Detection of Rhenium-Rich Particles at Grain Boundaries in Nickel-Base Superalloy Turbine Blades

Abstract

Ni-base single crystal superalloy turbine blades containing defect grains were investigated by high resolution electron microscopy. Several different types of defects, such as, stray grains, equiax grains and freckle chains which formed during casting, and recrystallized grains which formed during subsequent heat treatment, were selected. Regardless of the region of the turbine blade, many particles with large amounts of rhenium were detected at the boundary of the stray grain and matrix. The Re-rich particles were also detected at the boundary of the matrix and other defect grains, such as, equiax grains and freckle chain grains, and even at a low angle grain boundary. However, the boundary of the recrystallised grain and matrix which was formed after solution heat treatment showed just one Re-rich particle. Also, the composition of these Re-rich particles is different from any topologically close packed phases which have been reported in Ni-base superalloys. The results suggest that during solidi cation, the particles are formed from the melt and pushed ahead of each solidi cation front of the defect grain and the matrix, and piled up at the boundary of the matrix/defect grain.

General View of Rhenium-Rich Particles along Defect Grain Boundaries Formed in Nickel-Based Single-Crystal Superalloy Turbine Blades: Formation, Dissolution and Comparison with Other Phases

“…A magnified image of the cross section (Figure 2c) shows that there are several fine particles along the boundary regions of two grains which look like the same orientation of {001} of the γ' phase (exactly the misorientation of 7.1°). This observation is in good agreement with a previous study which showed a similar microstructure of the particles in a low-angle grain boundary of 6.1° [16].…”

“…Then, for reference, a new turbine blade with a high-angle boundary with a misorientation of 16.5° was observed. As already shown in previous studies [16,17], at the high-angle grain boundary, the intermediate layer becomes wider and clearer (Figure 2g) and most of all, the fine particles are easily recognized along the coherent interface. It should be noted that for more clear visualization of the distribution of the particles, a cross section was made along the intermediate layer, as shown in Figure 2h, which is different from the directions in Figure 2b,e.…”

“…Even though the TCP phases have been widely recognized in the community and are well understood, the rhenium-rich particles have had little investigation. Some previous studies by the authors showed that the particles have been easily detected in a high-angle grain boundary formed between defect grains, such as aforementioned stray, freckle chain and equiax and a matrix grain, even in a low-angle grain boundary [16,17].…”

“…In this general view paper of the particles, the formation of rhenium-rich particles has been introduced from a low-angle boundary to a high-angle grain boundary to allow a better understanding, although there are previous studies on the formation and detection of the particles [16][17][18]. Then, the particles have been compared to other phases or particles which can be found in the same base material.…”

Response to Discussion of “Formation of an Intermediate Layer Between Grains in Nickel-Based Superalloy Turbine Blades”

“…This sharp interface has also been confirmed in many other samples and with grain defects across a range of causes both thermal and solutal such as other stray grains, multigrain regions, freckle chain grains, and even low angle grain boundary. [5] Therefore, it is apparent that bi-films themselves did not exist in the samples examined in this and other studies.…”