JOURNAL OF RARE EARTHS, Vol. 29, No. 9, Sep. 2011, P. 902

Foundation item: Project supported by the National Natural Science Foundation of China (51001072)

Corresponding author: CHEN Bin (E-mail: steelboy@sjtu.edu.cn; Tel.: +86-21-34202765)

DOI: 10.1016/S1002-0721(10)60564-9

Characterization of microstructure in high strength Mg96Y3Zn1 alloy processed by extrusion and equal channel angular pressing

CHEN Bin ( ), LU Chen ( ), LIN Dongliang ( ), ZENG Xiaoqin ( ) (School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China)

Received 16 March 2011; revised 5 July 2011

Abstract: The Mg96Y3Zn1 alloy processed by extrusion and equal channel angular pressing (ECAP) was investigated. It was found that the Mg96Y3Zn1 alloy processed by extrusion and ECAP obtained ultrafine grains and exhibited excellent mechanical properties. After ECAP, the average grain size of Mg96Y3Zn1 alloy was refined to about 400 nm. The highest strengths with yield strength of 381.45 MPa and ultimate tensile strength of 438.33 MPa were obtained after 2 passes at 623 K. The high strength of Mg96Y3Zn1 alloy was due to the strengthening by the grain refinement, the long period stacking (LPS) structure, solid solution, fine Mg24Y5 particles, and nano-scale precipitates. It was found that the elongation was decreased with pass number increasing. It was because that the cracks were preferentially initiated and propagated in the interior of X-phase during the tensile test.

Keywords: Mg96Y3Zn1; extrusion; equal channel angular pressing; ultrafine grain; long period stacking structure; rare earths

Magnesium alloys are attractive engineering materials for use in a wide range of aerospace, military, automotive, elec-tronic industries and other applications. However, an impor-tant disadvantage of Mg alloys is their low strength. Signifi-cant efforts are being devoted lately to improve its strength to meet applied needs. In recent years, severe plastic defor-mation (SPD) has attracted increasing interest for its promi-nent advantages. The SPD is one of the effective methods used to produce bulk metal materials with submicron or nano-scale grain size. The equal channel angular pressing (ECAP) is the most well developed of all SPD processing techniques. The ECAP processed ultrafine grained (UFG) materials usually obtain ultrafine grain size and exhibit ex-cellent mechanical properties. There are some researches about the application of ECAP on magnesium and its alloys have been done[1–18]. The two-step process with extrusion and ECAP is devised for magnesium alloys[19–22]. The Mg97Y2Zn1 (at.%) alloy processed by extrusion and ECAP has been investigated, too[23]. The two-step ECAP processed Mg97Y2Zn1 alloy obtains ultrafine grain size and exhibits excellent mechanical properties. The highest strengths with yield strength (YS) of 400.31 MPa and ultimate tensile strength (UTS) of 449.97 MPa were obtained by two-step ECAP. It was found that the extruded Mg96Y3Zn1 alloy ex-hibited excellent mechanical properties both at ambient temperature and elevated temperature[24]. Compared with Mg97Y2Zn1 alloy, Mg96Y3Zn1 alloy presents the higher strength at elevated temperature. It is very interesting to investigate the possibility of producing UFG Mg96Y3Zn1 alloy by two-step ECAP and examine its microstructure and

mechanical properties.

1 Experimental

The experimental material Mg96Y3Zn1 (in at.%) was pre-pared by ingot metallurgy. The pure Mg, Zn and Mg-25%Y master alloys were molten in a crucible furnace under a pro-tecting gas (0.3%SF6 and 99.7%CO2). The ingots were ho-mogenized at 833 K for 12 h and then air cooling. Before ECAP, the hot extrusion was conducted at 673 K with an extrusion ratio of 12.25:1. The samples subjected to ECAP were machined into dimension of 10 mm in diameter and 80 mm in length. ECAP was conducted at 623 K using a die with the channel angle =90° and fillet angle =0°. This die configuration imposes an effective strain of approximately one per ECAP pass[25]. The samples were processed by route Bc in which the sample was rotated 90° clockwise along its longitudinal axis in each pass[26]. The graphite lubricant was used to decrease friction between samples and die. The pressing speed of plunger is 25.2 mm/min.

Tensile tests were carried out to evaluate the strength and ductility of the ECAP processed Mg96Y3Zn1 alloy. Tensile specimens parallel to the longitudinal axis with the gauge length of 18 mm were extracted from the center portion of the ECAP processed materials by using electro-discharge machining. Tensile testing was conducted on a Zwick elec-tronic universal material testing machine and specimens were stretched at room temperature under an initial strain rate of 5×10–4 s–1. Quantitative X-ray diffraction (XRD) measurement was performed with a D/max 2550V X-ray

CHEN Bin et al., Characterization of microstructure in high strength Mg96Y3Zn1 alloy processed by extrusion and … 903

diffractometer. The microstructure evolvement of Mg96Y3Zn1 alloy during ECAP process was characterized by an optical microscope (OM). Transmission electron micros-copy (TEM) observation was performed using JEOL JEM-2100 operating at 200 kV. The TEM samples were thinned at 233 K with a twin-jet polisher under conditions of 20 mA and 75 V using a solution of 5% HClO4 in ethanol. Further thinning to a thickness of electron transparency was carried out by using GATAN ion milling.

2 Results and discussion

2.1 Microstructure evolvement

Typical microstructures of the as-cast, solid solution treated, extruded and ECAP processed Mg96Y3Zn1 alloys are shown in Fig. 1. As can be seen in Fig. 1(a), the micrograph of as-cast Mg96Y3Zn1 is typical dendritic morphology. The eutectic phases disperse along grain boundary as networks. The X-ray diffraction (XRD) pattern of as-cast alloy indi-cates that the alloy consists of -Mg and X-Mg12ZnY, as shown in Fig. 2. It is obvious that the eutectic phase is X-Mg12ZnY. Fig. 1(b) shows the micrograph of Mg96Y3Zn1

alloy solution treated at 833 K. The eutectic phases are dis-solved partially after solution treatment. After extrusion at 673 K, the typical deformation microstructure can be ob-served, as shown in Fig. 1(c). The networks of secondary phase was broken into long sections and parallel to the direc-tion of extrusion. Many fine dynamic recrystallization (DRX) grains formed around secondary phases and distorted initial grain boundaries. It indicates that accumulated dislocations at initial grain boundaries during extrusion accelerated the DRX process. However, the DRX process is incomplete and large volume fraction of distorted initial microstructure is still visible. It suggests that the high yttrium addition re-strains the DRX process of the alloy.

Fig. 1 (d) shows the microstructure of Mg96Y3Zn1 alloy processed by ECAP for 1 pass at 623 K. After a single pass it is apparent that the microstructure consists of non-recrystallization zone surrounded by the recrystalliza-

tion zone. The grains in recrystallization zone are signifi-cantly refined. In non-recrystallization zone, however, the numerous deformation bands are developed instead of DRX. The non-recrystallization zone occupies a significantly larger area fraction. From Fig. 1(d) to Fig. 1(g), it shows that the deformation bands increase in width with further deforma-tion. The new grains are formed only along the initial grain boundaries and the deformation bands. After 4 passes, there is still a mix of non-recrystallization zone and recrystalliza-tion zone. It is obvious that the area fraction of the non-recrystallization zone is decreased progressively.

From what has been discussed above, we can draw the conclusion that the evolution of the microstructure of the Mg96Y3Zn1 alloy during ECAP depends critically upon the nature of the initial structure. It is evident that a homogene-ous ultrafine-grained structure develops readily after a single pass of ECAP in recrystallization zone. But there is still an inhomogeneous structure of non-recrystallization zone sur-rounded by recrystallization zone. With the increase of the strain, more ultrafine grains occur around the initial recrys-tallization zone and deformation bands. The microstructure evolves gradually into a more homogeneous structure with subsequent passes.

Fig. 3 shows the representative TEM micrographs after extrusion at 673 K and ECAP at 623 K by different passes. As can be seen in Fig. 3(a), the microstructure of extruded alloy consists of an array of inhomogeneous recrystallized grains with measured size from 0.3 to 2 m. The recrystalli-zation nucleated from grain boundaries and grain boundary corners (triple junctions) could be observed. From Fig. 3(b), it is apparent that the grain boundaries after one pass are ill-defined and diffuse in appearance. They are generally in-terpreted as representative of high energy non-equilibrium boundaries. The high density dislocation networks and dis-location tanglings are observed inside the grains. In Fig. 3(c), a number of new fine grains appear along the initial grain boundaries. After 3 passes, it is dominated by the homoge-neous equiaxed grains or sub-grains with an average grain size of 400 nm, as shown in Fig. 3(d). By contrast, the

Fig. 1 Microstructure evolvement of Mg96Y3Zn1 alloy

(a) As-cast; (b) Solid solution treated; (c) As-extruded; ECAP processed by 1 pass (d), 2 passes (e), 3 passes (f), 4 passes (g)

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Fig. 2 X-ray diffraction pattern of the as-cast Mg96Y3Zn1 alloy

boundaries after 4 passes are reasonably well-defined, as shown in Fig. 3(e). As a result, the ultrafine grain with sub-micron size of about 400 nm is obtained in ECAP processed Mg96Y3Zn1 alloy.

As mentioned above, there is the accumulation of disloca-tions within the grains during ECAP. The dislocations rear-ranged via glide and climb to form sub-grain structures. The sub-grain boundaries consist of low-angle grain boundaries (LAGBs). With the increase of the strain, the newly created dislocations in sub-grains were absorbed by the sub-boundaries. And the misorientation between sub- grain boundaries increased and evolved to high-angle grain boundaries (HAGBs). Therefore, the uniform grain distribu-tion and equiaxed ultrafine grains can be obtained. It is why the nucleation of new grains occurs mainly within the shear

bands and along grain boundaries during ECAP.

2.2 Tensile properties

Fig. 4 represents the comparison of the tensile properties of extruded Mg96Y3Zn1 alloy and ECAP processed Mg96Y3Zn1 alloy by different passes. It shows that extruded and ECAP processed Mg96Y3Zn1 alloy exhibits excellent mechanical properties. The YS, UTS, and elongation of the extruded Mg96Y3Zn1 alloy are 334.02, 400.32 MPa, and 7.3% respectively. After one pass of ECAP, YS and UTS of alloy increased significantly to 355.7 and 430.38 MPa. The YS of Mg96Y3Zn1 alloy increases with the strain increasing. The maximum YS of 381.45 MPa and UTS of 438.33 MPa were obtained by 2 passes at 623 K. With the further in-crease of the strain, the YS and UTS are decreased gradually. However, the uniform elongation of the alloy decreased con-tinuously from 7.3% to 3.2% with pass number increasing. It is due to that the cracks preferentially occur at the interior of X-phase instead of the interface of X-phase/ -Mg during tensile deformation[23]. Owing to the high volume fraction of coarse blocks of X-phase in the Mg96Y3Zn1 alloy, cracks are easier to initiate and propagate at the interior of X-phases during tensile test. The process of ECAP leads to more mi-cro-cracks in the X-phase, which accelerated the growth and coalescence of the cracks during tensile test and resulted in premature fracture and lower elongation.

2.3 TEM observation

Fig. 5 shows a bright-field TEM image of the Mg96Y3Zn1

Fig. 3 TEM micrograph of extruded Mg96Y3Zn1 alloy (a) and ECAP processed Mg96Y3Zn1 alloy by 1 pass (b), 2 passes (c), 3 passes (d), and

4 passes (e) at 623 K

CHEN Bin et al., Characterization of microstructure in high strength Mg96Y3Zn1 alloy processed by extrusion and … 905

Fig. 4 Mechanical properties of the extruded Mg96Y3Zn1 alloy and

ECAP processed Mg96Y3Zn1 alloy by different passes

alloy prepared by extrusion at 673 K. The fine lamellar structure can be observed inside grain, as marked by arrow. The corresponding selected area electron diffraction (SAED) pattern is also inserted in Fig. 5. In the pattern, it can be no-ticed that the extra reflection-spots are evident in c*-direction. According to the SAED pattern, the lamellar structure is proved to be long period stacking (LPS) structure.

In Fig. 6, the dispersing of fine Mg24Y5 phase along grain boundary is observed in extruded Mg96Y3Zn1 alloy. The dispersing of fine Mg24Y5 phase is also observed in ECAP processed Mg97Y2Zn1 alloy. These fine Mg24Y5 phases con-tribute significantly to the hindrance of grain boundary slip and matrix deformation[27]. Compared with Mg97Y2Zn1 alloy, the extra addition of yttrium in Mg96Y3Zn1 alloy is consid-ered to be consumed by formation of Mg24Y5 phase instead of formation of LPS structure. This is one of the prime rea-sons why extruded or ECAP processed Mg96Y3Zn1 alloys exhibit higher strength than that Mg97Y2Zn1 alloys.

In addition, several nano-scale precipitates along grain boundaries are also observed, as shown in Fig. 7. The crystal structure of the precipitates has not been identified yet. Chemical analysis by EDS reveals a significant composition difference between precipitate and matrix. As shown in

Fig. 5 TEM micrograph of LPS structure in extruded Mg96Y3Zn1

alloy and its SAED pattern

Fig. 6 TEM micrograph of the parallelepiped shape Mg24Y5 phase

in extruded Mg96Y3Zn1 alloy

Fig. 7 TEM micrograph of ECAP processed Mg96Y3Zn1 alloy by

two passes at 623 K (a), and the typical EDS spectra ob-tained from precipitate (b) and matrix (c)

Fig. 7(b), (c), the chemical compositions in precipitate and matrix are Mg-3.31 at.%Y-0.14 at.%Zn and Mg-1.34 at.%Y, respectively. It is obvious that the Y and Zn contents of the precipitates are higher than those of matrix.

From what has been discussed above, we may reasonably come to the conclusion that the high strength of Mg96Y3Zn1 alloy was due to the strengthening by the LPS structure, the grain refinement, solid solution, fine Mg24Y5 particles, and nano-scale precipitates.

3 Conclusions

(1) The ultrafine grains with grain size of about 400 nm

906 JOURNAL OF RARE EARTHS, Vol. 29, No. 9, Sep. 2011

were obtained after ECAP. (2) The ECAP processed Mg96Y3Zn1 alloy exhibited ex-

cellent mechanical properties. The highest strengths with YS of 381.45 MPa and UTS of 438.33 MPa were obtained. The high strength of Mg96Y3Zn1 alloy was thought to be due to the strengthening by the grain refinement, the LPS structure, solid solution, fine Mg24Y5 particles, and nano-scale precipi-tates. (3) The elongation was decreased with pass number in-

creasing. It was due to that the cracks preferentially occurred at the interior of X-phase and the process of ECAP intro-duced micro-cracks in the X-phase, which accelerated the growth and coalescence of the cracks during tensile test.

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