Microstructural characteristics and mechanical properties of Si-B alloys for functional and structural ultra-high temperature applications

Abstract

Silicon boron alloys have been recognized as important materials for e.g. a direct usage in ultra-high temperature latent heat thermal energy storage systems or as a batch materials for processing boron enhanced silicide-based composites. In this work, we put new experimentally driven insights on a structure of selected Si-B binary alloys. For this reason, four Si-xB alloys (x = 2.5; 8, 13.5; 86 (at%)) were fabricated from pure elements by using a crucible-less electric arc melting technique. Nominal compositions of the alloys were selected in accordance to Si-B phase diagram as hypoeutectic, eutectic, hypereutectic and corresponding to the stoichiometric composition of SiB6 intermetallic phase, respectively. The fabricated Si-xB alloys were subjected to a detailed structural characterization. Additionally, a hardness, elastic moduli and fracture toughness of each phase constituent were examined through a micro-indentation technique. A co-existence of various silicon borides, namely SiB4, SiB6 and SiBn, has been experimentally documented. Obtained results allows receiving a description of each alloy in terms of microstructure, crystal structure and mechanical properties of involved phases. It was found that mechanical properties (both microhardness and Young modulus) increases with raising boron content in recognized Si-B phases. The fracture toughness of silicon borides assessed through indentation experiments was found to be in the rage of 2.13–3.29 MPa·m1/2.