Magnesium alloys, twin boundaries and segregation

In the distant 1903, the notorious Wright brothers built the first aircraft equipped with an engine. Most of this amazing machine was made of spruce. Now wooden aircraft are museum exhibits, but at that time the use of this material was justified by its strength and lightness.

Now in aviation, shipbuilding and other industries much more complex materials are used, among which magnesium-based alloys are far from the last. Despite all its advantages, these alloys have a number of disadvantages that prevent their wider application. Today we will meet with you a study in which scientists from Monash University (Melbourne, Australia) discovered a new method to create a more durable and lightweight magnesium alloy. How did they succeed, what new physical and chemical properties were revealed, and what role did X-ray mapping play in this work? We will find answers to these questions in the report of the research group. Go.
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A bit of history

Pure magnesium was first isolated in 1828 by the French chemist Antoine Bussy. But this is not the first appearance of magnesium in the history of mankind. In 1695, in the town of Epsom (England), salt was isolated from mineral water, which is now known as magnesium sulfate heptahydrate (MgSO ₄ · 7H ₂ O). This substance was very bitter in taste and had laxative properties, which apparently were identified by the only possible method then - in practice. After almost 100 years in 1792, Anton von Ruprecht was able to isolate from MgO a substance that he called Austria. Austria, as it turned out, is magnesium, but with a very low degree of purity. And already in 1828, Bussy was able to get pure magnesium, restoring its molten chloride with metallic potassium. A little later, in 1830, Michael Faraday through the electrolysis of molten magnesium chloride (MgCl ₂ ) also received pure magnesium (Mg).


Antoine Bussy

However, magnesium gained industrial significance only in the early thirties of the twentieth century, after which the production of alloys based on it constantly grew.

Read more about magnesium alloys here .

In modern engineering, magnesium alloys are also widely used, but their range of applications can be expanded, as the researchers say.

In their work, they demonstrated the ability to visualize segregation * in magnesium alloys by applying atomic resolution X-ray dispersion spectroscopy at a much lower voltage than previously thought. Scientists also demonstrate joint segregation at the twin grain boundary * in a magnesium alloy with large and small dissolved substances forming alternating columns that completely occupy the twin boundaries * .

Segregation * - a change in the physical state of an inhomogeneous medium.

The grain boundary * is the interface between two grains in a polycrystalline material.

The boundaries of the twins * - the interface between the two parts of the crystal, which are mirrored to each other.

Generally speaking, this study shows that atomic analysis of the structure and chemical composition of dissolved substances in metal alloys with a complex composition is more than possible.

Study basis

Scientists note that grain boundaries play an important role in controlling the mechanical properties of many polycrystalline materials, in particular light magnesium alloys. The biggest obstacle to the wider use of magnesium in the aerospace and automotive industries is the difficulty of controlling deformation during thermomechanical processes.

At the moment, it is known that the addition of rare-earth (RE) elements to a magnesium alloy leads to a significant weakening of the recrystallization texture. And the addition of a combination of rare-earth and non-rare-earth elements can lead to an even weaker texture of recrystallization.

In addition, the addition of RE leads to a large number of deformation twins, which provide more nucleation sites for recrystallization grains with a random orientation.

Researchers note that the combination of large and small atoms of the corresponding alloy elements can lead to a much weaker texture and better formability * by maximizing joint segregation.

Formability * - the ability of a metal powder to acquire and maintain a given shape under the action of applied pressure and gravity.

However, obtaining sufficient information regarding these processes and their effect on the general structure of the alloy cannot be carried out at a sufficiently accurate level without experimental data on the atomic scale of the alloy structure, the chemical composition of twin boundaries, etc.

To solve this problem, one can use PEM - a transmission scanning electron microscope equipped with a regulator of spherical aberration. This device allows you to observe the distribution of heavy atoms using a visualization technique based on Z-contrast, as well as lighter atoms (oxygen, lithium or hydrogen) through circular bright field visualization.

However, the analysis of such Z-contrast images becomes problematic when the alloys have several alloying elements * .

Alloying element * - an element that is added to the metal and remains in it, while changing its structure and chemical composition.

Of course, it is possible to study the chemistry of the boundaries between grains using atomic probe tomography, but it is extremely difficult to determine in detail the location of the atom of the dissolved substance at the boundary.

Another problem in the study of alloying elements of light alloys is that segregation is damaged by an electron beam. For magnesium alloys, this problem is especially acute when the segregated atoms of the dissolved substance turn into one atomic column.

However, do not despair, because researchers in their work have found a way to solve this problem. All that is needed is energy dispersive X-ray spectroscopy (EDS) at a much lower voltage.

Using this method, scientists were able to discover the pattern of joint segregation of dissolved elements at the twin boundary, as well as the mechanism of migration of the twin boundary.

The test subject in this study was Mg-RE-Ag alloy, which has excellent mechanical properties at room temperature and at elevated temperatures. It is important that Nd has a larger atomic size than Mg, but Ag has a smaller atomic size than Mg.

Given that Nd and Ag have higher atomic numbers in the periodic table, they are unsuitable for Z-contrast imaging. That is, their distribution on an atomic scale can only be detected using EDS.

Research results

Image No. 1

Images 1a and 1b show dark-field PEM images of (1012) twin boundaries in a plastically deformed and annealed sample. All atomic columns within this boundary exhibit a brighter contrast than columns in a matrix or twin. Since the brightness of an individual atomic column in a dark-field PEM image is approximately proportional to the square of the average atomic number, a brighter contrast indicates the enrichment of the dissolved substance. At the same time, it is difficult to determine what exactly individual bright columns are rich in - Nd, Ag, or both, since the atomic numbers Nd (60) and Ag (47) are higher than that of Mg (12). For this reason, it was decided to apply atomic resolution EDS.

Figures 1c - 1e show the EDS images of the twin boundaries shown in 1b . These data were obtained using a significantly lower voltage (120 kV) than is usually required by this type of microscopy (300 kV).

EDS images clearly indicate that Nd atoms segregate exclusively in the places of expansion (circles on 1b - 1e ), but Ag atoms are concentrated exclusively in the places of compression. A similar pattern of segregation differs from that observed in Mg-Gd-Zn alloys, where larger and smaller atoms of the solute are concentrated only in the places of expansion.

It was also found that with continuous electron radiation, atomic columns enriched in Nd are much more stable than columns enriched in Ag. For this reason, the quality of EDS images for Nd is better than for Ag.

Next, it was necessary to establish the location of the jointly segregated atoms of the dissolved substance. For this, scientists studied the segregated (1012) twin boundary along the (1011) twin direction.

When viewed along (1011), the twin and matrix show identical projections of the atomic columns, and the diffraction patterns of these two crystals are also identical. And this makes it difficult to study the boundaries of twins at the atomic level. But the segregation of atoms of the dissolved element makes it possible to directly observe the boundary of twins in dark-field PEM images ( 1f - 1g ).

All columns at the twin boundary exhibit a brighter contrast, which indicates enrichment with solute along the studied direction. And, again, despite the fact that it is difficult to distinguish between Nd and Ag in the PEM images, the corresponding EDS images with atomic resolution clearly indicate that each atomic column contains Nd and Ag atoms ( 1h - 1j ).

Combining the data from the PEM and EDS images of the two aforementioned orthogonal directions, it was possible to obtain the distribution of Nd and Ag atoms within (1012) the twin boundaries ( 1k ). Along the direction (1210) corresponding to the blue arrow in the diagram, each atomic column contains Nd or Ag atoms. And along the direction (1011), i.e. red arrow, Nd and Ag atoms are distributed alternately in each column.

Image 1l schematically shows segregation layers along (1210) and (1011). A simulation was also carried out, the results of which are in excellent agreement with the experimental data ( 1n - 1o ).


Image No. 2

The phenomenon of joint segregation (co-segregation) was also observed at the (1011) twin boundary. Image 2a shows a PEM image (1011) of the twin boundary in a deformed and annealed sample. As in previous observations, the expansion sites and compression sites are filled with solute. Nd atoms segregate at the sites of expansion, and Ag atoms at the sites of compression ( 2b - 2e ). Thus, there is a segregation pattern similar to that on the (1012) twin boundary.


Image No. 3

Further, scientists performed calculations in order to identify the source of such an unusual picture of joint segregation, when alternating columns of large and smaller atoms of dissolved substances occupy the entire boundary of twins.

The graphs above show the calculated relative energies for the range of inclusions (fractions) of the solute at the (1012) and (1011) twin boundaries.

For the boundary (1012), it can be seen that for the place of expansion the most favorable is the full filling of the column with Nd atoms in the direction (1210) ( 3a ). In previous observations, the larger and smaller atoms of the solute together segregate only at the sites of expansion, but here we see the presence of mixed Nd and Ag atoms in one column of the site of expansion, which leads to an increase in energy.

A significant decrease in energy is observed if the compression site is completely occupied by Ag atoms (dashed line at 3a ), which is consistent with the experimental results.

Graph 3b shows the energy levels at the compression site. Here, the minimum energy level is also observed if the compression site is completely occupied by Ag atoms and the expansion site by Nd atoms.

For the boundary (1011), it can be seen that the most favorable scenario is filling the compression site with Ag atoms, and the expansion sites with Nd atoms ( 3c - 3d ).


Image No. 4

The next step in the study of joint segregation was the determination of the mechanisms of migration of the boundaries of the twins, which was done through calculations (image No. 4).

It is worth noting that the presence of Nd and Ag at the twin boundary leads to a change in the mechanism of border migration from the generally accepted regime to a completely new one.

Atoms within the boundary plane of twins and its nearest neighboring planes (first and second) behave differently when an external shear strain is applied. In a situation where there is no segregation of solute ( 4a ), the angle α associated with the initial twin boundary gradually decreases with increasing shear strain due to the fact that the Mg atoms of the compression site © move in the opposite direction from the Mg atoms of the expansion site (E) . The angle α decreases from 180 ° to 164 °. At this time, the angle β associated with the old layer increases up to 180 ° and becomes the next plane of the displaced twin boundary ( 4b ). There is also a slight change in the angle γ associated with the first layer.

Such a synchronous permutation of atoms leads to a mechanism of migration of the twin boundary, which includes the formation of discontinuities of two (1012) layers. However, when Nd and Ag are present at the twin boundary ( 4c ), the shuffling mechanism is reduced.

With an increase in the applied shear strain ( 4d ), the angle α remains close to 180 ° and prevents the shuffling movement in the places of compression and expansion, which occurs when the angle β increases with the applied strain in the absence of dissolved substance.

While the angles α and β remain relatively unchanged with increasing shear strain, the angle γ increases with the applied strain due to the motion of the Mg atom in the direction opposite to its two neighboring atoms in the first layer. As a result, the angle γ reaches 180 ° and becomes the next plane of the displaced boundary of the twins.

This mechanism of twin boundary migration through one layer (instead of two) is very different from the migration mechanism, where there is no segregation of solute ( 5a ).

To determine the general characteristics of the migration mechanism described above, cases were calculated in which segregation at the twin boundary occurred with Nd atoms or with Ag atoms, i.e., in a system of binary alloys.

In the case where only Nd ( 4e ) is present at the twin boundary, the tendency for atomic shuffling is similar to that observed in the presence of Nd and Ag at the twin boundary ( 4d ). The angles α and β practically do not change, and the angle γ increases with increasing shear strain ( 4f ).

Scientists have suggested that this new migration mechanism can occur in magnesium alloys containing rare earth elements or other impurities that supplement Mg-Nd. This is also evidenced by the fact that in the case of the presence of exclusively Ag ( 4g ) on the twin boundary, the mechanism of border migration is the same as in the absence of segregation of dissolved substances ( 4b ).

When the angles of the original plane of the boundary and the second layer change due to an increase in shear strain, the angle γ associated with the first layer changes slightly ( 4h ).


Image No. 5

There is also a theory that the combined segregation of Nd and Ag atoms at the twin boundary can significantly reduce the mobility of this very boundary. Thermodynamically, the segregation of the solute can reduce the boundary energy and, therefore, increase stability and at the same time reduce the mobility of the double boundary. Kinetically, the segregation of the solute at the twin boundary will have the effect of binding or resistance to migration of the boundary.

The calculated shear stress as a function of the deformation curve for the (1012) boundary of twins with / without segregated Nd and Ag atoms is shown in graph 5b .

In a situation where there is no segregation of the dissolved substance, that is, only Mg, the twin boundary begins to migrate at a shear stress above 116 MPa. When the boundaries of the twins are filled with Nd and Ag, a significant change in the shear stress and the appearance of the limit of elastic deformation are observed.

For a more detailed acquaintance with the nuances of the study, I recommend that you look into the report of scientists and additional materials to it.

Epilogue

In this study, scientists were able to demonstrate the possibility of studying the structure and chemical composition of the boundaries of twins in magnesium alloys at the atomic level, which was previously considered almost impossible. The technique they discovered made it possible to detect an unusual segregation pattern that causes a strong pinning effect on interfaces, and a migration mechanism that has not been previously studied.

The segregation data provide a more accurate picture of the thermal stability and mobility of the interfaces inside the alloys, which has a significant effect on their properties as a whole.

Thus, scientists were able to study in more detail what has been used for decades. The study of hidden properties, processes and phenomena allows us to expand our understanding of this or that object, whether it be a single element or alloy.

Thank you for your attention, remain curious and have a good working week, guys! :)

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