Electron microscopic study of the origin of coercivity in an Fe--Co--V alloy

To ascertain the origin of coercive force in an Fe--Co--V alloy (2.91 wt percent V, Fe = Co), a transmission electron microscopic study was made of the microstructures and domain wall configurations after the following treatments: A1 = cold rolled to 97 percent reduction + strand annealed for 30 s at 9500C + aging at 6000C for 2 h; A2 = treatment A1 + 5 s at 10500C; A3 = treatment A1 + 5 s at 9500C; A4 = treatment A1 + 5 s at 8500C. The values of coercive force are 25, 32, 38, and 13 Oe, respectively. The microstructure of A1 samples shows precipitation within some grains. Lorentz microscopy reveals that domain walls are pinned by these precipitates. No precipitation is observed in A2 samples, and instead they exhibit substructures characteristic of a structural transformation. The interaction between the transformation-induced substructure and domain walls appears to be weak. The structure of A3 and A4 samples consists of grains having relatively low and moderately high dislocation density. In A3 samples, domain walls tend to lie along the interfaces of the latter grains and rarely penetrate them. It is envisioned that the overall coercivity of A4 samples is lower because the volume fraction of the high dislocation density grains as well as the magnitude of internal strain are lower. (auth)