Emergence of Invar effect with excellent mechanical property by electronic structure modulation in LaFe11.6-xCoxSi1.4 magnetocaloric materials

Abstract

Materials with Invar effect, i.e. zero thermal expansion (ZTE), have important applications in precision manufacturing. Here we report the emergence of Invar effect with excellent mechanical property by modulating electronic structure in LaFe11.6-xCoxSi1.4, a family known for magnetocaloric effect. The observed linear thermal expansion coefficient (αl ∼ 1.5 × 10−8 K−1, 5–250 K, by XRD; αl ∼ 2.7 × 10−7 K−1, 109–250 K, by dilatometer) is superior to most other ZTE materials including the famous Invar alloy Fe0.65Ni0.35. By taking advantage of distinct resolution ability of neutrons and X-rays to Fe/Co adjacent elements, the site occupancy of Co atoms was determined by joint refinements of neutron powder diffraction (NPD) and X-ray diffraction (XRD). On the basis, ab initio calculations were performed on the atomically resolved electronic band structure. The incorporation of Co atoms alters the electron transfer properties by increasing 3d bonding electrons at Co sub-lattice, as a result, spontaneous magnetostriction is inhibited and Invar effect prevails in the ferromagnetic region of LaFe10.6Co1.0Si1.4. The electron local function (ELF) evidences the enhanced bonding strength and origin of excellent mechanical properties. The measured compressive strength can be up to ∼340 MPa, which endows the LaFe10.6Co1.0Si1.4 good machining ability. Further, the mode-mode coupling coefficient b calculated from density of states (DOS) explains the evolution of phase transition order with Co doping, which gives LaFe10.6Co1.0Si1.4 a significant advantage in its effective cooling capacity under low magnetic fields. The present study provides a solid foundation for exploiting multifunctional application of La(Fe,Si)13 alloys with Co incorporation from the perspective of electronic structure modulation.