machine described earlier [5], and at temperatures ranging from 508 to 578 K in 10 K increments with an accuracy of 71 K. The sensitivity of strain measurements was approximately 7 1.0 10 2 mm. Microstructural examinations were carried out using a light optical microscope (VERSAMET-2). The treated sheet specimens for metallographic examination were etched with Keller's reagent. The microstructure was also investigated using XRD measurements. A Shimadzu X-ray diffractometer (Dx-30) was used to trace the structural variations in the samples by analyzing the obtained patterns. The X-ray tube giving copper (Kα) radiation operated at 30 kV and 30 mA emits X-ray beam of wavelength λ ¼0.15406 nm.
3. Results To identify the initial phases, both slowly cooled and quenched sheet samples of Al–30 wt% Zn alloy were examined by XRD at room temperature (300 K), as shown in Fig. 1. It can be seen that the slowly cooled alloy (Fig. 1a) is primarily composed of two phases identified as the α-phase (Al-rich phase of fcc structure) and β-phase (Zn-rich phase of hcp structure). While the quenched alloy (Fig. 1b) consists of one phase α-phase (Al-rich phase of fcc structure). Isothermal creep curves for both slowly cooled and quenched Al–30 wt% Zn samples obtained under different deformation temperatures ranging from 508 to 578 K in steps of 10 K under constant stress of 21.20 MPa are illustrated in Fig. 2. The creep
Fig. 1. X-ray diffraction pattern of (a) the slowly cooled sheet and (b) the quenched sheet samples of Al–30 wt% Zn alloy.
Fig. 2. Creep curves of (a) the slowly cooled and (b) the quenched samples under constant stress of 21.20 MPa at different deformation temperatures as indicated.
Fig. 3. Relation between ln εtr and ln t for (a) the slowly cooled and (b) the quenched samples at different deformation temperatures as indicated.
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107
curves consist of three distinct stages: transient creep where the strain rate decreases with time, steady state creep with a constant strain rate, and lastly, tertiary creep where the strain rate accelerates to fracture [6]. It was found that the transient creep fits a power law expression of the form [7,8]:
εtr ¼ κ t n
ð1Þ
where εtr is the transient strain, n and κ are the transient creep parameters and ttr is the transient creep time in seconds. The parameters n and κ are correlated from the relation between ln εtr and ln t at different deformation temperatures. n and κ are determined from the slopes and the intercepts of the straight lines in Fig. 3, respectively. Fig. 4 demonstrates the effect of deformation temperature on the values of parameter n of both slowly cooled and quenched samples while Fig. 5 shows the same effect concerning the parameter κ. The dependence of the steady state creep rate, έst, (evaluated from the slopes of the linear parts of the creep curves shown in Fig. 2) for both slowly cooled and quenched samples on the deformation temperature is depicted in Fig. 6. It is clear from Figs. 4–6 that
Fig. 4. Dependence of the transient creep parameter n on the deformation temperature for the slowly cooled and quenched samples.
the slowly cooled samples have higher values of creep parameters than quenched samples,
the creep parameters (n, κ and έst) show two deformation
temperature regions, the low temperature region (below 548 K) and the high temperature region (after 548 K). The values of n, κ and έst increase with increasing deformation temperature and show an anomalous behavior in the vicinity of the transition temperature (548 K) for both slowly cooled and quenched samples.
The activation energy controlling the transient creep behavior in the present work could be calculated assuming that the transient creep parameter κ varies with the deformation temperature, T, according to an Arrhenius-type relation of the form [9]:
κ ¼ constant expð Etr =RTÞ
ð2Þ
while έst satisfied the following equation: έst ¼ constant expð Est =RTÞ
ð3Þ
where R is the universal gas constant and T is the deformation temperature in Kelvin. The activation energies of the transient creep, Etr, were calculated from Fig. 7(a) and (b) relating ln κ and 1000/T (K 1) and found to be 27 and 66 kJ/mol in the low temperature range (508–538 K) and the high temperature range (558–578 K), respectively. The values of the activation energy of steady state creep, Est, were calculated from Fig. 8(a) and (b) relating ln έst and 1000/T (K 1). The mean value of activation energy in the low temperature range was found to be 29 kJ/mol while a value of 73 kJ/mol was obtained in the high temperature range.
Fig. 5. Dependence of the transient creep parameter κ on the deformation temperature for the slowly cooled and quenched samples.
4. Discussion In the present study, the nature of creep behavior is found to be markedly dependent on the microstructural changes and the deformation temperature. The deformation temperature dependence of the calculated creep parameters (n, β and έst) of both slowly cooled and quenched samples shows a sole transition temperature at 548 K which is in good agreement with a previously reported conclusion obtained for the binary Al–Zn alloy [10]. The creep parameters (n, κ and έst) for the binary Al–30 wt% Zn alloy cooled by the two different cooling techniques are found to increase with increasing the deformation temperature and show
Fig. 6. Dependence of the steady state creep rate, έst, on the deformation temperature for the slowly cooled and quenched samples.
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Fig. 7. Relation between ln κ and 103/T in (a) the low temperature range and (b) the high temperature range for slowly cooled and quenched samples.
Fig. 8. Relation between ln έst and 103/T in (a) the low temperature range, (b) the high temperature range for slowly cooled and quenched samples.
two different deformation stages with respect to the temperature (Figs. 4–6). The increase in the creep parameters with deformation temperature has different mechanisms depending on the heat treatment technique, which affect greatly the microstructure of the samples. For slowly-cooled samples, the saturated solid solution decomposes on cooling by preferential precipitation on grain boundaries [11–13]. The cell boundaries move discontinuously forming the equilibrium α- and β-phases [14]. The vacancies and other lattice concentration of defects are, therefore, low for slowly-cooled samples compared with that of the quenched samples. The slower the quench, the greater is the possible vacancy migration distance to the grain boundary sink and the less will be the density of retained vacancies. The relative increase of the creep parameters of the slowly-cooled samples in comparison to that of quenched samples (Figs. 4–6) might be due to the high vacancy concentration existing in the quenched samples (α-phase) [15] which enhances the migration of Zn-vacancy pairs towards dislocations, and the formation of Zn atmospheres around them with the result of a decrease in the density of the mobile lattice defects. It is clear from Figs. 4–6 that by increasing deformation temperature, from 508 to 538 K, creep parameters were found to increase. This behavior could be explained as due to the fact that increasing deformation temperature causes a coarsening process for the β-phase. This process lead to a precipitation process at one of the inter-phase boundary and to a dissolution process at the other inter-phase boundary. These transformation processes are associated with movements of dislocations at the inter-phase boundaries [10]. The anomalous behavior in the creep parameters was found at the temperature 548 K which might be attributed to
the dissolution of the β-phase, thus causing free Zn atoms which consequently move towards the generated mobile dislocations and pinning them. During the entire second deformation temperature (558– 578 K), the increase in the creep parameters has been observed. This behavior is in accordance with what can be expected of thermodynamic grounds. Raising the deformation temperature enhances the dissolution of the incoherent β-phase in the αphase matrix [16]. The increased recovery of the vacancy-type defects enhances the rate of diffusion of solute atoms. Accordingly, β-phase dissolves completely and disappears [17]. Concerning our optical microscope observations, we have confirmed the effect of deformation temperature on mode of precipitation on slowly cooled Al–30 wt% Zn samples. We have observed that precipitation of β-phase on the grain boundaries at the temperature ranging from 508 to 538 K (see Fig. 9a). The heating at the temperature ranging from 548 to 578 K causes dissolution of the formed Zn-rich phase which completely disappears (see Fig. 9b) leading to the enhancement of the creep parameters. The above interpretation is strongly supported by XRD measurements. Fig. 10 shows the dependence of the half line width at maximum intensity, Δ2θ and the integral intensity, I, on the deformation temperature for quenched and slowly cooled crept samples. It was found that I and Δ2θ increased with increasing deformation temperature and exhibited minima at 548 K for all tested samples. The average activation energies of the transient and steady state creep stages were found to be 28 kJ/mol in the low temperature range. The former value indicates that the rate controlling
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Fig. 10. The deformation temperature dependence of (a) X-ray full width at half maximum intensity, Δ2θ, and (b) integral intensity, I, for slowly cooled and quenched crept samples.
Fig. 9. Optical micrographs of slowly cooled Al–30 wt% Zn samples heated at (a) 528 K and (b) 558 K.
mechanism is the dislocation mechanism involving grain sliding or grain migration [14]. The average energies activating both transient and steady state creep stages above the transition temperature was found to be 73 kJ/mol which is consistent with the activation energy for the dislocation climb [18].
5. Conclusions Creep characteristics of Al–30 wt% Zn alloy have been studied in the temperature range from 508 to 578 K for both quenched and slowly cooled specimens. The creep parameters of the quenched specimens were found to be smaller than those of the slowly cooled specimens. These parameters of both types of specimens were generally found to increase with increasing deformation temperature, showing two stages around the transformation temperature (548 K).
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