The effect of TCP- precipitates on surface pitting and cracking in a Ni-based superalloy turbine blade

The topologically close-packed phase (TCP) is a high-Cr, high-Mo component that precipitates from nickel-based superalloys under exposure to high temperatures, and is known to reduce the high-temperature strength and induce brittle cracking. The present study examines the TCP-a phase that causes pits and cracks to form on the surface of high-temperature turbine blades. Scanning electron microscopy (SEM), X-ray diffraction (XRD), and transmission electron microscopy (TEM) indicate the formation of the TCP-a phase as acicular or spherical precipitates with a tetragonal lattice structure (a = 8.8 angstrom, c = 4.5 angstrom) and high-Cr, high-Mo content. The high-Cr TCPa phase is formed by oxidation under the high-temperature operation of the turbine blade. That is, when a Cr oxide layer is formed on the blade surface, a Cr-depleted region and a region of TCP precipitation are continuously formed underneath the Cr oxide layer due to Cr accumulation. The TCP has a coarse morphology near the surface, and is needle-like at depth. In addition, a discontinuous zone without TCP precipitation exists between the coarse and acicular regions. As a cause of blade failure, pits are generated when part of the surface coating layer is cracked due to corrosion during use of the blade at high temperatures. When thermal fatigue is applied here after pitting, the TCP-a phase induces crack propagation in the acicular direction, thus leading to blade fracture. The cracks caused by these surface pits and TCP-a phase formation are also considered to be the main cause of blade failure due to overheating.