Study Of He Effects In Ferritic/Martensitic Steel

Large number of He atoms will be introduced into structural materials served in reactors due to(n, α) reaction. The existence of these He atoms will result in swelling and degradation of mechanical property of structural materials. Among the many candidate structural materials, special focuses are paid on ferritic/martensitic(F/M) steel because of its superior radiation swelling resistance, good thermal conductivity and good mechanical property. It is also the main reason that commercial F/M steel T91 and reently developed reduced activation F/M steel SIMP were chosen as the materials investigated in this work.TEM was employed to record the microstructural changes of T91 steel and SIMP steel, He bubbles morphology, distribution and size after He implantation under series of conditions. Discussions about the evolution of He bubbles morphology and size were made based on TEM analysis. It was found that the stable shapes of He bubbles in both steels are cube or cuboid under He implantation at 300℃, 450℃ and 550℃ to a dose of 1×1020ions/m2、5×1020ions/m2 and 1×1021ions/m2, respectively. The He bubbles are preferentially distributed along dislocations, grain boundaries and the surface of precipitations. After statistics made on He bubbles sizes, it was found that the average size of He bubbles in T91 is larger than the average size of He bubbles in SIMP under all the implantation conditions, which proves that SIMP steel has a superior ability to suppress the growth of He bubbles. SEM analysis shows that the surfaces of SIMP do not blister under He implantation at room temperature(RT), 300℃, 450℃ and 550℃ to a dose of 5×1021 ions/m2. However, the surfaces of T91 steel blister after He implantation at RT, 300℃ to a dose of 5×1021 ions/m2.In order to provide valuable information for development of nanosize-grain steel with a better radiation-resistant property, we investigate grain size effects on He bubbles morphology and distribution. SMAT treated T91 were implanted with He ions under the same condition before TEM analysis were made on them to investigate He bubbles morphology and distribution. It was found that after SMAT treatment, the coarse grains in T91 steel disappeared, the martensitic lath with width of 20-50 nm formed at a depth of 500 nm near the surface. The lath structure formed by SMAT treatment increases the density of boundaries and provides more sinks for He atoms. However, TEM analysis shows that almost all of the He bubbles were distributed along the lath boundaries, they connected with each other and fill with the lath boundaries and He bubbles within grains were hardly found. He bubbles distribution described above is very detrimental to material radiation resistant property and it is the main reason to cause the intergranular fracture. We thought that in the process of developing materials with better radiation resistant, grain size is not the smaller the better, it should be larger than the free mean path of He bubbles Brown motion. In order to prevent He bubbles from accumulating at the certain kind of defect, different kinds of defects that have different strength to absorb He atoms should be introduced into steels instead of increasing the density of just one kind of defect.In order to investigate the radiation induced hardening of SIMP steel, we investigated the hardness of SIMP after a series of He implantation by nano-indentation test. After nano-indentation test, TEM analysis was employed to make the statistic on the density and size of defects in samples. We calculated the contribution of different kinds of defects to materials hardness with DBH model based on TEM analysis. Compared with nanoindentation test, we found that the main reasons that cause the hardness change of SIMP steel are the existence of He bubbles and dislocation loops and He bubble is a kind of defect that has a larger barrier strength factor than dislocation loop has.