Tuning Mechanical Properties And Irradiation Resistance Of FCC-based Multi-principal Element Alloys Via Nano-precipitation

As a new kind of sustainable clean energy,nuclear power has been one of the important pillars of the global power supply.More importantly,the fourth generation fission reactor and nuclear fusion reactor are considered as one of the means that can completely solve the human energy problem.High-performance structural materials used in reactors play an important role in the safe and stable operation of nuclear power plants.Due to the complex and harsh service environment in future reactors,traditional nuclear reactor materials can no longer meet service requirements.Exploration of next-generation nuclear reactor materials is one of the key to the development of future nuclear energy technologies.Recent studies show that the face-centered cubic(fcc)multi-principal element alloys(MPEAs)containing strengthening pricipitates are expected to be suitable candidate materials because of their outstanding mechanical properties and radiation resistance.However,effects of the precipitate characteristics on the related properties,especially the correlation between the precipitates and the irradiation resistance,are still not well understood.In light of this,this dissertation focuses on tuning of mechanical properties and radiation resistance of several fcc MPEAs via nano-precipition.The main findings are as follows:(1)Radiation behavior of(CoCrFeNi)94Al4Ti2 MPEAs strengthed by the γ’nano-precipitates was studied.The results show that the γ’ precipitates in the MPEA became completely disordered,and their constituents were dissolved into the matrix due to the ballistic mixing effect during the room-temperature irradiation process.In addition,the maximum size of dislocation loops induced by the irradiation in the fcc single-phaseMPEA exceeds 100 nm,whilst that in the aged alloy containing theγ’ precipitates is only 26 nm,indicating that the high-density dispersed γ’precipitates effectively hinder the growth of dislocation loops,which is benefical for the improvement of the radiation resistance.(2)The Co-free(Cr0.7FeNi)94Al4Ti2 MPEA containing nano-precipitates and the(Cr0.7FeNi)91Al7Ti2 MPEA containing multiple kinds of precipitates were developed.By adjusting the heat treatment process,the types and characteristics(e.g.,volume,size and distribution)of the precipitates were regulated,leading to different mechanical properties.It is worth mentioning that the(Cr0.7FeNi)91Al7Ti2 MPEA exhibits a high yield strength up to 1673 MPa with decent ductility of 14%simultaneously.(3)Evolution behavior of three different types of precipitates in the(Cr0.7FeNi)91Al7Ti2 MPEA under high-temperature irradiation was studied.The results show that the interfacial relationship between the precipitates and the matrix determines the stability of the precipitates under the high-temperature irradiation environment.Due to its extremely low interfacial energy with the matrix,the γ’precipitates were quickly nucleate and re-precipitate after being bombarded by the high-energy particles and displacing the lattice.However,the incoherent disordered bcc(body-centered cubic)and ordered B2 precipitates were difficult to nucleate after the solute atoms diplacing the lattice due to excessive interfacial energy,and finally dissolved into the matrix.Therefore,the γ’ precipitates that can exist stably under high-temperature irraidaiton is an ideal strengthening meduim in radiationresistant MPEAs.(4)Both room-temperature and high-temperature irradiation behaviors of the(Cr0.7FeNi)94Al4Ti2 MPEA containing only the γ’ precipitates,were systematically studied,and the stability mechanism of γ’ under the high-temperature irradiation was explored.The results show that no voids formed in the(Cr0.7FeNi)94Al4Ti2 MPEA under the high-temperature and high-dose irradiation condition(i.e.,600℃and 100 dpa),whilest under the irradiation of 100 dpa at 400℃,only a few dislocation lines and small size dislocation loops(<5 nm)in the irradiated region were found.It can be seen that the(Cr0.7FeNi)94Al4Ti2 MPEA exhibits prominent radiation resistance under the high-temperature irradiation environment.It was found that,in the irradiation temperature range of 400-600℃,the γ’ precipitates underwent the dynamic process of "dissolution-reprecipitation" under the coupling effect of irradiation and high-temperature,and eventually reached the final irradiation equilibrium state.The dynamic evolution behavior of the γ’ precipitates can promote the displaced atoms and vacancies induced by the irradiation to shortrange reshuffle,thereby inducing the annihilation of lots of point defects,preventing the point defects from long-range diffusion and improving the radiation resistance of the alloy.In addition,experimental verification shows that the initial state of the precipitates does not affect the final irradiation equilibrium state of the alloy.Under different irradiation temperatures,the equilibrium concentration of γ’precipitates(i.e.,70 at.%Ni,15 at.%Al,10 at.%Ti and 5 at.%Fe+Cr)remains unchanged,whereas the final equilibrium size increases with the increase of the irradiation temperature.To sum up,a series of Co-free MPEAs strengthened by nano-precipitates were designed.The "dissolving-reprecipitation" dynamic evolution behavior of the coherent γ’ precipitates under the high-temperature irradiation environment was found,and the mechanism responsible for the improved radiation resistance was revealed,which provides a guidance for designing the novel reactor structural materials.