High-Entropy-Alloys

New Materials, Exceptional Properties

High-Entropy-Alloys are new materials that must be studied.

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This is a new research subject.

The flurry of activity caused by the High-Entropy Alloys  initial successes, were briefly introduced recently.

Look in section (7) of Issue 140 of Practical Welding Letter for April 2015.

Click on PWL#140.

It is essential that readers grasp a somewhat more detailed description of High-Entropy-Alloys. It is simply nothing less than a most influential Revolution in basic material science affecting most metallurgical research centers all over the world.

It is not something growing naturally within the traditional field of metal sciences, where small improvements brought substantial advancements during a time frame of more than a century.

The whole new idea of High-Entropy-Alloys sprung in capable minds and commanded immediately great attention as it was  showing unexpected potential to generate unexpected scores of new alloys from a limited number of available metals, capable of displaying most sought-after properties in very different physical fields.

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In a little longer than a decade, a mighty momentum was reached and substantial development efforts are currently being invested, limited only by availability of sufficient research grants, to bring about new materials with better properties for demanding applications in the least possible time.

The results confirmed up to the present time will change in practical ways the industry by making available a rich variety of High-Entropy-Alloys new metallic materials, unknown till now, whose characteristics and properties will permit the realization of constructions and machines capable of unheard of performance.

Most important centers of scientific knowledge like NASA are currently driving the developments and setting priorities for adequate solution of challenges still limiting operational capabilities.

High-Entropy-Alloys

High-Entropy-Alloys (HEA) are made by using five or more elemental metals in nearly equal proportions by melting them together to obtain stable random solid solutions.

High mixing entropy can enhance the formation of solution-type phases, and in general leads to simpler microstructure.

The name derives by the observation that their exceptionally high mixing entropy (a thermo-dynamical concept) favors stability, which means the capability to keep their microstructures fixed, without separating in different phases by ordering or segregation, as happens in traditional alloys.

An Article on Advances in High Entropy Alloys  was published (2) in Issue 158 of Practical Welding Letter for October 2016. 
Click on PWL#158

Properties

Components displaying high oxidation and corrosion resistance at extreme temperatures are sought for applications in electrical, magnetic and high-temperature applications.

Other new materials were studied for applications where elevate fracture toughness is required at cryogenic temperatures.

Multicomponents

The striking difference from traditional alloys is therefore the use of multicomponents for building these new materials.

A major benefit of HEAs is that they stimulate the study of compositionally complex alloys not previously considered.

Countless Promising Compositions

Another major benefit is that HEAs provide a way to design and produce a vast number of new alloys.

Just by playing with possible combinations, each with at least five metal elements as components, a huge number of compositions becomes available.

Most of them have great probability to bring about useful alloys, giving substantial potential for discoveries of important scientific and practical benefit.

Even by limiting the number of new alloys to study, only to those made with completely miscible metals, it has been proved nevertheless that enormous numbers of systems are possible, wherefrom only the most useful will be pursued.

Phases

Current HEA studies favor single-phase, disordered solid solution alloys.

It is believed that strengthening by solid solution is more effective in HEAs than in conventional alloys.

Many HEA microstructures have been produced, including single phase, multiple phase, nanocrystalline and even amorphous alloys.

Expectations

Several new alloys have been reported with such catching titles as "light as aluminum, strong as titanium", bringing the promise of future developments by painstaking work to characterize the properties and evaluate usability and economics.

One study affirms that:
"The field of high entropy alloys has exploded in the first 10 years. Vast opportunities for new compositions and microstructures are offered by this idea, but current efforts have become surprisingly focused on a narrow set of systems and on the search for single phase solid solution alloys.

This perspective outlines the challenge to re-engage the full range of compositional and microstructural complexity in the search for new structural metals"

Other studies propose special approaches to systematically screen and evaluate the vast composition space offered by HEA families.

Conclusions

From what is presently known about HEAs one should understand that in the coming years important development and progress will be made, as the promised benefits become familiar and the economics of production are better understood.

All the engineering fields at the forefront of human achievement will require detailed rethinking to find out which usage of new High-Entropy-Alloys will allow improved performance.

A new exciting era is opening for the delight and commitment of dedicated professionals who will have to study in depth the new knowledge as it becomes available, making the effort not to be left behind.

Interested readers are referred to the Mid April 2015 - PWL#140B - Bulletin_107 presenting Resources on High Entropy Alloys.


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