Effect of ZRB2-modifie on microstructure and mechanical properties of Mg-Zn-Y-Mn

Zhe Zhang,Yatong Zhang,Jinshan Zhang,∗,Yi Li,Yanbin Ma,Chunxiang Xu,∗

a College of Materials Science and Engineering,Taiyuan University of Technology,Taiyuan 030024,China

b Shanxi Key Laboratory of Advanced Magnesium-based Materials,Taiyuan 030024,China

Abstract The ZrB2 particles firstl modifie Mg94Zn2.5Y2.5Mn1 alloy.And the microstructures and mechanical performances of the modifie alloys were studied systematically.The results showed that the moderate addition of ZrB2 accelerated the development of long-period stacking ordered(LPSO)structure and refine the grain size.The grains in ZrB2-modifie alloys were nearly equiaxed with a homogeneous size.When 0.0075 wt%ZrB2 was added,the as-cast alloy with the fines grains(24.87μm)presented desirable mechanical properties(especially ductility)with maximum tensile strength and ductility of 225 MPa and 17.5%,respectively.

Keywords:Mg94Zn2.5Y2.5Mn1;ZrB2-modified LPSO;Microstructure;Mechanical properties.

1.Introduction

Due to the low density,good castability,high specifistrength and stiffness,magnesium alloys have valuable applications in industry[1–3].However,the poor plastic deformability of magnesium alloy due to the hexagonal structure greatly limits its further application.In recent years,high performance magnesium alloys have become a hotbed of research.Especially,Mg-Y-Zn alloys with long-period stacking ordered(LPSO)structure have drawn much attention due to their unique grain structure and excellent mechanical properties[4,5].As reported,the LPSO phase,W-phase and I-phase can be found in Mg-Y-Zn alloys according to the difference of Zn/Y atom ratios[6].And the LPSO phase can improve the mechanical properties of the magnesium alloys while the W-phase is deleterious to the ductility.Therefore,the improvement of the volume fraction of the LPSO phase and decrease the W-phase under the same conditions become the direction of our research.

Simultaneously,grain refinement as an effective way to enhance the alloy,can increase the density of grain boundary,thereby impeding the movement of dislocations[7].And it can also affect the formation of LPSO phase in Mg-Y-Zn alloys.Among the refinemen ways,modificatio is a convenient and effective method.The use of ceramic particles as a modifie has gradually attracted people’s attention.Based on the existing research on nanocomposites of magnesium alloy,better mechanical properties can be obtained since the good dispersion of nanoparticles in the magnesium matrix[8–10].ZrB2particles,as ceramic particles,have been widely applied in magnesium alloys.Dinaharan et al.[11]reported that in-situ formed ZrB2particles can act as outstanding reinforcements in AA6061 alloys thus increasing the hardness,strength and wear resistance.Paramsothy et al.[7]found that ZrB2particles can induce the formation of LPSO structure and increase the strength of Mg-RE alloy.Nevertheless,up to date,only few research has been completed on ZrB2-modifie Mg alloy with LPSO.In particular,the details about the impact of ZrB2on the development of LPSO phases are barely reported.

Accordingly,the different amounts(0.000,0.005,0.0075,0.01and 0.03 wt%)of ZrB2are added to Mg94Zn2.5Y2.5Mn1alloy,and this study aims to explore the impact of ZrB2on the microstructures and mechanical performance of Mg–Zn–Y–Mn alloys,particularly on the development of LPSO phases.

2.Experimental procedures

The rough material in this work was commercial pure Mg,Zn,Y,Mn and ZrB2powder(1μm average size)and the experimental magnesium alloys with different ZrB2addition were prepared in an electric resistance furnace.During melting,the alloy was in argon atmosphere.Then the molten magnesium alloys were cast into a preheated mold at 1003 K and the ingots were obtained.The Y-2000 X-ray diffraction(XRD)was used to analyze the phase constitution.The microstructures of the alloys were characterized by the DM2500M optical microscope(OM).Further observation and compositions were measured by the TESCANMIRA3 scanning electron microscope(SEM)equipped with energy dispersive spectrometer(EDS).The linear intercept method and image analysis technique were used to evaluate the grain size and the volume fraction of each phase,respectively.The tensile test was conducted by a DNS100 electronic universal material test machine with specimen size of 18 mm×4 mm×2.5 mm under a rate of 0.2 mm/min.At least three times repeated tests were carried out to obtain the average values of the tensile properties.The melting point of the alloy were obtained by an HCT-2 differential scanning calorimeter(DSC)with a heating rate of 20°C/min under nitrogen atmosphere.The nanomechanical properties of the alloys were conducted by Nano Indenter G200.

3.Results and discussion

3.1.Microstructure development

Fig.1a presents the SEM images of the as-cast Mg94Zn2.5Y2.5Mn1alloy with 0.0075 wt%ZrB2addition comparing to the matrix alloy without ZrB2addition(Fig.1b).Three main phases were observed,namely,α-Mg matrix,gray block second-phases,and white mesh eutectic phases.Further investigations of the EDS results(Fig.1c and d)and the XRD patterns(Fig.2)indicated that the gray block phase is the 18R-LPSO phase(Mg12YZn)and the white mesh phase is W-phase(Mg3Zn3Y2).

Fig.3 illustrates the OM images of the as-cast Mg94Zn2.5Y2.5Mn1alloys as function of ZrB2addition and the corresponding variation of the volume fraction of second phase.It can be seen that the grains were much fine with a uniform size when ZrB2content was increased from 0 wt%to 0.0075 wt%.Nevertheless,further increasing the addition level of ZrB2,the grains became coarse and transformed from uniform equiaxed dendrites to uneven dendrites.When 0.0075 wt%ZrB2was added,the homogeneous equiaxed dendrites with an average grain size of 24.87μm were observed(Fig.3c).Therefore,it is clear that the ZrB2particles can efficientl refin the Mg94Zn2.5Y2.5Mn1alloy.Many investigations have shown that in-situ ZrB2particles can refin the microstructure of the composites[12–14]and Al-4.99Zr-1.1B master alloy can refin the grains of AZ31 magnesium alloy due to the presence of ZrB2[15].

In order to understand the refinin mechanism of ZrB2particles,full attention should be paid to the characteristics of ZrB2particles.The ZrB2particle has a melting point of 2990°C which is higher than that of the matrix alloy so that it is stable in Mg melt.In view of the hexagonal structure of both ZrB2particles andα-Mg,the good lattice match between the ZrB2particles andα-Mg(～1.33)occurs only on the basal planes which their crystallographic orientation relationship is(0001)ZrB2‖(0001)Mg.The disregistry between the low index planes of them can be calculated by the following equa-and the result of the smallest disregistry is 5.11%[16],which is less than 6%(the critical value act as the effective heterogeneous nuclei).Therefore ZrB2particles can act as potential nucleator for matrix alloy.The test result is consistent with the result reported in previous research[15].

Fig.1.SEM images of as-cast Mg94Zn2.5Y2.5Mn1 alloy(a)with 0.0075 wt%ZrB2 addition and(b)without ZrB2 addition,EDS spectra for(c)block second-phase and(d)eutectic phase in as-cast Mg94Zn2.5Y2.5 Mn1 alloy.

Fig.2.XRD patterns of the as-cast Mg94Zn2.5Y2.5Mn1 alloys with and without ZrB2 additions.

As seen from Fig.3f,ZrB2particles can also induce the formation of LPSO phase.Without ZrB2addition,the volume fraction of LPSO phase is 9.93%.With the addition of ZrB2,the LPSO phase increases firs and then decreases,and even so the volume fraction of LPSO phase in the alloys with ZrB2addition are higher than that alloy free of ZrB2addition.When the amount of ZrB2is 0.0075 wt%,the volume fraction of LPSO phase is 25.62%,which is the highest.Previous research has revealed that stacking fault is a critical factor for the formation of LPSO phase[17].The lower the stacking fault energy(SFE)is,the more easily the stacking fault form.Also,the alloying elements have great influenc on stacking fault energy[18].With the addition of ZrB2,on the one hand,free Zr released from ZrB2particle can decrease the SFE of theα-Mg in(0001)α-Mgplane.As reported[17],the LPSO phase was formed on the habit plane of(0001)α-Mgand it grew along the direction of[01¯10]α-Mg.So the LPSO phase can form more easily with the increase of stacking faults which is due to the decrease of the SFE.On the other hand,it is known that the uniform and fin equiaxed dendrites are more unstable than the corresponding columnar dendrites by increasing the grain boundary surface energy and the grain boundary surface area per unit volume.When the original equiaxed dendrites comprised enough dissolved Y and Zn,the LPSO phase is more inclined to be formed firstl in the less stable equiaxed dendrites.Furthermore,the concentration of Zn and Y atoms decreases in the local area due to ZrB2refinement As is known to all,the Y and Zn atoms required to form the LPSO phase are much less than that for the W phase and the LPSO phase forms prior to the W phase[19].Therefore,the LPSO phase is easy to form compare to the W phase at a certain moment.So the volume fraction of the LPSO phase is significantl improved.

Fig.4a is the DSC curve of as-cast Mg94Zn2.5Y2.5Mn1alloy containing 0.0075 wt%ZrB2.Research showed that the melting point of the known three phases from high to low isα-Mg,LPSO structure and W phase,respectively,and the high melting phase was firs precipitated during solidificatio process[19].Therefore,the order of phase formation is accordinglyα-Mg,LPSO structure and W phase,respectively.

Fig.3.Optical micrographs and volume fraction of second phase of the as-cast Mg94Zn2.5Y2.5Mn1 alloys with different ZrB2 addition:(a)0.000 wt%ZrB2,(b)0.005 wt%ZrB2,(c)0.0075 wt%ZrB2,(d)0.01 wt%ZrB2,(e)0.03 wt%ZrB2,and(f)volume fraction of second phase.

As illustrated by Fig.4a,when the temperature of the melt down to about 629 °C,theα-Mg formed firstl.With the temperature of the melt decreasing,the Y and Zn solute atoms are continuously discharged from the melt alloy asα-Mg grows,consequently,leading to forming the atoms enrichment region in front of the liquid-solid interface.When the temperature drops to near 540°C,the LPSO phase formed due to the constituent fluctuatio in the melt and the remaining melt alloy continued to form the W phase during the process of temperature drop.Fig.4b and c are the schematic diagram of the constitutional supercooling(CS)and the corresponding changing process of crystal growth morphology(Fig.4d).The constitutional supercooling(Fig.4c)formed by the enrichment of the solute atoms at the front of the liquid-solid interface can change the growth mode of the crystal[20].When the critical undercooling is lower than the maximum undercooling of the CS zone,the CS can significantl inhibit the growth of the crystal[21].As illustrated by Fig.4c and d,with the increase of the degree of CS,the crystal growth morphology of the melt alloy is transformed from the plane crystal without the CS to the equiaxed dendrite in the wide CS zone during solidification

As evaluated from the crystallization theory,the number of grain in unit volume(Zv)and unit area(Zs)can be expressed by

Fig.4.The DSC curve of as-cast Mg94Zn2.5Y2.5Mn1 alloy with 0.0075 wt%ZrB2 addition(a)and the schematic diagram of grain refinemen by ZrB2 addition.

where N is the nucleation rate andVgis growth speed.The N andVgin the crystallization of metal increase with the increase of undercooling,and the growth rate of N is greater than that ofVg.So increasing the undercooling will increase the ratio of N/Vg,and the V value increases,thereby refinin the grain.

Fig.4e–k are the proposed schematic diagram of the solidificatio process.Without the addition of ZrB2,a certain CS zone is formed by the enrichment of the solute atoms in front of liquid-solid interface during the solidificatio process.Due to the limit of the CS zone in the growing process of the crystal,the crystal cell protuberance extends slightly to the melt alloy and its growth direction begins to shift to the preferential crystal growth direction.The secondary dendrite appears and the tertiary branch is split at the front end in the subsequent growth.The growth schematic diagram can be seen from Fig.4f–h.The continuous branching leads to the rapid formation of columnar dendrites skeleton.Fig.4i–k are the crystal growth mode of the melt alloy containing 0.0075 wt%ZrB2.ZrB2,consisting of Zr and B,which are superconstituent supercooling elements,has a strong nucleophilic ability.Research[15]showed that ZrB2can act as effective heterogeneous core forα-Mg matrix during solidification On the one hand,the addition of ZrB2is beneficia to the formation of atomic enrichment area,which makes it easier to form a wide undercooling region.The maximum value of the CS at the liquid-solid interface is higher than that required for heterogeneous nucleation.The nucleation occurs in the melt which resulting in the free growth of crystals and thus forming the equiaxed dendrites of various orientations.On the other hand,ZrB2is used as heterogeneous core ofα-Mg in molten alloy.A large number of heterogeneous nuclei lead to a strong dissociation of the grains,which is beneficia to the existence of free crystals in the melt.Also,the grain is inhibited to each other in the process of growing up[22].So the equiaxed grains is formed.At the same time,microfree Zr produced by the reaction of ZrB2with the Mg matrix also has vital grain refinemen influenc on the alloy due to its similar unit cell type and size as Mg[23].

Fig.5.(a)Load-depth curves and(b)corresponding elastic modulus and hardness for W-phase,18R-LPSO phase and α-Mg in as-cast Mg94Zn2.5 Y2.5Mn1 alloy containing 0.0075 wt%ZrB2.

Fig.6.Tensile stress-strain curves of the as-cast Mg94Zn2.5Y2.5Mn1alloys with different ZrB2 addition(a)and the corresponding fracture diagram:(b)0 wt%ZrB2,(c)0.0075 wt%ZrB2.

3.2.Mechanical properties analysis

The load-depth curves,corresponding elastic modulus and hardness for the W phase,18R-LPSO phase andα-Mg in the as-cast Mg94Zn2.5Y2.5Mn1alloy with 0.0075 wt%ZrB2addition are shown in Fig.5.The deformation of the three phases is different from each other.It can be seen in Fig.5a,when the load reaches the maximum(5 mN),the penetration depths of the W phase,18R-LPSO phase andα-Mg are up to 328 nm,385 nm and 474 nm,respectively.As shown in Fig.5b,the elastic modulus and hardness of the W phase are the highest while theα-Mg is the lowest.But the W phase is unfavorable to deformation owing to the characteristics of hard and brittle[24].Previous research have shown that the LPSO phase could effectively improve the mechanical performances due to its high elastic modulus and hardness by inhibiting the movement of dislocations[25].So the LPSO phase is recognized as a strengthening phase in Mg94Zn2.5Y2.5Mn1alloy.

The typical stress-strain curves of the as-cast matrix alloy with diverse ZrB2addition are shown in Fig.6a.As seen,the mechanical properties of the matrix alloy can be significantl improved due to the addition of ZrB2.When 0.0075 wt%ZrB2is added,the maximum tensile strength and ductility reach 225 MPa and 17.5%,respectively,which is the highest with 30.8%and 1.5 times higher than those of ZrB2-free matrix alloy respectively.However,further increase of the content of ZrB2leads to a slight decrease in strength and ductility.This is also consistent with the change regulation of as-cast microstructure.

Mg is a hexagonal close-packed structure and the crystal structure generally cleaves along the(0001)plane.Fig.6b reveals the tensile fracture morphology of the matrix alloy.It can be seen that the fracture has obvious cleavage steps and the larger cleavage surface.The fracture mode of the matrix alloy is cleavage fracture,and the fracture surface shows a river pattern.Fig.6c shows the fracture morphology of the matrix alloy containing 0.0075 wt%ZrB2.An obvious tearing edge appears in the fracture,the cleavage surface becomes smaller and the dimple appears,which presents a comprehensive fracture of brittle fracture and plastic fracture.As the dimples are the characteristics of plastic deformation,the more dimples are,the more plastic fracture is,thus increasing the plasticity.This is also consistent with the increase in the elongation of the matrix alloy after the addition of 0.0075 wt%ZrB2(Fig.6a).In addition,the cracks only initiate at the rupture of LPSO phase due to the axis-to-axis orientation relationship between the LPSO phase and Mg matrix[26].Therefore the strength and plasticity were increased owing to the strengthening effect of the increased LPSO phase.

In the present system,two main reasons can be used to explained the improvement of strength and elongation.On the one hand,the addition of ZrB2refin the grain size.The decrease of the grain size increases the resistance for the dislocations motion across grain boundaries.On the other hand,ZrB2can induce the formation of LPSO phase which is a high strength and toughness phase.The LPSO phase is able to withstand load transfer and can be bent and kinked during deformation.Therefore,the strength and ductility are increased.

4.Conclusions

The microstructure and mechanical properties of the ascast alloy change significantl with the addition of ZrB2particles:

(1)Moderate addition of ZrB2particles made the grains nearly equiaxed in shape with a homogeneous size in based alloys.As 0.0075 wt%ZrB2addition,the alloy presented fines grains with an average grain size of 24.87μm.

(2)ZrB2particles modifie the crystallography of the alloys in the as-cast condition,especially,the addition of ZrB2particles promotes the formation the 18R structure.

(3)The modificatio of ZrB2could effectively enhance the

ductility while benefithe strength of the Mg-Zn-Y-Mn alloys.The alloy with 0.0075 wt%ZrB2exhibits the best mechanical properties with ultimate tensile strength and plasticity of 225 MPa and 17.5%,respectively.

Acknowledgments

The authors acknowledge the financia support of Shanxi key laboratory of advanced magnesium-based material,National Natural Science Foundation of China(No.51474153 and 51574175)and Ph.D.Programs Foundation of Ministry of Education of China(20111402110004)