The article discusses the concept of high entropy alloys, introduced over the last decade and offering a methodology for development of new materials with enhanced properties, as well as the classification of such materials into a special category of multicomponent solutions. It is argued that the efficiency of this approach and applicability of the theory of regular solutions, on which the concept of high entropy alloys is based to materials science of multicomponent systems, are strongly exaggerated. The idea of random distribution of atoms in high entropy alloy structure is appropriate in fact only for ideal solid solutions, and there is no advantage in using it to understand the regularities of formation of complex alloys with special properties.

Phase transformations in Fe-Ni alloys produced by mechanical alloying have been investigated by X-ray diffraction, Mössbauer spectroscopy, and magnetometric methods. The critical temperatures of phase transformations during heating and cooling of the alloys in the concentration range of 12–26 at % Ni have been determined, and the intervals of existence of phases at room temperature have been established. In the alloys that contain less than 16 at % Ni, a “normal” (diffusion-controlled) γ-α transformation was observed. In some alloys, an isothermal γ-α2 martensitic transformation has been revealed, which was not earlier observed in cast Fe-Ni alloys. A considerable reduction in the start temperatures of the martensitic transformation in comparison with the cast alloys of the same nominal compositions has been found. Factors are considered that are responsible for the observed values of the temperatures of diffusionless transformations in the mechanically alloyed materials.

The evolution of the phase and structural states during heating of a powder mixture of composition Al 65 Cu 23 Fe 12 subjected to mechanoactivation has been investigated. The solid-state transformations have been studied using Mössbauer spectroscopy, X-ray diffraction, and differential scanning calorimetry. The sequence of phase formation during heating was compared with the data on the heat of formation of binary solutions and compounds in the system. The analysis performed showed that the nature of transformations in the process of the formation of the quasicrystalline phase was caused by the thermodynamic parameters of the system.

Compositions of Fe(100 − x)Mn x (x = 10 and 12 at. %) and Fe(100 − y)Ni y (y = 18 and 20 at. %) were produced by combined mechanical alloying of pure-metal powders and annealed in the austenitic field. After annealing and cooling to room temperature, the alloys had a single-phase austenitic structure. During deformation, the γ phase partially transforms into the α 2 phase (and/or ɛ phase in Fe-Mn alloys). The phase composition of the alloys after deformation depends on the amount of alloying elements and the predeformation annealing regime. The amount of martensite in the structure of a bulk alloy obtained by powder compacting grows proportionally to the degree of deformation of the sample.

The structure and properties of metal-matrix Al/Al-Cu-Fe-quasicrystal composites produced by compacting mechanically alloyed powders have been studied. An increase in the time of mechanical alloying was found to lead to an increase in the microhardness. Compression tests showed that the failure of samples occurred via the intense formation of cracks along the direction parallel to the applied stresses; the ultimate strength increases with increasing time of mechanical alloying. As the compacting temperature increases, the ultimate strength remains unchanged to temperatures of 300–400°C; at higher temperatures, a marked increase in the ultimate compression strength is observed. An increase in the compacting temperature leads to an improvement of frictional characteristic of samples.

Standard enthalpies of formation of quasicrystalline phase and the ternary solid solutions in the Al-Fe-Cu system and the intermetallic compound FeAl were determined by the means of solution calorimetry. The quasicrystalline phase was prepared using two different methods. The first method (1) consisted of ball milling the mixture of powders of pure aluminum copper and iron in a planetary mill with subsequent compacting by hot pressing and annealing. The second method (11) consisted of arc melting of the components in argon atmosphere followed by annealing. The latter method was used for preparing the compound FeAl and the solid solutions. The phases were identified using the XRD method. The enthalpy of the formation was determined for the quasicrystalline phase of the composition Al62Cu25Fe12.5 and the ternary BCC solid solutions Al35Cu14Fe51, Al40Cu17Fe43, and Al50.4Cu19.6Fe30. The measured enthalpy of formation of the intermetallic compound FeAl is in good agreement with the earlier published data. The enthalpies of formation of the quasicrystalline phases prepared using two different methods are close to each other, namely, -22.7 +/- 3.4 (method I) and -21.3 +/- 2.1 (method II) kJ/mol.

Hydrogen absorption in amorphous and quasicrystalline Ti45Ni17Zr38 powders, obtained both directly by mechanical alloying (MA) and after subsequent annealing, was studied. MA was performed in a planetary ball mill under protective argon atmosphere. The powders were hydrogenated at 503 and 573K under initial pressure of 2.9kPa. Hydrogen absorption of the as-milled amorphous samples led to formation of TiH2, ZrH2 and (ZrTi)H2 hydrides. Hydrogenation of quasicrystalline phase resulted in formation of hydrogen solution in quasicrystalline phase and ZrH2. The maximum registered concentration of H2 in the samples was 1.38H/M. Quasicrystalline phase disappeared after partial degassing up to 1.25H/M. Activation energies of hydrogen absorption and hydrogen evolution were measured, these values were found as 40.4±0.2 and 25.2±0.1kJ/mol correspondingly.

Formation of quasicrystalline phase in Ti45Zr38Ni17 and Ti41.5Zr41.5Ni17 systems both directly by mechanical alloying (MA) and after subsequent annealing was studied. Presence of icosahedral quasicrystalline phase together with initial metallic components was found in as-milled samples. Further MA processing resulted in almost complete amorphisation of the samples. Low temperature annealing of MA samples brought to increase of quasicrystalline phase content; beside the quasicrystalline phase the powders contained a Ti2Ni-type phase with fcc structure. Increase of temperature up to 525 degrees C resulted in formation of bcc crystalline approximant of quasicrystalline phase Ti11Zr10N4. Differential scanning calorimetry (DSC) showed extended exothermal effect starting from 300 degrees C, which corresponds to crystallization of the powders. Specific area measurements of the as-milled samples showed rather small specific surface (0.16-0.42 m(2)/g), which well agrees with the results of SEM study.

Метод механической активации использовали для получения металломатричных композитов. В настоящей работе в качестве наполнителя использовали квазикристаллический порошок Al 65Cu 23Fe 12, полученный механическим сплавлением компонентов с последующим отжигом. В качестве матричного материала использовали чистый порошковый Al. Для получения композита порошки матричного материала и наполнителя обрабатывали в планетарной шаровой мельнице в соотношениях Al-20 вес. % Al 65Cu 23Fe 12 и Al-10 вес. % Al 65Cu 23Fe 12. Фазовый состав исследовали методами рентгеноструктурного анализа и мессбауэровской спектроскопии. Показано, что нагрев композиционных образцов ведет к взаимодействию наполнителя и матрицы с образованием интерметаллических соединений. На шлифах компактированных образцов исследовали влияние условий обработки на микротвердость композиционных материалов. Показано, что увеличение как времени механоактивационной обработки, так и продолжительности выдержки под давлением при ком-пактировании ведет к росту микротвердости.

The Ni-Al phases were synthesized in the composition range 40-85 at. % Ni by mechanical alloying (MA) of elemental component mixtures. MA of Ni-Al powders produces the B2 phase in the range 40 - 61 at. % Ni and a solid solution Ni(Al) - in the range 65-85 at. % Ni. Milling of Ni62.5Al37.5 composition results in formation of the B2 and Ni(Al) phases mixture. Formation of a supersaturated solid solution for 65-75 at. % Ni were accompanied by appearance of B2 phase at the intermediate MA stage. Ball milling of Ni and Al powder mixtures of the compositions corresponded to intermetallics NiAl and Ni3Al, with a 5 at. % addition of X=Ti, Mo or Nib, where the third element was substituted for Ni (Ni45Al50X5 and Ni70Al25X5), Al (Ni50Al45X5 and Ni75Al20X5) or both (Ni47.5Al47.5X5 and Ni71.25Al23.75X5) was performed. The regularities of influence of the third components on the final structure of the as-milled and annealed alloys were observed. The preferred sublattices for X additions in B2 and L1(2) intermetallic phases were determined.

Mechanical alloying has been used to prepare metal-matrix composites. In this work, an Al65Cu23Fe12 quasicrystalline powder was produced by mechanical alloying of components and subsequent annealing of the mixture. This powder and pure Al powder were used as the filling and matrix of the composites, respectively. To produce composites, powder mixtures Al + 20 wt % Al65Cu23Fe12 and Al + 10 wt % Al65Cu23Fe12 were milled in a planetary ball mill. The phase composition of the prepared samples was studied using X-ray diffraction analysis and Mossbauer spectroscopy. It was shown that, upon heating, the components, Al and the Al65Cu23Fe12 quasicrystalline alloy, react with the formation of intermetallics. The effect of preparation conditions on the microhardness of composites was studied using polished sections of compacted samples. It was shown that the greater the time of both milling and holding under pressure upon compacting, the higher the microhardness of the composites obtained.

X-ray diffraction was used to study the kinetics of mechanical alloying of the alloy Cu50Cr50. The dependences of the phase composition, structural parameters (microdeformation, block sizes), and the lattice parameters on the time and conditions of milling were determined. Long-term milling results in a steady state corresponding to constant phase composition and structural parameters. The copper solubility in the chromium-based solid solution does not exceed 30 at. %. It is shown that an increase in the temperature of mechanical alloying hampers the formation of supersaturated solid solutions in the Cu-Cr system. The character of transformations upon mechanical alloying is discussed using the concepts of thermodynamic driving forces for the processes under consideration. The thermodynamic limitations on the mutual solubility in the Cu-Cr system in the course of mechanosynthesis are discussed. The results of this study are compared with those obtained for the Fe-Cu system, where no thermodynamic limitation on the mutual solubility of the components upon mechanical alloying was observed.

Aluminium-based Al-Fe alloys with Fe content of 2, 5, 8, 10 and I I wt. % were prepared by two techniques: rapid quenching (RQ) from the melt at the rate of 10(6) K/s and mechanical alloying (MA) of pure elements in a high-energy planetary ball mill. The structure of the alloys was examined using X-ray diffraction and Mossbauer spectroscopy. It is; shown that the crystalline structure refinement and the phase composition of the alloys essentially depend on the techniques used for the sample preparation. Phase transformations by high pressure torsion (HPT) of RQ and MA alloys were studied. The highest supersaturation of Fe in the aluminium-based solid solution can be reached using two subsequent techniques of alloy treatment: RQ and HPT. Microhardness measurements of HPT alloys show the significant stricture heterogeneity of specimens, the dependence of the microhardness on the radius of the pills was found. Phase composition and microhardness at beating were investigated. At the initial step of heating (120-150degreesC), an increase of microhardness was observed, whereas further heating results in decrease of the microhardness value.

The Fe100-xMnx alloys with an Mn content x = 6-30 at % obtained by mechanical alloying (MA) were investigated by X-ray diffraction and magnetic methods. It was established that MA results in a widening of the concentration interval of existence of the bcc (alpha) phase. Annealing of MA alloys leads to an increase in the stability of the gamma phase against the transformation into the alpha phase upon cooling. In the alloys with x less than or equal to 9 at %, a gamma --> alpha isothermal martensitic transformation was found at room temperature. The hexagonal close-packed E phase has not been observed in the alloys obtained by MA and after annealing. Room-temperature deformation of the gamma phase formed as a result of annealing leads to its transformation into the alpha and epsilon phases. The peculiarities of transformations of alloys obtained by MA are related to the low temperature of formation of the alloys upon MA and the high degree of their strain hardening.

Solid state transformations upon mechanical alloying (MA) of powder mixtures of compositions Ni45Al50Ti5, Ni50Al45Ti5, and Ni47.5Al47.5Ti5 to obtain NiAl(Ti) nickel monoaliminides were investigated by X-ray diffraction and differential scanning calorimetry. It is shown that MA occurs via the formation of supersaturated Al(Ni) and Ni(Al) solid solutions and of the Al3Ni and Ni2AlTi phases, which are revealed upon low temperature annealing after intermediate stages of MA. As follows from a comparison of the experimental lattice parameters and the ratios of the integrated intensities of the superlattice and fundamental lines with the calculated results obtained for different models of arrangement of atoms over the sublattices, the titanium atoms are located upon MA in that sublattice in which the deficit of atoms was predetermined by the composition of the initial mixture. When titanium is added over the stoichiometric composition of the initial mixture, it is preferentially distributed in the aluminum sublattice. The distribution of the titanium atoms over the sublattices of the NiAl intermetallide after MA is preserved upon heating up to 720degreesC.

Fe100-xMnx (6 less than or equal to x less than or equal to 30) and Fe100-xNiy (20 less than or equal to y less than or equal to 26) alloys (x and y given as at%) were prepared by mechanical alloying of elemental powders in a high-energy planetary ball mill. X-ray diffractometry and Mossbauer spectroscopy were used to determine structure and phase constitution of the samples. Thermo-magnetic measurements were used to determine the phase transformation temperatures. Mechanical alloying led to the formation of bcc alpha-Fe and fcc gamma-Fe based solid solutions with a high concentration of defects and fine crystallite size (10-20 nm). In the case of Fe100-xMnx samples, only the bcc phase was observed with x less than or equal to 6, while both bcc and fcc phases were present with 8 less than or equal to x less than or equal to 30. For the Fe100-yNiy system the single bcc phase was observed with y less than or equal to 20 and the two phases with 22 less than or equal to y less than or equal to 26. Only the fcc phase was observed after annealing and cooling to room temperature: for x greater than or equal to 10 in Fe100-xMnx, and for y greater than or equal to 22 in Fe100-yNiy samples. By cooling down to 77 K annealed samples of those compositions the Fe-Mn austenite resulted to be stable, whereas the Fe-Ni austenite partially transformed into martensite. Isothermal gamma --> alpha martensitic transformation was observed in Fe100-xMnx alloys for x greater than or equal to 9 at room temperature. The final phase constitution significantly depended on the annealing temperature. Partial transformation of austenite phase into bcc and/or hcp martensite could be also obtained by slight deformation.

X-ray diffraction analysis and Mossbauer spectroscopy were used to study the kinetics of phase and structural transformations occurring upon mechanical alloying of the Fe86.5Cu13.5 composition in a planetary ball mill. It was found that irrespective of milling conditions, an alpha-Fe-based solid solution is the final product of the two-stage process of mechanical alloying. At the first stage, the dissolution of Fe in the fcc lattice of copper takes place; the alpha-Fe-based solid solution is formed at the second stage of mechanical alloying. The two-stage character of the phase transformations is explained based on an earlier-suggested model that takes into account the thermodynamic driving forces of the mechanical alloying process.

Deformation mechanism of mechanical alloying (MA) process is discussed. Deformation considered as a superplastic flow-sliding of relatively hard nanocrystals along relatively soft grain boundaries (GB). In the steady state nanostructured MA materials consist of at least two phases: crystalline phase of grains and GB amorphous phase, which coexist in local equilibrium. Amorphisation of alloy can be described as an increase in the amount of GB phase during MA. At MA of immiscible systems the mutual solubility of components decreases with an increase in the mixing enthalpy. It is considered that in such systems the energy barrier for solid solution formation, which MA method allows to reach, is close to the heat of amorphous phase formation. For systems with low positive or negative enthalpies of solid solution formation, the final structure of MA materials most likely contains only one phase, which has the lowest Gibbs energy among all of possible phases.

X-ray diffraction (XRD) and differential scanning calorimetry (DSC) were used to study the structure and thermal stability of Ni-Al alloys produced by mechanical alloying. In the concentration range of 4085 at. % Ni, the following three phases were shown to form: at 40-61 at. % Ni, a B2 phase (CsCl type); at 6585 at. % Ni, a nanocrystalline solid solution Ni(Al); and at a composition of Ni62.5Al37.5, a nanocrystalline phase that could be interpreted as an ordered tetragonal phase L1(0). As a final product of mechanical alloying, singlephase structures with a minimum Gibbs energy of the competing phases are always formed, which indicates the key role of thermodynamic factor for phase formation upon mechanosynthesis.

The structure and thermal stability of Mo-doped Ni–Al intermetallic phases prepared by mechanical alloying were studied by x-ray diffraction and differential scanning calorimetry. The results demonstrate that mechanical processing of 50Ni + 45Al + 5Mo and 45Ni + 5Mo + 50Al powder mixtures yields nonstoichiometric aluminides and that a part of the Mo remains unreacted. The Mo introduced instead of Ni occupies some Ni sites in the structure of the forming aluminide, and this distribution persists during subsequent heating to 700°C. When Mo partially replaces Al, the Mo atoms also occupy some Ni sites, but subsequent heating to 700°C drives them to Al sites.