Abstract
The crystallization behaviour of metallic glasses (MGs) has been investigated since the discovery of these important functional materials in order to optimize their synthesis procedures and improve their performances. Methods including powder X-ray diffraction and transmission electron microscopy are usually combined to characterize the crystalline structure in these “amorphous” materials. Until now, these methods, however, have failed to show the crystallization of individual crystals in three dimensions. In this work, in-situ Bragg coherent X-ray diffraction imaging (BCDI) reveals the growth and the strain variation of individual crystals in the Fe-based MGs during annealing. There is preferential growth along the surface of the MG sample particles during the crystal formation and fractal structure formation around the developing crystal surfaces; there is also strain relaxation happening from the inner parts to the surfaces of the developing crystals while cooling. The work leads to propose that during the crystallization of Fe-based MGs, the growth of the individual crystals follows a two-step procedure; and at higher temperature after the first crystallization period of the Fe-based MGs, the crystallization of α-Fe could be a competitive process between the growth of α-Fe crystals and the erosion from other elements.
Highlights
The crystallization phenomenon in metallic glass (MG) has attracted tremendous attention, because of the interests in developing metallic glasses (MGs) composites with better performances [1,2], and in seeking critical insights of this complex material transformation process [3,4,5,6,7]
Synchrotron X-rays with an energy of 9 keV were selected for our Bragg coherent X-ray diffraction imaging (BCDI) measurements
In-situ BCDI experiments were performed on Fe-based MG samples during annealing in a vacuum environment with the annealing temperature in the vicinity of the crystallization transition to observe the formation of nano-crystallites
Summary
The crystallization phenomenon in metallic glass (MG) has attracted tremendous attention, because of the interests in developing MG composites with better performances [1,2], and in seeking critical insights of this complex material transformation process [3,4,5,6,7]. Some MGs have excellent elastic strain which could exceed 2% [10,11] Their lack of macroscopic plastic strain hinders the application of MGs as a general structural material. The nano-clusters or nano-scale precipitation with crystalline grains embedded in the amorphous matrix lead to an almost-constant, relatively high magnetic-entropy change which favors the Ericsson cycle [15]. Other properties such as multiple spin glass behavior may be a result of nano-scale nucleation [13]. Compared with the amorphous or crystalline materials, partially crystallized MG composites show unique mechanical and magnetic properties [16,17]. It is essential to investigate the relationship between the properties of MGs and their crystallization behaviour or structure which includes, for example, the crystal size and strain distribution within the nano-crystalline materials
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