Study On The Evolution Mechanism Of Laves Phase In CLAM Steel During Long-term Aging

With the continuous development of technology,energy depletion and environmental problems have become increasingly serious.Nuclear fusion energy is considered as a feasible solution for future energy development due to its advantages such as abundant nuclear fuel resources,large energy production and cleanliness.In the process of nuclear fusion reaction,the fusion reactor cladding/first wall need to withstand complex physical effects such as high temperature,cyclic thermal stress,and neutron irradiation.CLAM（China low-activation martensitic）steel,as one of independently developed low-activation martensitic（RAFM）steels by China,has great high-temperature mechanical properties,low thermal expansion coefficient and irradiation swelling rate,and is considered as a candidate structural materials for the fusion reactor cladding.After nearly 20 years of development,component design and heat treatment methods of CLAM steel have matured.However,researches of long-term aging have rarely been reported.This paper simulates the temperature characteristics of fusion reactor cladding service,conducts thermal aging experiments at 500℃,550℃,600℃,and 650℃for up to 30,000 h,and systematically studies the evolution of microstructures and new phases at different temperatures—The influence of Laves phase evolution on mechanical properties,reasonable assessment of the service life of CLAM steel,and provide data and theoretical support for the development of CLAM steel in the future.During the long-term aging,the Laves phase is nucleated and precipitated in the CLAM steel with the evolution of the microstructure,which is mainly A2B type intermetallic compound-Fe₂W.This paper mainly uses metallographic microscope and transmission electron microscope to observe the evolution of microstructure,analyzes the influence of different temperatures on the microstructure,and characterizes the crystal structure and alloy composition of Laves phase;the BSE imaging method of scanning electron microscope is used to characterize the Laves phase.Evolutionary behavior,quantitative analysis of Laves phase,and finally mechanical properties testing of aging specimens,including tensile,impact and microhardness testing,to establish the relationship between tissue evolution and new phase precipitation and changes in mechanical properties.Research indicates:.The results show that after heat treatment,CLAM steel is a typical tempered martensite structure;no Laves phase precipitates during aging at 500℃,and the tempered martensite structure is preserved intact,only a certain occurrence of martensite lath was observed.Widened,and the dislocation density did not decrease significantly.During long-term aging at 550℃and600℃,the Laves phase nucleates and precipitates at the grain boundaries according to the attachment of M₂₃C₆ carbides.With the extension of time,the Laves phase continuously aggregates and coarsens and swallows M₂₃C₆ carbides.Due to the Ostwald ripening effect,the number of Laves phase precipitates no longer increases,and only aggregation and coarsening between particles occurs.The final average size of the Laves phase is 420 nm（550℃）and 600nm（600℃）.The grain boundaries are distributed in clusters and large clusters,which seriously reduces the bonding force between grain boundaries and the ability to coordinate deformation.Laves phase was not found at 650℃,so Laves phase was not precipitated,but the microstructure was degraded rapidly,and the coarsening rate of carbides increased.In the early stage of aging at500℃,the tensile strength and hardness increased due to precipitation strengthening,and the strength gradually decreased after 5000 h,but still maintained a high strength value.At 550℃and600℃,the nucleation of the Laves phase grows,which reduces the solid solution strengthening effect of the W element,resulting in a decrease in strength and hardness.Due to the coarsening of the Laves phase at the interface,the bonding force between the grain boundaries decreases,and the impact absorption energy decreases.At 650℃,the martensite structure is degraded and replaced by sub-crystals.M₂₃C₆ carbides are coarsened.Martensite strengthening and precipitation strengthening disappear,resulting in varying degrees of strength and hardness reduction.1.The CLAM steel gets tempered martensite structure after normalizing and tempering heat treatment;its microstructure is fine,and the width of the martensite lath is about 200 nm.After aging for 30000 h at 500℃,Laves phase was not found in the structure of CLAM steel,and the tempered martensite structure remained intact.Only a little broadening of the martensite lath was observed.2.During the long-term aging process at 550℃and 600℃,the Laves phase nucleates and precipitates at the grain boundaries according to the attachment of M₂₃C₆ carbides,the conversion time is prolonged.As time goes on,it continuously aggregates and engulfs the attached M₂₃C₆carbides.Due to the Ostwald ripening effect in the late 20,000 h,the precipitation amount of Laves phase no longer increases,and only the aggregation and coarsening of particles occur;after30000 h,the average size of Laves phase reaches 420 nm（550℃）and 600 nm（600℃）,Along the grain boundaries and sub-grain boundaries are distributed in chains and large clusters,which seriously reduces the bonding force and coordinate deformability between the grain boundaries.3.During the long-term aging process,the diffusion and segregation of solid solution atoms-W toward the interface is the main reason for the nucleation of Laves phase;because of the influence of the grain boundary diffusion mechanism,the Laves phase mainly grows at the grain boundary and sub-grain boundary.The rate of conversion is large.The growth and coarsening of the Laves phase seriously reduces the solid solution strengthening,precipitation strengthening and interface strengthening effects of CLAM steel.4.650℃reached the dissolution temperature point of Laves phase,so no precipitation of Laves phase was found at this temperature,but the increase of temperature accelerated the degradation of microstructure,and the coarsening rate of M₂₃C₆ carbides increased.At the early stage of aging at 500℃,the tensile strength and hardness of CLAM steel increased due to precipitation strengthening,and then slowly decreased after 5000 h.The high strength value was still maintained after aging.At 550℃and 600℃,the nucleation of Laves phase grows,which reduces the solid solution strengthening effect of W,and the strength and hardness decrease.The coarsening of the Laves phase at the interface leads to a decrease in the bonding force between grain boundaries,which leads to a decrease in impact toughness.At 650℃,the martensite structure degradation was replaced by sub-crystals,and the coarsening of M₂₃C₆carbides led to a significant decrease in martensite strengthening and precipitation strengthening.Therefore,The strength and hardness decreased to varying degrees after aging.