Abstract
A survey of the cluster formation tendency and mechanism in transition metal-based glassy alloys is made with an emphasis on their manifestation in various physical properties. The cluster formation is partially inherited from the supercooling of the melt. However, it also develops due to the interaction between dissolved hydrogen and the frozen glassy structure. The glassy state as “cluster assembly” is regarded as a structural background for the interpretation of several anomalous concentration dependences of thermal and magnetic properties in these glasses. We will focus on the manifestation of alloying effects, the relation between irreversible and reversible structural relaxations both in the high, and low temperature range (observed near to the glass transition or after low temperature storage). The development of the cluster assembly is the consequence of the co-existence of various bonding types between the alloy components. These are brought together in the melt, ensuring sufficient glass-forming ability. The nucleation mechanism of the amorphous-nanocrystalline transformation is also explained as a cluster phenomenon, which significantly contributes to the evolution of magnetic ultra-softness in FINEMET-type alloys. Finally, the role of the quenched-in cluster structure in the mechanism of reversible and irreversible H-absorption is discussed. Irreversible H-induced structural rearrangements can appear as microphase separation in multicomponent systems, governed by the affinity difference between the metallic components and the absorbed hydrogen. This kind of H-induced reordering is responsible for the “volume activation” of amorphous H-storage alloys and it also causes the gradual breakdown of storage capacity during cyclic absorption–desorption steps. This article mainly focuses on the cluster phenomena in Fe-based glasses because of its unique combination of high mechanical strength, strong corrosion resistance, good thermal stability and excellent magnetic properties.
Highlights
In materials science and chemistry, the term “cluster” means an assembly of a few atoms [1], being in bonding relation during the time period of observation.This time period can be the measurement itself, being directed to the study of a given attribute or phenomenon
We focus rather on the metallurgical background of cluster formation as well as the experimental manifestation of the possible existence of clusters in various physical properties
Other types of clusters are the consequences of H dissolution in transition metal (TM)-based glasses
Summary
In materials science and chemistry, the term “cluster” means an assembly of a few (from several tens to few hundreds) atoms [1], being in bonding relation during the time period of observation. A lot of evidence exists on the size dependence of even such basic properties as the ionization energy [5], or the melting point [6] (both properties are coupled with the average bond strength between the constituent atoms) This difference provides a wide range of possibilities for property tailoring in several fields of materials science [7,8]. This concept is supported by several high resolution electron diffraction experiments [13,14,15,16] Such structural units are interpreted as “medium-range order”, suggesting that certain types of atomic ordering may extend far beyond the first neighbor atomic distance in the liquid-quenched glassy state. An attempt is made for the interpretation of extraordinary properties and phenomena in a few transition metal-based glasses, both in as-quenched state, and in their response to special heat treatments including sample storage at low temperature. Hydrogen-induced phase separation in the amorphous state will be discussed as a cluster phenomenon
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