Modeling the interface structure of type II twin boundary in B19′ NiTi from an atomistic and topological standpoint

Ahmed Sameer Khan Mohammed, Huseyin Sehitoglu

Research output: Contribution to journalArticle

Abstract

This study addresses fundamental quandaries in the understanding of Type II twin interface in B19′ NiTi. A combined atomistic-topological approach is proposed to resolve a longstanding debate on the interface structure, affirming the hypothesis of a semi-coherent ledged geometry comprising of disconnected terraces. Atomic registry across the terrace is shown to require interface coherence strains. The twinning plane is shown to be a non-crystallographic virtual boundary separating the strained twin variants. Consequently, the issue of lattice offset arises and is addressed by an atomistic evaluation of interface energetics upon parametric variation of an offset parameter. Required atomic movements for migration of the terrace are established from a crystallographic analysis of the strained interface structure, and validated by a Molecular Statics (MS) simulation of the twin migration segment in the Generalized Planar Fault Energy (GPFE) curve. The GPFE calculation estimates a twinning partial magnitude consistent with an earlier ab initio prediction. This twinning partial serves as a “perfect” interface dislocation which, along with the coherence strain, feed into a topological model causally explaining the known irrational indices of the effective Twin Boundary (TB). A complete mechanistic picture of diffusionless TB migration is presented, the importance of which is discussed.

Original languageEnglish (US)
Pages (from-to)93-109
Number of pages17
JournalActa Materialia
Volume183
DOIs
StatePublished - Jan 15 2020

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Keywords

  • Interface structure
  • NiTi
  • Shape memory alloys
  • Twin boundary migration
  • Type II twinning

ASJC Scopus subject areas

  • Electronic, Optical and Magnetic Materials
  • Ceramics and Composites
  • Polymers and Plastics
  • Metals and Alloys

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