High-performance lightweight materials are urgently needed, given the pressing quest for weight reduction and the associated energy savings and emission reduction. Here, by incorporating the multi–principal element feature of compositionally complex alloys, we develop the concept of lightweight steels further and propose a new class of compositionally complex steels (CCSs). This approach allows us to use the high solid solution strengthening and shift the alloys’ compositions into previously unattainable phase regions where both nanosized shearable κ-carbides and non-shearable B2 particles are simultaneously formed. The achievement of dual-nanoprecipitation in our CCSs leads to materials with ultrahigh specific tensile strength (up to 260 MPa·cm³ g⁻¹) and excellent tensile elongation (13 to 38%), a combination outperforming all other high-strength high-entropy alloys and advanced lightweight steels. Our concept of CCSs is thus useful for guiding the design of ultrastrong lightweight metallic materials.

Deformation twinning is rarely found in bulk face-centered cubic (FCC) alloys with very high stacking fault energy (SFE) under standard loading conditions. Here, based on results from bulk quasi-static tensile experiments, we report deformation twinning in a micrometer grain-sized compositionally complex steel (CCS) with a very high SFE of ~79 mJ/m², far above the SFE regime for twinning (<~50 mJ/m²) reported for FCC steels. The dual-nanoprecipitation, enabled by the compositional degrees of freedom, contributes to an ultrahigh true tensile stress up to 1.9 GPa in our CCS. The strengthening effect enhances the flow stress to reach the high critical value for the onset of mechanical twinning. The formation of nanotwins in turn enables further strain hardening and toughening mechanisms that enhance the mechanical performance. The high stress twinning effect introduces a so far untapped strengthening and toughening mechanism, for enabling the design of high SFEs alloys with improved mechanical properties.

High‐entropy alloys (HEAs) and metallic glasses (MGs) are two material classes based on the massive mixing of multiple‐principal elements. HEAs are single or multiphase crystalline solid solutions with high ductility. MGs with amorphous structure have superior strength but usually poor ductility. Here, the stacking fault energy in the high‐entropy nanotwinned crystalline phase and the glass‐forming‐ability in the MG phase of the same material are controlled, realizing a novel nanocomposite with near theoretical yield strength (G/24, where G is the shear modulus of a material) and homogeneous plastic strain above 45% in compression. The mutually compatible flow behavior of the MG phase and the dislocation flux in the crystals enable homogeneous plastic co‐deformation of the two regions. This crystal–glass high‐entropy nanocomposite design concept provides a new approach to developing advanced materials with an outstanding combination of strength and ductility.

Since its first emergence in 2004, the high‐entropy alloy (HEA) concept has aimed at stabilizing single‐ or dual‐phase multi‐element solid solutions through high mixing entropy. Here, this strategy is changed and renders such massive solid solutions metastable, to trigger spinodal decomposition for improving the alloys’ magnetic properties. The motivation for starting from a HEA for this approach is to provide the chemical degrees of freedom required to tailor spinodal behavior using multiple components. The key idea is to form Fe‐Co enriched regions which have an expanded volume (relative to unconstrained Fe‐Co), due to coherency constraints imposed by the surrounding HEA matrix. As demonstrated by theory and experiments, this leads to improved magnetic properties of the decomposed alloy relative to the original solid solution matrix. In a prototype magnetic FeCoNiMnCu HEA, it is shown that the modulated structures, achieved by spinodal decomposition, lead to an increase of the Curie temperature by 48% and a simultaneous increase of magnetization by 70% at ambient temperature as compared to the homogenized single‐phase reference alloy. The findings thus open a pathway for the development of advanced functional HEAs.