Brittle intermetallic compound makes ultrastrong low-density steel with large ductility

Alloying steel with aluminium improves the material’s strength-to-weight ratio, but the resulting formation of brittle intermetallic compounds within the steel matrix reduces its ductility; here the morphology and distribution of the intermetallic precipitates are controlled to alleviate this problem. Steel that's lighter — but stronger Despite having long been the metals of choice for many structural applications, iron and steel suffer from relatively low strength-to-weight ratios (specific strength). This has driven interest in the development of high-aluminium low-density steels, but such alloys tend to have poor ductility because of the formation of brittle precipitates within the steel matrix. Efforts have been made to suppress the formation of these precipitates, but now Sang-Heon Kim and colleagues show how instead their morphology and distribution in the matrix can be controlled to alleviate their undesirable influence on ductility. The resulting steel has a combination of specific strength and ductility that goes beyond that of titanium alloys, the lightest, strongest metallic materials known. As lightweight steel is also cheaper than titanium, the future for this new alloy could be bright. Although steel has been the workhorse of the automotive industry since the 1920s, the share by weight of steel and iron in an average light vehicle is now gradually decreasing, from 68.1 per cent in 1995 to 60.1 per cent in 2011 (refs 1 , 2 ). This has been driven by the low strength-to-weight ratio (specific strength) of iron and steel, and the desire to improve such mechanical properties with other materials. Recently, high-aluminium low-density steels have been actively studied as a means of increasing the specific strength of an alloy by reducing its density 3 , 4 , 5 . But with increasing aluminium content a problem is encountered: brittle intermetallic compounds can form in the resulting alloys, leading to poor ductility. Here we show that an FeAl-type brittle but hard intermetallic compound (B2) can be effectively used as a strengthening second phase in high-aluminium low-density steel, while alleviating its harmful effect on ductility by controlling its morphology and dispersion. The specific tensile strength and ductility of the developed steel improve on those of the lightest and strongest metallic materials known, titanium alloys. We found that alloying of nickel catalyses the precipitation of nanometre-sized B2 particles in the face-centred cubic ma