The effect of carbon content on the microstructures and stress rupture properties of a newly developed polycrystalline Ni-based superalloy with high Cr content has been studied.It was observed that both grain size and the number of carbides increased with an increase in carbon content.After heat treatment,granular M_(23)C_(6)carbides were dispersed around MC carbides along grain boundaries and inside grains.The quantity of granular M_(23)C_(6)carbides increased while their sizes decreased.These findings can be verified with the results of thermodynamic calculation and differential scanning calorimetry analysis.The stress rupture times(975℃/225 MPa)increased from 13.3 to 25.5 h with the carbon content increased from 0.1 to 0.2 wt.%.The improvement can be attributed to two primary factors.Firstly,grain boundary is typically weak region during deformation process and the grain size increased as carbon content increased in the alloy.Secondly,carbides act as hindrances to impede dislocation movement,leading to dislocation entanglement.As carbon content rose,the quantity of carbides in interdendritic regions and grain boundaries increased,providing a certain degree of strengthening effect and resulting in a longer stress rupture time.
Han WangShijie SunNaicheng ShengGuichen HouJinguo LiYizhou ZhouXiaofeng Sun
Understanding the evolution of microstructures in nuclear fuels under high-burn-up conditions is critical for extending fuel refueling cycles and enhancing nuclear reactor safety.In this study,a phase-field model is proposed to examine the evolution of high-burn-up structures in polycrystalline UO_(2).The formation and growth of recrystallized grains were initially investigated.It was demonstrated that recrystallization kinetics adhere to the Kolmogorov–Johnson–Mehl–Avrami(KJMA)equation,and that recrystallization represents a process of free-energy reduction.Subsequently,the microstructural evolution in UO_(2) was analyzed as the burn up increased.Gas bubbles acted as additional nucleation sites,thereby augmenting the recrystallization kinetics,whereas the presence of recrystallized grains accelerated bubble growth by increasing the number of grain boundaries.The observed variations in the recrystallization kinetics and porosity with burn-up closely align with experimental findings.Furthermore,the influence of grain size on microstructure evolution was investigated.Larger grain sizes were found to decrease porosity and the occurrence of high-burn-up structures.
Dan SunYanbo JiangChuanbao TangYong XinZhipeng SunWenbo LiuYuanming Li
