Refractory Metal to Nickel-Based Alloy Joining Technologies for High Temperature Applications

Abstract

This thesis provides a detailed description of efforts towards the joining of niobium (Nb) to nickel-based alloys in order to increase the overall efficiency of gas turbine-based power generation systems. The desired increase in efficiency can be achieved by allowing for higher operating temperatures. In addition to the thermal efficiency of the power generating systems, their long-term reliability is also a major concern. Nickel-based superalloys were developed for the sole purpose of satisfying requirements for high-temperature component materials. Some factors which contribute to the selection of nickel-based superalloys for use in these systems are their inherent high modulus of elasticity, excellent hot corrosion resistance, high creep resistance and superior strength at high temperatures. Refractory metals, or metals that have melting points that exceed 2450°C and their alloys also make excellent candidates for potential structural materials for high efficiency power generation systems due to their high melting temperatures and their good physical properties such as low thermal expansion coefficients, high thermal conductivities, and high Young’s moduli. Because both the strength and ductility of refractory metals are greatly affected by adverse microstructural changes that occur when their respective recrystallization temperatures are exceeded, conventional welding techniques are strength prohibitive for use in joining these metals. Therefore, joining these metals is best accomplished by a form of high temperature brazing known as transient liquid phase (TLP) bonding. With superalloys and refractory metals being strong potential candidates for high temperature and high efficiency power generation systems, a suitable technique is needed to produce an effective joint between these materials while retaining their base physical properties. Careful consideration must be used when designing the overall joint to avoid the formation of potentially harmful intermetallic phases. This can be accomplished by inserting diffusion barriers where needed. The final result is a joint that requires several substrate materials and successive TLP bonds to complete. The joint design, developed in this project, employs the use of several TLP bonds between a series of spacer interlayers linking pure nickel and niobium substrates. This design is used to prevent the substrates, Ni and Nb, from contacting to prevent formation of the intermetallic phases mentioned before. All of the TLP bonds are successful in terms of initial microstructure/microhardness and room-temperature mechanical testing.