Effect of Heat Treatment on the Structure and Properties of Sheet Semifinished Products Made of a Heat-Resistant Alloy Based on Titanium and Alloyed with Rare-Earth Metal

Abstract

This study is a continuation of investigations into the influence of microalloying with rare-earth metal gadolinium on the structure formation and properties of the titanium alloy under the thermal effect. It has been established previously that the introduction of gadolinium into an experimental heat-resistant alloy promotes the structural transformation in the cast state, leads to a decrease in size of structural components, and affects the growth and nucleation rates of the particles. It is revealed that additional alloying with gadolinium exerts no significant effect on the formation of the microstructure of hot-rolled sheets made of heat-resistant experimental alloy after annealing at 950°C. The structure is presented by equiaxial particles of the primary α-phase, secondary α-phase with lamellar morphology, and a small amount of the β-phase. It is determined that ordering proceeds in primary α-phase particles and precipitation α2-phase particles is observed during isothermal holding (t = 700°C, τ = 100 h), while silicide chemical compounds form at the α/β-boundary. It is shown that the α2-phase is formed in the body of the particles of the primary α-phase, while its near-boundary regions are free of inclusions, which is caused by their depletion with aluminum due to the β → α transformation. It is established that the sizes of precipitating silicide particles decrease with an increase in the gadolinium content in the alloy. The average particle size is 0.2–0.3 μm in the alloy with 0% Gd; on the contrary, it decreases to 0.05–0.1 μm in the alloy with 0.2% Gd. It is shown that the introduction of 0.2% Gd into the heat-resistant titanium alloy leads to a decrease in the depth of the “alpha-case” layer and an increase in cyclic durability and short-term strength at 700°C by 30%.