Research On The Phenomenon And Mechanisms Of Rapid Crack Propagation In The Higher Zone Of Service Temperature Range For Nickel-based Superalloys

Nowadays,the increasing demand for high performance of superalloy hot end components,such as aero engine and gas turbine compressor require hot end components will service at higher temperature,which put forward higher requirements for the fatigue performance of superalloy.Therefore,a fully understanding of the influence of temperature on the fatigue crack growth behavior of superalloy at the service temperature range and the dominant mechanism have become the urgent concern.Thus,this paper systematically studied the effect of temperature on the fatigue crack growth behavior of superalloy and its essential reason.Besides,the accumulation damage produced in long-term service has been studied.Experimental and theoretical basis for the damage tolerance design of superalloy at high temperature are proposed.A vast amount of the fatigue crack propagation experiments has been carried out to acquire the fatigue crack propagation behaviors.Powder superalloy(FGH4097,FGH4098 and FGH4096)and wrought superalloy(GH4738,GH4720Li)are tested in a wide temperature from room temperature to 800?.The result shows that the drop of fatigue life with the increase of temperature is not a linear tendency.It is obvious that there is an inflection point in the higher zone of the service temperature region.The phenomenon of rapid fatigue crack growth and rapid fatigue life decline have been widely observed near the inflection temperature.According to the observation result of the fatigue fracture features,crack propagation mode and damage zone characteristics at crack tip,the main reason of the existence rapid growth of fatigue crack at inflection temperature are explained.With the increasing of temperature,the fracture mode changes from transgranular fracture to intergranular fracture.At the inflection temperature,the obvious turning point presenting totally intergranular fracture morphology——?KT,decreases rapidly and close to the cracking point,due to the enhanced High temperature oxidation damage at grain boundary and result in the decreasing of grain boundary strength rapidly.Besides,the effect of hold time,stress,grain boundary misorientation and precipitate characteristics on grain boundary oxidation are studied.First principles are used to calculate and analyze theoretically the mechanism of grain boundary embrittlement induced by oxygen.Furthermore,the contribution of fatigue-mechanical damage term and grain boundary damage related term to fatigue crack growth is quantitatively characterized.The intersection point of two damage terms——equal damage load?Ke is proposed and characterized.Expressions of ?Ke(F-O)under pure fatigue condition and ?Ke(F-GB)under fatigue-creep condition are given.Basing on equal damage load ?Ke,the occurrence and existence of the rapid crack propagation is further revealed,it is essentially attributed to the damage load point ?Ke rapid drop to the cracking point.With regard to the long-term aged GH4738 alloy and the collected moving blades of flue gas turbine GH4738 experienced different service life,room-temperature fatigue crack growth behaviors of them manifest that the intergranular fracture turning point ?KT can characterize the damage accumulation of superalloys experienced long-term aging and long-term service.Furthermore,an evaluation method for long-term service life reduction caused by accumulation damage is proposed.According to the evaluation results on long-term aging and actual servicing flue gas turbine moving blades,the proposed method of service life reduction has theoretical basis and practical operability,especially for the assessment of service life reduction of flue gas turbine moving blades.In summary,this paper adopts a variety of experimental and theoretical calculation methods to clarify the commonality of rapid growth of fatigue cracks in the higher zone of service temperature range of Nickel-based superalloys,revealed the essence of the phenomenon and the contribution of the dominant mechanism.The evaluation method of long-term service damage accumulation and service life reduction is proposed,which provides the theoretical basis and feasible analysis method for the safe service and residual life assessment for the hot end components.