Thermomechanical processing of titanium, zirconium, magnesium, and zinc in the hcp structure

Abstract

In the hot working range, these hexagonal close-packed (hcp) metals undergo dislocation deformation with formation of a dynamically recovered substructure at low strains. The stress-strain rate relationship follows the sinh law and the Arrhenius temperature relationship has activation energies similar to creep. Because of adequate slip systems, the low (c/a) axial ratio metals, α-Ti and α-Zr, exhibit well-recovered substructures, with size inversely related to flow stress, up to high strain. Because of the transformation to β at about 0.6T m they do not exhibit dynamic recrystallization. The high c/a ratio metals, Mg and Zn, exhibit mainly basal slip at lower temperatures, but at higher, temperatures exhibit increasing amounts of prismatic slip, which is concentrated near grain boundaries. As the strain rises, subgrain misorientation increases, leading to dynamic recrystallization at the boundaries at which shear zones develop. The hot ductilities are much less than face-centered cubic (fcc) metals because neither dynamic recovery nor recrystallization are as effective in relieving intergranular sliding and fissuring.