Impact of Severe Deformation by HPT at Room Temperature on the Relaxation of the Glassy and the Supercooled Liquid States of PdNiP
Posted: 10 Dec 2019 Last revised: 24 Aug 2020
Room-temperature high-pressure torsion (HPT) of a Pd40Ni40P20 bulk metallic glass induces an exothermic heat release prior to the crystallization peak in the supercooled liquid state as it was found by differential scanning calorimetry. Subsequent annealing treatments below the glass transition temperature give rise to the appearance of an additional, glass transition-like endothermic peak in the supercooled liquid. In order to explore and identify the origin of these peaks, all relevant glass states were further investigated and compared by means of X-ray diffraction, low-temperature heat capacity measurements and transmission electron microscopy, which allowed an unambiguous description of the underlying structural transitions. Fluctuation electron microscopy revealed unique medium range order (MRO) characteristics of the individual glassy states. The calorimetric signals are traced back to the modifications of the short-range order including the MRO on an unprecedentedly detailed level. It is concluded that the exothermic peak in the supercooled liquid is attributed to the HPT-induced “crystal-like” nucleation sites that causes reordering of the MRO towards the global thermodynamic equilibrium which is probably accompanied with growth of pre-nucleation sites within the supercooled liquid. In terms of a potential energy landscape, each configurational metabasin forced by an external action is proposed to be identified by its unique MRO characteristics. These finding offer new perspectives for an MRO engineering concept in order to tailor and effectively control the properties of bulk metallic glasses.
Keywords: Bulk metallic glass (BMG); High-pressure torsion (HPT); Differential scanning calorimetry (DSC); Fluctuation electron microscopy (FEM); Medium range order (MRO)
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