Influence Of Long-term Aging On The Microstructure Evolution Behavior And Low Cycle Fatigue Of GH4169Alloy

GH4169alloy shows excellent performance below650℃such as high strength and ductility, so it has been used for various applications, like the production of engine turbine disk, and become the mainstay products in superalloy. Under actual servicing condition, the alloy undergoes high temperature and alternating load, and low cycle fatigue damage etc., which can lead to disastrous consequences. Therefore, it is very important to clear the low cycle fatigue behavior of GH4169alloy after long-term ageing.The environment in which turbine disk serviced is simulated to explore the evolutionary behavior of alloy structure during service process and to reveal the low cycle fatigue behavior under varying degrees of tissue deterioration state of the alloy, that is the influence of long-term aging on the micro structure evolution behavior and on low cycle fatigue of GH4169alloy. The purpose of the present research is to study the low cycle fatigue behavior of the alloy aged at different temperatures, in order to provide a fundamental data for the safety applications.The results show that the micro structure of GH4169alloy changes aged at750℃, the8phases gradually grow from the granular to short stick, long needle and its size and volume fraction increase with the increasing of ageing time. At the same time y" phases grow excessively and the volume fraction decrease.The low cycle fatigue behavior of the alloy are greatly affected by the evolution of the micro structure, which the fatigue strength decreased with increasing of aging time, because the y" phases coarsen with the increasing of aging time. The diameter of plate-like y" phases is about425nm, in length aged at750℃for500h. As a metastable phase, y" phases convert to8phases during long-term aging, causing down of the coherent strength, as well as the increasing of the δ phase. In addition,8phases mainly precipitate at grain boundaries which are replaced by δ phases boundaries then the precipitate free zones around δ phases, which also makes the decreasing of strength. Low cycle fatigue cyclic stress response of the alloy shows3steps:cyclic hardening, cyclic stability and cyclic softening. The low fatigue fractography consists of by the including fatigue crack initiation, slow, fast propagation and final fracture. Many fatigue striations are found in the steady expansion stage, the distance between these fatigue striations stay the same. Low cycle fatigue life tends to decrease.The micro structure of GH4169alloy is relatively stable when aged at650℃, the size of 8phases increase slightly. Cyclic response stress increases first with cyclic hardening, and then changes to stable, finally cyclic softening. This is mainly due to the multiplication between the counteract and interaction of dislocations. The alloy shows cyclic hardening obviously when aged at650℃for1500h.