Grain refining and improving mechanical properties of a warm rolledAZ31 alloy plate

Hua Zhang a,b,n, Wei Jin a,b, Jianfeng Fan a,b, Weili Cheng a,b, Hans Jørgen Roven c,d,Bingshe Xu a,b, Hongbiao Dong e

a Key Laboratory of Interface Science and Engineering in Advanced Materials, Ministry of Education, Taiyuan University of Technology, Taiyuan 030024, Chinab Shanxi Research Center of Advanced Materials Science and Technology, Taiyuan 030024, Chinac Department of Materials Science and Engineering, Norwegian University of Science and Technology (NTNU), Trondheim 7491, Norwayd Center for Advanced Materials, Qatar University, P.O. Box 2713 Doha, Qatare Department of Engineering, University of Leicester, Leicester LE1 7RH, UK

a r t i c l e i n f o

Article history:Received 11 June 2014Accepted 19 July 2014Available online 1 August 2014

Keywords:Metals and alloysPre-introduced twinsRecrystallizationMechanical properties

a b s t r a c t

The mechanical properties of an AZ31 magnesium alloy were improved after warm rolling. The studyinvolved a pre-rolling condition in which the material consisted of twins, playing a key role inrecrystallization grain refinement upon subsequent processing. The pre-existing twin boundariesincreased the number of nucleation sites for recrystallization during warm rolling, and resulted incomplete recrystallization and weakening of the basal texture during subsequent annealing. This led toimproved mechanical properties of the warm rolled plates.

& 2014 Elsevier B.V. All rights reserved.

1. Introduction

As the lightest structural material, magnesium alloys havepromising potential application in the fields of aerospace,automotive and electronic communication technologies [1,2,3].However, their poor plasticity and ductility restrict wide applica-tions. This unfavorable intrinsic characteristic is due to the limitednumber of operative slip systems, owing to a hexagonal closepacked (hcp) crystal structure [4]. In order to improve theirmechanical properties, there have been extensive efforts to modifythe microstructure by applying different technologies [5–7].Recently, a new method, namely pre-twinning, was applied toimprove the strength [8], rolling capability [9] and formability [10]of magnesium alloys. This is mainly due to the fact that changes inthe microstructure induced by twins can affect the deformationbehavior. First, a large number of existing twin boundaries canrefine the grain structure [8]. Second, grain rotations caused bytwinning translate texture [9,10].

Twinning not only affects the mechanical properties of magne-sium alloys but twins do also play an important role in recrys-tallization [11,12]. Recrystallization develops preferentially within

areas containing twins, but also at intersections between grainboundaries and twins. Hence one can claim that twins play a roleas preferred nucleation sites for recrystallization [12,13]. Being animportant softening and grain refining mechanism, recrystalliza-tion is vital for controlling the mechanical properties throughmicrostructure development. It is expected that, compared withtraditional magnesium alloy conditions, plates containing twinsare able to produce more potential nucleation sites for recrystalli-zation during subsequent rolling. This issue, however, has not beenreported so far. Therefore, the present work on a warm rolled AZ31alloy plate presents a novel method, in which pre-introducedtwins are utilized for recrystallization grain refinement andimproved mechanical properties.

2. Experiment procedures

As-received AZ31 (Mg-3%Al-1%Zn, by weight) alloy plateshaving a thickness of 30 mm were used. The as-received (AS)plate was machined to samples having the dimensions 100 mm(RD)�15 mm (TD)�30 mm (ND), where RD, ND and TD representthe rolling, the normal and the transverse direction, respectively.First, samples were pre-rolled along TD to a strain of 6% at roomtemperature to introduce twinning. Then, subsequently annealedat 200 1C for 6 h for removing dislocations while keeping the twinstructures [9]. The pre-rolled and subsequently annealed sampleswere labeled PR and PRA samples, respectively. In the next step,

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Materials Letters

http://dx.doi.org/10.1016/j.matlet.2014.07.1300167-577X/& 2014 Elsevier B.V. All rights reserved.

n Corresponding author at: Key Laboratory of Interface Science and Engineeringin Advanced Materials, Ministry of Education, Taiyuan University of Technology,Taiyuan 030024, China. Tel.: þ86 351 6014852; fax: þ86 351 6010311.

E-mail address: zhanghua2009@tyut.edu.cn (H. Zhang).

Materials Letters 135 (2014) 31–34

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more AS and new PRA samples were machined to specimenshaving the dimensions 100 mm (RD)�15 mm (TD)�3 mm (ND)for warm rolling. Before rolling, the AS and PRA samples were pre-heated in a furnace at 200 1C for 30 min. Then, the heated sampleswere directly rolled along RD by a reduction of 25%. After rolling, afinal anneal at 250 1C for 30 min were conducted in a similarfurnace. Subsequently, the established microstructures and tex-tures were examined by electron backscatter diffraction (EBSD) ina Zeiss Supra 55VP FEGSEM. Tensile specimens having nominalgage length dimensions 30 mm (length)�5 mm (width)�2 mm(thickness) were machined along RD by wire-cutting. Uniaxialtensile tests were carried out in a CMT6305–300KN electronicuniversal testing machine using a strain rate of 1�10�3 s�1 atroom temperature.

3. Results and discussion

Fig. 1 shows the (0002) pole figures and EBSD images of the AS,PR and PRA samples. It is seen that a completely recrystallizedmicrostructure, without twins, was present in the AS sample andthe average grain size, measured by an orientation imagingmicroscopy (OIM) software, was �26.2 μm. After pre-rolling alongTD, a large number of {10–12} tension twin boundaries formed inthe PR sample, and {10–11} compression twins and double twinboundaries were nearly absent, as shown in Fig. 1b. Generally, formagnesium alloys, {10–12} tension twinning easily takes placeunder compression load perpendicular to the c-axis of grains orunder tensile load parallel to the c-axis. This is due to the lowcritical resolved shear stress (CRSS) being of about 2–2.8 MPa [14].Hence, the compression load perpendicular to the c-axis of grainsduring the pre-rolling operation led to the formation of {10–12}tension twins. Furthermore, multiple tension twins having thesame orientation were parallel to each other within a grain. TheEBSD data indicated that the volume fraction of tension twins wasabout 57% for the PR sample. After annealing the rolled sample at200 1C for 6 h, no new grains were observed and the volumefraction of tension twins remained at 57% in the PRA sample(Fig. 1c). This indicates that the twin structure remained

unchanged during annealing, which is consistent with the resultsof previous studies [9,10].

Fig. 1a shows that the AS sample exhibited a typical basaltexture with the c-axis in the majority of grains being parallel toND. The occurrence of tension twinning caused grains to rotate by�861 during pre-rolling along TD, and thus resulted in theformation of a crystallographic texture having the c-axis // TD(Fig. 1b). This means that basal planes in twinned areas wereoriented nearly parallel to TD. Compared with the AS samplehaving a maximum basal texture intensity of 16.0, a much weakerbasal texture intensity (maximum 7.1) was obtained for the PRcondition. The reduced intensity was due to the formation ofc-axis // TD texture. Further, since the twin structure remainedunchanged after annealing (Fig. 1c), the distribution of texture inthe PRA sample was almost the same as that in the PR condition.

As shown in Fig. 2a and b, after rolling along RD at 200 1C, thedeformed microstructure consisted of numerous twins, i.e. both inthe rolled-AS and rolled-PRA samples and no new recrystallizedgrains were found. Due to the faceting-roughening phase transi-tions, the properties of twin grain boundaries strongly depend ontemperature suggested by Straumal et al. [15,16]. Actually, owingto the formation of a large number of twins, no clear grainboundaries were observed in the rolled-PRA sample (Fig. 2b).Compared with the rolled-AS sample (Fig. 2a), the rolled-PRAsample contained a higher density of deformation twins and alsotwin intersections resulting from a large number of pre-existingtwin boundaries, indicating that there are even more potentialnucleation sites for recrystallization [11–13]. This can lead to thedevelopment of finer recrystallized grains during subsequentannealing. Further, it is seen from Fig. 2c that a fine equiaxedgrain microstructure is accompanied by some coarse non-recrystallization grains in the annealed rolled-AS sample. More-over, some twins were observed in the coarse non-recrystallization grains in this condition. While as shown inFig. 2d, the annealed rolled-PRA sample had a fine and homo-geneous grain microstructure without twins. This is due to thecomplete recrystallization, facilitated by the large number ofnucleation sites at pre-existing twin boundaries formed duringwarm rolling. Moreover, the average grain sizes of the annealedrolled-AS and annealed rolled-PRA samples, measured by the OIM

Fig. 1. The (0002) pole figures and EBSD images of various samples: (a) as-received (AS), (b) pre-rolled (PR) and (c) pre-rolled and annealed (PRA) condition.

H. Zhang et al. / Materials Letters 135 (2014) 31–3432

software, were �12.3 and �7.5 μm, respectively. It can be statedthat the occurrence of complete recrystallization led to theobserved small average grain size of the annealed rolled-PRAsample. In addition, compared with the annealed rolled-AS sam-ple, the annealed rolled-PRA sample exhibited a somewhat wea-kened basal texture intensity being reduced from 9.7 to 6.3. This isprobably due to the fact that non-recrystallized grains have a moreintensive texture than recrystallized grains [17,18].

True stress-strain curves of various samples are shown in Fig. 3.The corresponding mechanical properties, i.e. 0.2% yield stress (YS),the ultimate strength (UTS) and failure elongation (FE) are summar-ized in Table 1. The table also contains the corresponding averagegrain sizes. The decrease in grain size will increase the resistance todislocation motion, and hence results in an increased YS. Butcompared with the AS sample, the annealed rolled-AS and annealedrolled-PRA samples exhibited slightly larger YS, which can beattributed to the observed weakened basal texture. Since a weakbasal texture can result in a larger Schmid factor for basal oa4 slip,and thus leads to a decreased YS [19], the improvement in the YS ishere less pronounced. The UTS of the annealed rolled-PRA sampleincreased from 302 to 320 MPa compared with that of the annealedrolled-AS sample, due to grain refinement strengthening resultingfrom the completely recrystallized fine grains. In addition, the FE ofthe annealed rolled-PRA sample increased to 21.7%, which can beattributed to the weakened basal texture intensity. This is because astrong texture places most grains in hard orientations, i.e. since theresolved shear stress in the basal plane is relatively small, resulting instress localization and premature shear failure (here condition AS)[20,21]. Furthermore, grain refinement can also contribute to theobserved increased FE in the annealed rolled-AS and annealedrolled-PRA samples.

4. Conclusions

Pre-existing twin boundaries introduced by pre-rollingincreased the number of nucleation sites for recrystallizationduring subsequent warm rolling. This resulted in a completelyrecrystallized structure and a texture weakening during subse-quent annealing. Grain refinement strengthening and a homoge-neous microstructure led to an increased tensile strength. Due tothe fine grain microstructure and accompanying weakened basaltexture, improved ductility was observed in the pre-rolled pluswarm rolled AZ31 alloy plate after annealing. Both the fine grain

Fig. 2. Optical microstructures of (a) rolled-AS sample and (b) rolled-PRA sample. Corresponding EBSD images and (0002) pole figures of (c) annealed rolled-AS sample and(d) annealed rolled-PRA sample.

Fig. 3. True stress-strain curves of various samples.

Table 1Mechanical properties of various samples.

Sample YS (MPa) UTS (MPa) FE (%) Grain size (μm)

AS 142 280 18.5 26.2Annealed rolled-AS 152 302 19.6 12.3Annealed rolled-PRA 157 320 21.7 7.5

H. Zhang et al. / Materials Letters 135 (2014) 31–34 33

structure and the weakened basal texture were induced by pre-introduced twins.

Acknowledgments

This work is supported by Scientific and Technological Innova-tion Programs of Higher Education Institutions in Shanxi(2014115), Program for New Century Excellent Talents in University(NCET-12–1040), National Natural Science Foundation of China50901048 and 51174143) and Shanxi Province Science Foundationfor Youths (2013021013-4).

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Grain refining and improving mechanical properties of a warm rolled AZ31 alloy plateIntroductionExperiment proceduresResults and discussionConclusionsAcknowledgmentsReferences