Microstructure and oxidation behavior of Ti–Si–B alloys

Abstract

Recently, the existence of a new intermetallic ternary phase, with a composition close to Ti6Si2B, was observed in the Ti–Si–B ternary system. The aim of this work is to identify the phase transformations during the oxidation experiments in air of Ti–Si and Ti–Si–B alloys containing Tiss+Ti5Si3 (eutectic), Tiss+Ti6Si2B+Ti5Si3 (eutectic) and Tiss+Ti6Si2B (after heat treatment at 1250°C for 16h). The alloy ingots were produced in an arc-melting furnace using water-cooled copper hearth and a non-consumable tungsten electrode gettered by titanium. In this sequence, oxidation experiments in air were carried out at 900 and 1100°C for 12, 24 and 48h. Samples were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and microanalysis via wave-length dispersive spectrometry (WDS). After oxidation experiments in air at 900°C during 48h, the following phases of the Ti–Si, Ti–B, Ti–Si–B systems were observed in the microstructures of the studied alloys: Tiss+Ti3Si in the binary eutectic (Tiss+Ti5Si3), Tiss+Ti3Si+Ti6Si2B in the ternary eutectic (Tiss+Ti6Si2B+Ti5Si3), and Tiss+Ti6Si2B in the two-phase heat-treated alloy (Tiss+Ti6Si2B). In all the alloys, the scale was composed of an outer layer of coarse grain TiO2 (rutile), an intermediate layer of a mixture of fine-grained TiO2 and SiO2 (tridymite) and other nitrite phases, and an inner layer formed by the TiSi and TiSi2 phases. Phases of Ti–Si, Ti–B and Ti–Si–B systems were not observed after oxidation experiments in air at 1100°C for 48h, due to the effects of atmospheric contamination.