DEVELOPMENT OF INTERNAL STRESSES IN NI-BASE SUPERALLOY DURING CYCLIC LOADING
Keywords:
LCFAbstract
Ni-base superalloy are extensively used as gas turbines and aero engine components because of their high temperature deformation resistance. In single crystal form Ni-base Superalloy SC16, consisting of 40% γ precipitates in the matrix of γ phase finds application in aero engines due to their improved high temperature resistance as compared to conventionally cast polycrystalline alloys. Deformation behaviour of this alloy under cyclic loading is interrelated in a complex manner with the state of internal stresses, material parameters such as volume fraction of γ precipitates, single crystal orientation, dislocation network at γ/γ′ interfaces as well as testing parameters such as temperature, strain rate, strain range etc. Keeping this in view, the state of internal stresses, characterized as back stress and friction stress developed during Low Cycle Fatigue (LCF) loading have been quantitatively estimated for the first time from the analysis of large volume of σ-ε hysteresis loops generated experimentally in the temperature range of 750oC to 950oC and total strain range of 0.8% to 2.0%, the strain rate of 10-3/s remaining constant. Analysis of σ - ε hysteresis loops generated during LCF loading revealed that both back stress and friction stress, as a function of number of load cycles, attains almost a steady state at a constant temperature and strain range after an initial short transition region. The steady state value of back stress increases with increasing total strain range and decreasing temperature. In contrast, the steady state friction stress almost remains unaltered with these parameters.
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