On the design of alloys and synthesis for composite steels

The design of metallic lightweight structural materials requires the improvement of the key parameters strength, stiffness, ductility and density, which are typically mutually exclusive. Composite steels in contrast allow by the blend of tough and ductile metallic matrices with stiff and low density particles the increase of materials’ stiffness while reducing its density. However, the assessment of suitable alloys and the possibility of controlled liquid metallurgy synthesis of these relatively novel materials is complex and often not well investigated. The present work focusses on research of steel composites regarding (i) the design of alloys and compositions and (ii) the in-situ liquid metallurgy synthesis methods to optimise performance and production costs of composite steels. The alloy design of composite steels comprised an extensive literature review to determine potential particles based on their intrinsic properties. As the intrinsic properties of particles alone are not sufficient to determine their suitability and impact on the composite steels, a bulk quasi-combinatorial screening was carried out to determine the possibility of in-situ synthesis of different borides within steel matrices and their effects on composites’ performance. The Fe – B – Ti system seemed to be especially attractive due to the outstanding performance obtained for the resulting Fe – TiB2 composites. Therefore this system was chosen to study the effect of Mn and C alloying on particles and matrix. Alloying of Mn to Fe – B – Ti melts led to a vast array of matrix microstructures and properties, while C strongly interfered with Ti and resulted in the formation of TiC particles besides TiB2 particles within the steel matrices. By adapting the liquid metallurgy synthesis techniques, steel composites’ performance and cost-efficiency was improved. Here, coarse and sharp edged particles formed in of Fe – B based composites were refined by remelting of the alloys followed by subsequent rapid solidification utilising laser remelting or the spray compaction technique. The cost-efficient synthesis of Fe – TiB2 based composites was realised by the reduction of Ti-oxide filled wires in liquid melts to replace costly Ti. A lab-scale continuous caster was built to study the possibility of composite synthesis towards its mass production. Lastly an innovative semi-continuous additive synthesis method was developed, which utilises atmospheric reactions to form particles within a metallic matrices. The derived knowledge provides detailed insights into the role of alloy compositions and liquid metallurgy synthesis on performance and cost-efficient design of steel composites