Microstructural changes and their influence on mechanical properties of vanadium-base alloys irradiated at moderate temperatures

A study of effects of low temperature ion and neutron irradiation ( ≤ 10 dpa at 200-420°C) on fusion candidate V-4Cr-4Ti alloy was performed. In TEM evaluations, cavities were not observed even after dual ion irradiations in which fusion relevant ratio of 5 appm He/dpa was injected. However, irradiation-induced formation of NaCl-structured platelet precipitates of (V,Ti)(C,N,O) was detected. These particles were uniformly distributed within the matrix, occupied {100} type habit planes, and caused significant hardening of the alloy. The size (<10 nm diameter) and the number density ( ≈ 2 x 1022 m-3) of the particles was independent of dose and irradiation temperature, within the investigated ranges. In evaluations of plastically deformed specimens, dislocation channels were observed to form by removal of (V,Ti)(C,N,O) particles from glide planes. Macroscopic slip localization and premature necking of tensile specimens is explained by this phenomenon. Models were developed to describe the effects of precipitation hardening on the deformation and fracture behavior of V-4Cr-4Ti. It was shown, that the processes of dislocation cutting and dissolution of the particles requires applied stresses on the order of the experimentally observed values. Channeling of dislocations could cause significant stress concentrations at tips of arrested dislocation pile-ups, which in turn, could lead to brittle cracks. It was shown, that in irradiation-induced precipitation hardened alloys conditions suitable for catastrophic propagation of such crack could be induced. The experimentally observed embrittlement of the alloy irradiated at low-temperatures, is a result of irradiation induced precipitation hardening promoting dislocation channel formation.