Stress relief annealing of super Invar alloy: Microstructure, soft magnetic and thermal expansion properties

Invar (Fe 64 Ni 36 ) alloy is regarded as a prospective contender for thermal structural materials and transformers utilized in aerospace vehicles, in addition to electrical, and other applications, largely due to its markedly low expansion properties. It is noteworthy that the optimization of permeability and coercivity in Invar alloys has the potential to meet the requirements of a broader range of applications. Consequently, enhancing the soft magnetic properties and extending the low expansion intervals is crucial. The effects of stress relief annealing on the microstructure, soft magnetic properties, and thermal expansion behavior of super Invar alloy (Fe 64 Ni 32 Co 4 ) are investigated. The findings indicate that the incorporation of Co has no impact on the phase structure of the Fe-Ni alloy, yet elevates the Curie temperature of the Fe-Ni alloy to 324.84°C. As the annealing temperature is increased, the grain diameter of the alloy increases gradually. Concurrently, the enhancement of density and magnetocrystalline isotropy diminishes the prevalence of lattice defects, which markedly influences the soft magnetic characteristics of super Invar alloy. At an annealing temperature of 1150°C, the material exhibits a saturation magnetic flux density of 1494 mT, a maximum permeability of 31.04 mH/m, a coercivity of 13.52 A/m, and a wide low expansion interval. This phenomenon can be attributed to random orientation, residual stress relief, and higher Curie temperatures. The results provide a comprehensive insight into the evolution of soft magnetic properties and their underlying mechanisms, covering a larger range of low expansion intervals.