Researchers conduct the first observation of a reentrant glass transition in metallic glasses using SANS
Metallic glass (MG), also known as amorphous alloy, has both the advantages of metal and glass. Thus it has been an interesting topic to study around the world for decades.
Polyamorphs are often observed in amorphous matters, and a representative example is the reentrant glass transition in colloid systems. For metallic amorphous alloys, however, the cases reported so far are limited to MG that undergo electronic transitions under gigapascal applied pressure, or the presence of two liquids at the same composition.
In this study researchers conduct the first observation of a reentrant glass transition in MG. This unusual reentrant glass transition transforms an MG from its as-quenched state (Glass I) to an ultrastable state (Glass II), mediated by the supercooled liquid of Glass I. Specifically, upon heating to above its glass transition temperature under ambient pressure, Glass I first transitions into its supercooled liquid, which then transforms into a new Glass II, accompanied by an exothermic peak in calorimetric scan, together with a precipitous drop in volume, electrical resistance and specific heat, as well as clear evidence of local structural ordering on the short-to-medium-range scale revealed via in-situ synchrotron X-ray scattering. Atomistic simulations indicate enhanced ordering of locally favored motifs to establish correlations in the medium range that resemble those in equilibrium crystalline compounds. The resulting lower-energy Glass II has its own glass transition temperature higher than that of Glass I by as much as 50 degrees.
As such, the present reentrant glass transition achieves an unconventional ultrastable MG and retains it to ambient conditions. Therefore, these findings not only expand our understanding of the states of amorphous metals, but also provide a new approach to access MG materials with interesting properties by transforming their atomic packing structures. Specifically, corresponding to its ultrastable state deep inside a PEL megabasin, as manifested by its drastically elevated glass transition temperature and atomic order as well as lower enthalpy, the reentrant MG offers a set of properties different from the conventional glass, such as higher density, electrical conductivity, hardness, and elastic modulus.
The full publication can be found here at https://www.sciencedirect.com/science/article/pii/S1369702119307655