Study On The Effect Of He Ion Irradiation On Microstructure And Properties Of CLAM Steel Weld

The structural material of the first wall of nuclear fusion reactor directly faces the plasma and needs to withstand 14.1 Me V high-energy neutron irradiation,so it must have excellent irradiation damage resistance.The first wall structure usually needs to be assembled by Tungsten inert gas welding,while the weld metal is one of the weak area in the welded joint.China low activation martensite（CLAM）steel is one of the preferred alloys for the first wall structure of the nuclear fusion demonstration reactor.It is of realistic significance to carry out research on the irradiation damage of the CLAM steel weld metal.In this study,He⁺ion irradiation would be used to simulate neutron irradiation,and to analyze the impact mechanism of irradiation dose(5×10¹⁵,5×10¹⁶,1×10¹⁷ ions/cm²)on microstructure and mechanical properties of as-welded and quenched and tempered welds.Use XRD,X-ray stress tester,SEM,LSCM and AFM to observe the crystal structure and surface morphology of the as-welded and quenched and tempered welds before and after irradiation;TEM and TEM-EDX were utilized to measure the distributed pattern of irradiated defects and the element segregation at grain boundaries;Nanoindentation technique and tensile test were applied to characterize irradiated hardening and embrittlement phenomena.This research work was listed as follows.（1）Research on the crystal structure and surface morphology of as-welded and quenched and tempered welds under three irradiation dosesAfter He⁺ion irradiation,the（110）main diffraction peaks of the as-welded and quenched and tempered welds both shifted to the small angle;when the irradiation dose was low,the diffraction peak deviation was larger,while the irradiation dose was over 5×10¹⁶ ions/cm²,the deviation decreased;under the same irradiation dose,the diffraction peak deviation of the quenched and tempered weld was smaller than that of as-welded weld.The X-ray stress measurement results showed that compressive stress was introduced into the irradiation damage area of the welds.The size and density of the black holes on the welds surface grew with the increasing irradiation damage;larger and denser holes were more likely to be emerged in the as-welded weld.When the dose was in the range of 0～5×10¹⁶ ions/cm²,the surface roughness value of the welds increased faster;under the same irradiation dose,compared with the as-welded weld,the growth rate of surface roughness value of the quenched and tempered weld was lower.（2）Research on the evolution mechanism of the microstructure of as-welded and quenched and tempered welds at high irradiation doses(1×10¹⁷ ions/cm²)After irradiation,the dislocation loops with b=<100>were formed in the welds,and the size and number density of the dislocation loops in the as-welded weld were both larger.Helium bubbles nucleated and grew at the grain boundaries and dislocations of the welds.The size of the helium bubbles in the welded weld was 2.47 nm,the number density was 2.7×10²³ m^-3,and the irradiation swelling rate was 1.7%;The size of the helium bubble in the quenched and tempered weld was 1.70 nm,the number density was 2.1×10²³ m^-3,and the irradiation swelling rate was0.4%.The size of the voids formed in the as-welded and quenched and tempered welds were approximately 21.15 nm and 14.12 nm,respectively.The solute elements Si and Ta enrichment and Fe depletion at the grain boundary of the as-welded weld were relatively larger,while the enrichment of Cr at both welds’grain boundaries were not obvious.（3）Research on the change law of mechanical properties of as-welded and quenched and tempered welds under three irradiation dosesAfter irradiation,the nano-hardness of the as-welded weld changed from 5.1 GPa to 7.6 GPa,and the nano-hardness of the quenched and tempered weld increased from 3.9 GPa to 5.21 GPa;the nano-hardness value gradually increased with the rising irradiation damage,but the growth trend tended to be saturated;at the same dose,the hardening degree of the quenched and tempered weld was lower than that of the as-welded weld.The fracture mechanism of the welded joints before and after irradiation were microporous aggregation type fracture.The tensile strength of as-welded welded joints changed from 875.88 MPa to 945.26 MPa,and the tensile strength of the quenched and tempered welded joints increased from 669.87 MPa to 694.09 MPa;from the dimple morphology,the plasticity of the quenched and tempered samples were better;the size and depth of the dimples of the two types of welded joints decreased with the increasing irradiation dose,which indicated that the ion irradiation would cause the plasticity of the material to deteriorate in some certain extent.