Modeling of twinning as a deformation mechanism in iron (bcc) crystals and iron-based alloys

Ojha, Avinesh

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https://hdl.handle.net/2142/44761 Copy

Description

Title

Modeling of twinning as a deformation mechanism in iron (bcc) crystals and iron-based alloys

Author(s)

Ojha, Avinesh

Issue Date

2013-05-28T19:18:49Z

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

Sehitoglu, Huseyin

Department of Study

Mechanical Sci & Engineering

Discipline

Mechanical Engineering

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

M.S.

Degree Level

Thesis

Keyword(s)

• body-centered cubic (bcc)
• twinning
• molecular dynamics
• energy landscape

Abstract

We develop an analytical expression for twin nucleation stress in bcc metals considering GPFE (generalized planar fault energy) and the dislocations bounding the twin nucleus. We minimize the total energy to predict the twinning stress relying only on parameters that are obtained through atomistic calculations, thus excluding the need for any empirical constants. We validate the present approach by means of precise measurements of onset of twinning in bcc Fe50Cr single crystals showing excellent agreement. The experimental observations of the three slip systems resulting in the twin formation was demonstrated via EBSD and TEM techniques along with DIC (Digital Image Correlation). In addition, the present work is geared towards understanding the twin-slip and twin-twin interactions in alpha -Fe crystals using Molecular Dynamics (MD) technique. Five types of twintwin and twin-slip intersections have been analyzed, namely <111>, <113>, <210>, <513>, <110>, and the magnitude of the residual dislocation left at the twin boundary for each type of intersection was identified. Further, the role of the residual dislocations in affecting the magnitude of the critical stress required for twin migration has been established for each intersection type. We are able to investigate the Schmid factor criteria for slip and twin nucleation under tensile and compressive loading orientations, and the results obtained are in close agreement with the theoretical critical resolved shear stress (CRSS) of the activated systems. Furthermore, the results obtained through MD simulations and experiments utilizing EBSD and DIC are used to investigate the effect of the residual dislocations on the energetics of twin-slip and twin-twin interactions. An analytical expression is developed based on the geometrical parameters such as twin width and length, and the magnitude of the residual dislocation that quantifies the critical stress required for twin migration. Energy analysis based on the total elastic energies of the interacting dislocations and the associated fault energies reveal a strong dependence of the twin migration stress as a function of the magnitude of the residual dislocation. A higher magnitude of the residual dislocation causes an increase in the twin-migration stress and makes the transmission of slip/twin more difficult through the coherent twin boundary (CTB).

Graduation Semester

2013-05

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http://hdl.handle.net/2142/44761

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Copyright 2013 Avinesh Ojha

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