Microstructure and mechanical properties of Mg-4Zn-xGd (x=0, 0.5, 1,2) alloys

Cui-ju Wng, Jin-wen Kng, Kun-kun Deng,?, Ki-o Nie, Wei Ling, Wei-guo Li

aShanxi Key laboratory of advanced magnesium-based materials, College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, PR China

b Engineering training center, Taiyuan University of Technology, Taiyuan 030024, PR China

Abstract The present work mainly focuses on the effect of minor content of Gd element on the microstructure and mechanical properties of Mg-Zn alloy.The Mg-4Zn-xGd (x=0, 0.5, 1, 2) alloys were fabricated and subjected to extrusion at 280°C with the ratio of 25:1.Results revealed that the addition of Gd could inhibit the dynamic recrystallization (DRX) of Mg-4Zn alloy, and both the size and volume fraction of DRXed grains demonstrated a decreasing tendency as the Gd content increased from 0.5 to 2 wt.%.It is the weakening effect on DRX behavior that lead to the smaller amount of fin precipitate and the stronger texture intensity.As compared with the Mg-4Zn alloy, the yield strength and ultimate tensile strength of Mg-4Zn-xGd alloys were improved obviously by the addition of minor content of Gd, however, at the expense of elongation.Excellent tensile properties with the yield strength of ～369.8 MPa and ultimate tensile strength of ～400.3 MPa were obtained when the Gd content was 2 wt.%.

Keywords: Magnesium alloy; Dynamic recrystallization; Dynamic precipitation; Microstructure; Mechanical properties.

1.Introduction

As the lightest metal material, the Mg alloys possess the advantages of low density,high specifi strength and stiffness,as well as the good casting performance [1,2], and Mg alloy is thought as the promising material in the field of electronic products, aeronautics and astronautics, automobile etc.However,the limited strength and elongation of Mg alloys at room temperature strongly restrict their commercial applications.

To overcome above shortcomings of Mg alloy, alloying is thought as an effective way[3-6].Moreover,the rare elements(Ce, La, Gd, Y, et al.) as the common adding element can obviously improve the casing properties, refin grain size and weaken the deformation texture of Mg alloy [7-9].To obtain extra-high strength Mg alloys, the amount of rare element must be very high (>10wt.%), which inevitably improve the price of Mg alloys owing to the expensive rare element[10-12].

To widen the application of Mg alloys, the development of Mg alloy with high strength and low price is urgently demand nowadays.Du et al [13,14] investigate the microstructure and mechanical properties of Mg-Zn-Ca alloy with minor Ce and La element, and the results indicated that the addition of minor rare element can improve the mechanical properties of Mg alloys.Similar results were also confirme by Li[15]and Sun[16]’s work, which demonstrate that the addition of only 0.5 wt.% Y can significantl refin grain size and improve the mechanical properties of Mg-3Zn-0.2Ca alloy.

As a common rare element, Gd can be soluted into Mg matrix and form the stable compound (Mg-Zn-Gd) with Mg and Zn elements,which would directly reflec on the mechanical properties of Mg alloy.Some work reveals that the minor content of Gd element has significan effect on refinin microstructure and improving mechanical properties of Mg-Zn series alloys [9].However, the DRX, dynamic precipitation as well as the mechanical properties of Mg alloy with minor content of Gd element is still unclear.

Table 1Chemical composition of the experimental alloys.

To clarify the influenc of minor content of Gd on the microstructure and mechanical properties of Mg alloy, the Mg-4Zn-xGd (x=0, 0.5, 1, 2) alloys are fabricated in present work, and subjected to extrusion at 280°C with the ratio of 25:1.Then the low content of Gd element on the effect of the microstructure and mechanical properties of the as extruded Mg-4Zn alloy samples is discussed in details

2.Experimental procedures

The Mg alloys with 4wt.% Zn and different Gd (0, 0.5 wt.%, 1 wt.%, and 2 wt.%) were denoted as Z4, ZW40,ZW41 and ZW42, respectively, were designed and fabricated in present work.To prepare the Mg-4Zn-xGd (x=0, 0.5, 1, 2)alloys, the pure Mg (99.9 wt.%), Zn (99.9 wt.%) and Mg-30(wt.%)Gd master alloy were used.All the alloys were fabricated in an electrical resistance furnace with the protection of anti-oxidizing gas atmosphere (2vol% SF6+98vol% CO2).Firstly, the pure Mg was added into a stainless steel crucible.At the melting temperature of 720°C, the minor (<0.05wt.%)Ca clothed with Al foil was added to avoid the oxidation of the melt.Ten minutes later, the pure Zn and Mg-30Gd master alloy were added into the melt in sequence.After 20 minutes at 720°C, the melt were poured into a steel mold preheated at 250°C.At last, the Mg-4Zn-xGd alloy with different Gd content (x=0, 0.5, 1, 2) were obtained, and the corresponded actual composition of the alloys tested by inductively coupled plasma emission spectrometer (ICP6300) is given in Table 1.

The as-casted Z4, ZW40, ZW41 and ZW42 alloys were homogenization treated at 430°C for 16h firstl , and then be held at 510°C for 20 h followed by water cooling.After that,all the alloys were machined toΦ40 mm×45 mm and extruded at 280°C with the ratio of 25:1 and the ram speed of 0.01 mm s?1.

The microstructures were observed by 4XC optical microscope (OM), MIRA3 LMH scanning electron microscope(SEM) equipped with energy spectrum scanner (EDS), electron backscattered diffraction (EBSD) and transmission electron microscope (TEM).For the as-extruded alloys, the observation position was selected at the center of the sample parallel to the extrusion direction (ED).The samples for OM and SEM were ground, polished and etched in oxalic [4g oxalic+96ml H2O].As to the EBSD specimens, they were ground and polished firstl , and then followed by ion etching using Leica RES101 with voltage of 4 V at 2 A electric current for 2 h.Date of EBSD were analyzed and processed by Channel 5 software.The size and volume fraction of both DRXed grains and secondary phase were statistically analyzed by Image Pro-Plus software[4].To make sure the accuracy of statistical results, at least four images were used for the measurement.TEM specimen was prepared by grinding-polishing to produce a foil with 40-50μm and then ion-milled at the voltage of 3kV.The secondary phase in the Z4,ZW40, ZW41 and ZW42 alloys were analyzed by DX-2700 X-ray diffractometer(XRD)with the2θranged from 20°to 80°and a scan rate of 2°/min.Then the obtained XRD pattern was analyzed by MDI Jade 6.0 software.

Table 2EDS results of the Mg-4Zn-xGd (x=0, 0.5, 1, 2) alloys.

Mechanical properties of the as-extruded Z4,ZW40,ZW41 and ZW42 alloys were tested by Instron5569 universal testing machine with the tensile rate of 0.5 mm/min.Three samples were selected for tensile test to ensure the accuracy and reliability of the test results, and then the average value was obtained.After that, the fracture surface of the alloys was observed by SEM.

3.Results

3.1.As-cast microstructure

The as-cast Z4, ZW40, ZW41 and ZW42 alloys were homogenization treated at 430°C for 16h and then held at 510°C for 20 h, and the SEM images are given in Fig.1.Almost all the secondary phases are dissolved into the Mg-4Zn alloy after homogenization, as shown in Fig.1(a).However, the dissolution of secondary phase is partially in the Mg-4Zn-xGd (x= 0.5, 1, 2) alloys, as shown in Fig.1(b)-(d).Fig.2 (a) is the XRD patterns of the as-cast Mg-4Zn-xGd (x=0, 0.5, 1, 2) alloys, which demonstrates that the alloys mainly containα-Mg matrix and W phase.According to the EDS results of Table 2, the point “A”, “B”and “C” in Fig.1 are 12Mg-49Zn-39Gd, 27Mg-52Zn-21Gd,and 26Mg-53Zn-21Gd, respectively, and the Zn/Gd ratio of those is similar to the W-phase [17,18].Based on EDS results of Table 2 and XRD results in Fig.2(a), the undissolved phase in Fig.1 could be recognized as W phase.

3.2.As-extruded microstructure

The OM images of the Mg-4Zn-xGd (x=0, 0.5, 1, 2) alloys after hot extrusion are given in Fig.3.It indicates that the dynamic recrystallization (DRX) occurs in all the alloys.However, the degree of DRX is different in the alloys with different Gd content.For the as-extruded Mg-4Zn-xGd (x=0,0.5, 1, 2) alloys, the measured volume fraction of DRXed grains (VDRX) is 82.6%, 71.4%, 66.3% and 60.7%, as the Gd content is 0, 0.5, 1 and 2 wt.% Gd, respectively.The inverse pole figur (IPF) maps of the as-extruded ZW40 and ZW42 alloys obtained by EBSD are given in Fig.4.It shows clearly that the DRXed grains in the as-extruded Z40 alloy are fine than that in the as-extruded ZW42 alloy.As indicated in Fig.3(i)-(l), the measured average size of DRXed grains(dDRX) in the as-extruded Z4 alloy is ～1.59μm, which is decreased to 0.96μm, 0.62μm and 0.56μm, respectively, when the 0.5, 1 and 2 wt.% Gd is added.It indicates that the existence of Gd element seems to inhibit the DRX behavior of Mg-4Zn alloy.

Fig.1.SEM images of the as-homogenized Mg-4Zn-xGd (x=0, 0.5, 1, 2) alloys.(a) Z4, (b) ZW40, (c) ZW41 and (d) ZW42.

Fig.2.The XRD patterns of the (a) as-cast and (b) as-extruded Mg-4Zn-xGd (x=0, 0.5, 1, 2) alloys.

The SEM micrographs of the as-extruded Z4, ZW40,ZW41 and ZW42 alloys are given in Fig.5.It can be found that the undissolved W phase distributes along extrusion direction after hot extrusion, as shown in Fig.1.Moreover,the measured volume fraction of the W phase are 0.7 %, 1.2% and 2.6 % for the as-extruded ZW40, ZW41 and ZW42 alloys, respectively, as shown in Table 3.Nevertheless, the fin precipitates can be observed clearly in Fig.5, which mainly exist in the DRXed regions of the as-extruded Z4, ZW40 and ZWX41 alloys, as depicted in Fig.5 (b), (f) and (j).The EDS results indicate that these phases contain Mg and Zn elements, which might be recognized as Mg-Zn phase by combining with the XRD results in Fig.2(b).Nevertheless,the rod-shaped precipitates exist in the ZW42 alloy, as shown by the yellow arrows in Fig.5 (p).The measured volume fraction of these precipitates (VP) is about 4.7 %, 5.8 %,3.6 % and 0.8 % for the as-extruded Z4, ZW40, ZW41 and ZW42 alloys, respectively, as shown in Table 3.That means the amount of precipitates in the as-extruded Mg-4Zn-xGd(x=0, 0.5, 1, 2) alloys decrease with the increasing amount of Gd element.Nevertheless,the measured average size of the precipitate is ～93 nm, ～98 nm, ～88 nm and ～79 nm for the as-extruded Z4, ZW40, ZW41 and ZW42 alloys, respectively.Both the amount and average size of precipitate decrease with the Gd amount increasing, which indicate that the addition of Gd element seem to inhibit dynamic precipitation of Mg-Zn alloy.

Fig.3.OM images of as-extruded Mg-4Zn-xGd (x=0, 0.5, 1, 2) alloys.(a)(e)(i) Z4, (b)(f)(j) ZW40, (c)(g)(k) ZW41 and (d)(h)(l) ZW42.

Fig.4.Inverse pole figur (IPF) maps of the as-extruded (a) ZW40 alloy and (b) ZW42 alloy.

Fig.6 shows the TEM microstructures of the as-extruded ZW42 alloy, which depicts that the as-extruded ZW42 alloy contains both the spherical precipitates and rod-shaped precipitates.The spherical precipitates usually exist at DRX region,and the morphology is given in Fig.6(a).The rod-shaped precipitates appear in unDRX region, as shown in Fig.6(b).Above results consist well with the SEM results of Fig.5(n).Corresponded results of EDS indicate that the composition of spheroidal precipitates contain Mg, Zn and Gd elements, and the atom percentage of Zn:Gd is about 6:1, which is close to the component of I phase.As combined with the XRD results in Fig.2(b), these spheroidal precipitates can be recognized as I phase.The rod-shaped precipitates only contain Mg and Zn elements, and they may belong to Mg-Zn phase.Considering the amount of the Mg-Zn phase is so little that its diffraction peak is hard to be found in Fig.2(b).

Fig.5.SEM images of as-extruded Mg-4Zn-xGd (x=0, 0.5, 1, 2) alloys.(a)(b)(c)(d) Z4, (e)(f)(g)(h) ZW40, (i)(j)(k)(l) ZW41 and (m)(n)(o)(p) ZW42.

Table 3Microstructure and tensile properties of the as-extruded Mg-4Zn-xGd (x=0, 0.5, 1, 2) alloys.

3.3.Texture

The pole figur of the as-extruded ZW40 and ZW42 alloys are given in Fig.7, it reveals that the intensity of basal plane texture changes from 20.54 to 32.04 as the Gd content increased from 0.5wt.% to 2wt.%.That means the content of Gd has a great influenc on the texture of as-extruded Mg-Zn alloy.As described in Fig.3,all the as-extruded alloys exhibit similar bimodal grain structure.To further analyze the texture of the as-extruded alloy, the inverse pole figur and pole figur of the DRXed and unDRXed regions in as-extruded ZW40 alloy are given in Fig.8.The results show that the intensity of basal plane texture in the unDRXed regions is obviously higher than that in the DRXed regions.Therefore,the as-extruded ZW42 alloys containing a larger amount of unDRXed regions possess higher texture intensity than the as-extruded ZW40 alloy.

3.4.Mechanical properties

The tensile engineering stress-strain curves of as-extruded Z4, ZW40, ZW41 and ZW42 alloys are given in Fig.9.The corresponded yield strength (YS), ultimate tensile strength(UTS) and elongation (EL) obtained from the Fig.9 are summarized in Table 3.It demonstrates that YS, UTS and EL of Z4 alloy is ～240MPa, ～307MPa and ～24.5%, respectively.With the Gd content increasing, the YS and UTS increase gradually accompanied by the decrease of EL.The excellent YS (～369.8 MPa) and UTS (～400.3 MPa) and moderate EL of ～7.8 % are obtained when the content of Gd is 2wt.%.

Fig.6.TEM microstructure of the as-extruded ZW42 alloys.(a)DRX regions, (c) unDRX regions, (b) and (d) are the EDS results of precipitated phase in (a)and (c), respectively.

Fig.7.Pole figur of the as-extruded ZW40 alloy and ZW42 alloy.

Meanwhile, Fig.10 summarizes the comparison of tensile properties in present work with the other high RE Mg alloys.It indicates that both the as-extruded ZW41 and ZW42 alloys in present work exhibit excellent strength-ductility match than the other high RE Mg alloys reported previously [19-23].The YS of both ZW41 and ZW42 alloys in present work are higher than that of the Mg-5Y-4Gd-0.4Zr (wt.%) alloys with different Zn additions [20] Furthermore, the YS of present ZWX410 and ZWX420 alloys are even higher than the Mg alloys with very high RE content (≥10wt.% RE), such as the as-extruded Mg-10Gd-3Y-(1Zn)-0.5Zr(wt.%)alloys[19],Mg-10Gd-2Y-0.5Zr [21], Mg-7Y-4Gd-1.5Zn-0.4Zr [22], and Mg-7.5Gd-2.5Y-3.5Zn-0.9Ca-0.4Zr [23], etc.

Fig.8.Pole figur and inverse pole figur of as-extruded ZW40 alloy.(a) DRXed regions, (b) unDRXed regions and (c) inverse pole figure

Fig.9.The tensile engineering stress-strain curves of as-extruded Mg-4ZnxGd (x=0, 0.5, 1, 2) alloys.

Fig.10.Comparison of tensile properties in present ZW41 and ZW42 alloys with other high RE Mg alloys.

4.Discussion

4.1.Microstructure of Mg-4Zn-xGd alloys influence by the minor content of Gd

4.1.1.DRX behavior

As mentioned above, addition of minor content of Gd results in the decrease of DRX ratio and refinemen of DRXed grains of the as-extruded Mg-4Zn alloy.In other word, the DRX behavior of Mg-Zn alloy seems to be inhibited by the addition of Gd element during hot extrusion process.Generally speaking,the factors affecting DRX include the large secondary phase[17,18,24],dynamic precipitates[4,25,26]extrusion parameters [4] and alloying component [27,28].Previous research had shown that the large particles (>1μm) could be used as the nucleation position for DRX in the process of thermal deformation, which was called as particle-stimulated nucleation (PSN) [17,24].Kim el at.’s [24] research showed that theVDRXof the as-extruded alloys was linearly proportional to the number of secondary phase larger than 1μm,indicating the larger particles could promote the nucleation of DRX.Even though the large particles (>1μm) in Mg-4Zn-xGd (x=0, 0.5, 1, 2) alloys increase with the increasing amount of Gd element, theVDRXof the alloys decreases in reverse, as shown in Table 3.That means the PSN is not the main factor affecting DRX behavior of present Mg-4Zn-xGd alloys.

Table 4The measured volume fraction (f) and average radius (r) of secondary phase particles,and calculated Zener limited grain diameter(Dz)for the as-extruded Mg-4Zn-xGd (x=0, 0.5, 1, 2) alloys.

It is known that the grain boundary migration can be restricted by fin dispersive particles through the Zener pinning effect [29-31].In the light of Zener pinning theory, the grain growth will be ceased once the driving force of grain growth counterpoise to the particle induced pinning force at certain average grain size [29].This grain size is called as the Zener limited grain diameter (Dz), which can be expressed by Eq.(2) [30,31]:

whererandfare the average radius and volume fraction of the precipitate, respectively.If the average grain size (D)is larger thanDZ, the Zener pinning will occur [29,31].The measuredfandrof precipitate in the as-extruded Mg-4ZnxGd alloys are shown in Table 4.According to Eq.(2),the calculatedDzfor the as-extruded Z4, ZW40, ZW41 and ZW42 alloys are about 0.013, 0.011, 0.016 and 0.066μm,respectively.The correspondeddDRXof the as-extruded Z4,ZW40, ZW41 and ZW42 alloys are about 1.59, 0.96, 0.62 and 0.56μm, respectively, which are much larger than the calculatedDzby Eq.(2).It indicates that the fin dynamic precipitates can pin the grain boundaries effectively to inhibit the growth of DRXed grains during extrusion process.As described in Section 3.2, the quantity of dynamic precipitates in Mg-4Zn-xGd alloy decrease in pace with the Gd content increasing, which demonstrate that the Zener pinning effect of these precipitates should be weakened as the amount of Gd increased from 0 to 2 wt.%.As a consequence, the DRXed grains are expected to grow up with the Gd content increasing.However, thedDRXof Mg-4Zn-xGd alloy decrease as the amount of Gd increased from 0 to 2 wt.% in present work,which indicates that dynamic precipitates are not the main factor that influencin the DRX behavior of present Mg-4ZnxGd alloys either.

Previous studies had shown that the solute segregation of RE atom and the stable RE-containing second phases can pin the grain boundaries to inhibit the growth of grains during the hot deformation process [9,21,32].Additionally, Hadorn et al.[33] found the segregation of Gd atoms at grain boundaries in the as-extruded Mg-0.39Gd alloy by using high-angle annular dark-fiel scanning transmission electron microscopy(HAADF-STEM).Basu et al.[34] also confirme severe segregation of Gd atoms at grain boundaries of the as-rolled Mg-1Gd (wt.%) alloy by using atom probe tomography (APT).In present work, the Gd content of Mg-4Zn-xGd alloys increase gradually as the amount of Gd increased from 0 to 2 wt.%.Thus, the increasing amount of Gd may make the grain boundaries and dislocations hard to move during DRX process [35], which is responsible for the decreasing volume fraction and refinin size of DRX grains.

4.1.2.Dynamic precipitation

As described in Section 3.2, the dynamic precipitates have been found in the DRXed regions of the as-extruded Z4,ZW40, ZW41 and ZW42 alloys, and the precipitates distributed mainly at the DRXed grain boundaries, as indicated by circles in Fig.5(c),(g),(k)and(o).The same phenomenon had also been found in AZ91-0.5Ca alloy [36].It should be noted that the DRXed region is more favorable for dynamic precipitation than that in unDRXed region [37].Generally speaking, the dynamic precipitation depends on the diffusion rate during hot deformation process.Usually, plastic deformation can produce massive dislocation and vacancy, which can provide extra channel for atom diffusion [4].As compared with dislocation diffusion or vacancy diffusion, the diffusion rate is much higher if the atoms diffuse along gain boundaries.Thus, the DRXed regions containing substantial grain boundaries are the ideal size for dynamic precipitation.As a consequence, dynamic precipitation occurs preferentially in the DRXed region as compared with that in the unDRXed region.

As illustrated in Table 3, both the volume fraction and average size of precipitates in the as-extruded ZW40 alloy are larger than that of Z4 alloy.This phenomenon indicates that the addition of 0.5 wt.%Gd is helpful to the dynamic precipitation and growth of precipitates.In fact, similar result had been found in Du et al’s investigation [17] that a little amount of Ce addition is propitious to the dynamic precipitation of Mg-6.2Zn-0.7Ca alloy.

However, the amount of precipitates shows a decreasing tendency as the Gd content increased from 0.5 wt.% to 2 wt.%, and it may result from the increasing amount of W phase accompanied by the increasing Gd content.The W phase may consume abundant Zn element and make it hard for the dynamic precipitation.On the other hand,it should be noted that the fin precipitation prefer precipitating at grain boundary in the DRXed region.That means theVPanddAPSare closely related to the number of grain boundaries.The higherVDRXand the finedDRXindicate more grain boundaries, which can further promote the precipitation.As described in Fig.3, thedDRXandVDRXdecrease gradually with the increasing content of Gd element.The gradual decrease ofdDRXwill lead to the gradual increasing area of total grain boundary, however, the gradual decrease ofVDRXwill play a contrary effect.In other words, the refinemen of DRXed grains is favorable for the precipitation, but the decreasingVDRXis not conducive to the precipitation.

TheVDRXanddDRXof Z4 alloy is about 82.6% and 1.59μm,respectively.Even though the average size of DRXed grains decrease as the Gd content increased from 0wt.% to 2 wt.%, the volume fraction of DRXed grains also decreased from 82.6%to 60.7%.Thus,the total fraction of DRXed grain boundaries demonstrates a decreasing tendency with the increasing Gd content, which is adverse to dynamic precipitation.The above two reasons may contribute to the decreasing amount and size of precipitate accompanied by the increasing Gd content.

4.2.Texture of the as-extruded Mg-4Zn-xGd alloy

As mentioned in Section 3.3, the basal plane texture is formed in the as-extruded Mg-4Zn-xGd alloys, and the texture intensity of which increase with the increasing Gd content.Recently, Stanford et al.’s [21] study revealed that the segregation of Gd atoms at grain boundaries had significantl effect on weakening basal plane texture.Furthermore, Yan et al.[9] found that the texture intensity of the as-rolled Mg-2Zn-xGd (x=0.1, 0.3 and 0.7 wt.%) sheets decreased gradually accompanied with the increasing Gd content.However,the basal texture intensity of present Mg-4Zn-xGd alloys increases with the increasing Gd content, which indicates that the content of Gd is not the main factor affecting the texture intensity.

The research shows that the dynamic precipitates can weaken the texture through stimulating DRX [4,38].That means the larger amount of precipitates the weaker of texture intensity.As mentioned in Table 3, the amount of the dynamic precipitate in the as-extruded ZW40 alloy is much higher than that in the as-extruded ZW42 alloy.Thus, the reducing amount of dynamic precipitates in the as-extruded ZW42 alloy may be one reason for its lower texture intensity.

Recently, many studies have shown that the texture intensity in the unDRXed regions of wrought magnesium alloys is higher than that in DRXed regions [3,18,38].Similar find ings are also proved in Fig.8 that the texture intensity of the unDRXed grains is obviously higher than the DRXed grains in as-extruded ZW40 alloy.Thus, the actual texture strength of as-extruded alloys is mainly determined by unDRXed regions.Based on Section 3.2,the quantity of unDRXed regions in Mg-4Zn-xGd alloys increase as the Gd content increased from 0 to 2 wt.%, which inevitably result in the improvement of texture intensity.

4.3.Mechanical properties of Mg-4Zn-xGd alloys

As described in Section 3.4, the increasing amount of Gd usually led to the improvement of YS and UTS of the as-extruded Mg-4Zn-xGd alloys, however, at the expense of elongation.As the Gd content increased to 2wt.%, the YS and UTS of the as-extruded ZW42 alloy are ～369.8 MPa and ～400.3 MPa, respectively, which are even superior to the other Mg alloys containing high RE element (≥10wt.%).Generally, the mechanical properties of the as-deformed magnesium alloy are related to the size of DRXed grains, precipitates and the texture intensity.

The relationship between YS and grain size can be expressed by Hall-Petch formula [15]:

whereσyis the YS,σ0is the material constants,kis the Hall-Petch slopes,dis the average grain size.The increment of YS caused by Gd additions could be expressed as follows:

wherex=0.5, 1 and 2,kis Hall-Petch slopes taking value of 271.6 MPa μm1/2in the light of the authors’ previous research on Mg-4Zn-0.5Ca alloy [25].According to the investigation in Section 3.2, thedDRXare ～1.59, ～0.96, ～0.62 and～0.56μm for the as-extruded Z4, ZW40, ZW41 and ZW42 alloys, respectively.Therefore, the calculatedΔσgbare 61.8 MPa, 129.5 MPa and 147.6 MPa when thexequal to 0.5, 1 and 2, respectively.So, it can be concluded that the decreasingdDRXwill be conducive to the increase of YS as the Gd content increased from 0 to 2 wt.%.

In addition, precipitates can pin dislocation effectively and result in obvious precipitation strengthening.The improvement of YS caused by precipitation strengthening (Δσps) can be expressed by the following formula [4,24]:

The specifi parameters are described in detail in Ref [25].Precipitation strengthening induced by precipitates is estimated to 36.5 MPa, 34 MPa, 39.2 MPa and 45.5 MPa for the as-extruded Z4, ZW40, ZW41 and ZW42 alloys, respectively, according to the VPanddAPSin Table 4.

In addition, previous work had shown that the strong basal plane texture in the as-extruded Mg alloy is beneficia to improve the strength of the alloy along extrusion direction[4,36].Therefore, the high texture intensity may lead to high YS.As described in Section 3.3, the texture intensity increase with the Gd content increasing, which is another reason for the increased YS of Mg-4Zn-xGd alloys as the Gd content increased from 0 to 2wt.%.

Fig.11 shows the OM near fracture surfaces and the SEM of fracture surfaces of the as-extruded Z4, ZW40, ZW41 and ZW42 alloys.As shown in Fig.11(a), (d), (g) and (j), the twinning is found in the unDRXed regions of the as-extruded Mg-4Zn-xGd alloys.However, it is negligible in the DRXed regions.As given in Fig.11(b), (e), (h) and (k), the microcracks are found along twinning boundaries, which indicates that the fracture of present alloys is related to the occurrence of twinning.Most of studies suggested that the elongation of the as-extruded Mg alloy is closely related to the unDRXed regions [4,25,29].Moreover, research had shown that the compressive twinning and double twinning could appear easily in unDRX grains as compared with DRXed grains[24,25].During the subsequent tensile process, the accumulation of dislocations at twinning boundary leads to stress concentration, and induce the initial of micro-crack at twinning boundary [4].Connection of micro-cracks would result in the instability and fracture of the alloy.Generally, the large amount of unDRXed regions in Mg alloys will be propitious to the initiation of twinning, which is adverse to their elongation [24,25].As shown in Fig.11(c), (f), (i) and (l), the dimples are also found in the fractured surface of as-extruded Z4, ZW40, ZW41 and ZW42 alloys, which reveals that plastic fracture is also a fracture mode.Accompanied with the increasing Gd content, the amount of dimples decrease, due to the decrease ofdDRXandVDRX.

Fig.11.Fracture surface of the as-extruded Mg-4Zn-xGd (x=0, 0.5, 1, 2) alloys.(a)(b)(c) Z4, (d)(e)(f) ZW40, (g)(h)(i) ZW41 and (j)(k)(l) ZW42.

5.Conclusions

In present work, the microstructure and mechanical properties of Mg-4Zn alloy influence by minor content of Gd is investigated and the main results can be described as follows.

(1) The occurrence of DRX of Z4 alloy is inhibited owing to the addition of Gd element, and both thedDRXandVDRXdecrease as the Gd content increased from 0 to 2 wt.%.

(2) The dynamic precipitation is influence by Gd element.The amount of fin precipitate in Mg-4Zn-xGd alloy increases slightly as the Gd content increased to 0.5 wt.%, and it decrease as the Gd content further increase to 2 wt.%.

(3) Strong basal texture is formed in the present Mg-4ZnxGd alloys, and the texture intensity increase with the increasing Gd content owing to the decreased area of DRXed regions.

(4) Addition of Gd element leads to the increase of YS and UTS of Mg-Zn alloy at the expense of elongation.Excellent mechanical properties with the YS of ～369.8 MPa, UTS of ～400.3 MPa and EL of ～7.8 % are obtained when the Gd content is 2 wt.%.

Acknowledgements

This work was supported by “National Natural Science Foundation of China” (Grant nos.51771128, 51771129 and 51201112), Projects of International Cooperation in Shanxi(Grant no.201703D421039),and Shanxi province science and technology major projects (20181101008).The authors also thank to the Support from Program for the Outstanding Innovative Teams of Higher Learning Institutions of Shanxi,and the Special Fund Project for Guiding Local Science and Technology Development by the Central Government(YDZX20191400002734).