Evolution of cold-expanded microstructure with aging temperature and its influence on fatigue performance of hole structure at elevated temperature

A surface-strengthened layer formed by cold expansion (CX) evolves at high temperatures. The evolution of surface integrity and microstructure after a CX-treated superalloy GH4169 center hole plate was investigated in the present study. Aging at 400 ℃ and 600 ℃ took place with a duration of 500 h (CX400 and CX600). In addition, the fatigue properties at 820 MPa/600 ℃/R= 0.1 were investigated. The results show that CX has a smooth surface, a compressive residual stress field with a depth of more than 1.4 mm and a non-improved surface hardness. The statistical fatigue life of the as-received specimens was 7015 cycles, and the fatigue lives after CX and CX400 treatment were 5.27 and 2.86 times higher, respectively, indicating an increase by CX and a decrease by 400 ℃ aging. In contrast, the fatigue life of the original sample decreased to 5576 cycles with 600 ℃ aging, while the corresponding fatigue life of the CX600-treated sample was 7979 cycles, representing a significant decrease in both cases. The transformation of the main strengthening γ″ phase to the δ-phase upon aging at 600 ℃/500 h are the main reasons for the weakening of the fatigue properties, which did not occur upon aging at 400 ℃ (see TEM).