Grain refinemen of Mg-Al alloys inoculated by MgAl2O4 powder

Hengbin Liao,Meiyan Zhan,Chengbo Li,Zhiqiang Ma,Jun Du

Department of Metallic Materials,School of Materials Science and Engineering,South China University of Technology,Wu Shan Road 381,Tianhe District,Guangzhou 510640,China

Abstract As a potent nucleating substrate for α-Mg grain,MgAl2O4 powder was used to inoculate the Mg-Al melt in this study.The effects of MgAl2O4 amount,holding time and Al content on the grain size and grain refinin ratio of the inoculated Mg-Al alloys are systematically investigated.The results show that the minimum grain size of Mg-3Al alloy is achieved by adding 2wt.% MgAl2O4 powder and this alloy exhibits higher grain refinin ratio than Mg-5Al and Mg-8Al alloys.The crystallographic misfi calculation indicates the wellmatching and possible orientation relationships (ORs) between α-Mg and MgAl2O4.Among these predicted ORs,[10-10]α-Mg//[110]MgAl2O4 in (0002)α-Mg//(1-13)MgAl2O4 possesses the smallest misfit i.e.,2.34% (fr).Both results of the experiment and crystallographic calculation demonstrate that the grain refinemen of Mg-Al alloys is attributed to the MgAl2O4 particles acting as the heterogeneous nucleation substrates for α-Mg grains.

Keywords: Grain refinement Inoculation;Heterogeneous nucleation;Mg-Al alloys;MgAl2O4.

1.Introduction

As the lightest metallic structural materials with low density and high specifi strength,Mg alloys have been applied in many industrial field such as aerospace and transportation vehicle [1,2].However,the poor tensile strength and low ductility limit their wider application,especially the Mg-Al alloys with good castability and high corrosion resistance [3].Generally,the metals with refine grains exhibit fine-scal distributions of second phase and they also possess enhanced tensile strength and ductility.Accordingly,the grain refine ment has been widely used in a large number of metallic materials including steel,Al and Mg alloys [4-6].Mg possesses a high Hall-Petch coefficien (K) due to the poor slip systems of the HCP structure.According to Hall-Petch relationship (σs=σ0+Kd-1/2),great improvement in the yield strength and ultimate tensile strength can be achieved for Mg alloys via grain refinemen [7].

In the past two decades,continuous effort has been devoted to grain refinemen of Mg alloy.Some methods were developed,such as superheating [8],Elfina process (FeCl3process) [9],carbon inoculation [10,11],addition of heterogeneous particles [12-14] and addition of solute [15].Among these methods,addition of heterogeneous particles is regarded as the most practical and effective approach.For Al-free Mg alloys,Zr particles can effectively refinα-Mg grain.However,Zr containing master metals are expensive and their utilization rate is relatively low [15,16].Moreover,adding Zr is invalid for grain refinemen of Al-bearing Mg alloys since Zr is easily reacted with Al to form Al3Zr.The currently available grain refiner for Mg-Al alloys include the Al4C3[17],Al2Ca [18] AlN [19] and TiB2[13].Among these grain refiners the Al4C3particles have been confirme to be the most effective one forα-Mg grains of Mg-Al alloys;however,they are easily poisoned by Fe/Mn element and hence the efficien y of grain refinemen is seriously debased [20].

Metal oxides as other kinds of effective grain refiner for Al or Mg alloys have attracted increasing interesting in the past decade.Several literature studies indicated that ZnO andCaO enabled the grain refinemen of Mg alloys,owing to the fact that the oxides in the melt could act as the nucleation substrates forα-Mg and the grain growth was also restricted by Zn and Ca solute[21,22].Moreover,introducing MgO into the melt of Al-Mg alloy with intensive shearing can greatly refin theα-Al grains,of which the nucleation sites were the MgAl2O4particles generated by the reaction of MgO and Al[23].The orientation relationship between MgAl2O4andα-Al was identifie as(111)||(111)[110]α-Al.In addition to the Al-Mg alloy,the MgO particles with the diameters of 100-200nm also promoted the heterogeneous nucleation ofα-Mg grain and thus remarkably refine the grains of AZ91D alloy [24].Our previous work demonstrated that the grains of Mg-Al alloys were effectively refine by the MgO inoculation,which created the MgAl2O4particles as the nucleation sites forα-Mg grains [25].

It is no doubt that the Mg melt at high temperature unavoidably suffers oxidation under air atmosphere,thereby resulting in the formation of MgO inclusion[26].The oxidation inclusion could serve as the nucleation substrates for Al or Mg alloys [23,24].For Mg-Al alloys,the MgAl2O4spinel would be another oxidation inclusion that forms in the course of melting and casting [27,28].In the research carried out by Fan et al.[24],the nucleation substrates(well dispersive MgO particles) were also derived from the oxide film (oxidation inclusions).Thus,it seems an interesting idea to refin grain by utilizing the oxidation inclusion.However,to our knowledge,there was less systemic study or experiment on the effect of oxidation inclusion (especially MgAl2O4) on the grain refinemen of Mg alloy.In previous work [25],we had put forward the possibility of heterogeneous nucleation ofα-Mg grain on MgAl2O4particle.Given that the MgAl2O4spinel is an important oxidation inclusion in Mg melt,the effect of MgAl2O4spinel on the grain refinemen of Mg alloys is urgently-needed to be clarified Accordingly,we focus on the MgAl2O4spinel as the grain refine of Mg-Al alloys in this work,and systematically investigate the effect of MgAl2O4amount,inoculation holding time,and Al content on the grain refinement

2.Experimental procedure

Three sets of experiments (as listed in Table 1) were designed in the present study.As for set I,the MgAl2O4powder with different addition amount from 0 to 2.5wt.% were added into the Mg-3Al melt (corresponding to No.1 to 7),which was held for 10min.To study the effect of holding time on the grain refinin efficien y,the Mg-3Al melt inoculated by 2.0wt.% MgAl2O4powder was held for 0 to 40min,and the specimens were marked as No.8 to 13 in the set II,respectively.Finally,the set III was carried out to study the effect of Al content on grain refinin efficien y of the Mg-Al alloys inoculated by MgAl2O4powder.In this set,Al content varied from 0 to 8wt.%,which are corresponding to the samples of No.14 to 19.The grain refinin efficien y was characterized by grain refinin ratio (δ) using the following equation:

Table 1 Experimental processes of Mg-Al alloys inoculated by MgAl2O4 powder.

The Mg-xAl alloys (x=0,1,2,3,5 and 8) were prepared by adopting the high purity Mg (99.95%,weight ratio) and high purity Al (99.99%).The raw materials were melted in an MgO crucible by using an electrical resistance furnace under the protection of mixed gas (99.5vol.% N2and 0.5vol.%SF6).The commercial MgAl2O4powder was used as the grain refiner its phase structure and morphology were characterized via X-ray diffraction (XRD,D8 Advance) and scanning electron microscope (SEM,Zeiss Merlin),as shown in Figs.1(a) and (b),respectively.Additionally,the size distribution of MgAl2O4particles analyzed by using Laser Scattering Particle Size Distribution Analyzer (HORIBA LA-960S)is given in Fig.1(c).The MgAl2O4powder was mixed with Mg powder according to a mass ratio of 3:7 and the mixture was pressed into a tablet at 100MPa.This tablet was plunged into the above Mg-Al melt at 760°C with slight stirring for dispersing the MgAl2O4particles.After the melt had been held for different period of time,it was poured into a cylindrical mild steel mold (?20mm × 25mm) that was preheated at 500 °C.

The metallographic samples were sectioned at 12mm from the bottom of that casting ingots;they were heat-treated at 420°C for 8h and cooled down to room temperature in the air.After these samples had been ground and polished,picric acid was adopted to reveal the grain boundaries.Their optical microstructures were observed by using an optical microscope (OM,Leica DFC320).The average grain sizes were measured via a linear intercept method introduced in ASTM standard E112-88.The compositions of the heterogeneous particles were analyzed with an electron probe microanalyzer(EPMA-1600) equipped with EDAX Genesis.The structural details were further characterized by using SEM.The tensile test was carried out on a test machine (Shimadzu,AG-X Plus 100KN) at a cross-head displacement rate of 1mm/min.

Fig.1.The morphology (a),XRD result (b) and size distribution (c) of commercially-available MgAl2O4 powder.

3.Results

3.1.Effect of MgAl2O4 amount on the grain refinemen of Mg-3Al alloy

Fig.2 shows the grain morphologies of the Mg-3Al alloy inoculated by MgAl2O4with different addition amount.The average grain size was evaluated from these OM pictures.Fig.3 illustrates the variation of average grain size and grain refinin ratio of the Mg-3Al alloy with different amount of MgAl2O4.It can be seen from Fig.2 that addition of MgAl2O4powder effectively reduces the grain size of Mg-3Al alloy.For Mg-3Al alloy,its average grain size is about 535 μm(Fig.2a),while this value decreases to 316 μm when 0.2wt.% MgAl2O4powder is added (Fig.2b).As the MgAl2O4amount increases,the average grain size continues to decline.With the addition of 0.5,1.0,1.5 and 2.0wt.%,the average grain sizes of Mg-3Al alloys are about 200,165,173 and 120 μm (shown in Fig.2c,d,e and f),respectively.However,there is no further refinemen when the amount of MgAl2O4increases from 2.0 to 2.5wt.%,as shown in Fig.2g.The average grain size of Mg-3Al alloy with 2.5wt.%MgAl2O4is 176 μm.

To show the improvement of mechanical properties,the tensile test of Mg-3Al alloy without/with inoculation was carried out.Fig.4 shows the strain-stress curves and mechanical properties of the Mg-3Al alloy and the Mg-3Al alloy inoculated by 2.0wt.% MgAl2O4.After being inoculated by 2.0wt.% MgAl2O4,the yield strength (YS) of the Mg-3Al alloy increases from 37.5 to 51.0MPa,and its ultimate tensile strength (UTS) increases from 169MPa to 183MPa.The elongation slightly decreases from 15.2 to 14.4%.

3.2.Effect of holding time on the grain refinemen of the inoculated Mg-3Al alloys

Fig.5 shows the grain morphologies of the Mg-3Al alloy inoculated by 2.0wt.% MgAl2O4with varying holding time.The effects of holding time on the average grain size and grain refinin ratio are illustrated in Fig.6.It is obvious that,theα-Mg grains of Mg-3Al alloy are effectively refine via 2.0wt.% MgAl2O4inoculation under the different holding time.After being inoculated by 2.0wt.%MgAl2O4powder for 2min,the average grain size of Mg-3Al decreases from 535 to 165 μm,as shown in Fig.5a.The uninoculated Mg-3Al alloy is shown in Fig.2a.When the holding time increases to 5 and 10min,the average grain size continuously declines to 152 and 120 μm (Figs.5b and 2f),respectively.However,no further grain refinemen is achieved as the holding time prolongs to 20 and 40min.On the contrary,the average grain size increases to 173 and 196 μm,as shown in Figs.5c and d,respectively,and this phenomenon is the socalled inoculation fading.According to Fig.6,a grain refinin ratio of 69.2%is obtained when the inoculated melt is held for 2min.As the holding time increases from 2 to 10min,there is a continuous increase in the grain refinin ratio.However,this value decreases when the holding time reaches 20 and 40min.Therefore,the optimal holding time is 10min for the inoculated Mg-3Al alloy,while prolonging the holding time results in inoculation fading.

Fig.2.Grain morphologies of the Mg-3Al alloy inoculated by MgAl2O4 with different addition amount:(a) 0wt.%,(b) 0.2wt.%,(c) 0.5wt.%,(d) 1.0wt.%,(e) 1.5wt.%,(f) 2.0wt.% and (g) 2.5wt.%.

Fig.3.Effect of MgAl2O4 addition amount on the average grain size (a) and grain refinin ratio of the Mg-3Al alloy (b).

Fig.4.Strain-stress curves (a) and mechanical properties (b) of the Mg-3Al alloy and Mg-3Al alloy with 2.0wt.% MgAl2O4 inoculation.

Fig.5.Grain morphologies of the Mg-3Al alloy with the MgAl2O4 inoculation in different holding time:(a) 2min,(b) 5min,(c) 20min,(d) 40min.

Fig.6.Effect of holding time on the average grain size (a) and grain refinin ratio (b) of the Mg-3Al alloy inoculated by MgAl2O4.

3.3.Effect of Al content on the grain refinemen of the inoculated Mg-Al alloys

Fig.7 shows the grain morphologies of the Mg-Al alloys and inoculated Mg-Al alloys with varying Al content.Fig.8 illustrates the effect of Al content on the average grain size and grain refinin ratio of the inoculated or uninoculated Mg-Al alloys.Comparing with Fig.7a0and a1,pure Mg possesses the typical coarse columnar structure.After being inoculated by 2.0wt.% MgAl2O4powder,the grain of pure Mg is effectively refined however,it still exhibits the columnar crystal structure.When Al is added into pure Mg,the grain morphology converts from the coarse columnar to equal-axial.The average grain size of Mg-1Al alloy is about 613 μm and that of the inoculated one is 327 μm,as shown in Fig.7b0and b1,respectively.For Mg-2Al alloy,the average grain size is 567 μm (Fig.7c0),which decreases to 170 μm (Fig.7c1) with the addition of MgAl2O4.As for the Mg-3Al alloy,its grain size decreases from 535 μm to 120 μm after being inoculated by 2wt.% MgAl2O4powder (as shown in Fig.2a and f) When Al content further increases to 5 and 8wt.%,the grain size decreases to 303 and 233 μm,as shown in Fig.7d0and e0,respectively.In inoculated Mg-5Al and Mg-8Al alloys,the average grain sizes are severally reduced to 93 and 124 μm,respectively.

Fig.7.Grain morphologies of the Mg-Al alloys:pure Mg (a0),Mg-1Al (b0),Mg-2Al (c0),Mg-5Al (d0),Mg-8Al (e0) and the inoculated Mg-Al alloys:pure Mg (a1),Mg-1Al (b1),Mg-2Al (c1),Mg-5Al (d1),Mg-8Al (e1).

From Fig.8,the grains size of Mg-Al alloys decreases as the Al content increases from 1 to 8wt.% and all Mg-Al alloys exhibit grain refinemen with the MgAl2O4inoculation.The grain size of the inoculated Mg-Al alloys decreases as the Al content decreases from 1 to 5wt.%;however,this value increases when the Al content increases from 5 to 8wt.%.The grain refinin ratio reaches the maximum value when Al content increases to 3wt.%,while it decreases as the Al content increases from 3 to 5 and 8wt.%.

Fig.8.Effect of Al content on the average grain size (a) of the uninoculated and inoculated Mg-Al alloys,and the grain refinin ratio (b) of the inoculated Mg-Al alloys.

Fig.9.EPMA BSE images of the inoculated Mg-Al alloys:pure Mg (a),Mg-1Al (b),Mg-2Al (c),Mg-3Al (d),Mg-5Al (e) and Mg-8Al (f).

4.Discussion

4.1.Grain refinemen mechanism of the Mg-Al alloys inoculated by MgAl2O4 powder

It is obvious that addition of MgAl2O4powder effectively refine theα-Mg grains of Mg-Al alloys.To reveal the grain refinemen mechanism of Mg-Al alloys inoculated by MgAl2O4,the potent nucleating particles were observed by utilizing EPMA,SEM and EDS analyses.

Fig.10.The atomic percent of oxygen and aluminum of the heterogeneous particles in the Mg-3Al alloy with 2wt.% MgAl2O4 inoculation.

Fig.9 shows the typical EPMA back-scattered electron(BSE) images of pure Mg and Mg-Al alloys inoculated by MgAl2O4.There are many tiny gray particles with the size of about 2-6 μm in these samples.Their compositions are detected by EDS,as summarized in Table 2.These particles are mainly composed of Mg,Al and O elements.The Al/O atomic ratios of these particles (marked as A,B,and F in Fig.9,respectively) are close to 1:2.Thus,they are referred to the added MgAl2O4particles.Moreover,high atomic percentage of Mg is attributed to the contribution from matrix effect.The particles (marked as C,D and E,respectively) are also the MgAl2O4particles because of their high Al and O element content even though the corresponding atomic ratios are not very close to 1:2.Additionally,Fig.10 shows the atomic percent of O and Al element of other heterogeneous particles in the Mg-3Al alloy with 2wt.% MgAl2O4inoculation.The Al/O atomic ratios of these particles are close to 1:2 and thus they are believed to be the added MgAl2O4particles.In other words,the MgAl2O4particles are stable in the pure Mg and Mg-Al melt.

Table 2 Compositions of the particles denoted by A,B,C,D,E and F.

Shih et al.[29] reported that the MgAl2O4spinel was found in oxide film of Al-bearing Mg in melt or heating at 700K.It is believed that this spinel (MgAl2O4) is created by the reaction of MgO with Al.The formation energy of MgAl2O4is -1878.75kJ/mol (at 1000K),which is lower than that of MgO (-493.09kJ/mol at 1000K).Furthermore,the X-ray diffraction indicated that the MgAl2O4existed in the MgO oxide layer of AZ91 alloy exposed at high temper-ature for 10h [28].This phenomenon can also confir that the MgAl2O4phase is stable in Mg-Al melt.

Fig.11.The secondary electron morphology of the inoculated Mg-8Al alloy and EDS analyses.

To further reveal the grain refinemen mechanism of Mg-Al alloys inoculated by MgAl2O4,their microstructures are examined by using the SEM equipped with the EDS analyses.Fig.11a shows the secondary electron morphology of the as-cast Mg-8Al alloy inoculated by 2.0wt.% MgAl2O4.According to the EDS analyses (Fig.11b and c),theα-Mg matrix and eutecticβ-Mg17Al12(marked by arrow B) are identified It is obvious in Fig.11a that theα-Mg grain is surrounded by eutecticβ-Mg17Al12.Moreover,the Mg-Al-O particle (marked by arrow C) is also found with EDS result,as shown in Fig.11d.This EDS result indicates that the Mg-Al-O particle is MgAl2O4phase since the atomic ratio of Al/O is close to 1:2.Furthermore,this MgAl2O4particle is mainly located at the center of theα-Mg grain.Based on the classical nucleation theory,the particles acting as the heterogeneous nucleation sites would be in the growth center of the grain [30].Moreover,there is no extra added solute affecting the grain size in this work.Therefore,the grain refinemen of the Mg-Al alloys inoculated by MgAl2O4powder is attributed to the heterogeneous nucleation ofα-Mg grains on the MgAl2O4particles.

Edge-to-edge matching (E2EM) model is a useful practical method to predict the nucleation potency of heterogeneous particles and the orientation relationships (ORs) of the given system.According to the E2EM model,the particles can act as the nucleating sites for Mg alloys only when low misfi exists between the matrix and heterogeneous particle.Normally,the D-value mismatch(fd)between the matching planes should be lower than 6% and the mismatch (fr) of matching directions should be lower than 10% [31].In addition,the matching directions must be the same type,i.e.,straight atom row to straight atom row or zigzag to zigzag.

To elucidate the grain refinemen mechanism of the Mg-Al alloys inoculated by MgAl2O4,crystallographic misfi betweenα-Mg and MgAl2O4is calculated based on E2EM model.According to the International Powder Diffraction File,the close-packed or nearly close-packed planes and directions associated with theα-Mg are {0002},{10-11},{10-10},<11-20>S,<10-10>Zand<11-23>Z.The close-packed or nearly close-planes and directions of MgAl2O4are {311},{004},{022},{222},<331>S,<110>S,and<110>Z.The misfi and mismatch (<10%) values of matching directions and planes betweenα-Mg and MgAl2O4are listed in Table 3.It is clear from Table 3 that the MgAl2O4possesses the lower misfi with theα-Mg and thus theα-Mg grains can nucleate on the MgAl2O4particles.Among the possible ORs,in (0002)α-Mg//(1-13)MgAl2O4has the smallest misfi of 2.34% (fr),and thus it is the most likely orientation relationship.Moreover,compared with the other effective heterogeneous particles like Al4C3with misfi of 7.5% (fr) and 3.4% (fd),and Al2CO with misfi of 3.6% (fr) and 1.23% (fd) [32],MgAl2O4has more suitable crystallographic configuratio with Mg matrix.Therefore,the MgAl2O4would be an idea and effective grain refine for Mg-Al alloys.

Table 3 Misfi and mismatch (<10%) of matching directions (rows) and planes between α-Mg and MgAl2O4 based on E2EM model.

4.2.Grain refinin efficienc of the inoculation of MgAl2O4 powder

It is obvious from Fig.3 that the grain size of the Mg-3Al alloy is reduced via the MgAl2O4inoculation.When 0.2wt.%MgAl2O4powder is added,the average grain size of the Mg-3Al alloy is reduced from 535 to 316 μm,with the grain refinin ratio of 41.1%.Furthermore,the grain refinin ratio increases with the amount of MgAl2O4increasing from 0.2 to 2.0wt.%.However,the grain refinin ratio of the Mg-3Al alloy decreases when MgAl2O4amount increases from 2.0 to 2.5wt.%.Therefore,the excessive addition of MgAl2O4will increase the grain size and debase the grain refinin efficien y.The optimal amount of MgAl2O4powder for the Mg-3Al alloy is 2.0wt.%.

The interdependence theory [33] implied that the distance of inoculant particles would impact the formational efficien y of grain nuclei.The particles located in the region (so called nucleation-free zone) of the pre-nucleated particles cannot induce the nucleation event.Additionally,the closer to the nucleation-free zone is,the larger the critical nucleation energy of the added inoculant particle is.This is not conduciveto the heterogeneous nucleation.In this work,the excessive addition of MgAl2O4reduces the average spacing of MgAl2O4particles,and thus adding the MgAl2O4from 2.0 to 2.5wt.% cannot continuously refin the grain.Additionally,it increases the possibility of agglomeration,thereby reducing the grain refinin efficien y.

For the grain refinemen via inoculation process,the holding time of treated melt could significantl affect the grain refinin efficien y [25].In this work,the matrix grains become fine and grain refinin ratio increases with the prolonged holding time from 2 to 10min.However,there is obvious inoculation fading as the holding time extends to 20 and 40min.This fading effect of MgAl2O4inoculant would appear when the holding period exceeds 10min.Generally,the grain refinin efficien y is related to the chemical stability and settling probability of the inoculant.As discussed in Section 4.1,the MgAl2O4particles are thermodynamically stable in Mg-Al melt and thus the inoculant fading is ascribed to the settlement of MgAl2O4particles.Reasonable increase of holding time from 2 to 10min is beneficia to the dispersal of inoculant particles and hence improves the grain refinin efficien y.However,such a long holding time (e.g.,20 and 40min) would increase the settlement events of the effective heterogeneous nucleation particles.Consequently,there is an optimal grain refinin efficien y in the inoculation under 10min holding.

From Fig.8a,the grain size of Mg-Al alloys slightly decreases as the Al content increases from 1 to 3wt.%,while it shows a significan decrease tendency when Al content increases from 3 to 5wt.%.Based on the classical nucleation and growth theory,for the Mg-Al alloys with low Al content (1 to 3wt.%),the decrease in grain size is mainly related to the growth restriction caused by Al segregation.For the Mg-5Al and Mg-8Al alloys,there would be stronger effect to restrict the growth of grain,and sufficien constitutional undercooling will promote the thermal nucleation.Accordingly,the grain size remarkably decreases as Al content increases from 3 to 5wt.%.Moreover,the pure Mg is also effectively refine via the MgAl2O4inoculation.This means that the critical nucleation undercooling of Mg grain on MgAl2O4particles is satisfie in pure Mg melt.The inoculated pure Mg still exhibits the columnar grain morphology due to the lack of solute restricting the growth of columnar grain.

All Mg-Al alloys can be effectively refine via the MgAl2O4inoculation and the grain refinin ratio varies with increasing the Al content,as shown in Figs.7 and 8,respectively.In other words,Al solute has a significan impact on the grain refinemen of Mg-Al alloys produced by the MgAl2O4inoculation.According to the classical nucleation theory,the constitutional undercooling is generated as the solute element precipitates and segregates at the ahead of solid/liquid interface.It will promote the nucleation events especially for the heterogeneous nucleation [34].Actually,the heterogeneous nucleation events are affected by the initial developing rate of the constitutional undercooling,which is define byat

wheremlis the gradient of the liquidus slope,c0is the concentration of the solute in binary alloy andkis the equilibrium partition coefficient Moreover,the growth restriction factor is also themlc0(k-1),designated Q [30].

In this work,the grain refinin ratio of the Mg-Al alloys inoculated by MgAl2O4raises as the Al content increases from 1 to 3wt.%.However,the grain refinin ratio decreases when Al content increases from 3 to 8wt.%.For the Mg-Al alloys with low Al content (1 to 3wt.%),they have the larger gran size (exceeds 500 μm) and can be remarkably refine with the MgAl2O4inoculation.When the Al content increases from 1 to 3wt.%,the initial developing rate of the constitutional undercooling caused during solidificatio will increase.This would promote the heterogeneous nucleation events ofα-Mg grains on the smaller MgAl2O4particles in the inoculated Mg-Al alloys,thereby increasing the grain refinin ratio.However,the grain refinin ratio exhibits a decreases tendency when Al content increases from 3 to 5wt.%even though the grain size decreases from 120 to 93 μm.It is no doubt that the smaller MgAl2O4particles can be activated as the Al content increases from 3 to 5wt.%.However,the grain size of Mg-Al alloys has been significantl reduced from 535 to 303 μm when the Al content increases from 3 to 5wt.%.Additionally,the most potent MgAl2O4particles has been activated to be the heterogeneous substrates in Mg-3Al alloy.Hence the grain refinin ratio decreases as the Al content exceeds 3wt.%.

As shown in Fig.8,the grain size of the inoculated Mg-Al alloys increases and the grain refinin ratio declines,as Al content increases from 5 to 8wt.%.This means that some effective MgAl2O4substrates cannot be activated when the Al content increases from 5 to 8wt.%.The reason might be related to the decreasing solidificatio range [35].Moreover,the latent heat produced by Al precipitation will rapidly increase as the Al content increases from 5 to 8wt.%,thereby accelerating the recalescence phenomenon.Therefore,the grain refinin efficien y of the MgAl2O4inoculation is continually debased when the Al content increases from 5 to 8wt.%.

5.Conclusions

1.A remarkable grain refinemen of Mg-Al alloys is achieved by the MgAl2O4inoculation.For Mg-3Al alloy,the grain size decreases from 535 to 120 μm when 2.0wt.%MgAl2O4is added.The optimal holding period of the inoculated Mg-3Al alloy with 2.0wt.% MgAl2O4is 10min;and grain refinin ratio will decrease when holding time exceeds 10min.

2.There are some possible orientation relationships betweenα-Mg and MgAl2O4based on the E2EM model,and hence the MgAl2O4particles are prone to act as the nucleation substrates forα-Mg grains.Among the predicted ORs,exhibits the smallest misfit i.e.,2.34% (fr).

3.The results show that the grain refinemen of Mg-Al alloys inoculated by MgAl2O4is attributed to the heterogeneous nucleation ofα-Mg grains on MgAl2O4particles.The Al solute content significantl impacts the grain refinin efficien y of the MgAl2O4inoculation.The grain refinin ratio increases as the Al content increases from 1 to 3wt.%,and it decreases when Al content exceeds 3wt.%.

Acknowledgement

This work was supported by the National Natural Science Foundation of China (51871100).