Development of Mg-Zn-Y-Zr casting magnesium alloy with high thermal conductivity

V.E.Bzhenov,A.V.Koltygin,M.C.Sung,S.H.Prk,Yu.V.Tselovlnik,A.A.Stepshkin,A.A.Rizhsky,M.V.Belov,V.D.Belov,K.V.Mlyutin

a National University of Science and Technology“MISiS”,Leninskiy pr.4,Moscow 119049 Russia

b LG Electronics Inc.,Yeoui-daero,Yeongdeungpo-gu.128,LG Twin Tower,Seoul 07336 Republic of Korea

Abstract Magnesium alloys are used in aircraft because of their light weight.However,for these alloys to be applied in electronic devices,high thermal conductivities are required.Several high-potential compositions of Mg-Zn-Y-Zr alloys were selected by phase composition and their freezing ranges calculated using Thermo-Calc software.The alloys were prepared,and their fluidit,hot tearing susceptibility,mechanical properties,and thermal conductivity were obtained and compared.The alloy composed of Mg-4 wt% Zn-3 wt% Y-0.3 wt% Zr was selected for further investigation,because of its high thermal conductivity and satisfactory mechanical properties.The Mg-4 wt% Zn-3 wt% Y-0.3 wt% Zr alloy’s fluidit and hot tearing susceptibility were similar to those of the widespread AZ91 commercial casting magnesium alloy.The influenc of a heat treatment regime on the microstructure,thermal conductivity,and mechanical properties of the developed alloy was also investigated.It was established that the room temperature thermal conductivity of the Mg-4 wt% Zn-3 wt% Y-0.3 wt% Zr alloy after aging at 300°C for 5h was 105W/mK.Additionally,the following tensile test results were obtained in aged condition:120MPa yield strength,200MPa ultimate tensile strength,and 4% elongation.The utilization of solid solution heat treatment at 520°C for 8h prior to aging can promote up to 9% increase in elongation.The Mg-4 wt% Zn-3 wt% Y-0.3 wt% Zr casting alloy can be used as a high thermal conductivity material with industrial applications.

Keywords:Magnesium alloy;Mg-Zn-Y-Zr;Thermal conductivity;Phase composition;Fluidity;Hot tearing susceptibility.

1.Introduction

Magnesium alloys are extensively applied as construction materials in the aircraft and aerospace industry because of their low density and high specifi strength[1-3].However,there is a current requirement for high thermal conductivity materials for applications in households,automobile electronics,and electronic devices.Consequently,high thermal conductivity magnesium alloys are promising candidates for these appliances.Currently,the majority of work is dedicated to the development of new high thermal conductivity magnesium alloys or increasing the thermal conductivity of existed alloys by adding reinforcements with high thermal conductivity[4].

All alloying elements that are soluble in magnesium reduce the thermal conductivity of magnesium alloys because these solute elements act as barriers that hinder the free path of electrons and phonons[5,6].The alloying elements in binary magnesium alloys are arranged in order of decreasing thermal conductivities as follows:Zn,Al,Ca,Sn,Mn,and Zr[7].Si and Ca do not alter the thermal conductivity of magnesium alloys due to their low solubilities in Mg[8,9].In a previous report,an Mg-Si-Zn-Ca casting alloy with high thermal conductivity was developed.However,the mechanical properties of this alloy are only high when the high pressure die casting technique is used[10].

There are several promising alloying systems for casting magnesium alloys with thermal conductivities>100W/mK such as Mg-RE-Ag-Zr(EQ21,QE22A alloys),Mg-Zn-REZr(EZ33A,ZE63A,ZE41A alloys),Mg-Zn-Th-Zr(ZH62A,HZ32A,HK31A alloys),Mg-Zn-Zr(ZK51A alloy),and Mg-Zn-Cu-Mn(ZC63 alloy)[11].Applying radioactive(Th)or expensive noble metals(Ag)in commercial alloys for households and electronic devices is inappropriate.The application of Cu has a detrimental effect on the Mg alloy’s corrosion rate[12].Consequently,we selected the Mg-Zn-RE-Zr system,where RE is a rare earth element.Furthermore,in this work demonstrated that Y has lower detrimental effects on Mg thermal conductivity than Gd and Nd.The Mg-Zn-Y-Zr system was ultimately chosen to develop a casting magnesium alloy with high thermal conductivity and strength.Zn and Y function as solution strengthening elements[13,14].In addition,the use of Y as an alloying element enhances the protective properties of the alloy oxide fil[15].It has been known that alloys with fin grain structure have good casting properties,and thus the addition of Zr,which is effective for the magnesium alloy’s grain refinemen in as-cast condition,was used[16].Numerous studies have investigated the Mg-Y-Zn-Zr system alloy’s properties[17-24].Herein,we focus on alloys with long-period stacking-ordered(LPSO)phases.Yamasaki et al.[5]observed that alloys with LPSO structures exhibit high thermal conductivities and strength.In addition to the LPSO phase,which corresponds to the Xphase(Mg12ZnY or Mg21Zn2Y2)in the Mg-Y-Zn-Zr alloys microstructure,the W-phase(MgYZn2or Mg3Zn3Y2)and Iphase(Mg3Zn6Y)can be found[20,25].Alloys with 14-18 vol% of W phase in the structure had a yield strength(YS)of 150-180MPa[20].Xu et al.also reported high mechanical properties of Mg-5.5 wt% Zn-1.1 wt% Y-0.8 wt% Zr alloy in the as-cast condition(YS=163MPa)[21].

The objective of this study is to develop a Mg-Zn-Y-Zr magnesium casting alloy with the best combination of thermal conductivity,castability,and mechanical properties.

2.Materials and methods

The following raw materials were used for the preparation of alloys:magnesium(99.95 wt% purity),zinc(99.98 wt%),aluminum(99.99 wt%),and Mg-30 wt% Ca,Mg-20 wt% Y,Mg-20 wt% Nd,Mg-15 wt% Zr,Mg-2.7 wt% Mn,and Mg-29 wt%Gd master alloys.The master alloys were prepared by melting in a steel crucible using a high-frequency induction furnace.

The ingots(300g)of magnesium-based binary alloys with Al,Ca,Mn,Zn,Zr,Y,Nd,and Gd were prepared to determine the influenc of different alloying elements on the thermal conductivity of magnesium.The alloys were melted in a steel crucible using a high-frequency induction furnace.Prepared melt was poured into the steel mold preheated to 150°C.The melt was protected with a cover of carnallite flux The compositions of the ingots obtained are shown in Table 1.

Table 1Chemical compositions of binary magnesium alloy samples and SSHT temperatures.

To facilitate the maximal dissolution of the alloying elements in the magnesium solid solution(α-Mg),solid solution heat treatment(SSHT)was conducted for 48h at the temperatures outlined in Table 1.The SSHT temperatures were 20-50°C lower than the solidus temperature of the alloys on the Mg-X binary phase diagrams,where X=Al,Ca,Mn,Zn,Zr,Y,Nd,and Gd.Subsequently,the samples were cut from the ingots and ground.The electrical conductivity was measured using a contact-free eddy current conductivity meter VE-27NC“Sigma”with a measurement range of 5.0-37.0 MS/m,and the thermal conductivity was calculated.In addition,the metallographic sections were prepared,and the element contents inα-Mg were measured by energy-dispersive X-ray spectroscopy(EDS).

The phase compositions and the freezing ranges of the Mg-Y-Zn-Zr alloys were calculated using Thermo-Calc Software[26]with version 4 of the TCMG4 magnesium-based alloy database[27].The alloys under investigation were prepared by melting in a steel crucible using a resistance furnace.The charge for each melt was 2.5kg.An Ar+2 vol% SF6gas mixture was used to ensure melt protection.The pouring temperature of the probes and samples used for further investigations was 740°C.

The microstructures of the alloys and the compositions of the phases were investigated using a Tescan Vega SBH3 scanning electron microscope(SEM)equipped with an Oxford Instruments AZtecEnergy EDS system.The chemical compositions of the prepared alloys were determined using EDS on areas with dimensions of 1mm×1mm.To determine the impurities quantity,the ARL-4460 Thermo Fisher Scientifi optical emission spectrometer was used.The chemical compositions of the investigated alloys are presented in Table 2.

Table 2Chemical compositions of investigated alloy samples.

Fluidity was measured using two steel molds with U-type channels with diameters of 10 and 8mm as shown in Fig.1a and b,respectively.The fluidit was calculated as the mean probe length after two pours.The hot tearing susceptibility of the investigated alloys was determined using a dog-bone test(Fig.1c).The hot tearing criterion for the test is the maximal dog-bone section length achieved without hot tears(measured in mm).A higher section length corresponds to a lower alloy hot tearing susceptibility.

For mechanical testing,the ingots were cast into a graphite mold,preheated to 150°C,as shown in Fig.1d.The cylindrical tensile samples with dimensions shown in Fig.1e were lathe-machined.The tensile tests were performed using the Instron 5569 universal testing machine with an advanced video extensometer(AVE).To evaluate the electrical conductivity,the alloy samples(45mm diameter and 45mm height)were cast into a steel mold.

The rectangular ingots(20×150×270mm)were poured into graphite molds,preheated to 150°C,to investigate the heat treatment schedules.Each ingot was cut into 20×150×20mm bars.One surface of each of the bars was ground.Then,the different heat treatment schedules were used for the bars and the electrical conductivity and hardness measured in each stage.The Brinell hardness was determined using an INNOVATEST Nemesis 9001 universal hardness tester.The ball(2.5mm diameter)was subjected to a 613N load for 30s.

The temperature dependences of the density,thermal diffusivity,and heat capacity were applied as third-order polynomials and the thermal conductivity(λ)was calculated using Eq.(1):

where,ais the thermal diffusivity,ρis the density,andCpis the heat capacity.

The density at room temperature(25°C)was measured by hydrostatic weighing.The temperature dependence of the density was calculated using the thermal expansion coefficien obtained by a NETZSCH DIL 402 C dilatometer(DIL).The thermal diffusivity was measured by the laser flas method(LFA)using NETZSCH LFA 447.The heat capacity was determined using a NETZSCH DSC 204F1 Phoenix calorimeter(DSC).

Fig.1.Schematic of the mold and probe for fluidit measurement(a)10mm channel and(b)8mm channel;(c)the mold and probe for hot tearing test;(d)the mold and ingot for the preparation of mechanical testing samples;(e)tensile test sample with dimensions.The all dimensions are provided in mm.

3.Results and discussion

3.1.Influenc of Al,Ca,Mn,Zn,Zr,Y,Nd,and Gd on Mg thermal conductivity

For choosing the alloying elements and their maximal concentration in developed alloy,the influenc of Al,Ca,Mn,Zn,Zr,Y,Nd,and Gd on Mg thermal conductivity was under investigation.The thermal conductivity(λ)of the alloys can be predicted from the measurements of the electrical conductivity(σ)using a linear empirical relationship suggested by Smith and Palmer[28,29].

where,L0is the Lorenz number,Tis the absolute temperature,andAandBare empirical constants.

There are noAandBcoefficient available for the Mg alloys.The linear relationship is obtained from the thermal and electrical conductivity data for 27 wrought and casting commercial magnesium alloys and pure Mg from previous reports[11,30].To differentiate between the thermal conductivity calculated using data obtained via DIL,LFA,and DSC and that calculated using the electrical conductivity,the former is designated asλand the latter asλ′.Fig.2 depicts a plot of the thermal conductivity vs.the electrical conductivity of magnesium alloys[11,30].The following equation was obtained:

Fig.2.Linear relationship between the thermal and electrical conductivities of commercial magnesium alloys[11,30].

Fig.3.Influenc of the element content inα-Mg of the binary magnesium alloys on thermal conductivity.

Fig.3 illustrates the calculated thermal conductivity(λ′)vs.the element content inα-Mg of the binary Mg alloys with Al,Ca,Mn,Zn,Zr,Y,Nd,and Gd.Theλ′was calculated using Eq.(3)based on the measured electrical conductivity.It was observed that Zn exhibited the lowest influenc on the thermal conductivity of Mg.The thermal conductivity of the alloy with 5 wt% Zn inα-Mg was～115W/mK.Hence,Zn can be the main alloying element in the developed alloy.Al and Y exhibit a similar influenc on the thermal conductivity of Mg.However,due to high solid solubility,the Al is inappropriate for the developed alloy.For example,the thermal conductivity of the AS41 casting magnesium alloy with only 4 wt% Al inα-Mg was 78W/mK[14].Because of that,the use of a Zn and Y combination is preferable to Zn and Al.Similarly,Mn,Ca,Zr,Gd,and Nd greatly decrease the thermal conductivity of Mg.Consequently,these elements can only be used as alloying elements for magnesium alloys with high thermal conductivities when their contents are not so high.It is known that Zr is the most powerful grain refine for Mg alloys that do not contain Al,Mn,and/or Si elements(since Al,Mn,and Si poison the grain refinin ability of Zr),and its addition can provide good alloy castability and increase strength[16].The analysis of the binary alloys’thermal conductivities confirm that the Mg-Zn-Y-Zr alloys are promising for developing an alloy that exhibits both high thermal conductivity and favorable mechanical properties.

3.2.Calculation of Mg-Zn-Y-Zr alloys’phase compositions and freezing ranges

The solidificatio pathways of Mg-Zn-Y-Zr alloys were calculated using the Scheil-Gulliver model in Thermo-Calc program.The compositions of the alloys for calculation were selected in the Mg-(1÷4 wt% Zn)-(1÷4 wt% Y)-0.6 wt% Zr range with a 0.5 wt% step-wise increase of Zn and Y.A fi ed content of Zr was chosen because it exhibited no significan influenc on the phase composition of the analyzed alloys and,in most cases,changed the initial temperature of the primaryα-Zr phase solidificatio as well as the quantity of this phase.As a result,the solidificatio pathways of 49 alloys were calculated.The calculated weight fraction of the eutectic in alloys after solidificatio is shown in Fig.4.It can be seen that at low Zn and high Y contents the eutectics with the LPSO and Mg24Y5phases are solidified However,if the Zn content is high and Y content is low,the eutectic with the W phase is formed in the majority of the alloys.Additionally,a low quantity of eutectics containing the I,Zn2Zr,MgZn,and Mg51Zn20phases can be observed in alloys with high Zn content.For alloys with almost equal Zn and Y contents,the binary and ternary eutectics with LPSO and W phases are observed.

The non-equilibrium freezing ranges of Mg-(1÷4 wt%Zn)-(1÷4 wt% Y)-0.6 wt% Zr alloys calculated using the Scheil-Gulliver solidificatio model are shown in Fig.5.The freezing range of the alloys with high Zn and low Y contents are 100-300°C.The longer freezing range can be explained by the formation of low-melting eutectics with MgZn and Mg51Zn20phases.Alloys with high Y and low Zn contents have a freezing range of 60-90°C.The most promising alloys with almost equal Y and Zn contents have a freezing range of 50-60°C.As shown previously,the eutectic phases in these alloys are LPSO and W.

Fig.5.The calculated via Thermo-Calc freezing ranges of the Mg-(1÷4 wt%Zn)-(1÷4 wt% Y)-0.6 wt% Zr alloys.

It is known that the higher volume fraction of eutectic phases increases the refillin ability during the later stage of solidificatio and that way improves the alloy hot tearing resistance.Also,a lower freezing range and high volume fraction of eutectic phases can promote increasing of fluidit.Therefore,the alloys with high volume fractions of LPSO and W phases can ensure high hot tearing resistance and fluidity these alloys will be considered further.

Fig.4.The calculated via Thermo-Calc weight fraction of the eutectic in the Mg-(1÷4 wt% Zn)-(1÷4 wt% Y)-0.6 wt% Zr alloys.

3.3.Mg-Zn-Y-Zr alloys microstructures

The as-cast microstructures of the Mg-Zn-Y-Zr alloys obtained using graphite mold casting are shown in Fig.6.In the microstructure of the MgZn3Y4Zr0.7 alloy(Fig.6a),theα-Mg grains are surrounded by the LPSO and W eutectic phases.The quantity of the LPSO and W phases are 10.1 and 1.0 vol%,respectively.The microstructure with the LPSO phase but with a lower amount is also observed for the MgZn2Y2 alloy.The microstructure of the MgZn4Y3Zr0.7 alloy(Fig.6b)is different since only theα-Mg+W eutectic was observed in the alloy microstructure,and the volume fraction of the eutectic phase was 4.4 vol%.In the other alloys with lower Zn and Y contents,theα-Mg+W eutectic with a lower amount of the W phase is observed.In the grain centers,theα-Zr particles are observed,which act as nuclei forα-Mg during alloy solidification despite both having the same type of crystal structure and nearly identical lattice parameter[16].An increase in the Zr content in the investigated alloys promotes a reduction in grain size.However,a meaningful reduction of grain size is observed after the addition of 0.3 wt% Zr.Further addition of Zr has minimal effect,and for alloys with 0.3 and 0.7 wt% Zr,the difference in the grain size is negligible.Overall,for some of the examined alloys,microstructures with large amounts of eutectic phases,up to 11 vol%,and fin grain structures are observed.

The thermal conductivity of the alloy depends on the elemental concentration inα-Mg.As a result,it is important to determine how the elements are distributed across theα-Mg grain.Using EDS analysis,the MgZn4Y3Zr0.3 alloy grain center and boundary composition are investigated.The Zn,Y,and Zr concentrations at the grain center are～1.3,0.8,and 0.5 wt% respectively.On the other hand,the Zn,Y,and Zr contents at the grain boundary are～2,1.5,and 0 wt%,respectively.This means that the partition coefficien of Zn and Y inα-Mg is<1,but that of Zr inα-Mg is>1.The same situation is observed for other alloys.Hence,Zn and Y promote a reduction in the thermal conductivity of the grain boundary area,and Zr decreases the thermal conductivity at the grain center.

3.4.Comparison of Mg-Zn-Y-Zr alloys and AZ91 alloy fluidit and hot tearing resistance

The most popular and widespread casting magnesium alloy is AZ91(Mg-Al-Zn)due to good casting characteristics[31].Hence,the castability of the investigated alloys can be compared to this well-known magnesium alloy.The fluidit of Mg-Zn-Y-Zr and AZ91 alloys is shown in Fig.7a.The lengths of the probes poured into the mold(channel diameter,D=8mm)are in the 330-380mm range except for that of the MgZn4Y3Zr0.7 alloy,which has a length of 290mm and large error bar.Similar results are observed for the probe withD=10mm.The probe lengths are in the 470-540mm range for the Mg-Zn-Y-Zr alloys but are slightly greater for the AZ91 alloy(570mm).Thus,we can conclude that the fluiditie of the Mg-Zn-Y-Zr and AZ91 alloys are almost the same.The reason for the similar fluidit observed in the investigated alloys is,in short,freezing range and a considerable amount of the eutectic.

Fig.6.As-cast microstructures of Mg-Zn-Y-Zr alloys:(a)MgZn3Y4Zr0.7 and(b)MgZn4Y3Zr0.7.

Fig.7.(a)Fluidity and(b)hot tearing criterion of the Mg-Zn-Y-Zr and AZ91 alloys.

The hot tearing criterion of the Mg-Zn-Y-Zr and AZ91 alloys,i.e.,the maximum length of the dog-bone section without tears,is shown in Fig.7b.The hot tearing test results indicate that the lowest hot tearing criterion and high hot tearing susceptibility is observed for the ternary Mg-Zn-Y alloys without Zr.A maximum hot tearing criterion is observed for the MgZn3Y4Zr0.7 alloy with a dog-bone length of 80mm(maximum used for the probe).These results are expected since the hot tearing tendency is strongly dependent on the alloy grain size[32].The grain size of the Zr-free alloys is the maximum that leads to high hot tearing susceptibility,but for Mg-Zn-Y-Zr alloys with small grain size,the hot tearing susceptibility is almost the same and low.Overall,the hot tearing susceptibilities of the Mg-Zn-Y-Zr alloys are almost the same as that of the AZ91 alloy.

3.5.Thermal conductivity of Mg-Zn-Y-Zr alloys

The thermal conductivities(λ′)of the Mg-Zn-Y-Zr alloys calculated using Eq.(3)are shown in Fig.8.The thermal conductivities of the MgZn4Y3 alloy at varying Zr contents of 0,0.3,0.5,and 0.7 wt% are 107,101,96,and 93W/mK,respectively.This means that the Zr content has a dramatic influenc on thermal conductivity,which is in agreement with the data presented in Section 3.1 and Fig.3.Ternary Mg-Zn-Y alloys have high thermal conductivities but are not appropriate for casting due to high hot tearing tendencies.Hence,the developed alloy can contain Zr,but at only 0.3 wt%.In this case,the thermal conductivity is still higher than 100W/mK and good castability is provided.The lowest thermal conductivity is observed for the MgZn3Y4Zr0.7 alloy due to the high Y and Zr content inα-Mg and its detrimental effect.This means that alloys with a high Y content and LPSO phase in the structure do not maintain high thermal conductivity.

Fig.8.Thermal conductivity(λ′)of the Mg-Zn-Y-Zr alloys calculated using Eq.(3).

3.6.Mechanical properties of Mg-Zn-Y-Zr alloys

The as-cast tensile properties of the Mg-Zn-Y-Zr alloys are shown in Fig.9.The highest yield strengths(YS)are observed for the MgZn2Y2Zr1,MgZn4Y3Zr0.5,and MgZn4Y3Zr0.7 alloys that are>135MPa.The YS is lowest for the MgZn4Y3 and MgZn2Y2 ternary alloys without Zr.All other alloys exhibit YSs in the 120-130MPa range.It is well-known that grain size significantl affects the mechanical properties of Mg alloys because of their high Hall-Petch strengthening coefficien[33].Therefore,the increase in Zr content promotes grain refinemen and enhanced YS.The ultimate tensile strengths(UTS)of the MgZn4Y3 alloys with Zr contents of 0,0.3,0.5,and 0.7 wt% are 203,208,242,and 224MPa,respectively.Zr only promotes an increase in the UTS until it reaches a composition of 0.5 wt%.At 0.7 wt% Zr,the UTS undergoes a slight decrease,possibly due to the formation of a large number of primaryα-Zr crystals at both the grain centers and the grain boundaries.A maximum elongation(El)of 8.8 and 10.3% is observed for the MgZn3Y4Zr0.7 and MgZn4Y3Zr0.5 alloys,respectively.As shown previously,the increasing of Zr content in the MgZn3Y4 alloy decreases its thermal conductivity,and only the MgZn4Y3Zr0.3 alloy is promising as a high thermal conductivity alloy.

Fig.9.Mechanical properties of Mg-Zn-Y-Zr alloys in as-cast condition.

3.7.Influenc of heat treatment on the microstructure,thermal conductivity,and mechanical properties of the MgZn4Y3Zr0.3 alloy

Based on the previously obtained properties,the MgZn4Y3Zr0.3 alloy was selected for further investigation.This alloy exhibits intermediate strength and low elongation,but a thermal conductivity>100W/mK.Also,its casting properties(fluidit and hot tearing susceptibility)are almost the same as those of the AZ91 magnesium alloy.Heat treatment can effectively improve the thermal conductivity of the alloys[34-36].Hence,heat treatment may improve the mechanical properties of the MgZn4Y3Zr0.3 alloy as well as its thermal conductivity.

Fig.10.Microstructures of the MgZn4Y3Zr0.3 alloy:(a)as-cast;(b)20h aging at 200°C;(c)20h aging at 300°C;(d)SSHT 8h at 520°C;(e)SSHT 8h at 520°C and 20h aging at 200°C;(f)SSHT 8h at 520°C and 20h aging at 300°C.

The solidus temperature of the alloy obtained using the Scheil-Gulliver model solidificatio calculation in Thermo-Calc was 577°C.Consequently,the SSHT temperature of 520°C was selected.Also,aging at 200,250,and 300°C,after casting or SSHT,was conducted.Fig.10 shows the microstructure of the MgZn4Y3Zr0.3 alloy in the as-cast condition,and after different heat treatment schedules.The microstructure after aging at 200 and 250°C for 20h has no visible changes in comparison with the as-cast alloy microstructure.However,aging at 300°C for 20h promotes the formation of precipitates,mostly at the grain boundaries.It was demonstrated previously that in the as-cast MgZn4Y3Zr0.3 alloy at the grain boundaries,the Zn and Y contents are higher;hence,the precipitates must be rich in these components.In the Mg-Zn-Y-Zr alloys,the LPSO,W,β1′,andβ2′precipitates were found previously[20,37,38].The microstructure of the alloys after SSHT at 520°C for 8h changed significantl(Fig.10d).The majority of the eutectic W phase had dissolved inα-Mg.The remaining eutectic phases were spheroid and a small amount of the Zr-rich precipitate was observed at the grain centers.Aging at 200°C for 20h after SSHT promoted an increased amount of Zr-rich precipitates.After SSHT(520°C,8h)and aging(300°C,20h),the microstructures of the alloys contained both the Zr-rich and Zn-/Y-rich precipitates,which were located in theα-Mg grain centers and throughout theα-Mg grain respectively.The observed significan changes in microstructure after heat treatment must have provided the changes in MgZn4Y3Zr0.3 alloy thermal conductivity and mechanical properties.

The hardness and electrical conductivity of the MgZn4Y3Zr0.3 alloy were measured in the as-cast condition,after SSHT and aging.Based on the results of the electrical conductivity measurements,the thermal conductivities of the alloys were calculated using Eq.(3).The as-cast samples were aged at 200,250,and 300°C.The hardness and thermal conductivity of the MgZn4Y3Zr0.3 alloy are shown in Fig.11a.The aging of as-cast samples at 200 and 250°C had minor effects on the hardness and thermal conductivity and its values were in the 70-74 HB and 103-107W/mK range,respectively.As shown previously,when aging was conducted at 200 or 250°C,no changes in the alloy microstructure were observed by SEM.When the alloy was aged at 300°C,the hardness decreased from 71 to 62 HB and the thermal conductivity increased from 103 to 116W/mK.The alloy hardness is obviously in connection with alloy strength.Hence,when aging was conducted at 300°C,overaging occurred and alloy strength decreased.These results were in agreement with the observed alloy microstructure because the Zn-/Y-rich precipitates were formed in the alloy at 300°C aging.Due to the formation of precipitates,theα-Mg solid solution is depleted with Zn and Y,and because of that,the alloy thermal conductivity is increased.

The as-cast samples were subjected to SSHT(520°C,8h)and aging(200 or 300°C,20h).The hardness and thermal conductivity of the MgZn4Y3Zr0.3 alloy are shown in Fig.11b.As expected,SSHT decreased the thermal conductivity from 103 to 93W/mK due to the dissolution of W eutectic phase and increasing of Zn and Y content inα-Mg.The slight increase in the hardness from 71 to 74 HB by solution strengthening was also observed after SSHT.Further aging at 200°C promoted an increase in the hardness to 78 HB and led to improved mechanical properties as a result of precipitation strengthening.The thermal conductivity was increased to 97W/mK during aging but remained lower than that of the as-cast condition(103W/mK).The minor change in electrical conductivity and hardness after aging showed that the amount of precipitates that formed during aging was very low.Following SSHT,aging at 300°C significantl altered the hardness and thermal conductivity.After the alloy was aged for 5h,the thermal conductivity and hardness were 116W/mK and 67 HB,respectively,as a result of overaging as arranged earlier.Further aging promoted a slight increase in the thermal conductivity and a decrease in the hardness.

Fig.11.Hardness and thermal conductivity of the MgZn4Y3Zr0.3 alloy:(a)as-cast samples aged at 200,250,and 300°C;(b)as-cast samples after SHHT at 520°C for 8h and aging at 200 or 300°C.

Fig.12 shows the results of the tensile test performed for the MgZn4Y3Zr0.3 alloy after different heat treatments.Aging at 300°C for 5h had no impact on the mechanical properties.The YS,UTS,and El were almost 120,200MPa,and 4%,respectively.Hence,this heat treatment schedule can only be recommended to improve the thermal conductivity of the alloy in the conditions where heat treatment at high temperatures are impossible,such as when casting is produced by high pressure die casting technique.However,the SSHT+aging heat treatment resulted in a slight decrease in the YS(110MPa)and an increase in El to 9%.The reason for a great increase in elongation is in the near-full dissolution and spheroidization of the weak bonded with the Mg matrix W phase[20,21].Both heat treatment schedules(SSHT+aging and aging)promoted the same thermal conductivity of the alloys.

Fig.12.Tensile properties of the MgZn4Y3Zr0.3 alloy in the as-cast condition,after aging(300°C,5h),after SSHT(520°C,8h)and aging(300°C,5h).

The calculation of alloy thermal conductivity using the results of electrical conductivity measurements is an express and not accurate method.Thus,the thermal conductivity of the MgZn4Y3Zr0.3 alloy was calculated based on the results of density,heat capacity,and thermal diffusivity measurements using Eq.(1).The thermal properties of the aged MgZn4Y3Zr0.3 alloy(300°C,5h)are shown in Fig.13.With increasing temperature,the density(ρ)of the alloy decreased,but the heat capacity(Cp)and thermal diffusivity(a)increased.The thermal conductivity increased from 105W/mK at room temperature to 120W/mK at 250°C.The phenomenon of a magnesium alloy’s thermal conductivity increasing with temperature was observed previously,and the reason for the increase in thermal diffusivity(and thus thermal conductivity)is the increased average velocities of electrons with temperature,which became dominant at high temperature[39,40].The thermal conductivity of pure Mg at 27°C is 156W/mK and the thermal conductivity of the developed MgZn4Y3Zr0.3 alloy is 67% that of pure magnesium[30].

Fig.13.Properties of the MgZn4Y3Zr0.3 alloy aged at 300°C for 5h.Cp is the heat capacity,ρis the density,a is the thermal diffusivity,and λ is the thermal conductivity.

4.Conclusions

(1)The analysis of the thermal conductivity of binary alloys shows that the alloying elements in magnesium can be arranged in order of decreasing thermal conductivity as follows:Zn,Al,Y,Mn,Ca,Zr,Gd,and Nd.

(2)The phase compositions and freezing ranges of Mg-(1÷4 wt% Zn)-(1÷4 wt% Y)-0.6 wt% Zr magnesium alloys by the Scheil-Gulliver solidificatio model were calculated using Thermo-Calc software.The most promising alloys,with Y and Zn in approximately equal compositions,exhibited a short freezing range(50-60°C).After solidification the LPSO(Mg21Zn2Y2)or W(MgYZn2)eutectic phases,or both,are formed.

(3)The alloy with a composition of Mg-4 wt% Zn-3 wt%Y-0.3 wt% Zr was selected based on its favorable thermal conductivity,castability,and mechanical properties.This alloy exhibits a similar fluidit and hot tearing tendency to the commercial AZ91 magnesium alloy.

(4)The aging conducted at 300°C for 5h promoted the precipitation that resulted in an increased thermal conductivity and decreased hardness in the developed alloy due to overaging.When the solution heat treatment at 520°C for 8h was conducted prior to aging,the thermal conductivity remained the same.

(5)The results of the tensile test for the chosen alloy obtained in as-cast condition were as follows:YS of 120MPa,UTS of 210MPa,and El of 4%.Aging the alloy at 300°C for 5h does not affect the mechanical properties.However,SSHT at 520°C for 8h and further aging at 300°C for 5h resulted in a decrease in the YS to 110MPa and an increase in the El to 9%.

(6)The thermal conductivity of the chosen alloy at room temperature was 105W/mK after aging at 300°C for 5h,which is 67% of the thermal conductivity of pure magnesium.

(7)In summary,the Mg-4 wt% Zn-3 wt% Y-0.3 wt% Zr alloy is the candidate for use as high thermal conductivity casting alloy for industrial applications,due to a good combination of thermal conductivity,castability,and mechanical properties.

Declaration of Competing Interest

None

Funding

This research did not receive any specifi grant from funding agencies in the public,commercial,or not-for-profi sectors.