Low-density and high-modulus steel achieved by hypereutectic TiB2 and high Al ferrite phase

High-aluminum low-density steels have been extensively studied due to their potential for lightweight and energy-saving applications. However, high aluminum content causes severe problems, such as a marked reduction in Young’s modulus and also carbide-induced challenges in casting and processing. This study aims to tackle these issues by advancing the concept of lightweight steels, offering a new class of carbon-free, austenite-based low-density triplex steel. This novel approach enables the utilization of ceramic and δ-ferrite to achieve an optimal balance of physical properties (low density and high Young’s modulus) and mechanical properties. Specifically, using conventional cost-effective steel fabrication routes, we produced an as-hot-rolled heterogeneous microstructure characterized by a fully equiaxed-austenitic matrix embedded with elongated δ-ferrite bands and micron-sized TiB₂ particles. Physically, this new low-density steel achieves an impressively low density of 6.88 g cm⁻³ while preserving a high Young’s modulus of 225 GPa, resulting in a high specific modulus. Mechanically, this new steel shows nearly a 100% increase in yield strength compared to the conventional Fe-TiB₂ steel, while retaining good ductility. The improved strength-ductility balance is attributed to the dual roles of δ-ferrite and its associated microstructural heterogeneity: the high dislocation density in δ-ferrite strengthens the steel through the composite effect, and the soft/hard phase contrast between δ-ferrite and austenite promotes additional strain hardening via the generation of geometrically necessary dislocations. This work presents a viable approach to producing low-density, stiff, strong, and ductile steels on a large scale for cost-effective lightweight applications.