Microstructural refinement of tantalum by severe plastic deformation
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Square bars of Ta containing as-cast, large and medium sized grains were deformed by multipass equal channel angular extrusion (ECAE) to strains of 4.6 at room temperature through 90 degree tooling. The ability of ECAE to break up the microstructure and produce a uniform, fine-grained recrystallized microstructure and the resulting mechanical properties are examined. Hardening and recrystallization behavior are characterized by Brinnell and Vickers microhardness measurements, and light and electron microscopy of the deformed, recovered and recrystallized materials. Optical microscopy was used to determine the grain size, and homogeneity of the recrystallized microstructure. Comparisons of four consecutive extrusions via ECAE routes C, Bc and E are made. Routes E and Bc are found to be more effective at breaking up the as-cast microstructure and development of a uniform recrystallized microstructure than route C. This is because routes Bc and E incorporate deformation schedules which include a higher number of intersecting shear planes than does route C. Multipass ECAE processing is shown to produce 100-400 nm diameter grains in material worked to a strain of 4.6, which when recrystallized forms grains 10-15 microns in diameter.
