Regularities of the formation and features of the influence of a fine structure on the properties of a new-generation magnesium alloy

The main advantage of magnesium alloys is their low specific weight. On average, the density of commercial magnesium alloys amounts to as little as 65–70 % of that of industrial aluminium alloys. Besides, magnesium alloys have other advantages, such as high specific stiffness, good damping properties and fatigue behaviour, and in some cases – producibility. The advantage of weight also remains for magnesium-based alloys, being the most lightweight structural materials.

The disadvantages that constrain the opportunities of wider application of magnesium alloys as structural materials include the following:

• low level of strength properties at room and increased (up to 200–300 ºC) temperatures;
• insufficient corrosion resistance;
• mechanical anisotropy in deformed semi-finished products (for the majority of highstrength deformable alloys).

It is known that from the point of view of metallurgical science, the structure and phase composition of an alloy are the prime consideration, which mechanical, technical, corrosion properties, and weldability of a metallic material directly depend on, along with some physical characteristics. Development of a fine-grained structure and effective phase composition is the most essential condition providing the opportunities for achieving high performance characteristics of structural materials. In their turn, phase composition and structure of an alloy are formed depending on chemical composition and processing factors of alloy production. In recent years, an opportunity to significantly improve the characteristics of magnesium alloys and substantially overcome their disadvantages through introducing certain proportions of some REE (rare-earth elements) together with known alloy components has become a matter of increasing interest [1–7]. Subject to meeting the required conditions, so called long period stacking ordered phases (LPSO phases) are formed inside magnesium alloy structure [8–11].

Magnesium alloys containing REE have been developed in our Institute and have found their industrial application. Of these, the most prospective is a new alloy of the VMD16 brand of the Mg–Zn–Zr–(REE) system, patented in the Russian Federation.

The work was completed within the framework of implementation of a complex scientific discipline 8.4: High-strength corrosion-resistant welded magnesium and cast aluminium alloys for new-generation aerospace equipment (“Strategic directions of the development of materials and technologies of their processing for the period until 2030”) [1].

The present piece of work was aimed at studying the regularities of formation and features of the infl uence of a fine structure (in particular, LPSO-phases) on the properties of a new-generation magnesium alloy of the VMD16 brand.

Ingots weighting up to 150 kg, Ø 350 mm, were manufactured under experimental-industrial conditions. After homogenizing, the ingots were pressed into a Ø 160 mm bar; cut-to-length sections were made of the bar. The sections were forged on a hydraulic press; thus, forgings having dimensions of 40×190×310 mm and weight of ~9 kg were obtained. Mechanical properties were defined as per GOST 1497 using Instron device; corrosion characteristics were obtained in accordance with GOST 9.913-90. The structure and phase composition were studied on the samples1 cut out of the alloy in the cast, homogenized, deformed and heat-treated states. The studies were carried out with the use of Leica DM IRM optical inverted microscope and JSM6490-LV scanning electron microscope with INCA450 attachment for microscopic X-ray spectrum analysis (MXRSA). When studying the samples on the scanning microscope, the photos were obtained using СОМРО mode ensuring image generation by back-scattered electrons. Besides, FEI Tecnai G2 F20 S-TWIN TMP transmission electron microscope was used. For local chemical analysis of structural components, Oxford X-max 80T EDS detector was used.

Based on the optical microscope study, there are no evident differences between the microstructure of REE alloys and that of alloys containing no REE (Fig. 1).

Specific features of the fine structure and presence of LPSO-phases were detected in the VMD16 alloy during an in-depth study using the transmission electron microscope at high resolution from ×30,000 to ×300,000.

In the deformed state (forging), the structure of the VMD16 alloy is characterized by fine grains (≤10 μm). A specific feature of the alloy phase composition is the presence of coarse grains of eutectic component (15–25 μm), as a rule located at the interface between several grains (Fig. 2а).

The grain boundaries are decorated with a chain of intermetallic phase nanoparticles (Fig. 1b). Another specifi c feature of the alloy, not common for standard magnesium alloys, is the presence of subgrain cellular structure in a deformed state. Mean size of the cells is 200–500 nm (Fig. 2b).

The presence of subgrain nanostructure in the alloy became detectable only due to a study using the scanning electron microscope at a resolution of ≥15,000.

A more extensive study of eutectic component grains using the scanning electron microscope, evidently shows two main areas in each eutectic component grain.

The darker area is a magnesium-based solid solution with an increased concentration of Zn and Y (Fig. 3c, d). The layers of the light area represent a magnesium-based solid solution with a high content of La, Nd.

This is confirmed by the results of qualitative microscopic X-ray spectrum analysis (MXRSA) of either of the two areas of eutectic component grain (Fig. 3а–d).

By way of profiling via intermetallic phase nanoparticles in the VMD16 alloy and carrying out MXRSA, zinc zirconides (Zr₃Zn₂ and Laves phase ZrZn₂) typical for the Mg–Zn–Zr system alloys were identified (transmission electron microscopy, Fig. 4).

Nanoparticles (50–70 μm) of zinc zirconides are arranged as shown above (Fig. 2а, b), on the grains boundaries.
Zinc zirconides can also be found in the volume of the grains. Besides, using microscopic X-ray spectrum phase analysis (XRSPA), the presence of Mg₁₂Nd phase, forming bright intermetallic phase nanoparticles having clear geometry and being ingrained into the eutectic components was identified.

Systematic studies of the fine structure of the alloy in all process conditions using the transmission microscope allowed to identify the presence of self-organizing LPSO-phases.

These phases are released in the form of nanoplates (having thickness from 1.5 to 25–30 nm) being parallel to each other and penetrating each grain volume (Fig. 5а–e). The nanoplates of LPSO-phases are parallel to the basal plane (0001) in the hexagonal closepacked lattice of α-solid solution.

The results of studying fine structure in the cast, homogenized, deformed, and heat-treated states have proved the presence of LPSO-phases in the alloy in any state (Fig. 5а–e).

A sort of “network” is formed by the plates of LPSO-phases, which is supposed to additionally reinforce the microvolume of each grain. At that, each plate represents a sort of a package consisting of a certain number of thin layers of magnesium-based solid solution enriched in alloying elements – yttrium and zinc. The same structure is common for the eutectic component containing, as was established earlier, the areas with different solid solution concentration (Fig. 5c). The X-ray profiling of LPSO-phase plates in transverse direction allowed to identify the pattern of elements distribution in the plates’ periodic layers. Both the modes of deformation and heat-treatment have impact on zinc and yttrium enrichment, which affects the thickness of an LPSO-phase plate.

After deformation, the thickness of LPSO-plates changes, as a rule, due to reduced number of layers. It was established that the number of periodic layers in an LPSO-plate was significantly reduced from 6–12 in cast and homogenized states down to 2–4 layers in the alloy structure after deformation and heat treatment (Fig. 5d, e). Such changes in the total number and content of elements in the layers of LPSO-plates can be most likely explained by diffusion processes and re-arrangement of Zn and Y atoms within the same under the impact of temperature and forces of deformation. LPSO-plates can be partially or completely dissolved in the course of hot deformation and their re-release at cooling of a deformed semi-finished product. Since after deformation the cooling conditions and the structure itself differ from those after casting, then LPSO-phase plates are also formed differently. X-ray profiling of the plates in transverse direction allows to identify the features of elements distribution in the plates of LPSO-phases in more detail. For the cast state of alloy, the content of zinc and yttrium in LPSO-phases amounts to 10 and 7 % (atom.) respectively, and for the optimally homogenized state – 5–6 % (atom.) of zinc and ~ 4 % (atom.) of yttrium [8]. After deformation, the content of elements (zinc and yttrium) in the periodic layers of LPSO-plates is approximately the same as after an optimum homogenization mode.

Thus, depending on the process conditions (temperature, exposure duration, deformation forces, etc.), a process occurs in the alloy implying re-arrangement of atoms of zinc and yttrium hardening elements immediately between the periodic layers in the plates of LPSO-phases and α-solid solution itself. Despite partial dispersion of alloy structural components at plastic deformation there is a migration of zinc and yttrium atoms from the periodic layers in the plates of LPSO-phases into the basic α-solid solution.

Under certain conditions, the migration of atoms is possible in the reverse direction: from α-solid solution into the periodic layers in the plates of LPSO-phases. The optimum deformation mode and subsequent heat treatment contribute to forming of periodic layers in the plates of LPSO-phases with yttrium content of 3.2–4.3 % (atom.), and zinc content of 3.8–4.2 % (atom.).

At that, the level of alloy mechanical properties shows an increase. Consequently, re-proportioning of zinc and yttrium elements in the periodic layers of LPSO-phases, which occurs on the nanolevel under the influence of process parameters, along with other factors, can eventually result in having impact on the macrolevel, promoting alloy characteristics improvement. The stability of phase composition of the VMD16 alloy (up to 500– 520 ºС), established based on the results of differential thermal analysis (DTA), is one of the reasons of high strength properties at increased temperatures (tabl.).

Table

Comparative mechanical properties of deformable magnesium alloys (forgings)

Based on research data, thermal linear expansion coefficient of LPSO-phases is significantly lower than that of the basic α-solid solution. Thus, LPSO-phases have higher thermal stability as compared with the basic α solid solution [6, 7].

A comparative table of properties of forgings made of the VMD16 alloy and analogues demonstrates the advantages of the new alloy with respect to all studied parameters: strength and corrosion characteristics, low mechanical anisotropy of deformed semi-finished products, increased strength at high test temperatures.

It should be noted that the presence of LPSO-phases was established only in the phase composition of the VMD16 alloy.

The advantages of the developed VMD16 alloy allow to recommend its use as a material for the loaded parts of airframe internals, parts of control system (brackets, arms, levers), hull parts, parts of external load system (supporting strut), in the products of aviation and rocketand-space industry.

Conclusion

The results of the conducted studies of the influence of regularities of formation and features of a fine structure (in particular, LPSO-phases) on the properties of a new-generation deformable magnesium alloy of the VMD16 brand confirm that a significant role in the improvement of magnesium-based deformable VMD16 alloy belongs to the self-organizing thermally stable nanosized LPSO-phases.

LPSO-phases are present in almost all alloy grains, they can be formed as blocks in multi-layer eutectic component grains, representing additional nanosized hardening elements randomly oriented in the material. These structural elements prevent dislocation motions and slow down differently directed diffusion processes in the alloy, which also contributes to a reduction of mechanical anisotropy and improvement of corrosion characteristics of the VMD16 alloy.