High Temperature Tensile Properties of Low-oxygen V-5Cr-5Ti Alloy
Li Qing Li Zengde Xie Xingcheng
GRIMAT Engineering Institute Co.,Ltd.
Abstract:
In the process of preparation and later deformation processing of vanadium based alloy,impurity elements such as O,C,N in the alloy were easy to react with matrix elements to form second phase particles,such as Ti(CON),which affected the processability and application performance of the alloy. The oxygen content of V-5Cr-5Ti alloy studied in many literatures was above 0.038%(mass fraction). The purpose of this paper was to study the microstructure evolution and high temperature tensile properties of V-5Cr-5Ti alloy with low oxygen content at different annealing temperatures. Low-oxygen V-5Cr-5Ti alloy with 0.018%~0.022%(mass fraction)oxygen content was prepared with vacuum electron beam melting method. The V-5Cr-5Ti alloy ingot was processed by vacuum packaging,and then rolled into 8 mm plate by multi pass rolling. The vacuum annealing tests(temperature of 980,1020,1060,1100 ℃;time of 1.5 h;vacuum degree less than 2×10-2 Pa)were carried out on the alloy plate. The microstructure of V-5Cr-5Ti alloy sheet after vacuum annealing was analyzed by optical microscope(OM). The alloy's tensile mechanics and fracture behavior characteristics at 25,300,900 and 1100 ℃ were studied by electronic universal testing machine and scanning electron microscope(SEM).The results showed that:(1)The recrystallization structure of the deformed low-oxygen V-5Cr-5Ti alloy was obtained after vacuum annealing at 1020 ℃ for 1.5 h. The grains were equiaxed with the smallest size.(2)The tensile mechanical properties of low-oxygen V-5Cr-5Ti alloy were similar to those of conventional alloy. The two alloys had high tensile strength,low yield ratio,and excellent application properties at 25 ℃ and 300 ℃;and they had good plasticity,high yield ratio and excellent plastic processing properties at 1100 ℃.(3)A large number of fine equiaxed dimples,a small number of elongated dimples,tearing edges and some cleavage steps were dispersed on the fracture surface of room temperature tensile alloy. The fracture surface of low-oxygen V-5Cr-5Ti alloy,which was stretched at 300,900 and 1100 ℃,showed equiaxed dimples of different sizes and depths,and the dimples grew from small and shallow to large and deep with temperature increasing. The second phase particles could be clearly seen at the bottom of some dimples.(4)During the tensile deformation of V-5Cr-5Ti alloy with low oxygen content at 300 and 900 ℃,Ti atoms reacted with a small amount of interstitial impurity atoms(C,O,N,etc.)to form new second phase particles,which increased the content of second phase particles in the alloy. Under the action of tensile stress,a large number of micropores were formed in the second phase,which grew up,gather and finally fracture,thus forming dimples on the fracture surface. The second phase grew rapidly and no recrystallization could be obtained after tensile deformation at 900 ℃. At 1100 ℃,the second phase particles decomposed and the second phase decreased. At the same time,the dynamic recrystallization occurred in the alloy at high temperature. The plastic deformation ability of the alloy was enhanced and large and deep dimples appeared. Therefore,compared with the V-5Cr-5Ti alloy with high oxygen content(about 0.038%),the tensile strength of the alloy with low oxygen content slightly decreased,but the plasticity increased significantly,so it had better comprehensive mechanical properties. The low oxygen content of V-5Cr-5Ti alloy reduced the content of the second phase particles in the matrix to a certain extent,which improved the plasticity of the alloy. The tensile fracture of low-oxygen V-5Cr-5Ti alloy at the experimental temperature was mainly ductile fracture. Tensile fracture was quasi-cleavage fracture at 25 ℃,and typical micropore aggregation fracture at high temperature.
Fig.3 Comparison of tensile properties of two different V-5Cr-5Ti alloys at different temperatures(A:low-oxygen V-5Cr-5Ti alloy;B:conventional V-5Cr-5Ti alloy)
(a)Ultimate tensile strength and yield strength;(b)Total elongation and area reduction at fracture
图4 低氧含量V-5Cr-5Ti合金在不同温度下的拉伸断口宏观形貌
Fig.4 SEM images of low-oxygen V-5Cr-5Ti alloy tensile specimens at different temperatures(a)25℃;(b)300℃;(c)900℃;(d)1100℃