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Title:Advanced experimental measurements of heterogeneous strain and temperature change in metals
Author(s):Pataky, Garrett James
Director of Research:Sehitoglu, Huseyin
Doctoral Committee Chair(s):Sehitoglu, Huseyin
Doctoral Committee Member(s):Sofronis, Petros; Ertekin, Elif; Lambros, John
Department / Program:Mechanical Sci & Engineering
Discipline:Mechanical Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Haynes 230
shape memory alloys
elastocaloric effect
high temperature
digital image correlation
fatigue crack growth
Abstract:This work investigates elevated temperature deformation in the nickel-based superalloy Haynes 230 and elastocaloric cooling in shape memory alloys. Three loading conditions were examined in the elevated temperature portion: fatigue crack growth, creep, and tension. Measurements were made on varying length scales using digital image correlation depending on the intended purpose of the study. The elastocaloric cooling study examines the temperature change in the reverse martensitic transformation utilizing infrared thermography and digital image correlation. The fatigue crack growth of Haynes 230 was investigated at room temperature and 900 °C using digital image correlation. As expected, the crack growth rates at high temperature were much faster than at room temperature. However, the crack closure levels, which were determined using digital image correlation analysis techniques, were found to be similar for the two cases studied. DIC strain fields and the corresponding plastic zone sizes were compared between the two cases. From these strain fields, the slip irreversibility, the difference between forward and reversed strains at the crack tip, was quantitatively measured. The high temperature case had an order of magnitude higher amount of slip irreversibility. Slip irreversibility measurements were determined to be an effective method to compare fatigue crack growth of cases with differing temperatures. Creep was studied in Haynes 230 at 800 °C and 900 °C. This study focused on the differences in the creep mechanisms and damage at the two temperatures. Using the microstructure, grain boundary serrations and triple junction strain concentrations were quantitatively identified. There was significant damage in the 900 °C samples and the creep was almost entirely tertiary. In contrast, the 800 °C sample exhibited secondary creep. Using an Arrhenius equation, the minimum creep rate exponents were found to be n and n for 900 °C and 800 °C respectively. The creep mechanisms were identified as solute drag for n and dislocation climb for n . Strain concentrations were identified at triple junctions and grain boundary serrations using high resolution digital image correlation overlaid on the microstructure. The grain boundary serrations restrict grain boundary sliding which may reduce the creep damage at triple junctions and extend the creep life of Haynes 230 at elevated temperatures. An experimental methodology has been developed to study materials deformed at high temperatures using high resolution ex-situ digital image correlation. This study is an advancement of techniques that have used sub-grain level strain measurements and linked them to microstructural data obtained with electron back-scatter diffraction. The approach utilized air blasting particles that were capable of remaining unchanged onto a sample surface and loading the sample at elevated temperatures in vacuum to prevent oxidation. Two tensile experiments were performed at temperatures of 700 °C and 800 °C on Haynes 230 using digital image correlation at high magnifications in order to study the heterogeneous strain fields. Overlaying the strain fields and microstructure data, global trends and areas of high strain were studied. This study represents an advancement of the usefulness of high resolution digital image correlation by expanding the loading conditions that can be studied and provided experimental results to further physics-based modeling. Solid state elastocaloric cooling, the endothermic reversible martensitic phase transformation in shape memory alloys, has the potential to replace vapor compression refrigeration. NiTi, Ni2FeGa, and CoNiAl shape memory alloys were experimentally investigated to measure the magnitude of temperature change using thermography during uniaxial tensile experiments. Consecutive tensile cycles were also performed, and they revealed a symmetric temperature profile between the two cycles. The unique, dual camera technique of digital image correlation and infrared thermography was utilized to track the transformation bands and temperature gradients to gain insight about the unloading, endothermic process. Fatigue implications, elevated temperature environments, and the theoretical maximum temperature based on entropy change were discussed.
Issue Date:2015-04-20
Rights Information:Copyright 2015 Garrett J. Pataky
Date Available in IDEALS:2015-07-22
Date Deposited:May 2015

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