Deformation mechanism of pure hafnium under high speed compression

The microstructures and mechanical properties of Hf compressed at the strain rates of 300 s-1,1290 s-1 and 2800 s(-1) were investigated. At a strain rate of 300 s(-1), dislocation slip dominated the deformation. Meanwhile, a small number of the face-centered cubic (FCC) lamellas formed via hexagonal close-packed (HCP) structure to FCC structure transformation. As the strain rate increased to 1290 s(-1), both the dislocation density and the number of the FCC lamellas increased significantly. When the strain rate reached 2800 s(-1), dislocation slip, phase transformation, twinning, dynamic recrystallization and shear banding occurred in the material. However, the number of the FCC lamellas decreased significantly at the strain rate of 2800 s(-1), indicating that the HCP to FCC phase transformation was restrained. The temperature rise inside the specimens compressed under the strain rate of 2800 s(-1) promoted deformation twinning and dynamic recrystallization. The microhardness in-creased gently for the specimens compressed at strain rates of 300 s(-1) and 1290 s(-1), while when the strain rate increased to 2800 s(-1), the microhardness increased substantially. The formation of various boundary structures, including microbands, twins and FCC bands, contributes to the hardness increase by acting as obstacles to dislocation motion. In addition, the formation of new grains by dynamic recrystallization also played a role in strengthening.