Processing of β-treated Zircaloy-4 by warm compressions along three orthogonal axes

The purpose of this work is to refine the grains of Zircaloy-4 before it is processed into cladding tubes containing the nuclear fuel. This raises the question of the mechanisms by which the Widmanstätten lamellae inherited from the β-quenching transform into smaller crystallites. With a view to accelerate this transformation, strain steps of compression along three orthogonal axes were performed in the upper α-range (i.e. 550–810 °C). The effects of the temperature, the size of the strain steps, the strain rate and the number of cycles of three compressions were investigated. All have a direct influence on the mechanical behavior, the global texture and the microstructural evolution. The latter was studied mostly by electron backscattering diffraction. The best conditions to break the initial lamellae were found at high strain rate (4 s⁻¹): small recrystallized grains around 10 μm were obtained after one cycle at 750 °C. But at moderate strain rates (0.1 s⁻¹), good results were also reached after two cycles at 650 °C. The mechanisms of globularization are different in both cases: in the first, germination of new grains prevails while in the second, a progressive evolution of the subgrains into grains was observed. Some consequences of heat treatments are also briefly described, pointing out the sensitivity of Zircaloy-4 to undesirable grain growth. The conclusion emphasizes the interest of multiaxial forging in the thermomechanical treatment of hexagonal alloys.