Effect of Rare Earth Elements on Mechanical Properties of Magnesium Alloys

Magnesium and its alloys, as the lightest metal structural materials at present, have the advantages of low density, high specific strength and stiffness, high damping, good thermal conductivity, excellent machinability, stable part size, and easy recovery.

Effect of Rare Earth Elements on Mechanical Properties of Magnesium Alloys 1

They are widely used in aviation, aerospace, automobile industry, transportation, electronics, communications, computers and other industries.

Due to the low mechanical properties and poor corrosion resistance of magnesium alloys, the wide application of magnesium alloys in production and life is limited.

When a small amount of rare earth is added, various properties of magnesium alloys can be greatly improved.

Rare earth elements are located in Group IIB of the periodic table.

The outermost electronic structure of the atom is the same, with two electrons.

The electronic structure of the second outer layer is similar, and the number of electrons on the 4f orbital of the penultimate layer is different from 0 to 14;

The chemical properties are not much different, and the chemical properties are very active.

Both magnesium alloys and rare earth elements have a close packed hexagonal crystal structure, so rare earth elements have a large solid solubility in magnesium alloys.

Except for Sc, the other 16 elements in the rare earth elements can form eutectic phase with mg. The solid solubility of most rare earth elements in Mg is very large.

Table 1 lists the maximum solid solubility of rare earth elements in magnesium and the compound phase coexisting with magnesium based solid solution.

Table 1 maximum solid solubility of rare earth elements in magnesium and compound phases coexisting with magnesium based solid solution

Rare earth element

(RE)

Atomic coefficient

Eutectic temperature / K

Maximum solid solubility (mass fraction) /%

Maximum solid solubility (atomic fraction) /%

Phase with compounds produced by Mg

Se

Y

La

Ce

Pr

Nd

Pm

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu

21

39

57

58

59

60

61

62

63

64

65

66

67

68

69

70

71

838

886

863

848

821

823

815

844

821

532

834

838

857

865

782

889

25.9

12.4

0.79

1.6

1.7

3.6

2.9

5.8

(≈0)

23.5

24

25.8

28.0

32.7

31.8

3.3

41.0

15.9

3.35

0.14

0.28

0.31

0.63

0.5

0.99

(≈0)

4.53

1.57

4.83

5.44

6.56

6.26

0.48

8.80

MgSc

Mg24 Y5

 Mg12La

Mg12 Ce

Mg12Pr

Mg12Nd

Mg41 Sm5

Mg17Eu2

Mg5Gd

Mg24Tb5

Mg24Dy5

Mg24Ho5

Mg24Er5 

Mg24Tm5

Mg2Yb

Mg24Lu

Effect of rare earth elements on Purification and grain refinement of Mg alloy

The chemical properties of magnesium element are active, and it is easy to react with O2 and H2O to form MgO, which makes the magnesium alloy contain oxide inclusions and reduces the quality and service performance of the magnesium alloy.

Oxide inclusions generally exist in the matrix or grain boundary of magnesium alloy castings, which lead to fatigue cracks and reduce mechanical properties and corrosion resistance.

The addition of rare earth elements can not only reduce the number of inclusions, but also refine the grain and improve the properties of the alloy.

When the rare earth element Ce is added to the AM50 magnesium alloy, Ce plays a role in purifying the alloy and reducing impurities such as Fe and Ni.

The addition of Y can reduce the grain size of extruded mg Zn Zr alloy.

The grain size decreases from 14.2μm without Y to 3.2μm (mass fraction), with a decrease of 77%.

Effect of rare earth elements on mechanical properties of Mg alloy

1. Mg Al RE system

Mg Al series magnesium alloys are the most abundant and widely used magnesium alloy series at present.

The rare earth elements added to mg Al series magnesium alloys mainly include Ce, Y, Nd, etc.

The Mg Al based alloys without rare earth mainly have α-Mg dendrites and intermetallic compound β-Mg17Al12 phase distributed among the dendrites;

However, when rare earth elements are added to Mg-3% Al based alloy, the α-Mg dendrite becomes finer and the intermetallic compound β-Mg17Al12 phase is replaced by Al11RE3 and Al2RE.

Al11RE3 and phase are basically stable at 200 ℃, and when the temperature continues to rise, Al11RE3 phase will change to Al2RE phase.

This also shows that the stability of Al11RE3 is conditional.

After the addition of rare earth elements, the strength of the alloy increases at room temperature or 200 ℃, and the elongation keeps at a high level.

The strength improvement after adding rare earth elements may be related to the following factors:

First, the formation of a large amount of intermetallic compound Al11RE3 plays a great role in strengthening the dendrite boundary;

Secondly, the addition of rare earth elements refined the dendrite arm and promoted the strength;

Finally, adding rare earth elements, especially Y, can improve the strength of Mg matrix through solid solution strengthening.

We introduce the effect of rare earth elements on the mechanical properties of Mg Al alloys through Mg Al Zn, Mg Al Mn and Mg Al Sn series.

Table 2 lists the States and mechanical properties of some typical Mg Al alloys added with rare earth.

Table 2 mechanical properties of Mg Al RE alloy

Alloy

 

State

Yield strength / MPa

Tensile strength / MPa

Elongation /%

Mg-3.0Al-1.8Ce-0.3Y-0.2Mn

Mg-3.0Al-2.2La-0.3Y-0.2Mn

Mg-9Al-Zn-2Y

Mg-12.55Al-3.33Zn-0.58Ca-1Nd

Mg-5A1-0.3Mn-1.5Ce

Mg-6Al-0.3Mn-0.9Y

Mg-4Al-2Sn-1Ca-1.0Ce

Mg-4Al-2Sn-0.5Y-0.4Nd

as cast condition

as cast condition

Extruded state

Extruded state

Rolling state

Rolling state

as cast condition

as cast condition

 

58

164

216.9

384

225

303

95

70

55

248

323.15

481

318

255

194

225

10

8

14.31

5

9

17.1

11.4

23.2

1. Mg Al Zn RE system

At present, the most commonly used mg Al magnesium alloys in industry are Mg Al Zn series, among which AZ91 cast magnesium alloy, AZ31 and AZ61 wrought magnesium alloy have better performance.

AZ91 magnesium alloy has good formability and is widely used in the die casting industry.

It can produce workpieces with complex structure by die casting;

AZ31 magnesium alloy and AZ61 magnesium alloy have strong deformation ability and are used to produce various magnesium alloy forgings and extrusions.

Rare earth element Y has great influence on the properties of AZ91 alloy.

The as cast AZ91 alloy without y is mainly continuous eutectic phase Mg17Al12.

When Y is added, the precipitates change greatly:

When the content of Y is 0.3% (mass fraction), no precipitate of Y is found in the alloy;

When the content of Y is between O 0.6% ~ 0.9% (mass fraction), a new Al2Y phase is formed, and the growth morphology of Mg17Al12 phase is changed;

When the addition amount of Y is further increased to 1.2% (mass fraction), the Al2Y phase is coarser, and the Mg17Al12 phase is transformed into a cotton like structure.

Fig. 1 shows the effect of Y addition on the strength of AZ91 alloy.

It can be seen from Fig. 1 that the strength of AZ91-Y alloy with Y addition is higher than that of AZ91 alloy without Y addition, whether at room temperature or at an effective temperature of 200 ℃.

Both yield strength and tensile strength increase with the increase of Y content;

When the content of Y is between 0.6% and 0.9%, the strength value reaches the maximum;

But when the content of Y exceeds 0.9%, the intensity tends to weaken.

The reasons for the strength enhancement may be:

The stress is effectively transferred from the relatively soft magnesium alloy matrix to the strengthening phase Al2Y phase, which increases the strength;

The stable Al2Y phase becomes an obstacle to the dislocation slip, which makes more dislocations gather near the Al2Y phase and enhances the dislocation strengthening.

Similarly, for AZ91D alloy extruded at 300 ° C and AZ91D + Y alloy added with rare earth element, rare earth element Y can improve the strength of the alloy.

When the content of Y is 2% (mass fraction), the mechanical properties of the alloy are the best.

Compared with AZ91 alloy, Mg-12.55Al-3.33Zn-0.58Ca-1Nd alloy has better tensile strength, and its tensile strength can reach 481MPa, but the elongation is low, only 5%.

Effect of Rare Earth Elements on Mechanical Properties of Magnesium Alloys 2

Fig. 1 Effect of Y addition on AZ91 alloy strength

When studying the effect of Gd on the properties of Mg-2Al-1Zn, it was found that Mg-2Al-1Zn-4Gd alloy had the highest yield strength and the lowest elongation at room temperature.

Mg-2Al-1Zn-4Gd alloy also showed the best yield strength and tensile strength at 200 ℃.

This indicates that mg-2Al-1Zn-4Gd alloy has good thermal stability.

On the whole, with the increase of temperature, the tensile property of the alloy weakens and the ductility improves.

2. Mg-Al-Mn-RE series

Mg-Al-Mn magnesium alloys mainly include AM60A, AM60B, AM50A and AM20 series.

Mg Al Mn magnesium alloys have low strength at room temperature, but their brittleness is low and their deformation ability is strong.

They are generally used to manufacture automobile wheels, steering wheels, seat frames and other important parts.

In order to improve its strength, rare earth elements such as Ce and Y can be added.

The mechanical properties of Mg-5Al-0.3Mn alloy are greatly affected by Ce.

The mechanical properties of Mg-5Al-0.3Mn alloy without Ce are very poor, and its tensile strength, yield strength and elongation are 158, 64MPa and 8%, respectively.

With the increase of Ce content, the tensile properties of the alloy are improved.

When the content of Ce is 1.5%, the tensile property of the alloy is the best.

Compared with the Mg-5Al-0.3Mn alloy without Ce, the increase of tensile strength, yield strength and elongation is 28.5%, 37.5% and 150%, respectively.

However, when the amount of Ce is further increased, the tensile properties of the alloy begin to weaken again.

When Ce is added to Mg-5Al-0.3Mn alloy, Al11Ce3 will be formed along the grain boundary, and Al11Ce3 phase can effectively hinder dislocation movement and grain boundary sliding;

In addition, with the addition of Ce, the morphology of β-Mg17Al12 phase is refined into granular shape and the volume fraction decreases, which are important reasons for the improvement of mechanical properties of Mg-5Al-0.3Mn-1.5Ce alloy.

However, when the amount of Ce added is large, the mechanical properties are weakened, because a large amount of Al11Ce3 phase with cluster structure is formed.

This cluster structure divides the α-Mg matrix into many small regions.

Therefore, cracks easily occur between the Al11Ce3 phase and the α-Mg matrix interface.

Therefore, it can be concluded that the morphology and content of Al11Ce3 phase have great influence on improving the mechanical properties of Mg-5Al-0.3Mn alloy.

The improvement of mechanical properties by simply adding rare earth elements is limited, and subsequent processing is an effective way to improve the strength.

After hot rolling the Mg-5Al-0.3Mn-1.5Ce alloy with the best mechanical properties, the tensile strength and yield strength of the alloy after hot rolling are increased by 318 and 225MPa (57% and 156% respectively), but the elongation is reduced to 9%.

The increase of tensile strength and yield strength is due to the dynamic recrystallization process during hot rolling, which makes the grain size significantly reduced;

The long needle like Al11Ce3 phase will break into many small parts during hot rolling to slow down the cutting effect;

And the strength of the alloy can be significantly improved by the interaction of the fractured Al11Ce3 with the dislocation and the pinning effect during the deformation.

It is found that rare earth element Y can also improve the tensile strength and microhardness of Mg-5Al-0.3Mn-xY (x = 0,0.3%, 0.6%, 0.9% (mass fraction)).

When the content of Y increases from 0 to 0.9% (mass fraction), the tensile strength, yield strength and elongation of as cast alloy increase from 179,56MPa and 11.8% to 192,62MPa and 12.6%, respectively;

The tensile strength, yield strength and elongation of the as rolled alloy increased from 293221MPa and 10.3% to 303255 MPa and 17.1% respectively.

The microhardness and tensile properties of the alloy are improved because al2y with high melting point (1758K) is the main precipitate of the alloy, compared with β- Mg17Al12 phase and Al2Y phase have higher thermal stability at high temperature.

In the hot rolling process, the Al2Y phase can effectively block the dislocation movement and grain boundary slip during the heating process;

During deformation, the dislocation density in the alloy increases due to dislocation proliferation and the formation of new dislocations.

With the increase of dislocation density, it becomes more obvious that other dislocations hinder dislocation movement.

Therefore, the applied pressure needs to increase according to the increase of the degree of metal deformation;

And the addition of Y and hot rolling refine the grain, so the mechanical properties, especially the yield strength, are improved.

3. Mg Al Sn RE series

It is very useful to add Sn to the magnesium alloy and combine it with a small amount of aluminum.

Sn can not only improve the ductility of magnesium alloy, but also reduce the cracking tendency during hot working, which is very beneficial to hammer forging.

The rare earth elements added to the Mg Al Sn magnesium alloy generally include Ce, Y, Nd and so on.

Ce can improve the tensile strength and elongation of Mg-4Al-2Sn-1Ca alloy at room temperature.

This may be due to the refinement of the camgsn phase in the alloy and the reduction of the grain size of the Ce containing alloy.

At room temperature, when the content of Ce is 1% (mass fraction), the alloy has the best mechanical properties, and its tensile strength, yield strength and elongation can reach 194.95MPa and 11.4%, respectively.

Fig. 2 shows the mechanical properties of Mg-4Al-2Sn, Mg-4Al-2Sn-0.9Y, Mg-4Al-2Sn-0.9Nd, Mg-4Al-2Sn-0.5Y-0.4Nd as cast alloys respectively.

It can be seen that the relative content of Y and nd also affects the mechanical properties of Mg-4Al-2sn-xY-yNd (x + y = 0.9% (mass fraction)).

It can be seen from Fig. 2 that the yield strength of all alloys is about 70MPa.

When the content of Y is 0.5% (mass fraction) and the content of Nd is 0.4% (mass fraction), the mechanical properties of the alloy are the best, and its yield strength, tensile strength and elongation are 70225MPa and 23.2% respectively.

Effect of Rare Earth Elements on Mechanical Properties of Magnesium Alloys 3

Fig. 2 mechanical properties of Mg-4Al-2Sn, Mg-4Al-2Sn-0.9Y, Mg-4Al-2Sn-0.9Nd and Mg-4Al-2Sn-0.5Y-0.4Nd alloys

2. Mg-Zn-REseries

Mg Zn alloys are widely used in wrought magnesium alloys and have good aging strengthening ability.

There are many kinds of rare earth elements added to mg Zn alloys, such as Y, Er, Gd, Nd, Ce, etc.

After adding rare earth elements, the mechanical properties of the alloy are improved, because the rare earth elements can refine the grain and form a strengthening phase in the alloy to improve the strength of the alloy.

Table 3 lists the state and mechanical properties of some typical mg Zn alloys added with rare earth.

Table 3 Mechanical Properties of Mg Zn RE system

Alloy

 

State

Yield strength / MPa

Tensile strength / MPa

Elongation /%

Mg-3.8Zn-2.2Ca-1.0Ce

Mg-3.8Zn-2.2Ca-1.0Gd

Mg-5.0Zn-0.9Y-0.16Zr

Mg-6.0Zn-1.0Mn-0.5Ce

Mg-2Zn-0.46Y-0.5Nd

Mg-4.3Zn-0.7Y

Mg-12Zn-1.5Er

Mg-3.5Zn-0.6Gd

as cast condition

as cast condition

Extruded state

Extruded state

Extruded state

Extruded state

Extruded state

Extruded state

 

119.2

114.2317

232

165.6

318

219

146.1

130.6

363

304

269

347

359

308

3.5

2.9

12

14.7

24

22

16.4

Adding Ce and Gd to as cast Mg-3.8Zn-2.2Ca alloy, the tensile strength of the alloy increased from 123.8MPa to 146.1 and 130.6MPa, and the elongation increased from 2.4% to 3.5% and 2.9%, respectively.

Adding rare earth elements to as cast alloys can not meet the strength requirements of alloys. More and more researchers began to study the effects of deformation and adding rare earth elements on the properties of alloys.

A comparative study of as cast and as extruded Mg-5.0Zn-0.9Y-0.16Zr alloy shows that the mechanical properties of the alloy after extrusion are greatly improved, and the tensile strength, yield strength and elongation are increased from 168105MPa and 1.8% to 363317MPa and 12%, respectively.

The improvement of mechanical properties is attributed to the effect of grain refinement after extrusion.

The mechanical properties of the extruded Mg-6Zn-1Mn-0.5Ce alloy were also improved, the yield strength was increased from 209MPa to 232MPa, the tensile strength was basically unchanged, and the elongation was increased from 11.5% to 14.7%.

Compared with the as cast Mg-12Zn-1.5Er alloy, the mechanical properties of the extruded alloy are significantly improved, as shown in Fig. 3.

The yield strength and tensile strength of the extruded alloy can reach 318mpa and 359mpa respectively.

In the stress-strain curve of the typical extruded mg-3.5Zn-0.6Gd alloy, it can be seen that the alloy has better strength and plasticity, that is, the tensile strength is 308mpa, the yield strength is 219MPa, and the elongation is 16.4%.

Effect of Rare Earth Elements on Mechanical Properties of Magnesium Alloys 4

Fig. 3 stress strain curves of as cast and extruded Mg-12Zn-1.5Er alloy at room temperature

In the process of extrusion, the extrusion ratio and extrusion temperature also have effects on the properties of the alloy with rare earth elements.

Qing Chen et al. prepared Mg-5.3Zn-1.13Nd-0.51La-0.28Pr-0.79Zr alloy, and studied the effects of extrusion ratio and extrusion temperature on the properties of the alloy.

It is found that the tensile strength, yield strength and elongation of the alloy are related to the extrusion ratio.

This change can be divided into two steps.

When the extrusion ratio is from 0 to 9, the tensile strength, yield strength and elongation change significantly, and the tensile strength increases from 169MPa to 309MPa;

When the extrusion ratio is changed from 9 to 100, the improvement of tensile strength, yield strength and elongation is very weak.

The author continued to study the effects of different extrusion temperatures on the mechanical properties of the alloy.

The results showed that the tensile strength, yield strength and elongation of the alloy decreased with the increase of extrusion temperature.

When the extrusion temperature is 250 ℃ ~ 350 ℃, the change is not obvious, but when the extrusion temperature is changed from 350 ℃ to 400 ℃, the tensile strength, yield strength and elongation are reduced from 324278MPa and 12% to 267208MPa and 5% respectively, and the change is more significant than that in other stages.

3. Mg-Li-RE series

Mg Li alloys are the lightest series of magnesium alloys.

After the addition of rare earth elements, the mechanical properties of Mg Li alloys are improved by solid solution strengthening and the formation of fine dispersed intermetallic compounds.

There are many kinds of rare earth elements added to the Mg Li alloy, such as Y, Ce, Nd, etc.

Table 4 lists the state and mechanical properties of some typical mg Li alloys added with rare earth.

Table 4 mechanical properties of Mg Li RE system

Alloy

 

State

Yield strength / MPa

Tensile strength / MPa

Elongation /%

Mg-7Li-3Y

Mg-5Li-3A1-2Zn-1.5RE

Mg-8Li-3Al-2.0Nd

as cast condition

as cast condition

as cast condition

 

144

160

206.5

185.95

22

14.4

9.25

The addition of Y to Mg-7Li alloy will form Y-rich α-Mg phase and Mg24Y5 precipitates, and with the increase of Y content, the α-Mg phase appears obvious refinement.

In terms of comprehensive strength and elongation, Mg-7Li-3Y alloy has the best mechanical properties, that is, its tensile strength, yield strength and elongation are 160,144MPa and 22% respectively.

When the content of Y is more than 3% (mass fraction), the strength increases slightly, but the elongation decreases significantly.

The effect of Y on the mechanical properties of Mg-8Li – (1,3) Al alloy was studied.

It was found that the tensile strength of the rolled LAY831 alloy reached 230 MPa, and the elongation of the extruded LAY811 alloy reached 60%.

Under the plastic deformation condition, the formation of ALY intermediate phase and the reduction of α phase obviously improved the mechanical properties of the alloy.

The addition of rare earth elements in Mg-5Li-3Al-2Zn alloy leads to the formation of Al2RE or Al3RE phase and the reduction of alli phase.

With the addition of rare earth elements, the tensile strength of the alloy increases with the increase of the addition amount, but when the addition amount is more than 1.5% (mass fraction), the tensile strength becomes weak.

The change trend of elongation is the same as that of tensile strength.

When the addition amount is 1.5% (mass fraction), Mg-5Li-3Al-2Zn-1.5RE has the best tensile strength and elongation, which are 206.5MPa and 14.4%, respectively.

Nd can also improve the tensile strength and elongation of the alloy.

When the content of Nd is 2.0% (mass fraction), the tensile strength of Mg-8Li-3Al alloy reaches the peak of 185.95MPa, and when the content of Nd is 1.6% (mass fraction), the elongation reaches the peak of 16.3%.

The improvement of mechanical properties is attributed to the reduction of the size of phase α by adding Nd and the binding slip of the new phase Al2Nd distributed at the phase boundary.

4. Others

Other properties of alloys with rare earth elements are listed in Table 5.

For Mg-4Y-4Sm-0.5Zr alloy, the tensile strength and yield strength slightly decreased with the increase of extrusion temperature;

On the contrary, the tensile strength and yield strength increase with the increase of extrusion temperature after aging.

When the alloy is aged at 200 ℃ for 16h, the alloy extruded at 400 ℃ has the best mechanical properties, that is, the tensile strength reaches 400MPa, the yield strength exceeds 300MPa, and the elongation reaches 7%.

The yield strength, tensile strength and elongation of Mg-10Gd-2Y-0.5Zr alloy increased by 20%, 8.2% and 150% respectively after 14 cycles of extrusion compression.

Table 5 mechanical properties of other alloys

Alloy

 

State

Yield strength / MPa

Tensile strength / MPa

Elongation /%

Mg-4Y-4Sm-0.5Zr

Mg-10Gd-2Y-0.5Zr

Mg-3Sn-2Ca-Ce

Mg-3Sn-2Ca-Y

Extrusion + aging

Extrusion

as cast condition

as cast condition

>300

270

144

137

400

330

158

150

7

15

3.3

3.2

When Ce is added to Mg-3Sn-2Ca alloy, the mechanical properties of the alloy are greatly improved when the content of Ce is 1.5% (mass fraction) or more.

When the content of Ce is 2% (mass fraction), the increase of tensile strength, yield strength and elongation at room temperature is 24.4%, 28.6% and 73.7% respectively, and the increase at 150 ℃ is 22.4%, 28.8% and 56% respectively.

Rare earth element Y can also improve the strength of the alloy.

When the content is 1.5% (mass fraction), the mechanical properties of the alloy are the best, that is, the tensile strength, yield strength and elongation at room temperature are 150137MPa and 3.2%, respectively, with an increase of 18.1%, 22.3% and 68.4%, and the corresponding increase at 150 ℃ is 19.8%, 24% and 54.9%.

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