Investigation On Composition Design,micro Structure And Property Of Low Cr ODS-FeCrAl Alloy

Nuclear power is an important low-carbon energy.However,the Fukushima nuclear accident in 2011 proved that the traditional Zr alloy cladding materials can not meet the safety requirements under the accident conditions.Countries are actively seeking accident-tolerant fuel systems and their cladding materials that can still work normally under accident conditions.Meanwhile,they are vigorously developing the Generation Ⅳ nuclear energy system and controllable fusion energy.Oxide Dispersion Strengthened(ODS)FeCrAl alloy has become an important candidate for accident tolerant fuel(ATF)cladding material due to its excellent mechanical properties and oxidation resistance.At the same time,it is also one of the important candidate materials for cladding of the Generation IV nuclear energy system cladding and fusion reactor blanket.The properties of ODS FeCrAl alloy are mainly affected by the content of alloying elements Cr and Al,whose addition is essential to improve the high temperature oxidation resistance by forming a dense and continuous protective alumina film.At the same time,its content also has a great influence on the high temperature aging brittleness of the alloy.Therefore,in order to develop ODS FeCrAl alloy that can meet the service requirements,it is necessary to investigate the mechanical properties,high temperature microstructure stability and oxidation resistance of the main alloy components(Cr and Al).Traditional ODS FeCrAl alloy generally has high Cr content to help form Al2O3 scale.However,the alloy with high Cr content is prone to form Cr-rich brittle phase within the working temperature range and under irradiation conditions,leading to a decrease in plasticity and fracture toughness,resulting in material failure and safety hazards.Therefore,it is necessary to minimize the Cr content and weaken the hardening and embrittlement of the alloy.In order to compensate for the oxidation resistance of low Cr alloy,some trace elements(Y,Zr,Si,etc.)were introduced into FeCrAl alloy to further improve the oxidation resistance and/or strengthening effect at high temperature.Therefore,a Zr-containing low Cr ODS FeCrAl alloy with a Cr content of 5wt.%,namely 5CrAlZr ODS steel,was prepared by high-energy ball milling mechanical alloying and hot isostatic pressing(HIP)process.At the same time,two representative ODS FeCrAl alloys with Cr content of 9wt.%and 16wt.%were prepared by the same process,namely 9CrAlZr ODS alloy and 16CrAlZr ODS alloy.The main research contents include:1)The effects of different Cr contents on the microstructure and mechanical properties of ODS FeCrAl alloy were investigated.The results show that as the Cr content increases from 5 wt.%to 16 wt%,the grain size of the alloy has little change.However,the grain size distribution of 5CrAlZr ODS alloy is narrower than that of the other two samples.The oxide dispersion particles in the three samples are mainly Y-Zr-O type particles,and also contain a small amount of Y-Al-O type particles.The average size and number density of oxide particles in 5CrAlZr ODS alloy and 16CrAlZr ODS alloy are similar,while the average size of particles in 9CrAlZr ODS is larger and the number density of particles is lower.In addition,with the increase of Cr content,the proportion of Y-Al-O particles in ODS FeCrAl alloy increases.In terms of room temperature ten sile strength,the tensile strength of 5CrAlZrO ODS alloy and 9CrAlZrO ODS alloy is similar,which is lower than that of 16CrAlZrO ODS alloy.In terms of high temperature tensile at 700℃,the tensile strength of the three samples has little difference,but the 5CrAlZr ODS alloy has the highest elongation,which is the best in the three alloys.In terms of impact toughness,DBTT of ODS FeCrAl steel increases with the increase of Cr content.The DBTT of 5CrAlZr ODS steel is only-81℃,which is significantly better than that of 9CrAlZr(-18℃)and 16CrAlZr ODS steel(-2℃).This indicates that the low chromium design with chromium content of 5 wt.%is helpful to improve the impact toughness of ODS-FeCrAl alloy.2)The aging behavior of ODS FeCrAl alloys with different Cr contents at 1000℃,1100℃ and 1200℃ was studied.The results show that the design of reducing the chromium content to 5 wt.%can make the microstructure of ODSFeCrAl alloy have better high-temperature aging stability.With the increase of aging temperature,the tensile strength of the three alloys decreased to different degrees.Compared with 9CrAlZr ODS and 16CrAlZr ODS alloys,the tensile strength and impact toughness of 5CrAlZr ODS steel after high temperature aging have the smallest decrease and the most stable performance.3)The effect of Cr content on the high temperature oxidation resistance of ODS FeCrAl alloy in air and water vapor environment was investigated.The results show that the pre-oxidation treatment significantly improves the oxidation resistance of 5CrAlZr ODS alloy in air and water vapor environment.The preoxidized 5CrAlZr and 16CrAlZr ODS steels showed almost the same oxidation weight gain curve after being oxidized in air at 1200℃ for 120h,while the 9CrAlZr ODS steel had a faster oxidation rate.For the unpreoxidized ODS FeCrAl alloy,when the temperature was 1000℃ and 1200℃,16CrAlZr ODS steel has the best high-temperature oxidation resistance in water vapor,followed by 5CrAlZr ODS steel,and 9CrAlZr ODS steel has the worst oxidation resistance.The pre-oxidized 16CrAlZr ODS steel still has the best resistance to high temperature water vapor oxidation after being oxidized in water vapor at 1200℃ for 120 min,followed by 5CrA]Zr ODS steel and 9CrAlZr ODS steel,respectively.4)The effects of Si on microstructure,mechanical properties and high temperature oxidation resistance of 9CrAlZr ODS steel were studied.The size of oxide particles in 9CrSiAl ODS steel with Si replacing Zr and 9CrAlZrSi added with Si and Zr obviously decreased,while the number density obviously increased.This shows that the introduction of Si is beneficial to refine the average size of oxide particles in 9CrAl ODS steel and increases their number density.In terms of mechanical properties,the tensile strength of the alloy with Si addition is higher than that of 9CrAlZr ODS steel,and 9CrSiAlZr has the highest tensile strength at room temperature and high temperature.From the perspective of impact properties,the difference between 9CrSiAl ODS steel and 9CrAlZr ODS steel is not significant,which are higher than that of 9CrSiAlZr ODS steel.In terms of oxidation resistance,the oxidation resistance of 9CrSiAl ODS steel and 9CrSiAlZr ODS steel is better than that of 9CrAlZr ODS steel.In summary,the strength and high temperature oxidation resistance of 9CrAl ODS steel can be significantly improved by introducing trace Si element instead of Zr.This paper also explored the optimization of the mixing process of graphene nanoplates and ODS steel powder,and studied the effect of graphene content on the microstructure and mechanical properties of 5CrAlZr ODS steel.The results show that through the exploration of the mixing process of ODS steel powder and 2 wt.%graphene nanoplates,it is found that the composite prepared by dry ball milling at 400 rpm for 24 h has the best combination of tensile strength and plasticity,and its tensile strength reaches 1250MPa,which is 37%higher than that of ODS steel without GNP addition.After adding 0.5-2wt.%GNPs,the strength and plasticity of bulk GNPs/ODS steel composites were improved.When the GNP content is above 2.0 wt.%,the strengthening effect and plasticity of the composites decrease sharply due to the aggregation or agglomeration of GNP.