Microstructure and mechanical properties of Mg-3.0Nd-0.4Zn-0.4Zr magnesium alloy

aMachinery and Electric Department,Heilongjiang Institute of Technology,999 Hongqi Street,Harbin 150050,PR China

bSchool of Materials Science and Engineering,Harbin University of Science and Technology,4 Linyuan Road,Harbin 150040,PR China

Microstructure and mechanical properties of Mg-3.0Nd-0.4Zn-0.4Zr magnesium alloy

L.H.Wena,b,*,Z.S.Jib,M.L.Hub,H.Y.Ninga

aMachinery and Electric Department,Heilongjiang Institute of Technology,999 Hongqi Street,Harbin 150050,PR China

bSchool of Materials Science and Engineering,Harbin University of Science and Technology,4 Linyuan Road,Harbin 150040,PR China

Mg-3.0Nd-0.4Zn-0.4Zr magnesium alloy were prepared by cast-extruding and chip-extruding.Microstructure,tensile and creep properties of the alloy were investigated.The results show that the alloy exhibit particle dynamic recrystallization during extrusion.The cast extruded-T6 rods at room temperature have a highest tensile strength of 258.5 MPa and a highest yield strength of 135.7 MPa.With the increase of test temperature,the strength of the alloy declines and the elongation increases.At 473 K,creep strain in the primary creep stage increases with increasing the creep stresses.Under 110 MPa,time spent during primary creep decreases with increasing the test temperatures.Stress exponent and creep activation energy of the alloy are 4.4 and 104 kJ/mol,respectively.Creep of the alloy can be controlled by dislocation climb mechanism.The morphology of the fracture surfaces was examined by employing scanning electron microscope.

Mg-3.0Nd-0.4Zn-0.4Zr alloy;Hot extrusion;Microstructure;Properties;Fracture behavior

1.Introduction

Magnesium alloy are the lightest alloy used as structural metallic alloy,and magnesium products have been applied for structural uses in the automotive,railway and aerospace industries[1-4].However,because of resource depletion in the future,recycling of materials is becoming more and more important in order to ensure the sustainable development ofsociety.In general,metallic scraps can be remelted to cast into ingot for subsequent processing.However,this liquid state recycling may not be eff i cient for magnesium alloy because of the needs of special protective environment and extra caution. Masaru Nakanishi et al.[5]proposed that extrusion is an effective processing method for eff i ciently reclaiming magnesium machined chips because it is not expensive.Extrusion of machined chips is conducted in a solid state,which does not need a special protective environment or extra caution. Because microstructural control such as grain ref i nement and dispersion of the oxide f i lm on the surface of scraps can be achieved due to the severe deformation during extrusion[6]. The solid recycled materials show excellent mechanical properties[7,8].

Many studies showed that magnesium alloys such as AZ31, ZK60 and AZ91 prepared by solid recycling process exhibited high tensile strength and high elongation to failure.In this paper,microstructure,tensile and creep properties of Mg-3.0Nd-0.4Zn-0.4Zr magnesium alloy produced byextrusion of cast and extrusion of chips are discussed.Coldpressing was employed to prepare extrusion billets of Mg-3.0Nd-0.4Zn-0.4Zrmagnesium alloy chips.Hot extrusion was carried out at 723 K.Extrusion ratio is 11.1:1.

2.Experimental procedures

Mg-3.0Nd-0.4Zn-0.4Zr magnesium alloy chips were prepared by machining an as-received ingot in a lathe.The size of a chip was 4 mm-5 mm in length,1.5 mm-2 mm in width and 0.5 mm in thickness,as shown in Fig.1.The machined chips were f i lled into a cylindrical container with a diameter of 40 mm and cold-pressed by slowly increasing pressure up to 300 Mpa for 30 s at room temperature into billets.Then chip billets and cast billets were hot extruded into rods.The extrusion temperature was 723 K with a speed of about 0.15 mm/s.The diameter of as-extruded rod was 12 mm with extrusion ratio of 11.1:1.Some of extruded specimens were solution treated at 803 K for 2.5 h in an electric resistance furnace under the mixed atmosphere of SO2and SF6with the ratio of 100:1,quenched into hot water at ～333 K,and then subsequently aged at 473 K for 16 h.

Tensile experiments at both ambient and elevated temperatures were performed in air using WDW-10 electron universal strength testing machine.Tensile creep tests were carried out using a constant load creep machine with a three zone furnace. The extension of the specimen was measured by a linear variable differential transformer mounted on the specimens. The temperature was maintained constant within ±2°C during testing.The tensile axis was parallel to the extrusion direction. Each datum was the average of results from more than three samples.Specimens were etched in 4%nitric acid liquor for microstructure observation.Microstructures were examined by using OLYMPUS-GX71-6230A optical microscope(OM).A FEI-SIRION scanningelectronmicroscope(SEM)was employed for the observation of the fracture surfaces of the specimens.

3.Results and discussion

3.1.Microstructure of cast

Fig.2 shows optical micrographs of as-cast materials in different states.Coarse grains with the eutectic intermetallic phase covering the primary α-Mg grains are seen in Fig.2(a),large precipitates are found along grain boundaries,which make its mechanical properties worse.During homogenizing annealing at 753 K for 48 h,segregation atom can dissolve into matrix structure or form strengthening phase,as shown in Fig.2(b),segregation microstructure is improved.It is obvious that the cast extrusion specimens exhibit dynamic recrystallization and the grains ref i ne greatly.It can be seen from Fig.2(c)that some second-phase particles were broken into small particles and moved from grain boundaries to grain interiors during the hot extrusion process.

Fig.1.Chips of the Mg-3.0Nd-0.4Zn-0.4Zr alloy.

3.2.Microstructure of chip cast after hot extrusion

Fig.3(a)and(b)shows the microstructure of the transverse section and the longitudinal section of the chip-extruded rods, respectively.It can be seen from Fig.3 that some coarse intermetallic phases were broken into small particles and moved from grain boundaries to grain interiors during the hot extrusion process.Fine equiaxed sub-grains exist in elongated grains.This kind of discontinuous particle morphology can improve the ductility of the alloy by reducing the nucleation, growth and coalescence of cracks and cavities at grain boundaries.The broken particles are gathered and distributed perpendicular to the extrusion direction.

3.3.Tensile properties of Mg-3.0Nd-0.4Zn-0.4Zr alloy

Table 1 presents the tensile properties at room temperature of the alloy in different conditions.For the cast specimens, improvements of ultimate tensile strength(UTS),tensile yield strength(TYS)and elongation are observed from as-cast to cast-extruded condition,by 63.3 MPa,31.8 MPa and 19.2%, respectively.In comparison with the as-cast condition,further obvious increases of UTS,TYS and elongation are achieved by extrusion of machined chips.After T6 heat-treatment,the UTS and the TYS have a increase,but the elongation decreases due to the precipitation hardening.

The maximum solid solubility of the neodymium in the α-Mg matrix is as high as 3.6 wt%at 825 K and it decreases rapidly with the decrease of temperature according to the phase diagram[9].At the ambient temperature the solubility of Nd in the α-Mg matrix is negligible.Due to nonequilibrium solidif i cation the α-Mg in the alloy studied is oversaturated and part of neodymium forms divorced eutectic Mg12Nd instead of precipitates in the as-cast alloy,which makesthepropertiesworse.During thehotextrusion process,Mg12Nd intermetallics was destroyed and broken into small particles,responsible for a substantial strengthening.Furthermore,hot extrusion may promote the formation of f i ner microstructure.According to the well-known Hall-Petch relation,the yield stress depends on the grain size as follows [10,11]:

Fig.2.Microstructures of the cast alloy in different states(a)as-cast(b)cast after homogenizing anneal(c)cast after extruding.

Fig.3.Microstructures of the chip-extruded rods of Mg-3.0Nd-0.4Zn-0.4Zr alloy(a)transverse section(b)longitudinal section.

where Δσ0.2is the increase in yield stress due to grain ref i nement,Kis a constant anddis the grain size.It is well known that the strengthening source of T6 specimens is precipitation strengthening.During aging,a large amount of the second phase particles precipitate from the α-Mg matrix. Precipitation of the second phase particles is likely to bring about additional barriers to the movement of dislocations, resulting in the enhancement of tensile strength[12].In addition,due to the uncoordinated deformation with the matrix,the precipitates may act as crack sources and in turn decrease the elongation of tensile samples.

Table 1Tensile properties of the alloy at room temperature.

Table 2Tensile properties of the alloy at 150,200 and 250°C.

Fig.4.Tensile properties of the Mg-3.0Nd-0.4Zn-0.4Zr alloy at different temperatures(a)tensile strengths(b)elongations.

Fig.5.(a)Creep strain versus 100 h plots tested under the stresses from 70 MPa to 130 MPa at 473 K(b)creep strain versus 100 h plots tested under the stresses from 130 MPa to 150 MPa at 473 K.

Table 2 lists the data of tensile tests of the alloy at elevated temperatures and the variations of tensile properties with the increase of temperature are shown in Fig.4.It can be seen that the alloy in both states exhibit lower strengths but higher ductilities than those at room temperature.The cast-extruded alloy has higher strengths and ductilities than those of the chip-extruded alloy.With the increase of temperature,the strength is decreasing and the elongation is increasing.Hot extrusion makes the magnesium alloy being in three-direction compressed stress state,which means that the alloy are in the high-plasticity.The dominant deforming process during extrusion is grain boundary sliding,which will break and disperse the surface oxide layer of chips[13].Dispersive oxide magnesia grains break continuity of the alloy,which leads to poor mechanical properties.During aging treatment,the tiny precipitates form from the oversaturated matrix.These precipitates have the effect on pinning dislocation movement [14].The β-phase(Mg12Nd)is thermally stable at elevated temperatures and has a certain orientation relationship with the α-Mg matrix.Moreover,the difference of atomic radii between magnesium and neodymium is possible to operate.

3.4.Creep properties of the chip-extruded(T6)alloy

3.4.1.Creep curves

Fig.6.(a)100 h creep curves at temperatures of 423,448 and 473 K under the applied stresses of 110 MPa(b)100 h creep curves at temperatures of 498 and 523 K under the applied stresses of 110 MPa.

Creep tests were conducted at f i ve temperatures of 423, 448,473,498 and 523 K under the applied stresses from 70 MPa to 150 MPa.Fig.5(a)and(b)shows the typical plot of creep strain versus time up to 100 h for the chip-extruded(T6) specimens tested under the stresses from 70 MPa to 150 MPa at 473 K.Creep Strain and steady-state creep rate of the specimens deformed at a stress of 150 MPa are higher than that of the specimens deformed at other stresses.It is seen from Fig.1 that creep resistance of the alloy is decreasing with increasing the applied stresses.The alloy which were tested under the stresses from 70 MPa to 110 MPa have excellent creep resistant properties.Steady-state creep rate of the specimen tested at 473 K/110 MPa is 5.73 × 10-8s-1. However,steady-state creep rate of the alloy at 473 K/ 150 MPa is added to 1.27 × 10-6s-1,near two order of magnitude higher than that of the alloy at 473 K/110 MPa. Creep curve of the alloy at 473 K/150 MPa ends in the secondary creep stage after 62 h creep deformation and the specimen ruptured after 76 h test.It can be seen from creep curves that creep strain in the primary creep stage increases with the creep stresses increasing.

Fig.7.The stress dependence on steady-state strain rate(dε/dt)at 473 K.

Fig.6(a)and(b)shows the 100 h creep curves of the chipextruded(T6)specimens at f i ve temperatures of 423,448,473, 498 and 523 K under the applied stresses of 110 MPa.Creep strain and steady-state creep rate of the specimen at 523 K are sharply higher than that of the specimens at other temperatures,and creep curve ends in the secondary creep stage after 9 h creep deformation and the specimen ruptured after 14 h test.Creep properties of the alloy decrease with increasing test temperature,which is shown in Fig.6.When test temperature is below 473 K,the alloy has excellent creep resistance. Steady-state creep rate of the specimen on the condition of 448 K/110 MPa is 5.14 × 10-8s-1.However,steady-state creep rate of the alloy at 498 K/150 MPa is increased to 2.33 × 10-7s-1,near one order of magnitude higher than that of the alloy at 448 K/110 MPa.Creep curve of the alloy at 498 K/110 MPa ends in the secondary creep stage after 60 h creep deformation and the specimen ruptured after 79 h test.It can be seen from creep curves that creep strain in the primary creep stage increases and time spent in the primary creep stage decreases with increasing test temperatures.

Fig.8.An Arrhenius plot for the creep test under 110 MPa.

Fig.9.SEM images of the tensile fracture surfaces of the alloy in different conditions(a)cast extruded-T6 at 250°C(b)chip extruded-T6 at 250°C.

Themainstrengtheningphaseinthealloyistheplate-shaped β phase,which has a high melting point of approximately 1293 K[15].Although the β phase has a bottom-centered square structure,which is incoherent with the h.c.p.lattice of the magnesium matrix,its high melting point and strong intermetallic bonding as well as the low diffusion rate of Nd element in magnesium all contribute to a thermally stable alloy. Therefore,the sliding of grain boundaries and the slip of dislocations in the matrix were effectively prevented at elevated temperatures,improving the properties at high temperature, especially the creep properties.

3.4.2.Creep mechanism

Generally,the steady-state creep rate(dε/dt)can be represented by the relationship between stress(σ)and temperature (T)as follows[16,17]:

whereAis a constant;nis the stress exponent andQcis the apparent activation energy for creep.Fig.7 illustrates the stress dependence on steady-state strain rate(dε/dt)at 473 K. Fig.8 shows an Arrhenius plot for the creep test of the alloy under 110 MPa.The stress exponent and the creep activation energy of the alloy are 4.4 and 104 kJ/mol,respectively,which suggests the creep of the alloy can be controlled by dislocation climb mechanism[18].

Fig.10.(a)SEM images of the creep fracture surface of the sample which failed after 76 h at 200°C/150 MPa(b)magnif i cation of pane in Fig.10(a).

3.5.Fractured surface of the specimen

3.5.1.Tensile fracture behavior

Fig.9(a)and(b)shows SEM images of the tensile fractured surfaces of the alloy in different conditions.It can be seen from Fig.9 that the failure surfaces are composed of some small dimples and a few cracked particles.The cracked particles are Nd-containing intermetallics.Due to the fragile characteristic of Nd-containing compounds,the particles were broken during the tensile test(or during extrusion)and become a cracking source.Moreover,apparently tearing ridges are observed in the fracture surface of the alloy.Cleavage planes, some dimples and tear ridges are observed in the failure surface.The cleavage planes mean that the direction of the cracks frequently changes during propagation and the crack extension resistance increases.The cast-extruded alloy present goodductility because the dimples in the fracture surface are more and bigger than that of the chip-extruded alloy.

3.5.2.Creep fracture behavior

Fig.10 shows SEM images of the creep fracture surface of the sample which failed after 76 h at 200°C/150 MPa.Many ellipse-like cracks are observed on the creep fracture surface as shown in Fig.10(a).During the creep test,vacancy of crystal lattice moves to the tensile direction at high temperature and stress and stops at grain boundaries that are pulled and f l ows inside grains.Then,voids on the grain boundaries and f l ows inside matrix are formed after a lot of vacancies congregated.Finally,ellipse-like cracks are formed.It can be seen from Fig.10(b)that some dimples and tear ridges are presented in the failure surfaces of the alloy. Some Nd-rich compounds are observed at the bottom of the dimples.

4.Conclusions

(1)The microstructure of the as-cast alloy consists of dendritic α-Mg and divorced eutectic Mg12Nd.The castextruded and chip-extruded alloy exhibit particle dynamic recrystallization.

(2)All the extruded alloy have better tensile properties than the cast alloy.The cast extruded-T6 rods at room temperature have a highest UTS of 258.5 MPa and a highest TYS of 135.7 MPa due to the precipitation hardening. With the increase of test temperature,the strength of the alloy declines and the elongation increases.Stress exponent and Creep activation energy of the alloy are 4.4 and 104 kJ/mol,respectively.Creep of the alloy can be controlled by dislocation climb mechanism.

(3)SEM observations of the fracture surfaces reveal that the fracture mode of the alloy at elevated test temperature is a mix mechanism with brittle fracture and gliding fracture.

Acknowledgments

The authors gratefully acknowledge the f i nancial support by the Chinese National Science Foundation(No.50674038; No.50974048)andtheHarBinScienceandTechnologyBurean (No.2011RFQXG020).

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Received 29 October 2013;accepted 6 January 2014 Available online 27 March 2014

*Corresponding author.Machinery and Electric Department,Heilongjiang Institute of Technology,999 Hongqi Street,Harbin 150050,PR China.Tel.: +86 451 88028776;fax:+86 451 86674840.

E-mail address:jmswlh2000@aliyun.com(L.H.Wen).

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Copyright 2014,National Engineering Research Center for Magnesium Alloys of China,Chongqing University.Production and hosting by Elsevier B.V.All rights reserved.