Recrystallization and Microstructural Evolution During ... - Springer Link

Met. Mater. Int., Vol. 20, No. 3 (2014), pp. 489~497 doi: 10.1007/s12540-014-3012-7

Recrystallization and Microstructural Evolution During Hot Extrusion of Mg97Y2Zn1 Alloy Bin Chen*, Xiaoling Li, Chen Lu, and Dongliang Lin Shanghai Jiao Tong University, School of Materials Science and Engineering, Shanghai 200240, China (received date: 25 March 2013 / accepted date: 4 October 2013) This study revealed that the extrusion temperature has a great influence on microstructure and mechanical properties of the Mg97Y2Zn1 alloy. The average grain sizes increased from 3 µm to 8 µm with increasing extrusion temperatures from 623 K to 773 K. Both dynamic recrystallization (DRX) and static recrystallization (SRX), which occur during and after deformation, respectively, were observed. The alloy, which extruded at a relatively high temperature, exhibited lower strength because the strain strengthening was balanced by the softening that originated from DRX. Three types of morphologies, namely, big recrystallized grains, fine recrystallized grains, and non-recrystallized grains, were observed in the extruded microstructures obtained at 623 K. The dislocation density was quite high in the fully recrystallized grain. The extruded microstructures obtained at 773 K were composed of large grains with more uniform size. Their degree of recrystallization was higher and the dislocation density also declined. All dislocation in the grain were distinguished as dislocations. Submicron scale precipitates were distributed along the newly formed recrystallized grain boundaries and had a remarkable pinning effect on the recrystallized grain growth after extrusion at 773 K. The precipitates can be divided into two main types: mixed type and single type. Key words: alloy, extrusion, dislocation, recrystallization, transmission electron microscopy (TEM)

1. INTRODUCTION Although Mg alloys are attractive as light-weight structural materials for the automotive and aerospace industries [1,2], few applications for wrought Mg alloys have been explored because of a number of drawbacks, such as lack of suitable alloys and lack of sufficient independent slip systems imposed by their hexagonal crystal structure [1]. Using more magnesium would significantly decrease the weight of automobiles, which is one of the important goals in automobile design [3]. However, usage of magnesium lags far behind that of aluminum. In order to compete with Al alloys, the 0.2% proof strength of a wrought Mg alloy needs to be improved to over 300 MPa with a tensile elongation of over 10% [4]. Rare-earth (RE) additions to Mg alloys have been widely investigated for the development of high-strength Mg alloys. Mg-Zn-RE alloys with a long period stacking order (LPSO) phase exhibits high strength and good ductility both at room and high temperatures [5]. Investigations on structural models and transformation of the LPSO phases have also been reported [6,7]. Much attention has been paid to the Mg97Y2Zn1 alloy in the past ten years. It was found that Mg97Y2Zn1 (at%) pro*Corresponding author: [email protected] ©KIM and Springer

cessed by rapid solidified powder metallurgy (RS/PM) has excellent mechanical properties with high yield strength (YS) of 610 MPa and elongation of 5% at room temperature [8]. Its high strength was thought to be due to strengthening by grain refinement, solid solution, fine Mg24Y5 compound particles, and introduction of a high density of plane faults caused by the formation of the LPSO structure [8]. Deformation kinking has also been found to play an important role in both the strength and plasticity of the Mg97Y2Zn1 alloy [9]. The Mg97Y2Zn1 alloy processed by extrusion and equal channel angular pressing (ECAP) obtains ultrafine grain size and exhibits excellent mechanical properties [10]. However, the potentially dangerous powder process of RS/PM is not acceptable for the production of commercial products, and ECAP cannot be easily scaled up to the commercial production of large structural products. In particular, these kinds of processing are costly compared with thermo-mechanical processing, which is a commonly used way to fabricate automobile and aerospace parts at a competitive cost. It is well known that thermomechanical processing of the Mg alloys, such as extrusion, forging and rolling, is very effective. In addition, hot extrusion is a practically promising way to produce Mg profiles. The strength and ductility of Mg alloys can be improved significantly due to the fine grains associated with dynamic recrystallization (DRX), texture, and dynamic precipitation [11].

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Grain refinement based on an understanding of the dynamic recrystallization mechanisms during thermo-mechanical processing has made a significant contribution to improving the strength of Mg alloys. The DRX of magnesium alloys has been investigated by many researchers [12-16], and an understanding of DRX is essential to obtaining desirable microstructures and mechanical properties. The aim of the present work is to study recrystallization and microstructural evolution during hot extrusion of the Mg97Y2Zn1 alloy. We discuss the influence of the extrusion temperature on the microstructure and mechanical properties of the Mg97Y2Zn1 alloy the relationship between the microstructure and the mechanical properties.

Fig. 1. Optical micrograph of as-cast Mg97Y2Zn1.

2. EXPERIMENTAL PROCEDURES The Mg97Y2Zn1 alloy was prepared by electric melting of high purity Mg, Zn and the Mg-Y master alloys under a protecting gas (0.3%SF6 and 99.7%CO2) in a steel crucible and casting them into a steel mold. The ingots were homogenized at 833 K for 12 h and then air cooled. Hot extrusion was done with an extrusion ratio of 10.24:1 at different temperatures from 623 K to 773 K. The tensile tests were conducted on a Zwick electronic universal material testing machine at room temperature. The compositions of the alloys were determined by an inductively coupled plasma-atomic emission spectrometry (ICP-AES). The phase analyses were carried out with a D/max 2550V X-ray diffractometer (XRD). The microstructure was characterized by optical microscopy (OM) and transmission electron microscopy (TEM). The specimens for TEM were prepared by grinding-polishing the sample to produce a foil of 50 μm thickness followed by punching 3 mm diameter disks. The disks were ion beam thinned to a thickness of electron transparency.

Fig. 2. An X-ray diffraction pattern of the as-cast Mg97Y2Zn1 alloy.

3. RESULTS AND DISCUSSION 3.1 Microstructure Figure 1 shows the optical micrographs of the as-cast Mg97Y2Zn1 alloy. It is obvious that the micrograph of the as-cast Mg97Y2Zn1 has typical dendritic morphology. The XRD pattern indicates that the Mg97Y2Zn1 alloy is mainly composed of a matrix phase and X-Mg12ZnY phase (a long-period 18R modulated structure), as shown in Fig. 2. The semi-continuous network XMg12ZnY phase is distributed along the Mg matrix phase boundary. Figure 3 shows the OM microstructures of the Mg97Y2Zn1 alloy extruded at different temperatures. It can be seen that grain refinement due to extrusion is achieved in both alloys compared to their billets with the grain sizes of 100 μm. The average grain sizes of the alloys increased from 3 μm to 8 μm with the increase of extrusion temperatures. The grain refinement is undoubtedly a consequence of dynamic recrystalliza-

Fig. 3. The microstructure of Mg97Y2Zn1 alloy after extrusion by different temperatures (a) 623 K, (b) 673 K, (c) 723 K, and (d) 773 K.

tion (DRX). It is obvious that the DRX process is almost completed and the high volume fraction of equiaxed fine grains were obtained after extrusion at 623 K, as shown in Fig. 3(a). The volume fraction of DRX grains increased with

Recrystallization and Microstructural Evolution During Hot Extrusion of Mg97Y2Zn1 Alloy

an increasing temperature. With an increased extrusion temperature, the grain size became larger because the high temperature drove the migration of the DRX grain boundaries. As Fig. 3 shows, there are two kinds of zones in the extruded alloys: X phase-rich and X phase-poor zones. The grain sizes in the X phase-rich zones are much smaller than in the X phase-poor zones (in Figs. 3(c) and (d)), which indicates that dynamic recrystallization in the Mg97Y2Zn1 alloy is sensitive to the X phase content on a local scale in the alloys. For instance, the grain size of the alloy extruded at 773 K is also inhomogenous with small grains of about 4 μm and large grains of 10 μm as shown in Fig. 3(d). Between closely packed phases in the X phase-rich zones, there are many recrystallized grains smaller than 2 μm, 3 μm. 3.2. Mechanical properties Figure 4 represents the comparisons of the tensile properties of as-cast and extruded Mg97Y2Zn1 alloys. It shows that the Mg97Y2Zn1 alloy after extrusion, as compared to the ascast alloy, exhibits excellent mechanical properties with both the strength and elongation increased significantly. The yield strength (YS) and the ultimate tensile strength (UTS) of the extruded Mg97Y2Zn1 alloy specimen are about 250 and 350 MPa, respectively, with an elongation to failure (ε%) of 7-12; these values are much higher than those of the as-cast alloy (YS = 96 MPa, UTS = 177 MPa, ε% = 5.8). The enhanced strengths have been attributed to the grain refinement, solid solution strengthening, strain strengthening, distribution of fine Mg24Y5 phase and high density of the long period stacking structures formed by hot extrusion [10]. The reason for strain strengthening is that the increased dislocations are interacted and tangled in non-recrystallized areas after extrusion [17]. The average strain strengthening seemed to be higher at lower temperatures, where strength was higher. This is due to the fact that the volume fraction of non-recrystallized areas of the alloys obtained at low temperatures was higher than that at high temperatures. The alloy that extruded at a relatively

Fig. 4. Comparison of the tensile properties of as-cast and extruded Mg97Y2Zn1 alloys after extrusion at different temperatures.

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Fig. 5. Typical appearance of the end from extruded sample.

high temperature exhibited lower strength because the strain strengthening was balanced by the softening that originated from full DRX. With an increasing temperature, the strength decreased and elongation increased because more and more tangled dislocations were decreased by DRX. 3.3. Microstructural evolution analysis In order to investigate the microstructural evolution during extrusion, the microstructures of nine areas were investigated at the end of the extruded sample, as shown in Fig. 5. The microstructural evolution of the samples extruded at 623 K and 773 K were analyzed as follows. Figure 6 shows the microstructural evolution of the Mg97Y2Zn1 alloy during the extrusion at 623 K. The figures from (a) to (i) in Fig. 6 correspond to areas 1 to 9 in Fig. 5, respectively. Figures 6(a) and (b) illustrate that, even before entering the deformation zone, significant microstructural changes had occurred: twins are clearly apparent, which was due to high pressure being applied by a plunger. The profuse twins appeared in original grains due to limited available slip systems at a relatively low temperature. At area 3, a few small recrystallized grains nucleated along some original grain boundary areas and twinned areas. The original grain boundary areas and twinned areas had much higher stored deformation energy than the matrix and therefore were favorable nucleation sites for DRX. However, the fraction of recrystallized grains was negligible and no significant grain refinement took place. Figures 6(d)(f) illustrate that, with increasing strain, the volume fraction of recrystallized grains increased progressively. Besides the equiaxed grains, the original grains that appear to be elongated toward the extrusion direction indicate that DRX remained incomplete even when extrusion processing was complete. However, the fraction of these original grains was much smaller than that of the recrystallized grains, and it did not exceed 20% on average. At areas 7-9, the material had left the deformation zone, but the grain size in some non-recrystallized areas was refined further due to the static recrystallization (SRX) [18].

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Fig. 6. The microstructure evolvement of Mg97Y2Zn1 alloy at (a) area 1, (b) area 2, (c) area 3, (d) area 4, (e) area 5, (f) area 6, (g) area 7, (h) area 8, and (i) area 9 during extrusion at 623 K.

Fig. 7. The microstructure evolvement of Mg97Y2Zn1 alloy at (a) area 1, (b) area 2, (c) area 3, (d) area 4, (e) area 5, (f) area 6, (g) area 7, (h) area 8, and (i) area 9 during extrusion at 773 K.

Therefore, we concluded that the main recrystallization mechanism was the DRX and SRX, which occurred during and after deformation, respectively. Figure 7 shows the microstructural evolution of the Mg97Y2Zn1 alloy during the extrusion at 773 K. The formation of twins was scarcely detected at area 1, while some fine lamellar structures were observed instead. It is obvious that the fine lamellar structures appeared during preheating at 773 K. The fact illustrates that the X-phase precipitated from the matrix on the basal plane (0001) because of the diffusion of yttrium and zinc at high temperature [19]. At areas 2 and 3, a few small

recrystallized grains appeared along some original grain boundary areas. Figure 7(d) reveals that only small scale recrystallization occurs in the alloy, while most parts of the sample exhibit a deformed microstructure and many grains are elongated. With an increasing strain, the volume fraction of the recrystallized grains increasesd progressively and the grain size also increased. It seems that the DRX sped up at 773 K in comparison with extrusion at 623 K. At the temperature, (a+c) dislocation slips and dislocation climb operated to accelerate the DRX. Besides the equiaxed grains, relatively large non-recrystallized grains that appear to be elongated


toward the extrusion direction appeared, indicating that DRX remained incomplete for some grains even when the extrusion processing was complete and left the deformation zone. However, the fraction of these non-recrystallized grains was much smaller than that of the recrystallized grains. The nonrecrystallized grains of the deformed microstructure were replaced by new recrystallized grains in the subsequent SRX process, as shown in Fig. 7(g)-(i). To investigate in more detail the recrystallization and microstructural evolution in the Mg97Y2Zn1 alloy, TEM was used to examine two samples extruded at 623 K and 773 K. The extruded microstructures obtained at 623 K were heterogeneous and appeared to be occupied by both non-recrystallized and recrystallized areas. The grain morphology varied dramatically in different areas. Three types of morphologies, namely, big recrystallized grains, fine recrystallized grains, and non-recrystallized grains, were observed, as shown in Fig. 8. In the sample extruded at 623 K, areas with a fully recrystallized microstructure were most frequently found, as shown in Fig. 8(a). However, in some other areas, the microstructure appears to be nanometer-sized grains of mostly about 50-100 nm surrounded by non-recrystallized grains. Figure 8(b) shows such a microstructure in the sample. The corresponding selected area electron diffraction (SAED) pattern represents the coexistence of very fine discontinuous ring patterns, brighter discontinuous ring patterns and spot patterns, as shown in the inset. This confirms the co-existence of recrystallized grains together with non-recrystallized grains in this region. Although the microstructure is less frequently observed, it serves as an

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indication that the extruded microstructure is inhomogeneous. Figure 9(a) shows a bright field image of a fully recrystallized grain that was obtained at 623 K. As can be seen from Fig. 9(a), there is fine lamellar structure inside the grain. The SAED pattern shows that the extra reflection-spots formed a lighter bright line in the c*-direction, which can be identified as an LPSO structure [19]. Its dark field image is shown in Fig. 9(b). Figures 9(c) and (d) are dark field images taken under two-beam diffraction conditions using g=0110 and 0002, respectively. The dislocation density is quite high in the grain with a well-developed LPSO phase. Once dynamic recrystallization had taken place and new grains had formed, the dislocation density decreased sharply at these locations. Due to the incomplete recrystallization and inhomogeneous deformation in the alloy, the density of the dislocation inside the recrystallized grains became larger with an increasing strain. When the dense LPSO phases and the tangled dislocations had formed in the whole grain, it was difficult to distinguish the types of dislocations. It is apparent from the interaction between the LPSO phase and the dislocation that the LPSO phase contributed to the strengthening of the alloy. Figure 10(a) shows a bright field image of a fully recrystallized grain in the specimen extruded at 773 K. The characteristic striations of the LPSO phase are seen in the whole grain as was observed in the alloy extruded at 623 K. Figures 10(b) and (c) are dark field images taken under two-beam diffraction conditions using g=0110 and 0002, respectively. It is notable that there is no dislocation and all dislocations can be distinguished as dislocations in the grain.

Fig. 8. Microstructures of Mg97Y2Zn1 alloy extruded at 623K. (a) a area with big recrystallized grains, (b) a area with fine recrystallized grains and non-recrystallized grains, and the corresponding SAED pattern.

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Fig. 9. (a) Bright field image of a fully recrystallized grain that obtained at 623K. (b) Dark field image of the same area in (a), showing the LPSO structure in contrast extra reflection-spots in SAED pattern. (c) and (d) Dark field images of the same area in (a), showing the high dislocation density in contrast g=0110 and 0002 with B//[1120], respectively.

That suggests that the motion of dislocation on a non-basal plane is more significantly activated at 773 K because dynamic recrystallization preferably took place at sites where the dislocation density was high. Once dynamic recrystallization had taken place and new grains were formed, the dislocation density decreased sharply at these locations. As the temperature of the deformation increased, the degree of recrystallization was higher and the dislocation density also declined. Therefore,

the 773 K extruded Mg97Y2Zn1 alloy exhibited lower dislocation density than the 623 K extruded Mg97Y2Zn1 alloys. Figures 11 and 12 shows the TEM images of precipitates formed along the grain boundaries after extrusion at 773 K. The dispersing of submicron scale precipitates along the grain boundary is observed. The grain boundary precipitates are in a discontinuous form. The precipitates on the grain boundaries are much bigger than the corresponding precipitates within


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Fig. 10. (a) Bright field image of a fully recrystallized grain that obtained at 773 K. (b) and (c) Dark field images of the same area in (a), showing the non-basal dislocations in contrast g=0110 and 0002 with B//[1120], respectively.

grains that are observed in the alloy extruded at 623 K. As shown in Figs. 11 and 12, in the samples examined by TEM in this study, the precipitates can be divided into two main types: mixed type and single type. Figure 11(b) shows the TEM micrographs of single type precipitates and its corresponding diffraction pattern. In the diffraction pattern, the 14 spots with regular intervals at areas of n/7 (0002) hcp (n is an integer) in the c*direction are observed between the spots corresponding to

the (0002) plane of pure Mg. Therefore, the precipitates can be identified as an LPSO structure with a hexagonal crystallographic structure. The mixed type precipitates reveal that the 14H-type LPSO structure coexisted in the 2H-Mg with certain crystallographic relationships of (0001)2H-Mg//(0014)14H-Mg. This was explained in a previous study by the authors [19,20]. The grain boundary precipitates have pining effects on grain boundary motions as shown in Fig. 12.

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Fig. 11. TEM analysis of extruded Mg97Y2Zn1 alloy. (a) TEM micrographs and corresponding SAED pattern of Mg97Y2Zn1 alloy after extrusion at 773 K, (b) is an enlarged part of (a) for clarity.

Fig. 12. TEM analysis of extruded Mg97Y2Zn1 alloy. (a) TEM micrographs of Mg97Y2Zn1 alloy after extrusion at 773 K, (b) is an enlarged part of (a) for clarity.

4. CONCLUSIONS In this study, we investigated the influence of extrusion temperature on the microstructure and mechanical properties

of the Mg97Y2Zn1 alloy. The microstructural evolution during extrusion was also analyzed. The results obtained in this study can be summarized as follows. (1) The average grain sizes of Mg97Y2Zn1 alloys increased


from 3 μm to 8 μm with increasing extrusion temperatures from 623 K to 773 K. Both DRX and SRX, which occur during and after deformation respectively, were observed. (2) The alloy that extruded at a relatively high temperature exhibited lower strength because strain strengthening was balanced by the softening that originated from full DRX. (3) Three types of morphologies, namely, big recrystallized grains, fine recrystallized grains, and non-recrystallized grains, were observed in the extruded microstructures obtained at 623 K. The dislocation density was quite high in the fully recrystallized grain. The extruded microstructures obtained at 773 K were composed of large grains with more uniform size. Their degree of recrystallization was higher and the dislocation density was also decreased. All dislocations in the grain were distinguished as dislocations. (4) The dispersing of submicron scale precipitates were distributed along the newly formed recrystallized grain boundaries and had a remarkable pinning effect on the recrystallized grain growth after extrusion at 773 K. The precipitates were divided into two main types: mixed type and single type.

ACKNOWLEDGEMENTS This research is supported by the National Natural Science Foundation of China (Grants No. 50471015).

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