| Titanium alloy is widely used in aerospace,shipbuilding,automotive,petrochemical,metallurgy and biological fields because of its high specific strength,good corrosion resistance and excellent biocompatibility.Alloying is a general method to improve the properties of titanium alloy,adding a certain amount and kinds of alloy elements to titanium to regulate the structure of titanium alloy,so as to improve the comprehensive mechanical properties of the alloy.Some studies have shown that the addition of some alloying elements can make the transformation of titanium alloy,thus significantly improving the strength and plasticity of titanium alloy.In this paper,the Ti-Cr-Al-Nb alloy was prepared with Cr as the main alloy element and Al and Nb as the auxiliary alloy elements.According to the principle of molybdenum equivalence,the alloy was determined to be Ti-x Cr-3Al-2Nb(x=2,4,6,8,9,10,11,12wt%).The as cast alloy ingots were melted in vacuum non consumable arc furnace,and the as rolled samples were obtained by hot rolling.OM,XRD,SEM,TEM and other characterization techniques were used to systematically analyze the microstructure and phase transformation of the alloy,and the mechanical properties were tested by universal testing machine.Firstly,the effects of different Cr contents on the microstructure and mechanical properties of the alloy were studied.Cr is?stable element,and the content of Cr directly affects the type of titanium alloy.After calculating the molybdenum equivalent,it is found that when the Cr content is less than 7wt%,the alloy is?+?titanium alloy.The rolled Ti-2Cr-3Al-2Nb alloy has the highest tensile yield strength,750MPa,851MPa,14%elongation,4wt%and 6%Cr content The work hardening phenomenon appears in the drawing process of wt%alloy after rolling,and the hardening rate increases with the increase of Cr content.The yield strength is 484MPa and 233MPa respectively,the tensile strength is 851MPa and 839MPa respectively,and the maximum elongation is 21%and 19%respectively.When Cr content is more than 7wt%,the alloy is a metastable?-type titanium alloy.It is found that there is a secondary yield platform in the tensile stress-strain curve of Ti-8Cr-3Al-2Nb alloy.The primary yield strength is 364MPa,the secondary yield strength is 537MPa,and the maximum elongation reaches 42%.When Cr content is 9wt%?12wt%,there is no secondary yield and the yield strength is strong The degree of plasticity increased from 622MPa(9 wt%)to 786 MPa(12 wt%),which showed a trend of decreasing and increasing.Secondly,based on the comparison of the mechanical properties of the alloy,Ti-8Cr-3Al-2Nb was selected as the object to analyze the microstructure of the alloy under different tensile values.It was found that hump appeared when?=2.5%,peaked when?=5%,and even plastic deformation occurred when??8%.The samples were made in the deformation region with?=5%,?=10%,as well as the rolling state and the region near the fracture.XRD analysis showed that the content of martensite?"phase increased rapidly from 14.72%to 42.64%in the region of?=0 to?=0.05,and the growth of martensite?"phase decreased in the region of?=0.05 to?=0.10,while the growth of martensite?"phase increased little from?=0.10 to complete fracture.Thirdly,it is possible to increase the strength of titanium alloy by adding appropriate amount of Si element.Ti-8Cr-3Al-2Nb alloy is selected as the object,and Si content of0.25,0.5,1.0wt%is added to the alloy.The results show that the secondary yield plateau disappears after adding Si,the yield strength of the alloy is obviously improved.The yield strength of the alloy is positively correlated with Si content and negatively correlated with plasticity.The addition of Si element will precipitate Ti3Si phase in the alloy matrix,which can improve the strength and reduce the plasticity of the alloy. |
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