Multi-Scale microscopy of Reactive sintered boride (RSB) neutron shielding materials

Abstract

• Porosity in RSB materials result of more outgassing during sintering than for cWCs. • Iron tungsten borides can form a cohesive network of bodies 10 - 100 µm diameter. • Cr behaves chemically as Fe with Cr segregating to any Fe and W-containing phase. • Cohesive, tractable RSB materials do not form when (B + C) at% < 40% . • Two distinct Fe-rich phases: A (Fe,Cr)(B,C) alloy and a Cr-depleted Fe phase. Protecting superconducting magnets from neutron irradiation is critically important when demonstrating the utility of spherical tokamaks. Reactive Sintered Borides (RSBs) are promising radiation-dense materials and excellent attenuators of slow (< 10 keV) neutrons. No experimental radiation data yet exists on RSBs, hence the need to establish a baseline of RSB microstructure prior to studies on radiation response and aging of RSBs in an active fusion environment. This work investigates the structure and composition of RSB materials over 5 orders of magnitude. SEM, TEM-EDX, atom probe tomography (APT) and XRD were evaluated the microstructure of a selection of RSB compositions. Sintered RSBs were observed to be dominated by FeWB/FeW 2 B 2 bodies, mixed tungsten borides and WC as the key hard phases present. TEM and SEM detected a complex Fe-rich alloy with near-pure Fe interstitial phases. Key findings from this work are that (i) carbon balance is as significant as boron content when considering microstructure and phase presence;(ii) FeWB/FeW 2 B 2 growth is highly temperature dependant and (iii) dense, coherent RSB formation is contingent on the total boron and carbon atomic percentage (B + C)at% is 40% < x ≤ 50%.