- Email: [email protected]
Effect of heat treatment on strain hardening of ZK60 Mg alloy
Materials and Design 32 (2011) 1526–1530
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Short Communication
Effect of heat treatment on strain hardening of ZK60 Mg alloy Xianhua Chen a,b,⇑, Fusheng Pan a,b, Jianjun Mao a, Jingfeng Wang a,b, Dingfei Zhang a,b, Aitao Tang a,b, Jian Peng a,b a b
College of Materials Science and Engineering, Chongqing University, Chongqing 400044, PR China National Engineering Research Center for Magnesium Alloys, Chongqing University, Chongqing 400044, PR China
a r t i c l e
i n f o
Article history: Received 21 July 2010 Accepted 8 October 2010 Available online 16 October 2010
a b s t r a c t Strain hardening behaviors of extruded ZK60 Mg alloy under different heat treatments (T4, T5 and T6) were studied using uniaxial tensile tests at room temperature. Hardening capacity, strain hardening exponent as well as strain hardening rate curve were obtained according to true plastic stress–strain curves. T5 and T6 treatments decrease strain hardening of extruded ZK60 alloy, and subsequently give rise to an obvious reduction in tensile uniform strain. While, as-T4 treated specimen shows the strongest strain hardening ability among these specimens, and its hardening capacity and strain hardening exponent are nearly twice those of as-T5 and T6 treated specimens. These effects were analyzed in terms of the microstructural variation and dislocation storage in ZK60 alloy. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction The low density of Mg and its alloys makes it an ideal choice for automotive and aerospace industries, such as body panels and space frames [1–3]. However, their extensive application is restricted due to their limited strength, ductility and formability. To improve the mechanical properties, many investigations have focused on optimizing heat treatment processing routes including solid solution, artificial aging etc. Great attention has been paid lately on the investigation of the effect of heat treatment on microstructures and room-temperature mechanical properties of Mg alloys [4–10]. Strain hardening is one of the most important considerations in the evaluation of plastic deformation of metallic materials [11]. The strength, ductility, toughness and deformability of materials are intimately related to strain hardening characteristics [12]. For this reason, abundant investigations have been carried out on the strain hardening behaviors and physical mechanism of conventional metallic materials [13–15]. The strain hardening behavior of cubic metals is fairly well understood, and the accumulation of a forest of dislocations is the dominant hardening mechanism [14,15]. Hexagonal metals present a more complex case due to their low symmetry, which restricts the number of slip systems, and their strong plastic anisotropy [14]. Hitherto some studies have been provided on strain hardening behavior of Mg and its alloys [10,16–26]. Most of the researchers focused on the effects of
⇑ Corresponding author at: College of Materials Science and Engineering, Chongqing University, Chongqing 400044, PR China. Tel./fax: +86 23 65102821. E-mail address: [email protected] (X. Chen). 0261-3069/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2010.10.008
texture, grain size, twinning, temperature, strain rate on the hardening characteristics of Mg alloys. However, only limited studies on the influence of heat treatment on the strain hardening behavior of Mg alloys are reported in the open literature [10]. The aim of the present work is to investigate the strain hardening response in ZK60 Mg alloy under different heat treatment conditions by means of tensile testing at ambient temperature. Our study will provide important basis for controlling the strength and ductility of Mg alloys by optimizing heat treatment conditions. 2. Experimental procedures Alloy ingots of ZK60 were prepared from high purity Mg (99.98 wt.%), Zn (99.99 wt.%) and Mg-27.85 wt.% Zr master alloy in an electric resistance furnace. When the temperature reached 780 °C, molten alloy was stirred for 8 min and subsequently held for 45 min. Then semi-continuous casting was used to prepare cylinder ingots of ZK60 magnesium alloy with a diameter of 90 mm. The actual chemical composition of the alloy was determined by a photoelectricity spectrum analyzer (APL4460). The ingots were homogenized at 420 °C for 18 h and then hotextruded into rods with diameter of 16 mm. Extrusion ratio was 27:1, and ingot temperature was 390 °C. Three different heat treatments were performed on the as-extruded specimen, namely solid solution at 420 °C for 8 h and cooled at air (T4), solid solution at 420 °C for 8 h and cooled in air plus artificial aging at 180 °C for 15 h (T6), and direct artificial aging at 180 °C for 15 h (T5). The extruded and heat-treated samples were machined into tensile specimens of 5 mm gauge diameter and 50 mm gauge length. Tensile testing was carried out on a CMT5105 material test machine at a
X. Chen et al. / Materials and Design 32 (2011) 1526–1530
1527
strain rate of 103 s1. As-cast samples were etched in a solution of 4 vol.% nitric acid and 96 vol.% ethanol. As-extruded and heat-treated samples were etched with a mixture of 1.5 g picric acid + 25 ml ethanol + 5 ml acetic acid + 10 ml water. Phase analyses were performed with a Rigaku D/MAX-2500PC X-ray diffractometer (XRD). Microstructures of the alloys were investigated with an optical microscope (OM) and scanning electron microscope (SEM, TESCAN VEGA II LMU) using an accelerating voltage of 20 kV.
supersaturated solid solution. Solid solution followed by artificial aging (T6) results in the precipitation of second phase again. Comparing Fig. 1b and d, it is found that T5 can bring on more second phase precipitates than T6 since lots of lattice defects in as-extruded specimen are beneficial for the nucleation of second phase particles.
3. Results
Typical engineering tensile stress–strain curves of four specimens in various states are shown in Fig. 2a. It is seen that heat treatment has a great effect on strength and ductility. The as-T5 treated specimen possesses a high 0.2% offset yield strength (ry) of 295 MPa, which is obviously higher than that of the as-extruded specimen (ry = 268). T4 treatment leads to fairly low yield strength of 203 MPa in ZK60 alloy. After T6 treatment, the yield strength increases to 258 MPa because of precipitate strengthening. It is noted that both ultimate tensile strength (rUTS) and elongation-to-failure (ef) exhibit a decreasing tendency when the as-extruded alloy is subjected to heat treatments, which could be related to the coarsening of grains and the precipitation of second phase. Because of the considerable uniform deformation, these engineering curves can be converted to true stress–strain curves using standard formula assuming uniform cross-sectional area along the gauge length, as shown in Fig. 2b. The corresponding true tensile properties including ry, rUTS, ef, and eu are summarized in Table 1. Obvious strain hardening happens in the plastic deformation re-
3.1. Microstructure Fig. 1 illustrates plan-view SEM micrographs of four ZK60 specimens in different conditions. Firstly, it reveals that grain size is various. As-extruded specimen consists of fairly fine recrystallized grains with an average size of 8 lm shown in Fig. 1a. The as-T5 treated specimen almost keeps the fine-grained structure, as shown in Fig. 1b. After T4 and T6 treatment, an obvious grain coarsening is detected in Fig. 1c and d. The mean grain sizes are up to about 30 lm and 40 lm for T4 and T6, respectively. Additionally, there is a difference in the precipitation of second phase (MgZn and MgZn2). For the as-extruded sample, some second phase precipitates are visible in a-Mg matrix. After the as-extruded specimen is subjected to T5, it is seen that a considerable number of precipitates are formed in the matrix. In T4 state, most of second phase constituents are decomposed and the matrix transforms to
3.2. Strain hardening behavior
Fig. 1. Typical SEM photographs showing the microstructure of the ZK60 alloy under different states: (a) as-extruded, (b) as-T5 treated, (c) as-T4 treated and (d) as-T6 treated.
1528
X. Chen et al. / Materials and Design 32 (2011) 1526–1530
Fig. 2. (a) Engineering tensile stress–strain curves of the four specimens (as indicated), (b) true tensile stress–strain curves.
Table 1 Yield strength (ry), ultimate tensile strength (rUTS), elongation-to-failure (Hc), uniform strain (ef), hardening capacity (eu) and strain hardening exponent (n) of the four specimens under various states. Specimen
ry (MPa)
rUTS (MPa)
Hc
ef (%)
eu (%)
n
As-extruded As-T5 treated As-T4 treated As-T6 treated
268 295 203 258
388 375 344 346
0.45 0.27 0.69 0.34
17.7 12.5 15.2 10.8
14.1 11.4 14.3 9.3
0.11 0.07 0.15 0.09
gime in these samples. The strain hardening of these samples can be analyzed by means of macroscopic strain hardening rate [14]:
H ¼ dr=de
ð1Þ
where r and e are the macroscopic true stress and true plastic strain, respectively. Fig. 3a illustrates the hardening rate H vs. net flow stress (rry) curves for the four samples. All the specimens show first a rapid strain hardening decrease after yielding due to a short elastoplastic transition [16], and further a relatively low reduction tendency of hardening associated with the stage III of strain hardening. It is seen that heat treatment affects the hardening response of ZK60 alloy remarkably. The initial H values are 12,650 MPa and 9540 MPa for T5 and T6, respectively, which are higher than that for extrusion (H = 7750 MPa). T4 treatment induces a lower initial H of about 6750 MPa in ZK60 alloy. However, when comparing the H values in stage III of hardening for the four ZK60 specimens, it is clear that T4 treatment enhances H, T5 and T6 treatments induce a decrease in H. At the net flow stress of
50 MPa, H of as-T4 treated specimen is approximately 2240 MPa higher than that of as-extruded specimen by 600 MPa, however H of as-T5 treated specimen is as low as 880 MPa. The Hep curves of ZK60 alloys are shown in Fig. 3b. The curves indicates that at the same plastic strain in stage III of hardening, asT4 treated specimen has the greatest H, and the H values of as-T5 treated and as-T6 treated specimens are less than that of as-extruded specimen. For a rod under uniaxial tension, necking initiates when the H value is less than the instantaneous flow stress, r [16,27]. According to this criterion, the uniform plastic strains of these alloys are evaluated. The tensile uniform elongation values are obtained to be 14.1%, 14.3%, 11.4% and 9.3% for extruded, T4, T5 and T6 treated states, respectively. The evaluated values are quite close to those measured according to tensile curves, as listed in Table 1. The hardening capacity, Hc, of a material is able to be considered as a ratio of (rUTS ry) to ry [17,28],
Hc ¼
rUTS ry ry
ð2Þ
Table 1 shows the hardening capacity of these ZK60 alloys in various conditions. It is observed that T4 treatment increases Hc from 0.45 to 0.69, T5 and T6 treatments nevertheless decrease Hc to 0.27 and 0.34, respectively. The variation of Hc with heat treatment may be related to the grain size, precipitates and dislocations in ZK60 alloys. In order to further quantify strain hardening behavior of ZK60 alloys, the uniform plastic deformation stage in uniaxial tensile curve is fitted by the equation [12]:
Fig. 3. Strain hardening rate (H) as a function of (a) net flow stress and (b) true plastic strain of the four specimens (as indicated).
X. Chen et al. / Materials and Design 32 (2011) 1526–1530
r ¼ K enp
ð3Þ
where K is the strength coefficient, and n is the strain hardening exponent. These tensile curves are fitted well, as shown in Fig. 4. The fitting results for ZK60 samples are listed in Table 1. It can be seen that n is intimately dependent on heat treatment condition. When the as-extruded specimen is subjected to T4 treatment, n is enhanced from 0.11 to 0.15, which is about double the value of as-T5 treated specimen. T6 treatment, similar with T5, is also detrimental to the n value. The n value of as-extruded samples is close to that of AZ31 alloy reported by Afrin et al. [17]. 4. Discussion The strain hardening of a material after yielding is related to the dislocation strain field interaction. An increase in dislocation density accompanying continued plastic deform is responsible for strain hardening in a metallic material; that is, dislocations themselves are obstacles to dislocation motion [29]. Thus, the Taylor dislocation contribution rd = MgGbq1/2 dominates the strain hardening effect; where q is the dislocation density; g is a constant; M is the Taylor factor; G is the shear modulus; and b is the Burgers vector [12]. Contribution to the stress from the dislocation density can be obtained by subtracting the yield stress from the total flow stress, and this can be written as q1/2 / rd = r ry [12,16,20]. The applied stress necessary to deform a material is then proportional to the dislocation density inside the material [17]. Consequently, the microstructural factors, leading to more dislocation accumulation inside grains, will facilitate the improvement of strain hardening in Mg alloys. Based on the above discussion, the variation of strain hardening behavior with heat treatment can be understood in terms of the microstructure change during heat treatment. In the initial hardening stage, the high density of second phase particles in the material precipitating during aging might have contributed to the enhanced initial strain hardening rate in the T5 and T6 state. Much higher activation energy is required to the plastic flow due to that the precipitation particles are strong barriers to dislocation movement [28,30]. Since solutionizing could reduce the number of particles, it is reasonable that obviously reduced initial strain hardening rate is observed in the T4 state. The strain hardening of a material at a higher hardening stage (stage III) is more related to the dislocation strain field interaction [17]. For the as-extruded state, fine grains and some second phase particles are present due to the hot plastic deformation
1529
and dynamic recrystallization. T5 heat treatment induced slight grain growth and structure relaxation (i.e., reduced internal lattice defects), but a number of second phase precipitates. Like conventional grain boundaries, abundant phase boundaries between precipitates and Mg matrix may act as dislocation sinks besides strengthening effect originating from blocking dislocation slip. During plastic deformation, the dislocations tend to high-energy phase boundaries and are then balanced out for the most part by reorganization and annihilation inside the boundaries [31]. At the same time, lower solute atom content in solid solution is expected to enhance dislocation recovery events at room temperature [10], which is probably connected with a rise of dislocation cross slip and climb ability due to the increasing of stacking fault energy [22]. Numerous precipitation particles and reduced solute atom content in solution are not beneficial to the storage of dislocations and consequently diminish strain hardening ability in ZK60 alloy obviously although the slight grain coarsening and reduced defects may enlarge the room for dislocation accumulation to some extent. Because of high temperature, complete static recrystallization, second phase decomposing and significant grain growth occurred in T4 treatment. This gives rise to a low initial dislocation density and big grain size in T4 state, which provide more space to accommodate lattice dislocations and dislocation debris can build up starting from a very low level. The studies by Afrin and del Valle et al., have testified that an increase in grain size does improve the strain hardening rate, harden capacity and strain hardening exponent in Mg alloy [16,17]. In addition, higher solute atom content in solid solution and much less precipitates are helpful to reduce dislocation recovery during tensile deformation [10]. Hence stronger strain hardening ability is observed in the stage III of hardening in as-T4 treated specimen than in as-extruded specimen. In fact, the as-T4 treated specimen exhibits the highest strain hardening among these studied ZK60 specimens. In contrast to the as-T4 specimen, it is found that a great amount of precipitates are distributed in the matrix in the asT6 specimen. As discussed above, the corresponding phase boundaries and decreased solution atom content facilitate considerable dislocation recovery during plastic straining, which brings on a lower strain hardening ability in T6 state as compared to T4 state. It is noted that T6 heat treatment even shows lower H, n and Hc values than those of the extruded state, indicating that the effect of precipitation should exceed the contribution of grain coarsening and reduced dislocation density to strain hardening in ZK60 alloy. 5. Conclusions
Fig. 4. True stress plotted as a function of true plastic strain and the corresponding fitting curves (dash) for as-T4 treated sample.
(1) T5 treatment makes the formation of numerous Mg–Zn precipitation particles and lower solute atom content in solution in ZK60 alloy, which enhances the yield strength and the dislocation recovery considerably. The strain hardening exponent and hardening capacity are about three-fifths of that in as-extruded state. (2) The dislocation storage ability is stronger for as-T4 treated specimen as compared to as-extruded specimen since solid solution treatment causes evident grain growth, second phase dissolution and more solute atoms in solution. The strain hardening rate H, exponent n and capacity Hc are noticeably higher than that in as-T5 condition. (3) Compared with as-extruded and T4 treated specimens, as-T6 treated specimen possesses obviously reduced strain hardening and tensile ductility, mainly originating from the precipitation of many second phase particles during aging.
1530
X. Chen et al. / Materials and Design 32 (2011) 1526–1530
Acknowledgement This work supported by the Fundamental Research Funds for the Central Universities (No. CDJZR10 13 00 01).
References [1] Mordike BL, Ebert T. Magnesium properties–applications–potential. Mater Sci Eng 2001;302:37–45. [2] Bohlen J, Letzig D, Kainer KU. New perspectives for wrought magnesium alloys. Mater Sci Forum 2007;546–549:1–10. [3] Kainer KU, von Buch F, In: Kainer KU, editors. Magnesium alloys and technologies, DGM; 2003. [4] Chen HM, Yu HS, Kang SB, Cho J, Min GH. Optimization of annealing treatment parameters in a twin roll cast and warm rolled ZK60 alloy sheet. Mater Sci Eng A 2010;527:1236–42. [5] Xu DK, Liu L, Xu YB, Han EH. The effect of precipitates on the mechanical properties of ZK60-Y alloy. Mater Sci Eng A 2006;420:322–32. [6] Chen HM, Kang SB, Yu HS, Cho J, Kim HW, Min GH. Effect of heat treatment on microstructure and mechanical properties of twin roll cast and sequential warm rolled ZK60 alloy sheets. J Alloys Compd 2009;476:324–8. [7] Beer AG, Barnett MR. Microstructure evolution in hot worked and annealed magnesium alloy AZ31. Mater Sci Eng A 2008;485:318–24. [8] Liang S, Guan D, Chen L, Gao Z, Tang H, Tong X, et al. Precipitation and its effect on age-hardening behavior of as-cast Mg–Gd–Y alloy. Mater Des 2011;32:361–4. [9] Peng Q, Hou X, Wang L, Wu Y, Cao Z, Wang L. Microstructure and mechanical properties of high performance Mg–Gd based alloys. Mater Des 2009;30:292–6. [10] Yakubtsov IA, Diaka BJ, Sager CA, Bhattacharya B, MacDonald WD, Niewczas M. Effects of heat treatment on microstructure and tensile deformation of Mg AZ80 alloy at room temperature. Mater Sci Eng A 2008;496:247–55. [11] Chen XH, Lu L. Work hardening of ultrafine-grained copper with nanoscale twins. Scripta Mater 2007;57:133–6. [12] Meyers MA, Chawla KK. Mechanical behavior of materials. Upper Saddle River, NJ: Prentice Hall, Inc.; 1999. [13] Chu D, Morris JW. The influence of microstructure on work hardening in aluminum. Acta Mater 1996;44:2599–610.
[14] Kocks UF, Mecking H. Physics and phenomenology of strain hardening: the FCC case. Prog Mater Sci 2003;48:171–273. [15] Kocks UF. The relation between polycrystal deformation and single-crystal deformation. Metall Trans A 1970;1:1121–33. [16] del Valle JA, Carreno F, Ruano OA. Influence of texture and grain size on work hardening and ductility in magnesium-based alloys processed by ECAP and rolling. Acta Mater 2006;54:4247–59. [17] Afrin N, Chen DL, Cao X, Jahazi M. Strain hardening behavior of a friction stir welded magnesium alloy. Scripta Mater 2007;57:1004–7. [18] Lin XZ, Chen DL. Strain hardening and strain sensitivity of an extruded magnesium alloy. J Mater Eng Perform 2008;17:894–901. [19] Jiang L, Jonas JJ, Luo AA, Sachdev AK, Godet S. Twinning-induced softening in polycrystalline AM30 Mg alloy at moderate temperatures. Scripta Mater 2006;54:771–5. [20] Cáceres CH, Blake AH. On the strain hardening behavior of magnesium at room temperature. Mater Sci Eng A 2007;462:193–6. [21] Wu HY, Lin FZ. Mechanical properties and strain-hardening behavior of Mg alloy AZ31B–H24 thin sheet. Mater Sci Eng A 2010;527:1194–9. [22] Máthis K, Trojanová Z, Lukácˇ P, Cáceres CH, Lendvai J. Modeling of hardening and softening processes in Mg alloys. J Alloys Compd 2004;378:176–9. [23] Knezevic M, Levinson A, Harris R, Mishra RK, Doherty RD, Kalidindi SR. Deformation twinning in AZ31: Influence on strain hardening and texture evolution. Acta Mater 2010;58:6230–42. [24] Wu HY, Yan JC, Tsai HH, Chiu CH, Zhou GZ, Lin CF. Tensile flow and strainhardening behaviors of dual-phase Mg–Li–Zn alloy thin sheets. Mater Sci Eng A 2010;527:7197–203. [25] Trojanová Z, Lukácˇ P. Compressive deformation behavior of magnesium alloys. J Mater Proc Technol 2005;162–163:416–21. [26] Máthis K, Trojanová Z, Lukácˇ P. Hardening and softening in deformed magnesium alloys. Mater Sci Eng A 2002;324:141–4. [27] Misra A, Zhang X, Hammon D, Hoagland RG. Work hardening in rolled nanolayered metallic composites. Acta Mater 2004;53:221–6. [28] Luo J, Mei Z, Tian WH, Wang ZR. Diminishing of work hardening in electroformed polycrystalline copper with nano-sized and uf-sized twins. Mater Sci Eng A 2006;441:282–90. [29] Courtney TH. Mechanical behavior of materials. Michigan: McGraw-Hill Companies, Inc.; 2000. [30] Dieter GE. Mechanical metallurgy. 3rd ed. Boston: McGraw-Hill; 1986. [31] Ma E, Wang YM, Lu QH, Sui ML, Lu L, Lu K. Strain hardening and large tensile elongation in ultrahigh-strength nano-twinned copper. Appl Phys Lett 2004;85:4932–4.
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Short Communication
Effect of heat treatment on strain hardening of ZK60 Mg alloy Xianhua Chen a,b,⇑, Fusheng Pan a,b, Jianjun Mao a, Jingfeng Wang a,b, Dingfei Zhang a,b, Aitao Tang a,b, Jian Peng a,b a b
College of Materials Science and Engineering, Chongqing University, Chongqing 400044, PR China National Engineering Research Center for Magnesium Alloys, Chongqing University, Chongqing 400044, PR China
a r t i c l e
i n f o
Article history: Received 21 July 2010 Accepted 8 October 2010 Available online 16 October 2010
a b s t r a c t Strain hardening behaviors of extruded ZK60 Mg alloy under different heat treatments (T4, T5 and T6) were studied using uniaxial tensile tests at room temperature. Hardening capacity, strain hardening exponent as well as strain hardening rate curve were obtained according to true plastic stress–strain curves. T5 and T6 treatments decrease strain hardening of extruded ZK60 alloy, and subsequently give rise to an obvious reduction in tensile uniform strain. While, as-T4 treated specimen shows the strongest strain hardening ability among these specimens, and its hardening capacity and strain hardening exponent are nearly twice those of as-T5 and T6 treated specimens. These effects were analyzed in terms of the microstructural variation and dislocation storage in ZK60 alloy. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction The low density of Mg and its alloys makes it an ideal choice for automotive and aerospace industries, such as body panels and space frames [1–3]. However, their extensive application is restricted due to their limited strength, ductility and formability. To improve the mechanical properties, many investigations have focused on optimizing heat treatment processing routes including solid solution, artificial aging etc. Great attention has been paid lately on the investigation of the effect of heat treatment on microstructures and room-temperature mechanical properties of Mg alloys [4–10]. Strain hardening is one of the most important considerations in the evaluation of plastic deformation of metallic materials [11]. The strength, ductility, toughness and deformability of materials are intimately related to strain hardening characteristics [12]. For this reason, abundant investigations have been carried out on the strain hardening behaviors and physical mechanism of conventional metallic materials [13–15]. The strain hardening behavior of cubic metals is fairly well understood, and the accumulation of a forest of dislocations is the dominant hardening mechanism [14,15]. Hexagonal metals present a more complex case due to their low symmetry, which restricts the number of slip systems, and their strong plastic anisotropy [14]. Hitherto some studies have been provided on strain hardening behavior of Mg and its alloys [10,16–26]. Most of the researchers focused on the effects of
⇑ Corresponding author at: College of Materials Science and Engineering, Chongqing University, Chongqing 400044, PR China. Tel./fax: +86 23 65102821. E-mail address: [email protected] (X. Chen). 0261-3069/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2010.10.008
texture, grain size, twinning, temperature, strain rate on the hardening characteristics of Mg alloys. However, only limited studies on the influence of heat treatment on the strain hardening behavior of Mg alloys are reported in the open literature [10]. The aim of the present work is to investigate the strain hardening response in ZK60 Mg alloy under different heat treatment conditions by means of tensile testing at ambient temperature. Our study will provide important basis for controlling the strength and ductility of Mg alloys by optimizing heat treatment conditions. 2. Experimental procedures Alloy ingots of ZK60 were prepared from high purity Mg (99.98 wt.%), Zn (99.99 wt.%) and Mg-27.85 wt.% Zr master alloy in an electric resistance furnace. When the temperature reached 780 °C, molten alloy was stirred for 8 min and subsequently held for 45 min. Then semi-continuous casting was used to prepare cylinder ingots of ZK60 magnesium alloy with a diameter of 90 mm. The actual chemical composition of the alloy was determined by a photoelectricity spectrum analyzer (APL4460). The ingots were homogenized at 420 °C for 18 h and then hotextruded into rods with diameter of 16 mm. Extrusion ratio was 27:1, and ingot temperature was 390 °C. Three different heat treatments were performed on the as-extruded specimen, namely solid solution at 420 °C for 8 h and cooled at air (T4), solid solution at 420 °C for 8 h and cooled in air plus artificial aging at 180 °C for 15 h (T6), and direct artificial aging at 180 °C for 15 h (T5). The extruded and heat-treated samples were machined into tensile specimens of 5 mm gauge diameter and 50 mm gauge length. Tensile testing was carried out on a CMT5105 material test machine at a
X. Chen et al. / Materials and Design 32 (2011) 1526–1530
1527
strain rate of 103 s1. As-cast samples were etched in a solution of 4 vol.% nitric acid and 96 vol.% ethanol. As-extruded and heat-treated samples were etched with a mixture of 1.5 g picric acid + 25 ml ethanol + 5 ml acetic acid + 10 ml water. Phase analyses were performed with a Rigaku D/MAX-2500PC X-ray diffractometer (XRD). Microstructures of the alloys were investigated with an optical microscope (OM) and scanning electron microscope (SEM, TESCAN VEGA II LMU) using an accelerating voltage of 20 kV.
supersaturated solid solution. Solid solution followed by artificial aging (T6) results in the precipitation of second phase again. Comparing Fig. 1b and d, it is found that T5 can bring on more second phase precipitates than T6 since lots of lattice defects in as-extruded specimen are beneficial for the nucleation of second phase particles.
3. Results
Typical engineering tensile stress–strain curves of four specimens in various states are shown in Fig. 2a. It is seen that heat treatment has a great effect on strength and ductility. The as-T5 treated specimen possesses a high 0.2% offset yield strength (ry) of 295 MPa, which is obviously higher than that of the as-extruded specimen (ry = 268). T4 treatment leads to fairly low yield strength of 203 MPa in ZK60 alloy. After T6 treatment, the yield strength increases to 258 MPa because of precipitate strengthening. It is noted that both ultimate tensile strength (rUTS) and elongation-to-failure (ef) exhibit a decreasing tendency when the as-extruded alloy is subjected to heat treatments, which could be related to the coarsening of grains and the precipitation of second phase. Because of the considerable uniform deformation, these engineering curves can be converted to true stress–strain curves using standard formula assuming uniform cross-sectional area along the gauge length, as shown in Fig. 2b. The corresponding true tensile properties including ry, rUTS, ef, and eu are summarized in Table 1. Obvious strain hardening happens in the plastic deformation re-
3.1. Microstructure Fig. 1 illustrates plan-view SEM micrographs of four ZK60 specimens in different conditions. Firstly, it reveals that grain size is various. As-extruded specimen consists of fairly fine recrystallized grains with an average size of 8 lm shown in Fig. 1a. The as-T5 treated specimen almost keeps the fine-grained structure, as shown in Fig. 1b. After T4 and T6 treatment, an obvious grain coarsening is detected in Fig. 1c and d. The mean grain sizes are up to about 30 lm and 40 lm for T4 and T6, respectively. Additionally, there is a difference in the precipitation of second phase (MgZn and MgZn2). For the as-extruded sample, some second phase precipitates are visible in a-Mg matrix. After the as-extruded specimen is subjected to T5, it is seen that a considerable number of precipitates are formed in the matrix. In T4 state, most of second phase constituents are decomposed and the matrix transforms to
3.2. Strain hardening behavior
Fig. 1. Typical SEM photographs showing the microstructure of the ZK60 alloy under different states: (a) as-extruded, (b) as-T5 treated, (c) as-T4 treated and (d) as-T6 treated.
1528
X. Chen et al. / Materials and Design 32 (2011) 1526–1530
Fig. 2. (a) Engineering tensile stress–strain curves of the four specimens (as indicated), (b) true tensile stress–strain curves.
Table 1 Yield strength (ry), ultimate tensile strength (rUTS), elongation-to-failure (Hc), uniform strain (ef), hardening capacity (eu) and strain hardening exponent (n) of the four specimens under various states. Specimen
ry (MPa)
rUTS (MPa)
Hc
ef (%)
eu (%)
n
As-extruded As-T5 treated As-T4 treated As-T6 treated
268 295 203 258
388 375 344 346
0.45 0.27 0.69 0.34
17.7 12.5 15.2 10.8
14.1 11.4 14.3 9.3
0.11 0.07 0.15 0.09
gime in these samples. The strain hardening of these samples can be analyzed by means of macroscopic strain hardening rate [14]:
H ¼ dr=de
ð1Þ
where r and e are the macroscopic true stress and true plastic strain, respectively. Fig. 3a illustrates the hardening rate H vs. net flow stress (rry) curves for the four samples. All the specimens show first a rapid strain hardening decrease after yielding due to a short elastoplastic transition [16], and further a relatively low reduction tendency of hardening associated with the stage III of strain hardening. It is seen that heat treatment affects the hardening response of ZK60 alloy remarkably. The initial H values are 12,650 MPa and 9540 MPa for T5 and T6, respectively, which are higher than that for extrusion (H = 7750 MPa). T4 treatment induces a lower initial H of about 6750 MPa in ZK60 alloy. However, when comparing the H values in stage III of hardening for the four ZK60 specimens, it is clear that T4 treatment enhances H, T5 and T6 treatments induce a decrease in H. At the net flow stress of
50 MPa, H of as-T4 treated specimen is approximately 2240 MPa higher than that of as-extruded specimen by 600 MPa, however H of as-T5 treated specimen is as low as 880 MPa. The Hep curves of ZK60 alloys are shown in Fig. 3b. The curves indicates that at the same plastic strain in stage III of hardening, asT4 treated specimen has the greatest H, and the H values of as-T5 treated and as-T6 treated specimens are less than that of as-extruded specimen. For a rod under uniaxial tension, necking initiates when the H value is less than the instantaneous flow stress, r [16,27]. According to this criterion, the uniform plastic strains of these alloys are evaluated. The tensile uniform elongation values are obtained to be 14.1%, 14.3%, 11.4% and 9.3% for extruded, T4, T5 and T6 treated states, respectively. The evaluated values are quite close to those measured according to tensile curves, as listed in Table 1. The hardening capacity, Hc, of a material is able to be considered as a ratio of (rUTS ry) to ry [17,28],
Hc ¼
rUTS ry ry
ð2Þ
Table 1 shows the hardening capacity of these ZK60 alloys in various conditions. It is observed that T4 treatment increases Hc from 0.45 to 0.69, T5 and T6 treatments nevertheless decrease Hc to 0.27 and 0.34, respectively. The variation of Hc with heat treatment may be related to the grain size, precipitates and dislocations in ZK60 alloys. In order to further quantify strain hardening behavior of ZK60 alloys, the uniform plastic deformation stage in uniaxial tensile curve is fitted by the equation [12]:
Fig. 3. Strain hardening rate (H) as a function of (a) net flow stress and (b) true plastic strain of the four specimens (as indicated).
X. Chen et al. / Materials and Design 32 (2011) 1526–1530
r ¼ K enp
ð3Þ
where K is the strength coefficient, and n is the strain hardening exponent. These tensile curves are fitted well, as shown in Fig. 4. The fitting results for ZK60 samples are listed in Table 1. It can be seen that n is intimately dependent on heat treatment condition. When the as-extruded specimen is subjected to T4 treatment, n is enhanced from 0.11 to 0.15, which is about double the value of as-T5 treated specimen. T6 treatment, similar with T5, is also detrimental to the n value. The n value of as-extruded samples is close to that of AZ31 alloy reported by Afrin et al. [17]. 4. Discussion The strain hardening of a material after yielding is related to the dislocation strain field interaction. An increase in dislocation density accompanying continued plastic deform is responsible for strain hardening in a metallic material; that is, dislocations themselves are obstacles to dislocation motion [29]. Thus, the Taylor dislocation contribution rd = MgGbq1/2 dominates the strain hardening effect; where q is the dislocation density; g is a constant; M is the Taylor factor; G is the shear modulus; and b is the Burgers vector [12]. Contribution to the stress from the dislocation density can be obtained by subtracting the yield stress from the total flow stress, and this can be written as q1/2 / rd = r ry [12,16,20]. The applied stress necessary to deform a material is then proportional to the dislocation density inside the material [17]. Consequently, the microstructural factors, leading to more dislocation accumulation inside grains, will facilitate the improvement of strain hardening in Mg alloys. Based on the above discussion, the variation of strain hardening behavior with heat treatment can be understood in terms of the microstructure change during heat treatment. In the initial hardening stage, the high density of second phase particles in the material precipitating during aging might have contributed to the enhanced initial strain hardening rate in the T5 and T6 state. Much higher activation energy is required to the plastic flow due to that the precipitation particles are strong barriers to dislocation movement [28,30]. Since solutionizing could reduce the number of particles, it is reasonable that obviously reduced initial strain hardening rate is observed in the T4 state. The strain hardening of a material at a higher hardening stage (stage III) is more related to the dislocation strain field interaction [17]. For the as-extruded state, fine grains and some second phase particles are present due to the hot plastic deformation
1529
and dynamic recrystallization. T5 heat treatment induced slight grain growth and structure relaxation (i.e., reduced internal lattice defects), but a number of second phase precipitates. Like conventional grain boundaries, abundant phase boundaries between precipitates and Mg matrix may act as dislocation sinks besides strengthening effect originating from blocking dislocation slip. During plastic deformation, the dislocations tend to high-energy phase boundaries and are then balanced out for the most part by reorganization and annihilation inside the boundaries [31]. At the same time, lower solute atom content in solid solution is expected to enhance dislocation recovery events at room temperature [10], which is probably connected with a rise of dislocation cross slip and climb ability due to the increasing of stacking fault energy [22]. Numerous precipitation particles and reduced solute atom content in solution are not beneficial to the storage of dislocations and consequently diminish strain hardening ability in ZK60 alloy obviously although the slight grain coarsening and reduced defects may enlarge the room for dislocation accumulation to some extent. Because of high temperature, complete static recrystallization, second phase decomposing and significant grain growth occurred in T4 treatment. This gives rise to a low initial dislocation density and big grain size in T4 state, which provide more space to accommodate lattice dislocations and dislocation debris can build up starting from a very low level. The studies by Afrin and del Valle et al., have testified that an increase in grain size does improve the strain hardening rate, harden capacity and strain hardening exponent in Mg alloy [16,17]. In addition, higher solute atom content in solid solution and much less precipitates are helpful to reduce dislocation recovery during tensile deformation [10]. Hence stronger strain hardening ability is observed in the stage III of hardening in as-T4 treated specimen than in as-extruded specimen. In fact, the as-T4 treated specimen exhibits the highest strain hardening among these studied ZK60 specimens. In contrast to the as-T4 specimen, it is found that a great amount of precipitates are distributed in the matrix in the asT6 specimen. As discussed above, the corresponding phase boundaries and decreased solution atom content facilitate considerable dislocation recovery during plastic straining, which brings on a lower strain hardening ability in T6 state as compared to T4 state. It is noted that T6 heat treatment even shows lower H, n and Hc values than those of the extruded state, indicating that the effect of precipitation should exceed the contribution of grain coarsening and reduced dislocation density to strain hardening in ZK60 alloy. 5. Conclusions
Fig. 4. True stress plotted as a function of true plastic strain and the corresponding fitting curves (dash) for as-T4 treated sample.
(1) T5 treatment makes the formation of numerous Mg–Zn precipitation particles and lower solute atom content in solution in ZK60 alloy, which enhances the yield strength and the dislocation recovery considerably. The strain hardening exponent and hardening capacity are about three-fifths of that in as-extruded state. (2) The dislocation storage ability is stronger for as-T4 treated specimen as compared to as-extruded specimen since solid solution treatment causes evident grain growth, second phase dissolution and more solute atoms in solution. The strain hardening rate H, exponent n and capacity Hc are noticeably higher than that in as-T5 condition. (3) Compared with as-extruded and T4 treated specimens, as-T6 treated specimen possesses obviously reduced strain hardening and tensile ductility, mainly originating from the precipitation of many second phase particles during aging.
1530
X. Chen et al. / Materials and Design 32 (2011) 1526–1530
Acknowledgement This work supported by the Fundamental Research Funds for the Central Universities (No. CDJZR10 13 00 01).
References [1] Mordike BL, Ebert T. Magnesium properties–applications–potential. Mater Sci Eng 2001;302:37–45. [2] Bohlen J, Letzig D, Kainer KU. New perspectives for wrought magnesium alloys. Mater Sci Forum 2007;546–549:1–10. [3] Kainer KU, von Buch F, In: Kainer KU, editors. Magnesium alloys and technologies, DGM; 2003. [4] Chen HM, Yu HS, Kang SB, Cho J, Min GH. Optimization of annealing treatment parameters in a twin roll cast and warm rolled ZK60 alloy sheet. Mater Sci Eng A 2010;527:1236–42. [5] Xu DK, Liu L, Xu YB, Han EH. The effect of precipitates on the mechanical properties of ZK60-Y alloy. Mater Sci Eng A 2006;420:322–32. [6] Chen HM, Kang SB, Yu HS, Cho J, Kim HW, Min GH. Effect of heat treatment on microstructure and mechanical properties of twin roll cast and sequential warm rolled ZK60 alloy sheets. J Alloys Compd 2009;476:324–8. [7] Beer AG, Barnett MR. Microstructure evolution in hot worked and annealed magnesium alloy AZ31. Mater Sci Eng A 2008;485:318–24. [8] Liang S, Guan D, Chen L, Gao Z, Tang H, Tong X, et al. Precipitation and its effect on age-hardening behavior of as-cast Mg–Gd–Y alloy. Mater Des 2011;32:361–4. [9] Peng Q, Hou X, Wang L, Wu Y, Cao Z, Wang L. Microstructure and mechanical properties of high performance Mg–Gd based alloys. Mater Des 2009;30:292–6. [10] Yakubtsov IA, Diaka BJ, Sager CA, Bhattacharya B, MacDonald WD, Niewczas M. Effects of heat treatment on microstructure and tensile deformation of Mg AZ80 alloy at room temperature. Mater Sci Eng A 2008;496:247–55. [11] Chen XH, Lu L. Work hardening of ultrafine-grained copper with nanoscale twins. Scripta Mater 2007;57:133–6. [12] Meyers MA, Chawla KK. Mechanical behavior of materials. Upper Saddle River, NJ: Prentice Hall, Inc.; 1999. [13] Chu D, Morris JW. The influence of microstructure on work hardening in aluminum. Acta Mater 1996;44:2599–610.
[14] Kocks UF, Mecking H. Physics and phenomenology of strain hardening: the FCC case. Prog Mater Sci 2003;48:171–273. [15] Kocks UF. The relation between polycrystal deformation and single-crystal deformation. Metall Trans A 1970;1:1121–33. [16] del Valle JA, Carreno F, Ruano OA. Influence of texture and grain size on work hardening and ductility in magnesium-based alloys processed by ECAP and rolling. Acta Mater 2006;54:4247–59. [17] Afrin N, Chen DL, Cao X, Jahazi M. Strain hardening behavior of a friction stir welded magnesium alloy. Scripta Mater 2007;57:1004–7. [18] Lin XZ, Chen DL. Strain hardening and strain sensitivity of an extruded magnesium alloy. J Mater Eng Perform 2008;17:894–901. [19] Jiang L, Jonas JJ, Luo AA, Sachdev AK, Godet S. Twinning-induced softening in polycrystalline AM30 Mg alloy at moderate temperatures. Scripta Mater 2006;54:771–5. [20] Cáceres CH, Blake AH. On the strain hardening behavior of magnesium at room temperature. Mater Sci Eng A 2007;462:193–6. [21] Wu HY, Lin FZ. Mechanical properties and strain-hardening behavior of Mg alloy AZ31B–H24 thin sheet. Mater Sci Eng A 2010;527:1194–9. [22] Máthis K, Trojanová Z, Lukácˇ P, Cáceres CH, Lendvai J. Modeling of hardening and softening processes in Mg alloys. J Alloys Compd 2004;378:176–9. [23] Knezevic M, Levinson A, Harris R, Mishra RK, Doherty RD, Kalidindi SR. Deformation twinning in AZ31: Influence on strain hardening and texture evolution. Acta Mater 2010;58:6230–42. [24] Wu HY, Yan JC, Tsai HH, Chiu CH, Zhou GZ, Lin CF. Tensile flow and strainhardening behaviors of dual-phase Mg–Li–Zn alloy thin sheets. Mater Sci Eng A 2010;527:7197–203. [25] Trojanová Z, Lukácˇ P. Compressive deformation behavior of magnesium alloys. J Mater Proc Technol 2005;162–163:416–21. [26] Máthis K, Trojanová Z, Lukácˇ P. Hardening and softening in deformed magnesium alloys. Mater Sci Eng A 2002;324:141–4. [27] Misra A, Zhang X, Hammon D, Hoagland RG. Work hardening in rolled nanolayered metallic composites. Acta Mater 2004;53:221–6. [28] Luo J, Mei Z, Tian WH, Wang ZR. Diminishing of work hardening in electroformed polycrystalline copper with nano-sized and uf-sized twins. Mater Sci Eng A 2006;441:282–90. [29] Courtney TH. Mechanical behavior of materials. Michigan: McGraw-Hill Companies, Inc.; 2000. [30] Dieter GE. Mechanical metallurgy. 3rd ed. Boston: McGraw-Hill; 1986. [31] Ma E, Wang YM, Lu QH, Sui ML, Lu L, Lu K. Strain hardening and large tensile elongation in ultrahigh-strength nano-twinned copper. Appl Phys Lett 2004;85:4932–4.