Dislocations Help Initiate the α–γ Phase Transformation in Iron—An Atomistic Study

Abstract

Using molecular dynamics simulation, we studied the influence of pre-existing dislocations on the austenitic and the martensitic phase transformations in pure iron. The simulations were performed in a thin-film geometry with (100) surfaces. We found that dislocations alleviate the transformation by lowering the austenitic transformation temperature and increasing the martensitic transformation temperature. In all cases, the new phase nucleates at the dislocations. The orientation relationships governing the nucleation process are dominated by the Burgers, Kurdjumov–Sachs, and Nishiyama–Wassermann pathways. However, upon growth and coalescence of the transformed material, the final microstructure consists of only few twinned variants separated by twin boundaries; this simple structure is dictated by the free surfaces which tend to form conserved planes under the transformation. After transformation, the material also contains abundant dislocations.

Phase Transition in Iron Thin Films Containing Coherent Twin Boundaries: A Molecular Dynamics Approach

“…The transition barrier is 7 meV/atom at this temperature [ 33 ]. In studies using the Meyer-Entel potential, inclusive of this work, the austenitic temperatures were always above the equilibrium temperature, and the martensitic temperatures were always below it [ 14 , 17 , 30 ]. In the phase transition induced by temperature changing, the finite heating/cooling rate should be the reason for this hysteresis as a result of the kinetic effect.…”

“…In the bulk material, crystal defects influent the transition behavior of Fe dramatically. Grain boundaries (GBs) [ 13 ], twin boundaries (TBs) [ 14 ] or dislocations [ 15 , 16 , 17 ] often act as the nucleation sites of the new phase. However, the influence of crystal defects on the transition in nanoscaled systems is still under discussion.…”

“…However, the influence of crystal defects on the transition in nanoscaled systems is still under discussion. Recently, it has been reported that the new phase prefers to nucleate at dislocation sites, in spite of the existence of the free surface in Fe thin slabs [ 17 ]. Here, the different defect structures, such as dislocations, stacking faults (SFs) and TBs, are important as they are present in the material in reality, depending on the alloy composition and applied heat or mechanical treatment [ 3 , 4 , 18 ].…”

Atomistic Simulation of the Strain Driven Phase Transition in Pure Iron Thin Films Containing Twin Boundaries

“…Only a few studies are related to this issue. In the nanoscale system, Meiser and Urbassek [29] simulated the Fe thin films containing dislocations and reported the thermally induced phase transition is initiated at the dislocation positions in despite of the existence of free surface. Here we should note that dislocation is only one defect type in the material.…”

“…The main reason for this choice is that TBs normally exhibit smaller energy compared to dislocations or grain boundary (GB). Since it has been confirmed that dislocations assist the phase transition in Fe thin films [29], it is meaningful to check whether another type of defect (TB), which is energetically more stable, has similar effect. In the present work, biaxial tensile strains will be applied to the bcc Fe thin films containing various TBs.…”