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Posted: June 25, 2010

Transformation-mediated ductility in metallic glasses

(Nanowerk News) Metallic glasses are -just as other types of glasses- brittle materials. This property sets a limit to the possibilities of their technical use. Researchers have now developed a mechanism that enables metallic glasses to become ductile under tensile loading ("Transformation-mediated ductility in CuZr-based bulk metallic glasses").
Metallic glasses are alloys that do not consist of regularly structured crystals like other metals but of an irregular atomic structure comparable to glasses or liquids. This highly uncommon structure causes a unique combination of physical properties. Metallic glasses are in general harder, firmer and more resistant to corrosion than ordinary metals.
However, forging and rolling require a greater degree of ductility which is characteristic for most metals but not for metallic glasses. Metallic glasses are brittle and breakable like window glass which limits possible technical application for this type of material.
A group of researchers has now developed a mechanism of deformation that avoids the brittleness of metallic glasses under tensile loading and makes them ductile.
For this purpose, the researchers have examined copper-zirconium-alloys that can be produced as either crystalline alloys or metallic glasses. In their crystalline condition, these alloys have a shape memory which means that they can "remember" a former shaping and assume this even after a deformation. However, this property also seems to affect the ductility in the glass state. Under mechanical stress, shape memory crystals with the size of a nanometre, that have a strong inclination to form so-called "forming twins" for their part, separate from the glass. The chemical composition of nano crystals is not different from the chemical composition of glass. For that reason only slight atomic rearrangements are necessary to crystallise the glass. The formation of "twins" is part of the shape memory effect that occurs preferentially via shear deformation on small length scales.
Both processes, the formation of crystals and the formation of twins need energy that derives from the energy used for deformation. This can delay the development of micro fissure that causes the brittle failure of the material. As a result, a macroscopic plastic stretching arises together with solidification.
A phase transformation under mechanical stress was also used for several ceramics to enhance ductility. Even though the structure and the atomic bonding of metallic glasses and ceramics are very different, that approach seems to be universal in application.
Source: Leibniz Institute for Solid State and Materials
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