Loading...

Study of Microstructure Evolution During Cyclic Martensitic Transformations in Shape Memory Alloys with Molecular Dynamics Simulation

Moravvej, Amin | 2016

789 Viewed
  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 49461 (08)
  4. University: Sharif University of Technology
  5. Department: Mechanical Engineering
  6. Advisor(s): Nejat, Hossein; Arghavani, Jamal
  7. Abstract:
  8. Shape memory alloys (SMA) exhibit a number of features which are not easily explained by equilibrium thermodynamics, including hysteresis in the phase transformation and ‘‘reverse’’ shape memory in the high symmetry phase. Processing can change these features: repeated cycling can ‘‘train’’ the reverse shape memory effect, while changing the amount of hysteresis and other functional properties. These effects are likely to be due to formations of localised defects and these can be studied by atomistic methods. Here we present a molecular dynamics simulation study of such behaviour employing a two-dimensional, binary Lennard–Jones model. Our atomistic model exhibits a symmetry breaking, displacive phase transition from a high temperature, entropically stabilised, austenite-like phase to a low temperature martensite-like phase. The simulations show transformations in this model material proceed by non-diffusive nucleation and growth rocesses and produce distinct microstructures. We observe the generation of persistent lattice defects during forward-and-reverse transformations which serve as nucleation centres in subsequent transformation processes. These defects interfere the temporal and spatial progression of transformations and thereby affect subsequent product morphologies. During cyclic transformations we observe accumulations of lattice defects so as to establish new microstructural elements which represent a memory of the previous morphologies. These new elements are selforganised and they provide a basis of the reversible shape memory effect in the model material
  9. Keywords:
  10. Shape Memory Alloy ; Molecular Dynamics ; Phase Transition ; Cyclic Loading ; Microstructure ; Molecular Dynamic Simulation ; Martensitic Transformation ; Hysteresis Functional Fatigue

 Digital Object List

 Bookmark

No TOC