Effects of service thermal cycles on the microstructure and mechanical property of K4648 superalloy

Investigation on the microstructural features of nickel-base K4648 superalloy was conducted on specimens undergone various numbers of service thermal cycles. Our results indicate that during the service thermal cycles, the blocky (TiNb)C phases remain in the K4648 superalloy, whereas the gamma' phases dissolve into the gamma matrix. There was no evidence of the existence of eta phase in the three different service states of K4648 superalloy specimens. Moreover, the amount of alpha-Cr precipitates decreases, but their size increases and the shape evolved from small particles into needle-like and then into rod-like long precipitates with increasing number of service thermal cycles, which was analyzed based on the deformation and thermal history. The mechanical property of the specimens decreased with increasing number of service thermal cycles. This is due to the decrease in the amount of (TiNb)C phases and the increase in the size of alpha-Cr phase, as well as the gamma' phase dissolution. Finally, fracture analysis suggests that the fracture mode of the alloy transforms from ductile mode to mixed mode at room and elevated temperatures. (C) 2016 Elsevier B.V. All rights reserved.