Study On Microstructure Formation And Creep Behaviour Of The Transient Liquid Phase Bonding Joints Of RAFM Steel

Reduced activation ferritic/martensitic(RAFM)steel has been intensively used as the preferred structural material for the first wall of the nuclear fusion reactors because of the characteristics of better thermo-physical and thermo-mechanical properties.The research of welding technology and process is crucial to the manufacturing and commercial application of nuclear reactors for safety,efficiency and stability.In this paper,transient liquid phase(TLP)bonding was adopted to obtain reliable joints of RAFM steel with Fe-Si-B amorphous foil.The microstructure of TLP joints with different bonding time was characterized by optical microscope(OM),scanning electron microscope(SEM),transmission electron microscope(TEM),electron back scatter diffraction(EBSD),and electron probe microanalyzer(EPMA).Furthermore,the shear test,Vickers hardness test and high temperature creep test were carried out to reveal the relationship of microstructure evolution and properties.The conclusions are as follows:The TLP joint region is mainly comprised of isothermal solidification zone(ISZ),diffusion affected zone(DAZ),and base material(BM).Block-like M₂₃C₆ carbides and?500 nm-size elliptical Fe₂B phase are precipitated in the isothermal solidification zone,while the larger-size rod-like Cr₂B phase with size of?3?m is distributed in the diffusion affected zone.With extending the homogenization time,the amount of Cr₂B phase reduces dramatically in the diffusion affected zone.The time required for the joint homogenization of chemical concentration largely depends on the diffusion of the Si atoms.The concentration distribution of the joint could be calculated by a finite difference time domain(FDTD)method to estimate homogenization time for TLP process.The shear failure occurred in the diffusion zone.With extending the homogenization time,the shear strength has a significant improvement,and the fracture transitions from brittle mode to ductile mode.In high temperature creep tests with different stress,failure of all TLP joints occurred at the base material.The main factors affecting the creep performance of TLP joints are the recovery of substructures and the coarsening and deformation of martensite laths.In addition,the M₂₃C₆ carbides in the base material were obviously coarsened,which weakens the pinning effect of M₂₃C₆ carbide on the grain boundary.Large-sized M₂₃C₆ carbides also accelerated the nucleation of the microvoids,and then microvoids continuously aggregated,merged,and expanded,further forming microcracks.It is worth noting that the microstructure of the joint changes obviously with the stress loading.The microstructure of the isothermal solidification zone of the TLP joint is ferrite and martensite.The size of ferrite is decreasing,the dislocation density and the proportion of the large angle grain boundary are continuously increasing with increasing the stress,which can effectively hinder crack propagation and improve the performance of the joint.