The effect of secondary heat treatment on the magnetic properties and thermal expansion coefficient of Super-Invar alloy

This study investigates the influence of secondary heat treatment on the magnetic properties and thermal expansion characteristics of finished Super-Invar alloy. By conducting the secondary heat treatment within the temperature range of 900°C to 1100°C, the microstructure of the alloy was optimized. XRD characterization results indicated that the alloy maintained the fcc phase structure before and after heat treatment, with no new phase generated, but more distinct crystal orientation. EBSD characterization results revealed significant grain growth, reduced twinning density, and improved grain uniformity after the secondary heat treatment. MFM characterization results showed that the magnetic domain size increased, while the intensity decreased. Fine domains underwent broadening and merging, and the number of domain walls decreased after the secondary heat treatment. The testing results of magnetic properties and coefficient of thermal expansion demonstrated that, as the temperature of the secondary heat treatment increased, the magnetic properties of the alloy improved significantly, accompanied by a noticeable increase in the coefficient of thermal expansion (CET). However, it was observed that the T1050 sample exhibited high magnetic performance but low CET, suggesting the existence of other factors influencing the low expansion characteristics of the alloy, in addition to its microstructure and magnetic properties.