Almost every alloy in use today follows the same recipe: take one main metal and add small amounts of others. Steel is mostly iron. Aircraft aluminium is mostly aluminium. Superalloys are mostly nickel.
High-entropy alloys break that recipe. Instead of one base metal, they combine five or more principal elements, each usually between 5 and 35 atomic percent. The idea was first published in 2004 and has since grown into one of the most active areas of metallurgy.
Where the name comes from
When many elements share a crystal lattice in similar amounts, there are an enormous number of ways the atoms can be arranged. That raises the alloy's configurational entropy. In some systems, this extra entropy helps stabilise a simple solid solution instead of the brittle intermetallic compounds you might expect from mixing so many elements.
A common working definition is a mixing entropy of at least 1.5R, where R is the gas constant. An equal mix of five elements reaches about 1.61R.
Why they behave differently
Researchers usually describe four effects. High entropy can favour simple phases. Atoms of different sizes distort the lattice, which makes it harder for dislocations to move and can raise strength. Diffusion tends to be slower, which can help stability at high temperature. And the combined behaviour of the mix can exceed what any single element would suggest, often called the cocktail effect.
Why they're hard to design
The design space is vast. With just five fixed elements, stepping each by 1% between 5 and 35 at.% gives 553,401 distinct compositions. No lab can make them all. That's why computational screening, the approach Trumet uses, matters: it narrows millions of possibilities down to the few worth making and testing.