Description
The whole is sometimes more than the sum of its parts. Certainly it is true for mesoscopic materials, when the assembly of particles acquires properties which are not the intrinsic ones of the constituent particles. An illustrative example is represented by the development of novel high-entropy alloys (HEAs) as novel bulk and coating materials, as well as their nitride and carbide coatings.
The conventional alloys are based on one principal host element which acts as a matrix, minor elements being added in order to improve their properties. Breaking away from the traditional alloy design concepts, since 2004 an alternative approach to alloy design, synthesis and processing has been proposed, based on at least five principal elements in near-equiatomic ratios, each at 5 – 35 at.%. These alloy systems have been called “high-entropy alloys”, because their configurational entropy, in the range 1.6 – 2.2 R (R - universal gas constant), exceeds that of an ordinary alloy (1 – 1.1 R). Apart from other "traditional" alloys, the inter-metallic compounds based on Fe-Al, Ni-Al and Ti-Al binary systems, have been found to possess high specific strengths and thermal resistance, but their brittleness was a severe disadvantage, rendering their application limited.
In hard coatings development, the pragmatic interest was linked with the increase of the speed and of the working temperature of the coated tools, widely used in the industry. As a result special attention was devoted to the behavior of the new coatings at elevated temperatures, and their tendency to phase segregation at these high temperatures. It is worth to note that roughly three decades ago, most materials scientists appreciated that above 1000 C even the best metal alloys turn into taffy and become useless for load-bearing applications, so that attention turned to another class of materials - the ceramics. Although there has been undeniable progress, the wide use of ceramics in the aerospace, automotive and chemical industries remains elusive, as they tend to be brittle.
A step further seems to be the synthesis of HEAs alloys; as a result of the combined effects of high mixing entropy, severe lattice distortion (due to the different atomic sizes of the elements) and sluggish diffusion (low efficiency in kinetics), these alloys easily yield the formation of simple and stable solid-solutions phases with FCC or BCC structures, rather than binary or ternary inter-metallic compounds, which are usually associated with the brittleness of the material. As a principle, one may design the alloys to exploit the merits of each element thus generating the so-called “cocktail effect”. The HEAs investigated so far exhibit remarkable properties, such as very fine structures with uniformly distributed nanoscale precipitates and even amorphous phases, high hardness even after annealing (up to 850 HV), high thermal stability, hydrophobicity, superelasticity, superior resistance to wear, corrosion and oxidation, high stiffness, strength and toughness, and good fatigue resistance in high-temperature water environments.
The purpose of this project is a fundamental understanding of the HEA-C phases in a close connection with a possible implementation of the material in industrial applications. Our approach consists in choosing different chemistry for HEA-C systems considering, e.g., phase stabilities, growth conditions, and different elements’ influence on mechanical and tribological properties, thermal and corrosion stabilities.