Combination strategy for high-performance Invar 36 soft magnetic alloy: Synergistic improvement of the heat treatment process

Invar alloys, recognized as dimensionally stable materials, face challenges in harsh environments due to their sensitivity, as low thermal expansion and magnetism are typically mutually exclusive properties. This paper describes the design of a stepped annealing process consisting of a higher annealing temperature, longer dwell time, and suitable quenching temperature to produce Invar 36 soft magnetic alloy with an excellent magnetic performance which the quasi-static soft magnetic properties were enhanced while maintaining a low expansion coefficient. By increasing the annealing temperature and extending the dwell time, the proportion and grain size of recrystallized grains are increased, thus the pinning force that grain boundaries and residual stress exert on the domain wall motion is reduced thereby improving the permeability and reducing the coercivity, the maximum permeability achieved is 11.91 mA/m, while the coercivity is as low as 11.37 A/m. Through meticulous optimization of the quenching temperature, a completely disordered magnetic configuration and special strip grains with larger grain sizes are obtained, thereby reducing lattice distortion and enhancing interface energy. This, in turn, offsets the displacement of lattice thermal vibration, resulting in a very low linear expansion coefficient of 0.8×10^-6/K, which is an order of magnitude lower than that of the traditional Invar alloy. This study provides a mechanism for regulating the magnetovolume effect (MVE) of spontaneous volume magnetostriction in Invar alloys and guides the development of novel alloys with balanced thermal expansive and soft magnetic properties.