Investigation of the kinetics of damage to refractory alloys under cyclic thermal loading in a gas flow

Abstract

We present the results of investigation of the kinetics of damage to structural elements made of refractory alloys (including the elements with protective coatings) under cyclic thermal loading. The influence of the stress-strain state on crack propagation under conditions of thermal fatigue loading is analyzed.The development of scientifically substantiated approaches and creation of methods capable of taking into account the actual kinetics of damage of the materials in the process of operation is one of the most urgent tasks in the solution of the problems of reliability of structural elements and evaluation of their residual service life and survivability.The main aim of the present work is to study the processes accompanying the initiation and development of thermal fatigue cracks in structural elements made of refractory alloys (with and without protective coatings) under conditions of cyclic thermal loading in high-temperature gas flows. In the proposed formulation, the present work is of primary importance for understanding the mechanisms of crack propagation under nonisothermal loads and substantiation of the possibility of application of new and well-known models of contemporary fracture mechanics to the phenomenon of thermal fatigue of structural materials of the indicated classes.In view of the fact that the properties of the protected material differ from the properties of the coating and the inevitable presence of a specific transition zone on the boundary of the coating and the protected material, the base-coating system should be studied as a structure.The formation of the fields of residual stresses (which can hardly be controlled) and low reliability of the available data on the character of changes in the properties of the base material in the transition zone significantly complicate the problem of correct determination of the actual stress-strain state (SSS) of the material and the proper choice of the criteria of limiting state and durability of the coatings.In the solution of the posed problems, we follow the following major directions: -the analysis and generalization of the experimental data on the mechanisms of thermal fatigue damage to different materials in the stage following the initiation of a thermal fatigue crack depending on the intensity of thermomechanical load and corrosion influence of the gas flow;-the investigation of the dynamics of thermal and stress-strain states of the material under cyclic loading in the presence of a crack by the experimental methods and methods of numerical simulation;-the analysis of the regularities of propagation of thermal fatigue cracks in alloys with protective refractory coatings in directionally crystallized materials; 386 0039-2316/06/3804-0386

Degradation of the surface layers of superalloys and thermal-fatigue damage to the structural elements of gas-turbine engines

“…The mechanisms of damage to the surface layers, initiation and propagation of thermal fatigue cracks in structural elements of GTE are investigated in [10][11][12]. In directionally crystallized alloys, the cracks are initiated from the grain boundaries transversely oriented relative to the line of action of the loads [10].…”

“…At the same time, in monocrystalline alloys, the cracks are initiated from the eutectic phase and carbides [10]. The results of experimental investigation of damage and the kinetics of thermal fatigue cracks carried out on wedge-shaped specimens depending on the thermal and stressed states of workpieces can be found in [11][12][13][14].…”