Microstructure evolution and recrystallization after annealing of tungsten heavy alloy subjected to severe plastic deformation

The microstructure evolution, recovery and recrystallization of the work hardening W phase for 93W-4.9Ni-2.1Fe alloy subjected to severe plastic deformation during annealing treatment are investigated. Microstructure stability is studied by heating at temperature ranging from 900 degrees C to 1400 degrees C. Results indicate that tungsten phase of a hot-hydrostatically extruded 93W-4.9Ni-2.1Fe alloy can hold good stability below 900 degrees C and static recovery of W phase occurs firstly. The recovery process may be attributed to the rearrangement and annihilation of dislocations. Secondly, when the annealing temperature goes beyond 900 degrees C, the micro-hardness drops sharply which indicates the onset of recrystallization. The micro-hardness starts to stay at a stable level at 1300 degrees C which indicates full recrystallization has been obtained. In this research, the plastic deformation amount is large (area reduction 75%), dislocation slip start within the elongated tungsten grain, resulting in the formation of high density dislocation walls consisting of dislocation lines and dislocation tangle to be proved by TEM. Then the high density dislocation walls are transformed into fine subgrain boundaries due to continuous and coordination deformation, which further evolve into refined boundaries and fine subgrains. The driving pressure for recrystallization arises from the crystal defects stored in the deformed material. These include high angle grain boundaries, dislocations and dislocation boundaries such as cell or subgrain walls. Thus, the static recovery process and recrystallization of tungsten phase during annealing occur successively. (C) 2016 Elsevier B.V. All rights reserved.