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Effect of various rare-earth metals in magnesium alloys on their strength properties

https://doi.org/10.38013/2542-0542-2020-3-38-44

Abstract

Based on the results of previously published studies, the main laws of the effect of various rare earth metals on the properties of magnesium alloys used as light structural materials are formulated. It is noted that the properties of magnesium alloys containing rare-earth metals are largely determined by their solubility in solid magnesium, which changes successively with an increase in the atomic number of these elements, as well as the tendency to harden during the decomposition of magnesium solid solutions. The possibility of improving the properties of magnesium alloys when doped with various rare-earth metals in a certain ratio is reported, and the examples of such new alloys are provided.

For citation:


Rokhlin L.L. Effect of various rare-earth metals in magnesium alloys on their strength properties. Journal of «Almaz – Antey» Air and Space Defence Corporation. 2020;(3):38-44. https://doi.org/10.38013/2542-0542-2020-3-38-44

Recent studies have shown that doping of magnesium alloys using rare-earth metals (REM) enables substantial improvement of their strength properties. Hereby, the REM affect magnesium in different ways [1][2][3]. The rare-earth metals are lo­cated in IIIB Group of the Mendeleev’s Periodic Table. Let us point out that 15 metals of them with atomic numbers from 57 to 71 are in one cell of the system comprising the so-called lanthanum row. In a number of properties the REM are divided into two groups: cerium with atomic numbers from 57 (lanthanum) to 63 (europium) and yttri­um with atomic numbers from 64 (gadolinium) to 71 (lutetium). Yttrium group also includes scan­dium (No.21) and yttrium (No. 39).

The effective magnesium strengthening in case of its doping with REM is determined mostly by their solubility in solid magnesium, which de­creases as the temperature reduces. As a result, by means of heat treatment including heating to high temperature followed by hardening, oversaturated magnesium-based solid solution is obtained, which then decomposes during ageing (low-tem- perature annealing), substantially improving the alloy strength characteristics. The higher the con­centration of the oversaturated solid solution is, the higher, albeit to a certain limit, alloy hardening will be due to its decomposition. Different influ­ence of certain REM on the strength of magnesium alloys is due to a wide range of the values of their solubility in solid magnesium.

Figure 1 provides typical parts of structural diagrams of binary alloys of magnesium and REM in the areas adjacent to magnesium, which show the variation of the REM solubility in solid magnesium as function of temperature and the dif­ference in the REM solubility in two systems of binary magnesium metals: with neodymium and with gadolinium.

Fig. 1. Structrural diagrams of binary alloys of Mg-Nd and Mg-Gd systems from magnesium side [4]: a - Mg-Nd, b - Mg-Gd

Figure 2 shows the variation of REM solu­bility in solid magnesium depending on their atomic number. The values of maximum solu­bility and the solubility at different temperatures are shown. Within the lanthanum row with atomic numbers from 57 to 71 the solubility values se­quentially increase with two exceptions: for eu­ropium and for ytterbium having abnormally low solubility compared to their neighbours. At the transition from cerium group REM to the yttrium group metals their solubility in solid magnesium grows up especially sharply. The values of yttrium solubility in solid magnesium are intermediate between the solubility of REM of each group in solid magnesium for all the temperatures.

Different influence of individual REM on magnesium is also manifested in the decompo­sition kinetics of the oversaturated solid solution during ageing and the alloy hardening accompa­nying the decomposition. This is proven by the results of hardness measurements of the binary al­loys with different REM as the isothermal ageing time increases, as shown in Figure 3. REM con­tent was close to the maximum solubility in solid magnesium. Due to the large difference in values the hardness variation curves are provided sepa­rately for the yttrium group (a) and for cerium the group (b).

The data provided show that the hardening effect due to the ageing of the alloys of magne­sium with yttrium group metals is significantly higher than the hardening effect due to the ageing of the alloys of magnesium with cerium group metals. It may be explained by higher solubility of the yttrium group metals in solid magnesium compared to that of cerium group metals. Among the alloys with cerium group REM, the maximum of hardening in ageing sequentially increases with the atomic number increase and somewhat shifts towards the ageing duration increase. Herewith, noticeable hardening is observed already at small exposure times.

The variation of hardness of magnesium al­loys with yttrium group REM with the ageing time increase is different in nature but similar for all metals. Initially an incubation period of hardening with minor hardness increase is observed and only then hardness sharply increases and reaches its maximum. As the atomic number of the lantha­num row REM increases, decomposition of the magnesium solid solution slows down and the hardness maximum shifts towards longer expo­sure times during ageing. Herewith, at the highest exposure time of400 h in case of magnesium alloy with erbium it is not attained and in the alloy with thulium it is not observed at all.

Maximum hardening during ageing with in­creasing atomic number in case of lanthanum row yttrium group REM has the decreasing trend. The ageing behaviour of magnesium alloy with yttri­um is close to the behaviour of the alloy with holmium. The comparison of hardening in ageing of magnesium alloys with different REM shows that in case of the yttrium group the highest hardening is attained but in this case longer ageing time is re­quired. This is distinctly seen in case of compari­son of the extreme systems in the alloy groups: Mg-Sm and Mg-Gd. The highest hardening in ageing is attained in magnesium alloys with Gd, Tb, Dy, and Y.

The hardening effect attained in ageing of magnesium alloys with the yttrium group REM depends on the ageing temperatures and decreases with the temperature increase. It may be seen in Figure 4 for magnesium alloy with dysprosium. In magnesium alloys with the cerium group REM the dependence of hardening in ageing on the tem­perature is minor.

Therefore, the alloys of the cerium and yttrium group REM have both advantages and disadvan­tages, if compared with one another in terms of different properties. In the alloys with the cerium group metals smaller strength properties are at­tained but to attain maximum hardening in them shorter ageing time is required. In the alloys with the yttrium group metals larger hardening may be attained but at longer ageing exposure times and higher content of the expensive rare-earth metals. In a number of cases it proves appropriate to use REM of both groups for magnesium doping. Joint interaction of two REM with magnesium is cha­racterized by the part of isothermal cross-section adjacent to magnesium in one of the structural dia­grams of such alloys shown in Figure 5.

Fig. 5. Isothermal cross-section of Mg-Sm-Dy structural diagram at 500 °C [6]

The part of the Mg-Sm-Dy structural dia­gram shown in Figure 5 shows that only two phases being the compounds of magnesium with each of the rare-earth metals in which the other rareearth metal is dissolved in substantial quantity are in equilibrium with the solid magnesium solution. Herewith, dissolution of one of the rare-earth metals in the other within the solution takes place by replacement of the atoms of the first metal. The convexity of the boundary of the area of the solid magnesium solution in the structural diagram is also noteworthy; it indicates that the joint solubi­lity of both REM in the solid magnesium solution is higher than the sum of solubility values of each of them in the same proportions.

The experience of studies of magnesium alloys with REM shows that in many cases it is appropriate to use them together to yield the best properties, as well as from the economic point of view. Hereby, the alloy may contain REM both of one group and of different groups.

In Baikov Institute of Metallurgy and Mate­rials Science of the Russian Academy of Sciences (IMET RAN) in cooperation with VIAM and VILS a deformable magnesium alloy IMV7-1 with two rare-earth metals - yttrium and gadoli­nium - was developed and is characterized by high strength properties at near-room and in­creased temperatures. The typical properties of this alloy containing Mg-4.7 % -4.6 %Gd-0.3 %Zr are provided in Table 1. As we can see, the highest strength properties of this alloy at room tempera­ture are attained after ageing at 200 °C, for 64 h, immediately after hot pressing with the ultimate strength 435 MPa and percentage of elongation 4.9 %. At the test temperature of 250 °C this alloy demonstrated the ultimate strength of 336 MPa at the percentage of elongation 16 % [7].

 

Table 1

Mechanical properties of hot-pressed plate of IMV7-1 alloy at room temperature.

Longitudinal Direction [7]

State

Ultimate strength, MPa

Yield strength, MPa

Elongation, %

Hot pressing

332

231

21.4

Ageing, 225 °C, 24 h

377

287

4.9

Ageing, 200 °C, 24 h

399

316

6.4

Ageing, 200 °C, 64 h

435

388

4.9

Table 2 provides typical properties for cast alloys of Mg-Y-Gd-Zr system with the content of doping elements close to their content in the deformable alloy IMV7-1. Like in case with the deformable alloy IMV-7-1, the highest strength properties of Mg-Y-Gd-Zr system were attained after the strengthening ageing. These particular values of the strength properties of cast alloys are provided in Table 2. We can see that the strength properties of cast alloys are significantly lower than those of the deformable magnesium alloy IMV7-1. However, compared with the cast mag­nesium alloys without rare-earth metals, such as standard alloys ML5 and ML12 widely used at near-room temperature, they are close in terms of ultimate strength and better in terms of yield strength. At temperature 250 °С the alloy with yttrium and gadolinium surpasses the standard heat-resistant magnesium alloy ML10 with neo­dymium in terms of ultimate strength and is close to it in terms of yield strength. For the said stand­ard magnesium cast alloys in aged condition the following values of the strength properties for the tensile test at room temperature are provided: for ML5 σВ – 255 MPa, σ0.2 – 120 MPa and for ML12 σВ – 250 MPa, σ0.2 – 150 MPa, and at 250 °С for ML10 alloy σВ – 165 MPa, σ0.2 - 130 MPa [8].

 

Table 2

Mechanical properties of cast alloys of Mg-Y-Gd-Zr system

Alloy composition, %

State

Test temperature, °C

σΒ, MPa

σ02, MPa

δ, %

Mg-3.0Y-4.6Gd-0.35Zr

Homogenized 515 °C, 6 h

20

217

135

21.1

Mg-3.2Y-4.6Gd-0.27Zr

Homogenized 515 °C, 6 h, ageing at 200 °C, 32 h

20

247

164

10.2

Mg-3.0Y-5.2Gd-0.35Zr

Homogenized 515 °C, 6 h, ageing at 200 °C, 24 h

250

193

136

27.2

The alloys of Mg-Y-Gd-Zr system demon­strate high strength properties only in the aged state. Hereby the ageing mode ensuring the highest strength properties provides rather long ageing time at a relatively low temperature - 200 °C. The ageing temperature increase over 200 °C in order to accelerate decomposition of the solid magne­sium solution accelerates it. However, in this case maximum hardening reduces. It is a peculiari­ty of magnesium alloys with the REM from the yttrium group, which includes both yttrium and gadolinium.

At IMET RAN studies were conducted which demonstrated that decomposition of the solid magnesium solution in the alloys of Mg-Y-Gd-Z system, like IMV7-1 alloy, may be accelerated using additional doping with one of the cerium group metals - samarium. One of the results of such studies is provided in Figure 6, which shows the curves of hardness variation in isothermal ageing of cast alloys of Mg-Y-Gd-Zr system, such as IMV7-1, without and with samarium. The pre­sented hardness variation curves show that sama­rium adding accelerates hardening at the expense of decomposition of solid magnesium solution; herewith, the maximum hardness attained in the aged state of alloys is also increased.

Conclusion

  1. In case of magnesium doping with rare- earth metals (REM) the laws of their influence on the strength properties of the alloys depending on their position in Mendeleev’s Periodic Table are distinctly manifested.
  2. REM solubility in solid magnesium is sequentially increased within a wide range with the increase of their atomic number, albeit with some exceptions (in case of europium and ytterbium).
  3. The increase of the REM solubility in solid magnesium promotes the improvement of their strength properties but only in certain ranges. The highest strength properties in the binary alloys of magnesium with REM were obtained in case of the first three elements of lanthanum row yttrium group: Gd, Tb, Dy, and Y.
  4. Similarity in the conversions during de­composition of the oversaturated solid solution in the alloys of magnesium with REM opens up the possibility of the use thereof in certain lim­its together, thus ensuring reduction of the alloy cost as well as improvement of certain proper­ties thereof.

References

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6. Lukyanova E. A., Rokhlin L. L., Dobatkina T. V, et al. Investigation of the Mg-rich part of the Mg-Dy-Sm phase diagram // Journal of Phase Equilibria and Diffusion. 2016. No. 37. P. 664-671.

7. Рохлин Л. Л., Добаткина Т. В., Никитина Н. И. и др. Исследование свойств высокопрочного магниевого сплава системы Mg-Y-Gd-Zr // Металловедение и термическая обработка металлов. 2010. № 12. С. 15-18.

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9. Лукьянова Е. А., Рохлин Л. Л., Добаткина Т. В. и др. Влияние самария на структуру и свойства сплава ИМВ7-1 системы Mg-Y-Gd-Zr //Металлы. 2018. № 1. C. 58-63.


About the Author

L. L. Rokhlin
Baikov Institute of Metallurgy and Materials Science, Russian Academy of Sciences
Russian Federation

Rokhlin Lazar Leonovich - Dr. Sci. (Engineering), Chief Researcher. Research interests: magnesium alloy metallurgy.

Moscow



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For citation:


Rokhlin L.L. Effect of various rare-earth metals in magnesium alloys on their strength properties. Journal of «Almaz – Antey» Air and Space Defence Corporation. 2020;(3):38-44. https://doi.org/10.38013/2542-0542-2020-3-38-44

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