Deposition of NiAl coating for improvement of oxidation resistance of cold-rolled Ni₃Al foils

Abstract

β-NiAl, an important intermetallic compound in Ni–Al systems, together with γ′-Ni₃Al, has been used as a coating material for Ni-base superalloys, because of its excellent oxidation resistance. In this study, NiAl-based coating through RF magnetron sputtering was selected for deposition on Ni₃Al thin foils which were cold-rolled up to 96% without any intermediate annealing steps. The thickness of the coating on Ni₃Al foils is restricted below 10 μm, which is much thinner than general coatings on bulk alloys. Insufficient coating thickness can induce rapid Al depletion in the NiAl coating layer. However, the addition of 3 at.% Ti was very effective in retarding atomic diffusion and Al depletion in the coating layer. The binary Ni–50Al coating did not form a stable α-Al₂O₃ oxide during cyclic oxidation tests at 1000 °C. Moreover, rapid Al diffusion between the coating layer and substrate was observed. Ni–47Al–3Ti–0.1Y was then selected as a modified coating composition. The Ni–47Al–3Ti–0.1Y coating showed superior oxidation resistance due to stable α-Al₂O₃ formation and effective retardation of Al diffusion from the coating layer. In addition, Ni-oxide and α-Al₂O₃ coexisted in the bare Ni₃Al foil without any coating, and the bare Ni₃Al foil exhibited more stable oxidation behavior than the bulk Ni₃Al alloy.

Introduction

Ni₃Al has received strong interest as a high temperature material because of its anomalous strengthening with increasing temperature [1], [2]. However, the very small room temperature ductility of polycrystalline Ni₃Al due to its severe intergranular brittleness [3] has been an obstacle to the development of this intermetallic compound. It is well known that the brittleness of the grain boundaries, which is a common problem in intermetallics, can be greatly improved by a small addition of boron [4].

As an alternative approach, directional solidification using the floating zone (FZ) method provides significant ductility improvement for Ni₃Al without any boron addition [5], [6]. The FZ method makes it possible to fabricate binary Ni₃Al thin foils below 100 μm in thickness with smooth and crack-free surfaces by cold rolling without any intermediate annealing steps [7], [8], [9]. Moreover, through annealing of the cold-rolled foils at temperatures over 1000 °C, the recrystallized foils can attain some room temperature ductility (3.0–14.6%) in air and they can be bent plastically around TD more than 100° without cracking [7]. These results demonstrate that Ni₃Al thin foils can be utilized for lightweight, high-temperature structural materials through forming honeycomb structures.

For the practical application of Ni₃Al foils, the oxidation properties at high temperature as well as their mechanical properties should be considered. In particular, the oxidation resistance of cast Ni₃Al alloys is inhibited because they form non-protective Ni-oxides such as NiO and NiAl₂O₄ at high temperature and spalling of the oxide scale occurs under a cyclic oxidation environment [11], [12], [13]. β-NiAl, an important intermetallic compound in Ni–Al systems, together with γ′-Ni₃Al, has been used for coating materials of Ni-base superalloys because of its excellent oxidation resistance. In addition to oxidation resistance at high temperature, coating materials should display chemical compatibility with the substrate [10]. Therefore, NiAl is considered a good candidate for a coating material of Ni₃Al foils.

When Ni₃Al foils are utilized for a honeycomb structure, the thickness of Ni₃Al foils should be less than 100 μm [14]. Therefore, it is generally considered that the thickness of the coating on the Ni₃Al foils is restricted below 10 μm, which constitutes a very thin deposition level relative to the general oxidation-resistant coating of over 100 μm in thickness on bulk alloys [10]. Insufficient coating thickness, however, can induce rapid Al depletion in the NiAl coating layer at high temperature because the scale-forming element Al is consumed continuously by the formation of Al₂O₃ scales and interdiffusion with the Ni₃Al substrate. In this study, we attempt to determine a suitable coating composition to address the limitation of thin coating. For the modification, the Al diffusion behavior from the coating to substrate as well as the oxidation-resistant property of the coating should be considered.