Isotropic negative thermal expansion in the multiple-phase La-Fe-Co-Si alloys with enhanced strength and ductility

The negative thermal expansion materials based on the magneto-volume transition are usually poor in mechanical properties, severely inhibiting their application. Here in this article, we present the enforcement of the compressive strength and ductility of La(Fe,Co,Si)₁₃ by directly adding extra Cu element and forming natural multiple phases alloys. Our study demonstrates that the extra Cu will facilitate the formation of LaCu₂ phase that is semi-coherent with the La(Fe,Co,Si)₁₃ phase, and the LaCuSi phase that exhibits a high density of stacking faults, which facilitates the enforcement of the strength and ductility of the alloy. Consequently, the natural alloys present increased strength from 70 MPa to 645 MPa, and the deformation behavior transforms from brittle to plastic, the ductility is simultaneously increased from 0.5 % to 4.4 %. The introduced phases exhibit positive thermal expansion behavior and compensate for the negative thermal expansion of the La(Fe,Co,Si)₁₃ phase, inducing a tunable isotropic negative to zero thermal expansion is achieved in the alloys. Moreover, the alloys have high mechanical and negative thermal expansion performance reliability after 10⁴ times thermal cycles. This work denotes that Cu is a functional element to enforce the mechanical properties of the La(Fe,Co,Si)₁₃ alloys, and also promotes a new possibility for obtaining low-cost and isotropic negative/zero thermal expansion materials with high application reliability.