Mechanism for Single Crystal Refinement in High Purity Niobium During Equal-Channel Angular Pressing
Keywords:niobium, single crystal, severe plastic deformation, softening-hardening
High-purity (99.99 wt.%) niobium (Nb) single crystal (SC) specimens as test material were received by electron-beam melting (EBM) technology and used as start material for equal-channel angular pressing (ECAP) of severe plastic deformation (SPD). First pressing was conducted at room temperature. The single crystal refinement via banded texture forming in the shear region of ECAP die during first pass and after fourth passes was studied. The dislocation ribbons and band morphology revealed in single crystal on intersection plane at first pressing. Primary grain boundaries initiation starts simultaneously. After four passes of ECAP the microstructure of intersecting bands in metal was formed and it was verified, that formed grain measures are 2 micrometers in mean.
The supplement microstructure and properties improving of Nb was conducted under hard cyclic viscoplastic (HCV) deformation. Results show that hardening rate during very first cycles was as maximal and decreased at next cycles. The viscoplastic behavior of Nb depends on tension-compression amplitude and microstructure, which was formed previously during ECA pressing. During HCV deformation Nb show: a) fully elastic behavior up to ±0.2 %, b) hardening – at compression and softening – at tension from ±0.5 % up to ±1 %, c) stable viscoplastic behavior at strain amplitude from ±1 % up to ±2 % and d) softening occurs at strain over ±2 % of cyclic straining amplitude. As a result of HCV deformation the UFG microstructure with low angle grain boundaries was preformed to high angle grain boundaries with lowering of dislocation density and metal has improved toughness and uniform elongation under tension loading.
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