The Direct Observation of Grain Refinement Mechanism in Advanced Multicomponent γ-TiAl Based Structural Intermetallics Doped with Boron
The synthesis of Ti–44Al–5Nb–2Cr–1.5Zr–0.4B–0.07La and Ti–44Al–5Nb–1Cr–1.5Zr–1B–0.17La (at.%) intermetallic alloys was performed from pure metals by the electron beam semi-continuous casting technique. Alloys are characterized by convoluted microstructure whose refinement is progressive with the boron content increase. The specimens were analyzed using JEOL JSM6610 scanning electron microscope (SEM ) equipped with electron backscatter diffraction and energy dispersive X-ray spectroscopy (EDX ) systems. The boron alloying causes precipitation of micro-dimensional complex (Ti,Nb)B borides within solidifying melt. These particles-inoculants promote the formation of finer microstructure. Auger spectrometry was applied for quantitative elemental analysis of borides, using PHI-680 device. It was proven that the origin of structure refinement consists in solid-phase seeding and germination of α2-Ti3Al-phase on (Ti,Nb)B ribbon-like colonies with subsequent growth of α2-laths through the volumetrically prevailing γ-TiAl domains. We fixed several consecutive stages of microstructure refinement process. The description of these stages is supported with the dimensional, crystallographic and compositional characterization of microstructural constituents. The work provides a basis for new techniques of microstructure/mechanical properties engineering of studied materials.
KeywordsQuantitative Elemental Analysis Lanthanum Hexaboride Boron Content Increase Boron Alloy Structure Refining Effect
Authors acknowledge the Russian Ministry of Education and Science grant 11.951.2014/K, and partly the Russian Foundation for Basic Research grant 13-03-00500.