Irrational twin interfaces in Shape Memory Alloys and relevance to functional fatigue

Mohammed, Ahmed Sameer Khan

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https://hdl.handle.net/2142/116049 Copy

Description

Title

Irrational twin interfaces in Shape Memory Alloys and relevance to functional fatigue

Author(s)

Mohammed, Ahmed Sameer Khan

Issue Date

2022-07-10

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

Sehitoglu, Huseyin

Doctoral Committee Chair(s)

Sehitoglu, Huseyin

Committee Member(s)

• Krogstad, Jessica
• Ertekin, Elif
• Sofronis, Petros

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)

• Shape Memory Alloys
• interfaces
• fatigue
• phase transformations
• mechanics
• dislocations

Abstract

Shape Memory Alloys (SMAs) are functional materials characterized by their ability to undergo large magnitudes of reversible mechanical deformation through a microstructural mechanism known as the martensitic phase transformation. This capacity makes SMAs attractive for several actuating applications spanning biomedical, aerospace, and automotive domains. However, such reversibility is not perfect and a finite magnitude of residual deformation remains trapped in the material upon unload. Over several cycles of deformation, this residual deformation accumulates and leads to failure by a phenomenon termed as functional fatigue. Functional fatigue occurs by the emission and accumulation of slip dislocations at the intersection of two critical interfaces in the phase transformation – the martensitic twin boundary and the “habit plane”. Both these interfaces are uniquely complicated by their tendency to exhibit irrational Miller-index identities that is atypical in crystalline materials. Such irrationality has precluded any understanding of the atomistic and topological structure of these interfaces, ultimately precluding an understanding of the source of fatigue. This research addresses this void from a modeling standpoint through anisotropic continuum mechanics, atomistic simulations and thermodynamics. This research begins by establishing the atomistic and topological structure of the Type II twin interface in NiTi, one of the most commercially successful SMAs. A Terrace-Disconnection (T-D) topology for the interface is proposed, deriving the necessity for a periodically spaced array of disconnections to relieve coherence strains on rational terraces. The atomic-structure of the rational terrace is determined through Molecular Statics (MS) simulations, establishing the existence of non-trivial lattice-offsets and the role of lattice-motif shuffles in the mechanism of twin migration, both of which are unique to the non-single-lattice structure of the martensitic phase in NiTi. The motion of twinning disconnections on the rational terraces was simulated within a MS framework. Following an early notional hypothesis on slip-emission proposed in the early 1980s, the interaction of the twinning disconnection with a migration barrier was simulated where the barrier mimics the role of the transformation front of the SMA. It was observed that a dislocation reaction occurs leading to the emission of a dislocation on the twin boundary, explaining the formation of residual slip dislocations. The structure of the type II twin interface continued to remain a matter of debate in the field with two contrasting propositions offered for its structure, one being the aforementioned T-D topology while the other was a Tilt-Wall (T-W) topology. Through a concerted modeling approach involving both MS simulations and continuum Eshelby-Stroh mechanics, this study establishes the energetic favorability of the T-D topology. It is theoretically shown that such a T-D topology attributes a capacity for the interface to evolve under microstructural strain explaining conflicting experimental observations in literature for the first time. Furthermore, it is shown that even type I and compound twins that are otherwise known to exhibit rational Miller-index identities can evolve under the influence of strain to exhibit irrational topologies in general. In this regard, an irrational T-D topology is proposed to be a norm rather than the exception for twin interfaces in SMAs. The influence of such strain-sensitive interface evolution on functional fatigue is discussed. A general framework to analyze irrational type II twin interfaces in other SMAs is then developed, applied to martensitic crystal structures of the B19-orthorhombic form in TiPd, TiPt and AuCd. It is shown that the irrational T-D topology is in effect a compromise achieved by the material to satisfy minimal continuum strain-energy and atomistic potential energy. Having resolved the irrational twin interface, the structure of the “habit plane” or the phase-transformation front was addressed next. An open problem to address was how the twin-variants in the martensitic phase are oriented with respect to the austenite and how the twin interface intersects the transformation front as that is the location of accumulating residual slip-dislocations. The study target of FeMnNiAl was chosen for its simpler cubic crystalline forms in the austenitic and martensitic phases. Experimental results for the first time observed multiple non-unique orientation relationships between the austenitic and martensitic phases, observing the Pitsch, Krudjumov-Sachs and Nishiyama-Wasserman orientation-relations independently. These observations were explained based on an energy-minimization theory bridging a gap between the predictions of the continuum theory with the crystallography of the transformation. The atomic-structure of the habit-plane is proposed as a coherent zone hosting large strain-gradients of transformation merging the martensitic twinned/faulted structure with the parent austenitic phase. The final contribution of this research is to provide a thermodynamic explanation of why slip-emission occurs in martensitic transformations at all. Prior work in the field attributed the necessity of residual slip to accommodate strain-mismatch at the habit-plane. However, these considerations were neither supported by an underlying understanding of interface structure nor by a complete consideration of the Gibbs’ free energy. This research proposes the spontaneity of slip-emission during reverse transformation by showing that it is the favored path of reversal for the material upon unload based on Gibbs’ free energy considerations. This path is preferred due to an addition of thermodynamic driving force afforded by the increasing transformation-strain of the martensitic inclusion, hypothesized and revealed for the first time. In conclusion, this study addresses multiple voids in revealing the cause of functional fatigue with the ultimate goal of providing tailorable design parameters to engineer superior fatigue-resistant SMAs.

Graduation Semester

2022-08

Type of Resource

Thesis

Copyright and License Information

Copyright 2022 Ahmed Sameer Khan Mohammed

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