High-Entropy Alloys ? Microstructures and Properties

There are relatively few revolutions in the venerable and rather staid field of metallurgy. One can count among them the advent of metallic glasses, of superplastic metals, of memory-alloys and - on the purely theoretical side - of quasicrystals. The latest revolution involves the relatively staid topic of alloy formulation, but is all the more startling because the resultant materials break every long-cherished rule of alloy design. In particular, the famous empirical rules of Hume-Rothery are completely ignored. That is, in the archetypal high-entropy alloy, five metals are alloyed together in equal proportions regardless of atomic-size difference, valence or crystal structure. Commonsense would tell any experienced metallurgist that that could result only in a uselessly brittle mass of intermetallic compounds. But in a truly paradigm-shifting manner, Professor J.W.Yeh of Taiwan correctly predicted that a high configurational entropy could suppress the appearance of detrimental intermetallic compounds and lead to simple familiar microstructures having very useful properties. High-Entropy Alloys can exhibit, for instance, astounding hardness and strength - often associated with a surprising ductility - and also have a very good corrosion resistance; often exceeding that of stainless steels. They have been the subject of exponentially increasing academic research for some two decades, and the time is not far off when commercial alloys will be marketed. The present book is therefore not aimed primarily at academic researchers already in the field, but rather at end-users in industry to whom this new class of alloy will undoubtedly be of great importance in the near future. It summarises the microstructures and properties of all of the high-entropy alloys which have been studied, and especially those of equimolar AlCoCrCuFeNi; surely the front-runner in the race to commercial success.