On the segregation of Re at dislocations in the γ' phase of Ni-based single crystal superalloys

Abstract

We report evidence of Re and Mo segregation (up to 2.6 at.% and 1 at.%) along with Cr and Co to the dislocations inside of γ' precipitates in a second generation Ni-based single crystal superalloy, after creep deformation at 750°C under an applied stress of 800 MPa. The observed segregation effects can be rationalized through bridging the solute partitioning behavior across the γ/γ' interface and the pipe diffusion mechanism along the core of the dislocation line. This understanding can provide new insights enabling improved alloy design. Tomography.Single crystal Ni-based superalloys have long been important engineering materials used in gas turbines and jet engines due to their superior creep, fatigue and oxidation properties at elevated temperatures [1][2][3][4][5][6][7]. These are associated with the two-phase microstructure, where a disordered face-centered cubic (FCC) γ matrix contains a high volume fraction of L1 2 ordered γ' precipitates, which have been considered initially as dislocation-free [3]. Many elements are typically added for solid solution strengthening of the γ matrix, such as W and Mo [8]. Re has attracted special attention due to the fact that additions of 2 to 6 wt.% Re significantly enhance the creep properties of Ni-based single crystal superalloys [1,3,[9][10][11]. How Re improves the creep properties is debated.The following possibilities have been suggested in the literature: (i) solid solution strengthening and distortions of the γ matrix lattice due to the large size of Re atoms [1,12], (ii) the slow diffusion rate of Re in Ni (slowest of all d-shell elements) [13,14], and (iii) the possible formation of Re clusters in the γ matrix that might hinder dislocation movement [15][16][17][18]. However, these Re

Deformation of Borides in Nickel-based Superalloys: a Study of Segregation at Dislocations

“…In summary, the present results show that nickel and cobalt segregate at dislocations in the borides, indicating that borides contribute to the deformation of superalloys and to the chemical alteration of the structure via solute drag at dislocations, as was already evidenced between the γ matrix and γ' precipitates [4,5]. We believe that the level of segregation depends on the composition of the borides, which in this study is different for the borides between the RRHT3 and STAL15-CC alloy (see Figure 1).…”

Imaging individual solute atoms at crystalline imperfections in metals

“…The third generation of superalloys contains only 5-6 wt% Re (2 at%) which increases creep life by a factor of almost two [14][15][16]. Re partitions strongly to γ [17][18][19], but experimental results on its distribution have been conflicting [15,20]. To answer the question of how Re can impart higher creep life by partitioning to γ, we selected a model Ni-Re binary alloy containing 2 at% Re.…”

On the effect of Re addition on microstructural evolution of a CoNi-based superalloy

“…Recent investigations [100,101] show that the Re atoms segregate to the interfacial dislocation cores as a Cottrell atmosphere and is responsible for stopping dislocation motion in particular at high temperatures. Further experiments by APT on creep deformed CMSX-4 alloy (3 wt.% Re) show significant segregation of Re atoms (~2 at.%) at the core of dislocations present inside the g 0 phase [102]. This indicates that the movement of dislocations during high temperature deformation experiences a Cottrell drag force by the slow diffusing Re atoms.…”