Characterization of Crack Tip Plasticity and Size Effect in High Temperature Material Using Nano-mechanical Raman Spectroscopy and Indentation
Alloy 617 (IN-617) is an alloy which mainly contains nickel (Ni), chromium (Cr), cobalt (Co) and molybdenum (Mo). IN-617 is widely used in applications that require high temperature operation due to its high temperature stability and strength as well as strong resistance of oxidation and carburization. Even though Alloy 617 and other nickel based super alloys have been studied before, some issues such as the very high temperature properties at the nano and micro scale have not been examined. In the current research, Alloy 617 was studied in the temperature range of room temperature to 1073 K (800 °C) for temperature dependent strength and crack resistance behavior. Combined high temperature nanoindentation and nanomechanical Raman spectroscopy methods were used to measure crack tip plasticity and stress distribution around a notch tip. Elastic modulus, hardness, creep exponent, creep strain rate and thermal activation volume of the different Alloy 617 samples were studied through nanoindentation method. Indentation size effect (ISE) was studied in terms of hardness variation as a function of loading depth and temperature. A relation between indentation depth and hardness was fitted at different temperatures. Using a combination of optical microscopy and SEM imaging, precipitate effect and oxidation effect were analyzed. A novel nano-mechanical Raman spectroscopy measurement platform was designed to measure crack tip stresses during in-situ mechanical deformation. Notch tip of Alloy 617 sample during 3-point bending tests with an initial notch was scanned during the 3-point bending test. Temperature field and stress distribution in the notch tip area were generated by combining measurement results at each scan point through the relation between the stress, temperature and Raman shift. Instead of considering actual grain structures with different material properties, a new finite element method was adopted to predict stress distribution applying the material properties which were obtained from indentation experiments around the same notch area as scanning. Predictions from theory and simulations match closely in stress concentration area with experimental measurements.
Tomar, Purdue University.
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