3d-visualization of magnesium strengthening mechanisms for a description of experimentally obtained data of alloying effect in Mg-Ga system

J.A.Khokhlova, M.A.Khokhlov

Paton Electric Welding Institute of National Academy of Sciences of Ukraine, Kyiv, Ukraine

Abstract Based on laws of theory of materials strengthening were discribed the experimentally obtained alloying effect in Mg-Ga system and shown using program for 3d atomic structures.

Keywords: Magnesium; Gallium; Alloying; Strengthening; Atom; 3d.

1.Introduction

The aim of the study is to attempt to visualize the experimentally fi ed effect of doping in the Mg-Ga system during the formation of intermetallic phases, which increase the hardness and elasticity.Classical theoretical descriptions of the mechanisms of intermetallic and intergranular hardening were used, as well as the capabilities of a three-dimensional atomic simulation program for constructing crystal structures of phases.It should be noted that gallium, from a fundamental point of view, is on the boundary between metals and semimetals, therefore it is a good model object for understanding the nature of the chemical bond in intermetallic compounds for research relationships between the structure and materials physical properties [1].

Such chemical interaction studied has applied values - gallium used as an activator in diffusion welding of Mg-alloys and other metals.

2.Experiments

Previous studies [2] have shown that when applying a thin layer of molten gallium and heating up to 300°C in a magnesium alloy of the Mg-Al-Zn system forms the diffusion zone with a width of up to 100μm (Fig.1) with a microhardness increasing of 1.5-2.0 GPa and Young’s modulus 50-72 GPa,at values for the base magnesium alloy respectively: 1.2 GPa and 43 GPa.The SEM chemical composition of the diffusion zone can be Mg2Ga and Mg5Ga2intermetalliсs according to binary diagram (Fig.2).

Fig.1.SEM maps of the Ga (a) distribution in Mg alloy (b).

Fig. 2. Binary diagram Mg-Ga.

According to Mendeleev’s Periodic Table,gallium in alloys of the Mg-Al-Zn system forms chemical compositions only in pairing with magnesium, therefore the process of strengthening is considered only in the Mg-Ga system.

Physical and computer simulations of such interaction were carried out for the exact determination of the physicochemical mechanisms of strengthening at the interaction of gallium with magnesium alloy.Physical modeling was to study the phase composition of the alloy formed by the interaction of magnesium with gallium in the remelting [3].Computer simulation will allow visualization at the atomic level of phase mechanisms of strengthening during the construction of crystalline lattice of phases, the type and crystallographic characteristics of which will be determined in the experimental part of this study by the X-ray method.

The presence of two types of structures is found on the cross-section of the ingot (Fig.3): globular "dark" grains of the primary structure and an intergranular "light" eutectic of a secondary structure with irregular morphology.Indentation test fi ed hardness 1.2-2 GPa and Young’s modulus 43-73 GPa.The X-ray diffractograms (Fig.4) showed the presence of the composition:

·32.54 wt.% Mg5Ga2(a=13.6935 °A; b=7,0220 °A;c=6.0284 °A);

·67,5 wt.% Mg (a=3.1941 °A; c=5.1879 °A).

From the fundamental positions [4-9] we know that the effect of strengthening during doping is associated with an increase in the density of atoms and their ordering after the crystallization of the alloy.The foreign atoms in the crystalline lattice of the base metal are the centers of the deformation which creates an elastic fiel of stress around them.More precisely, the effect of compaction of the crystal lattice is due to the difference in the size of the atoms and the increase in the frictional forces between them.

Fig.3.SEM maps of the Ga (a) distribution in Mg alloy (b).

Fig.4.Mg-Ga alloy X-ray diffraction.

3.Modeling

From the chemical point of view atom is the smallest electroneutral chemically indivisible particle of a substance that can be observed using electron microscopes.For atoms that form solid crystals the distance between adjacent nodes of a crystalline lattice can be approximated by their size.As a rule,it is difficul to fi the exact configuratio of the crystalline structure by experimentally obtained diffraction due to the presence of plenty of areas with sufficien atomic density to represent X-rays,which can cross the plane at different angles and fi the distorted diffraction image.Therefore, imaging of the Mg5Ga2phase which provides for the strengthening of the diffusion zone was carried out using the computer program ChemSite3.1, the theoretical and mathematical provisions of which are described by base physical law.

The program allows you to build images of two factors that contribute to strengthening:

1?crystallographic parameters;

2?growth of atomic structures of different volumetricspatial type.

The main properties of the atoms considered in the system are known:

·Mg(Fig.5a)?radius atom 160 pm,crystal lattice is hexagonal, Young’s modulus 43 GPa;

·Ga?radius atom 141 pm, crystal lattice is orthorhombic,Young’s modulus 40…50 GPa;

·Mg5Ga2(Fig.5b)?atoms ratio in orthorhombic crystal lattice [1] optimized to 8 atoms Mg and 4 Ga, Young’s modulus 50…73 GPa.

Well known that the strength of pure metals is determined mainly by the electronic structure of atoms and the type of their interaction [4,5].The effect of strengthening during alloying is associated with an increase in the density of atoms and their ordering after crystallization of the alloy.More clear description of the alloying effect - atoms compaction in the matrix crystal lattice by foreign atoms (Fig.6) and the difference in atom size forms increase of friction between them which leads to a decrease in the mobility of atoms and constrains the mobility of dislocations.Therefore, foreign atoms are the centers of the deformation of the lattice and creates an elastic fiel of stress around them in the crystalline lattice of the base metal (Fig.6).

Fig.5.Crystal cells: matrices - hexagonal (a) and phases - orthorhombic (b).

Fig.6.Volume and density differences for 10 periodical crystal cells of Mg hexagonal and Mg5Ga2 orthorhombic phase after its compaction.

For strengthening mechanism characterization of 67.5Mg+32.5Mg5Ga2(% wt.) alloy it’s should be considered the transition regions of the atomic structures of the matrix and phase.A completely arbitrary polycrystal without texture has a characteristic distribution of the orientations of boundaries, however, such cases are rare and most of the material will be different from this idealized representation to a greater or lesser side.

The boundary can be formed by single adjacent grains or crystallites [8,9].It is likely that these boundaries are classifie into a mixed type with an inclining component and torsion component.The thickness of the boundaries, as a rule,is estimated at 5 ...20 interatomic distances (～100 °A).Fig.7 shows a fragment of the consolidated boundaries of crystalline structures of Mg, Ga and Mg5Ga2.

Table 1Аb-initio Young’s modulus of Mg-Ga intermetalliсs [11].

Fig.8 shows how formed the boundary between Mg hexagonal and Mg5Ga2orthorhombic lattices with a significan angle of divergence, which occurs with the growth of crystals according to their volume-spatial type.The transition structure is formed by twin boundaries between the crystals when the atoms of one structure are on the surface of the partition.The twin layers at the coherent boundary of the reorientation of the crystal structure are very characteristic for a cast metal.The boundaries of two crystalline regions of different orientation are amorphous and contain fragments of atoms of both types.

On the contact surface of two different crystalline phases,the connection (cohesion) of crystalline lattices arises and remains.Areas with different spontaneous or proper deformations occurring in the solid phase when it is formed inside or on the surface of another solid phase are elastic domains.Due to the difference in the proper deformations of the phases,this surface is the source of internal stresses that extend over the distance, compared with the length of the contact surface(long-range field) Some another ab-initio result [10] confir the mechanical stability against small distortions in such intrinsic twin boundary structures.

Also, obtained in an experimental part of our work results in good correlated with other researcher’s data (Table 1),which predict the elastic and diffusion properties constants of Mg-based alloys using a stress-strain method.

Thus, 3d visualization of structures at alloying of magnesium alloy by gallium shows a significan reorientation of structural fragments (Mg and Mg5Ga2), significan density increases of the amorphous transition layer of interphase boundaries, which means contains a high level of internal stress and strengthening of the microstructure what was fi ed experimentally by indentation method.

Fig.7.A fragment of the consolidated boundaries of crystalline structures Mg, Ga and Mg5Ga2.

Fig.8.Fragments of the amorphous boundary of the hexagonal crystalline structure of Mg and orthorhombic Mg5Ga2 with a significan angle of reorientation,which arises when the crystals grow according to their volume-spatial type.

4.Conclusions

Computer model allowed to visualize the effect of strengthening consisting of two main factors: 1 - the volume formation of the intermetallic phase Mg5Ga2with the ordering and compaction of the crystalline structure of the Mg matrix at alloying by foreign Ga atoms; 2 - increasing the density of the amorphous transition layer at volumetricspatial changes with significan angle of divergence which forms between boundaries of Mg hexagonal and Mg5Ga2orthorhombic lattices according to their crystallographic characteristics.

Of course, obtained visualizations do not consider real and complex interactions at a distance (for example, electrostatic)and the laws of crystallization of multicomponent real chemical systems of the micro-macro levels.However, such results well explained fundamental key factors of experimentally obtained and studied alloying effect at crystallization Mg-Ga binary system.

Declaration of interests

The authors declare that they have no known competing financia interests or personal relationships that could have appeared to influenc the work reported in this paper.