Chosheng M,Wenbo Yu,?,Xufeng Pi,Antoine Guitton

a School of Mechanical and Electronic Control Engineering,Beijing JiaoTong University,Beijing,China

b Universitéde Lorraine–CNRS–Arts et Métiers ParisTech–LEM3,F-57000 Metz,France

c Laboratory of Excellence on Design of Alloy Metals for low-mAss Structures(DAMAS),Universitéde Lorraine,F-57073 Metz,France

Received 28 April 2020;received in revised form 27 July 2020;accepted 4 August 2020 Available online 22 September 2020

Abstract To investigate the effect of Al8Mn4Gd phase on microstructural and mechanical properties of Mg–Al–Ca magnesium alloy,two Mg–2.5Al2Ca and Mg–2.5Al2Ca-0.1Al8Mn4Gd alloys were designed and compared in this work.The results show that a small amount of Gd can significantly refineα-Mg grains and change the morphology of Al2Ca particles.Indeed,the formed Al8Mn4Gd phase could serve as a heterogeneous nucleation site for theα-Mg grains and Al2Ca particles.Furthermore,the introduction of Gd not only optimized the mechanical properties of Mg–Al–Ca alloy,but also facilitated the thermal deformation(such as hot rolling).

Keywords:Mg–Al–Ca alloy;Al8Mn4Gd phase;Microstructure and mechanical properties;Hot rolling.

1.Introduction

Magnesium alloy castings have a great advantage in vehicle light-weighting,and aerospace engineering due to the low density,short processing cycle and assembly costs,which have attracted numerous studies[1–8].As AZ series of magnesium alloy exhibits soften behavior at elevated temperature,Mg–Al–Ca alloys was developed to evaluate the mechanical properties due to the low price,texture weakening and good ignition resistance[9–11].However,the poor plastic deformation ability and mechanical properties of as-cast Mg–Al–Ca alloys are challenges still to be addressed yet.

In cast Mg–Al–Ca alloys,the typical microstructure contains two phases:α-Mg grains andβ-Al2Ca[12].Consequently,the most effective way to improve mechanical properties of Mg–Al–Ca based alloy is to modifyβ-Al2Ca by alloying with Mn,Zn,Si,Sr and Rare Earth(RE)elements,etc.[13–16].The modification mechanism is that the new formed particles could provide the nucleation site for the second phase.To fulfill this step,the particle should firstly form and exhibit one good crystallographic match with the second phase during solidification.For example,Li et al.[17]reported that introduction of manganese(Mn)into the as-cast Mg-5.5Al-3Ca wt.% alloys resulted in the formation of Al8Mn5phase and increased the yield strength of the alloy from 366MPa to 402MPa.However,Mn addition has no refining effect on the microstructure of the as-cast alloys.Nakata et al.[18]reported that only 1% of Zn addition could significantly improve both strength and ductility of rolled Mg-8Al-1Ca-0.3Mn(AX81)alloy sheet.This is because that introduction of Zn facilitated the formation of homogeneous grain structure and isotropic basal texture.Son et al.[19]found that samarium(Sm)addition resulted in a microstructure composed of equiaxed and refined grains and grain sizes in Mg-5 wt%Al-3 wt% Ca-based alloys.

Recently,Li et al.[20]found that the formation of Al8Mn4Gd phase in AZ91 alloy could significantly refineα-Mg grains and Mg17Al12particles,as Al8Mn4Gd phase can act as the heterogeneous nucleation site.However,so far,there are few reports about the effect of Al8Mn4Gd phase in Mg–Al–Ca alloy.Therefore,in the present work,two Mg-2.5Al2Ca and Mg-2.5Al2Ca-0.1Al8Mn4Gd alloys were designed and compared for investigating the effect of Al8Mn4Gd phase on microstructural and mechanical properties of Mg–Al–Ca magnesium alloy.

2.Experimental procedure

The mold and all tools of gravity casting were firstly preheated up to 250°C for removing the moisture.The electric resistance furnace was used to melt 1kg pure Mg and Al together in a steel crucible protected under the protection a mixture gas of CO2and SF6,pure Mg and Al were heated to be melt in one electric resistance furnace.Subsequently,MgGd30and MgMn was used as master alloys for introducing Gd and Mn element.Then Ca was introduced through CaAl20master alloys.The melt was continuously held and stirred at 720°C for 20min after complete melting,homogeneous melt Mg-2.5Al2Ca-0.1Al8Mn4Gd alloy was finally obtained and was poured into a pre-heated steel mold at 250°C.The nominal composition of experimental alloys was given in Table 1.

Table 1Nominal composition of experimental alloys(wt.%).

Samples for observation were ground with silicon carbide and then successively polished with 6μm,3μm,1μm and 0.25μm diamond suspensions.To avoid the work-hardening caused by conventional grinding,a chemo-mechanical polishing was performed using Al2O3(particle size:0.04μm)suspension.A diluted acetic acid solution comprising 50mL distilled water,150mL anhydrous ethyl alcohol and 1mL glacial acetic acid was used to etch the specimen surface.Scanning Electron Microscope(SEM)observation(Zeiss Merlin Germany)and X-Ray Diffraction(XRD)using Bruker D8 diffractometer(Karlsruhe,Germany)with Cu-Kαradiation were adopted to perform the microstructural observation and phase identification.

Tensile test was performed on a universal servo-hydraulic mechanical testing machine(Instron 5600,Norwood,MA)equipped with a knife-edge extensometer with a strain rate of 1mm/min at room temperature in air.The samples were fabricated following GB/T228.1–2010 flat specimen standard.The dimensions were 85mm in overall length and 35mm in gauge length,25mm width and 15mm in thickness.Each test was repeated six times for each composite in order to evaluate the corresponding mechanical properties.

Fig.1.Optical(top)and SEM(below)Morphologies of as-cast MgAlCa-0Gd(left)and Mg–Al–Ca-0.1Gd(right)alloys.

3.Results

3.1.Microstructural morphology and refine mechanism

The morphology and distribution of the particles in ascast alloys were respectively observed by optical and SEM micrographs,as shown in Fig.1,it is clear that Mg–Al–Ca alloy exhibit typical dendritic structure.(Fig.1a).A lot of gray plate particles(Al2Ca phase confirmed in Fig.2)are found around theα-Mg dendrites and inα-Mg inter-dendrites(Fig.1c).When Gd was added into Mg–Al–Ca alloy,the dendritic degree ofα-Mg strongly decreased(Fig.1b).Fig.1d shows that the plate-like Al2Ca changed into the spherical form and a new phase Al8Mn4Gd formed.It is confirmed by XRD shown in Fig.2a.

As found from XRD result given in Fig.2a,onlyα-Mg and Al2Ca two phases were detected in Mg–Al–Ca alloy,while the new phase Al8Mn4Gd appeared when Gd was introduced into Mg–Al–Ca alloy.In addition,Fig.2b shows the solidification paths of as-cast Mg-4Al2Ca and Mg-0.5Al8Mn4Gd alloys calculated by Scheil model.It reveals that the Al8Mn4Gd phase precipitated at the temperature of 640°C during the solidification process,while the precipitation ofα-Mg and Al2Ca phase starts below 600°C.

As Al8Mn4Gd phase precipitated earlier thanα-Mg and Al2Ca phases,its ability of acting as potent nucleation sites for theα-Mg grains and Al2Ca could be calculated by Bramfitt’s equation shown in Eq.(1)[21],the degree of potency of the nucleation catalysts based on the average disregistries along low-index directions within low-index planes between substrate and nucleation solid.

Fig.2.(a)XRD patterns of as-cast MgAlCa-0Gd and MgAlCa-0.1Gd alloys(b)the solidification route calculated of Mg-4Al2Ca and Mg-0.5Al8Mn4Gd alloys by JMatPro.

Fig.3.Schematic plans of the crystallographic relationships:(a)between Mg and Al8Mn4Gd.(b)between Al2Ca and Al8Mn4Gd.

Where(hkl)sis a low-index plane of substrate,[uvw]sis the low index direction in(hkl)s,d[uvw]sis the interatomic spacing along[uvw]s,(hkl)nis a low-index plane in the nucleation solid,[uvw]nis the low-index direction in(hkl)n,d[uvw]nis the interatomic spacing along[uvw]n,andθis the angle between[uvw]sand[uvw]n.If the disregistryδof lattice spacing is less than 15%,the substrate can promote the nucleation of melt material.Mg is hexagonal structure with the lattice parameters ofa=0.32093nm andc=0.52103nm.Al8Mn4Gd is tetragonal structure with the lattice parameters ofa=0.8929nm andc=0.512nm.Al2Ca is cubic structure with the lattice parameters ofa=0.802nm.

Three low-index planes of Al8Mn4Gd,including(100),(110)and(111)were selected as matching planes.The(0001)plane of Mg and the(001)plane of Al2Ca were used as nucleating plane.The schematic plans of the crystallographic relationships between Mg and Al8Mn4Gd,Al2Ca and Al8Mn4Gd were shown in Fig.3.By using the lattice parameters and Eq.(1),the disregistry between Mg and Al8Mn4Gd,Al2Ca and Al8Mn4Gd were calculated and summarized in Tables 2 and 3.It was found that the mismatch was just 7% between[10]direction on the(111)plane of Al8Mn4Gd and[20]direction on the(0001)plane of Mg.Thus,Al8Mn4Gd can act as nucleation site to refine theα-Mg grains,which was also reported during the alloy solidification process[22–25].Between Al2Ca and Al8Mn4Gd,the calculation indicates that the mismatch was 0.113(below 0.15)when the(110)plane of the Al8Mn4Gd was overlapped on the(001)plane of the Al2Ca.Thus,the refining effect of Al8Mn4Gd on Al2Ca as nucleation site is useful but not effective.This was consistent with the results shown in Fig.1,the size of Al2Ca phase only became a slightly smaller with the formation of Al8Mn4Gd phase.Moreover,the Al2Ca lamellar located at the grain boundaries evolved into granular from with the formation of Al8Mn4Gd phase in Mg–Al–Ca alloy.Therefore,we confirmed that formation of Al8Mn4Gd phase could lead to efficiently refineα-Mg and change Al2Ca phase morphologies,but the Al2Ca phase was slightly refined.

3.2.Mechanical properties of as-cast and hot rolled

The room temperature stress-strain curves of the ascast alloys and hot rolled ones are presented in Fig.4.For the as-cast specimens,the addition of Gd into Mg-2.5Al2Ca alloy only has one weak improvement on the tensile strength and ductility.However,Mg-2.5Al2Ca and Mg-2.5Al2Ca-0.1Al8Mn4Gd alloys exhibit obvious different improvement in mechanical properties with hot rolling deformation.With 50% hot-roll deformation degree,the Ultimate Tensile Strength(UTS)and ductility of as-cast Mg-2.5Al2Ca-0.1Al8Mn4Gd alloy were remarkably enhanced from 140MPa and 1.1% to be 225MPa and 1.6%.With further deformation to be 80% degree,UTS and ductility of Mg-2.5Al2Ca-0.1Al8Mn4Gd alloy could reach 260MPa and 2.1%.In contrast,the UTS and strain of Mg-2.5Al2Ca were only enhanced from 119MPa and 0.95% to be 140MPa and 1.1% with 50%hot-roll deformation degree.Subsequently,different from the continuous improvement found in hot rolled 2.5Al2Ca-0.1Al8Mn4Gd alloy,the UTS and strain of Mg-2.5Al2Ca alloy suddenly jumped to 230MPa and 1.6%with 80%deformation degree,even though these values are still lower than those of deformed Mg-2.5Al2Ca-0.1Al8Mn4Gd.

Table 2The results of planar disregistries between Mg and Al8Mn4Gd.

Table 3The results of planar disregistries between Al2Ca and Al8Mn4Gd.

Fig.4.Tensile stress-strain curve of as-cast and hot rolled specimens(a)Mg-2.5Al2Ca alloy and(b)Mg-2.5Al2Ca-0.1Al8Mn4Gd alloy.

Fig.5 presents the microstructural morphologies of hot rolled two alloys with different degree of hot rolling deformation treatment.It is evident that both two alloys exhibit different Al2Ca distribution.Even after hot rolling with 50% deformation degree,the lamellar Al2Ca phase still accumulated with one necklace form in Mg-2.5Al2Ca alloy.In contrast,Al2Ca and Al8Mn4Gd phases already uniformly dispersed in Mg-2.5Al2Ca-0.1Al8Mn4Gd alloy.When the deformation degree of specimens reached 80%,the lamellar Al2Ca phase was almost broken and well distributed in Mg-2.5Al2Ca alloy.Herein,one jump in mechanical properties occurred in hot rolled Mg-2.5Al2Ca alloy from 50% to 80% deformation degree.At the same time,the distribution of Al2Ca and Al8Mn4Gd particles become more uniform in Mg-2.5Al2Ca-0.1Al8Mn4Gd alloy.

Fig.5.The SEM morphologies of as-cast Mg-2.5Al2Ca and Mg-2.5Al2Ca-0.1Al8Mn4Gd alloys with different degree of hot rolling deformation treatment:(a and b)50% and(c and d)80%.

Fig.6a and b show the tensile fracture of two as-cast alloys.Theα-Mg dendrite form was found in tensile fracture of as-cast MgAlCa-0Gd alloy,the fracture surface was mainly occupied by cleavage surfaces.However,noα-Mg dendrite form was found in tensile fracture of as-cast MgAlCa-0.1Gd alloy,some tearing ridges occupied the fracture surface.Fig.6c and d present the tensile fracture surfaces of hot rolled specimens with 80%deformation degree.The tear edge accompanied with dimples were found in hot rolled Mg-2.5Al2Ca alloy,while only dimples were observed in hot rolled Mg-2.5Al2Ca-0.1Al8Mn4Gd alloy.Therefore,it can be concluded that addition of Gd into Mg–Al–Ca alloys facilitate the thermal deformation(such as hot rolling)and are conductive to improve their mechanical properties.

Fig.6.Fracture surface of specimens without hot roll treatment(a)and(b)and with hot rolled 80% degrees(c and d).

4.Conclusions

In this study,we proved that we could regulate the microstructure of Mg–Al–Ca magnesium alloy through the formation of Al8Mn4Gd phase.XRD and Bramfitt’s theory indicates that spherical and dispersed Al8Mn4Gd phase firstly precipitate during the solidification process by introduction of Mn and Gd into Mg–Al–Ca magnesium alloy,Al8Mn4Gd phase could efficiently restricted the growth of dendriticα-Mg and disperse lamellar Al2Ca in Mg-2.5Al2Ca alloy.This is because that Al8Mn4Gd phase can serve as heterogeneous nucleation sites for the formation ofα-Mg grains and Al2Ca particles,especially forα-Mg.In addition,the introduction of Gd into Mg–Al–Ca alloys could also facilitate the hot rolling deformation.With 50% hot-roll deformation,the UTS and ductility of as-cast Mg-2.5Al2Ca-0.1Al8Mn4Gd alloy were remarkably enhanced from 140MPa and 1.1% to be 225MPa and 1.6%.In contrast,the UTS and elongation of Mg-2.5Al2Ca were only enhanced from 119MPa and 0.95%to be 140MPa and 1.1%.

Acknowledgement

This work was financially supported by the National Science Foundation of China(No.51701010)and by Beijing Government Funds for the Constructive Project of Central Universities(No.353139535).Thanks to Gaomi Xiangyu Company(Shandong province,China)for the gravity casting.