Recrystallization of niobium single crystals deformed by ECAE

Author(s)

Bernardi, Heide H. ; Sandim, Hugo R. Z. ; Verlinden, Bert ; Raabe, D. ; Kang, SJL ; Huh, MY ; Hwang, NM ; Homma, H ; Ushioda, K ; Ikuhara, Y

• Document type: Journal article
• Source: Materials Science Forum; v. 558-559, p. 2-pg., 2007-01-01.

Abstract

High-purity niobium single crystals were deformed by equal-channel angular extrusion (ECAE) at room temperature to an equivalent Von Mises strain of about 1.15. Deformed samples were annealed in vacuum from 500 to 800 degrees C for I hour to investigate their microstructure evolution. The microstructure of deformed and annealed samples was characterized by scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and Vickers microhardness testing. The deformed structure after one ECAE pass is rather inhomogenous and consists of parallel sets of coarse shear bands whose spacing varies from one region to another in the cylindrical billet. the microstructure within the shear bands consists of elongated subgrains with sizes below 3 mu m and lamellar boundaries, The remaining non-sheared regions display a coarser subgrain structure. Recrystallization is virtually absent in samples annealed at 500 degrees C for 1 hour. Nucleation begins mostly within shear bands. The new grains with sizes ranging from 10 to 50 mu m are arranged in clusters rather than being homogenously distributed. The recrystallized volume fraction also varies from one region to another indicating an inhomogenous distribution of stored energy. At 700 degrees C, recrystallization is complete after annealing for 1 hour resulting in a structure with a mean grain size of about 100 mu m. (AU)