Strong silicon oxide inclusions in iron

Oxide inclusions are ubiquitous in steel and are known to affect, generally in a negative sense, the mechanical performance of steel products. Micromechanical properties of those phases are therefore important, yet they have remained largely unexplored. We present strength measurements performed on individual silicon oxide inclusions in iron. The inclusions are produced in laboratory-fabricated samples by silicon deoxidation of high-purity iron melts containing dissolved oxygen. Spherical silica inclusions of diameter ∼3 µm thus produced are notched using ion milling and then tested in bending for their strength, by loading under displacement control in-situ within a scanning electron microscope. Results show that silicon oxide inclusions thus precipitated within iron are amorphous in structure, exhibit a smooth outer surface, and have fracture strains in the range from 8 to 17 %, corresponding to fracture stress levels on the order of 10 GPa. The strength of silicon oxide inclusions precipitated in iron can hence approach the highest values so far measured in dry silica. The presence within iron and its alloys of precipitated silica thus need not be deleterious, and might perhaps even be exploited to produce novel high-strength materials.