On the onset of nano-ordered phase distributions in high-entropy alloys

This contribution to the Festschrift concentrates on 'nano-s', a topic which is central in Ilya Ovid'ko scientific work in the field of materials physics. In particular, we concentrate on a rather new alloying design concept of so-called High-Entropy Alloys (HEA) that was proposed by Yeh et al. in Adv Eng Mater. 2004;6:299-303. The original idea is that a negative Gibbs free energy of mixing can be achieved, even when the enthalpy of mixing is positive or close to zero, through an increasing mixing entropy contribution with increasing temperature. From theoretical analyses based on the density functional theory, no ordered nanostructures in the Al-Co-Cr-Fe-Ni high-entropy alloys were predicted. However, in contrast ordered fcc nanoparticles were discovered by us experimentally in the fcc matrix of Al-Co-Cr-Fe-Ni as well as in other systems. In the present paper, we also discovered a novel phenomenon, i.e., the onset of the phase distribution of the bcc phase in the fcc matrix of Al-0.3 CoCrFeNi. This paper concentrates on the initial stage of locally ordered distribution of bcc phase and reports on the crystallographic orientation relationship between them as (1-1 0)(bcc) //(200)(fcc), [001](bcc) //[001](fcc)