Double minimum creep in the rafting regime of a single-crystal Co-base superalloy

On the Precipitation-Strengthening Contribution of the Ta-Containing Co3(Al,W)-Phase to the Creep Properties of γ/γ′ Cobalt-Base Superalloys

“…By fitting Eq. [4] to the data of VF0, an apparent activation volume V app of 76 b 3 is obtained (assuming deformation in the fcc crystal structure by full a/2 h110i dislocations and a lattice parameter of 0.3637 nm as reported by Xue et al [16] for the c phase of Co-9Al-7.5W-2Ta at 1223 K (950°C)). This value is higher than the one from the c matrix of the Ni-base superalloy CMSX-4 with 36 b 3 and similar to the one of Nimonic 75 with 70 b 3 at 1223 K (950°C).…”

“…The compositions of the c and c¢ phases of a Co-9Al-7.5W-2Ta alloy with a c¢ volume fraction of approximately 50 pct [16] were determined by atom probe topography after aging at 1173 K (900°C) for 200 hours. By changing the alloy compositions along the resulting c/c¢ tie-line, the investigated alloys were designed to vary only in the c¢ volume fraction.…”

“…VF60 at 1223 K (950°C) and 200 MPa (see Figure 3(b)). Since only the second creep minimum is associated with the completion of rafting at temperatures above 1223 K (950°C) [16,30] and the first minimum is reached after a plastic strain of less than 0.5 pct, the effect of rafting is considered negligible in the following. By fitting Eq.…”

“…Shearing of the c¢ precipitates was also reported in a quaternary Co-9Al-7.5W-2Ta SX superalloy after 0.5 pct plastic strain under a compressive load of only 150 MPa at a temperature of 1223 K (950°C). [16] Recently, Lenz et al [17] studied the tension-compression asymmetry in a CoNi-base superalloy at 1123 K (850°C) and 400 MPa. Even though the shearing mechanisms differ depending on the sign of the applied load, the authors calculated that nearly 10 pct of the total plastic creep strain of 0.3 pct can be attributed to shearing events in the c¢ precipitates for both tensile and compressive creep.…”

Correlative Microscopy—Novel Methods and Their Applications to Explore 3D Chemistry and Structure of Nanoscale Lattice Defects: A Case Study in Superalloys

“…These are visible because of shearing of c¢, which is the primary active deformation mechanism operating during high temperature creep in this type of alloy. 25 From the stereographic projection in Fig. 2b Once an ROI, highlighted in yellow, is identified in the bulk sample as shown in Fig.…”