Structural and magnetic properties of metastable phases and undercooled liquids in Fe83B17

Abstract

<p>Containerless processing techniques allow the exploration of several largely unexplored realms of materials science and physics. In particular, the formation of transient metastable phases is typically challenging to study \textit{in-situ}, and so information on high-temperature solidification pathways are often incomplete at best. Similarly, the structure and properties of the undercooled liquid regime are just now being explored. The wide variety of optical instrumentation and scattering techniques available for use with levitation based systems allow novel, multi-faceted approaches to \textit{in-situ} materials characterization.</p> <p>Metastable phase formation in the Fe$_{83}$B$_{17}$ system was investigated through a wide variety of techniques using multiple electrostatic levitators. Two primary phase selection pathways were observed via \textit{in-situ} x-ray diffraction studies. Samples either solidified into the equilibrium Fe$_2$B + $fcc$-Fe phase or formed a coherently intergrown metastable Fe$_{23}$B$_6$ + $fcc$-Fe structure, which either transformed during the solidification plateau to the equilibrium phase or persisted through cooling down to room temperature. The metastable solidification featured a kinetically suppressed allotropic transformation, as well as the irreversible precipitation of a primitive tetragonal Fe$_3$B structure at low temperatures.</p> <p>Both solidification pathways were probed with the use of the newly developed ISU-ESL tunnel diode oscillator to observe magnetic transitions through the dynamic susceptibility. In addition to observing the ferromagnetic transition of $bcc$-Fe, the ferromagnetic transition temperature of the Fe$_3$B phase was identified to be 795 K. Fe$_2$B was seen to experience a ferromagnetic transition at 1015 K, which appeared to be characteristic of local moment magnetism. A new transition temperature of 850 K was established for the metastable Fe$_{23}$B$_6$ + $fcc$-Fe structure.</p> <p>The newly developed Neutron Electrostatic Levitator, in conjunction with x-ray scattering results from the Beamline Electrostatic Levitator, was used in combination to probe the liquid structure of Fe$_{83}$B$_{17}$ and compared to that of Fe$_{83}$C$_{17}$. Reverse monte carlo simulations were carried out to model the structure of the liquid, which was then characterized using Voronoi tesselation and Honeycutt-Andersen indexing. In doing so, qualitative evidence for a greater degree of similarity to the Fe$_{23}$B$_6$ structure in the Fe-B liquid was demonstrated and used to construct an argument for the comparative rarity of observation of the Fe$_{23}$C$_6$ in the Fe-C binary.</p>