First observe the air cooling at different heating temperatures. 1000 ℃ is already in the β phase region of TC4 titanium alloy, and its air-cooled structure is equivalent to β-annealed structure. The microstructure is the coarse original β grain boundaries and the needle-like α inside the β grains, which corresponds to a higher tensile strength and lower tensile plasticity. 950 ° C and 930 ° C are in the upper part of the TC4 titanium alloy two-phase zone. The samples obtained different proportions of the primary α-phase and β-phase during heat preservation. During the subsequent air cooling, the β-phase undergoes multiple plastic transformations to precipitate the α-needle. Both are equiaxed primary α-phase and β-phase. Obviously, the amount of primary α-phase is higher on the sample held at 930 ° C. At this time, the tensile strength is similar to that of the sample at 1000 ° C, but the tensile plasticity and impact toughness are almost the highest. At 830 ℃, it is at the lower part of the two-phase zone of TC4 titanium alloy. The number of primary α phases is so large that the transformed β phase morphology cannot be seen on the microstructure. Only the intercrystalline β between the equiaxed primary α phases is seen. The tensile strength decreased slightly, but the tensile plasticity was higher.
In addition, the original structure of the TC4 titanium alloy before heat treatment has a great influence on the microstructure and mechanical properties after heat treatment. For TC4 titanium alloy, the original structure can be roughly divided into two categories. One type is similar to the above-mentioned structure after 1000 ° C furnace cooling, that is, a basket-like structure, which is generated after air firing or processing with a small amount of deformation at a temperature higher than the phase transition point; the other type is α and β Or the equiaxed α plus the transformed β structure is produced by processing a large amount of deformation above the transformation point or by processing different amounts of deformation below the transformation point. Experiments show that the former is difficult to be changed by the subsequent heat treatment. This kind of structure, after annealing (700-800 ℃, 1 hour of air cooling), is usually lower in tensile plasticity and section shrinkage than the equiaxed (primary α plus transformed β) structure, but high temperature creep resistance and fracture Toughness and hot salt stress corrosion resistance are high.
Obviously, the properties of TC4 titanium alloy are various. In order to ensure the required stability of the final product, the original structure and heat treatment process should be considered comprehensively.