High temperature oxidation of Ti–46Al–6Nb–0.5W–0.5Cr–0.3Si–0.1C alloy

Highlights

• Ti–46Al–6Nb–0.5W–0.5Cr–0.3Si–0.1C (at%) alloy was newly developed.

• The high-temperature oxidation behavior of the new TiAl-base alloy was described.

• The good isothermal and cyclic oxidation resistance was discussed.

Abstract

A newly developed Ti–46Al–6Nb-0.5W-0.5Cr-0.3Si-0.1C alloy was oxidized isothermally and cyclically in air, and its high-temperature oxidation behavior was investigated. When the alloy was isothermally oxidized at 700 °C for 2000 h, the weight gain was only 0.15 mg/cm². The parabolic rate constant, k_p (mg²/cm⁴·h), measured from isothermal oxidation tests was 0.002 at 900 °C and 0.009 at 1000 °C. Such excellent isothermal oxidation resistance resulted from the formation of the dense, continuous Al₂O₃ layer between the outer TiO₂ layer and the inner (TiO₂-rich, Al₂O₃-deficient) layer. The alloy also displayed good cyclic oxidation resistance at 900 °C. Some noticeable scale spallation began to occur after 68 h at 1000 °C during the cyclic oxidation test.

Introduction

The two phase (γ-TiAl+α₂-Ti₃Al) alloys based on Ti-(45–48%)Al are attracting considerable attention from the aerospace and automobile industries, because of their low density, high melting point, and high specific strength. Of major concern are however their poor ductility at room temperature, their low strength, and insufficient oxidation resistance at high temperatures. Much effort has been directed toward improving these shortcomings by alloy modifications. The addition of Nb significantly increased the oxidation resistance of TiAl-based alloys by suppressing the TiO₂-rich scale growth and increasing the scale adherence [1]. Tungsten also increased the oxidation resistance, and reduced the oxygen solubility or diffusivity in TiAl-based alloys [2], [3]. Silicon further increased the oxidation resistance as dissolved ions in the oxide scale [4] or by forming SiO₂ islands at the scale/alloy interface [3]. Chromium however decreased the oxidation resistance by increasing the TiO₂-rich scale growth [3], unless added by a quantity of more than 10% [5]. The compositions in this study are in atomic percentages, unless otherwise stated. Recently, we developed a new TiAl alloy, of which the composition was Ti–44Al–6Nb–2Cr–0.3Si–0.1C [6]. This alloy exhibited superior tensile strength at room temperature and 900 °C, and good oxidation resistance at 900 °C compared to the commercially available Ti–47.21Al–6.28Nb–0.49Cr–0.28Si–0.15Ni–0.17V alloy. In this study, another new TiAl alloy with a composition of Ti–46Al–6Nb–0.5W–0.5Cr–0.3Si–0.1C was developed. This new TiAl alloy displayed better oxidation resistance than our previous alloy [6] and other TiAl-X alloys (here, X = Si, refractory metals (viz. Mo, W, Ta), noble metals (viz. Pt, Au), transition metals (viz. Ni, Fe, Co, Mn, V), reactive metals (viz. Y, Zr, Hf), Nb-(Mn, Y, Ta, Zr, Hf, B), Nb–Cr–(W, Cu, Si, Mo), and Cr–(Si, Zr, Y, W, Ta)) [7]. The design concept of Ti–46Al–6Nb–0.5W–0.5Cr–0.3Si–0.1C was as follows: (i) control the amount of Al to have the duplex structure consisting of γ-grains and lamellar colonies of alternating layers of γ and α₂ platelets for superior fracture toughness and creep resistance [8]; (ii) prepare a high Nb-containing alloy for good oxidation resistance [9], [10], high creep resistance, and good ductility at room temperature [11]; (iii) add W for oxidation resistance [2], [3], [9]; (iv) add Cr to improve ductility [6]; (v) add Si to improve high-temperature oxidation resistance [9] and mechanical properties [12]; and (vi) add C to improve high-temperature mechanical properties [12]. The aim of this study is to describe the oxidation characteristics of our new alloy at 700–1000 °C. The high-temperature oxidation kinetics obtained from short- and long-time isothermal and cyclic oxidation tests as well as the scale microstructure of the Ti–46Al–6Nb–0.5W–0.5Cr–0.3Si–0.1C alloy were discussed.