Evolution of microstructure, texture and mechanical properties of ZK60 magnesium alloy in a single rolling pass

Author’s Accepted Manuscript Evolution of microstructure, texture and mechanical properties of ZK60 magnesium alloy in a single rolling pass Wenke Wang, Guorong Cui, Wencong Zhang, Wenzhen Chen, Erde Wang www.elsevier.com/locate/msea

PII: DOI: Reference:

S0921-5093(18)30455-6 https://doi.org/10.1016/j.msea.2018.03.096 MSA36289

To appear in: Materials Science & Engineering A Received date: 22 December 2017 Revised date: 23 March 2018 Accepted date: 23 March 2018 Cite this article as: Wenke Wang, Guorong Cui, Wencong Zhang, Wenzhen Chen and Erde Wang, Evolution of microstructure, texture and mechanical properties of ZK60 magnesium alloy in a single rolling pass, Materials Science & Engineering A, https://doi.org/10.1016/j.msea.2018.03.096 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.

Evolution of microstructure, texture and mechanical properties of ZK60 magnesium alloy in a single rolling pass Wenke Wang, Guorong Cui*, Wencong Zhang, Wenzhen Chen*, Erde Wang School of Materials Science and Engineering, Harbin Institute of Technology, Weihai 264209, PR China [email protected] (Guorong Cui), [email protected] (Wenzhen Chen) *

Corresponding author. Postal Address: Room A301, School of Materials Science and Engineering, Harbin

Institute of Technology, Weihai 264209, PR China. Tel.: +86 631 5672167; Fax: +86 631 5672167.

Abstract: The evolution of microstructure and texture in a single rolling pass was investigated at rolling temperature of 250℃. Based on the analysis of microstructure evolution, thickness reduction of 31% was identified as a critical deformation strain for the occurrence of continuous dynamic recrystallization (CDRX) in this work. CDRX strikingly refined the grain size and improved the microstructure homogeneity. The results based on intragranular misorientation axis analysis showed that the activation of abundant stochastic stored dislocations with a broad range of orientations developed in CDRX was the main reason of the transformation from <10-10>//RD texture to randomized texture along <10-10>-<11-20> arc in inverse pole figure. Microhardness distribution was mainly influenced by the fraction of deformed grains. Grain refinement increased yield stress and uniform elongation, while weaker texture only increased uniform elongation. Keywords: Rolling; Continuous dynamic recrystallization; Texture; Dislocation; Yield stress

1 Introduction Magnesium alloys have received significant attention in the automotive and electronics industries due to their low density and high specific strength [1, 2]. However, their inherent hexagonal close packed (HCP) structure generally leads to poor ductility and sheet formability, which limits their widespread commercial usability [3, 4]. Various techniques have been proposed and investigated to improve the formability of magnesium alloys based on grain refinement and texture modification [5, 6]. Rolling was recognized as the most suitable method for industrial sheet fabrication, and moreover could significantly refine the grains owing to its multiple dynamic recrystallizations (DRX) [7, 8]. Typically, Kim et al. fabricated AZ31 magnesium

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alloy sheets with a maximum elongation of 35% due to its fine grain size of 1.4μm [9]. In terms of texture, because of stress state of rolling, a strong (0002) basal texture was usually inevitable in which the c-axes in both deformed grains and DRXed grains were parallel to the normal direction (ND) of magnesium alloy sheets [7]. This only demonstrated the relationship of crystal plane between deformed grains and DRXed grains, but ignored their relationship of crystal orientation which was equally important as crystal plane. Recently, Aidin Imandoust et al. [10] reported that during extrusion process continuous dynamic recrystallization (CDRX) transformed a sharp <10-10> fiber into a randomized texture, which was mainly ascribed to the stochastic relaxation of basal , pyramidal , and prismatic dislocations into low-angle grain boundaries. It is well known that there exists difference in stress state between extrusion process and rolling process, and to our best knowledge, the transformation of crystal orientation from deformed grains to DRXed grains has not been reported in rolling process. Importantly, this misorientation relationship has been well understood for cubic materials [11-13], and with its help a desirable microstructure was successfully produced, which was far more successful for cubic materials than for magnesium alloys [11, 14, 15]. Understanding this transformation mechanism could help to design better materials in term of mechanical properties and material performance. Moreover, it was particularly important for magnesium alloys due to its non-cubic symmetry. Therefore, in this work, the evolution of microstructure and texture in a single rolling pass was investigated in order to unravel the relationship of crystal orientation between deformed grains and DRXed grains. Meanwhile, the relationships between the microstructure and mechanical properties during this single rolling process were analyzed in detail.

2 Experimental procedures The initial material used in this work was Mg-6.63wt.%Zn-0.56wt.%Zr alloy (ZK60 magnesium alloy). The rolling process ceased in the middle region of the plate and a terminated sample was obtained. The total thickness reduction was about 42% (from 7mm to 4.06mm in thickness). This rolling process was performed on a two high mill with 220mm in roll diameter at a roll velocity of 5m/min and its rolling temperature was about 250℃. In this work, thickness reduction of 42% was selected based on the following consideration. First, according to our previous work, this thickness reduction was beneficial to realize DRX and produce a finer uniform microstructure on the premise of no cracking in the sheets [7, 16]. Second, the microstructure variation at different thickness reduction was easily obtained in a single rolling pass when the thickness reduction achieved 42%, and under this case, other factors influencing DRX, especially the initial material and surrounding environment, could be well eliminated. In terms of the

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rolling temperature, our previous work suggested that 230℃-250℃ (thickness reduction of 40%) were the optimal rolling temperatures because a finer and more homogeneous microstructure was easily obtained at these temperatures due to the completed DRX [31]. Therefore, 250℃ was chosen as the rolling temperature in this work. Fig. 1a shows the metallographic observation which was carried out using an OLYMPUS GX71 optical microscope on RD-ND plane (RD: rolling direction) of terminated sample. In order to characteristic the evolution of microstructure and texture during current rolling process in detail, five equidistant areas along RD on RD-ND plane were examined by electron backscatter diffraction (EBSD), and were denoted as PA, PB (deformation strain: 18%), PC (31%), PD (39%) and PE (42%), respectively (Fig. 1b-f). The statistical results about the above microstructures were summarized in Fig. 2. This terminated sample was prepared by soft diamond polishing, followed by electropolishing in a 5:3 solution of C2H5OH and H3PO4 for 8min at 0.25A. EBSD measurements were performed on scanning electron microscopy (Zeiss) equipped with EBSD system. The indexing of Kikuchi patterns was identified by an accelerating voltage of 20keV, together with a working distance of 15mm and a sample title angle of 70º. In OIM software, the grain orientation spread (GOS) in each grain was determined by calculating the average misorientation between all points within the grain [17]. In this work, DRXed grains were identified by GOS values smaller than 2º, while deformed grains greater than 5º. Meanwhile, the grains were outlined by grain boundaries including the LAGBs (LAGBs: low angle grain boundaries ranging from 5º to 15º) and HAGBs (HAGBs: high angle grain boundaries ranging from 15ºto 180º) [7]. In addition, intragranular misorientation axis (IGMA) analysis was used to identify the geometrically necessary dislocation (GND) content in both deformed and DRXed grains, which was frequently described in detail elsewhere [10, 17, 18]. According to the literature [17], the dislocation type could be roughly defined by the rotation axis, and hence the activity Taylor axes [19] including basal/ and prismatic dislocations were investigated using IGMA analysis in this work. For example, the red lines in the legend of Fig. 1 represented the boundary misorientation with <10-10> rotations for basal dislocations, while blue lines with <0001> rotations for prismatic dislocations, and their rotation angles were 3º. Uniaxial tension tests were carried out using an Instron 5967 machine with an initial strain rate of 6.7×10-4 s-1 at room temperature. Tension samples whose gauges were 15mm in length and 4mm in width, were only extracted from PA and PE of ZK60 magnesium alloy sheets along RD and TD (TD: transverse

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direction). In order to ensure the repeatability of the results, three experiments were conducted for each condition. In addition, microhardness tests were performed on a Vickers indenter with a load of 200g and a loading time of 15s. The measure regions were on RD-ND plane and were 0.5mm from the surface of rolling plane in order to avoid the region of severe plastic deformation in the surface.

3 Results and discussion 3.1 Microstructure characteristics Fig. 1a shows the macroscopical metallographic results on RD-ND plane of terminated sample in which the shear bands increased gradually after PC. The details about microstructure evolution could be obtained in GOS maps (Fig. 1b-f). Clearly, both PA and PB presented a duplex structure characterized by massive coarse grains surrounded by a small quantity of small grains. Until PC, the yellow areas attained maximum, indicating that there existed abundant deformation microstructure in this area. With further strain in PD (39%) and PE (42%), a distinct necklace structure of small grains formed along the boundaries of coarse grains, which was a symbol of continuous dynamic recrystallization (CDRX) [10, 20]. After rolling, the grain size was refined from 16.4μm in PA to 7.9μm in PE (Fig. 2a), and the microstructure homogeneity was strikingly improved via CDRX based on the microstructure observation and error bar of average grain size (Fig. 2a). Interestingly, the variations of GOS values including ＜2º (DRXed grains) and ＞5º (deformed grains) suggested that the deformation strain of 31% in PC was a critical strain. More specifically, the fractions of deformed grains (GOS＞5º) gradually increased before PC but decreased after PC, while the fractions of DRXed grains (GOS＜2º) had a steep increase after PC. In rolling process, the deformation was mainly accommodated by basal slip before PC due to its lower critical resolved shear stress (CRSS). However, after PC, a larger deformation strain incompatibility occurred, which made the local internal stress exceeded the CRSS for non-basal slip. Thus non-basal slip was activated and its fractions would increase with the strain increased. It is well known that initiation of non-basal slip was a prerequisite condition for CDRX occurrence [21]. This finally led to the occurrence of CDRX and the sharply increased fractions of DRXed grains after PC. In order to demonstrate the formation of CDRX, the higher magnification of Area 1 indicated by white dashes in PD was shown in Fig. 2c. Clearly, there existed massive LAGBs at DRXed grains front and the sub-grains bounded partly by LAGBs and partly by HAGBs [22], which was the emblematic of CDRX. In addition, EBSD results indicated a small amount of boundary misorientation angles of 86.3º appeared in the microstructure (Fig. 2c). This suggested some

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fine grains were formed due to the DRX mechanism associated with twinning (TDRX) [23], which was completely understandable because of the complex stress state of rolling. It is well known that DRX behavior is a comprehensive outcome of many process parameters instead of depending on single parameter. That is to say, the critical process parameter for DRX was not a constant, but changed as another parameter changed. However, understanding DRX behavior under different process parameters was of vital importance, especially the determination of critical process parameter. This could help to obtain a finer and homogeneous microstructure, and finally improve the properties of magnesium alloys. In this work, the aim focused on the evolution of microstructure and texture in a single rolling pass, and under this rolling condition, the thickness reduction of 31% could be identified as a critical deformation strain for DRX. However, for other rolling process parameters, what the exact critical process parameters was not clear, and this would be the focus of magnesium alloys in the future.

3.2 Texture characteristics Fig. 3a shows the characteristics of (0002) basal plane in five areas. In PA, the (0002) basal plane spread along ND with small texture intensity of 4.73. With thickness reduction increased, the distribution of (0002) basal planes along ND strengthened and its texture intensity gradually increased. As a result, a strong (0002) basal texture formed with the maximum texture intensity of 7.82 in PE in which the c-axes within vast majority of grains were parallel to ND [7]. The above results about texture evolution was more directly observed in the ND inverse pole figures of Fig. 3b in which the texture intensity of <0001>//ND gradually increased from 3.66 in PA to 6.69 in PE. Fig. 4 shows the characteristics of (0002) basal plane distribution in five areas along various directions (RD, TD or ND) in detail. The process of obtaining the data in Fig. 4 could be divided into two steps: first, the (0002) Times Random along various directions was taken from pole figure (Fig. 3a); second, these values were subjected to normalization processing in which the values of Times Random divided by π

∫02 Iintensity cos θ dθ (θ is the angle away from RD, TD or ND) to acquire normalized (0002) pole density. Clearly, all the (0002) basal pole density distributions in five areas along RD were obviously strengthened in the angle range of 80º-90º, indicating that along RD the texture characteristics in these five areas varied little. In contrast, the (0002) basal pole density distributions along TD and ND varied greatly. More specially, with the increase of deformation strain, the maximum basal peak along TD obviously increased, especially in PE, which suggested that the preferred orientation was more pronounced in PE. For ND (Fig. 4c), although the maximum basal peaks in five areas varied little, their inclination angles of the maximum

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basal peak gradually approached the angle of 0º with the rolling process. This fully demonstrated that more and more c-axis was parallel to ND, which was consistent with the above results and prior literatures [24, 25]. Additionally, all the maximum basal peaks in five areas along RD were obviously larger than TD and ND, suggesting RD possessed a stronger basal texture. Fig. 5a shows the discrete inverse pole figures along RD in which the texture components were divided into DRXed grains (red dots) with GOS＜2º and deformed grains (blue dots) with GOS＞5º. The texture patterns in deformed grains were quantified by the spread angle away from the <0001> direction in inverse pole figures [26]. Clearly, the spread angles gradually changed from 71º-90º in PA to 81º-90º in PC, indicating the texture transformation from a relative weak texture in PA to an <10-10>-<11-20> arc texture in PC. With further deformation strain, this arc texture in PC transformed a hot spot around <10-10> in PD and PE, i.e. <10-10>//RD texture component. Analogous phenomenon of this texture in deformed grains was described by Honniball et al. [27] who reported that the stability of <10-10> grains towards prismatic slip locked them in place and then led to the formation of a single <10-10> fiber instead of <10-10>-<11-20> fiber in deformed state. In contrast, the texture component in DRXed grains with equiaxed morphology didn’t exhibit distinct preferred orientation, but a random orientation along the <10-10>-<11-20> arc in inverse pole figures which was more obvious at higher strain (PD and PE). In order to clearly describe the relationship of crystal orientation between the deformed grains and DRXed grains, the analyses of pole density were again applied to the (10-10) and (11-20) plane. In Fig. 5b and c the <10-10> of grain was parallel to RD, while in Fig. 5d and e the <11-20> of grain parallel to RD. Clearly, for deformed grains in Fig. 5b, two maximum pole density peaks appeared at 7º and 60ºaway from RD. However, DRXed grains exhibited a random distribution of (10-10) pole density in Fig. 5c. It could be seen from the sketch figure in Fig. 5c that the angles of 60º away from RD was parallel to the <10-10> of grains. Therefore, the two maximum pole density peaks in Fig. 5b represented that the <10-10> of deformed grains was parallel to RD (<10-10> //RD texture component), and this relationship became more obvious at the latter stage of rolling process (PE). Moreover, Fig. 5c demonstrated the <10-10>-<11-20> randomized texture component in DRXed grains. This results could also be obtained in Fig. 5d and e in which the <11-20> of deformed grain away from RD at the angle of about 30º and 84º (Fig. 5d) and DRXed grains still exhibited a random distribution of (11-20) pole density (Fig. 5e). In addition, what caused the 7º (Fig. 5b) and the 84º (Fig. 5d) instead of 0º and 90º might be ascribed to the angular

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deviation between the normal direction of rolling sheet and the compression direction of rolling mill during rolling process. C.D. Barrett et al. [17] reported that during extrusion process DRXed grains had two different types of fiber textures: a newly preponderant <11-20>-axis fiber and an inborn <10-10>-axis fiber. They also pointed out that the formation of <11-20>-axis fiber was ascribed to the gradual lattice rotation around the <0001> pole due to the activation of prismatic slip, while the formation of <10-10>-axis fiber came from abundant basal and/or GNDs by gradual rotation around their common <10-10> pole [17]. In view of this, in this work, the fractions of basal/ and prismatic dislocations were summarized according to Fig. 1b-f and shown in Fig. 6a. Clearly, the fractions of basal/ dislocations were always higher than prismatic dislocations in entire rolling process, fully indicating that the basal/ slip was the main deformation mechanism in rolling process. Meanwhile, this explained two aspects: first, basal plane gradually rotated towards RD-TD plane (Fig. 2a) with the rolling, which could be analyzed via Wang’s conclusion that the slip plane would gradually rotate toward the rolling plane [28]; second, because the activation of basal/ dislocations rotated the grain around <10-10> pole, their higher fractions retained the stable existing of <10-10>//RD texture in Fig. 5a [17]. Moreover, the fractions of prismatic dislocations maintained an average above 20% before PC, suggesting the lattice rotation around <0001> pole occurred during rolling process. Accordingly, the crystal orientation of new DRXed grain embryos originated by prismatic dislocations had a rotation angle of approximately 30º around <0001> pole with respect to their parent grains [17]. Therefore, the gradually increased fractions of misorientation angle (Fig. 6b) at around 30º with thickness reduction increased (from PA to PE) corroborated what prismatic slip contributed to the formation of DRXed nucleation. Importantly, both the fractions of basal/ and prismatic dislocations decreased after PC. It is well known that in CDRX the dislocations whose initiation dominates plastic deformation are rearranged into high-density LAGBs [29]. These LAGBs then progressively rotate such that the misorientation increases continuously and subsequently HAGBs form [29]. Thus, the decreased fractions of basal/ and prismatic dislocations after PC (Fig. 3a) demonstrated that these dislocations were gradually trapped by LAGBs which eventually converted to HAGBs (sign of the formation of new DRXed grain). The decreased fractions of misorientation angle at around 10º from PC to PE (Fig. 3b) verified the formation of HAGBs from LAGBs. In order to analysis the relationship of crystal orientation between deformed grains and DRXed grains, a higher magnification of Area 2 indicated by white dashes in

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PC was shown in Fig. 7 of inverse pole figure map. Clearly, this selected area was undergoing CDRX as the process continued by formation of sub-grains in deformed magnesium matrix. In Fig. 7, arrays of basal/ dislocations (red) and prismatic dislocations (blue) constructed the LAGBs (gray) and these dislocations exhibited a stochastic combination. As mentioned previously, the basal/ dislocations and prismatic dislocations produced misorientation by rotating the crystals around their Taylor axes. Many literatures reported that the total lattice rotation arising from active dislocations could be estimated to be an ⃗⃗𝑖 , where 𝑟𝑖 is the shear strain induced by arithmetic summation of all active dislocations: ⃗⃗⃗⃗⃗⃗⃗ 𝑇𝑛𝑒𝑡 = ∑𝑖 𝑟𝑖 𝑇 active dislocations, and ⃗⃗⃗⃗⃗⃗⃗ 𝑇𝑛𝑒𝑡 is a vector in the direction of the rotation axis and with a magnitude equal to the total amount of rotation [10, 30]. Therefore, the nucleation formed by abundant stochastic stored dislocations with a broad range of orientations, which was named “SSDs” by Aidin Imandoust [10]. This led to the nucleation with stochastic orientations and eventually a randomized texture developed. However, grain growth occurred after stage of nucleation. Aidin Imandoust et al. contrasted the growth behavior between <11-20>//RD grain and <10-10>//RD grain by Schmid factor analysis [10]. They pointed out that <11-20>//RD grain had lower Schmid factor value and lower dislocation density in deformed state, while <10-10>//RD grain higher Schmid factor and higher dislocation density [10]. This produced a dislocation density gradient that applied a force to the boundary toward the grain with higher dislocation density [10]. Therefore, the <11-20>//RD grain grew easily and this type of crystal orientation would be dominated texture component in the case of static recrystallization [31]. But in DRX, the DRXed grain continued to experience plastic deformation, and this caused its <10-10> gradually to turn to RD [29, 31]. This prediction could be verified in Fig. 7 including a deformed grain and four DRXed grains born from this deformed grain. The (0002) pole figure demonstrated that the deformed grain and four DRXed grain had same orientation of basal plane, while their crystal orientation spread along <10-10>-<11-20> arc in inverse pole figure. Here, DGA could be regarded as a newborn DRXed grain due to its fine grain size and its <11-20> was parallel to RD (<11-20>//RD grain). Moreover, the degree of experiencing plastic deformation of DRXed grain could be characterized by its grain size, i.e. the larger grain size was, the greater degree of experiencing plastic deformation occurred. Therefore, in Fig. 7, the sequence in grain size DGA＜DGB (DGB≈DGC) ＜DGD indicated the sequence in the degree of experiencing plastic deformation DGA＜DGB (DGB≈DGC) ＜DGD. This was roughly proved by the dislocation amounts in these grains in which DGD grain had most dislocations (Fig. 7). Clearly, with deformation increased, the

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crystal orientation of DRXed grain gradually rotated toward <10-10> pole from <11-20> pole and finally established an arc <10-10>-<11-20> in RD inverse pole figure coupled with deformed grain. Therefore, the <11-20>//RD texture arising from CDRX together with the effect of deformation on crystal orientation led to the occurrence of randomized texture along <10-10>-<11-20> arc, especially at the latter stage of rolling process. Moreover, activation of abundant stochastic stored dislocations with a broad range of orientations into LAGBs and HAGBs in CRDX should be the essential reason.

3.3 Mechanical properties Fig. 8 shows the microhardness distribution on RD-ND plane along RD. Clearly, the microhardness gradually increased before PC but decreased after PC, which agreed well with the evolution of average GOS from PB to PE. As mentioned above, the fraction of deformed grains exhibited similar variation. It is well known that the deformed grains generally possess high density dislocations, and then lead to a higher 𝜂

strain hardening. According to the literatures [32, 33], the microhardness could be calculated as 𝐻𝑣 = 𝐾𝜀𝑒𝑞 , where 𝐾 is materials constant, 𝜀𝑒𝑞 is the true strain and 𝜂 is the hardenability exponent as the reference of the degree of strain hardening or strain softening. Therefore, the increased fraction of deformed grains (the increased strain hardening) would increase the microhardness before PC while the decreased fraction of deformed grain would decrease the microhardness after PC. Interestingly, it could be seen that the microhardness in PA and PE didn’t agree well with the above trend. This could be ascribed to the grain size effect. Jiang had reported that the fine grains in adiabatic shear band of ZK60 magnesium alloys produced higher microhardness [34]. As a result, the lower microhardness in PA was mainly ascribed to the coarse grains and the higher microhardness in PE the fine grains. Fig. 9a shows the stress-strain responses of ZK60 magnesium alloy sheets (PA and PE) along RD and TD in tension test at room temperature. Their yield stress (YS, MPa), ultimate tension stress (UTS, MPa), uniform elongation (UE-determined using considére construction: dlnσ/dε=1, %), and fracture elongation (FE, %) are summarized in the insert of Fig. 9a. Both yield stress along RD in PA and PE were higher than TD. This was ascribed to the texture difference. As mentioned above, RD possessed a stronger basal texture than TD, and so the basal slip or (10-12) tension twin activated easily during tension test along TD, which would result in lower stress (YS and UTS) but higher elongations (UE and FE) along the TD [35]. It could also be seen from Fig. 9a that the increase degree of YS was different between RD (about 10MPa) and TD (about 26MPa). As commonly reported, the YS in magnesium alloy had close relation to the grain size and texture [26, 36]. In this work, the grain size was significantly refined after rolling

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(16.4μm in PA and 7.9μm in PE). Therefore, the fine grain size in PE would make the YS increase in both directions simultaneously [37]. Therefore, the difference in the increase degree of YS between RD and TD was mainly ascribed to their different texture variation. Texture results demonstrated that the basal texture in PE along RD varied little, but that along TD was obviously strengthened (Fig. 4). Strong basal texture would increase the difficulty of activation of basal slip or (10-12) tension twin. Thus, the greater increase in PE along TD also benefited from the formation of strong basal texture besides the effect of grain refinement. Compared to the YS, the UTS exhibited a negligible dependence on grain size and texture variation in present work. In terms of the variation of the elongation (FE and UE), the FE increased along RD (from 12% to 16.2%) but decreased along TD (from 19.2% to 16.7%), while the UE varied little along RD (about 9%) but decreased along TD (from 16.5% to 12.2%). It was usually considered that the fracture elongation could be divided into two parts: uniform elongation and post-uniform elongation [38]. In addition, according to the literature [39], uniform elongation was enhanced with the increase of strain hardening rate. Moreover, with weakened basal texture, the softening behavior of dynamic recovery arising from the cross-slip of dislocations could be restricted, which led to the increase in the strain hardening ability [40]. In order to clearly describe the influence of microstructure (mainly texture) on uniform elongation, the strain-hardening curves using the plot (Fig. 9b) were derived from flow curves in Fig. 9a, where ,  and =d/dreferred to stress, strain and strain-hardening rate, respectively. Many literatures had analyzed the deformation mode using this method [7, 26, 41] in which the curve shapes could be quantified as the negative slop Ksl whose lower values generally corresponded to higher strain hardening ability. Clearly, the negative slops in Fig. 9b demonstrated two important points: first, the slops along TD were higher than RD; second, the slops in PA were higher than PE, especially along TD. Chen suggested that the negative slop exhibited a strong dependency upon texture and became gentler (lower Ksl value) with basal texture weakening [5]. As a result, the weaker texture along TD possessed lower Ksl value and higher strain hardening ability which made its UE higher than RD. Meanwhile, with the rolling process, the stable basal texture state along RD resulted in a small change in UE, while obviously strengthened basal texture along TD decreased the UE. In addition, fine grain size had a positive influence on post-uniform elongation by affecting the contribution of grain boundary sliding [42]. The finer grain size in PE promoted the post-uniform elongation, finally leading to the increase of fracture elongation along RD and the stable fracture elongation along TD by mitigating the decreased uniform elongation due to the effect of strong

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basal texture.

4 Conclusions In this work, a terminated sample was obtained by ceasing the rolling process in the middle region of ZK60 magnesium alloy sheet. The evolution of microstructure and texture of this terminated sample was investigated, especially the relationship of crystal orientation between deformed grains and DRXed grains. Meanwhile, the effect of the microstructure and texture variation on microhardness and mechanical properties were analyzed. The main conclusions were as follows: (1) Thickness reduction of 31% was a critical strain for the occurrence of CDRX under current rolling condition. CDRX strikingly refined the grain size and improved the microstructure homogeneity. (2) The basal plane gradually rotated toward the rolling plane along TD during rolling process. The results based on IGMA analysis indicated that the activation of abundant stochastic stored dislocations with a broad range of orientations into LAGBs and HAGBs in CDRX was the main reason of the transformation from <10-10>//RD texture mainly arising from deformation to randomized texture along <10-10>-<11-20> arc in inverse pole figure. (3) Fraction of deformed grains with high density dislocations in conjunction with grain size had remarkably influence on microhardness distribution and the former was the dominant. Grain refinement and strong basal texture had positive effect on the increase of yield stress. Weaker texture produced a higher strain hardening ability, and then improved the uniform elongation, while grain refinement mainly contributed to the post-uniform elongation.

Acknowledgement This work was supported by National Natural Science Foundation of China (Grant No. 51401064), Sci–tech Development Project in Shandong Province (Grant No. 2014GGX10211), Sci–tech Major Project in Shandong Province (Grant No. 2015ZDJQ02002) and Fundamental Research Funds for the Central Universities (Grant No.HIT.NSRIF.2016109).

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Fig. 1 Microstructure evolution during a single rolling process: (a) metallographic observation on the RD-ND plane of terminated sample with five equidistant areas along RD for EBSD; (b)-(f) grain

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orientation spread (GOS) maps in corresponding five areas

Fig. 2 Variation of average grain size in five areas (a); fractions variation of deformed grains (GOS＞5º) and DRXed grains (GOS＜2º) in five areas (b) and higher magnification of Area 1 indicated by white dashes in Fig. 1e (c)

Fig. 3 (0002) basal pole figures (a) and corresponding ND inverse pole figures (b) in five areas

Fig. 4 (0002) pole density distributions of ZK60 magnesium alloy sheets in five areas along RD (a), TD (b)

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and ND (c)

Fig. 5 Discrete inverse pole figures along RD (a); (10-10) pole density distribution in deformed grains (b) and DRXed grains (c); (11-20) pole density distribution in deformed grains (d) and DRXed grains (e) (In discrete inverse pole figures, deformed grains (GOS＞5º) and DRXed grains (GOS＜2º) were denoted in blue and red, respectively.)

Fig. 6 Fractions of basal/ and prismatic dislocations according to IGMA analysis (a) and distribution of misorientation angle in five areas (b)

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Fig. 7 High magnification of Area 2 indicated by white dashes in Fig. 1d with inverse pole figure map (In RD inverse pole figures, Red dots represent the crystal orientation of DGA; yellow dots of DGB and DGC; green dots of DGD and blue dots of deformed grain)

Fig. 8 Microhardness distribution on RD-ND plane along RD and the evolution of average GOS values in five areas

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Fig. 9 Room temperature strain-stress curves along RD and TD in PA and PE (a) and their corresponding hardening curves (b) (YS: yield stress; UTS: ultimate tension stress; UE: uniform elongation; FE: fracture elongation)

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