| Mo alloy has high melting point,high hardness and high temperature creep resistance.It has been developed and utilized as a new generation of cladding material.Mo alloy will undergo catastrophic oxidation in severe loss of coolant accident.It is expected to solve the oxidation problem of Mo by preparing Zr coating on the surface of Mo pipe.However,the cracking oxidation effect of Zr alloy at high temperature has limited protective effect on the matrix.The mutual diffusion of coating and matrix elements at high temperature could not be ignored.The protection of Mo-Zr system relative to the matrix is worth exploring.Nb element is a common trace element in cladding.It is very important to study the effect of Nb element addition on the high-temperature phase relationship of Mo-Nb-Zr alloy,further study the effect of alloy composition on the microstructure,thickness,phase composition and element distribution of Mo-Nb-Zr alloy oxide layer,and explore the high-temperature oxidation process and high-temperature oxidation mechanism to meet the service requirements of Mo based alloy cladding materials.The main work and research results of this subject are as follows:Six kinds of Mo-Zr binary as-cast alloys were prepared by arc melting.The high temperature oxidation behavior of Mo-Zr as cast alloy at 1100 oC was studied by isothermal oxidation,including the change of oxidation kinetic curve,oxide surface,cross-section microstructure and so on.After isothermal oxidation for 1h,the mixed oxide layer of m-Zr O2/Mo O3 with multi holes and crack defects is formed on the alloy surface,which has little protection to the matrix.Through the intermediate corrosion layer of the alloy,it is found that the preferential oxidation of alloy phase（Mo）,（Zr）and Laves（Zr）and the subsequent selective oxidation of Laves phase and（Mo）.Various factors affecting the formation of defects in the oxide layer were investigated,including Zr O2 martensitic transformation,volume expansion of oxide formation and growth,and mismatch between cooling process and matrix.The 950 oC and 1500 oC isothermal sections phase diagram and vertical liquidus of Mo-Nb-Zr system were measured by SEM-EDS,EPMA and infrared high temperature measurement.It is found that compared with the phase diagram drawn by the latest Mo-Nb-Zr thermodynamic database,the miscibility gap of BCC phase near the Nb-Zr end of 950 oC isothermal section has a wider solid solubility range,the BCC+Laves two-phase region measured by 1500 oC isothermal section is narrower,and the liquidus of x（Mo）+x（Nb）=0.67,x（Mo）+x（Zr）=0.6 vertical section is slightly lower than the calculated liquidus.Mo-Nb-Zr ternary as cast alloys with eight components were prepared by arc melting technology,and Nb（0.8%,1%and 15%）content gradients were set in Laves rich alloy S4（65Mo-35Zr,at%）.The 1100 oC isothermal oxidation behavior of as-cast Mo-Nb-Zr alloy was studied.It was found that the high temperature oxidation resistance of Mo-Nb-Zr alloy with Nb（8%～15%）was worse than that of Mo Zr alloy,and that of S4 alloy with 0.8%Nb was worse than that of 1%Nb and 15%Nb.The oxide layer is a mixed oxide layer composed of m-Zr O2,c-Zr O2,Mo O3 and Nb2O5.It is found that the intermediate corrosion layer has a good role in preventing the diffusion of O in the matrix,and the possible causes of catastrophic oxidation of oxide layer failure are put forward:additional stress accumulation of intermediate corrosion layer,expansion mismatch caused by polymorph and anisotropy of Nb2O5,volume expansion and thermal expansion mismatch of Zr O2 polymorph,etc. |
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