Abstract
High temperature creep and dwell-fatigue properties of the new nickel-based superalloy AD730™ have been investigated. Three microstructures have been studied in creep (850 °C and 700 °C) and dwell-fatigue (700 °C stress control with trapezoidal signals, and dwell times ranging from 1 s to 3600 s): a coarse grains microstructure, a fine grains one, and single crystalline samples. The aim of this study is to assess the influence of the grain size on creep and creep-fatigue properties. It is demonstrated that fine and coarse grains microstructures perform similarly in creep at 700 °C, showing that the creep properties at this temperature are controlled by the intragranular precipitation. Moreover, both the coarse grains and the fine grains microstructures show changes in creep deformation mechanisms depending on the applied stress in creep at 700 °C. At higher creep temperatures, the coarse grains microstructure performs better and almost no effect is observed by suppressing grain boundaries. During dwell-fatigue tests at 700 °C, a clear effect of the mechanical cycling has been evidenced on the time to failure on both the coarse and the fine grains microstructures. At high applied stresses, a beneficial effect of the cyclic unloading to the lifetime has been observed whereas at lower applied stresses, mechanical cycling is detrimental compared to the pure creep lifetime due to the development of a fatigue damage. Complex creep-fatigue interactions are hence clearly evidenced and they depend on the pure creep behavior reference.
Highlights
Increasing aero-engine efficiency and reducing NOx emissions is generally achieved in gas turbines by increasing the operating temperatures
The viscoplastic behavior of the AD730TM superalloy under pure creep and dwell-fatigue loading has been investigated at 850 °C and 700 °C on three different microstructures in order to investigate the influence of several microstructural parameters
The creep behavior mainly depends on the grain size, and on the intragranular γ’ volume fraction
Summary
Increasing aero-engine efficiency and reducing NOx emissions is generally achieved in gas turbines by increasing the operating temperatures. AD730TM is a new nickel-based superalloy, developed by Aubert & Duval for turbine disks or seal ring applications, with the particularity of being less expensive than its direct challengers due to its chemical composition and its good workability, enabling it to be manufactured through the cast and wrought route [3,4,5,6]. During service operations, such high temperature components are subjected to several mechanical solicitations, already widely studied, such as creep and fatigue. In an aim to achieve more representative conditions, one would prefer to use dwell-fatigue tests to mimic real operation conditions of high-pressure turbine disks (e.g., take off, cruise, and landing)
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