Dislocation slip and twinning stress in shape memory alloys- theory and experiments

Wang, Jifeng

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Description

Title

Dislocation slip and twinning stress in shape memory alloys- theory and experiments

Author(s)

Wang, Jifeng

Issue Date

2014-01-16T17:58:39Z

Director of Research (if dissertation) or Advisor (if thesis)

Sehitoglu, Huseyin

Doctoral Committee Chair(s)

Sehitoglu, Huseyin

Committee Member(s)

• Ertekin, Elif
• Hsia, K. Jimmy
• Bellon, Pascal

Department of Study

Mechanical Sci & Engineering

Discipline

Mechanical Engineering

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Dislocation slip
• twinning
• stress
• shape memory alloys
• theory
• experiments

Abstract

Slip and twinning are two important deformation mechanisms governing Shape Memory Alloys (SMAs) plasticity, which results in affecting their pseudoelasticity and shape memory performance. Precisely determining Peierls stress in dislocation slip and critical twin nucleation stress in twinning is essential to facilitate the design of new transforming alloys. This thesis presents an advanced energetic approach to investigate the attributes of phase transformation, slip and twinning in SMAs utilizing Density Functional Theory based ab initio calculations, and the role of energy barrier is characterized. Through different length scales incorporating atomistic simulations into dislocation-based mechanics, an extended Peierls-Nabarro (P-N) model is developed to establish flow stresses in SMAs and the predicted Peierls stresses are in excellent agreement with experiments. In addition, a twin nucleation model based on P-N formulation is proposed to determine the critical twin nucleation stresses in SMAs, and the validity of the model is confirmed by determining twinning stresses from experiments. The first part of the thesis presents an energetic approach to comprehend a better understanding of phase stability, martensitic transformation path and dislocation slip in SMAs utilizing first principle simulations. In particular, we discovered energy barriers in transformation path from austenite B2 to martensite and B33 of NiTi, and studied phase stability of and under effect of hydrostatic pressure. The results provide a more authoritative explanation regarding the discrepancy between the experimental observations and theoretical studies. In addition, we calculated energy barriers associated with martensitic transformation from austenite L21 to modulated martensite 10M of Ni2FeGa incorporating shear and shuffle and slip resistance in [111] direction as well as in [001] direction of austenite L21. The results show that the unstable stacking fault energy barriers for slip by far exceeded the transformation transition state barrier permitting transformation to occur with little irreversibility. This explains the experimentally observed low martensitic transformation stress and high reversible strain in Ni2FeGa. Furthermore, we established the energetic pathway and calculated the theoretical shear strength of several slip systems in B2 NiTi. The results show the smallest and second smallest energy barriers and theoretical shear strength for the and the cases, respectively, which are consistent with the experimental observations. This study presents a quantitative understanding of plastic deformation mechanism in B2 NiTi, and the methodology can be applied for consideration of a better understanding of SMAs. In the second part of the thesis, we developed an extended P-N model to precisely predict dislocation slip stress in SMAs utilizing atomistic simulations and mesomechanics. We validated our model by conducting experiments and the results show that this model provides precise and rapid results compared to traditional experiments. This extended P-N model with Generalized Stacking Fault Energy curves provides an excellent basis for a theoretical study of the dislocation structure and operative slip modes, and an understanding to discovery of new compositions avoiding the trial-by-trial approach in SMAs. Further, we developed a twin nucleation model based on the P-N formulation to precisely predict twin nucleation stress in SMAs. We classified different twin modes that are operative in different crystal structures and developed a methodology by establishing the Generalized Planar Fault Energy to predict the twinning stress. This new model provides a science-based understanding of the twin stress for developing SMAs.

Graduation Semester

2013-12

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Copyright 2013 Jifeng Wang

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