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Title:Critical stresses for twinning, slip and transformation in shape memory alloys: modeling and experiments
Author(s):Ojha, Avinesh
Director of Research:Sehitoglu, Huseyin
Doctoral Committee Chair(s):Sehitoglu, Huseyin
Doctoral Committee Member(s):Ertekin, Elif; Bellon, Pascal; Sofronis, Petros
Department / Program:Mechanical Sci & Engineering
Discipline:Mechanical Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Shape memory alloys
Density functional theory
Ti-based alloys
Digital image correlation
Abstract:There are four important parameters-(i) transformation stress (ii) twinning stress (iii) austenite slip stress and (iv) martensite slip stress that govern shape memory and superelastic behavior in shape memory alloys (SMAs). However, it is a tremendous experimental effort to determine these parameters for multiple alloys. In addition, slip, twinning and transformation events could be activated simultaneously or require in-situ high-resolution image correlation techniques. At this time, these quantities are inferred from experiments on a case by case basis. In this study, we believe that a significant benefit could be realized if these parameters are determined theoretically based on atomistic considerations. This has not been a simple task especially for the case of transformation stress where the martensite nucleation formation involves complex atomic level displacements and shuffles on different planes. For stress-induced martensite, the derivation of transformation stress is of paramount importance, but it has not been achieved in early work because of the crystallographic complexity. This thesis is geared towards developing such theoretical models using advanced energetic approach to predict these quantities precisely. In the first section of the thesis, we investigate the bcc-fcc transformation in the new FeMnAlNi SMA utilizing density functional theory. A modified Peierls Nabarro (PN) formalism is developed that incorporates the transformation shear energy at an atomic scale in conjunction with the heterogeneous dislocation based martensitic transformation theory to predict transformation stresses. We confirm the validity of our formulations by comparing the data obtained from experiments and literature showing excellent agreement. The formulation so developed will facilitate the development of future novel shape memory alloys. In order to have a better shape memory and superelastic performance, the difference between the transformation/twinning stress and the austenite/martensite slip stress must be high. In the second section of the thesis, we focus on the role of alloying on twinning, slip and transformation stress levels. We take Ti-based SMAs as examples to establish the critical slip, twinning and transformation stresses as a function of alloying contents such as Ta and Zr addition. We establish the energy barriers-the Generalized Stacking Fault Energy (GSFE) and the Generalized Planar Fault Energy (GPFE) for slip and twinning respectively, and the transformation energy barriers encompassing a wide composition range of Ta and Zr. It is found that alloying Ta in Ti-Nb alloys increases all of these quantities. However, the rate of increase of CRSS with an increase in Ta content is much higher for slip compared to transformation and twinning, thus the difference between the two increase with an increase in Ta content. The higher CRSS of austenite slip compared to transformation stress is beneficial to improve the shape memory response. In such a case of high austenite slip stress, the transformation proceeds at a stress much lower compared to the austenite slip stress, and hence the possibility of any plastic strain accumulation is minimized. Overall, we find that higher alloying content exhibits higher difference between the austenite slip and the transformation stress, exhibiting better shape memory performance. The other class of alloys we focus on in this work are the Fe3Al and Fe3Ga-B alloys that exhibit superelastic performance without undergoing martensitic transformation. These alloys with D03 austenite structures exhibit pseudoelasticity due to reversible slips and twinning-detwinning mechanisms upon loading and unloading. The advantage of these alloys compared to transforming shape memory alloys is the potentially large pseudoelastic temperature range. Reversible slip in these alloys is due to the to and fro motion of the a/4<111> superpartials associated with the anitphase boundary energy, and is commonly referred to as “APB pseudoelasticity”. The backstress generated by the APB energy provides the driving force for the reversal of the deformation upon unloading, resulting in a superelastic phenomenon as in shape memory alloys. Using density functional theory simulations, we obtain the energy barriers for [110}<111> and {112}<111> slips in D03 Fe3Al and the elastic moduli tensor, and undertake anisotropic continuum calculations to obtain the backstress and the frictional stress responsible for reversible slip. We extend our formulation using Eshelbian approach to incorporate the effects of interstitial boron solutes in elevating the twinning stress level in Fe3Ga-B. We find that boron solutes occupy the octahedral sites by reducing the structural energy, and making dislocations difficult to move, thus contributing to solute strengthening. We compare the theoretically obtained slip stress magnitudes (friction and back stress) with experimental measurements disclosing excellent agreement.
Issue Date:2017-09-13
Rights Information:Copyright 2017 Avinesh Ojha
Date Available in IDEALS:2018-03-13
Date Deposited:2017-12

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