High-temperature diffusion in couple of electrodeposited iridium/rhenium

Highlights

• The diffusion activation energy of ED Re into Ir is much lower than that of CVD Re.

• ED Re has a higher defect concentration and thermodynamically unstable preferred orientation.

• The defect concentration and preferred orientation of Re affects its diffusion behavior in Ir/Re.

Abstract

Ir coated Re (Ir/Re), combining the outstanding high-temperature mechanical property of Re and excellent high-temperature oxygen permeability resistance of Ir, is a promising high-temperature material in high temperature applications, such as rocket engine combustion chambers and crucibles for crystal growth, etc. The elemental diffusion behaviors in the Ir/Re material are closely related to its failure mode and lifetime. Therefore, the diffusion behavior investigation on the Ir/Re material is essential for understanding the failure mechanism and building the lifetime prediction model of this material. In this work, a three-layered CVD Re/ED Ir/ED Re sample was fabricated and its diffusion behaviors at the temperatures ranging from 1400 to 2000 °C were investigated. The diffusion coefficients and activation energy of ED Re diffusing into Ir using a semi-infinite diffusion model were obtained and compared with those of the CVD Re. The obviously lower diffusion activation energy of ED Re (0.98 eV) compared with the CVD Re is due to their microstructure difference. The higher defect concentration, thermodynamically unstable grain structure and preferred orientation of the ED Re layer promotes the dominant diffusion of Ir into Re, resulting in a thicker diffusion layer with a frame-shaped diffusion front.

Introduction

Iridium coated rhenium (Ir/Re) combustion chamber for the radiation-cooled chemical rocket engine has an operation temperature up to 2200 °C, providing an outstanding specific impulse performance for the satellite's apogee engine [1,2]. The extremely low oxygen permeability of Ir combined with excellent high-temperature mechanical properties of Re is responsible for its outstanding performance in high temperature applications [3,4]. However, the element diffusion at high temperatures through the Ir/Re interface is believed to be the main cause of the degradation of the Ir/Re material [5]. Therefore, the study of diffusion behavior in couple of Ir/Re is beneficial to the life prediction of the Ir/Re combustion chamber.

Researchers have performed the investigations on the diffusion behavior between Ir and Re prepared by different methods, among which CVD and electrodeposition in molten salts (ED) are two most important preparation methods for Ir/Re combustion chambers [[6], [7], [8]]. Hamilton obtained the diffusion coefficients and activation energy of the CVD Ir/CVD Re diffusion couples by investigating their diffusion behavior in the temperature range of 1400–1900 °C using the semi-infinite diffusion model based on the judgment that Re diffuses predominantly into Ir [5]. Chen selected a mutual diffusion model to study the diffusion behavior of CVD Ir/CVD Re couple, and confirmed the predominance of the diffusion of Re into Ir [9]. Smirnov evaluated the diffusion kinetics and microhardnesses of the ED Ir/ED Re couples in 1950 °C and concluded that the Ir plays a dominant part in interdiffusion of the Ir/Re couple. Bai studied the diffusion kinetics of a CVD Re/ED Ir/CVD Re couple at 1800–2200 °C using the semi-infinite diffusion model to avoid the distortion in elemental measurement in the diffusion zone by the mandrel elements like Mo and C [10]. It could be seen that there remains controversial about the diffusion behaviors of the Ir/Re couple, especially for those prepared by different methods.

In recent years, the Ir/Re combustion chambers produced by ED have been considered to have an advantage over those produced by CVD in fabricating cost and mechanical properties of Re [11]. Therefore, the diffusion behavior of ED Ir/ED Re is essential to a better understanding of the failure mechanism of the Ir/Re combustion chamber in the service conditions, which is of great significance for the engineering application of this method.

In this work, the samples of three-layered CVD Re/ED Ir/ED Re have been prepared and their diffusion behaviors have been studied over the temperature range of 1400–2000 °C. The differences in structure dependent diffusion behavior between the ED Ir/ED Re and the ED Ir/CVD Re were comparatively studied and discussed.