Toughest material ever is an alloy of chromium, cobalt and n

An alloy of chromium, cobalt and nickel (CrCoNi) is the toughest material ever tested, and its characteristics could make it useful in the construction of aircraft or spacecraft.

CrCoNi is a subset of a class of metals called high entropy alloys (HEAs). All the alloys in use today contain a high proportion of one element with lower amounts of additional elements added, but HEAs are made of an equal mix of each constituent element.

These balanced atomic recipes appear to bestow some of these materials with an extraordinarily high combination of strength and ductility when stressed, which together make up what is termed "toughness." HEAs have been a hot area of research since they were first developed about 20 years ago, but the technology required to push the materials to their limits in extreme tests was not available until recently.

A CrCoNi single crystal was grown by a Bridgman technique using a Ni-base superalloy seed. (...)

A Bridgman furnace of type KZV-A40-400/161G-V from Gero GmbH (Neuhausen, Germany) was used to grow a cylindrical single-crystalline specimen (diameter: 12 mm, height: 85 mm) of the equiatomic Cr33.3Co33.3Ni33.3 (composition in at.%) MEA, Figs. 1a,b. A <100>-oriented single crystal of CMSX-4 type superalloy (16 mm in height and 11.6 mm in diameter) was used for seeding as both alloys have the same structure type and similar lattice parameters [16]. Details on the chemical and microstructural characterization of this superalloy can be found in the literature [17]. A cold-worked polycrystalline feedstock was vacuum induction melted starting with pure metals, homogenized, processed by rotary swaging and finally machined on a lathe to fit the dimensions of the Bridgman crucible. Details regarding feedstock processing can be found elsewhere [18]. Melting was carried out in an Al2O3 crucible under vacuum (4.5 × 10-4 mbar). The Bridgman furnace was heated to 1843 K above the liquidus temperature of CrCoNi (1690 K) [2], and hold at this temperature for ∼80 min. For solidification, a withdrawal rate of 180 mm/h was chosen. Further information concerning the Bridgman melting procedure can be found in Ref. [19].

Porous materials, such as aerogels, facilitate the fabrication of materials with density less than that of air. However, their apparent density increases because of the air inside the materials, and therefore, they cannot levitate in air under normal conditions.