Ultrafine grained titanium alloy has a series of outstanding advantages, its room temperature strength can be improved to a certain extent, and it has a great elongation during high temperature stretching. The refined grains are usually obtained by large deformation methods, such as equal-diameter corner extrusion, high-pressure torsion, multi-axis forging, and cumulative roll welding. In addition, hydrogen treatment can also be used for titanium alloys.
In the 1970s, the Moscow Aircraft Manufacturing Research Institute studied the effect of hydrogen on the processing performance of titanium alloys, and proposed the concept of “hydrogen plasticization”. Using hydrogen as a temporary alloy element, through hydrogen permeation, eutectoid decomposition, vacuum dehydrogenation and other processes It uses hydrogen-induced plasticity, hydrogen-induced phase transformation, and the reversible alloying of hydrogen in titanium alloys to improve the processing performance and refine the microstructure of the material.
Hydrogen treatment method can be used to refine the grain structure of titanium alloy castings and forgings to improve their mechanical properties. It has been reported in the literature that the microstructure of TiAl alloy can be refined by hydrogen treatment, and its compressive strength and yield strength have been significantly improved. In practical applications, it is usually possible to combine the hydrogen treatment technology with the corresponding subsequent heat treatment and thermal deformation treatment to obtain a very fine grain structure. Studies have shown that the high-temperature large-scale deformation of the opposed hydrogen titanium alloy can form equiaxed fine grains with a grain size of about 1 μm, and even form nano-scale grains. The study of Ti-6.3Al-3.5Mo-1.7Zr (%, mass fraction) alloy shows that the hydrogen atom fraction in hydrogen treatment is 14% to 16%, the deformation temperature is reduced to 550 ℃, and then through the deformation process and metastable phase During the decomposition process, nano-grains with a grain size of 40 nm were finally obtained. Comparing the engineering stress-strain curves of Ti-6Al-4V alloys with different grain sizes, it can be seen that ultrafine grain materials exhibit high yield strength and high elongation compared with coarse grain or general fine grain materials.
Let the titanium alloy absorb a large number of hydrogen atoms (protium), and then let these hydrogen atoms (protium) be desorbed under high temperature vacuum. This process is called protium process. For α + β-type titanium alloys, the protium treatment includes the following three processes: (1) protium absorption in a hydrogen atmosphere; (2) martensite transformation and thermal processing eventually cause dispersed hydride precipitation; (3) final protium Desorption treatment and recrystallization. According to reports, Ti-6Al-4V alloy is treated with protium, the alloy absorbs 0.5% of protium, and desorbs at 873K. It shows ultrafine equiaxed grain structure, and has a large angle grain boundary. Studies have shown that the protium treatment method will increase the content of β phase in the α matrix. The tensile test shows that the yield strength of the alloy has been improved at room temperature, while the maximum elongation of the alloy at 1123K reaches 9000%. It was also reported that the Ti-6Al-4V sheet was subjected to protium treatment with a protium content of 0.5%, then quenched at 1223K, hot rolled at 1023K to a thickness reduction rate of 80%, and desorbed at 873K, and successfully produced with ultrafine The uniform structure of equiaxed grains, the grain size is 0.3 ~ 0.5μm. The test results show that the superplastic elongation and other mechanical properties of the alloy increase obviously as the grain size decreases.
Although the hydrogen treatment method shows great potential for refining titanium alloys, compared with other conventional methods, the hydrogen treatment method is more costly, and for larger structural parts, the treatment method also has uneven hydrogen distribution and equipment requirements Higher issues need further research and solution.