Galfenol (Fe100-xGax, x = 6, 12.5, 17, 18.4, 19, 22) and Alfenol (Fe81Al19) alloy rods (~50 mm x 6 mm dia.) were annealed under compressive stresses up to 219 MPa at temperatures from 100°C to 700°C for 10 to 100 minutes. Because of the magnetostriction of the alloys, these heat treatments build in a uniaxial magnetic anisotropy that depends upon annealing stress, annealing temperature, and alloy composition. This built-in uniaxial magnetic anisotropy extends the high power capability of these alloys to operate both in tension and in compression.
Magnetization and magnetostriction measurements of both unannealed and annealed alloys were taken from −100 MPa to +40 MPa. To fit the magnetization and magnetostriction as a function of magnetic field and stress, an energy expression containing a fourth order anisotropy term (cubic term) plus a second order uniaxial term was utilized. The computed magnetizations and magnetostrictions are found by an energy-weighted average using the Armstrong smoothing constant. Excellent fits to the magnetostriction and moment data were obtained. From the model fits m, d33 and k33 were calculated. Since the built-in stresses can be found simply from the model, it is possible to predict the amount of prestress built into the alloys.