Study On The Preparation,Microstructure And Property Of SiC Nanoparticle Dispersion Strengthening Ni-based Alloys

Molten Salt Reactor?MSR?with advantageous features in inherent safety,economy,fuel cycle capabilities and a low primary loop pressure,has been selected as one of the advanced Generation IV concept reactors.The structural materials used for the core of MSR are subject to a great challenge of high-temperature strength,strong irradiation and corrosive molten fluoride salts.Hastelloy N is considered as the candidate material in a number of MSR designs due to its outstanding corrosion resistance.However,the weakness in high-temperature strength and irradiation resistance limits its application in the core of MSR.In this study,we successfully prepared a novel Ni-SiC composite for the application in the core of MSR via a powder metallurgy route including high-energy ball mill,spark plasma sintering,etc.It is found that SiC nanoparticles significantly improve the mechanical strength of Ni matrix,while keeping its good ductility when an appropriate amount of the SiC nanoparticle is added.This strengthening capacity is improved with a decrease in the grain size of the Ni matrix.In addition,SiC nanoparticles remain stable in the Ni matrix during sintering,annealing and high-temperature aging due to the incoherent interfacial relationship between SiC nanoparticles and Ni matrix.However,SiC nanoparticle coarsening happens with an increase in the initial amount of SiC nanoparticles and ball milling time,thus reducing mechanical properties of the Ni-SiC composite.Furthermore,it is shown that the grain growth of the Ni-SiC composite happens during high-temperature aging.However,long-time-milled Ni-SiC composite keeps its mechanical property during aging due to the good stability of its ultra-fine microstructure at high temperatures.Finally,it is found that low-temperature aging effectively refines the Ni matrix and decreases the internal defects,resulting in an improvement of microstructure and mechanical property.Unfortunately,the mechanical property of Ni-SiC composite is lower than that of Hastelloy N and still not satisfactory for the application in the core of MSR.Based on the preparation of Ni-SiC composite,we chose Mo as an additive and prepared a new class of NiMo alloy,which keeps the advantages of carbide dispersion strengthening in addition to precipitation strengthening by nano-precipitates.A new dispersion strengthening mechanism,namely Dispersion and Precipitation Strengthening?DPS?,is proposed.It is shown that these novel NiMo-SiC DPS alloys possess superior mechanical properties to the Ni-SiC composites developed using a similar processing route.The NiMo matrix of the new alloy is reinforced by dispersion strengthening of residual?unreacted?SiC nanoparticles from the initial powder mixture in addition to the precipitation strengthening of Ni₃Si nano-precipitates precipitated during the sintering/annealing process.Furthermore,the matrix is also strengthened by the solid-solution of Mo in Ni.The improvement of mechanical properties and the evolution of microstructure have reached designed expectations.On the other hand,well-dispersed Mo₂C particles produced during sintering effectively inhibit the grain growth of NiMo matrix.However,Mo₂C easily agglomerates together in the NiMo matrix,leading to a decrease in the ductility of the NiMo-SiC DPS alloy.It is found that the longer time ball milling effectively optimizes the distribution of Mo₂C and limits the agglomeration of Mo₂C,resulting in a much finer grain size and higher Hall-Petch?grain boundary?strengthening on the NiMo matrix.However,the existence of micro-crack caused by cold welding during ball milling results in a poor density of the long-time-milled NiMo-SiC DPS alloy.The poor density leads to an unexcitedly low total elongation of the alloy.As a result,NiMo-SiC DPS alloys possess a superior mechanical strength to Hastelloy N and other corrosion resistive alloys.This indicates that DPS mechanism possess a high performance in strengthening efficiency.Moreover,the successful preparation of the SiC nanoparticle dispersion strengthening Ni-based alloys?Ni-SiC composite and NiMo-SiC DPS alloy?provides a new way for the development of new structural materials,which aim to be used in the high-temperature MSR in future.