A commercial-purity Zr702 was grain-refined from 20 m to 0.2–0.5m by equal channel angular pressing.
Grain refinement was most evident in the first pass but was insignificant during the subsequent passes. Two microstructural characteristics evolved: a lamellar structure
and an equiaxed subgrain structure during the route A pressing and the route BC pressing, respectively. Due to the rotation of specimen in
between passes, two sets of geometrically necessary boundaries were formed during the route BC pressing. The mechanism of grain refinement
during the route A pressing was evolution of high-angle geometrically necessary boundaries from the low-angled ones, while that during
the route BC pressing was decomposition and rearrangement of pre-existing boundaries. The yield stress of severely deformed specimens
increased with the reducing grain size according to a Hall–Petch relationship.
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