Magnetostrictions and transformation process in Fe73.5Cu1Ta2Nb1Si13.5B9

Finemet-based alloy with substitution of 2 at.% of niobium by tantallum is studied from the viewpoint of transition to (nano)crystalline state and its influence on magnetostriction properties. The measurements have been performed on samples annealed isothermally at different temperatures for 1 h, annealing temperatures ranging from room temperature up to 750°C. After annealing, the magnetostriction has been measured at room temperature using the capacitance method. The results of saturation magnetostriction measurements λS, volume magnetostriction ω and force magnetostriction ∂ω∂H and their evolution with annealing reflect the transformation process. Saturation magnetostriction λS changes slightly (by 10%) in the course of relaxation and crystallization of metastable phases. Formation of borides in the final phases of transformation decreases strongly the value of λS to 1/12 of the value for the as-quenched sample. Force magnetostriction ∂ω∂H reflects the process of volume changes in the course of crystalline phase formation accompanied by changes of internal stresses which influence the domain structure of the material. X-ray and transmission electron microscopy (TEM) investigations identify the type of crystalline phases formed at selected annealing steps of the transformation process.