2D X-ray diffraction method for evaluating the local crystallization of Fe-based amorphous alloy ribbon induced by ultrashort pulsed laser local heating

Abstract

Fe-based amorphous alloys, which exhibit excellent soft magnetic properties, are difficult to machine, owing to their high strength, toughness, and hardness resulting from their unique structures. The authors proposed a novel blanking method that improves the machinability by crystallizing only the local regions involved in machining, thereby reducing the local strength and toughness. Because the micro-region mechanical properties of amorphous alloys vary greatly depending on the precipitated crystal species and volume fraction, it is necessary to nondestructively investigate the type of microstructure achieved by local heat treatment to determine the appropriate blanking conditions. However, an appropriate method for investigating crystals precipitated locally in an amorphous matrix is yet to be established. In this study, we used the 2D X-ray diffraction method to investigate the local crystallization of an Fe-based amorphous alloy ribbon after local heat treatment using an ultrashort pulsed laser. Furthermore, the 2D method was used to investigate the residual stresses in the fully crystallized amorphous alloy ribbons, and the results were compared using the sin2(ψ) method. The 2D method detected the slightly precipitated crystals in a local area with a radius of about 40 µm in the amorphous matrix. The integrated intensity of the diffraction peaks can be used to predict the internal structure after local heat treatment. Furthermore, the 2D method could measure residual stress with smaller error than the sin2(ψ) method. The core-shell crystals precipitated by heat treatment exhibited a compressive residual stress of −82 MPa to −67 MPa in the core Fe3B crystals and tensile residual stress of 39–60 MPa in the shell α-Fe crystals, approximately. The residual stress values of the precipitated crystals depended on the direction of rotation of the quenching roll used in the production of the amorphous alloy ribbons.