Effect of neodymium on mechanical behavior of Mg-Zn-Zr magnesium alloy

J O U R N A L O F M A T E R I A L S S C I E N C E L E T T E R S22,2003,445–447

Effect of neodymium on mechanical behavior of Mg-Zn-Zr magnesium alloy

WEI WU,YUE WANG,XIA ZENG,LIJIA CHEN,ZHENG LIU

School of Material Science and Engineering,Shenyang University of Technology,Shenyang110023, People’s Republic of China

Magnesium alloys are attractive for many structural ap-plications in automobile and aeroplane industries as a result of their high isotropic modulus and strength-to-weight ratio in both the cast and wrought forms. It has been documented that some alloying additions can in?uence the microstructure and performance of magnesium-based alloys.For example,the addition of Mn can lead to a signi?cant improvement in corrosion resistance[1].The magnesium alloy containing Zn can be grain re?ned effectively by Zr,which is rendered ineffectively by the presence of Al and Mn[2].In ad-dition,it has been well known that rare-earth additions such as Ce Nd,Pr to Mg can improve the creep perfor-mance[3,4].

In this paper,neodymium,as an additional alloy-ing element,was added to one commercial Mg-Zn-Zr alloy,namely ZK40magnesium-based alloy.The ten-sile properties of the Mg-Zn-Zr alloy with and with-out an addition of3%Nd at different temperatures is reported.The effect of neodymium on mechanical be-havior of the magnesium alloy was investigated.

The Mg-Zn-Zr alloys with and without an addition of3%Nd used in this investigation was produced by induction melting in magnesia crucibles under an argon atmosphere and cast into round bars.Pure neodymium power was added to the molten magnesium alloy im-mediately prior to pouring.The nominal chemical com-position of the unmodi?ed Mg-Zn-Zr alloy in weight percent is as follows:Zn4.0,Zr0.7,Si0.2,Ni0.01, Cu0.03,and the balance magnesium.The tensile spec-imens were machined from the as-cast bars,and had a gage length of25mm and a gage diameter of5mm.The displacement-controlled tensile tests were performed from room temperature to180?C and at a constant ini-tial strain rate of3×10?31/s in laboratory air.

Fig.1illustrates the0.2%offset yield strength at dif-ferent temperatures for the Mg-Zn-Zr alloys with and without Nd.It is obvious that for the alloy contain-ing Nd,there is almost no difference in yield strength at room temperature and120?C,while increasing tem-perature from120?C to180?C leads to a signi?cant de-crease in the yield strength.For the unmodi?ed ZK60 alloy,however,its yield strength shows a continuous decrease with increasing test temperature.Moreover,it can be seen that at all temperatures used in this investi-gation,an addition of3%Nd can signi?cantly enhance the yield strength of the alloy.Especially at the higher temperatures,this increase in yield strength will be-come more considerable.

The variations in the ultimate tensile strength as a function of test temperature are shown in Fig.2for the Mg-Zn-Zr alloys with and without Nd.The alloy con-taining3%Nd exhibits a different varying tendency in ultimate tensile strength with test temperature,com-pared with the unmodi?ed Mg-Zn-Zr alloy.Again,

the Figure1Yield strength as a function of temperature for the alloys with and without

Nd.

Figure2Ultimate tensile strength as a function of temperature for the alloys with and without Nd.

0261–8028C 2003Kluwer Academic Publishers445

Figure 3Fracture elongation versus temperature for the alloys with and without Nd.

ultimate tensile strength of the ZK60magnesium alloy shows a continuous decrease with temperature.In all of the tension tests performed at three temperatures,the alloy containing 3%Nd shows the highest ultimate tensile strength at 120?C.In addition,it can be noted that at room temperature,the ultimate tensile strength of the Mg-Zn-Zr alloy decrease as a result of the addi-tion of Nd.In contrast,at two higher temperatures of 120?C and 180?C,the alloy with an addition of 3%Nd can give the higher ultimate tensile strength than the alloy without Nd.The above-mentioned fact suggests that the addition of 3%Nd to the Mg-Zn-Zr alloy can effectively enhance its ultimate tensile strength at the higher temperatures.

The fracture elongation of the Mg-Zn-Zr alloys with and without the addition of Nd at different temperatures is presented in Fig.3.The variation in the elongation to failure is observed to be similar for the two alloys,that is,the elongation increases with an increase in test temperature.Unexpectedly,the alloy containing Nd shows lower elongation than the unmodi ?ed alloy.Grain size of the as-cast Mg-Zn-Zr alloys was deter-mined using optical metallography.It was found that the average grain size of the alloys with and without Nd were about 0.25mm and 0.55mm,

respectively.

Figure 4Fractographs of the tested specimens at room temperature:(a)unmodi ?ed ZK40alloy and (b)ZK40alloy containing 3%Nd.

It means that adding Nd of 3%can re ?ne the grains of the Mg-Zn-Zr alloy.The grain-re ?ned effect due to the addition of 3%Nd can partly contribute to the im-provement in the yield strength of the alloy containing https://www.360docs.net/doc/ed18484796.html,ing a standard Hall-Petch equation [5],the in-?uence of the grain size on the yield strength can be expressed as σGS =k /d 1/2,in which σGS is the in-crease in yield strength due to grain re ?nement,K is a constant and d is the grain size.It is obvious that the material with the ?ner grains will give the higher yield strength.It should be pointed out that the neodymium solid-dissolving in the α-Mg matrix phase also plays a role of strengthening,and can contribute to the higher yield strength shown by the Mg-Zn-Zr alloy containing Nd at room temperature.

It has been documented that in the rare earth con-taining magnesium alloys,some Mg-RE intermetallic compounds can precipitate during heat treatment [6].Thus,for the Nd containing Mg-Zn-Zr alloy used in this investigation,it may be expected that the Mg 12Nd particles can precipitate on grain boundaries and dislo-cations inside grains due to dynamic strain aging dur-ing deformation at the higher temperatures.Actually,it has been identi ?ed by X-ray diffraction analysis that the precipitates of Mg 12Nd is present in the specimens tested at 120?C and 180?C.It may be thought that those precipitates inside the grains can serve as barriers to dislocation motion to hinder the movement of existing dislocations,and subsequently,enhance the production of additional dislocations to accommodate the applied strain.The intergranular precipitation of Mg 12Nd can pin the grain boundaries and restrict the growth process of the grains.Thus,the high strength of the neodymium-containing alloy may be attributed to the dislocation-precipitate interaction and the ?ner grain size during deformation at the higher temperatures.

Fracture surfaces in the Mg-Zn-Zr alloys with and without an addition of 3%Nd tensile tested at different temperatures.Fig.4shows the fractographs of the spec-imens tested at room temperature for the two alloys.It can be seen from Fig.1that the fracture surfaces of the two alloys are characterized by brittle fracture.More-over,a small amount of dimples can be found on the fracture surface of the unmodi ?ed ZK40alloy,while some cleavage feature can be observed.It implies that the addition of Nd will increase the brittleness of the

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Figure5Fractograph of the ZK40alloy containing3%Nd at120?C. unmodi?ed alloy at room temperature.This observa-tion is consistent with fact that the Nd containing alloy exhibits the lower fracture elongation at room temper-ature.At120?C and above,however,only dimples can be seen on the fracture surfaces of the Nd containing alloy,as shown in Fig.5.The same failure feature is also observed on the fracture surfaces of the unmodi-?ed ZK40alloy tested at120?C and180?C.It means that the two alloys show the ductile fracture at the tem-peratures of120?C and above.

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