Microstructure Evolution during Thermal Processing : Insight from In-Situ Time-Resolved Synchrotron Radiation Experiments

Abstract

Abstract. The magnetic, mechanical or chemical properties of nanocrystalline materials strongly differ from the ones of their coarse-grained counterparts. Moreover, significant changes of the phase diagrams were already evidenced for nanostructured alloys. Thermal processing with or without applied pressure controls the microstructure development at the nanometer scale and thus essentially decides upon the final nanomaterial behaviour and properties. A common route for the synthesis of metallic nanomaterials is the devitrification of amorphous precursors obtained via non-equilibrium processing, e.g. by rapid solidification or high-energy ball-milling. Time-resolved in-situ X-ray diffraction experiments may nowadays be performed at high-brilliance synchrotron radiation sources for a variety of temperature-pressure conditions. The temperature-time evolution of the grain-size distribution and microstrain can be monitored in detail at specimen-relevant scales. Together with local information from electron microscopy and chemical analysis, in-situ X-ray experiments offer a complete set of tools for engineering of the microstructure in nanomaterials. The effect of individual processing steps can be distinguished clearly and further tuned. An example is provided, concerning the high-temperature microstructure development in Co-rich soft magnetic nanostructured alloys. IntroductionModern nanostructured soft-magnetic alloys typically consist of nanocrystalline ferromagnetic grains embedded in an amorphous phase. This type of nanocomposite microstructure provides an efficient averaging of the magneto-crystalline anisotropy and magnetostrictive coefficients of the nanophase over a large number of grains within the nanomaterial.During the past years, several Fe-based soft-magnetic nanostructured alloys were developed, e.g. Fe 73.5 Si 13.5 B 9 Nb 3 Cu 1 (FINEMET, [1] (HITPERM, [3]). The recent research on soft-magnetic nanocomposites was reviewed by several authors [4][5][6][7][8]. The search for bulk amorphous metallic glasses with good soft magnetic properties [8], further led to the discovery of Co-rich bulk amorphous alloys in the Co-Fe-M-B alloy system (M: Zr, Nb) [9]. The new Co-based amorphous alloys exhibit low coercivity and stable high permeability in the frequency range up to 1 MHz. Wide supercooled liquid regions of up to 80 K were evidenced for Co-Fe-Zr-B alloys with Fe content in excess of 14 at.%. Cobalt-rich alloys with nearly zero magnetostriction were as well obtained at lower Fe concentrations, near to 2 at.% [10]. There is an increasing interest in the further development of these alloys, in view of their unique combination of soft-magnetic and mechanical properties. In-situ high-temperature X-ray diffraction using synchrotron radiation and thermal analysis were used to investigate the nano-