Review of alloys developed using the entropy approach

OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 23 No. 2 2021 The analysis of the structure of heat treated and deformed alloys consisting of refractory components (TiZrTaHf, TiZrTaNb, TiZrHfNbV) indicates that, like many other HEAs, they can be considered as meta- stable materials [90-93]. Rogachev concluded that most high-entropy alloys consist of several phases, the number of which increases as a result of annealing [17]. CoFeNiMnCu [94], AlNbTiVZrx [95], CoCrFeN- iMnTi0.1 [96], ZrTiHfCuNiFe [97] and some other alloys are classified as stable or limited stable HEAs. At the same time, it should be kept in mind that the assessment of the degree of the HEAs stability, in many cases, is a methodically complex task. This is especially relevant, for example, in the case of observation of particularly fine precipitations of phases, the volume fraction of which is small. It is likely that during characterization of structure, some of these phases may be overlooked. It should be noted that the concept of “metastability”, which contradicts the initial concept of stable high-entropy alloys, currently is not considered as a fundamental disadvantage of real multicomponent systems. The positive effect caused by the precipitation of several phases may lead to effect of dispersion hardening of materials [17]. At the same time, in order to avoid embrittlement of HEAs it is suggested to prevent the formation of s -phases. The analysis of structural transformations HEAs presented in [17] allowed us to conclude that it is dif- ficult to interpret the phenomena associated with the stability of multicomponent systems. It is believed that the relationship between the stability of the HEAs exclusively with the level of configurational entropy is oversimplified. A more reasonable approach to solving this problem is associated with the development of semi-empirical criteria for the stability of HEAs, the quantum mechanical calculations [98-100], and the thermodynamic analysis of multicomponent systems. The severely deformed alloys, as well as alloys with a crushed grain structure are expectedly less stable [101]. The problems solved by Ivchenko in his thesis were related to the structure and properties of the AlCrFeCoNiCu high-entropy alloy [102]. Of particular interest were the experimental data on the structural-phase transformations occurring in the alloys fabricated by melt spinning and splatting, and on the effect of severe plastic deformation and heat treatment on the structure and phase composition of the AlCrFeCoNiCu alloy. When cooled at a rate of 10 K/s, a complex dendritic structure is formed in the AlCrFeCoNiCu alloy, each of the phases of which consists of six components. The nanoscale phases isolated within the dendrites and in the inter-dendritic space are uniformly distributed over the volume of the ingot. They are character- ized by equiaxed and lamellar morphology, and have ordered (structural types B2 and L1 2 ) and disordered (A1, A2) structures [102]. Rapid solidification by the melt splatting (~10 6 K/s), as well as by melt spinning (~10 5 K/s), leads to the formation of an ultra-fine-grained (560 nm) dendritic structure, which consists of nanoscale six-component phases. One of the features of six-component AlCrFeCoNiCu alloys after rapid quenching and severe plastic deformation is the formation of local nanoscale precipitations. The corresponding rearrangement of the alloy components results in dimensional-spatial periodicity of the elemental and phase composition in the volume of the ingot [102]. Concentration fluctuations in the form of clusters in the size range from one to several tens of nanometers were observed in the AlCrFeCoNiCu alloy by 3D-AP tomography. Dislocation hardening mechanisms of high-entropy alloys are much less studied than those in classical alloyed steels and alloys. Nevertheless, many works pay special attention to this problem. The solid solution hardening mechanism, hardening by grain boundaries, dislocation pileups, and dispersion hardening are considered as the main mechanisms which increase the strength of HEAs. One of the problems discussed in [21] was related to the study of the structure and properties of the CoCrFeMnNi alloy alloyed with carbon and aluminum. It was found that the introduction of 0.7 at. % C and 3.4 at. %Al is accompanied by an increase in stacking fault energy and a suppression of the twinning process at the initial stages of deformation. The higher strength of CoCrFeMnN(Al,C) alloy after reduc- tion to 80% compared to the equiatomic five-component CoCrFeMnNi alloy is due to the large contribu - tion of solid-solution hardening caused by carbon and aluminum. It is established that grain boundaries have the most significant effect to hardening of the CoCrFeMnN(Al,C) alloy annealed after cold rolling ( e = 80%). The effect on strengthening of nanoscale carbides formed during annealing at 700 - 900 °C of a cold-rolled alloy is comparable with that of the grain boundaries.