Search

UO2-BeO is an important composite nuclear fuel with high thermal conductivity. The impact of grain boundaries between UO2-UO2 and phase boundaries between UO2-BeO on the bubble evolution and the thermal conductivity in nuclear fuels is significant. In this study, we utilize a 3D phase field model to examine the influence of these boundaries on the nucleation and growth of bubbles in a bicrystalline UO2-BeO. Our simulations reveal that the diffusion of vacancies and gas atoms towards the phase boundaries results in bubbles preferentially forming at the phase boundaries. Intergranular bubbles possess an asymmetrical lectical shape due to the long-range diffusion-controlled boundary migration and the different defect production rate on the both sides. Spherical intragranular bubbles appear after the intergranular bubbles with high defects production rates and large grain size. The effective thermal conductivity decreases with the irradiation time and shows three distinct stages for systems that have both inter- and intragranular bubbles. [ABSTRACT FROM AUTHOR]

Cooling after solutionizing or aging plays a crucial role in heat treatment and can greatly influence the final precipitate microstructure and mechanical properties of titanium (Ti) alloys. In this study, we employ phase field simulations to strategically engineer heterogeneous precipitate microstructures in near β-Ti alloys by manipulating the cooling rate, which leads to different phase transition mechanisms. By varying the cooling rate within the temperature range of 900 ∘ C to 500 ∘ C , we identify four distinct microstructures: uniformly distributed fine congruent α c plates and fine α precipitates, heterogeneous and hierarchical α precipitates, and uniformly distributed coarse α precipitates. Further analysis reveals that these diverse precipitate microstructures originate from three distinct phase transition mechanisms, i.e., congruent transition, pseudospinodal decomposition, and classical nucleation and growth, each activated at different temperatures during continuous cooling. Utilizing the insights from the simulations, we successfully produce microstructures in Ti-5Al-5Mo-5V-3Cr-1Zr (Ti55531) with heterogeneous α precipitates by applying intermediate cooling rates (∼0.01 ∘ C /s), which activate classical nucleation and growth as well as pseudospinodal decomposition. These samples exhibit exceptional comprehensive mechanical properties, including an ultimate strength of approximately 1260 MPa and a total elongation of around 14 %. This investigation provides valuable guidance for designing novel heterogeneous precipitate microstructures by simply controlling the cooling rate, leading to significant enhancements in the mechanical properties of β-Ti alloys. UM: Uniform precipitate microstructure, HM: Heterogeneous precipitate microstructure [Display omitted] [ABSTRACT FROM AUTHOR]

The formation of ultra-fine α precipitates in β-titanium alloys can significantly improve strength but may lead to rupture and low ductility. Here, we propose a simple two-step aging heat treatment to design a bimodal microstructure with distinctively different populations of fine-scale and coarse α precipitates in Ti-5Al-5Mo-5V-3Cr-1Zr (Ti-55531, wt. %) by involving two transformation mechanisms, i.e., classical nucleation and growth, and pseudo-spinodal decomposition. Such a multi-scale α microstructure exhibits a synergistic combination of yield strength (∼1.1 GPa) and ductility (∼19.5% elongation). TEM characterization shows that the appearance of deformation twins in coarse α precipitates contribute to increased ductility, and the higher strength may be attributed to dislocation tangles in fine-scale α precipitates. Our work provides a new strategy to overcome the strength-ductility trade-off by designing a heterogeneous microstructure. • Bimodal lamellar α precipitate microstructures in Ti-alloys are designed via two-step aging. • Such a heterogeneous microstructure exhibits a superior combination of strength and ductility. • The good combination can be attributed to the deformation twins in coarse α precipitates and the dislocation tangles in fine α precipitates. [ABSTRACT FROM AUTHOR]