Ultrastrong Negative Thermal Expansion Compositionally Complex Alloy

Abstract Negative thermal expansion (NTE) materials, which are crucial for fabricating strong metallic composites with temperature‐invariant volumes, face significant challenges: they not only need to maintain a large thermal expansion over a wide temperature range but also need to harness strength and ductility for load‐bearing applications. Unfortunately, most of these materials are brittle (compressive strength < 1 GPa), while the few ductile materials available have a narrow temperature range and significant thermal hysteresis. Herein, a compositionally complex Fe–Co–Ni–Ti alloy is reported exhibiting an excellent combination of large NTE over a wide temperature range with narrow thermal hysteresis, high compressive strength, and modest ductility. This unusual set of properties stems from the unique microstructure of the alloy, in which the matrix phase enables a unique kinetically sluggish thermoelastic martensitic transformation with a pronounced volumetric change, while the mechanically hard secondary phases contribute to the strengthening effect. An ultrahigh strength of up to 2.64 GPa could be achieved by manipulating the nanoscale local chemical ordering, accompanied by tunable thermal expansion behavior. This study opens a new design strategy for high‐performance functional materials.