Controlling the Phase Composition, Structure, and Complex of Properties of the High-Modulus Titanium Alloy by Thermohydrogen Processing

Abstract

The possibilities and efficiency of applying the thermohydrogen processing (THP) of the high-modulus Ti–8.7Al–1.5Zr–2.0Mo titanium alloy with an aluminum content exceeding the solubility limit in α-titanium are considered. Experimental data on the influence of hydrogen on the alloy phase composition and structure are acquired. Regularities of phase transformations in the hydrogen-containing alloy under various thermal impacts are analyzed. The phase diagram of the alloy–hydrogen system in the hydrogen concentration range from the initial one to 1.0 wt % and the temperature range from 20 to 1100°C is constructed. It is shown that a single-phased β-structure is fixed with the concentration of introduced hydrogen of 0.6 wt % and higher by means of quenching from the β-region. Hydrogen saturation up to 0.8–1.0% leads to the implementation of the β → δ shear hydride transformation during quenching from temperatures below 750°C, and to the partial eutectoid transformation of the β-phase under slow cooling. It is established that hydrogen extends the stability region of the β-phase, lowering the temperature of the β/(α + β) transition by 210°C (at 1.0% H), and increases the stability temperature of the α₂-phase by 50°C. The process flowsheets and THP modes forming two structural types—submicrocrystalline and bimodal—are developed and approved for alloy samples. The formation mechanisms of these structures during THP are analyzed and the mechanical properties of the alloy are determined. It is established that THP leads to an increase in strength and hardness when compared with the initial state. The THP forming the microcrystalline structure causes a decrease in plasticity characteristics at the maximal hardness.