Chunlong Cheng,Xiaoqiang Li,Qichi Le,Ruizhen Guo,Qing Lan,Jianzhong Cui

Key Laboratory of Electromagnetic Processing of Materials,Ministry of Education,Northeastern University,Shenyang 110819,People’s Republic of China

Received 7 July 2019;received in revised form 18 August 2019;accepted 19 September 2019 Available online 24 June 2020

Abstract The oxidation behaviors of AZ80,AZ80-0.32 Y and AZ80–0.38 Nd(wt.%)alloys were researched at 413°C,420°C,427°C and 433°C for up to 6h in air environment via a high precision analytical balance,a laser confocal microscope,differential scanning calorimeter(DSC)analysis,X-ray diffraction(XRD)analysis,scanning electron microscope(SEM)observation,and X-ray photoelectron spectroscopy(XPS)analysis.The results show that the weight gain and oxidation rate of AZ80 are reduced significantly,the initiation form and propagation of cracks in oxide layer are changed.Compact and protective oxide layer forms on alloy surface with Y or Nd addition.And the activation energies of AZ80,AZ80-0.32Y and AZ80-0.38Nd alloys calculated via Arrhenius equation are 82.556kJ/mol,177.148kJ/mol and 136.738kJ/mol,respectively.

Keywords:Magnesium;Rare-earth;Oxidation kinetics;Activation energy.

1.Introduction

Using of magnesium alloys is becoming increasingly dominant in equipment manufacturing industry,especially in aerospace,automobile and 3C(communication consumer electronics and computer)industries,due to characteristics of low density,good castability,high rigidity,machinability,excellent specific strength and stiffness,and high electrical and thermal conductivity[1–4].So magnesium alloys are arousing interest in experts and scholars nowadays.It’s particularly disturbing that the high chemical activity,high affinity with oxygen and nitrogen,restrict application of Mg alloys since they represent flammability to high temperature above 450°C and poor oxidation resistance even in room temperature[5–8].Magnesium alloys application in commercial aircraft was banned by Federal Aviation Administration(FAA)before 2014 for a reason of stability lack at an open flame or other heat source[9].But effort has been made by FAA to lift the existing ban on magnesium alloys using,which increases application opportunities of magnesium alloys[10].Although the mechanical properties of magnesium alloys have been enhanced significantly by process improvement or other means,the development of magnesium alloy oxidation resistance is stagnant.While in view of many excellent properties of magnesium alloys,experts and scholars have repeatedly called for theirs application in aircraft,high-speed rail,subway and automobile hubs for energy saving and emission reduction purpose[11].Such extensive application opportunities exert pressure on developing ignition-proof magnesium alloys[9].

After exposed to elevated temperature in air environment,magnesia,MgO formed on magnesium alloys surface with an fcc cubic structure,is the main component of the reaction products.MgO has a lattice parameter of 0.421nm and Mg ions occupy all octahedral sites.It also has a high degree of stoichiometry with bond type of ionic.What’s more,MgO has plenty of bulk properties,for example its diffusion is isotropic owing to the cubic structure.And thermally grown MgO offers insufficient protection to Mg matrix against elevated temperature exposures to oxidizing environments[9,12].Effective protection from environmental degradation relies on the integrity of oxide as diffusion barriers[13].Coalescence of new aluminum oxide islands doesn’t form any surface grooves and glass-glass interface,but cracks,pores,and spallation generate after coalescence of new MgO islands owing to the low Pilling–Bedworth Ratio(PBR)of 0.81[14].Oxide layer of Mg shows poor deformation plasticity and can’t provide competent protection for matrix against high-temperature oxidation.Subscale occurs when Mg alloys exposure to oxidizing environments.So the long-term oxidation kinetics existing in Mg alloys show linear rate law which means a phaseboundary process is the rate-determining step for the reaction[15].

Therefore,it is most important to improve Mg alloys oxidation resistance to meet the demands of high temperature environment for its widespread application.Generally,two methods are normally adopted to improve oxidation resistance of Mg alloys.Firstly,surface modification is an effective technique to offer protection against oxidation for magnesium alloys,such as ion plating,electroless plating,chemical conversion coating,vacuum evaporation plating and electrochemical plating et al.[2,6,8].While it is commonly considered that the disadvantages of surface modification are high cost and poor self-repairing ability of the protective film.It is reported that alloying,for instance rare-earth elements(REs)addition in magnesium alloys,is an effective solution to elevate Mg alloys oxidation resistance even at high temperature[16–18].Rare-earth elements are famous for“reactive element effects”,which can enhance alloys oxidation resistance even at high temperature[9].The reactive-element effects include(1)improvement of scale adhesion or crack resistance,(2)change of oxidation kinetics,(3)reduction of oxidation rate,related to change of scale-growth mechanism,(4)modification of oxidation layer microstructure,and(5)occurrence of selective oxidation[19–21].Different interpretations have been proposed to understand the role and mechanism of rare earth elements,but a number of bulk studies have focused on specific metals under a particular situation,and universally accepted or applicable theory hasn’t been developed.In general,experts are inclined to use the dynamic-segregation theory to explain the“reaction element effects”,which indicates that rare earth ions segregating at grain boundaries and the metal/oxide interface can prevent cation outward diffusion and interfacial void growth.Therefore,better adhesion oxide layer and lower oxidation rate can be obtained for matrix against elevated temperature exposures to oxidizing environments[22,23].And many researchers have reported Mg-Nd and Mg-Y binary alloys oxidation behaviors[5,7,24],effect of Y or Nd adding on oxidation behavior of commercial magnesium alloys AZ80 with more complex chemical compositions has rarely been reported.And this study is of great theoretical significance and engineering significance.For commercial applications,small amount of rare earth adding is necessary,which meets the low cost requirements.

In this paper,AZ80,AZ80-0.32 Y,and AZ80-0.38 Nd(wt.%)magnesium alloys were prepared.And the influence of REs(Y,Nd)on high temperature oxidation behavior(oxidation kinetics)and activation energy of AZ80 alloy was investigated.Besides,the microstructure characteristic of ascast alloys,onset melting temperature of second phases and characteristic of oxide layers formed on alloys surface were systematically researched.

Table 1Chemical compositions of alloys(wt.%).

Fig.1.The experimental apparatus of isothermal oxidation testing device.

2.Experimental methods

2.1.Materials

Alloys of AZ80,AZ80-0.32wt.% Y,and AZ80-0.38wt.%Nd(hereinafter referred to as AZ80,AZ80-0.32Y,and AZ80-0.38Nd)are used in this paper.The chemical composition of alloys is displayed in Table 1.The tested alloys were melted in a steel crucible,using an electric resistance furnace.The magnesium melt was protected with shielding gas of 99vol.%CO2–1vol.% SF6.Commercial pure Mg,pure Al,pure Zn,(purity over 99.99% of all),Mg-30wt.% Y,Mg-25wt.% Nd master alloy and MnCl2particles were used in this research.Firstly,rusts removing bulk Mg ingots were melted at 715°C in the steel crucible,and simultaneously shielding gas was provided on cavity of electric resistance furnace to maintain nonflammable atmosphere and protect melting from flaming.Afterwards,surface cleaning treatment pure Zn ingots,pure Al ingots,Mg-Y and Mg-Nd master alloys were poured into the melt once Mg ingots melted completely.After all ingots dissolving absolutely,magnesium alloy melt was heated up to 735°C for the adding of MnCl2.Then the melt was held at 735°C with refine agent for about 30min for complete diffusion of alloying elements.Finally,the melt was poured into a steel mold at 685°C with shielding gas protected.Three kinds of ingots(AZ80,AZ80-0.32Y,and AZ80-0.38Nd)were acquired.

2.2.Microstructural observation

Cuboid block samples cut from ingots,mechanical ground by SiC abrasive papers,polished and etched in oxalic acid were used for microstructural observation.A ULTRA PLUS field emission scanning electron microscopy(FESEM)equipped with energy dispersive X-ray spectroscopy(EDS)is used for the observation of as-cast alloys microstructure.A JEM-2100F transmission electron microscopy(TEM)is used for phases identify.And an X Pertpro X-ray diffraction(XRD)apparatus is used for the phase identification.

Fig.2.XRD patterns of as-cast AZ80,AZ80-0.32Y,and AZ80-0.38Nd alloys.

2.3.Isothermal oxidation evaluation

Differential scanning calorimeter(DSC)analysis and isothermal oxidation evaluation including,isothermal oxidation test,oxidation kinetics analysis,and calculation of activation energies via Arrhenius equation are conducted.Samples,20mm×15mm×15mm in size,were cut from ingots.And after ground up to 3000 grit SiC papers,they were used to examination isothermal oxidation behavior using a device equipped with a tube type resistance furnace and a high precision analytical balance named METTLER TOLEDD-XS at standard atmospheric pressure,and the weight change was recorded by the balance in the whole oxidation process.The isothermal oxidation apparatus is demonstrated in Fig.1.The tube type resistance furnace in Fig.1 is a cavity with an upper opening and a lower opening,so the resistance furnace cavity is connected with atmosphere to provide the same air environment.The isothermal oxidation tests are undertaken at four temperatures(413°C,420°C,427°C,and 433°C)for up to 6h.And the oxidation experiment is carried out three times each temperature at the same air environment to prevent the deviation from the reasonable values of the weight gain.In fact,the extent of the effect of REs addition on the oxidation kinetics may be different every time.So the average weight gain of the three tests is given.And before the test,the specimens were cleaned in an ultrasonic cleaner with ethanol and dried with a blower.For DSC analysis NETZSCH STA449F3 apparatus is used to research onset melting temperature of second phase in alloy with heating rate of 10°C/min and samples for test are5×2mm in size.

Fig.3.FESEM/EDS microstructures of as-cast alloys:(a)AZ80,(b)AZ80-0.32Y,and(c)AZ80-0.38Nd.

2.4.Analysis of the oxide scales

The morphologies,chemical compositions and the crosssection of oxide scales generating on experimental alloys surface are also observed by the FESEM,XRD mentioned above.Besides,for purpose of determining the compositions of oxide layers,an ESCALAB 250Xi X-ray photoelectron spectroscopy(XPS)is used for oxide product analysis.And also an OLS4100 laser confocal microscope was used to examine morphologies and surface roughness of oxide layers.

3.Results

3.1.Microstructural observations

Fig.2 gives the XRD patterns of as-cast AZ80,AZ80-0.32Y,and AZ80-0.38Nd alloys.The results show that AZ80 alloy mainly consisted ofα-Mg,β-Mg17Al12,and trace Al8Mn5phases.With Y or Nd adding in AZ80 alloy,Al2Y or Al2Nd phases appear.Fig.3 demonstrates the phase distribution and morphologies of as-cast alloys with EDS mapping.The microstructures of alloys are difference.As shown in Fig.3(a),a large amount of chain-like divorcedβ-Mg17Al12distribute mainly along theα-Mg dendrite boundaries and many eutectics scatter along the divorcedβ-Mg17Al12in AZ80 alloy.While in AZ80-0.32Y alloy,chain-shapedβ-Mg17Al12are eliminated significantly and new phase Al2Y generate which is consistent with the findings of Tang[25].Dotted fine Al2Y particles and residual small island-likeβ-Mg17Al12uniformly distribute in the magnesium matrix as depicted in Fig.3(b).However,the effect of Nd onβ-Mg17Al12elimination is not good as Y even though new phase Al2Nd generate also as seen from Fig.3(c).There are large numbers of needle-like Al2Nd and residual island-likeβ-Mg17Al12in the matrix,and some needle-like Al2Nd coexist withβ-Mg17Al12.

To further ascertain the existence of Al2Y and Al2Nd in as-cast AZ80-0.32Y and AZ80-0.38Nd alloys,TEM analysis was carried out.Fig.4 shows the bright field images of Al2Y and Al2Nd phases with selected area electron diffraction(SAED)patterns.As shown in Fig.4(a),dotted fine Al2Y particle was confirmed by TEM bright field image with the[?111]selected area electron diffraction which is consistent with reports of Zhang et al.[26].As shown in Fig.4(b)needle-like Al2Nd particle coexisting withβ-Mg17Al12was also confirmed by TEM bright field image with the[112]selected area electron diffraction which is consistent with reports of Liu et al.[27].

Fig.4.TEM micrographs and SAED patterns of second phases:(a)Al2Y and(b)Al2Nd.

Fig.5.The DSC curves for AZ80,AZ80-0.32Y,and AZ80-0.38Nd alloys.

Fig.6.The weight gain curves for 6h and fitted curves for first hour of alloys during exposure at different temperatures in air:(a)AZ80 6h,(b)AZ80 1h,(c)AZ80–0.32Y 6h,(d)AZ80-0.32Y 1h,(e)AZ80-0.38Nd 6h,and(f)AZ80-0.38Nd 1h.

Fig.7.The parabolic law oxidation kinetics and activation energies of alloys:(a)AZ80,(b)AZ80-0.32Y,(c)AZ80–0.38Nd,and(d)oxidation activation energies.

3.2.DSC analysis

High temperature oxidation of Mg alloys is typically accompanied with a change of the microstructure and even partially melting of alloy.DSC curve gives the heat change resulting from change of the microstructure and even partially melting of alloy,for example the onset point of DSC valley represents the melting of second phase and the area of valley represents the total heat absorbed to melt the second phase[28,29].Fig.5 illustrates the DSC curves of as-cast alloys.The onset melting temperature of DSC valley indicates the melting ofβ-Mg17Al12phase in alloy and the valley area portrays the total heat absorbed to melt them.The onset melting temperatures ofβ-Mg17Al12phases in AZ80,AZ80-0.32Y and AZ80-0.38Nd alloys are all 421°C which is close to 437°C,the theory melt point ofβ-Mg17Al12[30].While the absolute area of alloys DSC valleys are different.The absolute areas of AZ80,AZ80-0.32Y,and AZ80-0.38Nd DSC valleys are 0.664J/g,0.063J/g,and 0.082J/g,respectively.The DSC results indicate that the addition of REs(Y,Nd)don’t increase melt point ofβ-Mg17Al12,but eliminate them remarkably.

3.3.Oxidation kinetics and activation energy

Fig.6 shows the isothermal oxidation weight gain data of all alloys at different temperatures for 6h or 1h in air.The weight gain increases with the increase of oxidation time and oxidation temperature.The overall mass gain at exposure temperatures between 413°C and 427°C for 6h in air are listed in table 2.The results indicate that the addition of Y or Nd in AZ80 alloy could reduce the total weight gain for a certain extent,which means the improvement of high temperature oxidation resistance of alloy.Besides,for AZ80 alloy,catastrophic oxidation occurred after exposed 13270s at 433°C,with weight gain of 13.591mg/cm2.While even exposed for 6h at 433°C,catastrophic oxidation didn’t occur in AZ80-0.32Y and AZ80-0.38Nd alloys,and when the weight gain are 14.000mg/cm2,exposed time of AZ80-0.32Y and AZ80-0.38Nd alloys are 19978s and 16287s,respectively.The results also indicate the improvement of oxidation resistance for Y or Nd addition.What is more,oxidation kinetics can reflect rate of Mg alloy oxidation,which is illustrated by the oxidation behavior of Mg alloy.The oxidation kinetics of alloy can be expressed by Eq.(3.1)[31]:

Fig.8.The optical macrographs of AZ80,AZ80-0.32Y,and AZ80-0.38Nd alloys after 6h exposure at different temperatures in air.

Where,,the weight gain per unit area during oxidation;k,the oxidation rate constant;t,the time of alloy exposed to the oxidizing environment;n,the reactivity index,and the higher index a reaction has,the quicker the reaction is[31].The first hour weight gain data are selected and fitted according Eq.(3.1).The oxidation reactivity indexes for all alloys at different exposure temperatures are obtained as given in Fig.6(b),(d),(f)and summarized in Table 3.The data suggest that the oxidation reactivity indexes increase with increasing of exposure temperature for all alloys,and AZ80-0.32Y alloy shows the lowest reactivity index at each exposure temperature which also indicates that addition of Y in AZ80 restrains oxidation rate of alloy effectively.

Fig.9.The three dimensional morphology of oxide film surface oxidized at different temperatures for 6h in air:(a–c)AZ80,(d–f)AZ80-0.32Y,and(g-i)AZ80-0.38Nd.

Table 2The overall weight gain of alloys after oxidized 6h.

Table 3The oxidation reactivity indexes of all alloys at different exposure temperatures.

The oxidation reactivity indexes are almost less than 1 for all alloys exposed below 433°C as shown in Fig.6.So oxidation kinetics shed light to common parabolic rate law which means cationic transport across the growing oxide layercontrols the rate of scaling.And the following equations are applied to research oxidation behavior and activation energy of alloy[14,32]:

Table 4The values of kp and Q.

Fig.10.The surface roughness of oxide scale oxidized at different temperatures for 6h in air.

and

And then Eqs.(3.4)and(3.5)are obtained,

and

Where,Eq.(3.3)is Arrhenius equation;x,the weight gain per unit area during oxidation;kp,the parabolic rate constant;t,also the time of alloy exposed to the oxidizing environment;cp,the pre-exponential constant;Q,the oxidation activation energy;R,the gas constant;T,the absolute temperature.Alloy are exposed at three temperatures(413°C,420°C and 427°C),T,and then the corresponding three parabolic rate constants,kpwill obtain.Then puttingTand the correspondingkpinto Eq.(3.5)and fitting according univariate linear law oxidation activation energy is obtained.Qis the absolute value of the slope in Eq.(3.5).Fig.7 depicts the parabolic rate fitted curves and the calculated oxidation activation energies of alloys.The values ofkpandQare summarized in Table 4,from which it could be seen that Y or Nd addition in AZ80 alloy could reduce the parabolic rate constant for a certain extent and increase the oxidation activation energy significantly.The oxidation activation energies calculated via Arrhenius equation of AZ80,AZ80-0.32Y,and AZ80-0.38Nd alloys are 82.556kJ/mol,177.148kJ/mol,and 136.738kJ/mol,respectively.

Fig.11.SEM morphologies of oxide layers for the experimental magnesium alloys at 420°C for 6h in air:(a,b)AZ80,(c,d)AZ80-0.32Y,and(e,f)AZ80-0.38Nd.

3.4.Surface morphology and oxide product analysis

Fig.8 shows the optical macrographs of alloys after 6h exposed at different temperatures in air.It could be found that gray cauliflower-like oxide nodules and oxide burr arise on surface and edges of AZ80 alloy after 6h oxidized at 413°C.As the oxidation temperature reached 420°C,oxide nodules increase and slight shedding of oxide scale occurs.And increasing of oxidation temperature exacerbates oxide exfoliation.Catastrophic oxidation occurs when the oxidation temperature reached 433°C,and AZ80 block collapses as seen in Fig.8.While oxide scales of AZ80-0.32Y and AZ80-0.38Nd are smooth and even without nodules and burr after oxidation at 413°C.But shedding of AZ80-0.32Y oxide scale layer by layer occurs with oxidation temperature increase.Even the oxidation temperature reached 433°C,catastrophic oxidation don’t occur in AZ80-0.32Y and AZ80–0.38Nd alloys.It is worth noting that AZ80-0.32Y shows slight oxidation,compared with AZ80-0.38Nd alloy.

Fig.9 gives the three dimensional morphologies of the oxide surface oxidized at different temperatures for 6h in air.The area of studied is a square of dimensions 1280×1280(μm2),and the change in height of oxide is described by the color variation.AZ80-0.32Y alloy presents most homogeneous oxide surface,follows by AZ80-0.38Nd which could also be seen in Fig.10,the surface roughness(Sa,μm)of oxide scale oxidized at different temperatures for 6h in air.Surface roughness refers to the small pitch of the machined surface and the unevenness of tiny peaks and valleys,which is a micro geometry error.The smaller the surface roughness,the smoother the surface[33].Gray cauliflower-like oxide nodules appears on AZ80 alloy surface.Due to the non-film forming character of AZ80 alloy,it make no sense to investigate its surface roughness.While the surface roughness of oxide scales increase with increasing of oxidation temperature both in AZ80-0.32Y and AZ80–0.38Nd alloys as shown in Fig.10.But the Sa values of AZ80-0.32Y less than AZ80-0.38Nd at all corresponding oxidation temperatures.

Fig.11 shows the SEM morphologies of surface oxide on alloys after oxidation at 420°C for 6h in air.The oxide layer formed on AZ80 is rough and loose with lots of porous seen from Fig.11(a)and(b).It is noticeable that nodular-like oxides with many burrs on surface formed on sample.The oxide layer of AZ80-0.32Y is more homogeneous and compact than AZ80 as shown in Fig.11(c)and(d).While compact outerlayer shed,and loose inner-layer exposed.The nodular-like oxides of inner-layer are smaller and show less burrs on surface than AZ80.Although the oxide layer of AZ80-0.38Nd is more compact and with less porous,it is uneven.Loose and compact structures appear alternately on oxide layer.And the oxide morphology is similar to that of inner oxide of AZ80-0.32Y alloy.XRD is used to identify oxides.The results show that MgO and MgCO3formed on all experimental magnesium alloys as shown in Fig.12.The appearance of MgCO3in oxide film of magnesium alloys after exposure at temperature above 400°C in air has been reported by many literatures[34,35,24].And the EDS results corresponding to the squares denoted in Fig.11A–D are shown in Table 5,which indicates that the oxygen contents of oxide layers are various,and the higher MgO and lower MgCO3contents,the looser oxide layers is.

Fig.12.XRD patterns of AZ80,AZ80–0.32Y,and AZ80–0.38Nd specimens after oxidation at 420°C for 6h.

Table 5The EDS results corresponding to the squares denoted in Fig.11(at.%).

For purpose of determining the compositions of oxide layers,X-ray photoelectron spectroscopy(XPS)is used.The results in Fig.13 show that the oxide layers of three alloys are composed of not only MgO,MgCO3,but also Mg(OH)2and Al2O3.While no Y2O3or Nd2O3are detected in AZ80-0.32Y or AZ80-0.38Nd alloys.It has been reported by Yu et.al[34]that when Al/Y ratio(wt.%)in Mg-2.5Y-xAl alloy reached 1.68,no Y2O3appeared in alloy oxide layer due to the formation of Al2Y which is considered as high melting temperature phase and would not dissolve in matrix during oxidation.And in AZ80-0.32Y,the Al/Y ratio(wt.%)is 25 and far more than 1.68,so Y is consumed by Al for the formation of Al2Y which results in the absence of Y2O3in alloy oxide layer.As for AZ80-0.38Nd,the absence of Nd2O3in alloy oxide layer is also for the formation of high melting temperature phase Al2Nd.

Fig.14 illustrates the cross section morphologies of experimental magnesium alloys oxidized at temperatures of 413°C and 420°C for 6h in air.Fig.15 shows the oxide layers thickness of alloys.The thickness of the scale is measured by Image-Pro Plus software.The values in Fig.15 are the average thickness of the oxide scale obtained from seven positions in Fig.14(a–f).The oxide layers thickness of AZ80,AZ80-0.32Y,and AZ80-0.38Nd alloys oxidized at 413°C are 29.3μm,4.4μm,and 7.6μm,respectively.And the thickness at 420°C are 41.6μm,10.6μm,and 27.5μm,respectively.So the thickness increases with increasing of temperature.After oxidation,AZ80-0.32Y alloy shows the most compact and protective oxide layer with fewest defects,then follows by AZ80-0.38Nd.It is worth noting that longitudinal cracks generate in AZ80 oxide layer as shown in Fig.14(a)and(d),transverse cracks appear in AZ80-0.32Y oxide layer as seen in Fig.14(g),and oblique cracks arise in AZ80-0.38Nd oxide layer as given in Fig.14(f).What is more,subscale in AZ80 and AZ80-0.38Nd alloys shows plane spread,while in AZ80-0.32Y alloy shows line spread.

Fig.14.The cross-sectional morphologies of the experimental magnesium alloys oxidized at different temperatures for 6h in air:(a)AZ80,413°C;(b)AZ80-0.32Y,413°C;(c)AZ80-0.38Nd,413°C;(d)AZ80,420°C;(e,g)AZ80-0.32Y,420°C;(f)AZ80-0.38Nd,420°C.

Fig.15.Plot of oxide layer thickness of alloys oxidized at 413°C and 420°C for 6h in air.

4.Discussion

4.1.Effect of Y or Nd on alloys microstructure

Thermostability reflects oxidation resistance of alloys due to changing of microstructure and even partially melting of alloys when heating at high temperature[29].Theβ-Mg17Al12,with melting temperature of 437°C in theory,is considered as low melting temperature phase and will malacia and melt when exposure at about 437°C.While in this research lots ofβ-Mg17Al12with onset melting temperature of 421°C in practice are contained in AZ80 as seen in Fig.3(a).So negative effect ofβ-Mg17Al12on AZ80 oxidation resistance is getting due to its partially melting during oxidation.Adding Y or Nd in AZ80 don’t increase melting temperature ofβ-Mg17Al12,but eliminate them for a certain extent.Compared with Nd,Y alloying has a better effect.Fine Al2Y particles generate and uniformly distribute in alloy matrix.Al2Y reported[36]directly formed from Mg-1Al-9Y melt at 692.4°C is considered thermostable and can improve thermostability of surrounding area.So alloying Y in AZ80 enhance oxidation resistance of alloys to a great extent.As for alloying Nd,the same benefit on improving alloy oxidation resistance obtains,owing to appearing of Al2Nd and eliminating ofβ-Mg17Al12.

4.2.Oxidation kinetics and activation energy analysis

REs have influence of enhancing high temperature oxidation resistance of metals,which has been previously researched[37–39].In this study,the effect of Y or Nd on high temperature oxidation behavior of AZ80 alloy is researched but few amount of rare-earth element is detected in the oxide layers,consequent to the forming of stabilized Al2Y or Al2Nd which results in the extremely low solid solution of Y or Nd in AZ80.And the effect of Y or Nd on the high-temperature oxidation behavior of AZ80 alloy is elevated from other aspects,for instance oxidation kinetics,activation energy,and oxide layer characteristics.

Though weight gain increase with the increase of oxidation time and temperature,alloying Y or Nd in AZ80 alloy can significantly alleviate the total weight gain,and improve high temperature oxidation resistance of alloys.The oxidation resistance of AZ80 is remarkably promoted by alloying～0.32wt.% Y.The first hour weight gain curve of AZ80-0.32Y alloy shows the lowest and all below 1 reactivity index at each exposure temperature,so in first hour AZ80-0.32Y alloy shows the common parabolic rate law oxidation kinetics which means cationic transport across the growing oxide layer controls the rate of scaling[15].AZ80 shows reactivity index of about 1 at each exposure temperature in first hour,and the oxidation kinetics obeys linear rate law which means a phase-boundary process is the rate-determining step for the reaction[15].While as for AZ80-0.38Nd,reactivity index increases from 0.75 to 1.68 with oxidation temperature increases from 413°C to 433°C.So the effect of Y or Nd on high temperature oxidation behavior could be crystallized by the parabolic rate fitted curves with the parabolic rate constant and the calculated oxidation activation energy as shown in Fig.7 and Table 4.For comparing,oxidation behavior of AZ80 is crystallized still by the parabolic rate fitted curves though linear rate law oxidation kinetics it obeys.The results indicates that alloying～0.32wt.% Y and～0.38wt.% Nd in AZ80 alloy increase the activation energy of AZ80 alloy from 82.556 kJ mol?1to 177.148 kJ mol?1and 136.738 kJ mol?1,respectively.But they are still far less than 393 kJ mol?1,the activation energy of pure aluminum powder[40].So adding Y or Nd in AZ80 alloy could improve oxidation resistance of alloy significantly and adding Y has better effect,compared with Nd.

4.3.The characteristics and compositions of oxide layers

The oxidized alloys consist of the oxidized systems and the metallic systems.No intact oxide layer generates in AZ80 alloy surface for the reason that a large number ofβ-Mg17Al12distribute in AZ80 alloy matrix and lead to selective oxidation during exposure in high temperature.The process could be described as follows with oxidation temperature increase continuously,β-Mg17Al12phases begin softening and swelling,then break alloy surface and protuberances form.After that,the Mg within the alloy contacts oxygen and a sharp reaction begins and releases a large amount of heat,resulting in collapsing of alloy.While gray cauliflower-like oxide nodules don’t appear in AZ80-0.32Y alloy,and oxide scale of alloy generated in 413°C is smooth and compact,while with exposure temperature increase,thermal stresses come into being,and oxide layer spalls layer by layer due to the thermal expansion mismatches between the oxide and the alloy matrix.But catastrophic oxidation don’t occur eventually.As for AZ80-0.38Nd alloy,the oxide layer is not compact and homogeneous as AZ80-0.32Y,but it is more adherent than AZ80.And it is interesting that the outer-layer is more compact than inner-layer.Component analysis shows that the outer-layer consists of more MgCO3than inner-layer and the AZ80 alloy oxide layer.Besides,this research shows that the more MgCO3content in oxide layer,the better tightly coherent of oxide layer is.According to the report[41],MgCO3is very stable in nature.The solubility of MgCO3in water at ambient pressure and temperature is 0.02g per 100ml and MgCO3can remain intact for quite a long time in air and even water.Besides,it has a PB ratio of 2.04 which means compact enough of MgCO3layer to protect matrix against elevated temperature exposures to oxidizing environments,and the reaction of CO2with MgO to obtain MgCO3can occur at atmospheric pressure as heating to 400°C[42].In this experiment,the lowest exposure temperature is 413°C,so MgCO3exists in all oxide layers of the experimental magnesium alloys.What’s more,it is found that alloying rare-earth element of Y or Nd can facilitate generating of MgCO3maybe via activating the chemical reaction,MgO+CO2=MgCO3,to obtain protective MgO/MgCO3composite layer.This is similar to report of Wang et al.[43],who found that compact MgCO3protective surface layer could be obtain via reaction of MgO with excited CO2excited by either high energy electron beam or glow discharge.

4.4.The effect of Y or Nd on cracking behavior of oxide layers

Fig.14 shows cracks generating due to the thermal expansion mismatches between the oxide and the alloy matrix after oxidation.It is interesting that crack types are different.Longitudinal cracks generate in AZ80 oxide layer as shown in Fig.14(a)and(d).According to Wagner theory of oxidation[15],the growth of oxide film is related to diffusion of magnesium ions and oxygen ions through the film.As oxidation proceeds and longitudinal cracks generating,the area across which cations and anions can be supplied is,therefore,increased.So the longitudinal cracks at the scale-metal interface or scale-gas interface could act as channel for outward diffusion of cations and anions as seen in Fig.16(a),resulting in oxidation accelerated.Transverse cracks appear in AZ80-0.32Y oxide layer as seen in Fig.14(g),and they act as barrier for outward diffusion but channel for transverse diffusion of cations and anions as seen in Fig.16(b),which promotes densification of oxide film and results in oxidation suppression.As for AZ80-0.38Nd alloy,oblique cracks arise in oxide layer as given in Fig.14(f),which also act as barrier for outward diffusion of cations and anions as seen in Fig.16(c),resulting in oxidation suppression for a certain extent.And all cracks sprout in scale-metal interface owing to the high thermal expansion mismatches here and then expand in different forms with thickening of oxide film.Finally,longitudinal cracks appear in AZ80 alloy oxide scale,transverse cracks generate in AZ80-0.32Y alloy oxide layer which can explain why the oxide layer of AZ80-0.32Y alloy peels off layer by layer,and oblique cracks arise in AZ80-0.38Nd alloy oxide film.The results indicate that Y or Nd addition in AZ80 alloy could change the initiation form of cracks,and facilitate generating of MgCO3,which makes the oxide layers denser than that of AZ80.

Fig.16.The schematic representation of role of oxide layer cracks on outward diffusion of cations and anions in alloys:(a)AZ80,(b)AZ80-0.32Y and(c)AZ80-0.38Nd.

5.Conclusions

In this research,the effect of～0.32wt.% Y and～0.38wt.% Nd addition on high temperature oxidation resistance of AZ80 was studied.The main conclusions are as follows:

(1)The addition of Y or Nd in AZ80 alloy can eliminate the low melting temperature phaseβ-Mg17Al12for a certain extent due to the formation of uniformly distributed fine Al2Y or needle-like Al2Nd phases respectively,which enhances thermostability of AZ80 alloy.

(2)The weight gain and oxidation rate of AZ80 alloy are reduced significantly with Y or Nd addition,for the formation of compact and protective oxide layer on surface,which means the improvement of alloy oxidation resistance.And compared with addition of Nd,Y has better effect owing to the formation of more smooth and compact oxide layer.

(3)Longitudinal cracks,transverse cracks and oblique cracks generate on oxide layer of AZ80,AZ80-0.32Y and AZ80-0.38Nd alloys,respectively.Y or Nd addition changes the initiation form and propagation of cracks in AZ80 alloy oxide layer.Transverse and oblique cracks act as barrier for outward diffusion but channel for transverse diffusion of cations and anions,promoting densification of oxide film.

(4)The activation energies of AZ80,AZ80-0.32Y and AZ80-0.38Nd alloys calculated via Arrhenius equation are 82.556kJ/mol,177.148kJ/mol and 136.738kJ/mol,respectively.

Acknowledgments

The authors are grateful to the National Key Research and Development Program of China(No.2016YFB0301104),Nation Natural Science Foundation of China(No.51771043),Foundation of State Key Laboratory of Baiyunobo Rare Earth researches and Comprehensive Utilization,and Programme of Introducing Talents of Discipline Innovation to Universities 2.0(the 111 Project 2.0 of China,No.BP0719037).