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Title:Advanced experimental characterization of deformation mechanisms and temperature changes on novel alloys systems
Author(s):Wu, Yan
Director of Research:Sehitoglu, Huseyin
Doctoral Committee Chair(s):Sehitoglu, Huseyin
Doctoral Committee Member(s):Bellon, Pascal; Krogstad, Jessica; Ertekin, Elif
Department / Program:Mechanical Sci & Engineering
Discipline:Mechanical Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Shape Memory Alloys
High Entropy Alloys
Abstract:The human civilization continues to benefit from novel alloys systems ascribed to their unprecedented functionalities. Among many, shape memory alloys (SMAs) and high entropy alloys (HEAs) are presently of great research interest in material science and engineering community. Unlike conventional alloys, the former exhibits astonishing recoverability from excessive deformation and the latter possesses exceptional strength and ductility extending to cryogenic temperatures. This work investigates the high temperature response and elastocaloric effect of SMAs and strain hardening behavior of HEAs using advanced experimental techniques, i.e. digital image correlation (DIC), optical microscopy, electron microscopy, etc. In the first portion of this dissertation, the mechanical responses of the NiTiHf high temperature SMAs are investigated. This class of materials represent a significant advancement in extending the functionality of binary NiTi to elevated temperatures above 100 °C. Despite this potential, the previous results in the literature point to a disappointingly low transformation strains with addition of Hf. On the other hand, based on theoretical analysis using the lattice constants, the transformation strains should increase substantially with increase in Hf content. The present study addresses this discrepancy. Using DIC, the transformation strain is established with very careful strain measurements at small scales in both isobaric and isothermal experiments. Because of the heterogeneity of strain distributions, the results depend on the sub-region considered. By scrutinizing the alloy systems with Hf content in the range from 12.5 to 25 at.%, we show that the experimental transformation strains in NiTiHf indeed increase with increasing Hf to unprecedented strain levels near 20%. In the second part of this dissertation, the elastocaloric (EC) effect of SMA is examined. The EC effect refers to the rapid cooling in SMAs during reverse transformation from martensite to austenite under adiabatic conditions. We present a very comprehensive study of the EC response far extending the existing literature by studying the effect of loading states (tension and compression), long-term cycling, strain localization, and deformation temperatures in several alloy systems including CuZnAl, NiTi, NiTiCu, Ni2FeGa and NiTiHf13.3. We found a substantial temperature change of 14.2 °C in CuZnAl, 18.2 °C in NiTi, 15.2 °C in NiTiCu, 13.5 °C in Ni2FeGa, and 6.95 °C in NiTiHf13.3 upon reverse transformation depending on the entropy change (as high as 60 J/kg K), the stress hysteresis, the inhomogeneity of the transformation and the number of superelastic cycles. A gradual deterioration of the EC effect in tension develops, while in compression the EC effect can be sustained much longer (in excess of 104 cycles). The Ni2FeGa SMAs possess an operational EC temperature window of nearly 200 °C, which is the widest among the chosen SMAs. With over one hundred experiments reported, the current study represents an authoritative summary of the EC capabilities of a wide range of SMAs. The last part of this dissertation deals with the equi-atomic FeMnNiCoCr HEAs due to their exceptional strain hardening behavior extending to large strains and to low temperatures (77K). We analyze the nano- to macroscale deformation response of FeMnNiCoCr single crystals and explain variations in strain hardening based on the activation of different twin and slip systems and their interactions. We experimentally determine the latent and the self hardening moduli upon twin-twin, slip-twin, twin-slip and slip-slip interactions. Choosing single crystal orientations that isolate these interactions enables the evaluation of the pertaining hardening moduli without ambiguity. Differing from the earlier experimental approaches employed, which necessitate sample reorientation to quantify the self and latent hardening coefficients, in this work, we demonstrate a novel framework where plastic straining is implemented in a monotonic fashion entailing the latent and primary systems operate simultaneously. To extract the hardening moduli and to characterize different interactions on experimental grounds, <111>tension, <001>compression, <122>tension, <144>tension and <149>compression single crystalline samples are studied by high resolution DIC, electron backscatter diffraction and transmission electron microscopy techniques. The results demonstrate that the magnitude of residual Burgers vectors play a key role in explaining the experimental hardening trends.
Issue Date:2019-03-18
Rights Information:Copyright 2019 Yan Wu
Date Available in IDEALS:2019-08-23
Date Deposited:2019-05

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