Creep of an oxidation resistant coated Mo-9Si-8B alloy

Highlights

• Tensile creep testes were carried out at 1200 °C for coated for Mo-9Si-8B alloys.

• Full retention of the coating after 6% plastic strain.

• The coating structure was maintained during creep deformation.

• Observed creep rates for coated samples are consistent with reported values for uncoated material.

• Coating has a negligible effect on the creep behavior.

Abstract

In order to evaluate the impact of an oxidation resistant coating on the structural performance of a Mo-9Si-8B alloy tensile creep experiments were conducted at 1200 °C. After a plastic strain of 6% the creep rates of the coated samples compared favorably with the reported values for uncoated samples. Moreover, the coating structure was maintained during creep deformation and the coating exhibited a self-healing capability.

Introduction

The current paradigm of nickel-based superalloys has approached the very limits of high-temperature strength and stability, often operating at 0.9 T_m, of the incipient melting temperature [1]. In order to realize an increase in hot section intake temperatures alternative material systems must be explored. The Mo–Si–B alloy system is a promising candidate due to a number of exceptional properties possessed by the multiphase microstructures. For example, at 1200 °C the flow stress at 4% strain in a three phase Mo-9Si-8B alloy is more than twice that for the commercial TZM alloy [2]. In earlier studies, oxidation-prone molybdenum was shown to be protected from oxidizing environments by the Mo–Si–B based coating structure to temperatures as high as 1600 °C [3]. In order for such a coating to be implemented into a gas turbine engine, it must demonstrate stability and robustness in a wide variety of environmental and operation conditions.

Numerous studies on high temperature oxidation behavior in Mo-Si-B alloys reveal that the alloy compositions with the best performance do not have satisfactory mechanical properties (notably low temperature ductility and fracture toughness) due to the high volume fraction of silicide phases needed for the oxidation resistance [4]. Similarly, alloy compositions with satisfactory mechanical properties usually exhibit poor oxidation resistance due to the continuous Mo_ss phase [5]. One strategy to address this dilemma is to apply an oxidation resistant coating. A successful coating has been developed based upon a co-deposition of B and Si by pack cementation that exhibits thermodynamic and mechanical compatibility and self-healing capability [6]. The coating has been demonstrated to provide a robust environmental resistance to attack by oxidation, CMAS, hot corrosion by molten salt and water vapor [3]. Besides the environmental resistance it is important to assess the influence of the coating on the mechanical behavior. In this work it is demonstrated that the high temperature creep performance of a Mo-9Si-8B alloy is essentially unaffected by this coating, proving the protective capability of the coating under application relevant conditions with superimposed plastic deformation.