Slip-Induced Directional Order in Fe-Ni Alloys. II. Experimental Observations

Abstract

The results of an extensive investigation into the nature of slip-induced directional order in Fe-Ni alloys near the 75% Ni composition are reported in this paper. Single crystal rods of 4–79 moly-Permalloy were swaged in the [111] and [001] orientations, as were crystals of the following orientations subjected to plane strain compression: (001) [100], (001) [110], (110) [001], (110) [1̄12], (110) [1̄10], (111) [1̄12], (112) [1̄10] and (112) [1̄1̄1]. Magnetic torque measurements show that all crystals developed a uniaxial anisotropy upon deformation, with the easy magnetic direction in good agreement with our previous theoretical deductions. In the [001] swaged crystal, deviation of the observed easy axis from the calculated value was rationalized in terms of unequal participation of the expected slip systems. In all cases of plane strain compression, the induced anisotropy constant rose to a maximum value, and then decreased, as the thickness reduction was increased. In one experiment, crystals of a 2%Mo-76% Ni-22%Fe alloy received a long-range-ordering treatment at 475°C for 190 h prior to plane strain compression in the (110) [1̄12] orientation. The anisotropy energy reached a maximum value of 460 000 erg/cm3, the highest value of slip-induced anisotropy thus far reported in Fe-Ni alloys. As for the anisotropy decrease at large reductions, reorientation of slip-induced atom pairs as a result of lattice rotation, is undoubtedly a factor. However, this explanation is not valid for the (110) [1̄12] orientation, which remained stable even after 98% reduction. A more widely applicable mechanism is advanced, whereby the decrease in anisotropy is attributed to randomization of the atom pairs through slip across antiphase ordered domain boundaries, both those grown-in and those created by intersecting slip. Additional observations have been made of the crystallographic texture and magnetic squareness in cold-formed polycrystalline Permalloy tapes, which are currently used as magnetic memory elements in storage devices. The results are satisfactorily explained in terms of slip-induced directional-order theory. Implications of seeded single crystals to these results are discussed.