Study On Grain Refinement And Performance Of U-5.5 Wt.% Nb Alloy

U-Nb alloys have been studied widely due to their good corrosion resistance and mechanical properties.So far,U-Nb alloys are usually prepared by casting,which will inevitably lead to grain coarsening and inhomogeneous.It is known that grain size has a very important influence on the properties of materials.Generally,strength of the material increases with the decreasing grain size.Besides,different grain size leads to alteration of the bulk and surface of a material,leading to different electrochemical behaviors.Hitherto,there are few reports on the grain refinement and its effect on properties of U-Nb alloys.Therefore,systematic reserch on grain refinement and its effect on properties of U-Nb alloys is of great instructive to both academic and engineering.In this paper,U-5.5 wt.%Nb alloys with different deformation were prepared by cold rolling.The micro structural evolution of U-5.5 wt.%Nb alloys were studied by metallographic microscope,X-ray diffraction(XRD),focused ion beam and transmission electron microscope(TEM)during grain refinement.The recrystallization and grain growth behaviors were investigated by microhardness and metallographic microscope.The quasi-static tesion tests of U-5.5 wt.%Nb alloys with different grain size were carried out using mechanical testing mechine,and the fracture morphologies were analyzed by scanning electron microscope(SEM).The effect of grain refinement on the micro-mechanical properties of U-5.5 wt.%Nb alloy were investigated by nanoindentation The open circuit potential,electrochemical impedance spectroscopy and potentiodynamic polarization of U-5.5 wt.%Nb alloys with different grain size were measured,the corrosion morphology and composition were analyzed by SEM and EDS,respectively.The effects of grain refinement on its mechanical and electrochemical properties were studied.The main conclusions are as follows:During the rolling process,grains were flattened and pulled into strips along the rolling direction.With the increase of deformation,grains and inclusions were gradually crashed,grain boundaries became blurred,and substructures appeared in grains.The diffraction peak of the rolled samples have been broadened.The greater the deformation,the higher the annealing temperature or the longer the annealing time,the easier the recrystallization was.The larger the deformation,the smaller the recrystallized grains.With the increase of annealing temperature or time,grains were grown?p gradually.The a" U-5.5 wt.%Nb alloy with an average grain size of 4.3 ?m has been prepared by rolling and heat treatment.The martensite and crystal structure in the samples did not change after grain refinement.However,the solute atoms were enriched near the grain boundaries,and the relative concentration of uranium decreased with the decreasing distance from grain boundary.After isothermal annealing at 750 ?,the recrystallization kinetics of the alloys coincide with the classical S curve,the recrystallization index n was 1.584,and the recrystallization activation energy of 314.47?555.84 kJ/mol.The grain growth exponents of alloy at 700?and 750 ? were 2.24 and 2.21,respectively.The activation energy of grain growth for alloy samples annealed at 30 min and 60 min were Q30min=211.72 kJ/mol and Q60min?186.56 kJ/mol,respectively.After grain refinement,the yield strength of U-5.5 wt.%Nb alloy was reduced by 5.75%,tensile strength decreased by 5.60%,elongation increased by 30.06%,and the reduction of area increased by 33.75%.The results of nanoindentation show that the hardness and elastic modulus decreased by 11.8%and 21.8%,respectively.In neutral and alkaline solutions,both samples showed typical activation-passivation-overpassivation corrosion behavior.After grain refinement,the U-5.5 wt.%Nb alloy has a wider passivation area and higher breakdown potential.In acidic solution and 3.5 wt%NaCl solutions,no passivation behavior was observed in both samples.Pitting corrosion occurred quickly in NaCl solutioa After grain refinement,pitting potential and the impedance increased,and the corrosion current density decreased,and the corrosion resistance was increased.