Interface Healing Mechanism Of Oxide Dispersion Strengthened Steels Produced By Additive Forging

Oxide-dispersion-strengthened(ODS)steels have been considered as candidate materials for nuclear reactors because of their advanced creep properties,excellent irradiation tolerance,and superior corrosion resistance based on nanoscale oxides with unparalleled stability at elevated temperatures.It widely reported that conventional fusion welding leads to the agglomeration of nanoparticles and disruption of the fine grain distribution.Therefore,in order to use ODS steels for structural components with large,complex structures,robust joining techniques are urgently needed.Additive forging(AF)can take advantage of the coupled effects of temperature and deformation to facilitate interfacial grain boundary migration and atomic diffusion.What's more,it does not create a local overheating zone at the bonding interface,which can prevent the disruption of the original microstructures and the distortion of fabricated components.Hence,in this work,in order to understand the correlation between the DRX mechanism and the bonding efficiency,ferrite steels with 14 wt.%Cr(hereinafter referred as high Cr ferrite steels)were studied since it is the parent material of 14YWT alloy.And then AF was successfully applied to ODS steels.The main research contents and conclusion can be summarized as follows.(1)AF was applied to join 14Cr ferrite steels in the temperature range of 950?1200 ? under the strain rate of 0.01?30s-1 with strain from 0.11 to 0.51.At low strain rate of 0.01 s-1,it can be found that the bonding interface remain distinct and unreliable.The restoration mechanism within this materials is classified as CDRX,featured by progressive rotation of subgrains until the transformation from strain-induced sub-boundaries to regular grain boundaries.In addition,SIP of Ti(C,N)is sufficient,but detrimental to the elimination of bonding interface.(2)This alloy could be successfully bonded at a temperature of at least 1100?with strain of 0.51 when applied high strain rate(10 s-1 and 30 s-1),which exhibits nearly the same tensile properties compared to the base material.In this case,it can be found there is transition from CDRX to DDRX by applied increasing strain rate,contributing to the healing of bonding interface.Due to the extreme rapid deformation,the formation of sub-boundaries is severely suppressed,resulting in the enhanced pile-up of dislocation and therefore higher stored energy,Meanwhile,the SIP process is sluggish,these factors can provide favorable condition for the occurrence of DDRX.DDRX mechanism involves the nucleation along the pre-existing grain boundaries and subsequent growth via long-distance migration of HAGBs,which can eliminate high-density defects at the original interface.(3)AF were carried out for 14YWT alloy and the effects of surface roughness,deformation temperature(750??1100?)and deformation(0.11?0.51)on the bonding quality were investigated.The results shows that the inclusions along the bonding interface are significantly reduced when the surface roughness is lowered,which improve the interfacial bonding quality.And the Cr23C6 inclusions at the interface are gradually reduced with the temperature increasing.As the strain level increases,recrystallization within the material occurs sufficiently,which is beneficial to the healing of the interface.14YWT steels can be successfully bonded at a temperature of at 950? under strain rate of 0.01 s-1 with a true strain of 0.22 and surface roughness of 0.113?m,without degrading the fine grain and nanoparticle distribution,and the presence of inclusions or micro-voids along the bonding interface.Moreover,the joints had nearly the same tensile properties compared to that of the base material.(4)Subsequent micro structure characterization demonstrates that CDRX characterized by progressive subgrain rotation occurred in this alloy,but DDRX characterized by strain-induced grain boundary migration(SIBM)and subsequent bridging sub-boundary rotation take place concurrently at the vicinity of the bonding interface.The activation of interfacial DDRX can be ascribed to the nonuniformity of the nanoparticles,and thereby the grain size,which result in the deformation incompatibility,therefore stored energy differences across the bonding boundaries.Hence,This would promote the occurrence of SIBM along the bonding boundaries.And then the bulging areas of the boundaries are potential sites for the formation of DRX nuclei and it will grow with ongoing deformation by long-distance migration of grain boundaries,contributing to the healing of the original bonding interface.Therefore,it can inferred the operation of DDRX at bonding interface play a dominant role in achieving robust and effective joints during AF process.