| Nuclear power has rapidly developed in these decades due to the shortage of conventional energy and the consequent environment problem.The ? generation of nuclear reactor has been proposed and will be planned to build in 2030.Hastelloy X(HX)alloy is a solid solution-strengthened nickel-based superalloy,which possesses a good combination of excellent oxidation resistance,formability and high-temperature strength.It has been recommended as a potential candidate material for high-temperature gas-cooled reactor.However,intergranular corrosion and internal oxidation along grain boundaries happened occasionally during service life,which accelerated crack initiation and performance degradation of components.It severely restricts the application of HX alloy.To ensure the reliable operation of structural components,improvement of intergranular performance of HX alloy under extreme environmental conditions has become a burning issue.Since it was proposed,grain boundary engineering(GBE)has played an important role on intergranular performance improvement of crystal materials,which also can be introduced to HX alloy.In this dissertation,based on the conception of grain boundary engineering,research has been made on the evolution of grain boundary character distribution(GBCD)during thermomechanical processing in HX.The effect of GBCD on precipitation behavior,intergranular corrosion,grain boundary oxidation and creep behavior has been studied.The conclusions of this research have been drawn as following:Cold rolling and annealing treatment were employed in thermomechanical processing(TMP),and the effect of reduction ratio,annealing temperature,annealing time and processing times on GBCD was investigated.The results showed that the evolution of grain boundary network was depended on dislocation density induced by pre-strain.The microstructure recovery process at high-temperature annealing for short time in low-strain sample was beneficial for special grain boundary regeneration.Multiple processing could not only optimize GBCD,but also control grain growth effectively.The GBCD could be effectively optimized through four times of 5%reduction followed by annealing treatment at 1160? for 5min,then solution at 1175? for 15min.The fraction of special grain boundaries increased by 15%and the random boundary network was significantly dispersive.Grain boundary evolution was driven by dislocation decomposition and the migration of mobile dislocation.Precipitation behavior of HX alloy and thermal stability of microstructure were studied through long-term aging at 850?.M6C carbides were found to precipitate in the matrix,while ? phases tended to precipitate on random grain boundaries(RGB).Compared with the base material(BM),GBCD in grain boundary engineered material(GBEM)effectively suppressed the nucleation and coarsening of the harmful ? phases on the grain boundaries.The formation of ? phase was closely related to the grain boundary structure.Due to Mo depletion in vicinity of RGB,the formation of the precipitates was inhibited on the adjacent grain boundaries.The thermal stability of GBCD was confirmed at 850? for 720h without significant microstructural degradation.Assessment of intergranular corrosion resistance of BM and GBEM was carried out by electrochemistry impedance spectrum and H2SO4+Fe2(SO4)3 corrosion tests.The effect of special grain boundary on intergranular corrosion was investigated.The result showed that the capacitive reactance of GBEM was much larger than that in BM,leading to much lower corrosion sensitivity in GBEM than in BM.During H2SO4+Fe2(SO4)3 corrosion test,the ratio of corrosion of BM kept much higher compared with that of GBEM,which showed that the weight loss ratio of BM was almost 3 times of GBEM.The surface morphologies revealed that the BM sample was seriously destroyed through grain dropping.In contrast,the intergranular corrosion was significantly depressed in GBEM where few grains dropping could be observed.It was attributed to low ? CSL which could effectively suppress ion diffusion and corrosion crack propagation.Effect of GBCD on oxidation resistance is studied by early stage oxidation and long term oxidation tests.The relationship among void,oxygen diffusion and oxidation was further investigated.Early stage oxidation results at or above 900? indicate that the ratio of weight gain in GBEM is much lower that of BM.Results of oxidation test at 1000? for 5d indicate that the optimized GBCD is good for the growth of oxidation film with less defects and the fraction of corroded boundaries in GBEM is markedly decreased than that in BM.Internal oxidation along grain boundaries is closely related to boundary energy.Low angle boundaries and ?3 boundaries are immune to oxidation,while slight oxidation is observed on some special boundaries such as ?5 and ?9.And the random boundaries are prone to be oxidized.The result showed that the voids were oxygen carrier during inward diffusion.Precipitates on grain boundaries and diffusion voids would accelerate oxidation,which were related to grain boundary structure.The creep behaviors at 600?-300MPa and 950?-30MPa of HX alloy were studies.The relationships between precipitates and dislocation at different temperature were investigated and the effect of GBCD was evaluated through creep tests at 950? with tensile stress of 30MPa.The results showed that intergranular fracture is the main fracture characteristics of HX alloy,where all the.cracks initiation on random boundaries.Precipitates could effectively hinder dislocation slipping at low stress,but are inclined to become crack cores at high stress.At high temperature,the hindering effect of precipitates on dislocation is weak.Due to the size effect of GBCD processing,the grain boundary network of GBEM creep samples weren't optimized undergoing the same TMP.Meanwhile,the residual dislocation in GBEM creep sample induced nucleation of numerous precipitates during sensitization.It significantly makes the strengthening effect of solute atoms in the matrix weaken and sharply deteriorated the creep property of sensitized GBEM sample.With the modifications of longer annealing time during TMP and smaller creep sample size,the creep life of GBEM sample was significantly increased. |
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