Control of high-modulus titanium alloy phase composition, structure and complex of properties using thermohydrogen processing methods

Abstract

This paper considers the possibility and efficiency of thermohydrogen processing of the high-modulus Ti–8,7Al–1,5Zr–2,0Mo titanium alloy with aluminum content exceeding its solubility limit in α-titanium. Experimental data on the effect of hydrogen on the alloy phase composition and structure are obtained. Regularities of phase transformations in the hydrogen-containing alloy are analyzed under different thermal effects. An alloy–hydrogen system is diagramed in the hydrogen concentration range from the initial content up to 1,0 wt.% and temperature range from 20 up to 1100 °C. It is shown that a β single-phase structure forms in the alloy via quenching from the temperatures of β range at a hydrogen content of 0,6 wt.% or more. Hydrogen saturation up to 0,8–1,0 wt.% causes β → δ hydride shear transformation during quenching from the temperatures below 750 °C and results in partial eutectoidal β phase transformation at slow cooling. It is stated that hydrogen extends the region of β phase stability by decreasing the temperature of β / (α + β) transition by 210 °C (at 1,0 wt.% of hydrogen) and increases the temperature of α2 phase stability by 50 °C. Technological schemes and modes of thermohydrogen processing are developed and tested using the alloy specimens in order to form the two types of structure – submicrocrystalline and bimodal, and formation mechanisms of these structures under thermohydrogen processing are analyzed as well. Mechanical properties of the alloy specimens are determined. It is stated that thermohydrogen processing results in growth of strength and hardness as compared with the initial state. The thermohydrogen processing forming submicrocrystalline structure leads to decrease of plasticity characteristics at maximum hardness.