Metals

Irradiation creep of engineering alloys in nuclear reactor cores differs from the creep that is observed outside of the irradiation environment. It exhibits characteristics like high temperature thermal creep because it occurs in an environment of elevated vacancy point defect concentrations, but one must also consider the effect of interstitial point defects and the effect of both vacancy and interstitial concentrations, which are greater than the thermal equilibrium values, on an evolving microstructure. Irradiation creep is dependent on the point defect flux to different sinks and can be modelled using conventional rate theory. The net interstitial or vacancy point defect flux to different sinks determines the strain rate in a direction that can be considered perpendicular to the plane of the sink, which is the extra half plane of an edge dislocation or the plane of a grain boundary. There has been increasing evidence that, for complex alloys such as Zr-2.5Nb pressure tubing in CANDU reactors, the irradiation creep is largely dependent on the grain structure (size and shape). While the maximum amount of thermal creep by dislocation slip will be proportional to the distance a dislocation travels, i.e., proportional to the grain dimension in the direction of slip, observations indicate that the magnitude of irradiation creep is inversely proportional to the grain dimensions, indicating a creep mechanism dependent on diffusional mass transport. Mechanistic modelling of irradiation creep based on rate theory is described and used to account for high diametral creep rates observed for pressure tubes with unusual microstructures fabricated by non-standard fabrication routes.

The effects of different finishing rolling temperatures on the microstructure and mechanical properties of a 580HE hole expansion steel were systematically investigated using optical microscopy, scanning electron microscopy, electron backscatter diffraction, and transmission electron microscopy. The results show that the yield strength increases with decreasing finishing rolling temperature, whereas the tensile strength and total elongation exhibit relatively small variations. Significant changes in phase fraction, grain size, spatial distribution, and NbC precipitation behavior are observed under different finishing rolling temperatures. The microstructure mainly consists of polygonal ferrite and granular bainite, while acicular ferrite is formed at higher finishing rolling temperatures. With decreasing finishing rolling temperature, the ferrite and bainite grains are markedly refined and become more uniformly distributed. Meanwhile, the ferrite fraction slightly increases, the crystallographic texture is weakened, and, more importantly, the number density of precipitates increases while their size is significantly reduced. The hole expansion ratio increases noticeably with decreasing finishing rolling temperature, which is mainly attributed to grain refinement, improved microstructural and strain homogeneity, and the selective strengthening effect of fine NbC precipitates. These factors effectively reduce stress concentration and hardness mismatch between soft and hard phases, thereby delaying crack initiation during hole expansion.

This study examines the effect of vanadium addition on the microstructure and mechanical properties of low-cost powder metallurgy Ti-4Al-2Fe-3Cu alloys. Alloys with and without 6 wt.% V were fabricated by hot extrusion of blended elemental powders followed by vacuum heat treatment. Microstructural analysis revealed that the base alloy exhibits a coarse lamellar α/β structure, while vanadium addition promotes a refined basketweave morphology with a significantly higher β-phase fraction, increasing from 28.1% to 46.2%. Energy-dispersive spectroscopy confirmed preferential partitioning of Fe, Cu, and V into the β phase. Mechanical testing showed that the addition of 6 wt.% V markedly enhances strength, increasing yield strength and ultimate tensile strength from 1122 MPa and 1214 MPa to 1291 MPa and 1349 MPa, respectively, while maintaining comparable tensile ductility (~3.5%). The strength improvement is attributed to α-plate refinement, increased β-phase fraction, and solid-solution strengthening of the β phase. These results demonstrate that vanadium addition is an effective approach for improving the strength of low-cost PM titanium alloys without compromising ductility.