Physico-Chemical Nanomaterials Science 

Formation of nanocrystalline structure in high-entropy alloys 
Sklyrchuk V., Plevachuk Yu.O., Dufanets M., Kulyk Yu.
Department of Metal Physics, Faculty of Physics, Ivan Franko National University of Lviv, Kyrylo & Mephodiy str. 8, Lviv-79005, Ukraine.
E-mail: sklyarchuk@ukr.net

Nanocrystalline high-entropy alloys (HEA) containing up to six metallic components are among the most promising materials, characterized by a unique combination of mechanical properties such as high hardness, resistance to temperature disordering and an increased level of strength characteristics at high temperatures. Among HEA special attention is attracted by the multicomponent Al- and Cu- based systems with transition elements (Fe, Ni, Co, Cr, etc.), in which formation of a mixture of solid solutions based on bcc and fcc structures with a high level of strength and plastic characteristics takes place. 
In this work, X-ray diffractometry, microstructural analysis and microhardness measurements have been carried out to study phase formations in the equiatomic high entropy alloys of the AlCoCuFeNiCr system of different chemical compositions. In accordance with the thermodynamic and structural criteria and based on the X-ray diffraction analysis, it was shown that the structure of the equivalent atomic high-entropy alloys  AlCoCuFe and AlCoCuFeNi has two phases and contains solid solutions with bcc (type B2) and fcc-lattice (type A1) with nanosized particles of about 20 and 80 nm. According to the microstructural analysis, the primary bcc phase reveals dendritic morphology, enriched with transitional elements, while the copper-enriched fcc-solid solution is released in the interdendritic space. Phase analysis of the AlCoNiCr alloy revealed formation of nanodispersed particles of metallic σ-phase enriched with Co and Cr, along with an ordered solution of the bcc phase, which is in agreement with the calculations of the enthalpy of mixing. An increase of the alloys microhardness contributes to both increasing the volume content of a solid solution with the bcc lattice and the separation of the nanodispersed intermetallic σ-phase. Correlation of microhardness of alloys with volume fractions of phase components and their thermodynamic characteristics has been revealed.