Unique high-temperature deformation dominated by grain boundary sliding in heterogeneous necklace structure formed by dynamic recrystallization in HfNbTaTiZr BCC refractory high entropy alloy

Abstract

Microstructural evolution of dynamically recrystallized (DRX) grains and grain boundary sliding (GBS) in the heterogeneous necklace structure of HfNbTaTiZr refractory high entropy alloy (RHEA) was studied systematically during high temperature deformation. Uniaxial compression testing was carried out to different strains at 1000 °C and a strain rate 10−3 s−1. Significant bulging of grain boundaries led initially to the formation of DRX grains. The fraction of DRX grains increased with strain, and typical necklace structures of fine (d ≤ 1.5 µm) DRX grains formed at strain ε ≥ 0.3. The DRX grains showed very limited grain growth, and heterogeneous microstructures composed of coarse unrecrystallized regions surrounded by the characteristic DRX necklace structure were formed at larger strains. Interrupted testing with marker grids revealed that DRX grains deformed by a GBS mechanism. The DRX necklace regions connected mesoscopically and also displayed diamond network morphologies, with unique “Y-shaped”, “T-shaped” and “X-shaped” junctions. The formation of different types of junctions were rationalized on the basis of GBS accommodated by local dislocation slip in unrecrystallized regions. The unrecrystallized regions showed preferred <001>/<111> micro-texture, consistent with conventional dislocation slip in the BCC crystals. On the other hand, the newly formed DRX grains initially had similar orientations to those of the parent grains, but they displayed a random texture with increasing strain, as expected from GBS. The randomized texture of DRX grains and the stability of DRX grains size represented GBS in the DRX necklace regions.