Ultrafine-grained titanium alloys offer a number of outstanding advantages, including a degree of increased room-temperature strength and great elongation in high-temperature tensile applications. Grain refinement is usually achieved by large deformations, such as equal-angle extrusion, high-pressure torsion, multi-axis forging, and cumulative coil welding. In addition to this, hydrogen treatment can be used for titanium alloys.
In the 1970s, the Moscow Institute of Aircraft Manufacturing studied the effect of hydrogen on the processing properties of titanium alloys, put forward the concept of "hydrogen plasticization", with hydrogen as a temporary alloying element, through hydrogen penetration, azeotropic decomposition, vacuum dehydrogenation and other processes, the use of hydroplasticization, hydrogen phase transition, and reversible alloying of hydrogen in titanium alloys, to improve the processing properties, refine the material microstructure, and refine the material. Processing performance, refine the microstructure of the material.
Hydrogen treatment can be used to refine the grain organization of titanium alloy castings and forgings and improve their mechanical properties. It has been reported in the literature that hydrogen treatment can refine the microstructure of TiAl alloy, so that its compression strength and yield strength have been significantly improved. In practice, hydrogen treatment can often be combined with subsequent heat treatment and heat distortion to achieve very fine grain size. It has been shown that high-temperature, large-scale deformation of hydrogenated titanium alloys can result in the formation of equiaxed fine crystals with grain sizes of about 1 μm, or even nanometer-sized grains. Studies on Ti-6.3Al-3.5Mo-1.7Zr (%, mass fraction) alloy showed that: hydrogen treatment in the hydrogen atom fraction of 14% to 16%, the deformation temperature is reduced to 550 ℃, and then through the deformation process and the decomposition process of the substable phase, the final grain size of 40 nm nano-grains were obtained. Comparing the engineering stress-strain curves of Ti-6Al-4V alloys with different grain sizes, it can be seen that the ultrafine-grained material presents high yield strength and high elongation compared with coarse-grained or general fine-grained materials.
The treatment of allowing titanium alloys to absorb a large number of hydrogen atoms (protium) and then allowing these hydrogen atoms (protium) to desorb at high temperatures in a vacuum is known as protium treatment. For α + β type titanium alloys, the protium treatment consists of the following three processes: (1) protium absorption in a hydrogen atmosphere; (2) martensitic transformation and thermal processing ultimately inducing diffuse hydride precipitation; and (3) final protium desorption treatment and recrystallization. It was reported that protium treatment of Ti-6Al-4V alloy, which absorbed 0.5% protium and desorbed at 873 K, showed an ultrafine isotropic crystalline organization with large angular grain boundaries and grain sizes in the range of 300-500 nm. It was shown that the protium treatment method increases the content of the β phase in the α matrix. Tensile tests showed an increase in the yield strength of the alloy at room temperature, while the maximum elongation of the alloy at 1123 K reached 9000%. It was also reported that protium treatment of Ti-6Al-4V sheet with 0.5% protium content, followed by quenching at 1223 K, hot rolling at 1023 K to a thickness reduction of 80%, and protium desorption at 873 K successfully produced a homogeneous organization with ultrafine isometric crystals with grain sizes ranging from 0.3 to 0.5 μm. Tests showed that mechanical properties such as superplastic elongation of the alloy improved significantly with the reduction of grain size.
Although the hydrogen treatment method shows great potential for refining titanium alloys, it is more costly than other conventional methods, and for larger structural parts, the method also has problems such as uneven hydrogen distribution and high equipment requirements, which need to be further investigated and solved.
