Unveiling the Effect of Rare Earth Elements on Mechanical Properties of Magnesium Alloys

At present, magnesium and its alloys are considered as the lightest metal structural materials. They offer several advantages such as low density, high specific strength and stiffness, high damping, good thermal conductivity, excellent machinability, stable part size, and easy recovery.

Magnesium alloys find extensive use in various industries such as aviation, aerospace, automobile, transportation, electronics, communications, and computers. However, their widespread application is limited due to their low mechanical properties and poor corrosion resistance.

The addition of rare earth elements in small quantities can significantly enhance the properties of magnesium alloys.

Rare earth elements occupy Group IIIB in the periodic table, with a similar electronic structure in the outermost layer consisting of two electrons. The electronic structure of the second outer layer is also similar, with the number of electrons in the 4f orbital of the penultimate layer ranging from 0 to 14. Although the chemical properties are relatively similar, rare earth elements are highly reactive.

Since both magnesium alloys and rare earth elements have a close-packed hexagonal crystal structure, rare earth elements have a high solid solubility in magnesium alloys. With the exception of Sc, all other 16 rare earth elements can form a eutectic phase with Mg, and the solid solubility of most rare earth elements in Mg is quite significant.

Table 1 presents the maximum solid solubility of rare earth elements in magnesium, along with the compound phases coexisting with the magnesium-based solid solution.

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

Magnesium is a highly reactive element, readily reacting with O2 and H2O to form MgO. This results in the presence of oxide inclusions in magnesium alloys, which can negatively impact their quality and performance.

Oxide inclusions typically exist in the matrix or grain boundaries of magnesium alloy castings, leading to fatigue cracks, reduced mechanical properties, and diminished corrosion resistance.

The addition of rare earth elements can significantly improve the properties of magnesium alloys by reducing the number of inclusions and refining the grain. When Ce is added to the AM50 magnesium alloy, it acts as a purifying agent, reducing impurities like Fe and Ni.

Similarly, Y can also reduce the grain size of extruded mg Zn Zr alloy, decreasing it from 14.2μm without Y to 3.2μm (mass fraction), representing a decrease of 77%.

Effect of rare earth elements on mechanical properties of Mg alloy

1. Mg Al RE system

The Mg-Al series of magnesium alloys is presently the most abundant and widely used series of magnesium alloys.

Rare earth elements added to Mg-Al series magnesium alloys mainly include Ce, Y, Nd, among others.

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

However, with the addition of rare earth elements to Mg-3% Al based alloys, the α-Mg dendrites become finer, and the intermetallic compound β-Mg17Al12 phase is replaced by Al11RE3 and Al2RE.

Al11RE3 and Al2RE phases are essentially stable at 200 ℃, and with a further increase in temperature, the Al11RE3 phase will transform to Al2RE.

This also implies that the stability of the Al11RE3 phase is conditional.

After adding rare earth elements, the strength of the alloy increases at room temperature and 200 ℃, while the elongation remains at a high level.

The improvement in strength due to the addition of rare earth elements can be attributed to the following factors:

Firstly, the formation of a significant amount of intermetallic compound Al11RE3 greatly strengthens the dendrite boundary.

Secondly, the addition of rare earth elements refines the dendrite arm and promotes strength.

Finally, the addition of rare earth elements, particularly Y, can enhance the strength of the Mg matrix through solid solution strengthening.

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

Table 2 presents the states and mechanical properties of some typical Mg-Al alloys that have been added with rare earth.

Table 2 mechanical properties of Mg Al RE alloy

1. Mg Al Zn RE system

Currently, the most widely used magnesium alloys in industry are the Mg-Al-Zn series, particularly the AZ91 cast magnesium alloy, and the AZ31 and AZ61 wrought magnesium alloys, which exhibit better performance.

The AZ91 magnesium alloy has excellent formability, making it suitable for use in the die casting industry. It can be used to produce workpieces with complex structures via die casting.

On the other hand, both the AZ31 and AZ61 magnesium alloys exhibit strong deformation abilities, making them ideal for producing various magnesium alloy forgings and extrusions.

The properties of the AZ91 alloy are greatly influenced by the rare earth element Y. Without Y, the as-cast AZ91 alloy consists primarily of a continuous eutectic phase of Mg17Al12.

The addition of Y causes significant changes in the precipitates:

When the mass fraction of Y is 0.3%, no Y precipitate is observed in the alloy.

When the mass fraction of Y is between 0.6% and 0.9%, a new Al2Y phase is formed, and it alters the growth morphology of the Mg17Al12 phase.

When the mass fraction of Y is further increased to 1.2%, the Al2Y phase becomes coarser and the Mg17Al12 phase transforms into a cotton-like structure.

Figure 1 illustrates the impact of Y addition on the strength of AZ91 alloy.

Figure 1 clearly shows that the strength of AZ91-Y alloy with Y addition is higher than that of AZ91 alloy without Y addition, both at room temperature and at an effective temperature of 200 ℃.

Both yield strength and tensile strength increase as the Y content increases.

The strength value reaches its maximum when the Y content is between 0.6% and 0.9%.

However, when the Y content exceeds 0.9%, the strength tends to weaken.

The reasons for the strength enhancement may be:

1. The stress is effectively transferred from the relatively soft magnesium alloy matrix to the strengthening Al2Y phase, which increases the overall strength.
2. The stable Al2Y phase becomes an obstacle to the dislocation slip, which makes more dislocations accumulate near the Al2Y phase and enhances dislocation strengthening.

Similarly, at a temperature of 300°C, the addition of rare earth element Y in AZ91D alloy and AZ91D+Y alloy can improve their strength.

The mechanical properties of AZ91D+Y alloy are at their best when the mass fraction of Y is 2%.

Compared to AZ91 alloy, the Mg-12.55Al-3.33Zn-0.58Ca-1Nd alloy has better tensile strength, reaching 481 MPa. However, its elongation is low, at only 5%.

Fig. 1 Effect of Y addition on AZ91 alloy strength

When investigating the effect of Gd on the properties of Mg-2Al-1Zn, it was observed that the Mg-2Al-1Zn-4Gd alloy exhibited the highest yield strength and the lowest elongation at room temperature. Furthermore, the Mg-2Al-1Zn-4Gd alloy displayed the highest yield strength and tensile strength at 200 ℃, indicating good thermal stability.

Overall, as the temperature increases, the tensile strength of the alloy weakens while its ductility improves.

2. Mg-Al-Mn-RE series

The Mg-Al-Mn magnesium alloys consist of the AM60A, AM60B, AM50A, and AM20 series.

Although these magnesium alloys have low strength at room temperature, they possess low brittleness and high deformation ability, making them suitable for producing crucial components like automobile wheels, steering wheels, and seat frames.

The addition of rare earth elements such as Ce and Y can enhance their strength.

Ce has a significant impact on the mechanical properties of Mg-5Al-0.3Mn alloy.

In the absence of Ce, the mechanical properties of Mg-5Al-0.3Mn alloy are extremely poor, with a tensile strength, yield strength, and elongation of 158, 64MPa, and 8%, respectively.

The tensile properties of the alloy improve with an increase in Ce content.

The best tensile property of the alloy is observed when Ce content is at 1.5%.

Compared to the Mg-5Al-0.3Mn alloy without Ce, the addition of Ce results in an increase in tensile strength, yield strength, and elongation by 28.5%, 37.5%, and 150%, respectively.

However, further increase in Ce content results in a weakening of the alloy’s tensile properties.

When Ce is added to the Mg-5Al-0.3Mn alloy, Al11Ce3 is formed along the grain boundary, effectively impeding dislocation movement and grain boundary sliding.

Moreover, the addition of Ce results in the refinement of the morphology of β-Mg17Al12 phase into a granular shape, and a decrease in volume fraction. These are the main reasons for the improvement in the mechanical properties of the Mg-5Al-0.3Mn-1.5Ce alloy.

However, a large amount of Al11Ce3 phase with a cluster structure is formed when the amount of added Ce is high, resulting in a weakening of the mechanical properties.

The cluster structure of this alloy divides the α-Mg matrix into numerous small regions, making it susceptible to cracks at the interface between the Al11Ce3 phase and the α-Mg matrix.

Hence, it can be concluded that the morphology and content of Al11Ce3 phase play a crucial role in enhancing the mechanical properties of the Mg-5Al-0.3Mn alloy.

However, the improvement in mechanical properties by merely adding rare earth elements is limited, and subsequent processing is an effective way to enhance the strength.

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

The increase in strength can be attributed to the dynamic recrystallization process during hot rolling, which significantly reduces the grain size, breaks down the long needle-like Al11Ce3 phase into smaller pieces, and slows down the cutting effect.

Moreover, the interaction between the fractured Al11Ce3 phase and dislocation, along with the pinning effect during deformation, significantly improves the strength of the alloy.

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

As the Y content increases from 0 to 0.9% (mass fraction), the tensile strength, yield strength, and elongation of the as-cast alloy increase from 179.56 MPa and 11.8% to 192.62 MPa and 12.6%, respectively. Similarly, the tensile strength, yield strength, and elongation of the as-rolled alloy increase from 293.221 MPa and 10.3% to 303.255 MPa and 17.1%, respectively.

The microhardness and tensile properties of the alloy are improved because the high-melting-point Al2Y precipitate is the main phase in the alloy, compared with the β-Mg17Al12 phase and Al2Y phase, which have higher thermal stability at high temperatures.

During the hot rolling process, the Al2Y phase effectively blocks dislocation movement and grain boundary slip during heating. With deformation, the dislocation density in the alloy increases due to dislocation proliferation and the formation of new dislocations. As the dislocation density increases, other dislocations hinder dislocation movement, requiring an increase in applied pressure to achieve the desired degree of metal deformation.

In addition, the addition of Y and hot rolling refines the grain, resulting in improved mechanical properties, particularly the yield strength.

3. Mg Al Sn RE series

Adding Sn to magnesium alloy and combining it with a small amount of aluminum is highly beneficial. Sn not only improves the ductility of magnesium alloy, but also reduces the cracking tendency during hot working, making it ideal for hammer forging.

Rare earth elements such as Ce, Y, Nd, etc., are generally added to the Mg-Al-Sn magnesium alloy. Ce can improve the tensile strength and elongation of the Mg-4Al-2Sn-1Ca alloy at room temperature. This is due to the refinement of the camgsn phase in the alloy and the reduction of grain size in the Ce-containing alloy.

At room temperature, an alloy with 1% (mass fraction) of Ce shows the best mechanical properties, with its tensile strength, yield strength, and elongation reaching 194.95 MPa and 11.4%, respectively.

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

The mechanical properties of Mg-4Al-2Sn-xY-yNd (where x + y = 0.9% (mass fraction)) are influenced by the relative content of Y and Nd.

As shown in Fig. 2, all alloys have a yield strength of approximately 70 MPa.

The alloy exhibits the best mechanical properties when the content of Y is 0.5% (mass fraction) and the content of Nd is 0.4% (mass fraction), with a yield strength, tensile strength, and elongation of 70, 225, and 23.2%, respectively.

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

Magnesium-zinc (Mg-Zn) alloys are extensively used in the production of wrought magnesium alloys and possess excellent aging strengthening ability.

Various rare earth elements, including Y, Er, Gd, Nd, and Ce, among others, are commonly added to Mg-Zn alloys.

The addition of rare earth elements enhances the mechanical properties of the alloy by refining the grain and forming a strengthening phase, which improves its strength.

Table 3 provides details of the state and mechanical properties of some typical Mg-Zn alloys that have been added with rare earth elements.

Table 3 Mechanical Properties of Mg Zn RE system

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

However, adding rare earth elements to as-cast alloys alone cannot meet the strength requirements. As a result, researchers have begun studying the effects of deformation and rare earth element addition on alloy properties.

A comparative study of the as-cast and as-extruded Mg-5.0Zn-0.9Y-0.16Zr alloy revealed that the alloy’s mechanical properties greatly improved after extrusion. Specifically, the tensile strength, yield strength, and elongation increased from 168 MPa and 1.8% to 363 MPa and 12%, respectively. This improvement 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, with the yield strength increasing from 209 MPa to 232 MPa and the elongation increasing from 11.5% to 14.7%, while the tensile strength remained basically unchanged.

Furthermore, when compared with the as-cast Mg-12Zn-1.5Er alloy, the extruded alloy’s mechanical properties showed significant improvement, as shown in Fig. 3, with the yield strength and tensile strength reaching 318 MPa and 359 MPa, respectively.

The stress-strain curve of the typical extruded Mg-3.5Zn-0.6Gd alloy showed that the alloy had better strength and plasticity, with a tensile strength of 308 MPa, yield strength of 219 MPa, and elongation of 16.4%.

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

The extrusion process is known to be influenced by factors such as extrusion temperature and extrusion ratio, which can affect the properties of alloys containing rare earth elements.

A study conducted by Qing Chen et al. involved the preparation of Mg-5.3Zn-1.13Nd-0.51La-0.28Pr-0.79Zr alloy, and investigated how extrusion ratio and extrusion temperature impact the properties of the alloy.

The results of the study indicated that the tensile strength, yield strength, and elongation of the alloy were dependent on the extrusion ratio, which exhibited a two-step change.

The first step occurred when the extrusion ratio was between 0 to 9, resulting in a significant change in the tensile strength, yield strength, and elongation. Specifically, the tensile strength increased from 169MPa to 309MPa.

The improvement in tensile strength, yield strength, and elongation is negligible when changing the extrusion ratio from 9 to 100. The author conducted further research on the mechanical properties of the alloy by examining the impact of different extrusion temperatures.

The results indicated that the tensile strength, yield strength, and elongation of the alloy decreased as the extrusion temperature increased. While the changes were not noticeable between 250 ℃ to 350 ℃, there was a significant reduction in the tensile strength, yield strength, and elongation when the extrusion temperature was increased from 350 ℃ to 400 ℃.

Specifically, the tensile strength decreased from 324278 MPa to 267208 MPa, and the elongation decreased from 12% to 5%. This change was more pronounced than in other temperature ranges.

3. Mg-Li-RE series

Mg-Li alloys are the lightest among the series of magnesium alloys.

The mechanical properties of Mg-Li alloys are improved through solid solution strengthening and the formation of fine, dispersed intermetallic compounds, which are achieved by adding rare earth elements.

Several rare earth elements such as Y, Ce, and Nd can be added to Mg-Li alloys.

Table 4 shows the state and mechanical properties of some typical Mg-Li alloys with added rare earth elements.

Table 4 mechanical properties of Mg Li RE system

The addition of Y to Mg-7Li alloy results in the formation of Y-rich α-Mg phase and Mg24Y5 precipitates. With increasing Y content, the α-Mg phase becomes noticeably refined.

Mg-7Li-3Y alloy exhibits the best mechanical properties in terms of comprehensive strength and elongation, with a tensile strength, yield strength, and elongation of 160, 144 MPa, and 22%, respectively.

When the Y content exceeds 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 also studied. The tensile strength of the rolled LAY831 alloy was found to reach 230 MPa, while the elongation of the extruded LAY811 alloy reached 60%.

Under plastic deformation conditions, the formation of ALY intermediate phase and the reduction of α phase significantly improved the alloy’s mechanical properties.

The addition of rare earth elements to Mg-5Li-3Al-2Zn alloy results in the formation of Al2RE or Al3RE phase and a reduction in alli phase. The tensile strength of the alloy increases with the addition of rare earth elements, but when the addition amount exceeds 1.5% (mass fraction), the tensile strength weakens. The elongation exhibits a similar trend.

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

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

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

4. Others

Table 5 lists additional properties of alloys containing rare earth elements.

For the Mg-4Y-4Sm-0.5Zr alloy, the tensile strength and yield strength decrease slightly as the extrusion temperature increases. However, after aging, the tensile strength and yield strength increase as the extrusion temperature increases.

After aging the alloy at 200 ℃ for 16 hours, the best mechanical properties were observed in the alloy extruded at 400 ℃, with a tensile strength of 400 MPa, a yield strength exceeding 300 MPa, and an elongation of 7%.

After 14 cycles of extrusion compression, the Mg-10Gd-2Y-0.5Zr alloy exhibited a 20% increase in yield strength, an 8.2% increase in tensile strength, and a 150% increase in elongation.

Table 5 mechanical properties of other alloys

The addition of Ce to the Mg-3Sn-2Ca alloy leads to significant improvements in its mechanical properties, with the greatest enhancement observed when the Ce content reaches 1.5% (mass fraction) or higher.

At a Ce content of 2% (mass fraction), the tensile strength, yield strength, and elongation at room temperature increase by 24.4%, 28.6%, and 73.7%, respectively. Similarly, at 150 ℃, the increases are 22.4%, 28.8%, and 56%, respectively.

In addition to Ce, the rare earth element Y can also enhance the strength of the alloy. At a Y content of 1.5% (mass fraction), the alloy exhibits the best mechanical properties, with a tensile strength of 150 MPa, yield strength of 137 MPa, and elongation of 3.2% at room temperature. This represents an increase of 18.1%, 22.3%, and 68.4%, respectively. The corresponding increases at 150 ℃ are 19.8%, 24%, and 54.9%.