Magnetic precipitation hardening and Bloch‐wall pinning in an 85Co‐12Fe‐3Nb alloy

摘要

The development of intrinsic coercivity during isothermal aging of a solution treated and quenched 85Co‐12Fe‐3Nb alloy was studied. The hysteresis loops, initial susceptibility, and magnetization were also measured and provided information for identifying the mechanism of coercivity development. Successive hysteresis loops clearly demonstrate that Bloch‐wall pinning dominates the reversal mechanism in this alloy. The maximum coercivities found during isothermal aging were 21.5, 33, and 37 Oe at 800, 750, and 700 °C, respectively. The microstructure at maximum coercivity consisted of Widmanstätten plates of a phase similar to Co3Nb which formed along the {111} planes of the ferromagnetic matrix. At the lowest aging temperature, 700 °C, a metastable ordered L12 γ′ phase formed during the early aging times resulting in a significant increase in the coercivity compared to the solution‐treated samples. The periodic microstructure associated with the γ′ phase also resulted in an extremely low initial susceptibility. After prolonged aging, the γ′ was replaced by the Widmanstätten precipitates. The decrease in coercivity for long aging times at 750 and 800 °C was the result of continued coarsening of the Widmanstätten morphology. The alloys magnetically hardened by the periodic γ′ microstructure exhibited hysteresis behavior indicative of homogeneous wall pinning, whereas the alloys hardened by the Widmanstätten precipitate showed the behavior characteristic of inhomogeneous wall pinning.