Aluminide Coatings on Mo-Si-B-Ti-Fe Alloy

ABSTRACT

Molybdenum-silicon-boron-titanium-iron (Mo-Si-B-Ti-Fe) alloys are of interest for high temperature applications primarily due to their superior creep resistance. However, these alloys have high corrosion rates in high temperature environments, thereby necessitating the need for protecting the surface of these alloys against oxidative attack. Aluminizing is one approach that could be effective. A Mo-12.5Si-8.5B-25Ti-2Fe (atom %) alloy was aluminized using halide activated pack cementation (HAPC). The process was carried out at 750°C, for times ranging from 1 to 25 hours in an argon environment. The aluminized specimens were characterized using X-ray diffractometry (XRD), optical microscopy and scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS). Coating thickness, structure, phase identification and elemental compositions were obtained using these techniques. The morphologies for different coating times in the 1-25 h range demonstrate a single layer coating that increases in thickness with processing time. Cyclic oxidation tests of both aluminized and as-received Mo-Si-B-Ti-Fe were conducted for several cycles with each cycle consisting of a 1-hour hold at 800°C followed by room temperature exposure for 10 minutes. Aluminized coupons exhibited superior oxidation resistance relative to the as-received alloys of Mo-Si-B-Ti-Fe.

INTRODUCTION

As the need for higher efficiencies in airborne and marine gas turbines increases, operating temperatures need to increase. This requires new alternatives to the currently used nickel-base superalloys.¹ Molybdenum-silicon-boron (Mo-Si-B) alloys are novel creep-resistant alloys containing Mo_ss, Mo₃Si and Mo₅SiB₂. However, they are denser than nickel-base superalloys and have poor oxidation resistance. The density of UNS N06230 is 9.0 g/cm³ while the density of Mo-Si-B is 9.6 g/cm³.² Titanium additions to Mo-Si-B lead to a reduction in density, i.e., 7.8 g/cm³. These alloys contain Ti₅Si₃ in addition to the Mo_ss, Mo₅Si₃ and Mo₅SiB₂ found in the base alloy. The addition of titanium also increases the strength, creep resistance, and oxidation resistance of the base alloy.¹ Iron additions stabilize the creep and oxidation resistant Mo₅Si₃ phase.³ Despite the increase in oxidation resistance due to the addition of titanium, bare Mo-Si-B-Ti-Fe has an unacceptable corrosion rate for turbine applications at high temperatures. However, the addition of a protective coating can provide improved oxidation resistance.