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twin recrystallization during back extrusion of AZ31 magnesium
Author’s Accepted Manuscript Microband/twin recrystallization extrusion of AZ31 magnesium
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S.M. Fatemi, A. Zarei-Hanzaki
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PII: DOI: Reference:
S0921-5093(17)31304-7 https://doi.org/10.1016/j.msea.2017.09.134 MSA35601
To appear in: Materials Science & Engineering A Received date: 14 August 2017 Revised date: 29 September 2017 Accepted date: 30 September 2017 Cite this article as: S.M. Fatemi and A. Zarei-Hanzaki, Microband/twin recrystallization during back extrusion of AZ31 magnesium, Materials Science & Engineering A, https://doi.org/10.1016/j.msea.2017.09.134 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Microband/twin recrystallization during back extrusion of AZ31 magnesium S.M. Fatemia,1, A. Zarei-Hanzakib a
School of Mechanical Engineering, Shahid Rajaee Teacher Training University, 136-16785, Tehran, Iran b
Department of Metallurgical & Materials Engineering, University of Tehran, 515-14395, Tehran, Iran
Abstract The occurrence of dynamic recrystallization of microband and twins during severe plastic deformation of an AZ31 magnesium alloy was studied through optical and scanning electron microcopy as well as electron back scatter diffraction analyses. Microstructural observations indicated that the wavy boundaries of the microbands had diffused out and generated new boundaries within parent bands. Also the grains formed through twin fragmentation mechanism
tend to acquire random orientations. Moreover, transverse boundaries segmented the twins into regions of ultra-fine and nano-scale size. A new so-called “twin-assisted” grain refinement mechanism was also recognized which adopted the orientations different from those produced by conventional recrystallization mechanisms. The latter mechanism is useful to assist texture weakening during thermomechanical processes of magnesium alloys.
Keywords: magnesium; recrystallization; texture
Introduction In order to exploit the benefits of magnesium alloys, it is important to develop a variety of secondary processing routes which can effectively improve their formability without reducing their strength.
As is well established this may be achieved through microstructural grain
refinement. Accordingly numerous investigations have been conducted to generate ultrafine grained magnesium alloys by employing severe plastic deformation (SPD) techniques. Considering low symmetry HCP crystal structure of magnesium alloys, the post-SPD mechanical properties result from the combined effects of grain refinement and crystallographic texture
1
Corresponding author: [email protected] (SM Fatemi) Tel:+98-21-22970052, fax:+98-21-22970037
changes [1]. It has been reported that the effect of grain size on the strength and ductility of magnesium is challenging due to a change in the texture [2, 3]. Fundamental investigations illustrated that the grain refinement mechanisms may effectively influence the final texture. A strong basal texture deteriorates final mechanical properties, particularly material’s formability. Accordingly, considerable effort is currently being directed to weaken the basal texture or obtain a random texture in Mg products. Hence, it is desirable to identify the microstructural mechanisms by which the basal texture of the deformed material may be alleviated. To promote the latter mechanisms, one can design the alloy (e.g. [4]) and/or deformation parameter [5, 6] to elaborate final texture of SPD-processed materials. Certain recrystallization mechanisms have been known to induce a desired texture changes during deformation of magnesium alloys, like shear band nucleation [7], particle stimulated nucleation [8], and twin recrystallization [9-12]. Moreover, discontinuous dynamic recrystallization was found as the dominant grain refinement mechanism in AZ31 alloy at low strains, during which the new grain usually mimic the texture of parent grain. The dynamically recrystallized grains, in turn, were repetitively refined through continuous dynamic recrystallization upon applying further deformation [13]. {10-12} extension and {10-11} contraction twins are the most commonly observed twin types in magnesium alloys. Contraction twins are much harder to nucleate due to energetic prospects of atomic shuffling [12]. Twinning reorients the crystal such that a mirror symmetry relative to the parent grain is created. In contrast to slip, twinning provides shear strain via reorientation of the lattice. Such reorientation in turn drastically modifies the crystallographic texture. Nevertheless, it was illustrated that contraction twins do not cause remarkable texture changes during deformation, due to their low volume fraction [14]. Extension twins were observed to contribute at low strains and grow gradually with increasing cumulative strain, eventually encompassing the whole grain [6]. The study of related microtexture showed that basal texture became weak due to the propagation and coalescence of tensile twins [6]. In addition to the latter notion, there is another twin-related determining factor which contributes in texture evolution, particularly at higher deformation temperatures. This factor is related to twin recrystallization mechanism, in which new grains formed through subdivision of twin bands by transverse boundaries [10]. However, previous results showed that contraction and secondary twins are more favorable nucleation sites for recrystallization of single crystal [15]
and polycrystalline magnesium [14, 15] at high temperatures. It has been hypothesized that the reorientations by the two aforementioned twinning types are favorable for easy glide, leading to a high dislocations density around the twins at high temperatures [16]. Similar results were also reported for single crystals at room temperature [9-11]. In this work, the occurrence of recrystallization in microband and extension twins during severe deformation of a wrought AZ31 magnesium alloy was traced and studied. Moreover, a novel twin-assisted grain boundary (GB) nucleation was recognized and discussed.
Experimental Procedure Severe deformation was imposed using back extrusion (BE) method. A commercial rolled AZ31 alloy was employed as experimental alloy, which was received as a 22 mm-thick plate with initial average grain size of 25-30 µm. Very low fraction of Mg17Al12 particles was present in the initial microstructure. However, no traceable evolution of the particles or their effect on the microstructure development was evidence during microstructural observation. The initial material possessed a common rolling texture including basal poles mainly aligned toward plate normal [17]. The cylindrical BE work-pieces with the dimensions of H8×Φ18 mm2 were machined from the as-rolled plate, where the BE axis was selected to be parallel to the initial rolling direction. To minimize the friction during the process, MoS2 lubricant was pasted to the surface of the workpieces and dies. A punch diameter of 12 mm was selected and this yields a cup shape product of 3 mm wall thickness. The workpieces were heated to the predetermined temperatures in a resistant furnace and held for 10 min to equalize the temperature. The actual working temperature was controlled using a contact thermocouple kept in touch with the inner die wall. Finally, the workpieces were back extruded at temperature of 150 °C using a computerized universal testing machine at a constant ram speed of 10 mm/min. All the BE tests were performed through penetrating the punch by depth of 6 mm into the specimen. The BE processing has been successfully carried out with no danger of crack or discontinuity. After deformation, to preserve the obtained microstructure, the processed specimens were immediately quenched into water.
The as-processed work-pieces were then sectioned along the extrusion
direction and studied for optical and SEM analysis utilizing scanning electron microscope (FEGSEM, Zeiss Ultra Plus) equipped with EBSD. Prior to microstructural analysis, the sections were mechanically ground with SiC papers of grit sizes down to 4000. The polishing was done by
colloidal silica slurry containing particles of 0.05 mm. Then, the specimens were etched with a solution composed of 4 gr picric acid,10 ml water,10 ml acetic acid and 70 ml ethanol for about 3 s. The microstructural maps showing grains and sub-grains were extracted orientation data from the EBSD.
Results & Discussion The high shear strain imposed during BE processing is mostly accommodated through the formation of shear bands in magnesium [18]. A non-homogenous strain pattern is introduced across the specimen during BE processing, where a maximum strain magnitude of about 2 is estimated at the product inner corner [19]. Microstructural observations showed that the adjacent grains to the shear localization areas would experience a net rotation toward the shear direction to satisfy the material continuity during deformation. To accommodate strain concentration within neighboring areas, an increase in the dislocation density, twins, and the GBs area fraction may be invoked. Moreover, as a consequence of either inhomogeneous stresses transmitted by neighboring grains or the intrinsic instability of the grains during BE, transition bands were developed within individual grains (as is arrowed in Fig. 1a).
The generation of such
microbands satisfies grain compatibility requirements, as magnesium possesses a limited number of active slip systems, though different slip systems may operate in neighboring areas of a grain [20]. The grains located in the region between the shear bands have been elongated in the shear direction, while micro-bands and twins are frequently formed within the grains (Fig. 1b). Each of the two latter features could stimulate different grain refinement mechanisms as discussed in what follows.
a
b
Fig. 1. a) Transition bands formed in a deformed grain interior, b) deformed grains between two shear bands including microbands (white arrow) and twins (dark arrow).
Figure 2a shows the micro-sheared regions (micro-shear band), developed in the adjacent areas of a macro shear band. The micro-shear bands were fragmented into ultra-fine grains by splitting the prior grains into ultra-fine ones.
Figure 2b and c depict the ultra fine grains
developed possibly by deformation-induced splitting process, instead of nucleation and growth. Figure 2d reveals early stages of this process in the boundary region of a microscopic shear band after BE process (arrowed in Fig. 2d). As a result of the localized shear deformation in the micro shear bands and the softening of the rotated adjacent grains, the formation of the extrusions and wavy boundaries may be promoted [21]. Some regions appear to be locally extruded out to form a bulge (some examples are marked by large arrows in Fig. 2e). The bulge formation tends to accelerate the splitting process. The bulged areas are separated to form a new grain upon increasing the shear strain. An analogous evolution in the adiabatic shear bands has been also reported in severely deformed titanium alloy [22], where the splitting process was justified through the formation of longitudinal dislocation walls. This mechanism may generate new boundaries within the parent bands. b a
c
d
e
Fig. 2. a) The FESEM micrograph of micro-shear band in the region adjacent to the shear bands, b) and c) the magnified images of the rectangular area in (a), d and e ) initiation of bulged area at the microbands boundaries.
10 µm
10 µm
Fig. 3. The optical micrographs of the microstructure obtained after first BE step.
The
fragmented twins by transverse boundaries are seen.
A detailed analysis showed newly formed transverse boundaries which partitions the twin into different areas (Fig. 3). The transverse segmentations were also realized in ultrafine/nano twins (Fig. 4), which contribute to accommodate large shear strain imposed by BE [23]. Some examples of segmented twins are marked by dotted lines in Fig. 4. These segments of nano and ultra-fine scale may be considered as new grains.
The latter structure appears to bear a
considerable resemblance to the microstructures within lathes observed by Yang et. al during cold rolling of Ti alloys [24]. It is believed that the dislocations within the twin tend to
accumulate at some locations to form transverse dislocation walls, leading to the breakdown of the band into several segments. Twinning in magnesium reorients the lattice to ‘‘hard’’ crystallographic orientations [25], and the transformation of dislocations as they pass through the twinning front can lead to the establishment of higher hardening rates within the twin interior [26]. The latter may cause the activation of nonbasal slip thereby resulting in dislocations pile up [27]. Moreover, incoherency energy of the twin boundaries with matrix [28] ties up the boundaries to migrate.
Therefore, the rearrangement of the dislocations may end up the
formation of transverse sub-boundaries containing a high density of boundary dislocations. This is confirmed by EBSD measurements (arrowed in Fig. 5), where transverse low angle GBs have been developed inside the extension twins. The twin boundaries are not straight line and contain large number of defects after severe deformation. As the strain is increased, the subgrains inside the twins may rotate to develop a high angle boundary. This process fragments the twin band into new grains, some examples of which are denoted with dotted rectangles in Fig. 5a. This was identified relying on the grains morphology as well as boundary misorientation corresponding to twinning rotation (~86°). Myshlyaev et al. [29] also observed subdivision of primary twins by transverse low angle boundaries during hot torsion testing of pure magnesium. The similar subdivision of mechanical twin lamellae into the segments separated by high-angle boundaries has been also reported in Refs. [30, 31].
Fig. 4. The FESEM micrograph of ultra-fine twins developed during BE processing.
The aforementioned fragmentation process suggests that the grain refinement can be completed via an athermally assisted shear deformation-induced mechanism instead of a thermally controlled growth of new grains. It is obvious that such a process lacks the nucleation/growth period and is not typical of the regular dynamic recrystallization process; rather, it may reflect some kind of deformation-induced grain refinement. To explore the orientations relationship of the new grains, the related crystallographic components developed through the latter mechanism are presented on the (0001) pole figure in Fig. 5b. It is clear that the new grains tend to acquire random orientations, showing different angles and axes of rotation from the parent matrix component (and also from basal texture which is typical for tension twins). These evidences suggest that a kind of rotational recrystallization mechanism is operative during fragmentation of twin bands. (a)
10 µm ED
(b)
(0001)
TD
Fig. 5. a) EBSD maps of the experimental AZ31 alloy deformed to the first BE step at 150ºC showing low angle boundaries in white and high angle boundaries in black with misorientation
between 2 and 15 º and >15º, respectively (b) (0001) pole figure illustrating the crystallographic orientations of the different grains marked in (a).
Based on the microstructural observations of the specimen deformed through BE, a new morphology of DRX nuclei has been recognized. As arrowed in Fig. 6, some triangular shape nuclei have been formed at the prior GBs. To provide further understanding of the observed nucleation mechanism, the EBSD analysis of the material was performed. The obtained maps are given in Fig. 7. Figure 7a displays some examples of the individual triangular nuclei attached to the GBs which may then grow to produce a recrystallized grain. The color differences and low angle boundaries (white lines) suggest that the nuclei are parts of twins which crossed the GB. To verify the latter speculation, the boundaries characterized by 86° misorientation (corresponding to characteristic misorientation of 10-12 twins) are marked in Fig. 7b. The result confirms that the aforementioned nuclei are twin segments. The formation of a twin leads to shear in the grain, and when the twin grows and intersects GBs, it could cause twin nucleation in another grain. The higher angle GBs (usually above 40°) don’t allow twins to pass through. In contrast, a twin can cause twinning in the neighboring grain at which boundary misorientation is low (below 40°). The latter may be achieved through dislocation pile up at the tip of a twin intersecting low angle-GBs [32].
10 µm
Fig.6. Optical microstructure of the experimental alloy deformed through BE showing triangular shaped new grains.
The twin segment locates in an adjacent grain generates geometrically a new grain nucleus. This geometrical type nucleation at GBs is termed “twin-assisted” recrystallization. Therefore, the appearance of triangle-shape grains may be attributed at least partly to the occurrence of “twin-assisted” recrystallization. The part of twin in parent grain is suggested to be affected by extended dynamic recovery resulting in either consuming the twinning texture or partly formation of new grains depending on the grain orientation and active slip systems. Such mechanism may also end up to the formation of new grains in parent grain interior, the boundary of which has no connection with the parent GBs (indicted by white dashed lines in Fig. 7a). The presence of low angle GBs (white lines) denoted the operation of dynamic recovery. In line with present evidence on the twin restoration, previous research on low alloy titanium [33] has concluded that the twining orientations may be consumed at the early stages of static recrystallization.
b)
a)
10 µm
c)
10 µm Fig. 7. EBSD maps of the experimental AZ31 alloy deformed through BE at 150ºC showing a) through “twin assisted” mechanism b) Band Contrast map showing boundaries of 86º misorientation in red, c) inverse pole map including new grains along the prior GBs. boundaries in white, red, green and black with misorientations 2-5º, 5-10º, 10-15º and >15º, respectively showing low GBs (2-15º) in white and high GBs (>15º).
It is of interest to compare the texture change associated with the novel “twin-assisted” recrystallization mechanism and the conventional discontinuous recrystallization.
Figure 7c
shows the EBSD map of the new grains nucleated through conventional mechanism. In order to examine the influence of nucleation mechanisms on the texture evolution, the microtexture of the new grains together with the texture of their parent grains are presented for “twin assisted” and conventional recrystallization in Fig. 8. Conventionally recrystallized nuclei and their parent grains exhibit a relatively similar near-basal texture (Fig. 8a), though the orientation of recrystallized grains show wider spread of 10–20° around normal direction. This spread in grain orientations is consistent with the previous investigation [34] showing texture weakening during early stages of conventional static recrystallization in magnesium alloys. It is generally believed that the characteristic feature of discontinuous mechanism is the exhibition of similar orientation of the new grains with the old ones from which they have been grown [35]. This is well supporting the obtained result in the present work. For the “twin-assisted” nuclei (Fig. 8b), the (0002) pole figures showed a near 90° rotation in the basal planes for the new grains in
comparison to the original (parent grains) texture. The amount of observed texture rotation is close to the rotation associated with {
̅ } extension-twinning (86°). This result is a further
confirmation for the role of twins in the new grains formation. An important feature of the grains nucleated by “twin assisted” mechanism is that they generally adopt orientations which are different from those produced by conventional recrystallization mechanisms, and thus it offers a route to manipulate the texture. This is particularly interesting for magnesium alloys, since “twin assisted” may be useful in producing components to assist texture weakening during thermomechanical processes of magnesium alloys.
{0002}
{11-20} {0002}
{10-10} {11-20}
(a)
{0002}
{10-10}
{0002} {11-20}
{11-20} {10-10}
(b)
{11-2-3}
Parent Grains
{11-2-3}
Recrystallized Grains
{11-2-3}
Parent Grains
{11-2-3}
Recrystallized Grains
Fig. 8. Basal pole figure of recrystallized and parent grains for (a) conventional DRX grains along GBs shown in Fig. 7c, b) “twin-assisted” nucleation mechanism shown in Fig. 7a.
Conclusions The grain refinement mechanisms related to microband and twins was systematically investigated in an AZ31 magnesium alloy deformed through accumulative back extrusion. Microshear bands and twins were found to form in the rotated grains adjacent to the shear bands, within which the formation of ultrafine and nano grains was observed.
The boundary of
microbands may bugle due to the shear stress, leading to extruded portion, which generate new boundaries within parent bands. Moreover, transverse boundaries fragmented the twins into segments of ultra-fine and nano sizes. A new DRX mechanism called “twin-assisted” was observed during BE processing, the associated texture of which is in twin relation with the matrix. The latter may be useful in producing texture components other than the usual strong basal texture.
References
[1] S. Fatemi, A. Zarei-Hanzaki, J. Cabrera, Microstructure, Texture, and Tensile Properties of Ultrafine/Nano-Grained Magnesium Alloy Processed by Accumulative Back Extrusion, Metallurgical and Materials Transactions A 48(5) (2017) 2563-2573. [2] R.B. Figueiredo, S. Sabbaghianrad, A. Giwa, J.R. Greer, T.G. Langdon, Evidence for exceptional low temperature ductility in polycrystalline magnesium processed by severe plastic deformation, Acta Materialia 122 (2017) 322-331. [3] M. Gzyl, A. Rosochowski, S. Boczkal, L. Olejnik, The role of microstructure and texture in controlling mechanical properties of AZ31B magnesium alloy processed by I-ECAP, Materials Science and Engineering: A 638 (2015) 20-29. [4] J.P. Hadorn, R.P. Mulay, K. Hantzsche, S. Yi, J. Bohlen, D. Letzig, S.R. Agnew, Texture weakening effects in Ce-containing Mg alloys, Metallurgical and Materials Transactions A 44(3) (2013) 1566-1576. [5] E. Ball, P. Prangnell, Tensile-compressive yield asymmetries in high strength wrought magnesium alloys, Scripta Metallurgica et Materialia 31(2) (1994) 111-116. [6] D. Hou, T. Liu, D. Shi, H. Chen, H. Chen, Study of twinning behaviors of rolled AZ31 magnesium alloy by interrupted in situ compressive tests, Materials Science and Engineering: A 653 (2016) 108-114. [7] I. Basu, T. Al-Samman, G. Gottstein, Shear band-related recrystallization and grain growth in two rolled magnesium-rare earth alloys, Materials Science and Engineering: A 579 (2013) 50-56. [8] A. Sadeghi, M. Pekguleryuz, Recrystallization and texture evolution of Mg–3% Al–1% Zn– (0.4–0.8)% Sr alloys during extrusion, Materials Science and Engineering: A 528(3) (2011) 1678-1685. [9] K.D. Molodov, T. Al-Samman, D.A. Molodov, G. Gottstein, Mechanisms of exceptional ductility of magnesium single crystal during deformation at room temperature: Multiple twinning and dynamic recrystallization, Acta Materialia 76 (2014) 314-330. [10] I. Basu, T. Al-Samman, Twin recrystallization mechanisms in magnesium-rare earth alloys, Acta Materialia 96 (2015) 111-132. [11] K.D. Molodov, T. Al-Samman, D.A. Molodov, G. Gottstein, On the ductility of magnesium single crystals at ambient temperature, Metallurgical and Materials Transactions A 45(8) (2014) 3275-3281. [12] M. Yoo, Slip, twinning, and fracture in hexagonal close-packed metals, Metallurgical Transactions A 12(3) (1981) 409-418. [13] S. Fatemi-Varzaneh, A. Zarei-Hanzaki, J. Cabrera, P. Calvillo, EBSD characterization of repetitive grain refinement in AZ31 magnesium alloy, Materials Chemistry and Physics 149 (2015) 339-343. [14] T. Al-Samman, K.D. Molodov, D.A. Molodov, G. Gottstein, S. Suwas, Softening and dynamic recrystallization in magnesium single crystals during c-axis compression, Acta materialia 60(2) (2012) 537-545.
[15] N. Dudamell, I. Ulacia, F. Gálvez, S. Yi, J. Bohlen, D. Letzig, I. Hurtado, M.T. PérezPrado, Twinning and grain subdivision during dynamic deformation of a Mg AZ31 sheet alloy at room temperature, Acta Materialia 59(18) (2011) 6949-6962. [16] L. Jiang, J.J. Jonas, A.A. Luo, A.K. Sachdev, S. Godet, Twinning-induced softening in polycrystalline AM30 Mg alloy at moderate temperatures, Scripta Materialia 54(5) (2006) 771775. [17] S. Fatemi-Varzaneh, A. Zarei-Hanzaki, H. Paul, Characterization of ultrafine and nano grained magnesium alloy processed by severe plastic deformation, Materials Characterization 87 (2014) 27-35. [18] S. Fatemi-Varzaneh, A. Zarei-Hanzaki, J. Cabrera, Shear banding phenomenon during severe plastic deformation of an AZ31 magnesium alloy, Journal of Alloys and Compounds (2011). [19] S. Fatemi-Varzaneh, A. Zarei-Hanzaki, M. Naderi, A.A. Roostaei, Deformation homogeneity in accumulative back extrusion processing of AZ31 magnesium alloy, Journal of Alloys and Compounds 507(1) (2010) 207-214. [20] Q. Guo, H. Yan, Z. Chen, H. Zhang, Grain refinement in as-cast AZ80 Mg alloy under large strain deformation, Materials characterization 58(2) (2007) 162-167. [21] Q. Xue, G. Gray, Development of adiabatic shear bands in annealed 316L stainless steel: Part II. TEM studies of the evolution of microstructure during deformation localization, Metallurgical and Materials Transactions A 37(8) (2006) 2447-2458. [22] Y. Yang, F. Jiang, B. Zhou, X. Li, H. Zheng, Q. Zhang, Microstructural characterization and evolution mechanism of adiabatic shear band in a near beta-Ti alloy, Materials Science and Engineering: A (2010). [23] S. Fatemi-Varzaneh, A. Zarei-Hanzaki, M. Naderi, A.A. Roostai, Deformation homogeneity in accumulative back extrusion processing of AZ31 magnesium alloy, Journal of Alloys and Compounds (2010). [24] D. Yang, P. Cizek, P. Hodgson, C. Wen, Microstructure evolution and nanograin formation during shear localization in cold-rolled titanium, Acta materialia 58(13) (2010) 4536-4548. [25] S. Agnew, M. Yoo, C. Tome, Application of texture simulation to understanding mechanical behavior of Mg and solid solution alloys containing Li or Y, Acta materialia 49(20) (2001) 4277-4289. [26] Z. Basinski, M. Szczerba, M. Niewczas, J. Embury, S. Basinski, The transformation of slip dislocations during twinning of copper-aluminium alloy crystals, Revue de Metallurgie, Cahiers d'Informations Techniques(France) 94(9) (1997) 1037-1043. [27] A. Galiyev, R. Kaibyshev, G. Gottstein, Correlation of plastic deformation and dynamic recrystallization in magnesium alloy ZK60, Acta materialia 49(7) (2001) 1199-1207. [28] P. Partridge, E. Roberts, The formation and behaviour of incoherent twin boundaries in hexagonal metals, Acta Metallurgica 12(11) (1964) 1205-1210. [29] M. Myshlyaev, H. McQueen, A. Mwembela, E. Konopleva, Twinning, dynamic recovery and recrystallization in hot worked Mg-Al-Zn alloy, Materials Science and Engineering A 337(12) (2002) 121-133. [30] N. Stanford, U. Carlson, M. Barnett, Deformation Twinning and the Hall–Petch Relation in Commercial Purity Ti, Metallurgical and Materials Transactions A 39(4) (2008) 934-944. [31] P. Cizek, M. Barnett, Characteristics of the contraction twins formed close to the fracture surface in Mg-3Al-1Zn alloy deformed in tension, Scripta Materialia 59(9) (2008) 959-962.
[32] A. Khosravani, Application of High Resolution Electron Backscatter Diffraction (HREBSD) Techniques to Twinning Deformation Mechanism in AZ31 Magnesium Alloy, (2012). [33] Y. Wang, J. Huang, Texture analysis in hexagonal materials, Materials chemistry and physics 81(1) (2003) 11-26. [34] J. Robson, D. Henry, B. Davis, Particle effects on recrystallization in magnesiummanganese alloys: Particle-stimulated nucleation, Acta Materialia 57(9) (2009) 2739-2747. [35] F.J. Humphreys, M. Hatherly, Recrystallization and related annealing phenomena, Pergamon2004.
during
back
S.M. Fatemi, A. Zarei-Hanzaki
www.elsevier.com/locate/msea
PII: DOI: Reference:
S0921-5093(17)31304-7 https://doi.org/10.1016/j.msea.2017.09.134 MSA35601
To appear in: Materials Science & Engineering A Received date: 14 August 2017 Revised date: 29 September 2017 Accepted date: 30 September 2017 Cite this article as: S.M. Fatemi and A. Zarei-Hanzaki, Microband/twin recrystallization during back extrusion of AZ31 magnesium, Materials Science & Engineering A, https://doi.org/10.1016/j.msea.2017.09.134 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Microband/twin recrystallization during back extrusion of AZ31 magnesium S.M. Fatemia,1, A. Zarei-Hanzakib a
School of Mechanical Engineering, Shahid Rajaee Teacher Training University, 136-16785, Tehran, Iran b
Department of Metallurgical & Materials Engineering, University of Tehran, 515-14395, Tehran, Iran
Abstract The occurrence of dynamic recrystallization of microband and twins during severe plastic deformation of an AZ31 magnesium alloy was studied through optical and scanning electron microcopy as well as electron back scatter diffraction analyses. Microstructural observations indicated that the wavy boundaries of the microbands had diffused out and generated new boundaries within parent bands. Also the grains formed through twin fragmentation mechanism
tend to acquire random orientations. Moreover, transverse boundaries segmented the twins into regions of ultra-fine and nano-scale size. A new so-called “twin-assisted” grain refinement mechanism was also recognized which adopted the orientations different from those produced by conventional recrystallization mechanisms. The latter mechanism is useful to assist texture weakening during thermomechanical processes of magnesium alloys.
Keywords: magnesium; recrystallization; texture
Introduction In order to exploit the benefits of magnesium alloys, it is important to develop a variety of secondary processing routes which can effectively improve their formability without reducing their strength.
As is well established this may be achieved through microstructural grain
refinement. Accordingly numerous investigations have been conducted to generate ultrafine grained magnesium alloys by employing severe plastic deformation (SPD) techniques. Considering low symmetry HCP crystal structure of magnesium alloys, the post-SPD mechanical properties result from the combined effects of grain refinement and crystallographic texture
1
Corresponding author: [email protected] (SM Fatemi) Tel:+98-21-22970052, fax:+98-21-22970037
changes [1]. It has been reported that the effect of grain size on the strength and ductility of magnesium is challenging due to a change in the texture [2, 3]. Fundamental investigations illustrated that the grain refinement mechanisms may effectively influence the final texture. A strong basal texture deteriorates final mechanical properties, particularly material’s formability. Accordingly, considerable effort is currently being directed to weaken the basal texture or obtain a random texture in Mg products. Hence, it is desirable to identify the microstructural mechanisms by which the basal texture of the deformed material may be alleviated. To promote the latter mechanisms, one can design the alloy (e.g. [4]) and/or deformation parameter [5, 6] to elaborate final texture of SPD-processed materials. Certain recrystallization mechanisms have been known to induce a desired texture changes during deformation of magnesium alloys, like shear band nucleation [7], particle stimulated nucleation [8], and twin recrystallization [9-12]. Moreover, discontinuous dynamic recrystallization was found as the dominant grain refinement mechanism in AZ31 alloy at low strains, during which the new grain usually mimic the texture of parent grain. The dynamically recrystallized grains, in turn, were repetitively refined through continuous dynamic recrystallization upon applying further deformation [13]. {10-12} extension and {10-11} contraction twins are the most commonly observed twin types in magnesium alloys. Contraction twins are much harder to nucleate due to energetic prospects of atomic shuffling [12]. Twinning reorients the crystal such that a mirror symmetry relative to the parent grain is created. In contrast to slip, twinning provides shear strain via reorientation of the lattice. Such reorientation in turn drastically modifies the crystallographic texture. Nevertheless, it was illustrated that contraction twins do not cause remarkable texture changes during deformation, due to their low volume fraction [14]. Extension twins were observed to contribute at low strains and grow gradually with increasing cumulative strain, eventually encompassing the whole grain [6]. The study of related microtexture showed that basal texture became weak due to the propagation and coalescence of tensile twins [6]. In addition to the latter notion, there is another twin-related determining factor which contributes in texture evolution, particularly at higher deformation temperatures. This factor is related to twin recrystallization mechanism, in which new grains formed through subdivision of twin bands by transverse boundaries [10]. However, previous results showed that contraction and secondary twins are more favorable nucleation sites for recrystallization of single crystal [15]
and polycrystalline magnesium [14, 15] at high temperatures. It has been hypothesized that the reorientations by the two aforementioned twinning types are favorable for easy glide, leading to a high dislocations density around the twins at high temperatures [16]. Similar results were also reported for single crystals at room temperature [9-11]. In this work, the occurrence of recrystallization in microband and extension twins during severe deformation of a wrought AZ31 magnesium alloy was traced and studied. Moreover, a novel twin-assisted grain boundary (GB) nucleation was recognized and discussed.
Experimental Procedure Severe deformation was imposed using back extrusion (BE) method. A commercial rolled AZ31 alloy was employed as experimental alloy, which was received as a 22 mm-thick plate with initial average grain size of 25-30 µm. Very low fraction of Mg17Al12 particles was present in the initial microstructure. However, no traceable evolution of the particles or their effect on the microstructure development was evidence during microstructural observation. The initial material possessed a common rolling texture including basal poles mainly aligned toward plate normal [17]. The cylindrical BE work-pieces with the dimensions of H8×Φ18 mm2 were machined from the as-rolled plate, where the BE axis was selected to be parallel to the initial rolling direction. To minimize the friction during the process, MoS2 lubricant was pasted to the surface of the workpieces and dies. A punch diameter of 12 mm was selected and this yields a cup shape product of 3 mm wall thickness. The workpieces were heated to the predetermined temperatures in a resistant furnace and held for 10 min to equalize the temperature. The actual working temperature was controlled using a contact thermocouple kept in touch with the inner die wall. Finally, the workpieces were back extruded at temperature of 150 °C using a computerized universal testing machine at a constant ram speed of 10 mm/min. All the BE tests were performed through penetrating the punch by depth of 6 mm into the specimen. The BE processing has been successfully carried out with no danger of crack or discontinuity. After deformation, to preserve the obtained microstructure, the processed specimens were immediately quenched into water.
The as-processed work-pieces were then sectioned along the extrusion
direction and studied for optical and SEM analysis utilizing scanning electron microscope (FEGSEM, Zeiss Ultra Plus) equipped with EBSD. Prior to microstructural analysis, the sections were mechanically ground with SiC papers of grit sizes down to 4000. The polishing was done by
colloidal silica slurry containing particles of 0.05 mm. Then, the specimens were etched with a solution composed of 4 gr picric acid,10 ml water,10 ml acetic acid and 70 ml ethanol for about 3 s. The microstructural maps showing grains and sub-grains were extracted orientation data from the EBSD.
Results & Discussion The high shear strain imposed during BE processing is mostly accommodated through the formation of shear bands in magnesium [18]. A non-homogenous strain pattern is introduced across the specimen during BE processing, where a maximum strain magnitude of about 2 is estimated at the product inner corner [19]. Microstructural observations showed that the adjacent grains to the shear localization areas would experience a net rotation toward the shear direction to satisfy the material continuity during deformation. To accommodate strain concentration within neighboring areas, an increase in the dislocation density, twins, and the GBs area fraction may be invoked. Moreover, as a consequence of either inhomogeneous stresses transmitted by neighboring grains or the intrinsic instability of the grains during BE, transition bands were developed within individual grains (as is arrowed in Fig. 1a).
The generation of such
microbands satisfies grain compatibility requirements, as magnesium possesses a limited number of active slip systems, though different slip systems may operate in neighboring areas of a grain [20]. The grains located in the region between the shear bands have been elongated in the shear direction, while micro-bands and twins are frequently formed within the grains (Fig. 1b). Each of the two latter features could stimulate different grain refinement mechanisms as discussed in what follows.
a
b
Fig. 1. a) Transition bands formed in a deformed grain interior, b) deformed grains between two shear bands including microbands (white arrow) and twins (dark arrow).
Figure 2a shows the micro-sheared regions (micro-shear band), developed in the adjacent areas of a macro shear band. The micro-shear bands were fragmented into ultra-fine grains by splitting the prior grains into ultra-fine ones.
Figure 2b and c depict the ultra fine grains
developed possibly by deformation-induced splitting process, instead of nucleation and growth. Figure 2d reveals early stages of this process in the boundary region of a microscopic shear band after BE process (arrowed in Fig. 2d). As a result of the localized shear deformation in the micro shear bands and the softening of the rotated adjacent grains, the formation of the extrusions and wavy boundaries may be promoted [21]. Some regions appear to be locally extruded out to form a bulge (some examples are marked by large arrows in Fig. 2e). The bulge formation tends to accelerate the splitting process. The bulged areas are separated to form a new grain upon increasing the shear strain. An analogous evolution in the adiabatic shear bands has been also reported in severely deformed titanium alloy [22], where the splitting process was justified through the formation of longitudinal dislocation walls. This mechanism may generate new boundaries within the parent bands. b a
c
d
e
Fig. 2. a) The FESEM micrograph of micro-shear band in the region adjacent to the shear bands, b) and c) the magnified images of the rectangular area in (a), d and e ) initiation of bulged area at the microbands boundaries.
10 µm
10 µm
Fig. 3. The optical micrographs of the microstructure obtained after first BE step.
The
fragmented twins by transverse boundaries are seen.
A detailed analysis showed newly formed transverse boundaries which partitions the twin into different areas (Fig. 3). The transverse segmentations were also realized in ultrafine/nano twins (Fig. 4), which contribute to accommodate large shear strain imposed by BE [23]. Some examples of segmented twins are marked by dotted lines in Fig. 4. These segments of nano and ultra-fine scale may be considered as new grains.
The latter structure appears to bear a
considerable resemblance to the microstructures within lathes observed by Yang et. al during cold rolling of Ti alloys [24]. It is believed that the dislocations within the twin tend to
accumulate at some locations to form transverse dislocation walls, leading to the breakdown of the band into several segments. Twinning in magnesium reorients the lattice to ‘‘hard’’ crystallographic orientations [25], and the transformation of dislocations as they pass through the twinning front can lead to the establishment of higher hardening rates within the twin interior [26]. The latter may cause the activation of nonbasal slip thereby resulting in dislocations pile up [27]. Moreover, incoherency energy of the twin boundaries with matrix [28] ties up the boundaries to migrate.
Therefore, the rearrangement of the dislocations may end up the
formation of transverse sub-boundaries containing a high density of boundary dislocations. This is confirmed by EBSD measurements (arrowed in Fig. 5), where transverse low angle GBs have been developed inside the extension twins. The twin boundaries are not straight line and contain large number of defects after severe deformation. As the strain is increased, the subgrains inside the twins may rotate to develop a high angle boundary. This process fragments the twin band into new grains, some examples of which are denoted with dotted rectangles in Fig. 5a. This was identified relying on the grains morphology as well as boundary misorientation corresponding to twinning rotation (~86°). Myshlyaev et al. [29] also observed subdivision of primary twins by transverse low angle boundaries during hot torsion testing of pure magnesium. The similar subdivision of mechanical twin lamellae into the segments separated by high-angle boundaries has been also reported in Refs. [30, 31].
Fig. 4. The FESEM micrograph of ultra-fine twins developed during BE processing.
The aforementioned fragmentation process suggests that the grain refinement can be completed via an athermally assisted shear deformation-induced mechanism instead of a thermally controlled growth of new grains. It is obvious that such a process lacks the nucleation/growth period and is not typical of the regular dynamic recrystallization process; rather, it may reflect some kind of deformation-induced grain refinement. To explore the orientations relationship of the new grains, the related crystallographic components developed through the latter mechanism are presented on the (0001) pole figure in Fig. 5b. It is clear that the new grains tend to acquire random orientations, showing different angles and axes of rotation from the parent matrix component (and also from basal texture which is typical for tension twins). These evidences suggest that a kind of rotational recrystallization mechanism is operative during fragmentation of twin bands. (a)
10 µm ED
(b)
(0001)
TD
Fig. 5. a) EBSD maps of the experimental AZ31 alloy deformed to the first BE step at 150ºC showing low angle boundaries in white and high angle boundaries in black with misorientation
between 2 and 15 º and >15º, respectively (b) (0001) pole figure illustrating the crystallographic orientations of the different grains marked in (a).
Based on the microstructural observations of the specimen deformed through BE, a new morphology of DRX nuclei has been recognized. As arrowed in Fig. 6, some triangular shape nuclei have been formed at the prior GBs. To provide further understanding of the observed nucleation mechanism, the EBSD analysis of the material was performed. The obtained maps are given in Fig. 7. Figure 7a displays some examples of the individual triangular nuclei attached to the GBs which may then grow to produce a recrystallized grain. The color differences and low angle boundaries (white lines) suggest that the nuclei are parts of twins which crossed the GB. To verify the latter speculation, the boundaries characterized by 86° misorientation (corresponding to characteristic misorientation of 10-12 twins) are marked in Fig. 7b. The result confirms that the aforementioned nuclei are twin segments. The formation of a twin leads to shear in the grain, and when the twin grows and intersects GBs, it could cause twin nucleation in another grain. The higher angle GBs (usually above 40°) don’t allow twins to pass through. In contrast, a twin can cause twinning in the neighboring grain at which boundary misorientation is low (below 40°). The latter may be achieved through dislocation pile up at the tip of a twin intersecting low angle-GBs [32].
10 µm
Fig.6. Optical microstructure of the experimental alloy deformed through BE showing triangular shaped new grains.
The twin segment locates in an adjacent grain generates geometrically a new grain nucleus. This geometrical type nucleation at GBs is termed “twin-assisted” recrystallization. Therefore, the appearance of triangle-shape grains may be attributed at least partly to the occurrence of “twin-assisted” recrystallization. The part of twin in parent grain is suggested to be affected by extended dynamic recovery resulting in either consuming the twinning texture or partly formation of new grains depending on the grain orientation and active slip systems. Such mechanism may also end up to the formation of new grains in parent grain interior, the boundary of which has no connection with the parent GBs (indicted by white dashed lines in Fig. 7a). The presence of low angle GBs (white lines) denoted the operation of dynamic recovery. In line with present evidence on the twin restoration, previous research on low alloy titanium [33] has concluded that the twining orientations may be consumed at the early stages of static recrystallization.
b)
a)
10 µm
c)
10 µm Fig. 7. EBSD maps of the experimental AZ31 alloy deformed through BE at 150ºC showing a) through “twin assisted” mechanism b) Band Contrast map showing boundaries of 86º misorientation in red, c) inverse pole map including new grains along the prior GBs. boundaries in white, red, green and black with misorientations 2-5º, 5-10º, 10-15º and >15º, respectively showing low GBs (2-15º) in white and high GBs (>15º).
It is of interest to compare the texture change associated with the novel “twin-assisted” recrystallization mechanism and the conventional discontinuous recrystallization.
Figure 7c
shows the EBSD map of the new grains nucleated through conventional mechanism. In order to examine the influence of nucleation mechanisms on the texture evolution, the microtexture of the new grains together with the texture of their parent grains are presented for “twin assisted” and conventional recrystallization in Fig. 8. Conventionally recrystallized nuclei and their parent grains exhibit a relatively similar near-basal texture (Fig. 8a), though the orientation of recrystallized grains show wider spread of 10–20° around normal direction. This spread in grain orientations is consistent with the previous investigation [34] showing texture weakening during early stages of conventional static recrystallization in magnesium alloys. It is generally believed that the characteristic feature of discontinuous mechanism is the exhibition of similar orientation of the new grains with the old ones from which they have been grown [35]. This is well supporting the obtained result in the present work. For the “twin-assisted” nuclei (Fig. 8b), the (0002) pole figures showed a near 90° rotation in the basal planes for the new grains in
comparison to the original (parent grains) texture. The amount of observed texture rotation is close to the rotation associated with {
̅ } extension-twinning (86°). This result is a further
confirmation for the role of twins in the new grains formation. An important feature of the grains nucleated by “twin assisted” mechanism is that they generally adopt orientations which are different from those produced by conventional recrystallization mechanisms, and thus it offers a route to manipulate the texture. This is particularly interesting for magnesium alloys, since “twin assisted” may be useful in producing components to assist texture weakening during thermomechanical processes of magnesium alloys.
{0002}
{11-20} {0002}
{10-10} {11-20}
(a)
{0002}
{10-10}
{0002} {11-20}
{11-20} {10-10}
(b)
{11-2-3}
Parent Grains
{11-2-3}
Recrystallized Grains
{11-2-3}
Parent Grains
{11-2-3}
Recrystallized Grains
Fig. 8. Basal pole figure of recrystallized and parent grains for (a) conventional DRX grains along GBs shown in Fig. 7c, b) “twin-assisted” nucleation mechanism shown in Fig. 7a.
Conclusions The grain refinement mechanisms related to microband and twins was systematically investigated in an AZ31 magnesium alloy deformed through accumulative back extrusion. Microshear bands and twins were found to form in the rotated grains adjacent to the shear bands, within which the formation of ultrafine and nano grains was observed.
The boundary of
microbands may bugle due to the shear stress, leading to extruded portion, which generate new boundaries within parent bands. Moreover, transverse boundaries fragmented the twins into segments of ultra-fine and nano sizes. A new DRX mechanism called “twin-assisted” was observed during BE processing, the associated texture of which is in twin relation with the matrix. The latter may be useful in producing texture components other than the usual strong basal texture.
References
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