Influence of Dy and Ho on the Phase Composition of the Ti-Al System Obtained by ‘Hydride Technology’

The manuscript describes the phase composition, microstructure, some physical and mechanical properties of the Ti-Al system with addition of 2 at. % Dy (TAD) and Ho (TAH) obtained by “hydride technology”. Phase diagrams for Ti-Al-Dy and Ti-Al-Ho at a temperature of 1150 °C and basic properties for ternary phases Dy₆Ti₄Al₄₃ and Ho₆Ti₄Al₄₃ were calculated. A crystallographic database of stable and quasistable structures of the known elemental composition was created in the USPEX-SIESTA software by means of an evolutionary code. The calculations show that adding REM leads to a significant stabilizing effect in each Ti-Al-Me (Me = Dy, Ho) system without exception. It has been established that the lattice energies of AlTi3Ho and AlTi3Dy are, respectively, equal to: EAl4Ti12Dy3 = −32,877.825 eV and EAl4Ti12Dy3 = −31,227.561 eV. In the synthesized Ti49Al49Ho2 compound, the main phases include Al-Ti, Al3Ti3 and Al4Ti12Ho3 and the contributions to the theoretical intensity are equal to 44.83, 44.43 and 5.55%, respectively. Ti49Al49Dy2 is dominated by the Al-Ti, Al3Ti3 and Al4Ti12Dy phases, whose contributions are equal to 65.04, 16.88 and 11.2%, respectively. The microhardness of TAD and TAN specimens is 1.61 ± 0.08 and 1.47 ± 0.07 GPa, respectively.


Introduction
Strict requirements imposed on the characteristics of the final material result in the necessity to develop new or improve existing alloys with increased physical and mechanical properties. This is associated with the widespread demand for such alloys in shipbuilding, mechanical engineering, aerospace, and other industries [1][2][3]. Titanium-aluminum (Ti-Al) alloys with additives of various rare earth metals (REM) are attractive materials meeting these requirements.
Obviously, the final properties of the material will depend on the chemical composition of the initial components. The search for a production technology depends on the set tasks, which include a high level of mechanical properties, manufacturability, resistance to corrosion and flammability. The increased operating temperatures of various products actualize the task of creating alloys with high strength and heat-resistant characteristics. Such properties can be achieved by adding various REM. REM with incomplete d-shells and having close atomic diameters with other components of the alloy allow forming complex-alloyed solid solutions and contribute to the formation of a large number of intermetallic compounds [4]. It should be noted that the addition of REM in alloys is an

Obtaining Alloy
This work investigates the structural state and quantitative phase analysis of the 49%Ti-49%Al-2%Dy (at.%, hereinafter referred to as TAD) and 49%Ti-49%Al-2%Ho (at %, hereinafter referred to as TAH) system, synthesized by the HT. The HT allows obtaining binary Ti-Al systems of a specified composition, as well as ternary Ti-Al-REM systems with a controlled content of REM.
To obtain Ti-Al-Me (Me = Dy, Ho) systems, powders of Ti (purity 99.2%), Dy (purity 99.9%) and Ho (purity 99.9%) were used as raw materials. Hydrides were obtained in a hydrogen stream (purity 99.9%) at 420 • C. Dy or Ho hydride was ground in an agate mortar before grinding. The obtained Ti, Dy, and Ho hydrides were re-hydrogenated for complete hydrogenation after grinding. Then, the resulting hydrides were thoroughly mixed with the Al powder (purity 99%). To obtain metal hydrides in a compact state, hydride powders were received using a laboratory extruder (LabTools, St. Petersburg, Russia, 2019) at a pressure of 4 × 10 6 Pa. The specimens were placed in the reactor at a vacuum of 0.01 Pa and heated to 1150 • C.

Research Methods
X-ray diffraction analysis (XRD) of the obtained specimens (Ti-Al-REM) was performed using a DRON4-07 diffractometer with CuKα radiation in accordance with the Bragg-Brentano scheme in increments of 0.020 • C. The structural state and quantitative content of phases were identified by the Rietveld method using reflex [31]. The GSAS, PSW and XPOWDER were used as databases to determine phases. In the case of the Rietveld method, the maximum possible number of parameters was varied. The background radiation on the diffraction patterns was approximated by a polynomial of the 20th degree. The full-profile integral intensity of the reference phases was evaluated in a self-consistent manner. The reference lattices were selected from the crystallographic COD database [32].
Since there were no Ti-Al-REM compounds in COD database, the list of reference standards was supplemented with predicted structures obtained in the USPEX code with the SIESTA interface [33]. The lattices of the fixed Ti 49 Al 49 REM 2 composition, used later for qualitative analysis of the phase content of synthesized Ti-Al-REM alloys, were predicted in the work. The stability of the reference lattices was assessed both in the USPEX-SIESTA software and in the CASTEP code [34]. The total energy of the lattices was determined at 0K. Calculations of the orbitals of the electronic states in CASTEP, the distribution of the one-electron density and the energy of the ground state were self-consistent. The valence electron wave functions of Ti-Al-REM atoms were analyzed in the plane wave cutoff parameters of kinetic energy, equal to 330 eV. The convergence in the total energy was~0.5 × 10 −6 eV/atom.
Computational modeling tools such as density functional theory (DFT) methods implemented in the Vienna Ab Initio Simulation Package (VASP) program (version VASP.6.X.) were used to calculate the phase diagrams. Having calculated the energies of known compounds in a certain chemical system, one can construct a phase diagram at a temperature T = 0 K and a pressure p = 0 atm. It should be noted that it is possible to draw approximate conclusions about the final phase diagrams at a given temperature and pressure for a system consisting mainly of solid phases that are free with respect to the gaseous element.
The microstructure of the specimens (Ti-Al-REM) was determined using a "JEM-2100F" (JEOL Ltd., Tokyo, Japan) microscope with energy-dispersion spectral (EDS) analysis. The results of morphology and elemental composition of the specimen surface were presented in the form of energy-dispersive X-ray spectra (EDX) using a QUANTA 200 3D (FEI Company, Hillsborough, OR, USA). The microhardness of the alloy specimens was determined using a PMT-3M microhardness tester (LOMO, JSC, St. Petersburg, Russia) at a load of 200 g by the Vickers method. Crystal structures Ho 6 Ti 4 Al 43 (mp-1212360) and Dy 6 Ti 4 Al 43 (mp-567159) were described using the open database Materials Project.

Results and Discussion
A crystallographic database of stable and quasistable structures of the known elemental composition was created in the USPEX-SIESTA software (version 10.5) by means of an evolutionary code. The reference standards from the COD database, as well as super cells (Al 4 Ti 12 Ho 3 and Al 4 Ti 12 Dy 3 ) found based on Ti 49 Al 49 REM 2 composition, were used for the quantitative phase analysis (QFA) by the Rietveld method. QFA with embedded REM is complicated by the absence of crystallographic information in the COD database and literature that would allow identifying their concentration. Tables 1 and 2 show the reference and refined states of Al-Ti, Al 3 Ti 3 and Ti 49 Al 49 REM 2 (structural parameter, volume and lattice energy, space group, fraction and degree of reliability (R wp )). The reference and refined lattices are slightly different. Lattice predictions in the USPEX-SIESTA software showed that the Al 3 Ti 3 lattice with embedded REM in the interstices [0.5:0.5:0.5] could be attributed to quasistable structures based on Ti 49 Al 49 REM 2 elemental composition. Additionally, quantum chemical calculations of the energy were carried out both in the initial state and by embedded REM in the CASTEP code. It has been established that the lattice energies are equal to: EAl 4 Ti 12 Dy 3 = -32,877.825 eV and EAl 4 Ti 12 Dy 3 = -31,227.561 eV. The calculations show that adding REM into these interstices is possible and leads to a significant stabilizing effect in each Ti-Al-REM system without exception. It is interesting to note that the increase in the binding energy is accompanied by a significant polarization of Millikan charges. The Al-Ti crystal lattice has a P4/MMM (Tetragonal) space group, Al 3 Ti 3 crystal lattice has a P1 (Triclinic) space group, and Ti 49 Al 49 REM 2 has a P6/MMM (Hexagonal) space group.
The QFA of contributions to the integral intensity of individual phases (Tables 1 and 2 and Figure 1) showed that with a high degree of reliability (R wp < 7.2%), the experimental diffractograms of synthesized compounds Ti 49 Al 49 Ho 2 and Ti 49 Al 49 Dy 2 are well approximated by the integral (theoretical) intensity. This is also evidenced by the intensity differences ( Figure 1 and Diagrams 3). However, the contributions of individual phases differ in different systems. Figure 1b,d also show the calculated intensities of single phases. In the synthesized Ti 49 Al 49 Ho 2 compound, the main phases include Al-Ti, Al 3 Ti 3 and Al 4 Ti 12 Ho 3 . The contributions to the theoretical intensity are equal to 44.83, 44.43 and 5.55%, respectively (Table 1). Ti 49 Al 49 Dy 2 is dominated by the Al-Ti, Al 3 Ti 3 and Al 4 Ti 12 Dy phases, whose contributions are equal to 65.04, 16.88 and 11.2% (Table 2), respectively. The 3D lattices with the spatial distribution of atoms are shown in Figure 2.
It has been established that Ti49Al49Ho2 and Ti49Al49Dy2 compounds are formed during the synthesis of Ti-Al-REM systems with a controlled content of REM (Ho2, Dy2). It was found that phases based on Al-Ti predominate in the studied specimens. The total proportion of phases is 0.89 and 0.76. The ternary Ti49Al49Ho2 and Ti49Al49Dy2 alloys have a lower content. The total proportion of which was 0.05 and 0.16. The existence and quasistability of ternary systems are predicted in the USPEX-SIESTA and CASTEP codes.  The microstructure of the TAD and TAH alloys with characteristic X-ray radiation spectra is shown in Figure 3. The composite materials contain pores. A color gradient is observed in the SEM patterns. Gray areas correspond to the alloy and dark areas correspond to the porous space. The obtained results indicate that the synthesized Ti 49 Al 49 Ho 2 and Ti 49 Al 49 Dy 2 compounds are dominated by binary AlTi-based compounds, whose proportion exceeds 0.90 of the total content. The parameters of the refined lattices and the volume of Al-Ti and Al 3 Ti 3 differ slightly from the values in the reference state. In the Al 4 Ti 12 Ho 3 alloy, the lattice volume increased mainly due to the growth of the z coordinate. Complete structural information is known for all the reference and predicted lattices.
It has been established that Ti 49 Al 49 Ho 2 and Ti 49 Al 49 Dy 2 compounds are formed during the synthesis of Ti-Al-REM systems with a controlled content of REM (Ho 2 , Dy 2 ). It was found that phases based on Al-Ti predominate in the studied specimens. The total proportion of phases is 0.89 and 0.76. The ternary Ti 49 Al 49 Ho 2 and Ti 49 Al 49 Dy 2 alloys have a lower content. The total proportion of which was 0.05 and 0.16. The existence and quasistability of ternary systems are predicted in the USPEX-SIESTA and CASTEP codes.
The microstructure of the TAD and TAH alloys with characteristic X-ray radiation spectra is shown in Figure 3. The composite materials contain pores. A color gradient is observed in the SEM patterns. Gray areas correspond to the alloy and dark areas correspond to the porous space. The microstructure of the TAD and TAH alloys with characteristic X-ray radiation spectra is shown in Figure 3. The composite materials contain pores. A color gradient is observed in the SEM patterns. Gray areas correspond to the alloy and dark areas correspond to the porous space. In the material, there are unevenly distributed areas with a predominance of Dy (luminous areas in black-white image) up to 300 μm in size. The consequence of the modifying effect of Dy is a relatively larger average grain size (150 μm) of the studied experimental Dy-containing gamma alloys in contrast to alloys with Sc and Y microadditives (100 μm) [35,36]. Grains with an average size of 110-130 μm were formed on the TAH specimen surface. The addition of Ho to the Ti-Al system leads to the formation of smaller grains compared to Dy. Rounded Dy-containing phases are observed in different parts of the matrix during EDS analysis directly in the places of accumulation of various kinds of In the material, there are unevenly distributed areas with a predominance of Dy (luminous areas in black-white image) up to 300 µm in size. The consequence of the modifying effect of Dy is a relatively larger average grain size (150 µm) of the studied experimental Dy-containing gamma alloys in contrast to alloys with Sc and Y microadditives (100 µm) [35,36]. Grains with an average size of 110-130 µm were formed on the TAH specimen surface. The addition of Ho to the Ti-Al system leads to the formation of smaller grains compared to Dy. Rounded Dy-containing phases are observed in different parts of the matrix during EDS analysis directly in the places of accumulation of various kinds of inclusions ( Figure 3). It was established that the TAD alloy matrix consists only of Al and Ti particles during local EDS analysis. The results obtained are in good agreement with the data [37]. Dy and Ho are concentrated at grain boundaries as shown in Figure 3.
It should be noted that the presence of carbon is due to the use of a conductive compound into which the specimen is pressed. Since the specimens have a porous structure, the compound fills these pores during processing (grinding-polishing) and is identified as carbon during mapping.
The particles morphology formed at grains boundary has been studied in detail. Figure 4 shows that dark particles with an average size of 0.51 µm are located on the surface, and larger particles (approximately 1.14 µm) are located at the grain boundary for the TAD specimen. For the TAH specimen, the average size of the formed particles are 1-1.3 µm. Moreover, the particles enriched with Ho (in the TAH specimen) form clusters (chains) among themselves.  The EDS results for the TAD and TAH specimens are shown in Table 3. The elemental analysis showed that the grains of the TAD system differed a little in chemical composition. It is shown that the grains richest by Dy are located at the boundary (Spectrum 1-3). According to the elemental analysis of local areas in the TAH specimens, individual dark particles located in a chain are compounds of Dy, Ti and Al. Individual particles formed  The EDS results for the TAD and TAH specimens are shown in Table 3. The elemental analysis showed that the grains of the TAD system differed a little in chemical composition. It is shown that the grains richest by Dy are located at the boundary (Spectrum 1-3). According to the elemental analysis of local areas in the TAH specimens, individual dark particles located in a chain are compounds of Dy, Ti and Al. Individual particles formed sequentially are Ho particles enriched in Ti and Al. The brighter particles are enriched with oxygen and are compounds of the formed oxides. On Figure 5 shows a bright-field pattern obtained in the volume of a TAD specimen. The interpretation of the microdiffractogram was carried out for more detailed identification ( Figure 5, regions 1-3). It can be seen that a lamellar structure begins to form in the TAD pattern, which is not clearly represented. The phases Ti (300), Ti (112), Ti 3 Al (402), Ti 3 Al (224), TiAl (301), DyAl 2 (551) and Dy (114) were found during the surface interpretation. According to the interpretation of the obtained data in the spectral region (2), the TiAl 2 (Cmmm, orthorhombic), Dy 3 Al 2 (P42nm, tetragonal), and Dy 3 Al 2 (3m, rhombohedral) phases were identified. The following phases were found in spectral region (3): Ti-Al (P4/mmm, tetragonal), Ti 3 Al (P63/mmc, hexagonal), Ti (Im-3m, cubic), Dy (P63/mmc, hexagonal), and also Dy 6 Ti 4 Al 43 (P63/mmc, hexagonal). On Figure 5 shows a bright-field pattern obtained in the volume of a TAD specimen. The interpretation of the microdiffractogram was carried out for more detailed identification ( Figure 5, regions 1-3). It can be seen that a lamellar structure begins to form in the TAD pattern, which is not clearly represented. The phases Ti (300), Ti (112), Ti3Al (402), Ti3Al (224), TiAl (301), DyAl2 (551) and Dy (114) were found during the surface interpretation. According to the interpretation of the obtained data in the spectral region (2), the TiAl2 (Cmmm, orthorhombic), Dy3Al2 (P42nm, tetragonal), and Dy3Al2 (3m, rhombohedral) phases were identified. The following phases were found in spectral region (3): Ti-Al (P4/mmm, tetragonal), Ti3Al (P63/mmc, hexagonal), Ti (Im-3m, cubic), Dy (P63/mmc, hexagonal), and also Dy6Ti4Al43 (P63/mmc, hexagonal). A detailed analysis of the TAH specimen showed the presence of dislocations on the surface ( Figure 6). In this way, phases TiAl2 (712), Ti3Al (004), Ho (114) Ho (203), HoAl3 (2215), HoAl2 (551) were identified in the surface composition of the spectral region (1). The authors think that the formation of dislocations is associated with the product crystallization. In the process of which the formation of dispersed particles and lamellar structures occurs that causes stress and increased defectiveness of the alloy. Analysis in  A detailed analysis of the TAH specimen showed the presence of dislocations on the surface (Figure 6). In this way, phases TiAl 2 (712), Ti 3 Al (004), Ho (114) Ho (203), HoAl 3 (2215), HoAl 2 (551) were identified in the surface composition of the spectral region (1). The authors think that the formation of dislocations is associated with the product crystallization. In the process of which the formation of dispersed particles and lamellar structures occurs that causes stress and increased defectiveness of the alloy. Analysis in spectral region (2) showed the presence of the Ho 3 Al 2 (P42nm, tetragonal) and Ho 6 Ti 4 Al 43 (P63/mmc, hexagonal) phases. The similar composition of the phases is typical for the spectral region (3). These particles are located close to the grain edge and are mainly represented by phases consisting of Ho and Al. The particle in the volume differs in phase composition. In addition to Ho 3 Al 2 (P42nm, tetragonal) and Ho 6 Ti 4 Al 43 (P63/mmc, hexagonal), the presence of Ti-Al (P4/mmm, tetragonal) and Ti 3 Al (P63/mmc, hexagonal) was detected during identification of TEM results. spectral region (2) showed the presence of the Ho3Al2 (P42nm, tetragonal) and Ho6Ti4Al43 (P63/mmc, hexagonal) phases. The similar composition of the phases is typical for the spectral region (3). These particles are located close to the grain edge and are mainly represented by phases consisting of Ho and Al. The particle in the volume differs in phase composition. In addition to Ho3Al2 (P42nm, tetragonal) and Ho6Ti4Al43 (P63/mmc, hexagonal), the presence of Ti-Al (P4/mmm, tetragonal) and Ti3Al (P63/mmc, hexagonal) was detected during identification of TEM results. The phase diagrams were calculated and plotted at a temperature of 1150 • C (temperature of vacuum annealing of the specimens) in the case of ternary TAD and TAN systems. This was done to determine the compounds formation area of isotypes Dy 6 Ti 4 Al 43 and Ho 6 Ti 4 Al 43 . Figure 7 shows that the Dy 6 Ti 4 Al 43 compound is stable. In addition, the formation of stable phases DyAl 3 and DyAl 2 in this area is theoretically feasible. Moreover, the Dy 6 Ti 4 Al 43 compound has common faces: DyAl 3 -Dy 6 Ti 4 Al 43 -Al, TiAl 3 -Dy 6 Ti 4 Al 43 -Al and DyAl 3 -TiAl 3 -Dy 6 Ti 4 Al 43 . Similarly, it is theoretically plausible to formation the stable phases HoAl 3 and HoAl 2 with common faces: HoAl 3 -Ho 6 Ti 4 Al 43 -Al, TiAl 3 -Ho 6 Ti 4 Al 43 -Al and HoAl 3 -TiAl 3 -Ho 6 Ti 4 Al 43 in the TAH specimen (Figure 8). The obtained diagrams are consistent with the data for the TAD and TAH systems [38][39][40][41]. The phase diagrams were calculated and plotted at a temperature of 1150 °С (temperature of vacuum annealing of the specimens) in the case of ternary TAD and TAN systems. This was done to determine the compounds formation area of isotypes Dy6Ti4Al43 and Ho6Ti4Al43. Figure 7 shows that the Dy6Ti4Al43 compound is stable. In addition, the formation of stable phases DyAl3 and DyAl2 in this area is theoretically feasible. Moreover, the Dy6Ti4Al43 compound has common faces: DyAl3-Dy6Ti4Al43-Al, TiAl3-Dy6Ti4Al43-Al and DyAl3-TiAl3-Dy6Ti4Al43. Similarly, it is theoretically plausible to formation the stable phases HoAl3 and HoAl2 with common faces: HoAl3-Ho6Ti4Al43-Al, TiAl3-Ho6Ti4Al43-Al and HoAl3-TiAl3-Ho₆Ti₄Al₄₃ in the TAH specimen (Figure 8). The obtained diagrams are consistent with the data for the TAD and TAH systems [38][39][40][41].  Theoretical calculations were carried out and the main properties of the Ho6Ti4Al43 and Dy6Ti4Al43 compounds were described ( Table 4). The Ho6Ti4Al43 and Dy6Ti4Al43 phases crystalize in the hexagonal P6₃/mcm space group (No. 193). They have almost equal formation energy of ≈ −0.340 eV/atom and are non-magnetic materials. The total magnetization is 0.33 and 0.41 μB/f.u., respectively. They are thermodynamically stable. The calculated density is 4.12 (Ho6Ti4Al43) and 4.08 g·cm⁻ 3 (Dy6Ti4Al43). The phase diagrams were calculated and plotted at a temperature of 1150 °С (temperature of vacuum annealing of the specimens) in the case of ternary TAD and TAN systems. This was done to determine the compounds formation area of isotypes Dy6Ti4Al43 and Ho6Ti4Al43. Figure 7 shows that the Dy6Ti4Al43 compound is stable. In addition, the formation of stable phases DyAl3 and DyAl2 in this area is theoretically feasible. Moreover, the Dy6Ti4Al43 compound has common faces: DyAl3-Dy6Ti4Al43-Al, TiAl3-Dy6Ti4Al43-Al and DyAl3-TiAl3-Dy6Ti4Al43. Similarly, it is theoretically plausible to formation the stable phases HoAl3 and HoAl2 with common faces: HoAl3-Ho6Ti4Al43-Al, TiAl3-Ho6Ti4Al43-Al and HoAl3-TiAl3-Ho₆Ti₄Al₄₃ in the TAH specimen (Figure 8). The obtained diagrams are consistent with the data for the TAD and TAH systems [38][39][40][41].  Theoretical calculations were carried out and the main properties of the Ho6Ti4Al43 and Dy6Ti4Al43 compounds were described ( Table 4). The Ho6Ti4Al43 and Dy6Ti4Al43 phases crystalize in the hexagonal P6₃/mcm space group (No. 193). They have almost equal formation energy of ≈ −0.340 eV/atom and are non-magnetic materials. The total magnetization is 0.33 and 0.41 μB/f.u., respectively. They are thermodynamically stable. The calculated density is 4.12 (Ho6Ti4Al43) and 4.08 g·cm⁻ 3 (Dy6Ti4Al43). Theoretical calculations were carried out and the main properties of the Ho 6 Ti 4 Al 43 and Dy 6 Ti 4 Al 43 compounds were described ( Table 4). The Ho 6 Ti 4 Al 43 and Dy 6 Ti 4 Al 43 phases crystalize in the hexagonal P6 3 /mcm space group (No. 193). They have almost equal formation energy of ≈−0.340 eV/atom and are non-magnetic materials. The total magnetization is 0.33 and 0.41 µB/f.u., respectively. They are thermodynamically stable. The calculated density is 4.12 (Ho 6 Ti 4 Al 43 ) and 4.08 g·cm −3 (Dy 6 Ti 4 Al 43 ).  Such compounds belong to isotype compounds of the Ho6Mo4Al43 (P63/mcm, hexagonal) type. There are data on the magnetic properties of Dy6Ti4Al43 and Ho6Ti4Al43 systems [44]. According to [45], the Dy6Ti4Al43 compound has paramagnetic properties and Ho6Ti4Al43 ferro-or paramagnetic properties. The calculations presented in Table 4 show that the Dy6Ti4Al43 and Ho6Ti4Al43 structures are stable. They do not have magnetic ordering, but the total magnetization is 0.33 and 0.41 μB/f.u, respectively.
The microhardness was measured for TAD and TAN specimens. It has been established that the microhardness for TAD is 1.61 ± 0.08 GPa, and for TAH is 1.47 ± 0.07 GPa. The microhardness for the Ti-Al system obtained by HT is 1.23 ± 0.06 GPa [35]. The increased microhardness may be associated with a change in the phase composition [41].  Such compounds belong to isotype compounds of the Ho6Mo4Al43 (P63/mcm, hexagonal) type. There are data on the magnetic properties of Dy6Ti4Al43 and Ho6Ti4Al43 systems [44]. According to [45], the Dy6Ti4Al43 compound has paramagnetic properties and Ho6Ti4Al43 ferro-or paramagnetic properties. The calculations presented in Table 4 show that the Dy6Ti4Al43 and Ho6Ti4Al43 structures are stable. They do not have magnetic ordering, but the total magnetization is 0.33 and 0.41 μB/f.u, respectively.
The microhardness was measured for TAD and TAN specimens. It has been established that the microhardness for TAD is 1.61 ± 0.08 GPa, and for TAH is 1.47 ± 0.07 GPa. The microhardness for the Ti-Al system obtained by HT is 1.23 ± 0.06 GPa [35]. The increased microhardness may be associated with a change in the phase composition [41].
Such compounds belong to isotype compounds of the Ho 6 Mo 4 Al 43 (P63/mcm, hexagonal) type. There are data on the magnetic properties of Dy 6 Ti 4 Al 43 and Ho 6 Ti 4 Al 43 systems [44]. According to [45], the Dy 6 Ti 4 Al 43 compound has paramagnetic properties and Ho 6 Ti 4 Al 43 ferro-or paramagnetic properties. The calculations presented in Table 4 show that the Dy 6 Ti 4 Al 43 and Ho 6 Ti 4 Al 43 structures are stable. They do not have magnetic ordering, but the total magnetization is 0.33 and 0.41 µB/f.u, respectively.
The microhardness was measured for TAD and TAN specimens. It has been established that the microhardness for TAD is 1.61 ± 0.08 GPa, and for TAH is 1.47 ± 0.07 GPa. The microhardness for the Ti-Al system obtained by HT is 1.23 ± 0.06 GPa [35]. The increased microhardness may be associated with a change in the phase composition [41].

Conclusions
In this work, materials from the Ti-Al-Me (Me = Dy, Ho) system were obtained by "hydride technology". The phase composition, microstructure, some physical and mechanical properties of the Ti-Al system with addition of 2 at % Ho or Dy were studied. 1.

3.
A crystallographic database of stable and quasistable structures of the known elemental composition was created in the USPEX-SIESTA software by means of an evolutionary code. The calculations show that adding REM leads to a significant stabilizing effect in each Ti-Al-REM system without exception. It has been established that the lattice energies are equal to: EAl