Analysis of the Microstructure, Phase Composition, and Oxidation Kinetics of Heat-Resistant Titanium Alloys with Gadolinium

Abstract

The structure, phase composition, and local chemical composition of half-finished products (rods) made from Ti–Al–Mo–Zr–Si- and Ti–Al–Mo–Zr–Sn–Si-based alloys additionally microalloyed with 0.4 wt % gadolinium have been studied in this work. The structure of the rod made from the Gd-alloyed Ti–Al–Mo–Zr–Si composition was found to be characterized by the presence of Gd2O3 gadolinium oxide particles. The structure of the rod made from the Gd-alloyed Ti–Al–Mo–Zr–Sn–Si composition, which contains tin as a low-melting element, is characterized by the presence of Gd–Sn–O complex oxide particles, whose cores and shells are Gd5Sn3 intermetallics and Gd2O3 gadolinium oxide, respectively. The formation of such particles in both alloys occurs at the stage of ingot solidification, and the particles remain in the materials at all stages of their subsequent treatment. The oxidation kinetics of the alloys upon isothermal holding in a temperature range of 600–800°С is studied depending on the structure of half-finished products. The oxidation processes of the Ti–Al–Mo–Zr–Sn–Si alloy are less intensive than those of the Ti–Al–Mo–Zr–Si alloy. The alloying with gadolinium up to 0.4 wt % leads to the accelerated oxidation of the tin-free alloy, which is due to the presence of Gd2O3 particles precipitated at grain boundaries and interfaces. The alloying of the tin-containing composition with gadolinium almost does not affect the oxidation kinetics at 600–700°С and slightly decelerates the oxidation at 750–800°С; this is related to the presence of the Gd–Sn–O oxide particles in the structure, which are uniformly distributed over the grain body. The laws of oxidation of both the compositions free from gadolinium and alloyed with gadolinium were determined. It was shown that the oxide film formed upon the oxidation of both alloys is multilayer and consists of alternating layers of aluminum and titanium oxides. In this case, the progressive growth of oxide film layers alters the film exfoliation owing to the high brittleness of aluminum oxides. The presence of gadolinium oxide particles in both alloys leads to the porosity of diffusion zone of the base metal upon oxidation.