Light, strong and cost effective: Martensitic steels based on the Fe – Al – C system

Abstract

We introduce a novel alloy design concept for density reduced ultra-high strength steels. It is based on the effects of C to increase martensite strength and the Al-solubility in austenite, in conjunction with Al to increase the martensite start temperature and to reduce density. This alloy combination results in inherently strong but light martensitic microstructures, whose mechanical properties (i.e. strength and ductility) can be readily adjusted over a wide range by applying straightforward and established heat treatments. The concept is validated on an Fe – 8 Al – 1.1C (wt.%) alloy subjected to quench and tempering treatments. The steel exhibits relatively low yield strength (~600 MPa) and reasonable ductility (~ 15% elongation) after hot rolling and soft annealing, but can be brought to a maximum hardness of 62 HRC after quenching and tempering at 250 °C. These attractive mechanical properties are coupled with a low density (6.95 g cm–3) and high elastic stiffness (Young's modulus 199 GPa). These lightweight martensitic steels are cost-effective structural materials for weight-critical applications, promising unprecedented specific strength. Advanced characterisation presented herein allows us to derive the fundamental underpinnings of carbide precipitation and phase transformation, and outline and discuss perspectives for refining the alloy composition and processing parameters.