Targeting Superior Zero Thermal Expansion Material by the Concept of TRIP-Invar

A key challenge for spin-dominated functional materials is their suboptimal structural properties, a problem that restricts their widespread applications. Here, this limitation is addressed by introducing additional lattice degree of freedom. This is exemplified in a novel cobalt-based alloy, which is targeted to demonstrate both the spin-state transformation-induced zero thermal expansion (Invar effect, ensuring precision) and lattice transformation-induced plasticity (TRIP effect, enhancing safety), referred to as TRIP-Invar. An unusual martensitic transformation exhibiting three-phase coexistence has been observed under stressing at 77 K, which results in pronounced work hardening behavior and exceptional cryogenic toughness. Notably, reversible spin/lattice transformations enable intrinsic thermal repairability. This findings not only expand the categories within the Invar family, but also provide a reference for the discovery of other integrated structural and functional materials, enabling humanity's exploration of extreme environments like the poles and deep space.