Composition Design And High-temperature Microstructural Stability Of Fe-Cr-Al-based Alloys Based On A Cluster Model

Fe-Cr-Al ferrite stainless steel has the advantage of higher temperature oxidation resistance,corrosion resistance and high temperature mechanical properties than Zr alloys,which is expected to be the next generation accident-tolerant fuel(ATF)cladding material.However,the microstructural stability of this material at high temperatures is critical for its use as fuel cladding materials in nuclear reactors,while is closely related to the types and contents of microalloyed elements.In this dissertation,the characteristics of Fe-Cr-Al-M alloy composition and the addition of trace elements M(M = Mo,Nb,Ti,Zr,Ta,etc.)were firstly studied under the guidance of the "cluster-connected atom" structural model,so the Fe-Cr-AlM series alloys were designed according to the cluster model.And then,the structure and characterization of the series alloy were systematically characterized,thus the effects of addition of Fe-Cr-Al-M series alloying elements on the microstructure stability and the evolution of microstructures with temperatures were revealed,and the influence of microstructure change on the mechanical properties of the alloy were also studied.Thus providing a new ferrite stainless steel as the candidate material for accident-tolerant fuel(ATF)cladding.Alloy ingots were prepared by vacuum arc melting,and then solution-treated at 1200 oC for 2 h before hot-rolled at 800 oC into sheets.The sheet samples were aged at 800 oC for 24 hrs,followed by re-solution treatments at 1000 oC,1100 oC and 1200 oC for 1 h.The microstructures of samples under various treatments were compared to study the HT microstructural stability of this series of alloys.Structural identification,microstructure and elemental distributions were investigated by means of XRD,OM,SEM and EPMA respectively.Mechanical property results were carried out on HV micro-hardness facility and MTS universal test machine.The results showed that the fine precipitates(Laves phase)are distributed homogeneously in the ferritic matrix of the aged Mo/Nb alloyed samples.When the Mo / Nb ratio is 2: 1,the Laves phase precipitates more uniformly and disperses.On this basis,the volume fraction of Laves phase decreases obviously when Ti alloy is added,while the Laves phase distribution occurs when Mo/Ti/Zr is alloyed,and Volume fraction is also significantly reduced;On the basis of the composition of Mo / Nb = 2: 1,Ta was alloyed,Ta microalloying did not change the size of the second phase particles.However,these particles begin to re-dissolve into the matrix after 1000 oC/1h solution treatment.Moreover,these particles disappeared in the Mo/Nb containing alloy of M1-1 after 1200 oC/1h solution,while Ta further minor-alloying could still ensure a certain amount of precipitates distributed on the grain boundaries,which effectively suppresses the abnormal growth at HT.From the results of EPMA,it could be seen that the content of Ta in the complex Laves phase of Fe2(Mo,Nb,Ta)increased with the increase of Ta content.On the basis of the composition of Mo / Nb = 2: 1,the size of the second phase particles couldn't changed with Zr alloyed samples.After 1000 °C to 1200 °C solution,Zr further minor-alloying could still have more precipitates on the grain boundary.These precipitates could effectively prevent the growth of ferrite grains.The size of precipitations would increase with the increasing of the content of Zr,and the Zr content should be controlled at 0.1 wt.%.