Behavior of Super Alloys Subjected to Fatigue Loads Under High Temperature


This study is evaluating what initial conditions can activate cubic slip planes, then the level of accommodation and strain homogenization within the grain, and how a given initial condition affects the material behavior when subjected to operational cyclic loads under high temperature. Ni-based super alloys are widely used in turbine engines mainly due to its high strength and fatigue resistance at elevated temperatures. One hypothesis to explain its atypical characteristic among metals is that a cross-slip mechanism is in place. The activation of {100} cubic slip systems along of the octahedral slip planes {111} in Ni-based superalloys has been verified when under high strain and temperature. The material would exhibit a more homogeneous strain distribution and less strain localization. We seek for the ideal precondition that will improve the endurance of Ni-based superalloy (IN 718) samples subjected to operational loading. We evaluate the initial conditions that activate cubic slip planes, the level of accommodation, and strain homogenization within the grain. With focus on the deformation mechanism, the sample microstructure can be fully characterized by electron backscatter diffraction (EBSD) and the slip systems, after the applied pre-condition, can be tracked via digital image correlation (DIC). Accomplished tasks: (a) samples’ manufacturing, (b) sample polishing and preparation, (c) furnace installation and operational tests, (d) development of laboratory procedures, equipment and microscopes (optical and SEM), (d) calibration and controller fine tuning for the MTS tensile testing machine, and (e) fatigue test with several specimens, including control samples and modified pre-conditions Next steps: Characterization of the microstructure of tested specimens under special conditions via EBSD to identify the slip planes and confirm or not the activation of cubic slip systems.

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