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The different effects of solute segregation at twin boundaries on mechanical behaviors of twinning and detwinning
Materials Science & Engineering A 644 (2015) 365–373
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Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
The different effects of solute segregation at twin boundaries on mechanical behaviors of twinning and detwinning Yunchang Xin n, Yuanyuan Zhang, Huihui Yu, Houwen Chen, Qing Liu nn School of Materials Science and Engineering, Chongqing University, Chongqing 400044, China
art ic l e i nf o
a b s t r a c t
Article history: Received 15 June 2015 Received in revised form 16 July 2015 Accepted 17 July 2015 Available online 18 July 2015
¯ } twin In the present study, a comparative study about the hardening effect of solute segregation at {1012 ¯ } twinning predominant deformation and a detwinning predominant one boundaries (TBs) on a {1012 was carried out. The influence of the pre-straining and subsequent annealing conditions on mechanical behavior was systematically addressed. Our results show that solute segregation at TBs can occur even at 100 °C. The annealing at 100 °C for 20 min induces a partial segregation at TBs, while that at 150 °C or higher temperature for 20 min can induce a complete solute segregation. The annealing conditions and pre-strain levels generate quite different effects on deformation by twinning and that by detwinning. Both annealing hardening and annealing softening might happen during the twinning predominant recompression. Annealing hardening occurs only with pre-strains of 3.0% and 5.5% after annealing at 100 °C for 6 h. A higher pre-strain or a higher annealing temperature or a longer annealing time generate a higher annealing softening effect. However, during the detwinning predominant recompression, all used annealing treatments generate hardening effect in all the pre-strained samples. With a complete solute segregation at TBs, a hardening of about 11–25 MPa is generally achieved. It is also found that solute segregation at TBs reduces the strain hardening rate of deformation by TBs migration. & 2015 Elsevier B.V. All rights reserved.
Keywords: Mg alloy Solute segregation Twinning Detwinning Hardening
1. Introduction Twin boundaries (TBs) play an important role in plastic deformation and ultimately in controlling the working ability and mechanical properties of many engineering materials [1]. Notable examples are Mg alloys which are attractive for weight saving constructions in transportation and aerospace industries. Due to the limited active slip systems at room temperature, twinning is one of the main deformation modes. The main role of twinning in plastic deformation involves accommodating strain along the caxis direction [2,3], rotation of crystallographic orientations [4] and relaxing stress concentration [4,5]. Recently, pre-straining and subsequent annealing are extensively used to tailor mechanical performance of Mg alloys [6–8]. It was found that a large number ¯ TBs generated by pre-straining can effectively enhance of 1012 yield strength of a hot-rolled AZ31 plate without a compromise of ¯ ductility [6]. Xin et al. reported that reorientation by 1012
{
}
{
}
twinning can greatly improve the hot rolling ability or reduce the tension-compression yield asymmetry [9,10]. Pre-straining was also used to modify damping capacity, fatigue performance, n
Corresponding author. Fax: þ 86 23 65106407. Corresponding author. Fax: þ 86 23 65111295. E-mail addresses: [email protected] (Y. Xin), [email protected] (Q. Liu).
nn
http://dx.doi.org/10.1016/j.msea.2015.07.049 0921-5093/& 2015 Elsevier B.V. All rights reserved.
stretch ability and extrusion microstructure of Mg alloys [7,8,11,12]. Recently, Nie et al. reported that a periodic segregation of solute at coherent TBs of Mg alloys would take place after suitable annealing treatments. The solute segregation at TBs will pin TBs, ¯ } leading to hardening during subsequent reloading (a {1012 twinning predominant deformation) [13]. In our previous work, it ¯ } TBs of a prewas also found that this solute segregation at {1012 twinned Mg AZ31 plate also resulted in an enhanced activation ¯ } twins. Some atomic simulations stress for detwinning of {1012 were also carried out to understand the solute segregation process at TBs and its strengthening effect on twinning dislocations [14,15]. A twinning deformation is constituted of both twin nucleation and twin growth, while detwinning is only a TBs migration process. In fact, it was considered that the TBs migration during a twin growth also differs from that during detwinning. Generally, the twin growth involves the nucleation of new twinning dislocations, twinning dislocations subsequently gliding along coherent TBs and climbing along basal-prismatic facets [16]. However, the nucleation of twinning dislocations is not necessary during detwinning. Although the experimental results have shown that solute segregation at TBs can harden both twinning and detwinning, this solute segregation may generate different hardening effects on the deformation dominated by twinning and that controlled by detwinning. To our best knowledge, there is no
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comparative study addressing this possible different effects. In addition, solute segregation at TBs is closely related to annealing temperature and time. However, a systematical investigation about the influence of annealing conditions on annealing hardening effect was not conducted yet. In the present study, a comparative study about the hardening ¯ } TBs on the {1012 ¯ } twinning effect of solute segregation at {1012 predominant recompression and the detwinning predominant one was carried out. The influence of pre-strain levels and annealing conditions on mechanical behavior was systematically addressed. The corresponding mechanisms were studied and discussed. This study deepens understanding about deformation mechanism of Mg alloy containing pre-existing twins and contributes to the applications of TBs to tailor mechanical behavior.
2. Experiments and methods
samples. 2.2. Microstructure and texture measurement For microstructure examination by optical microscopy, the specimens were carefully ground and chemically etched in an acetic picral solution (2 ml acetic acid þ1 g picric acid þ2 ml H2Oþ16 ml ethanol). Pole figures were measured using an X-ray diffraction (XRD, Rigaku D/max-2500PC) meter. The measured incomplete pole figures were analyzed to determine the orientation distribution function and the complete pole figures were reconstructed. To reveal the microstructure and their crystallographic orientations, electron back-scattered diffraction (EBSD) mapping using a step size of 0.5 μm was conducted on a scanning electron microscope (SEM, Zeiss AURIGA) equipped with a HKLEBSD system. Samples for EBSD mapping were mechanically ground followed by electrochemical polishing in the AC2 electrolyte. The EBSD data was analyzed using the Channel 5 software.
2.1. Pre-straining, annealing and mechanical tests A hot-rolled Mg AZ31 thick plate with fully recrystallized grains was used. As seen in Fig. 1, the initial plate has a mean grain size of about 16 μm and a typical basal texture with the (0002) poles ¯ } TBs, blocks of 20 mm largely parallel to the ND. To generate {1012 (ND) 30 mm (TD) 24 mm (RD) were compressed along the TD to 1.5%, 3.0% and 5.5% at room temperature and designated as PR1.5%, PR3.0% and PR5.5%, respectively. Here, RD, TD and ND represent rolling direction, transverse direction and normal direction of the initial plate, respectively. Different pre-strains were ¯ } used to prepare the samples with different fractions of {1012 twins. Blocks with a dimension of 10 mm 8 mm 8 mm were cut from the center of pre-strained samples for subsequent reloading. A part of the pre-strained specimens were annealed using 6 different treatments: 100 °C for 20 min, 100 °C for 6 h, 150 °C for 20 min, 150 °C for 6 h, 200 °C for 20 min and 200 °C for 6 h. Mechanical behavior of pre-strained samples under recompression ¯ } twinning) and that along the ND along the TD (to continue {1012 ¯ } twins) at room temperature were (to initiate detwinning of {1012 both tested on a Shimadzu AG-X mechanical testing system using a strain rate of 0.001 s 1. Each test was repeated three times. Graphite was used to reduce friction between the heap and the
3. Results 3.1. Mechanical behavior Stress–strain curves illustrating the compression behaviors along the ND and along the TD are shown in Figs. 2 and 3. The yield stresses derived from those stress–strain curves are listed in Tables 1 and 2. Under ND-compression (Fig. 2a–c), all the curves have a plateau shape, the typical feature of detwinning predominant deformation in Mg alloys [8,17] and the annealing treatment greatly changes the shape of stress–strain curves. An obvious yield elongation exist in annealed samples under ND compression except in those annealed at 100 °C for 20 min. As seen in Tables 1 and 2, the annealing at 100 °C for 20 min only generate a slight hardening during ND recompression, that at 150 °C for 20 min or 200 °C for 20 min generate more pronounced and similar hardening effects (about 15–25 MPa). Increasing the annealing time from 20 min to 6 h at 100 °C further enhances the annealing hardening effect in ND-compression. However, increasing annealing durations at 150 °C or 200 °C hardly varies the hardening effect. The influence of annealing conditions on mechanical properties in TD recompression seems more complex. For PR1.5%, no hardening is observed in all the annealing conditions. For both PR3.0% and PR5.5%, a 20 min annealing at 100 °C or 150 °C hardly changes the yield stress. A slight hardening is noticed after annealing at 100 °C for 6 h. However, the annealing treatments at 200 °C for 6 h or at 150 °C for 6 h generate an obvious softening of PR5.5%. An annealing at 200 °C for 20 min already reduces yield stress of PR5.5% by 9 MPa. The influence of annealing on strain hardening response of PR3.0% during TD recompression and ND re-compression were analyzed and the results are given in Fig. 4. All the strain hardening rate curves are characterized by a peak. Although annealing treatments hardly change the peak hardening rate, it seems that annealing treatments reduce hardening rate at the early stage after yielding (the regions denoted by the green arrows in Fig. 4). 3.2. Microstructure and texture
Fig. 1. (a) Inverse pole figure maps and (b) pole figures (acquired by XRD) of the hot-rolled Mg AZ31 plate used in the present study.
In order to confirm whether the as-used annealing treatments affect twins in pre-strained samples, optical microstructures of PR1.5% and PR5.5% before and after annealing were both examined and are presented in Fig. 5. Obviously, the twins well remain after annealing at 200 °C for 6 h. Therefore, it can be inferred that all the as-used annealing treatments do not damage the twin structure in
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Fig. 2. Stress–strain curves under (a, b, c) compression along the ND and (d, e, f) compression along the TD of the pre-strained samples annealed at different temperatures for 20 min, respectively.
pre-strained samples. Microstructure of the pre-strained samples was further studied by EBSD and is shown in Fig. 6. Many thin ¯ } twins exist in bands (bands in blue or green) indentified as {1012 ¯ ¯ PR1.5%. Contraction twins ( {1011} or {1013}) and double twins ¯ ¯ }) are hardly detected. Increasing the pre-strain ( 1011 − {1012
{
}
along the TD, more twins appear and thickening of those twins takes place. As can be seen in Fig. 6e, many wide twins form and some grains are completely twinned. Pole figures of the pre-twinned samples derived from EBSD ¯ } twinning rotates the maps are given in Fig. 7. Generally, {1012 basal poles toward the compression direction by about 86° [18]. As basal poles of matrix are largely parallel to the ND as shown in
¯ } Fig. 1, the (0002) poles around TD are orientations of the {1012 twins generated during TD-compression. In PR1.5%, there is only a small fraction of basal poles close to the TD. Increasing the prestrain along the TD, more (0002) poles incline from the ND to the TD. Deformation behavior of PR3.0% sample (annealed at 200 °C for 6 h) during 1.3% recompression along the TD was investigated by an in-situ EBSD mapping and the results are shown in Fig. 8. Largely thickening of a large number of twins (the green and blue bands) takes place after 1.3% TD-recompression. A few new twins also appear as denoted by the white arrows in Fig. 8b. It can be seen that twin growth is the predominant mechanism during TDrecompression. As an in-situ EBSD experiment in our previous
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Fig. 3. Stress–strain curves under (a, b, c) compression along the ND and (d, e, f) compression along the TD of the pre-strained samples annealed at different temperatures for 6 h. Table 1 Yield stress (YS), peak stress (PS) and the maximum compression ratio (E) of samples under recompression along the ND. Sample PR1.5%
PR3.0%
PR5.5%
YS (MPa) PS (MPa) E (%) YS (MPa) US (MPa) E (%) YS (MPa) PS (MPa) E (%)
Before annealing
100 °C for 20 min
100 °C for 6 h
150 °C for 20 min
150 °C for 6 h
200 °C for 20 min
200 °C for 6 h
49 277 7.3% 55 281 8.7% 77 288 10.3%
55 282 10.0% 64 277 9.3% 85 284 11.6%
71 283 11.4% 75 276 13.4% 92 280 11.2%
69 274 10.4% 80 277 13.0% 91 282 11.6%
72 272 12.1% 81 274 13.3% 91 278 12.8%
71 275 11.8% 78 275 12.3% 91 279 12.1%
72 273 11.2% 78 273 12.8% 88 280 13.8%
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Table 2 Yield stress (YS), peak stress (PS) and the maximum compression ratio (E) of samples under recompression along the TD. Sample PR1.5%
PR3.0%
PR5.5%
YS (MPa) PS (MPa) E(%) YS (MPa) PS (MPa) E(%) YS (MPa) PS (MPa) E(%)
Before annealing
100 °C for 20 min
100 °C for 6 h
150 °C for 20 min
150 °C for 6 h
200 °C for 20 min
200 °C for 6 h
85 294 11.0% 101 295 10.8% 140 290 8.5%
82 290 12.4% 102 293 10.7% 139 292 9.9%
86 292 12.9% 107 290 10.8% 144 296 10.3%
84 289 12.2% 103 293 12.6% 138 295 10.6%
84 295 12.7% 99 293 12.0% 131 296 10.0%
82 289 11.6% 98 294 13.0% 131 293 9.0%
80 295 15.0% 94 291 12.6% 125 295 10.7%
publication confirmed that detwinning dominated recompression along the ND of pre-strained sample [19], deformation behavior of the pre-twinned samples during ND recompression was not carried out in present study.
can be achieved after annealing at 100 °C for 6 h or at 150 °C for 20 min.
4. Discussion
Pre-strain levels greatly affect the twin fraction and TB density ¯ } twins. In order to identify whether twin fraction and TB of {1012 density affect annealing hardening effect, the twin fractions and TB densities in pre-strained samples were measured and are listed in Table 3. The twin fraction was measured as the area fraction of twins in EBSD maps and the TB density was measured as the mean length of TBs per unit area. For each sample, more than 500 grains acquired from 2 individual EBSD maps were used for those measurements. When the pre-strain increases from 1.5% to 5.5%, the twin fraction rises from 17% to 64%. However, the TB density is not proportional to the pre-strain levels. The PR3.0% has a much higher TB density than both the PR1.5% and PR5.5%. However, the much higher TB density in PR3.0% than in PR1.5% does not result in an obvious increasing of annealing hardening effect during NDrecompression. It can be inferred that increasing of yield stress by solute segregation at TBs during a detwinning predominant deformation is generally independent of twin fraction and TB density. As the annealing at 100 °C for 20 min only generates a partial solute segregation at TBs, its hardening effect during ND recompression is much lower than that by a complete solute segregation, i.e. annealing at 100 °C for 6 h or annealing at 200 °C for 6 h. However, with a complete segregation at TBs, annealing treatments and pre-strain levels do not greatly affect the hardening effect during a detwinning predominant recompression,
4.1. The influence of annealing conditions on solute segregation at TBs Nie et al. directly observed the segregation of Zn and Gd at ¯ }, {1012 ¯ } and {1013 ¯ } TBs of Mg alloys using a High-angle {1011 annular dark-field scanning transmission electron microscopy (HAADF-STEM) [13]. Using a STEM-EDS mapping, we also ob¯ } TBs of AZ31 after served the co-segregation of Zn and Al at {1012 annealing at 200 °C for 6 h [19]. Solute segregation at TBs is related to the annealing temperature, annealing time as well as diffusion coefficients of solutes. Nie et al. found that an annealing at 275 °C ¯ } TBs for 5 min can induce a complete segregation of Gd at {1012 [13]. Our previous study demonstrated that a complete segrega¯ } TBs can be finished after annealing at tion of Al and Zn at {1012 170 °C for 15 min. The present work shows that an annealing at 100 °C for 20 min can induce hardening of ND-recompression, ¯ } TBs can take indicating that segregation of Al and/or Zn at {1012 place even at 100 °C. Increasing the annealing time to 6 h further enhances yield stress of ND-recompression. Evidently, annealing at 100 °C for 20 min only generates a partial segregation. An annealing at 150 °C for 20 min and or at 100 °C for 6 h generates a similar hardening effect to that of annealing at 150 °C for 6 h or at 200 °C for 6 h, which implies that a complete solute segregation
4.2. The influence of pre-strain levels and annealing conditions on hardening effect
Fig. 4. Strain hardening curves as a function of plastic strain during (a) compression along the ND and (b) compression along the TD of PR3.0% annealed at different temperatures for 6 h, respectively. Here, ε and ε0.2 refers to the strain and that at yielding point, respectively. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.
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Fig. 5. Optical microstructures of the pre-strained samples before and after anneling at 200 °C for 6 h: (a) PR1.5% before annealing, (b) PR1.5% after annealing, (c) PR5.5% before annealing and (d) PR5.5% after annealing.
Fig. 6. Inverse pole figure maps and boundary misorientation maps of (a, b) PR1.5%, (c, d) PR3.0% and (e, f) PR5.5%. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
ND-recompression (see Tables 1 and 2). On the contrary, annealing hardening effect during the twinning predominant deformation (TD-recompression) is highly dependent on the annealing
conditions and the pre-strain levels. For example, no annealing hardening is observed for PR1.5% in all the annealing conditions. For PR3.0%, hardening only takes place after annealing at 100 °C
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Fig. 7. Pole figures (acquired by EBSD) of (a) PR1.5%, (b) PR3.0% and (c) PR5.5%.
for 6 h, while obvious softening appears after annealing at 200 °C for 6 h. In PR5.5%, an annealing at 200 °C for 20 min already induces an obvious softening. As discussed previously, twinning contains both twin growth and twin nucleation, while detwinning is a process undergoing by only TBs migration. Hong et al. reported that the early stage (below strain 2%) during compression along ¯ } twin nucleation the TD of a AZ31 plate was dominated by {1012 and followed by a twin growth predominant mechanism up to 8% [20]. Therefore, it can be inferred that twin nucleation and propagation rather than twin growth is the main mechanism during yielding of PR1.5% under TD recompression. Solute segregation at TBs can harden the twin migration, while cannot strengthen the twin nucleation. In this case, the pinning of TBs by solute cannot generate an strong hardening effect during TD recompression of PR1.5%. On the other hand, annealing hardening appears during TD-recompressions of PR3.0% with annealing at 100 °C for 6 h. This implies that the twin growth is important at this stage. This can be confirmed in Fig. 8, too. The softening during TD-recompression of PR3.0% and PR5.5% by annealing at 200 °C for 6 h is probably related to reducing the dislocation density by annealing. It can be seen that the annealing softening effect is more pronounced with a higher pre-strain or a higher annealing temperature or a longer annealing time. Although the TD-compression of a basal textured AZ31 plate is a ¯ } twinning predominant deformation, a small amount of {1012 dislocations slip is initiated, too [21]. Theoretically, the maximum ¯ } twinning can generate along the cstrain that a completely {1012 axis is about 6.5%. In the present study, the twin fractions in PR1.5%, PR3.0% and PR 5.5% are about 17.4% and 39.8% and 64%, respectively, and those twins can generate ND-strains about 1.2%, 2.5% and 4.2%, respectively. That is strains about 0.3%, 0.5% and 1.3% in PR1.5%, PR3.0% and PR5.5%, respectively are accommodated by slips. A higher pre-strain generates a higher accumulation of dislocations. Those slips during pre-compression can accumulate near the TBs and grain boundaries or within twins or matrix or interact with TBs [22,23]. The dislocation accumulation within matrix and twins and the TB-dislocation interaction will lead to a general work hardening, i.e. a higher activation stress for TBs motion and dislocations sliding. However, removing of accumulated dislocations by annealing seems to generate a higher softening of TD-recompression, compared to ND-recompression. However, the reason is not clear yet.
4.3. The influence solute segregation at TBs on strain hardening The presence of a peak in strain hardening curves of a twinning or detwinning predominant deformation in Mg alloys is extensively reported [20,24]. It is considered that three main me−
chanisms might contribute to strain hardening of a {1012} twinning predominant deformation: (i) a Hall–Petch effect from grain refinement by twins, (ii) a glissile-to-sessile transformation of dislocations in the region experiencing twinning, and (iii) rotation from soft orientations to hard orientations by twinning [25]. However, a recent experimental study showed that the Hall–Petch mechanism seemed not important [21]. The dislocation related mechanisms were found to be the main hardening mechanisms below a strain of 2.7%, generating a hardening about 8–10 MPa. At a strain of 7.2%, the hardening from dislocations associated me¯ } chanism was up to 22 MPa [21]. The texture hardening by {1012 twinning was confirmed to be another important mechanism and its contribution was dependent on twin fraction [21]. Similar to twinning, detwinning transforms the twin orientation (favorable for detwinning) into the matrix orientation (favorable for slip), and, hence, the mechanism (iii) is considered to be the most important mechanism in strain hardening of a detwinning predominant deformation. Regarding the peak hardening rate, Wu et al. considered that a complete detwinning/twinning required the activation of slip to accommodate the strain, and this transition from detwinning/ twinning with a lower activation stress to slip that requires a higher activation stress generated this peak [26]. As seen in Fig. 3, the transition from detwinning to slip during ND-recompression is sharper than that from twinning to slip during TD-recompression. Therefore, the peak hardening rate of ND recompression is much higher than that of TD recompression (see Fig. 4). Although annealing hardly varies the shape and peak hardening rate, while reduces hardening rate at the early stage after yielding (the regions denoted by the green arrows). The appearance of a flat region in stress–strain curve (denoted by the arrow in Fig. 2) after yielding is the main reason for the lower hardening rate at this stage. Reducing dislocation density in pre-strained sample by annealing may decrease strain hardening rate. However, the present study shows that increasing the annealing temperature from 100 °C to 200 °C hardly changes this flat stage in stress–strain curve. Barnett et al. and Min et al. reported that twin propagation
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Fig. 9. Stress strain curves under compression along the ND of PR3.0% samples without annealing and with annealing at 100 °C for 6 h.
Fig. 8. In-situ EBSD analyses of PR3.0% during 1.3% recompression along the TD: inverse pole figure maps (a) before TD recompression and (b) after TD recompression.
TB motion. However, the depinning of TBs from solute atoms results in a lower activation stress for subsequent TBs migration. This pinning–depinning process will generate a yield point elongation. In addition, annealing of PR1.5% with a much lower dislocation density generates a similar flat stage to that of PR5.5% with a much higher dislocation density. Therefore, the solute segregation at TBs reduces strain hardening rate of deformation by ¯ } TBs are not considered to be TBs migration. Generally, {1012 strong barriers for dislocation sliding. Song et al. however, found that the solute segregation at TBs can further increase the strain hardening performance of deformation dominated by slips [29]. It can be seen that solute segregation at TBs poses the opposite effects on strain hardening of deformation by TB migration and that by dislocations slip. As the strain hardening during detwinning mainly comes from the rotation from soft orientations (favorable for detwinning) to hard orientations (favorable for slip). It can be seen in Fig. 9 that solute segregation at TBs enhances the yield stress up to the stress at ε1 of the sample without annealing. That is, below the strain ε1, the hardening by solute segregation is higher than that from texture hardening by detwinning. Therefore, below the strain ε1, deformation of the annealed sample can undergo without further increasing the stress. Hence, a lower hardening rate appear at this stage.
5. Conclusion Table 3 The twin fraction and the average length of TBs per unit area (TB density) in the pre-strained samples. Sample
PR1.5%
PR3.0%
PR5.5%
Twin fraction (%) TB density (μm 1)
17.3% 0.084
39.8% 0.14
64.0% 0.09
in a Lüders front might generate this yield point elongation or flat stage in stress–strain curve [27,28]. However, the present study shows that this flat stage appears only after annealing, which indicates that twin propagation mechanism is not important in the present study. It is suspected that the solute segregation at TBs by annealing mainly results in this flat stage. As the solute segregation will pin TBs migration, leading to a higher activation stress for
In the present study, a comparative study about the hardening ¯ } twin boundaries (TBs) on the effect of solute segregation in {1012 ¯ } twinning predominant recompression and the detwinning {1012 predominant one was carried out. The influence of the pre-strain levels and annealing conditions on mechanical behavior was systematically addressed. Several conclusions are reached as follows:
¯ } TBs can take place even at 100 °C. (1) Solute segregation at {1012 The annealing at 100 °C for 20 min generally induces a partial segregation, while that at 100 °C for 6 h or at 150 °C or higher temperature for 20 min can generate a complete solute segregation. (2) The annealing conditions or pre-strain levels generate quite ¯ } twinning predominant deformadifferent effects on a {1012 tion and a detwinning predominant one. In the detwinning
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predominant recompression, all used annealing treatments generate obvious hardening effect in all the pre-strained samples. With a complete solute segregation at TBs, a hardening by about 11–25 MPa is generally achieved. (3) Both annealing hardening and annealing softening might happen during the twinning predominant recompression, highly depending on the annealing temperature, annealing time and pre-strain level. All the annealing treatments generate no annealing hardening in sample with a 1.5% pre-strain. Obvious annealing hardening occurs only in PR3.0% and PR5.5% after annealing at 100 °C for 6 h. A higher pre-strain or a higher annealing temperature or a longer annealing time generate a higher annealing softening effect. ¯ } TBs hardly changes the peak (4) Solute segregation at {1012 hardening rate, but reduces strain hardening rate of deformation by TBs motion.
Acknowledgment This study is co-supported by National Key Basic Research Program of China (2013CB632204) and National Natural Science Foundation of China (51371203, 51131009 and 51421001)
References [1] K. Lu, L. Lu, S. Suresh, Science 324 (2009) 349–352. [2] M.R. Barnett, Mater. Sci. Eng. A 464 (2007) 8–16. [3] R.H. Wagoner, X.Y. Lou, M. Li, R.K. Boger, S.R. Agnew, Int. J. Plast. 23 (2007)
373
44–86. [4] B. Clausen, C.N. Tomé, D.W. Brown, S.R. Agnew, Acta Mater. 56 (2008) 2456–2468. [5] L. Jiang, J.J. Jonas, A.A. Luo, A.K. Sachdev, S. Godet, Scr. Mater. 54 (2006) 771–775. [6] Y. Xin, M. Wang, Z. Zeng, M. Nie, Q. Liu, Scr. Mater. 66 (2012) 25–28. [7] Y. Cui, Y. Li, S. Sun, H. Bian, H. Huang, Z. Wang, Y. Koizumi, A. Chiba, Scr. Mater. 101 (2015) 8–11. [8] S.H. Park, S.-G. Hong, C.S. Lee, Scr. Mater. 62 (2010) 666–669. [9] Y. Xin, M. Wang, Z. Zeng, G. Huang, Q. Liu, Scr. Mater. 64 (2011) 986–989. [10] Y. Xin, X. Zhou, Q. Liu, Mater. Sci. Eng. A 567 (2013) 9–13. [11] S.H. Park, H.S. Kim, J.H. Bae, C.D. Yim, B.S. You, Scr. Mater. 69 (2013) 250–253. [12] B. Song, R. Xin, A. Liao, W. Yu, Q. Liu, Mater. Sci. Eng. A 627 (2015) 369–373. [13] J.F. Nie, Y.M. Zhu, J.Z. Liu, X.Y. Fang, Science 340 (2013) 957–960. [14] M. Ghazisaeidi, L.G. Hector Jr, W.A. Curtin, Acta Mater. 80 (2014) 278–287. [15] J. Zhang, Y. Dou, Y. Zheng, Scr. Mater. 80 (2014) 17–20. [16] A. Ostapovets, R. Groeger, Model. Simul. Mater. Sci. Eng. 22 (2014). [17] D.W. Brown, A. Jain, S.R. Agnew, B. Clausen, Thermec 2006 539–543 (2007) 3407–3413. [18] S.H. Park, S.G. Hong, C.S. Lee, Scr. Mater. 62 (2010) 202–205. [19] Y. Xin, X. Zhou, H. Chen, J.-F. Nie, H. Zhang, Y. Zhang, Q. Liu, Mater. Sci. Eng. A 594 (2014) 287–291. [20] S.G. Hong, S.H. Park, C.S. Lee, Acta Mater. 58 (2010) 5873–5885. [21] L. Lv, Y. Xin, H. Yu, R. Hong, Q. Liu, Mater. Sci. Eng. A 636 (2015) 389–395. [22] E. Ma, Y.M. Wang, Q.H. Lu, M.L. Sui, L. Lu, K. Lu, Appl. Phys. Lett. 85 (2004) 4932. [23] C.J. Shute, B.D. Myers, S. Xie, T.W. Barbee, A.M. Hodge, J.R. Weertman, Scr. Mater. 60 (2009) 1073–1077. [24] Y. Xin, X. Zhou, L. Lv, Q. Liu, Mater. Sci. Eng. A 606 (2014) 81–91. [25] M. Knezevic, A. Levinson, R. Harris, R.K. Mishra, R.D. Doherty, S.R. Kalidindi, Acta Mater. 58 (2010) 6230–6242. [26] L. Wu, S.R. Agnew, D.W. Brown, G.M. Stoica, B. Clausen, A. Jain, D.E. Fielden, P. K. Liaw, Acta Mater. 56 (2008) 3699–3707. [27] J. Min, L.G. Hector Jr, J. Lin, J.T. Carter, A.K. Sachdev, Int. J. Plast. 57 (2014) 52–76. [28] M.R. Barnett, M.D. Nave, A. Ghaderi, Acta Mater. 60 (2012) 1433–1443. [29] B. Song, R. Xin, G. Chen, X. Zhang, Q. Liu, Scr. Mater. 66 (2012) 1061–1064.
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Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
The different effects of solute segregation at twin boundaries on mechanical behaviors of twinning and detwinning Yunchang Xin n, Yuanyuan Zhang, Huihui Yu, Houwen Chen, Qing Liu nn School of Materials Science and Engineering, Chongqing University, Chongqing 400044, China
art ic l e i nf o
a b s t r a c t
Article history: Received 15 June 2015 Received in revised form 16 July 2015 Accepted 17 July 2015 Available online 18 July 2015
¯ } twin In the present study, a comparative study about the hardening effect of solute segregation at {1012 ¯ } twinning predominant deformation and a detwinning predominant one boundaries (TBs) on a {1012 was carried out. The influence of the pre-straining and subsequent annealing conditions on mechanical behavior was systematically addressed. Our results show that solute segregation at TBs can occur even at 100 °C. The annealing at 100 °C for 20 min induces a partial segregation at TBs, while that at 150 °C or higher temperature for 20 min can induce a complete solute segregation. The annealing conditions and pre-strain levels generate quite different effects on deformation by twinning and that by detwinning. Both annealing hardening and annealing softening might happen during the twinning predominant recompression. Annealing hardening occurs only with pre-strains of 3.0% and 5.5% after annealing at 100 °C for 6 h. A higher pre-strain or a higher annealing temperature or a longer annealing time generate a higher annealing softening effect. However, during the detwinning predominant recompression, all used annealing treatments generate hardening effect in all the pre-strained samples. With a complete solute segregation at TBs, a hardening of about 11–25 MPa is generally achieved. It is also found that solute segregation at TBs reduces the strain hardening rate of deformation by TBs migration. & 2015 Elsevier B.V. All rights reserved.
Keywords: Mg alloy Solute segregation Twinning Detwinning Hardening
1. Introduction Twin boundaries (TBs) play an important role in plastic deformation and ultimately in controlling the working ability and mechanical properties of many engineering materials [1]. Notable examples are Mg alloys which are attractive for weight saving constructions in transportation and aerospace industries. Due to the limited active slip systems at room temperature, twinning is one of the main deformation modes. The main role of twinning in plastic deformation involves accommodating strain along the caxis direction [2,3], rotation of crystallographic orientations [4] and relaxing stress concentration [4,5]. Recently, pre-straining and subsequent annealing are extensively used to tailor mechanical performance of Mg alloys [6–8]. It was found that a large number ¯ TBs generated by pre-straining can effectively enhance of 1012 yield strength of a hot-rolled AZ31 plate without a compromise of ¯ ductility [6]. Xin et al. reported that reorientation by 1012
{
}
{
}
twinning can greatly improve the hot rolling ability or reduce the tension-compression yield asymmetry [9,10]. Pre-straining was also used to modify damping capacity, fatigue performance, n
Corresponding author. Fax: þ 86 23 65106407. Corresponding author. Fax: þ 86 23 65111295. E-mail addresses: [email protected] (Y. Xin), [email protected] (Q. Liu).
nn
http://dx.doi.org/10.1016/j.msea.2015.07.049 0921-5093/& 2015 Elsevier B.V. All rights reserved.
stretch ability and extrusion microstructure of Mg alloys [7,8,11,12]. Recently, Nie et al. reported that a periodic segregation of solute at coherent TBs of Mg alloys would take place after suitable annealing treatments. The solute segregation at TBs will pin TBs, ¯ } leading to hardening during subsequent reloading (a {1012 twinning predominant deformation) [13]. In our previous work, it ¯ } TBs of a prewas also found that this solute segregation at {1012 twinned Mg AZ31 plate also resulted in an enhanced activation ¯ } twins. Some atomic simulations stress for detwinning of {1012 were also carried out to understand the solute segregation process at TBs and its strengthening effect on twinning dislocations [14,15]. A twinning deformation is constituted of both twin nucleation and twin growth, while detwinning is only a TBs migration process. In fact, it was considered that the TBs migration during a twin growth also differs from that during detwinning. Generally, the twin growth involves the nucleation of new twinning dislocations, twinning dislocations subsequently gliding along coherent TBs and climbing along basal-prismatic facets [16]. However, the nucleation of twinning dislocations is not necessary during detwinning. Although the experimental results have shown that solute segregation at TBs can harden both twinning and detwinning, this solute segregation may generate different hardening effects on the deformation dominated by twinning and that controlled by detwinning. To our best knowledge, there is no
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comparative study addressing this possible different effects. In addition, solute segregation at TBs is closely related to annealing temperature and time. However, a systematical investigation about the influence of annealing conditions on annealing hardening effect was not conducted yet. In the present study, a comparative study about the hardening ¯ } TBs on the {1012 ¯ } twinning effect of solute segregation at {1012 predominant recompression and the detwinning predominant one was carried out. The influence of pre-strain levels and annealing conditions on mechanical behavior was systematically addressed. The corresponding mechanisms were studied and discussed. This study deepens understanding about deformation mechanism of Mg alloy containing pre-existing twins and contributes to the applications of TBs to tailor mechanical behavior.
2. Experiments and methods
samples. 2.2. Microstructure and texture measurement For microstructure examination by optical microscopy, the specimens were carefully ground and chemically etched in an acetic picral solution (2 ml acetic acid þ1 g picric acid þ2 ml H2Oþ16 ml ethanol). Pole figures were measured using an X-ray diffraction (XRD, Rigaku D/max-2500PC) meter. The measured incomplete pole figures were analyzed to determine the orientation distribution function and the complete pole figures were reconstructed. To reveal the microstructure and their crystallographic orientations, electron back-scattered diffraction (EBSD) mapping using a step size of 0.5 μm was conducted on a scanning electron microscope (SEM, Zeiss AURIGA) equipped with a HKLEBSD system. Samples for EBSD mapping were mechanically ground followed by electrochemical polishing in the AC2 electrolyte. The EBSD data was analyzed using the Channel 5 software.
2.1. Pre-straining, annealing and mechanical tests A hot-rolled Mg AZ31 thick plate with fully recrystallized grains was used. As seen in Fig. 1, the initial plate has a mean grain size of about 16 μm and a typical basal texture with the (0002) poles ¯ } TBs, blocks of 20 mm largely parallel to the ND. To generate {1012 (ND) 30 mm (TD) 24 mm (RD) were compressed along the TD to 1.5%, 3.0% and 5.5% at room temperature and designated as PR1.5%, PR3.0% and PR5.5%, respectively. Here, RD, TD and ND represent rolling direction, transverse direction and normal direction of the initial plate, respectively. Different pre-strains were ¯ } used to prepare the samples with different fractions of {1012 twins. Blocks with a dimension of 10 mm 8 mm 8 mm were cut from the center of pre-strained samples for subsequent reloading. A part of the pre-strained specimens were annealed using 6 different treatments: 100 °C for 20 min, 100 °C for 6 h, 150 °C for 20 min, 150 °C for 6 h, 200 °C for 20 min and 200 °C for 6 h. Mechanical behavior of pre-strained samples under recompression ¯ } twinning) and that along the ND along the TD (to continue {1012 ¯ } twins) at room temperature were (to initiate detwinning of {1012 both tested on a Shimadzu AG-X mechanical testing system using a strain rate of 0.001 s 1. Each test was repeated three times. Graphite was used to reduce friction between the heap and the
3. Results 3.1. Mechanical behavior Stress–strain curves illustrating the compression behaviors along the ND and along the TD are shown in Figs. 2 and 3. The yield stresses derived from those stress–strain curves are listed in Tables 1 and 2. Under ND-compression (Fig. 2a–c), all the curves have a plateau shape, the typical feature of detwinning predominant deformation in Mg alloys [8,17] and the annealing treatment greatly changes the shape of stress–strain curves. An obvious yield elongation exist in annealed samples under ND compression except in those annealed at 100 °C for 20 min. As seen in Tables 1 and 2, the annealing at 100 °C for 20 min only generate a slight hardening during ND recompression, that at 150 °C for 20 min or 200 °C for 20 min generate more pronounced and similar hardening effects (about 15–25 MPa). Increasing the annealing time from 20 min to 6 h at 100 °C further enhances the annealing hardening effect in ND-compression. However, increasing annealing durations at 150 °C or 200 °C hardly varies the hardening effect. The influence of annealing conditions on mechanical properties in TD recompression seems more complex. For PR1.5%, no hardening is observed in all the annealing conditions. For both PR3.0% and PR5.5%, a 20 min annealing at 100 °C or 150 °C hardly changes the yield stress. A slight hardening is noticed after annealing at 100 °C for 6 h. However, the annealing treatments at 200 °C for 6 h or at 150 °C for 6 h generate an obvious softening of PR5.5%. An annealing at 200 °C for 20 min already reduces yield stress of PR5.5% by 9 MPa. The influence of annealing on strain hardening response of PR3.0% during TD recompression and ND re-compression were analyzed and the results are given in Fig. 4. All the strain hardening rate curves are characterized by a peak. Although annealing treatments hardly change the peak hardening rate, it seems that annealing treatments reduce hardening rate at the early stage after yielding (the regions denoted by the green arrows in Fig. 4). 3.2. Microstructure and texture
Fig. 1. (a) Inverse pole figure maps and (b) pole figures (acquired by XRD) of the hot-rolled Mg AZ31 plate used in the present study.
In order to confirm whether the as-used annealing treatments affect twins in pre-strained samples, optical microstructures of PR1.5% and PR5.5% before and after annealing were both examined and are presented in Fig. 5. Obviously, the twins well remain after annealing at 200 °C for 6 h. Therefore, it can be inferred that all the as-used annealing treatments do not damage the twin structure in
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Fig. 2. Stress–strain curves under (a, b, c) compression along the ND and (d, e, f) compression along the TD of the pre-strained samples annealed at different temperatures for 20 min, respectively.
pre-strained samples. Microstructure of the pre-strained samples was further studied by EBSD and is shown in Fig. 6. Many thin ¯ } twins exist in bands (bands in blue or green) indentified as {1012 ¯ ¯ PR1.5%. Contraction twins ( {1011} or {1013}) and double twins ¯ ¯ }) are hardly detected. Increasing the pre-strain ( 1011 − {1012
{
}
along the TD, more twins appear and thickening of those twins takes place. As can be seen in Fig. 6e, many wide twins form and some grains are completely twinned. Pole figures of the pre-twinned samples derived from EBSD ¯ } twinning rotates the maps are given in Fig. 7. Generally, {1012 basal poles toward the compression direction by about 86° [18]. As basal poles of matrix are largely parallel to the ND as shown in
¯ } Fig. 1, the (0002) poles around TD are orientations of the {1012 twins generated during TD-compression. In PR1.5%, there is only a small fraction of basal poles close to the TD. Increasing the prestrain along the TD, more (0002) poles incline from the ND to the TD. Deformation behavior of PR3.0% sample (annealed at 200 °C for 6 h) during 1.3% recompression along the TD was investigated by an in-situ EBSD mapping and the results are shown in Fig. 8. Largely thickening of a large number of twins (the green and blue bands) takes place after 1.3% TD-recompression. A few new twins also appear as denoted by the white arrows in Fig. 8b. It can be seen that twin growth is the predominant mechanism during TDrecompression. As an in-situ EBSD experiment in our previous
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Fig. 3. Stress–strain curves under (a, b, c) compression along the ND and (d, e, f) compression along the TD of the pre-strained samples annealed at different temperatures for 6 h. Table 1 Yield stress (YS), peak stress (PS) and the maximum compression ratio (E) of samples under recompression along the ND. Sample PR1.5%
PR3.0%
PR5.5%
YS (MPa) PS (MPa) E (%) YS (MPa) US (MPa) E (%) YS (MPa) PS (MPa) E (%)
Before annealing
100 °C for 20 min
100 °C for 6 h
150 °C for 20 min
150 °C for 6 h
200 °C for 20 min
200 °C for 6 h
49 277 7.3% 55 281 8.7% 77 288 10.3%
55 282 10.0% 64 277 9.3% 85 284 11.6%
71 283 11.4% 75 276 13.4% 92 280 11.2%
69 274 10.4% 80 277 13.0% 91 282 11.6%
72 272 12.1% 81 274 13.3% 91 278 12.8%
71 275 11.8% 78 275 12.3% 91 279 12.1%
72 273 11.2% 78 273 12.8% 88 280 13.8%
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Table 2 Yield stress (YS), peak stress (PS) and the maximum compression ratio (E) of samples under recompression along the TD. Sample PR1.5%
PR3.0%
PR5.5%
YS (MPa) PS (MPa) E(%) YS (MPa) PS (MPa) E(%) YS (MPa) PS (MPa) E(%)
Before annealing
100 °C for 20 min
100 °C for 6 h
150 °C for 20 min
150 °C for 6 h
200 °C for 20 min
200 °C for 6 h
85 294 11.0% 101 295 10.8% 140 290 8.5%
82 290 12.4% 102 293 10.7% 139 292 9.9%
86 292 12.9% 107 290 10.8% 144 296 10.3%
84 289 12.2% 103 293 12.6% 138 295 10.6%
84 295 12.7% 99 293 12.0% 131 296 10.0%
82 289 11.6% 98 294 13.0% 131 293 9.0%
80 295 15.0% 94 291 12.6% 125 295 10.7%
publication confirmed that detwinning dominated recompression along the ND of pre-strained sample [19], deformation behavior of the pre-twinned samples during ND recompression was not carried out in present study.
can be achieved after annealing at 100 °C for 6 h or at 150 °C for 20 min.
4. Discussion
Pre-strain levels greatly affect the twin fraction and TB density ¯ } twins. In order to identify whether twin fraction and TB of {1012 density affect annealing hardening effect, the twin fractions and TB densities in pre-strained samples were measured and are listed in Table 3. The twin fraction was measured as the area fraction of twins in EBSD maps and the TB density was measured as the mean length of TBs per unit area. For each sample, more than 500 grains acquired from 2 individual EBSD maps were used for those measurements. When the pre-strain increases from 1.5% to 5.5%, the twin fraction rises from 17% to 64%. However, the TB density is not proportional to the pre-strain levels. The PR3.0% has a much higher TB density than both the PR1.5% and PR5.5%. However, the much higher TB density in PR3.0% than in PR1.5% does not result in an obvious increasing of annealing hardening effect during NDrecompression. It can be inferred that increasing of yield stress by solute segregation at TBs during a detwinning predominant deformation is generally independent of twin fraction and TB density. As the annealing at 100 °C for 20 min only generates a partial solute segregation at TBs, its hardening effect during ND recompression is much lower than that by a complete solute segregation, i.e. annealing at 100 °C for 6 h or annealing at 200 °C for 6 h. However, with a complete segregation at TBs, annealing treatments and pre-strain levels do not greatly affect the hardening effect during a detwinning predominant recompression,
4.1. The influence of annealing conditions on solute segregation at TBs Nie et al. directly observed the segregation of Zn and Gd at ¯ }, {1012 ¯ } and {1013 ¯ } TBs of Mg alloys using a High-angle {1011 annular dark-field scanning transmission electron microscopy (HAADF-STEM) [13]. Using a STEM-EDS mapping, we also ob¯ } TBs of AZ31 after served the co-segregation of Zn and Al at {1012 annealing at 200 °C for 6 h [19]. Solute segregation at TBs is related to the annealing temperature, annealing time as well as diffusion coefficients of solutes. Nie et al. found that an annealing at 275 °C ¯ } TBs for 5 min can induce a complete segregation of Gd at {1012 [13]. Our previous study demonstrated that a complete segrega¯ } TBs can be finished after annealing at tion of Al and Zn at {1012 170 °C for 15 min. The present work shows that an annealing at 100 °C for 20 min can induce hardening of ND-recompression, ¯ } TBs can take indicating that segregation of Al and/or Zn at {1012 place even at 100 °C. Increasing the annealing time to 6 h further enhances yield stress of ND-recompression. Evidently, annealing at 100 °C for 20 min only generates a partial segregation. An annealing at 150 °C for 20 min and or at 100 °C for 6 h generates a similar hardening effect to that of annealing at 150 °C for 6 h or at 200 °C for 6 h, which implies that a complete solute segregation
4.2. The influence of pre-strain levels and annealing conditions on hardening effect
Fig. 4. Strain hardening curves as a function of plastic strain during (a) compression along the ND and (b) compression along the TD of PR3.0% annealed at different temperatures for 6 h, respectively. Here, ε and ε0.2 refers to the strain and that at yielding point, respectively. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.
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Fig. 5. Optical microstructures of the pre-strained samples before and after anneling at 200 °C for 6 h: (a) PR1.5% before annealing, (b) PR1.5% after annealing, (c) PR5.5% before annealing and (d) PR5.5% after annealing.
Fig. 6. Inverse pole figure maps and boundary misorientation maps of (a, b) PR1.5%, (c, d) PR3.0% and (e, f) PR5.5%. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
ND-recompression (see Tables 1 and 2). On the contrary, annealing hardening effect during the twinning predominant deformation (TD-recompression) is highly dependent on the annealing
conditions and the pre-strain levels. For example, no annealing hardening is observed for PR1.5% in all the annealing conditions. For PR3.0%, hardening only takes place after annealing at 100 °C
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Fig. 7. Pole figures (acquired by EBSD) of (a) PR1.5%, (b) PR3.0% and (c) PR5.5%.
for 6 h, while obvious softening appears after annealing at 200 °C for 6 h. In PR5.5%, an annealing at 200 °C for 20 min already induces an obvious softening. As discussed previously, twinning contains both twin growth and twin nucleation, while detwinning is a process undergoing by only TBs migration. Hong et al. reported that the early stage (below strain 2%) during compression along ¯ } twin nucleation the TD of a AZ31 plate was dominated by {1012 and followed by a twin growth predominant mechanism up to 8% [20]. Therefore, it can be inferred that twin nucleation and propagation rather than twin growth is the main mechanism during yielding of PR1.5% under TD recompression. Solute segregation at TBs can harden the twin migration, while cannot strengthen the twin nucleation. In this case, the pinning of TBs by solute cannot generate an strong hardening effect during TD recompression of PR1.5%. On the other hand, annealing hardening appears during TD-recompressions of PR3.0% with annealing at 100 °C for 6 h. This implies that the twin growth is important at this stage. This can be confirmed in Fig. 8, too. The softening during TD-recompression of PR3.0% and PR5.5% by annealing at 200 °C for 6 h is probably related to reducing the dislocation density by annealing. It can be seen that the annealing softening effect is more pronounced with a higher pre-strain or a higher annealing temperature or a longer annealing time. Although the TD-compression of a basal textured AZ31 plate is a ¯ } twinning predominant deformation, a small amount of {1012 dislocations slip is initiated, too [21]. Theoretically, the maximum ¯ } twinning can generate along the cstrain that a completely {1012 axis is about 6.5%. In the present study, the twin fractions in PR1.5%, PR3.0% and PR 5.5% are about 17.4% and 39.8% and 64%, respectively, and those twins can generate ND-strains about 1.2%, 2.5% and 4.2%, respectively. That is strains about 0.3%, 0.5% and 1.3% in PR1.5%, PR3.0% and PR5.5%, respectively are accommodated by slips. A higher pre-strain generates a higher accumulation of dislocations. Those slips during pre-compression can accumulate near the TBs and grain boundaries or within twins or matrix or interact with TBs [22,23]. The dislocation accumulation within matrix and twins and the TB-dislocation interaction will lead to a general work hardening, i.e. a higher activation stress for TBs motion and dislocations sliding. However, removing of accumulated dislocations by annealing seems to generate a higher softening of TD-recompression, compared to ND-recompression. However, the reason is not clear yet.
4.3. The influence solute segregation at TBs on strain hardening The presence of a peak in strain hardening curves of a twinning or detwinning predominant deformation in Mg alloys is extensively reported [20,24]. It is considered that three main me−
chanisms might contribute to strain hardening of a {1012} twinning predominant deformation: (i) a Hall–Petch effect from grain refinement by twins, (ii) a glissile-to-sessile transformation of dislocations in the region experiencing twinning, and (iii) rotation from soft orientations to hard orientations by twinning [25]. However, a recent experimental study showed that the Hall–Petch mechanism seemed not important [21]. The dislocation related mechanisms were found to be the main hardening mechanisms below a strain of 2.7%, generating a hardening about 8–10 MPa. At a strain of 7.2%, the hardening from dislocations associated me¯ } chanism was up to 22 MPa [21]. The texture hardening by {1012 twinning was confirmed to be another important mechanism and its contribution was dependent on twin fraction [21]. Similar to twinning, detwinning transforms the twin orientation (favorable for detwinning) into the matrix orientation (favorable for slip), and, hence, the mechanism (iii) is considered to be the most important mechanism in strain hardening of a detwinning predominant deformation. Regarding the peak hardening rate, Wu et al. considered that a complete detwinning/twinning required the activation of slip to accommodate the strain, and this transition from detwinning/ twinning with a lower activation stress to slip that requires a higher activation stress generated this peak [26]. As seen in Fig. 3, the transition from detwinning to slip during ND-recompression is sharper than that from twinning to slip during TD-recompression. Therefore, the peak hardening rate of ND recompression is much higher than that of TD recompression (see Fig. 4). Although annealing hardly varies the shape and peak hardening rate, while reduces hardening rate at the early stage after yielding (the regions denoted by the green arrows). The appearance of a flat region in stress–strain curve (denoted by the arrow in Fig. 2) after yielding is the main reason for the lower hardening rate at this stage. Reducing dislocation density in pre-strained sample by annealing may decrease strain hardening rate. However, the present study shows that increasing the annealing temperature from 100 °C to 200 °C hardly changes this flat stage in stress–strain curve. Barnett et al. and Min et al. reported that twin propagation
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Fig. 9. Stress strain curves under compression along the ND of PR3.0% samples without annealing and with annealing at 100 °C for 6 h.
Fig. 8. In-situ EBSD analyses of PR3.0% during 1.3% recompression along the TD: inverse pole figure maps (a) before TD recompression and (b) after TD recompression.
TB motion. However, the depinning of TBs from solute atoms results in a lower activation stress for subsequent TBs migration. This pinning–depinning process will generate a yield point elongation. In addition, annealing of PR1.5% with a much lower dislocation density generates a similar flat stage to that of PR5.5% with a much higher dislocation density. Therefore, the solute segregation at TBs reduces strain hardening rate of deformation by ¯ } TBs are not considered to be TBs migration. Generally, {1012 strong barriers for dislocation sliding. Song et al. however, found that the solute segregation at TBs can further increase the strain hardening performance of deformation dominated by slips [29]. It can be seen that solute segregation at TBs poses the opposite effects on strain hardening of deformation by TB migration and that by dislocations slip. As the strain hardening during detwinning mainly comes from the rotation from soft orientations (favorable for detwinning) to hard orientations (favorable for slip). It can be seen in Fig. 9 that solute segregation at TBs enhances the yield stress up to the stress at ε1 of the sample without annealing. That is, below the strain ε1, the hardening by solute segregation is higher than that from texture hardening by detwinning. Therefore, below the strain ε1, deformation of the annealed sample can undergo without further increasing the stress. Hence, a lower hardening rate appear at this stage.
5. Conclusion Table 3 The twin fraction and the average length of TBs per unit area (TB density) in the pre-strained samples. Sample
PR1.5%
PR3.0%
PR5.5%
Twin fraction (%) TB density (μm 1)
17.3% 0.084
39.8% 0.14
64.0% 0.09
in a Lüders front might generate this yield point elongation or flat stage in stress–strain curve [27,28]. However, the present study shows that this flat stage appears only after annealing, which indicates that twin propagation mechanism is not important in the present study. It is suspected that the solute segregation at TBs by annealing mainly results in this flat stage. As the solute segregation will pin TBs migration, leading to a higher activation stress for
In the present study, a comparative study about the hardening ¯ } twin boundaries (TBs) on the effect of solute segregation in {1012 ¯ } twinning predominant recompression and the detwinning {1012 predominant one was carried out. The influence of the pre-strain levels and annealing conditions on mechanical behavior was systematically addressed. Several conclusions are reached as follows:
¯ } TBs can take place even at 100 °C. (1) Solute segregation at {1012 The annealing at 100 °C for 20 min generally induces a partial segregation, while that at 100 °C for 6 h or at 150 °C or higher temperature for 20 min can generate a complete solute segregation. (2) The annealing conditions or pre-strain levels generate quite ¯ } twinning predominant deformadifferent effects on a {1012 tion and a detwinning predominant one. In the detwinning
Y. Xin et al. / Materials Science & Engineering A 644 (2015) 365–373
predominant recompression, all used annealing treatments generate obvious hardening effect in all the pre-strained samples. With a complete solute segregation at TBs, a hardening by about 11–25 MPa is generally achieved. (3) Both annealing hardening and annealing softening might happen during the twinning predominant recompression, highly depending on the annealing temperature, annealing time and pre-strain level. All the annealing treatments generate no annealing hardening in sample with a 1.5% pre-strain. Obvious annealing hardening occurs only in PR3.0% and PR5.5% after annealing at 100 °C for 6 h. A higher pre-strain or a higher annealing temperature or a longer annealing time generate a higher annealing softening effect. ¯ } TBs hardly changes the peak (4) Solute segregation at {1012 hardening rate, but reduces strain hardening rate of deformation by TBs motion.
Acknowledgment This study is co-supported by National Key Basic Research Program of China (2013CB632204) and National Natural Science Foundation of China (51371203, 51131009 and 51421001)
References [1] K. Lu, L. Lu, S. Suresh, Science 324 (2009) 349–352. [2] M.R. Barnett, Mater. Sci. Eng. A 464 (2007) 8–16. [3] R.H. Wagoner, X.Y. Lou, M. Li, R.K. Boger, S.R. Agnew, Int. J. Plast. 23 (2007)
373
44–86. [4] B. Clausen, C.N. Tomé, D.W. Brown, S.R. Agnew, Acta Mater. 56 (2008) 2456–2468. [5] L. Jiang, J.J. Jonas, A.A. Luo, A.K. Sachdev, S. Godet, Scr. Mater. 54 (2006) 771–775. [6] Y. Xin, M. Wang, Z. Zeng, M. Nie, Q. Liu, Scr. Mater. 66 (2012) 25–28. [7] Y. Cui, Y. Li, S. Sun, H. Bian, H. Huang, Z. Wang, Y. Koizumi, A. Chiba, Scr. Mater. 101 (2015) 8–11. [8] S.H. Park, S.-G. Hong, C.S. Lee, Scr. Mater. 62 (2010) 666–669. [9] Y. Xin, M. Wang, Z. Zeng, G. Huang, Q. Liu, Scr. Mater. 64 (2011) 986–989. [10] Y. Xin, X. Zhou, Q. Liu, Mater. Sci. Eng. A 567 (2013) 9–13. [11] S.H. Park, H.S. Kim, J.H. Bae, C.D. Yim, B.S. You, Scr. Mater. 69 (2013) 250–253. [12] B. Song, R. Xin, A. Liao, W. Yu, Q. Liu, Mater. Sci. Eng. A 627 (2015) 369–373. [13] J.F. Nie, Y.M. Zhu, J.Z. Liu, X.Y. Fang, Science 340 (2013) 957–960. [14] M. Ghazisaeidi, L.G. Hector Jr, W.A. Curtin, Acta Mater. 80 (2014) 278–287. [15] J. Zhang, Y. Dou, Y. Zheng, Scr. Mater. 80 (2014) 17–20. [16] A. Ostapovets, R. Groeger, Model. Simul. Mater. Sci. Eng. 22 (2014). [17] D.W. Brown, A. Jain, S.R. Agnew, B. Clausen, Thermec 2006 539–543 (2007) 3407–3413. [18] S.H. Park, S.G. Hong, C.S. Lee, Scr. Mater. 62 (2010) 202–205. [19] Y. Xin, X. Zhou, H. Chen, J.-F. Nie, H. Zhang, Y. Zhang, Q. Liu, Mater. Sci. Eng. A 594 (2014) 287–291. [20] S.G. Hong, S.H. Park, C.S. Lee, Acta Mater. 58 (2010) 5873–5885. [21] L. Lv, Y. Xin, H. Yu, R. Hong, Q. Liu, Mater. Sci. Eng. A 636 (2015) 389–395. [22] E. Ma, Y.M. Wang, Q.H. Lu, M.L. Sui, L. Lu, K. Lu, Appl. Phys. Lett. 85 (2004) 4932. [23] C.J. Shute, B.D. Myers, S. Xie, T.W. Barbee, A.M. Hodge, J.R. Weertman, Scr. Mater. 60 (2009) 1073–1077. [24] Y. Xin, X. Zhou, L. Lv, Q. Liu, Mater. Sci. Eng. A 606 (2014) 81–91. [25] M. Knezevic, A. Levinson, R. Harris, R.K. Mishra, R.D. Doherty, S.R. Kalidindi, Acta Mater. 58 (2010) 6230–6242. [26] L. Wu, S.R. Agnew, D.W. Brown, G.M. Stoica, B. Clausen, A. Jain, D.E. Fielden, P. K. Liaw, Acta Mater. 56 (2008) 3699–3707. [27] J. Min, L.G. Hector Jr, J. Lin, J.T. Carter, A.K. Sachdev, Int. J. Plast. 57 (2014) 52–76. [28] M.R. Barnett, M.D. Nave, A. Ghaderi, Acta Mater. 60 (2012) 1433–1443. [29] B. Song, R. Xin, G. Chen, X. Zhang, Q. Liu, Scr. Mater. 66 (2012) 1061–1064.