Effects Of Impurity On The Microstructure And Mechanical Properties Of Hafnium

Hafnium has many advantages,such as high temperature resistance,large neutron absorption cross-section area,excellent corrosion resistance,strong absorption capacity and good comprehensive mechanical properties,and thus have attracted more and more attention,research and application at home and abroad.As an important strategic material,hafnium has been widely used in national economy and national defense development,especially in nuclear industry and modern ceramics.The mechanical properties of hafnium play a key role in its application.There are many factors affecting the mechanical properties of hafnium,such as chemical composition,texture,strain rate,and temperature.The influence of chemical composition on its mechanical properties plays a critical role and can not be ignored.Therefore,it is important to understand and master the influence of non-metallic impurity elements(O,N,C,H,etc.)on the microstructure and mechanical properties of hafnium.It is the focus of this paper.In this paper,the influence of non-metallic impurity elements on the microstructure and properties of high-purity metals at home and abroad is studied by combining first-principles simulation calculation with basic experiments.The influence of stable occupancy and content of non-metallic impurity atoms in hafnium on the microstructure and mechanical properties of hafnium is investigated.Firstly,the occupancy of O,N and C in hafnium was simulated by first-principles calculation based on density functional theory(DFT).It was determined that the stable interstitial positions of O,N and C atoms in hafnium were octahedral and hexahedral positions.With the increase of O,N and C content,the axial ratio of hafnium increases slightly,which indicates that high non-metallic impurity content may weaken the ductility of hafnium.From the density of states diagram,it can be seen that the width of the pseudogap is wider than that of pure hafnium when the content of non-metallic impurities increases,which indicates that the bond type tends to more covalent.At the same time,the related elastic constants are calculated,and the ratio of bulk modulus(B)to shear modulus(G)changes.It can be seen that the B/G value decreases with the increase of the content of non-metallic impurities,that is,the plasticity of hafnium decreases with the increase of the content of non-metallic impurities.In the experimental part,hafnium with different contents of non-metallic impurities was prepared by high vacuum non-consumable arc melting technology.Among them,the contents of O,N and C in samples ranged from 50 to 360 ppm,30 to 630 ppm,30 to 100 ppm respectively,and the H content remained 10 ppm all the time.As-cast,rolled and annealed samples with different impurity contents were prepared.The effects of non-metallic impurity elements on the microstructure and mechanical properties of hafnium were studied by optical microscopy,hardness,compression test,tensile test and SEM.The experimental results show that as the content of non-metallic impurity elements increases,no obvious changes are observed in the as-cast and deformed microstructures,while the grain size of the annealed samples decreases to some extent.But compared with fine-grained strengthening,it is said that solid solution strengthening plays a major role,so the hardness,compressive yield strength,tensile yield strength and tensile strength of each sample are significantly improved,and the elongation is reduced.It is mainly attributed to the lattice distortion caused by non-metallic impurity atoms dissolved in hafnium and the formation of Cottrell atmosphere,which pins and hinders the movement of dislocations,thus greatly enhancing the strength and hardness of materials.At the same time,from the point of view of compression and tension fracture,the compression fracture is basically river pattern,which is typical cleavage fracture;while the tensile fractures are mostly cleavage surfaces,and there are obvious tear marks in the section.With the increase of impurity content,the tearing edges in the section increase gradually.When the oxygen equivalent exceeds 280 ppm,the fracture mode of annealed samples changes from quasi-cleavage fracture to complete brittle fracture,which further indicates that the higher is the content of non-metallic impurities,the worse is the plasticity and toughness of hafnium.