Alloy’s hardness versus crystal lattice distortion.
Source: BAM
The strength of an alloy depends on several factors such as its composition, its processing and ins internal microstructure. The simplest composition, i.e. a single-crystalline, single-element one, is not very strong. Strength can be introduced by adding one or more additional elements that mix with the first one.
In this paper there is no “first one” element but instead a carefully chosen mix of Al, Co, Cr, Cu, Fe and Ni in close to equimolar composition, and a group of five sub-alloys of which one element has been retracted (with the exception of Ni). This gives us the opportunity of studying the influence of each of the elements on the materials’ strength, which (in this paper) is highlighted via the hardness.
Even before this study it has been known that Ni is the element that serves as a guidance for the material’s homogeneity and crystallographic structure and can thus not be removed from the composition.
A more surprising observation has been made for the elements Fe and Co. These two elements, especially Co, seem to “glue” the other elements together and their removal drives the remaining elements towards the formation of a second phase, thus destroying the homogeneity via a phase separation. While this has already been assumed in former works, it could now be confirmed for the Co and shown for Fe for the first time. Once Fe or Co are removed and the second phase is formed, the material’s hardness increases significantly but the originally planned comparison in the homogeneous state is no longer possible.
The removal of Cr and Cu does not have a significant influence on the alloy’s hardness. These two elements are the least interested in neighbouring with other elements than themselves, so their removal does not influence the rest of the alloy in terms of hardness.
Al, being the only element with a very different arrangement of electrons, does have a great influence on the alloy’s behaviour. Its presence is a disturbance to the other five elements, leading to a re-arrangement of the atoms in the crystal lattice which increases the hardness of the alloy. In the given composition Al8Co17Cr17Cu8Fe17Ni33 it can thus be concluded that Al is the strongest alloy strengthener.
How Atomic Bonding Plays the Hardness Behavior in the Al–Co–Cr–Cu–Fe–Ni High Entropy Family
Andrea Fantin, G. O. Lepore, M. Widom, S. Kasatikov, Anna Maria Manzoni
published in Small Science, 2023, essay number: 2300225, pages 1 bis 12
BAM Department Materials Engineering
BAM Division Materialography, Fractography and Ageing of Engineered Materials