Influence of thermal exposure on microstructure and stress rupture properties of a new Re-containing single crystal Ni-based superalloy

Abstract

S ingle-crystal (SC) nickel-based superalloys are widely used as turbine blade materials because of their excellent mechanical properties [1] . The superior high-temperature behavior is mainly attributed to the two-phase microstructure consisting of a γ-matrix (Ni) and a large volume fraction of γ′-precipitates (Ni 3 Al) in the range of 65% to 70%. Within this optimum γ′ volume fraction range, the strengthening effect of the γ′-precipitates depends largely on their size and morphology [2][3][4] . During high-temperature service, the γ/γ′ two-phase microstructure will be degraded due to a coarsening process, during which the γ′ precipitates evolve from cuboidal into plate-like morphology, accompanied by a loss of the precipitate coherency and deterioration Abstract: In this study, the long-term thermal microstructural stability and related stress rupture lives of a new Re-containing Ni-based single-crystal superalloy, DD11, were investigated after high-temperature exposure for different lengths of time. The results show that the γ' precipitates retained a cuboidal morphology and the γ' size increased after short thermal exposure for 50 h at 1,070 °C. As the thermal exposure time was prolonged to 500 h, the cuboidal γ' gradually changed into irregular raft-like morphology due to particles coalescence, and the morphology of the microstructure was almost unchanged after further thermal exposure up to 3,000 h. The stress rupture experiments at 1,070 °C and a tensile stress of 140 MPa showed that the rupture lives increased significantly after thermal exposure for 50 h and dropped dramatically with increasing exposure time up to 500 h but decreased slowly after exposure for more than 500 h. These results imply that stress rupture properties did not decrease when the γ' remained cuboidal but degraded to different extents during the γ' coarsening process. The coarsening of the γ' precipitates and change in morphology were regarded as the main factors leading to the degradation of the stress rupture lives. This study provides fundamental information on the high-temperature longterm microstructural stability and mechanical performance, which will be of great help for DD11 alloy optimization and engineering aeroengine applications.Key words: Ni-based superalloy; thermal exposure; microstructure; coarsening; stress-rupture properties of the mechanical properties [5,6] . The driving forces for coarsening are the reduction of the γ/γ′ interfacial area, the decrease of the lattice mismatch strain, and reduction of the modulus misfit [7,8] . The γ′ coarsening process is strongly affected by service temperature and time and vary with compositional differences in Ni-based superalloys due to the diffusion-related coarsening process. Since single-crystal blades and vanes work for a long time at elevated temperatures during service, a reliable mechanical performance is essential for engineering application as gas turbine blades. Consequently, the stability of the microstructure and mechanical properties after long-term exposure at e...

Numerical Analysis of Microstructure Development during Laser Welding Nickel-Based Single-Crystal Superalloy Part II: Multicomponent Dendrite Growth

“…Dendrite size in different growth regions is substantially finer in order to mitigate the solidification cracking. Theoretical predictions are in reasonable agreement with the experiment results under similar dendrite growth conditions [16][17][18].These theoretical predictions provide accurate and reliable way to evaluate the solidification cracking susceptibility and optimize the single-crystal growth conditions Laser welding conditions (laser power, welding speed and welding configuration)-microstructure map (dendrite selection, morphology stability and microstructure development) is useful for microstructure control and therefore provide the prerequisite for laser welding conditions optimization.…”

“…Yang [17] analyzed the effect of the withdrawal velocity and thermal gradient on stray grain formation by using cellular automaton-finite difference (CA-FD) model during nickel-based single-crystal superalloys casting. Zhang Weiguo [18] analyzed the influence of solidification rate on primary dendrite morphology and arm spacing of Ni-based superalloy DZ125 during directional solidification. M.Qian [19,20] evaluated the effect of grain size and special grain boundaries on weldability of wrought Waspaloy and Alloy 718.…”