Effects of tensile loading during annealing of alnico melt spun ribbons

Conventional magnetic annealing (MA) of the permanent magnet alloy alnico involves application of an external magnetic field at temperatures within the spinodal decomposition range. This field biases the growth of the Fe-Co rich, ferromagnetic alpha (1)-phase in an energetically favorable 001 direction in alignment with the applied field within an Al-Ni rich, paramagnetic alpha (2)-phase. Utilizing a magnetic field to bias the alpha (1)-phase may limit alnico from reaching theoretical coercivity due to (1) the field having maximum biasing ability at temperatures near the Curie temperature where large alpha (1)-phase nanorods form and (2) connectivity of the alpha (1)-phase occurs unavoidably during MA. Both decrease the effective shape anisotropy of the alpha (1)-phase, thereby reducing coercivity. Herein, we explore tensile-loading as a biasing mechanism to control and optimize the final alnico nanostructure beyond that achieved by MA. Two samples of melt-spun alnico were heat-treated at 860 degrees C for 5 minutes: one sample was subjected to 10 MPa tensile stress for comparison with a stress-free control sample. Structural and magnetic characterization revealed that the stress-annealed ribbon sample possessed expected phase assemblages, but was distinguished by a similar to 2x larger grain diameter and an elongated anisotropic alpha (1)-phase within grains that were oriented to a shear stress along 001 directions at an angle of similar to 45 degrees relative to the loading direction. Both types of annealing produced a similar increase in the coercivity and remanence, but a decrease in saturation magnetization.

Automated summary

Conventional magnetic annealing of alnico limits its coercivity due to the biasing effect of a magnetic field. This study explores tensile-loading as a new biasing mechanism to optimize the final nanostructure of alnico.