Investigation on microstructures and mechanical properties of Mg-6Zn-0.5Ce-xMn (x=0 and 1) wrought magnesium alloys

Cihong Hou, Hongshui Co, Fugng Qi,*, Qing Wng, Linhui Li, Nie Zho,*,Dingfei Zhng, Xioping Ouyng

a School of Materials Science and Engineering, Xiangtan University, Xiangtan 411105, PR China

b China Railway Eryuan Engineering Group Co.Ltd., Chengdu 610000, PR China

cCollege of Materials Science and Engineering, Chongqing University, Chongqing 400045, PR China

Abstract The microstructure evolution and mechanical properties of Mg-6Zn-0.5Ce-xMn (x=0 and 1wt.%) wrought magnesium alloys were researched, and the morphologies and role of Mn element in the experimental alloys were analyzed.The research shows that all of Mn elements form the α-Mn pure phases, which do not participate in the formation of other phases, such as the τ-phases.The mechanical properties of Mn-containing alloys in as-extruded and aged states are superior to Mn-free alloys.During the hot extrusion process, the dispersed fin α-Mn particle phase hinders the migration of grain boundaries and inhibits dynamic recrystallization, which mainly takes effect of grain refinin and dispersion hardening.During the aging treatments, the dispersed fin α-Mn particle phase not only hinders the growth of the solution-treated grains, but also becomes the nucleation cores of β′1 rod-like precipitate phase, which is conducive to increasing the nucleation rate of the precipitate phase.For the aged alloy, the Mn addition mainly takes effect of grain refinin and promoting aging strengthening.

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Keywords: Mg-6Zn-0.5Ce alloy; Microstructure evolution; Mechanical property; Mn element; Aging precipitation.

1.Introduction

As the lightest metal material, Mg alloys have good thermal conductivity, good shock absorption and good electromagnetic shielding, so they are widely used in automotive,aerospace and electronics field [1-3].Nowadays, most Mg alloy structural components are processed by die casting, and 80% of die castings are used in the automotive industry [4].Compared with cast Mg alloys, wrought Mg alloy materials have more potential for development.Plastic deformation can produce various sizes of bars, pipes, profiles plates and forgings, etc.Through the control of material structure and the application of heat treatment process, wrought Mg alloy can obtain better strength and ductility, which is more suitable for the production of large-scale structural parts and meet the requirements of diverse structures.However, the absolute strength of Mg alloys is low and the plasticity at room temperature is poor, which greatly limits their development and industrial applications.In order to expand the application,it is necessary to develop novel wrought Mg alloys with excellent comprehensive mechanical properties.

Mg-Zn based alloys with high Zn content are one of the most commonly used Mg alloy systems.In this alloy system, the content of Zn element generally ranges from 4 to 9wt.%, and its age hardening effect is remarkable, so it has great potential for improving property through alloying and heat treatment [5,6].Rare earth elements have a unique extranuclear electronic structure and play a unique role in met-allurgy and materials, such as purifying alloy melts, refinin alloy structures, improving mechanical properties and corrosion resistance, etc.[7].They are considered to be the most valuable and developmental alloying elements in Mg alloys.Among them, Ce element is a cheaper RE element, which can be used as a modificatio element of Mg-Zn binary alloy [8,9].It cannot only refin the grain of Mg-Zn alloy,but also serve as a texture modifie [10].Therefore, for the Mg-Zn-Ce system alloy, it has great potential to develop into the high plastic wrought Mg alloy [11,12].At present, some scholars have studied the influenc of Ce content on the microstructure and property of Mg-Zn alloys [13-18].It's worth noting that the elongation improvement is not obvious for alloys with higher alloying levels of Ce,or even worse.The reason is that the Ce-containing intermetallic compounds fracture early [19].Chen et al.[18]found that the superior mechanical properties could be obtained by the addition of 0.5wt.%Ce in Mg-6Zn-0.5Zr alloy.

It is well known that adding alloying elements is one of the most effective methods to improve the properties of Mg alloys.The scholars have found that Zr element could improve the performance of Mg-Zn-Ce alloy [20-22].Mn is also one of the important alloying elements in Mg alloys,and the maximum solubility in Mg matrix is close to 2.2wt.%.In General, a small amount of Mn (generally not more than 0.3%)is added to Mg alloys, and its main function is to remove impurity elements.However, in recent years, some researchers have found that higher Mn content can refin grains and improve mechanical properties [23-26].In the early stage, our research team studied the effect of different Mn content (0.4,0.68 and 1.02wt.%) on the structure and properties of Mg-6Zn-xMn wrought Mg alloy and found that Mn can refin the grains, and the mechanical properties of the alloy are better when the content of Mn is 0.68wt.% and 1.02wt.% [27].In addition, our research team also studied the microstructure and mechanical properties of Mg-xZn-1Mn (x=4, 5, 6, 7, 8,9wt.%) alloy and found that when the Zn content is 6wt.%,the alloy has the best mechanical properties [28].In order to reduce the cost, we can try to replace the Zr element with the Mn element, and then add it to the Mg-Zn-Ce ternary alloy to improve the performance of the alloy, with a view to developing a novel type of high-strength wrought Mg alloy.Therefore, in the present work, we study that the microstructure and mechanical properties of Mg-6Zn-0.5Ce alloy with the 1wt.% Mn addition, and explore the morphologies and role of Mn element in experimental alloys.

2.Experimental

The raw materials of Mg-6Zn-0.5Ce-xMn (x=0 and 1) alloys were industrial Mg(≥99.9wt.%), industrial Zn(≥99.9wt.%), Mg-20.82wt.% Ce and Mg-4.10wt.%Mn master alloys.The experimental alloys were prepared by a domestically produced ZG-01L vacuum induction melting furnace.The melting process of the alloys was mainly divided into the following three steps.First, according to the burnout loss rate, calculated the alloy element ratio andprepared the alloy raw materials.Next, put the graphite crucibles containing the alloy raw materials into the melting furnace, and pumped into argon (Ar) as a protection gas,and then started heating to 690-750 °C, kept warm until the raw materials were fully melted.Finally, the alloy melt was cast in a metal mold, and the mold is cooled and demolded in the air.The composition of the as-cast alloys obtained by melting was tested.The experimental equipment was 1800CCDE X-ray fluorescenc spectrometer (XRF).The test sample was processed into a cylindrical shape with a diameter of 33mm, and the sample was ground for chemical composition testing.The actual chemical composition results of the as-cast experimental alloys are shown in Table 1.

Table 1Chemical composition of the as-cast experimental alloys.

Fig.1.Schematic map of tensile test specimen at room temperature.

The ingots were then subjected to homogeneous annealing for eliminate the non-equilibrium eutectic compounds, that is,firs heating at 330°C for 16h and second heating at 420°C for 2h.The as-homogenized alloy ingots were extruded on an XJ-500 horizontal extruder into cylindrical bars with a diameter of 16mm, and the extruded bars were air-cooled.The selected extrusion parameters were listed in Table 2.For the purpose of age hardening, the extruded alloy bars were treated with solid solution (T4) at 420°C for 2h, and then quenched by water.Then, the solid solution Mg alloy bars were artificiall aged, and the parameters were shown in Table 3.

Table 2Extrusion parameters of Mg-6Zn-0.5Ce-xMn wrought Mg alloys.

Table 3Heat treatment parameters of Mg-6Zn-0.5Ce-xMn wrought Mg alloys.

A CMT-5105 microcomputer-controlled electronic universal testing machine was used to test the mechanical properties of the as-extruded and aged alloy bars at room temperature.It was used to analyze important mechanical property parameters of alloys such as yield strength, ultimate tensile strength and elongation.The standard parts of tensile specimens were prepared according to the national standard GB/T228-2002.The schematic map of tensile sample was shown in Fig.1.The tensile test adopted uniform speed unidirectional displacement stretching, and the stretching rate was 3mm/s.

The structural constituents of the alloys were analysed with a Rigaku D/MAX-2500PC X-ray diffractometer using Cu-Kαradiation with a scanning angle from 10° to 90° and a scan-ning rate of 4°/min.The metallographic microstructures of the experimental alloys in different states were systematically studied by an NEISS NEOPHOT 30 optical microscope.The secondary electron (SE) probe, backscattered electron (BSE)probe and energy dispersive spectrometer (EDS) provided by the scanning electron microscope(SEM)were used to observe the morphology, quantitative or semi-quantitative analysis of the second phase, and analysis of the tensile fracture.The SEM equipment model was TESCAN VEGA Ⅱ, and the EDS model was OXFORD INCA Energy 350.The transmission electron microscope (TEM) was used to perform bright fiel(BF) image and high resolution (HR) electron microscopy analysis on the alloy,and the morphology,composition,structure and distribution of the precipitated phase were analyzed.The test equipment was a Zeiss LIBRA 200 FE TEM.

3.Results and discussions

3.1.Microstructures of as-cast alloys

The X-ray diffraction (XRD) patterns of the as-cast Mg-6Zn-0.5Ce and Mg-6Zn-0.5Ce-1Mn alloys are shown in Fig.2.As can be seen from Fig.2a, the Mg-6Zn-0.5Ce alloy is mainly composed ofα-Mg phase, Mg7Zn3phase andτ-phase.During the solidificatio of the alloy, theτ-phase rich in Zn and Ce forms which is identifie as a C-centered orthorhombic crystal structure by Wei et al.[29].After the addition of Mn, theα-Mn phase diffraction peaks are also detected in the Mg-6Zn-0.5Ce-1Mn alloy, as shown in Fig.2b, indicating that Mn addition has no effect on the phase composition of Mg-Zn-Ce alloy.

Fig.2.XRD patterns of the as-cast (a) Mg-6Zn-0.5Ce and (b) Mg-6Zn-0.5Ce-1Mn alloys.

Fig.3 (a and b) are the as-cast metallographic structures for Mg-6Zn-0.5Ce-xMn (x=0 and 1) alloys.It can be seen from the figur that the metallographic structure is mainly composed of theα-Mg matrix,the compounds which are concentrated on the grain boundary and between the dendrite,and the dispersed secondary phases particles.Compared with Fig.3a and Fig.3b, it is found that the network compound becomes continuous, and the area fraction of spherical secondary phases increases significantl after the Mn addition.Meanwhile, it indicates that the dendrites are partly refined and the dendrites spacing is reduced with the addition of Mn,although it is not dramatic.The reason is that the solid solubility of Mn in Mg is only 2.2wt.%, and most of the Mn element is dissolved in the Mg matrix.This means that the Mn element cannot play a nucleation role in the melting.Therefore, the as-cast dendritic microstructure cannot be significantl refine by the Mn addition.Fig.3c shows the SEM and mapping scanning images of Mg-6Zn-0.5Ce-1Mn alloy.It indicates that the fishbone-li e compounds are made up of Mg element, Zn element, Ce element and Mn element.According to the elemental mapping scanning images from the alloy, Mg atoms are uniformly distributed in the matrix, Zn atoms and Ce atoms are enriched in the eutectic phases, and the Mn atoms are uniformly distributed in the eutectic dendrites.Based on the EDS analysis, the particle should be theτ-phases.

3.2.Microstructures of as-homogenized alloys

Fig.4 (a and b) are the as-homogenized metallographic structures for Mg-6Zn-0.5Ce-xMn (x=0 and 1) alloys.It is shown that a small amount of spherical secondary phase particles distributes in the grains and a majority of semicontinuous network eutectics locates on the grain boundaries.Compared with the as-cast optical microstructures, during the homogenization treatment, the continuous network compounds between the dendrites and the granular compounds inside the dendrites have partly dissolved.According to the EDS analysis, it is found that the homogenization treatment can dissolve the as-cast Mg-Zn eutectic compounds, but the Mg-Zn-Ce rare-earth compounds remain unchanged.The dendritesize of the two as-homogenized alloys is almost the same,and it has not changed significantl with the addition of Mn.Fig.4 (c and d) are the line scanning and mapping scanning images for the as-homogenized Mg-6Zn-0.5Ce-1Mn alloy.According to the line scanning image (Fig.4c), it can be known that the Mn elements in the matrix are uniformly distributed.According to the surface scanning image of eutectic compounds in Fig.4d,it is shown that Mn elements distribute uniformly at the compounds and matrix.

Fig.3.Optical images of the as-cast Mg-6Zn-0.5Ce-xMn alloys: (a) x=0 and (b) x=1.(c) SE-SEM and mapping scanning micrographs of as-cast Mg-6Zn-0.5Ce-1Mn alloy.

3.3.Microstructures of as-extruded alloys

Fig.5 is the XRD patterns of the as-extruded Mg-6Zn-0.5Ce and Mg-6Zn-0.5Ce-1Mn alloys.The results show that the Mg-6Zn-0.5Ce alloy consists ofα-Mg phase, Mg7Zn3phase andτ-phase, as shown in Fig.5a.After the Mn adding,theα-Mn phase diffraction peaks are also detected in the Mg-6Zn-0.5Ce-1Mn alloy (Fig.5b), indicating that Mn may mainly exist as a pureα-Mn phase.

Fig.6 (a and b) are the as-extruded metallographic structures for Mg-6Zn-0.5Ce-xMn(x=0 and 1)alloys.The grains are significantl refine by dynamic recrystallization, and the fin particles are formed through the hot extrusion deformation.As seen from the figure the grains are obviously refine with the addition of Mn.On the one hand,the Mg-6Zn-0.5Ce alloy is completely dynamic recrystallized.The recrystallized grains are very uniform, with the grain size of about 24μm.On the other hand, the mixed crystal structure appears in the alloy with adding 1wt.% Mn.The crystal grains are remarkably refined and the grain size is about 5μm.It is due to that the saturated Mn elements will precipitate during homogenization and hot extrusion, then theα-Mn precipitated particles phase plays a role in hindering grain boundary migration, inhibiting dynamic recrystallization and refinin grains.Fig.6c shows the SEM micrograph of the as-extruded Mg-6Zn-0.5Ce-1Mn alloy.As seen from the line scanning image,the Mn elements distribute uniformly on the Mg matrix.The Mn-rich particle which contain impurities such as Fe element are formed during the casting process, and the Mn-rich particles are the nucleation core of the divorced eutectic compound Mg7Zn3during the eutectic reaction, then the Mn-rich particles are wrapped in a eutectic compound.The eutectic compound can be dissolved due to the homogenization treatment, and a small number of Mn-rich particles will emerge.The high angle annular dark fiel scanning TEM (HAADFSTEM) micrograph of Mg-6Zn-0.5Ce-1Mn alloy is shown in Fig.6d.It is worth noting that most of the rich-Mn particles are combined with the Mg-Zn compounds, which are crushed and dispersed in the matrix during the hot extrusion treatment.According to the EDS analysis, the content of Mnwhich is belong to these Mg-Zn compounds particles phase is higher than the addition of Mn (0.74wt.%).

Fig.4.Optical images of the as-homogenized Mg-6Zn-0.5Ce-xMn alloys: (a) x=0 and (b) x=1.(c and d) SE-SEM, line scanning and mapping scanning micrographs of as-homogenized Mg-6Zn-0.5Ce-1Mn alloy.

Fig.5.XRD patterns of the as-extruded (a) Mg-6Zn-0.5Ce and (b) Mg-6Zn-0.5Ce-1Mn alloys.

3.4.Microstructures of solution-treated and aged alloys

The supersaturated solid solution obtained by high temperature solution treatment is mostly metastable.When placed at room temperature or heated to a certain temperature, and then kept for some time,the second phases or solute atom aggregation area and metastable transition phase will precipitate out from the supersaturated solid solution.The process is called precipitation.Due to the precipitation of the dispersed new phase, aging treatment can significantl improve the strength and hardness of the alloys, which is called aging hardening or precipitation hardening [30,31].Therefore, the essence of aging treatment is the dissolution of supersaturated solid solution.

Identificatio of the secondary phases present in the solid solution and aged condition is performed by XRD analysis,and the results are shown in Fig.7.It is found that theα-Mg,MgZn2andτphases are detected for Mg-6Zn-0.5Ce alloy(Fig.7 (a and c)), and theα-Mg, MgZn2,α-Mn andτ-phases are detected for Mg-6Zn-0.5Ce-1Mn alloy(Fig.7(b and d)).It is worth noting that the Zn element exists in the form of MgZn2phase for solid solution and aged alloys.The reason is that Zn element is completely dissolved in the matrix to form a solid solution rather than forming intermetallic compounds under the present heat treatment conditions.

Fig.8 shows the solid solution metallographic structures for Mg-6Zn-0.5Ce-xMn (x=0 and 1) samples.Compared to the as-extruded alloy, the grain grows significantl after the solid solution treatment at 420°C for 2h.Most of the streamline second phases in the as-extruded structure are dissolved in the matrix, but there are still a small amount of residual second phases distributed on the grain boundaries.Comparing Fig.8a and Fig.8b, it is clearly shown that the degree of grain refinemen is significantl improved after the addition of Mn, that is, the secondary phase particle size of Fig.8b is significantl smaller than that of Fig.8a.The reason is that theα-Mn phase particles take the effect of hindering dynamic recrystallization, inhibiting grain boundary growth and refinin grains.

Fig.6.Optical images of the as-extruded Mg-6Zn-0.5Ce-xMn alloys: (a) x=0 and (b) x=1.(c) SE-SEM and (d) HAADF-STEM micrographs of as-extruded Mg-6Zn-0.5Ce-1Mn alloy.

Fig.7.XRD patterns of the (a and b) solid solution and (c and d) two-step aged Mg-6Zn-0.5Ce-xMn alloys: (a, c) x=0 and (b, d) x=1.

According to the above analysis, it is known that the supersaturated Mn element is mostly precipitated in the form ofα-Mn phase during the extrusion process.Therefore, theα-Mn precipitated phase will grow and have a certain coarsening during the solid solution treatment.Fig.9 is the BF-TEM micrographs for solid solution Mg-6Zn-0.5Ce-1Mn alloy.As seen from Fig.9, only theα-Mn precipitated particles with an average diameter of 50-100nm are detected at the matrix.According to statistics, the morphologies of theα-Mn precipitated phases can be generally classifie into three types,namely regular polygonal (mainly hexagonal), rod shape and spherical shape.

Theα-Mn phase has a complex body-centered cubic structure.Based on the experimental and computational methods,Zhang et al.found that there is no definit orientation relationship betweenα-Mn and matrix in Mg-Mn alloys, and there is no coherent or semi-coherent interface relationship between the two [32].A high-resolution TEM (HR-TEM) micrograph of the sphericalα-Mn precipitate phase for the solid solution Mg-6Zn-0.5Ce-1Mn alloy is shown in Fig.10a.By calibrating the two-phase superposition diffraction spots, the result indicates that the orientation relationship between matrix andα-Mn phase is [2ˉ1ˉ10]α//[001]Mn,(0ˉ111)α//(110)Mn, as shown in Fig.10b.Fig.10 (c and d) shows the position of the electron beam incident direction in the matrix lattice and theα-Mn lattice.It can be confirme that the electron beam is incident along [2ˉ1ˉ10]of the matrix and incident along [001]of theα-Mn precipitate phase.

Fig.9.(a, b and c) BF-TEM micrographs of solid solution Mg-6Zn-0.5Ce-1Mn alloy.

Fig.10.Crystallographic features of spherical-like α-Mn precipitate phase for solid solution Mg-6Zn-0.5Ce-1Mn alloy, (a) HR-TEM image, (b) superimposed diffraction pattern, (c) position of Mg [2ˉ1ˉ10]in HCP unit cell and (d) position of Mn [001]in BCC unit cell.

Fig.11 (a and b) are the two-step aged metallographic structures for Mg-6Zn-0.5Ce-xMn (x=0 and 1) alloys.As shown in the figure the microstructure is consisting of Mg matrix, closed grain and dispersed secondary phase particle.Some blocky particles are distributed in the grain and boundary, and their volume fraction increases as the addition of Mn.It is due to that the Mn element exists in the form ofα-Mn particle in the matrix.At the same time, it shows that the grains are refine after the adding of Mn.Fig.11 (c and d) are the SE-SEM micrographs of two-step aged Mg-6Zn-0.5Ce-xMn(x=0 and 1)alloys.It can be seen from the image that the alloys have a mass of crushed residual particles with different size.The small particles are within 1μm, and thelarge particles are up to 100μm.The second phases particles are mainly distributed in the grain boundary.Based on the EDS analysis, the second phases particles can be identifie asτ-phases.

Fig.11.(a and b) Optical and (c and d) SE-SEM images of two-step aged Mg-6Zn-0.5Ce-xMn alloys: (a, c) x=0 and (b, d) x=1.

Fig.12a shows the two-step aged BF-TEM micrograph for Mg-6Zn-0.5Ce-1Mn alloy.It can be seen that the rod-like phase is attached to a hexagonal phase to nucleate and grow.Based on the elemental mapping scanning images, it is found that the Zn atoms are enriched in the rod-like phase, and the Mn atoms are enriched in the hexagonal phase.Combined with previous research [33], we can confir that the rod-like phase isβ′1phase, and the hexagonal phase isα-Mn phase.

In order to further research the relationship ofβ′1phase andα-Mn phase, the HR-TEM micrograph of the two-step aged Mg-6Zn-0.5Ce-1Mn alloy is shown in Fig.13a.Theα-Mn phase acts as heterogeneous nucleation site ofβ′1phase.Fig.13b is the superimposed diffraction points after fast Fourier transform (FFT) of Fig.13a.And three sets of points are separated from Fig.13b.It can be known that the electron beam is incident along the [1ˉ21ˉ3]αof the matrix after calibration of the points.Due to the orientation relationship ofβ′1phase andα-Mg matrix is[01ˉ12]β'1//[1ˉ21ˉ3]α,(20ˉ2ˉ1)β'1//(10ˉ10)α, the electron beam is incident along the [01ˉ12]β'1of theβ′1phase.Meanwhile,the orientation relationship ofβ′1phase andα-Mn phase is[01ˉ12]β'1//[011]Mn,(2ˉ1ˉ10)β'1//(ˉ200)Mn.Therefore, a coherent interface is formed betweenβ′1phase andα-Mn phase.In other words, theα-Mn phase could act as heterogeneous nucleation site ofβ′1phase.Fig.13 (c and d) show the position of the electron beam incident direction in theβ′1lattice and theα-Mn lattice.It can be confirme that the electron beam is incident along [01ˉ12]of theβ′1phase and incident along[011]of theα-Mn precipitate phase.

3.5.Mechanical properties of as-extruded and aged alloys

Fig.14 shows the mechanical properties of Mg-6Zn-0.5Ce-xMn (x=0 and 1) wrought Mg alloys in different states.Compared with the as-extruded alloys, the two-step aged alloys has higher strength, especially the yield strength.The reason is that the Mg-Zn eutectic compounds are dissolved during the solid solution treatment, and precipitated at the subsequent aged treatment.At the same time, whether the as-extruded or two-step aged states, the ultimate tensile strength and yield strength of Mn-containing alloys are greater.For the extruded alloy, theα-Mn precipitate phases are dispersed distribution and take the effect of grain refinin and dispersion hardening.Therefore, the as-extruded Mg-6Zn-0.5Ce-1Mn alloy has higher strength, especially the yield strength is significantl improved 40MPa.For the twostep aged alloy, theα-Mn precipitate phases play a role of refinin grain during solid solution treatment.Therefore, the strength of the alloy is improved after the addition of Mn.

As mentioned above, due to fin grain strengthening and dispersion hardening after the addition of Mn, the yield strength of the as-extruded and aged Mn-containing alloys is higher than that of the Mn-free alloys.It is well known that the relationship between the yield strength of the alloy and its grain size generally conforms to the Hall-Petch formula[34].

wherekis a locking constant anddis the average grain size.For the experimental alloys, due to the relatively low solid solubility of Mn in the matrix, the effect of Mn onkcan be ignored [35].The grain size of Mn-containing alloy is much smaller than that of Mn-free alloy.According to the formula,the Mg-6Zn-0.5Ce-1Mn alloy has a higherσthan the Mg-6Zn-0.5Ce alloy.

Metal materials undergo elastic deformation, plastic deformation, crack initiation and propagation, and even fracture under the action of quasi-static load.Fractures are the main evidence for failure analysis.Fig.15 (a and c) are the as-extruded SE-SEM fracture micrographs of Mg-6Zn-0.5Ce-xMn (x=0 and 1) samples.The fracture surfaces consist of multiple cleavage surfaces, cleavage steps and some tearing ridges for Mg-6Zn-0.5Ce alloy, as shown in Fig.15a.It indicates that the fracture surface has the characteristic of quasi-cleavage fracture for Mg-6Zn-0.5Ce alloy.After adding of Mn,the fracture mode changes to ductile fracture, due to the fracture surface is mainly rough and dimpled.The corresponding BSE-SEM fracture micrographs of the asextruded Mg-6Zn-0.5Ce-xMn (x=0 and 1) alloys are shown in Fig.15 (b and d).It is shown that the number of second phases has increased significantl after adding of Mn.The secondary phase has a double effect on the property.Firstly,the secondary phase which locate on the grain and boundarycan initiate crack initiation and promote crack propagation during the extrusion process.Secondly, the secondary phase can effectively hinder the dislocations movement and inhibit the cracks propagation,thereby forming smaller cleavage fracture.According to the EDS analysis, the coarse secondary phase particles are mainlyτ-phases.

Fig.12.(a) BF-TEM and (b) HAADF-STEM micrographs for two-step aged Mg-6Zn-0.5Ce-1Mn alloy.Mapping scanning images of the elements: (c) Mg,(d) Zn and (e) Mn.

Fig.13.TEM analysis of the rod-like β′1 phase nucleated on α-Mn phase for two-step aged Mg-6Zn-0.5Ce-1Mn alloy.(a) HR-TEM micrograph, (b) FFT pattern from (a), (c) position of β′1 [01ˉ12]in HCP unit cell and (d) position of Mn [011]in BCC unit cell.

Mg alloy can cause dislocation sliding or twinning deformation during the tensile process.The twins will change the crystal orientation and cause greater stress concentration.The stress concentration region will preferentially become crack nucleation locations.Fig.16 is the longitudinal SE-SEM images near the fracture surface of Mg-6Zn-0.5Ce-xMn (x=0 and 1) samples.It can be seen that a large number of twins are produced after tensile fracture, some of the twins are parallel to each other, and multiple groups of parallel twins are interlaced to form a certain angle.Compare Fig.16a with Fig.16b, the fraction of interlaced twins increased after Mn adding.The high-density interlaced twins effectively reduce the grain size and weaken the texture,resulting in the strength increases.At the same time, due to the proportion of the ma-trix without twinning is reduced, the coordinated deformation ability of the Mg-6Zn-0.5Ce-1Mn alloy is weakened, and the elongation is decreased.

Fig.14.The room-temperature mechanical properties of Mg-6Zn-0.5CexMn (x=0 and 1) wrought Mg alloys at as-extruded and two-step aged states.

Fig.15.(a and c) SE-SEM fracture micrographs and (b and d) BSE-SEM fracture micrographs of as-extruded Mg-6Zn-0.5Ce-xMn samples: (a, b)x=0 and (c, d) x=1.

Fig.16.SE-SEM images from longitudinal sections near the fracture surface of the as-extruded Mg-6Zn-0.5Ce-xMn alloys: (a) x=0 and (b) x=1.

Fig.17.BF-TEM micrographs adjacent to fracture surface of two-step aged Mg-6Zn-0.5Ce-xMn alloys: (a) x=0 and (b) x=1.(c) HR-TEM image for Mg-6Zn-0.5Ce-1Mn alloy, (d) FFT pattern from (c).

The BF-TEM micrographs from adjacent to fracture surface of two-step aged Mg-6Zn-0.5Ce alloy and Mg-6Zn-0.5Ce-1Mn alloy are shown in Fig.17a and Fig.17b, respectively.The matrix, second phases, twin and dislocations can be observed in the figures The dislocations are distributed inside twins, with a small quantity in the matrix.And the ends of dislocations either terminate in the twin or are connected to the twin boundary.The dislocations are hindered by grain boundary and the second phase during tensile process, and higher stress is required to activate dislocation slip,thereby increasing the strength of the alloy.In order to relax these stresses, the dislocations are emitted from the boundary toward the grain, and disappear into the other end grain boundary by dislocation slip.It's worth noting that the number of precipitated phases in Mg-6Zn-0.5Ce alloy (Fig.17a)is more than that in Mg-6Zn-0.5Ce-1Mn alloy (Fig.17b).The reason is that a large number of dislocation and dislocation tangles provide nucleation positions for the second phase, which promotes the precipitation of the second phase and improves the dispersion hardening.The hardening weakens the improvement of mechanical properties of Mn addition.Some precipitate phases are observed to be sheared or engulfed by the twins in the BF-TEM micrographs (Fig.17a and Fig.17b).To clearly study the phenomenon,the HR-TEM image of rod-likeβ′1phase and twin in two-step aged Mg-6Zn-0.5Ce-1Mn alloy is shown in Fig.17c.It is shown that theβ′1phase is sheared by twins, and some lattice fringes are bending deformation.The reason is that the lattice dis-location is generated and slipped during the direct twin-β′1interaction.The interaction plays a critical role in the yield strength of high strength Mg alloys.However, the underlying mechanisms have not been understood due to the complexity and diversity of the nano-scale crystal defect-precipitate interactions in the hcp alloys.The FFT transformation from the HR-TEM image (Fig.17c) is shown in Fig.17d.A set of spots can be separated and calibrated after findin the matrix and the rod-likeβ′1phase, which indicates that after tensile deformation the orientation relationship between matrix and rod-likeβ′1phase is [11ˉ20]α//[0001]β'1,(ˉ1100)α//(01ˉ10)β'1.

4.Conclusion

In this paper, the effects of 1wt.% Mn addition on the microstructures and mechanical properties of Mg-6Zn-0.5Ce alloy were studied by means of experimental analysis methods such as metallography, SEM and TEM, combined with room temperature tensile test.The main conclusions are as follows:

(1) All of Mn elements form theα-Mn pure phases, which do not participate in the formation of other phases, such as theτ-phases.

(2) The Mn element has an obvious effect on the microstructure of Mg-Zn-Ce alloy.Theα-Mn phases can hinder grain boundary migration and inhibit dynamic recrystallization, so the average grain size of the Mncontaining alloy is significantl smaller than that of the Mn-free alloy.

(3) Theα-Mn phase acts as heterogeneous nucleation site ofβ′1phase for the aged Mg-6Zn-0.5Ce-1Mn alloy.The orientation relationship betweenβ′1andα-Mn is[01ˉ12]β'1//[011]Mn.

(4) The Mn element can improve the room-temperature mechanical property of Mg-Zn-Ce alloy, mainly because Mn element takes effect of grain refining dispersion hardening and aging strengthening.

Author contributions

Caihong Hou and Hongshuai Cao contributed equally.

Declaration of Competing Interest

The authors declare that they have no known competing financia interests or personal relationships that could have appeared to influenc the work reported in this paper.

Acknowledgement

This work was funded by National Natural Science Foundation of China (Project No.51701172), Foundation of China Railway Eryuan Engineering Group Co.Ltd.(Project No.KYY2020035(21-21)), Natural Science Foundation of Hunan Province (Project No.2018JJ3504), and China Postdoctoral Science Foundation (Project No.2018M632977).