A Study Of Fatigue Meso-mechanical Properties Of A Nickel-based Superalloy By Depth-sensing Indentation Technique

Further development of Fatigue Life Prediction Techniques requires meso/micro research of mechanical behavior of fatigue and damage. A procedure for measuring and calculating the meso-mechanical properties of polycrystalline solids has been summarized in this thesis around depth-sensing indentation techniques and by this procedure the changing characteristic features of the meso-mechanical properties of a nickel-based superalloy GH4145/SQ during low-cycle fatigue at elevated temperature is investigated.Firstly, the original sample of the nickel-based superalloy is measured using DSI to determine the specific testing conditions of the DSI measurement, from which the indentation size effect (ISE) is further analysed.It is shown that when choosing such measuring conditions as the indentation load of 500mN, holding time of 15s and 17 measured numbers, the non-destructive measurements of the meso-mechanical properties as well as a neglectable ISE can be realized, the indentation hardness appear normal probability distributions.A calculating analysis of five representative methods for determing the mechanical properties based on load vs depth (P-h) curve is performed. The calculated meso-mechanical properties is then compared with the macro-tensile testing results and a method developed by M.Dao based on the maximum penetration depth that agrees well with the macro-mechanical data is suggested to calculate the meso-mechanical properties of the nickel-based superalloy during low-cycle fatigue at elevated temperature.The statistic changing characteristics of the indentation parameters including the maximum intendation depth, residual depth, plastic work and elatic work as well as the meso-mechanical properties including the hardness,yield stress,elastic modulous and strain hardening exponent of the Nickel-based superalloy during low cycle fatigue at elevated temperature are investigated by using DSI technique and it is further compared with the cyclic stress response curves obtained from macro cyclic tests. Meanwhile, the microstructural characteristic features such as the slip band spacing at different stages of fatigue damage process are measured by means of metallurgical microscopy. It is shown that during the whole process of fatigue, the meso-mechanical parameters such as the hardness, elastic modulus, yield stress and strain hardening exponent appear normal probability distributions. When the strain amplitude is held, the hardness and yield stress increase with increasing cycles, while the elastic modulus, strain hardening exponent and slip band spacings decrease with increasing cycles. When the cyclic number is held, the hardness and yield stress increases with the increasing strain amplitude, while the elastic modulus, strain hardening and slip band spacings decrease with the increasing strain amplitude. It is also proven that the variation in mechanical properties of the material is in correlation with the microstructural characteristics such as the slip band spacings in the statistical sense.