Carbide Inclusions In Uranium-niobium Alloys And Their Effects On Mechanical Properties

U-Nb alloys have special usage because of their high density,idiosyncratic nuclear properties and excellent corrosion resistance.Elemental addition of Nb is the key for enhancing the corrosion resistance.In the high-temperature y-U phase,the Nb element is fully soluble and this condition can be retained to room temperature by quenching from the y-U phase.Different metastable phases(?',?" or ?°)can be obtained depending on the Nb contents.As the niobium content is varied between 3.5 wt.%and 6.5 wt.%.a monoclinic ?"metastable phase is produced.Among the three metastable martensitic phases(?',?" or ?°),the ?" phase U-Nb alloys have been studied extensively,because the excellent ductility and superior corrosion resistance of this particular composition qualifies it for use in several applications.Uranium and niobium elements have a huge difference in density,therefore,in order to dissolve the niobium in the liquild uranium,long time holding at high temperature or electromagnetic stiring is applied during uranium metallurgy process.However,carbon impurity element contamination still can't be avoided,although the crucible protected by the coating during uranium metallurgy process.Carbon impurity element are easy to react with uranium and niobium to produce carbide inclusions,which have a significant disadvantage on plasticity and toughness.Therefore,the aim of this paper is investigating the formation mechanism of carbide inclusions,clarifying the variation of morphologies in primary Nb2C carbide inclusions,obtaining the effect of carbide inclusions on mechanical properties of U-5.5Nb alloy,and discussing the microvoid formation mechanism during tensile deformation at different strain.This study can provide basic support for carbide inclusions control and promotation of mechanical properties in U-5.5Nb alloy.Multi-means are used to investigate the microstructure,phase structure and mechanical properties,the main results are as follows:(1)Effect of carbon impurty element on the microstructure and characteristic of inclusions is investigated by different carbon addition doping.The result shows that the carbon impurity element has a significant effect on carbide inclusion morphology and the matrix microstructure.At the low carbon addition,Nb2C inclusions show a polygonal shape and the matrix has a columnar grain morphology,while with the increase of the carbon addition,columnar grains transform into equiaxed grains and the average grain size decreases from 107 ?m to 24?m.Moreover,at a high carbon addition condition,the polygonal Nb2C inclusions grow into the rectangle shape and aggragate with each other to form clusters.(2)Mechanical properties of carbide inclusions in U-5.5Nb alloy are investigated by the mean of nanoindentation.The results show that the elastic modulus and hardness in Nb2C inclusion are 246.5±23.7 GPa and 24.6±3.1 GPa,respectively,while the corresponding values in U(N,C)inclusion are 153.5±3.0 GPa and 4.3±0.1 GPa respectively,and the corresponding values in matrix are 72.1±3.0 GPa and 2.6±0.1 GPa,respectively.The elastic modulus and hardness of Nb2C inclusion are higher than those in U(N.C)and matrix shows that the Nb2C inclusion is hard spot in the matrix.The variations of mechanical properties between carbide inclusions and matrix result in different microvoid nucleation mechanisms in U-Nb allov during deformation(3)The difference of mechanical properties between U(N.C)and Nb2C inclusions with matrix results in different microvoid formation mechanism.The results of the microstructure analysis after interrupted testing show that no voids form during twinning deformation However,after twinning deformation,voids first nucleate from U(N,C)inclusion cracks,while Nb2C inclusions maintain their original shape without cracking or undergoing decohesion at the interface.With a further increase in strain.Nb2C inclusions become involved in void nucleation through the decohesion of the Nb2C-matrix interface.Nb2C inclusions tend to generate voids due to decohesion from the matrix interface,which is ascribed to the hardness and strength of Nb2C inclusions being higher than those of the matrix The Nb2C-matrix interfacial strength was calculated to be 0.9-1.2 GPa(4)The two U-5.5Nb alloys with different carbon impurity levels show a typical single?" phase structure in the matrix.carbon impurity element has little effect on the tensile strength,but has a significant effect on alloy ductility and toughness.With increasing of carbon impurity elements,elongation and Charpy impact toughness significantly decrease The results can ascribe to the formation of more microvoid promoted by increasing of carbide inclusions.More microvoids lead to easy formation of cracks,which result in the early fracture of matrix with a reduced ductility and toughness.