Allotropic phase transformation of pure zirconium by high-pressure torsion

Pure Zr is processed by high-pressure torsion (HPT) at pressures in the range of 1-40 GPa. A phase transformation occurs from α to ω phase during HPT at pressures above ∼4 GPa while the total fraction of ω phase increases with straining and saturates to a constant level at higher strain. This phase transformation leads to microstructural refinement, hardness and strength enhancement and ductility reduction. Lattice parameter measurements confirm that c for α phase is expanded about 0.6% by the presence of ω phase. The temperature for reverse transformation from ω to α phase increases with straining and thus, straining under high pressure increases thermal stability of ω phase. The ω phase obtained by HPT is stable for more than 400 days at room temperature.