Amir Mahyar Majd,Mohammad Farzinfar,Matin Pashakhanlou,Mohammad Javad Nayyeri

Department of Materials Engineering,Science and Research Branch,Islamic Azad University(IAU),Tehran,Iran

Abstract In the present article,the effect of Rare Earth elements on the microstructural development and mechanical properties of as cast and age treated Mg–4Al–2Sn(AT42)alloy is studied.Investigation has been conducted by optical and scanning electron microscope,XRD and tensile tests.Analysis of the data showed that alloy’s dendrites turn into larger dendritic structure with sharp and narrow arms from equiaxed rosette type by addition of RE elements.In contrast to the base alloy,aging treatment shows a positive effect on the mechanical properties(yield strength,tensile strength and elongation)of AT42+1RE alloy mainly because of retention of the thermally stable RE containing intermetallics as strong barriers to grain growth.Also,increase of solute aluminum due to the decomposition of Mg17Al12 along with saturated RE elements led to formation of blocky shape Al2RE in the microstructure during aging which enhanced the mechanical properties.It was found that the best result(yield of 70 MPa,tensile strength of 168 MPa and elongation of 14%)could be achieved by aging the AT42+1RE alloy at 443 K(170°C)for 8 h.However,mechanical properties of AT42+1RE alloy starts to decrease after exceeding its optimum aging conditions due to the coarsening of intermetallics.

Keywords:Magnesium alloys;Aging;Rare earth elements;Mechanical properties;Microstructure.

1.Introduction

Magnesium alloys as the lightest structural metals are attractive for many applications such as automobile,aerospace and railway industries[1].Aluminum constitutes the main alloying element in commercial magnesium alloys,chieflbecause of its low price,high availability,low density and advantageous effect on corrosion and strength properties.However,Mg–Al alloys suffer from relatively poor mechanical properties at elevated temperatures,mainly due to the low thermal stability of Mg17Al12precipitates along grain boundaries[2–4].Vast number of studies have been performed to address these disadvantages by improving structural stability of Mg alloys at elevated temperature(over 400 K(127°C));precipitation hardening is a promising mechanism for developing high strength magnesium alloys[3,5–7].Most of the attempts for improving the high-temperature mechanical properties of Mg alloys have concentrated on suppressing discontinuous precipitation of Mg17Al12and creating thermally stable intermetallics in the magnesium matrix.This can be achieved by introducing alloying elements such as RE(Rare Earth),Ca,Si,Sn and Sb[4,8–11].Among them,Sn is relatively inexpensive and cause an enhancement of high temperature mechanical properties by formation of thermally stable Mg2Sn phase,which is mainly distributed along grain boundaries in the as-cast condition[11–13].Also,as the solubility of Sn in the Mg drastically decreases from eutectic to the room temperature,it is expected that the Mg2Sn phase will be redistributed homogeneously in the Mg matrix through the solutionizing and aging-treatments,which will lead to increased strength properties at both ambient and elevated temperatures.Further needs for better performance of Mg–Al–Sn alloys necessitate the use of other alloying elements such as Ca and RE elements.It has been reportedthat RE containing alloys show effective age hardening due to the significan decrease of the solubility of RE elements with decreasing temperature[1,8].Rare earth elements have two positive effects on Mg–Al–Sn system.Firstly,due to the high affinit of aluminum atoms to RE elements,they cause suppression of Mg17Al12through the formation of highly thermally stable Al11RE3and Al2RE intermetallics.Secondly,these particles act as strong barriers to the dislocation movement during deformation.In other word,improvement in high temperature strength has been ascribed to the branchshape compounds together with the low diffusion rate of RE elements in Mg matrix.Similar results have been obtained for high-temperature mechanical properties of RE-containing AM60,AZ91,and AE42 alloys[4,8].

Table 1 Chemical composition of mischmetal(wt%).

A glimpse review of the literature reveals that only a few studies have examined the Mg–Al–Sn–x quaternary systems with the majority of them focusing on Mg–Al–Sn–Ca[14,15].The aim of the present study is to investigate the effect of RE additions on microstructural evolution and mechanical properties of as-cast and age treated Mg–4Al–2Sn(AT42)alloy.To this end,new Mg alloys based on AT42 and AT42+1RE were prepared,and the effects of the intermetallic compounds on the microstructures and tensile properties of as-cast structures were investigated.Furthermore the effect of various aging conditions,3 different temperatures and 4 different durations,on precipitations and properties of alloys were examined.

2.Experimental procedure

The two alloys used in this investigation were Mg–4Al–2Sn(AT42)and Mg–4Al–2Sn–1RE(AT42+1RE).High purity Mg,Al and Sn were used to prepare the base alloy(AT42).Melting was performed in a mild steel teapot at 993 K(720°C)in an electric resistance furnace(8 kW)under covering flu (50%MgCl2,20%KCl,15%MgO and 15%CaF2)to protect molten magnesium from oxidation.In order to investigate the effect of RE elements on the microstructure and mechanical properties of AT42 magnesium alloy,1 wt%of RE elements were added to the molten alloy as mischmetal(Table 1)at 993 K(720°C).The melt was mechanically stirred and held for 5 min in a furnace to ensure that the alloying elements were completely dissolved.Another 5 min were allowed to ensure a homogenous composition and to settle the oxides and other contaminants before pouring the melt into a 473 K(200°C)preheated permanent mold.Pouring was accomplished by a tilt-casting technique in order to minimize the melt turbulence and the quick emergence of entrapped air.In order to evaluate mechanical properties,as cast specimens were machined into samples with the ASTM E8M dimension.Also,for structural characterization,specimens were prepared from cutting of the cast one.To study the effect of aging treatment on the microstructural and mechanical behavior of the both alloys,samples were solution treated at 800 K(527°C)for 2 h in a resistance furnace under an argon atmosphere,before being water quenched at room temperature.For precipitation hardening,samples were aged at various temperature(443 K(170°C),473 K(200°C)and 523 K(250°C))for different durations(1,2,4 and 8 h).For both cast and aged samples,the microstructures and phase distribution were characterized by Olympus BX51M optical microscopy and VEGA II TESCAN scanning electron microscopy(SEM).The observed samples for metallographic and SEM investigations were ground,polished and etched by nital 1%solution.The chemical compositions of phases and RE-rich particles were determined with an energy dispersive spectrometer(EDS).Hardness was measured with a Vickers hardness tester(HXD-1000)under a load of 500 g and the fina results were taken from an average of 10 measurements.Tensile properties were tested at room temperature using a 25 kN Instron 8502 uniaxial test machine.X-ray diffraction(XRD)observation was performed using EQuinox 3000,INEL,France.

3.Results and discussion

SEM and optical micrographs of the investigated alloys in the as-cast and age treated are represented in Figs.1 and 2.It can be concluded from Fig.1a and b that the microstructure of the AT42 alloy mainly consists ofα-Mg,β(Mg17Al12)phase,the lamellar eutectic structure and the Mg2Sn precipitates at the grain boundaries(Fig.1b).Theα-Mg dendrites form in AT42 alloys are rosette type(Fig.2a)while addition of RE elements led to the formation of augmented dendritic structure with narrow and sharp arms(Fig.2b).In addition to the aforementioned phases,Al11RE3 and Al2RE intermetallic compounds are formed in AT42+1RE alloy(Fig.1c and d).Due to the high tendency of the RE elements to combine with aluminum[16],a large amount of latent heat is released during solidificatio [17]which increases solidificatio time during casting.As a result,α-Mg dendrites have more time to grow into coarse dendritic grains(grain size of 30,34 μm for AT42 and AT42+1RE,respectively).On the other hand,the amount of solute aluminum diminishes because of the addition of RE elements,i.e.constitutional undercooling increases,thus equiaxed morphology of dendrites is replaced with sharp and narrow dendritic arms.

Fig.1.As cast SEM micrograph of(a)as-cast AT42 alloy,(b)constituents of AT42 alloy,(c)AT42+1RE alloy and(d)constituents of AT42+1RE alloy.

Fig.2.Optical image of as-cast structure of(a)AT42 and(b)AT42+1RE alloys.

Fig.3.XRD patterns of(a)AT42 and(b)AT42+1RE alloys in the as-cast and aged conditions.

The XRD patterns of the AT42 and AT42+1RE alloy in the as-cast and aged conditions are exhibited in Fig.3.Examination of the XRD patterns of age treated AT42+1RE alloy indicates that the intensity of theβphase decreases after being solution and age treated which shows dissolution ofβphase.On the other hand,solute Al was consumed by RE elements to form Al2RE and to a lesser extent Al11RE3intermetallics.This changes were confirme through the XRD and EDS analysis(Table 2).Moreover,according to K.N.Braszczy′nska-Malik et al.[4,8]while the weight ratio of Al/RE is above 1.4,all of the aluminum will be tied up as Al11RE3;otherwise,intermetallics appear in the form of Al2RE blocky phases.Therefore,during casting process,the precipitates form as Al11RE3.On the other hand,as the solubility of RE in Mg is limited[18],solid solution of RE in the Mg during casting is very low.Thus the precipitates appear as blocky Al2RE phase during aging process(Table 2 and Fig.5).Also,the intensities of the Mg2Sn peaks show more increases in AT42 alloy compared to AT42+1RE alloy.The XRD results have good consistency with the SEM micrographs which is shown in Fig.4.Review of the microstructure of both age treated alloys revealed that aging treatment have a great effect on the volume fraction of the Mg2Sn phase.This phase is present in the form of isolated spherical particles and mainly distributed near the grain boundaries and to a lesser extent within the grains(Fig.4).Also,during aging treatment of AT42+1RE alloy,new RE containing intermetallic phases appeared in the structure.

Addition of RE elements have direct effect on the hardness of as-cast alloys which is due to the formation of hard Al11RE3intermetallic phases and solid solution strengthening(Fig.5)of RE elements(35±3 and 50±2 Hv for AT42 and AT42+1RE alloy).

Fig.4.SEM micrograph of(a)AT42 and(b)AT42+1RE alloy after aging at 523 K(250°C)for 2 h.

Table 2 Average chemical composition of the intermetallics in the AT42 and AT42+1RE alloys.

It was indicated that after 8 h of aging at 523 K(250°C),the average gran size of the AT42 alloy measured 250 μm.This observation is in contrast to the microstructure of the RE-containing alloy which experience less grain growth,as exhibited in Figs.6 and 7.Grain size of AT42+1RE alloy which loses its initial dendritic microstructure to equiaxed structure during solid solution treatment increases to average grain size of 88 μm after 8 h of aging at 523 K(250 °C).This structure is fine than the grain structure of AT42 alloy which experienced extraordinary grain growth during aging.It has been pointed out that the RE large atoms have a hindering effect on diffusion process in Mg alloys[1,19].Therefore,the diffusion process of grain growth would be postponed by addition of RE elements.On the other hand,due to the high thermal stability of RE containing intermetallics,they act as strong barriers to grain growth during aging which enhance the tensile properties.

In as–cast specimens it was revealed that AT42+1RE have 11%higher yield strength compared to AT42(59 and 66 MPa for AT42 and AT42+1RE,respectively)while its elongation decreased by 35%(12.8%and 8.1%for AT42 and AT42+1RE,respectively).This improvement may be attributed to the distribution of rod-like Al11RE3intermetallic phases at intragranular and triple grain-boundary,which plays an important role in strengthening theα-Mg matrix and hindering the grain growth of the alloy[20].In contrast to[21],RE elements led to decrease of elongation in as-cast structure.This might be due to the different amount of Ce,Al and Zn content in both studies[22].Due to the brittle nature of RE containing intermetallics and their acicular shape which rises the stress concentration at the tip of them during tension,addition of RE elements led to decrease of elongation.The results are in agreement with those reported by[14].

Also,the mechanical properties of the age treated alloys at ambient temperature are presented in Fig.8.As it can be seen in Fig.8a and c,tensile properties of AT42 alloy decrease drastically by aging treatment.Although the Mg2Sn phase has a positive effect on tensile properties of the alloy,they becomes coarser during aging treatment,which will lower the mechanical properties to the alloy.In addition,extraordinary grain growth of AT42 alloy during aging led to decrease of tensile properties of the age treated alloy.Totally,it can be concluded that the deteriorating effect of grain growth is dominant in weakening of tensile properties of AT42 alloy.Although increase of aging time lead to increase of elongation at 423 K and 473 K,it would result in decrease of elongation by aging at 523 K.This might be due to formation of new phases which enhance hardness and tensile strength which lead to decrease of elongation(Fig.8e).

Fig.5.Elemental distribution of age treated AT42+1RE at 523 K(250°C)for 8 h.

Fig.6.Optical micrograph showing the microstructure of AT42 alloy after(a)aging at 443 K for 2 h,(b)aging at 523 K(250°C)for 8 h,and AT42+1RE alloy(c)aging at 443 K(170°C)for 2 h,(d)aging at 523 K(250°C)for 8 h.

Fig.7.Average grain size of as-cast and age treated(a)AT42 and(b)AT42+1RE alloys.

In contrast,yield and tensile strength of the AT42+1RE alloy increased as the aging time and temperature increases(Fig.8b and d).Two reasons may be suggested for these observations.The firs one might be due to the dispersion of fin granular Mg2Sn at grain boundaries(Fig.4b).The intermetallic morphology affect the tensile properties[18].The granular particles are in favor of the tensile properties while because of the easy crack formation caused by the stress concentration at the tip of acicular particles,they have harmful effect on the tensile properties.It is worth mentioning that coarsening of the particles depend on the diffusion of the alloying elements in the Mg matrix.It is reported that diffusion rate of alloying elements hindered in the presence of RE elements[23].Thus,intermetallic particles have very slow coarsening kinetics.Therefore,AT42+1RE specimens aged at 443 K(170°C)for 8 h exhibited the highest mechanical properties(YS=70 MPa,UTS=168 MPa and Elongation of 14%)while increasing of temperature to 523 for the same time results in decrease of mechanical properties(YS=65 MPa,UTS=160 and Elongation of 11%).It worth to be mentioned that this reduction is more drastically in AT42 alloy.In addition,the RE containing acicular particles act as a barrier in front of dislocation movement during tensile test and results in enhancement of tensile properties.Moreover,during aging of AT42+1RE alloy,the formation of blocky Al2RE intermetallic phase lead to the increase of tensile properties.On the other hand,due to the formation of Al11RE3and Al2RE,solute aluminum in Mg matrix decreases which results in decrease of solid solution hardening effect of aluminum.The interaction of aforementioned strengthening and weakening mechanisms leads to the enhancement of tensile properties of AT42+1RE alloy during aging.Also,it has to be mentioned combination of moderate grain growth and increase of intermetallics volume fraction led to enhancement of elongation of AT42+1RE alloy(Fig.8f).

Fig.8.Comparison of yield strength of(a)AT42,(b)AT42+1RE,ultimate tensile strength of(c)AT42,(d)AT42+1RE and elongation of(e)AT42 and(f)AT42+1RE at various aging time and temperature.

4.Conclusion

The microstructure evolution and mechanical properties of AT42 and AT42+1RE magnesium alloys were investigated in the as-cast and variety of age treated conditions.Addition of RE elements were detectable through variation of equiaxed rosette dendrite to augmented dendritic structure and enhanced mechanical properties of as-cast structures.Solution and aging treatment led to vanishing of Mg17Al12phases in both alloys while it cause increase of Al2RE intermetallic in AT42+1RE alloy.Furthermore,aging treatment showed deteriorating effect on mechanical properties of AT42 alloy,while it had a positive effect on RE containing alloy.This improvement in strength has been ascribed to the formation of the high thermally stable RE containing intermetallic phases as strong barriers to grain growth and dislocation movement of AT42+1RE alloy.It was found that the best result was achieved via aging of AT42+1RE alloy at 443 K(170°C)for 8 h.However,elongation,yield and tensile strength decrease after exceeding its optimum condition due to the intermetallics coarsening.