Description：Four kinds of TC4 titanium alloy rolled plates with different thicknesses were selected and studied for its structure and mechanical properties by using metallographic microscope and mechanical properties test.
Keywords: TC4 titanium alloy plate; metallographic organization; mechanical properties; anisotropy; fracture morphology
Abstract: The tissue of the TC4 titanium alloy sheet after rolling and annealing is a mixed tissue composed of α phase and residual β phase, α The morphology of the phase presents the characteristics of outgoing strip, equal axis and small group; The overall strength of TC4 titanium alloy plate shows a trend of decreasing first and then stabilizing with the increase of thickness, And the plasticity shows a trend of first rising and then stabilizing; At the thickness of 0.8mm, TC4 titanium alloy sheet, Tensile strength of 1075 MPa and yield strength of 1027 MPa, And the strength and plasticity of TC4 titanium alloy plate along RD and TD directions after rolling and annealing; The microscopic fracture morphology of TC4 titanium alloy plates of different specifications is mainly toughness, In addition to the fracture morphology of the TC4 titanium alloy plate with a thickness of 0.8mm, Also has a certain number of small planes, There are a large, particularly small microcracks inside the tough nest.
TC4 titanium alloy (the nominal component is TI-6Al-4V) is a typical α + β two-phase alloy with excellent corrosion resistance, strong toughness, high temperature mechanical properties and many other excellent properties. It has been widely used in aero-engine, ocean engineering, chemical engineering and other fields. This alloy is also known as universal titanium alloy.
The production process of TC4 titanium alloy mainly includes smelting, forging, rolling, etc. At present, the application of TC4 titanium alloy rolling plate is widely used 3-4]. Han Ying et al. studied the influence of the rolling process on the texture evolution and performance of TC4 titanium alloy sheet, and showed that after the forward rolling process and forward rolling refine the grains, the α grains are stretched; after the forward rolling process, the grains break more evenly, and a large number of isoaxial α grains were formed; the two rolling processes are plastic, but the strength is low. Wang wei et al 6] studied the rolling fire of EB melting TC4 titanium alloy microstructure, texture and mechanical properties, the study shows that the increase of rolling fire makes the cast tissue of original coarse grain broken, the grain in the axial, and small α phase formation, the small Angle boundary increases, the alloy after three fire rolling small Angle boundary increased by 35.1%. This paper analyzes and studies TC4 titanium alloy plates with different rolling specifications, and explores the corresponding relationship between the organization and mechanical properties of TC4 titanium alloy plates with different rolling thickness, which provides reference for actual production.
1. Test materials and methods
The specification of TC4 titanium alloy slab selected in this test is 250 mm 1000 mm 2000mm, After heating up through tunnel natural gas heating furnace and roll bottom resistance furnace, At temperatures ranging from 850 to 1,020℃, By φ 900 / φ840 mm 2450 mm four-roll reversible hot rolling mill, The first fire rolling is billet rolling, The rolling thickness of the first to third times is 50.0,6.0 and 3.5mm, respectively, The 4th fire rolling is finished rolling, The semi-finished TC4 titanium alloy plates with a thickness of 3.5 mm are rolled into TC4 titanium alloy plates with a thickness of 2.0,1.5,1.0,1.0 and 0.8 mm, Then, it was annealed for 780℃ 2h, Their specific components are shown in Table Table 1. The original metallographic structure of TC4 titanium alloy slab is shown in Figure 1. Its longitudinal and transverse tissues are isoaxial structure. This tissue is dominated by large primary α phase (α,), and the β transition tissue (β) composed of thin strip secondary α phase and secondary α phase on the substrate.
TC4 titanium alloy plate phase change point test GB / T 23605-2009 standard requirements, using the phase change point of TC4 titanium alloy plate is 995~1000℃. Then the four specifications of TC4 titanium alloy plates were cut and processed, and the properties tests of metallographic tissue and room temperature stretching were carried out. Figure 2 shows the direction marks of rolling TC4 titanium alloy plate: rolling direction (RD direction) and transverse (TD direction), where Figure 2a shows the sampling position of metallographic tissue, Figure 2b shows the sampling position of tensile sample and the observation position of tensile fracture. Use the OLYMPUS optical microscope to observe the metallographic structure of TC4 titanium alloy plate, the room temperature of TC4 titanium alloy plate and high temperature tensile test using INSTRON-5580 universal testing machine, use Image pro5 to measure the diameter of the grains in the tissue, observe the micromorphology of the tensile fracture by using Quanta scanning electron microscope, take 3 groups of samples for each test, and finally take the average value.
2 Results and Discussion
2.1 Metallographic Organization
Figure 3 shows the metallographic organization of TC4 titanium alloy plates with different specifications.
According to FIG. 3, the TC4 titanium alloy plates rolled and annealed are organized as a mixed tissue composed of α phase and residual β phase. The morphology of the α phase of the TC4 titanium alloy plates with different thicknesses has different shapes, showing the outgoing strip, isoaxial shape and small shape, and the residual β phase exists between the α phases. Compared with the original TC4 titanium alloy plate, the grain refinement of TC4 titanium alloy is obvious after rolling annealing, and it has obvious signs of rolling, which is due to the organization of complete dynamic recrystallization of TC4 titanium alloy plate. In the RD of TC4 titanium alloy plates of 0.8 0,1.0 and 1.5mm, among, Band organization was most obvious in TC4 titanium alloy plates of 1.0mm thickness, This is because: during the rolling process, On the cylinder and base surface of TC4 titanium alloy plate, The alignment and orientation of the grains does not change, Only around the c axis (c axis is from the transverse (TD) tendency to the normal (ND) and close to the normal (ND), To duce a strong base surface texture), But the cone slide is different, It can cause the grain to produce a tilt phenomenon, And the tilt can recrystallize or rotate the grains in the tissue. However, the band tissue of TC4 titanium alloy sheet with a thickness of 2.0 mm is not obvious, and its α grains are obviously elongated, showing long strips and unevenly distributed in the tissue. The four tissues of TC4 titanium alloy plates are mainly small isoaxial α grains, and no obvious ribbon structure was found in the metallographic tissue.
TC4 titanium alloy plate RD and TD to tissue exist in a large number of fine α grains, indicating that the stress state of TC4 titanium alloy plate changed in the process of rolling, the original TC4 titanium alloy slab grain is not twist along a fixed direction, storage and dislocation density in the deformation area, for the core of the organization provides a large drive energy, cause TC4 titanium alloy plate is easy to crystallization in the process of annealing, makes a large number of small isoaxial α grains produced in the tissue 8. This is consistent with the results of Wang Niujun et al. 9 on TC4 titanium alloy plate tissue under different processing conditions, that is, the rolling process has a refining effect on the grains in the tissue.
2.2 Tensile properties
Figure 4 shows the mechanical properties of TC4 titanium alloy plates with different thicknesses, and Figure 5 shows the engineering stress-engineering strain curve during the stretching process of TC4 titanium alloy plates with different thicknesses. As can be seen from Figure 4 and Figure 5, with the increase of thickness, the strength of TC4 titanium alloy plate shows a trend of decreasing first and then stabilizing, while the plasticity shows a trend of increasing first, and then stabilizing. The strength of TC4 titanium alloy plate of different specifications is low and the plasticity is higher, because: annealing will produce the processing hardening and dislocation density reduction in the rolling process, so that the rolling stress can be fully released. When the thickness is 0.8 mm, the TC4 titanium alloy sheet has the greatest strength and the maximum tensile strength R.1075 MPa, maximum yield strength R, 1027MPa, while for plasticity, the elongation A of TC4 titanium alloy plates of different specifications is roughly the same, with the maximum value of 17.5%.
TC4 titanium alloy plate at room temperature, when the tensile stress along the interface expansion, the dislocation formed in the expansion process will cause plug on the α / β phase interface, under the action of dislocation product force and the crack tip force, the α / β phase layer, the microcrack expansion in microhole polymerization, eventually lead to fracture 10. And the thickness of 0.8 mm TC4 titanium alloy plate strength is larger, this is due to TC4 titanium alloy plate in the process of rolling deformation is larger, the internal dot matrix distortion is larger, form the storage can more, which will have more recrystallization core position, more tiny, fine particles in the process of stretching to the obstruction of dislocation motion is stronger, dislocation in small grain area is blocked and form the dislocation accumulation group, must apply more external force can start dislocation again, lead to TC4 titanium alloy plate strength is higher.
As can be seen from FIG. 4, the TC4 titanium alloy plate along the RD and TD directions, among which the 0.8 mm TC4 titanium alloy plate is the most obvious, with its tensile strength R. The maximum difference is 33 MPa, the maximum difference of yield strength R is 32MPa, while the elongation A is almost the same, the maximum difference is 3%, indicating that the 4 specifications TC4 titanium alloy plates have a certain degree of anisotropy.
Four specifications of TC4 titanium alloy plate along the RD direction of tensile strength is high, but also has good plasticity, mainly because: rolling grain refinement, fine grain will increase the total area, and the boundary strength at room temperature is greater than the crystal strength, and can hinder the dislocation slip, and improve the tensile strength, in TC4 titanium alloy plate fracture crack extension direction deflect, increase crack extension path, increase its elongation. At the same time, the TC4 titanium alloy plate along the RD direction yield strength is lower than the TD direction, this is due to stretching, the tensile stress of TC4 titanium alloy plate RD direction and {0001} crystal surface smaller Angle, sliding slip is easy to start, leading to the TC4 titanium alloy plate yield strength drop, then in the fracture process of processing hardening and dislocation intersection phenomenon, the TC4 titanium alloy plate has low yield strength and high tensile strength. In the tensile direction for TD direction, the tensile stress and | 0001} surface between the Angle is larger, the slip system is difficult to start, need larger external force to open the sliding system, TC4 titanium alloy plate increase the yield strength, after yield, the organization is more likely to produce stress concentration phenomenon, leading to reduce the tensile strength of TC4 titanium alloy plate.
2.3 Break morphology
Figure 6 shows the microscopic fracture morphology of TC4 titanium alloy plates of different specifications after stretching. The microscopic morphology of the fracture is roughly the same, all of which are mainly toughness, with the isoaxial toughness (position A ‘). A few small toughness nests are distributed in the larger toughness, which has obvious characteristics of toughness fracture, and the macroscopic performance is a large elongation. The formation of the toughness pit is due to the internal tissue of the alloy using the isaxial α phase as the core source of micropore formation, and then the microporous nucleus, growth and polymerization of the alloy tissue, and the toughness pit gradually grows in the process of microporous polymerization. The plasticity of the alloy is usually determined by the depth and size of the toughness. When the toughness is deep and large, the alloy has good plasticity, and when the toughness is shallow and small, the alloy plasticity is poor.
When the TC4 titanium alloy sheet is 0.8mm thick, Its RD direction (FIG. 6a) and TD direction (Fig. 6b) have a large number of toughness nests, And a certain number of small planes (position B), With a rounded and smooth bottom, And have tiny voids present, And the sharper edges, There is also a lot of particularly fine microcracks inside the nest (positions C and D), After the application applying tensile stress, Greater tip stress when the crack is extended in the tissue, Causes the crack when extending forward, Also extend in the direction perpendicular to the main crack extension, Form a secondary crack, Thus increasing the twists zicity of the crack extension, Resulting in increased strength and decreased plasticity of the alloy. At the same time, there are a certain number of tear edges (position E) in the fracture morphology shown in Figure 6a. And the appearance of the tear edges indicates increased strength and decreased plasticity, which is consistent with the macroscopic tensile properties of TC4 titanium alloy plate. When TC4 titanium alloy plate thickness of 1.0mm, the TD direction (figure 6d) hole hole (position F), this is because: uniform small grain in the tissue, sample in the process of stretching, tissue cracks encounter small α grain, stress concentration phenomenon, if the crack continues to expand, the expansion direction will deflection, the hole along the α / β grain boundary junction, in this position, lead to plastic decline. When the thickness of TC4 titanium alloy plate is 1.5 and 2.0mm, the fracture morphology of RD direction and TD direction is basically the same without obvious difference, indicating that the mechanical properties of TC4 titanium alloy plate are close and the difference is small.
According to Figure 6, in the RD direction, when the TC4 titanium alloy plate is thick (1.5 and 2.0 mm), the fracture micromorphology is composed of a large number of toughness nests. When the thickness is thin (0.8 mm), the fracture micro morphology tear edges and micro cracks appear due to the increased deformation, which means that the strength and plasticity of the TC4 titanium alloy plate increase. TD direction changes tend to be similar to the RD direction, are with the TC4 titanium alloy plate thickness thinning, fracture from a single toughness morphology to the toughness and other morphology direction, the difference is when TC4 titanium alloy plate thickness is thin, the RD direction of fracture morphology is given priority to with the toughness, micro cracks and tear edges, and the TD direction of fracture morphology is given priority to with toughness, micro cracks and holes.
Conclusion
(1) The TC4 titanium alloy plate after rolling and annealing is organized as a mixed tissue composed of α phase and residual β phase, and the morphology of α phase shows the outgoing strip, isoaxis and small mass. Compared with the original plate, the grains of TC4 titanium alloy plate are refined obviously, and there are obvious signs of rolling.
(2) The strength of TC4 titanium alloy plate shows a trend of decreasing first and then stabilizing first with the increase of thickness, while the plasticity shows a trend of increasing first and then stabilizing first. When the thickness is 0.8 mm, the TC4 titanium alloy sheet has the greatest strength and the maximum tensile strength R. For 1075 MPa, maximum yield strength R, for 1027 MPa. In terms of plasticity, the elongation A of different specifications of TC4 titanium alloy plate is roughly the same, with the maximum value of 17.5%. The strength and plasticity of the four TC4 titanium alloy plates differ between RD and TD directions, indicating that the four TC4 titanium alloy plates have a certain degree of anisotropy.
(3) different specifications of TC4 titanium alloy plate after tensile fracture morphology is given priority to with the toughness, has obvious toughness fracture characteristics, the thickness of 0.8 mm of TC4 titanium alloy plate along the TD direction of the fracture morphology, also has a certain number of small plane, toughness nest inside there are a large number of particularly small micro cracks.
