Pure metals are made up of regular layers of atoms neatly stacked together. This means that the rows can easily slide past each other, so the metal is soft and not very useful. Normally harder materials, called alloys, are made by adding small amounts of other substances to disrupt the regular pattern and stop the rows from sliding so easily. These are used for many purposes, from weaponry to aircraft.
But what happens when many metals are mixed in roughly the same quantities? Well, this is what scientists at Ames Laboratory in Iowa are trying to determine. By mixing a number of different metals in about equal ratios, lighter and stronger alloys could be created. Scientists may be able to control many properties, like melting point, resistance to corrosion and radioactivity and even magnetic and electrical properties. These new alloys are called ‘high entropy alloys’; they get their name from the high level of disorder caused by having so many different metal nuclei present. This high entropy means that the metal won’t just rearrange into a new ordered structure, such as alternating rows of each type of metal atom, which would make the metal softer and more brittle.
There are thousands of potential alloys that could be made this way. It has been estimated that there are about 313,560 possible alloys just from using three to six out of a selection of 26 metals. In actual fact there are over 80 metallic elements, and the number predicted could be extended even more if varying proportions are used. This means there are far too many alloys to make and test, so how do we know which the useful ones are? Teams will focus on using blends elements that already have properties close to those desired in the alloy. For example, teams making aircraft engines and aeroplane frames need alloys which are light, corrosion resistant, and able to maintain their strength at high temperatures, so they focus on elements like niobium, tantalum and chromium, which already have high melting points. Scientists may also try to replicate existing structures or even use algorithms to predict which the most useful combinations will be.
In a new method, about 30 one to two centimetre tall samples can be produced in an hour. Different mixes can be used for each pillar and their properties tested to find the most useful alloys. The pillars are formed by a ring of nozzles which fire jets of powdered metal into a laser beam, so that the colliding grains collide and build up a platform.
Alloys much harder than stainless steel have already been produced, as well as materials much stronger, more ductile, more resistant to friction and less brittle than the corresponding pure metals. They are yet to be produced or used in industry, but could be useful for aircraft, low temperature pipelines and cryogenic vessels.
XiaoZhi Lim, Nature, 18th May 2016, Mixed-up metals make for stronger, tougher, stretcher alloys, http://www.nature.com/news/mixed-up-metals-make-for-stronger-tougher-stretchier-alloys-1.19942 (accessed 24/05/2016), Nature, 533, 306–307 (19 May 2016)