| With the increasing energy consumption,traditional energy sources alone cannot meet the needs of society,and the development and utilization of new energy sources have received wide attention from various countries.Nuclear fusion energy is an important clean energy source,and nuclear fusion reactor is the key device for nuclear fusion energy.The blanket is an important structure for energy conversion and irradiation shielding in nuclear fusion reactors.The blanket is close to the plasma and is exposed to high temperatures,high neutron irradiation and other complex service environments,where high temperatures can lead to organizational changes and affect the stability of the blanket material.Vanadium alloys are highly promising candidates for fusion reactor blanket.Therefore,it is important to study the microstructure and mechanical properties of vanadium alloys at high temperatures by simulating the service environment of vanadium alloys through heat treatment,and to analyze the stability of vanadium alloys at high temperatures.In this thesis,the effect of indentation size effect on the hardness of vanadium alloy is investigated by testing the nanoindentation hardness of vanadium alloy under different loads,and finding ways to eliminate the effect of indentation size effect on the nanoindentation hardness in order to obtain accurate nanoindentation hardness of V-4Cr-4Ti alloy.The aging treatment parameters were adjusted to simulate the service environment of the vanadium alloy in a nuclear fusion reactor,and the vanadium alloy was aged at 773-973 K/10-50 h.The precipitation was formed in the matrix by the aging treatment,and the composition of the precipitation was determined,and the interface between the precipitation and the matrix was analyzed from a crystallographic point of view using high-resolution lattice images under the orthocrystalline band axis to determine the orientation relationship between the precipitation and the matrix,and further The mechanism of precipitation formation and its effect on the mechanical properties of V-4Cr-4Ti alloy were further investigated.The hardness and strength of the vanadium alloy were analyzed by testing to establish a modified relationship between hardness and strength.The main conclusions are as follows:(1)In order to study the effect of indentation size on the hardness of vanadium alloys,nano indentation methods were used to test V-4Cr-4Ti alloys in cold rolled and solid solution states at different peak loads.The results show that for both materials,the indentation depth increases with the increase of peak load.Under the same load,the pressing depth of solid solution V-4Cr-4Ti alloy is larger,and the trend of increasing the pressing depth is greater with the increase of load.Both cold rolled and solid solution V-4Cr-4Ti alloys exhibit indentation size effects(ISE),which are excluded by the Nix-Gao model.In addition,the study also found that grain size also affects the indentation size effect,and analyzed using the plastic deformation theory of grain boundary sliding.(2)The mechanism of precipitation formation and its effect on mechanical properties were studied by aging treatment of V-4Cr-4Ti alloy.At 773-873 K/10 h,the precipitates are lath like,and at 973 K/10 h,the precipitates change from lath-like to polygon.The results of calibrated selected area electron diffraction and elemental analysis indicate that the precipitates of both shapes are composed of Ti(CN).Using high-resolution lattice images under the orthorhombic belt axis,it was found that after773-973 K/10 h aging treatment,Ti,C,and N elements formed a face-centered cubic structure together,and there was a semi-coherent interface between the precipitated phase and the matrix.The shape,size,and density of precipitates have significant effects on the mechanical properties of V-4Cr-4Ti alloy.The hardness,tensile strength and conditional yield strength increase with the increase of size and density,and the lath-like precipitates reduce the total elongation.Compared with lath-like precipitates,although the size and density of polygon precipitates have decreased,the hardness,tensile strength,conditional yield strength and total elongation have increased.Compared with V-4Cr-4Ti alloy,polygon precipitation is more obvious in strengthening than lath precipitation.The formation of precipitates can be affected by prolonging the holding time at 873 K.Lath-like are formed in V-4Cr-4Ti alloy after holding for 10 h,and polygon precipitates are formed after holding for 20 h and 50 h.The size and density of precipitates increase with the prolongation of holding time.The precipitates in both shapes are Ti(CN).Both precipitates can increase the hardness and strength of the matrix.As the size and density of lath-like like precipitates increase,the hardness and strength of the matrix increase,and the total elongation decreases.As the size and density of the polygon precipitates increase,the hardness of the matrix first increases and then decreases,and the strength decreases.By testing the mechanical properties of V-4Cr-4Ti alloy after different aging treatments,it was found that the Vickers hardness and nanoindentation hardness of V-4Cr-4Ti alloy after 873 K/20 h were the highest,and the total elongation was the highest;When the aging treatment parameter is 873 K/10 h,the ultimate tensile strength and conditional yield strength of the sample are the highest.After 873 K/20 h,V-4Cr-4Ti alloy has the best comprehensive properties.(3)By combining the data of V-4Cr-4Ti alloy after different aging treatments,the relationship between Vickers hardness,tensile strength,and conditional yield strength was analyzed.By establishing the correlation between Vickers hardness,ultimate tensile strength,and Vickers hardness,conditional yield strength,and conducting experimental verification,it was found that the experimental results can better match the modified relationship between Vickers hardness,ultimate tensile strength,and Vickers hardness,conditional yield strength.The relationship established in this thesis provides a good reference for deriving ultimate tensile strength and conditional yield strength through hardness. |
PDF Full Text Request