Abstract
In polycrystalline materials, grain boundaries are known to be a critical microstructural component controlling material’s mechanical properties, and their characters such as misorientation and crystallographic boundary planes would also influence the dislocation dynamics. Nevertheless, many of generally used mechanistic models for deformation twin nucleation in fcc metal do not take considerable care of the role of grain boundary characters. Here, we experimentally reveal that deformation twin nucleation occurs at an annealing twin (Σ3{111}) boundary in a high-Mn austenitic steel when dislocation pile-up at Σ3{111} boundary produced a local stress exceeding the twining stress, while no obvious local stress concentration was required at relatively high-energy grain boundaries such as Σ21 or Σ31. A periodic contrast reversal associated with a sequential stacking faults emission from Σ3{111} boundary was observed by in-situ transmission electron microscopy (TEM) deformation experiments, proving the successive layer-by-layer stacking fault emission was the deformation twin nucleation mechanism, different from the previously reported observations in the high-Mn steels. Since this is also true for the observed high Σ-value boundaries in this study, our observation demonstrates the practical importance of taking grain boundary characters into account to understand the deformation twin nucleation mechanism besides well-known factors such as stacking fault energy and grain size.
Highlights
In polycrystalline materials, grain boundaries are known to be a critical microstructural component controlling material’s mechanical properties, and their characters such as misorientation and crystallographic boundary planes would influence the dislocation dynamics
A recent computational study indicates that intergranular interactions could influence the local strain distribution and strain transfer near grain b oundary[31], leaving open questions, i.e., whether the grain boundary character such as misorientation and boundary plane structure would regulate (i) the deformation twinning nucleation mechanisms associated with a grain boundary and (ii) dislocation dynamics and the deformation twining precursor structure near/at the grain boundary
A histogram in Supplementary Fig. S1 indicates the population of coincidence site lattice (CSL) boundaries having a certain axis/angle pair obtained by electron backscattered diffraction (EBSD) analysis[32]
Summary
Grain boundaries are known to be a critical microstructural component controlling material’s mechanical properties, and their characters such as misorientation and crystallographic boundary planes would influence the dislocation dynamics. There are five proposed deformation twin nucleation mechanisms applicable to high-Mn TWIP steels: that are Venables pole mechanism[15], Fujita-Mori stair-rod cross-slip mechanism[16], Cohen-Weertman-Frank cross-slip mechanism[17], Miura-Takamura-Narita primary slip mechanism[18], and Mahajan-Chin three-layer faults mechanism[19] They are based on microstructure investigations by transmission electron microscopy (TEM) and commonly indicate (a) a sufficient dislocation density in a grain and/or local stress concentration as essential prerequisites, and (b) an arrangement of highly coordinated Shockley partial dislocations glide on {111} slip planes, are the key features of the deformation twinning process. A recent computational study indicates that intergranular interactions could influence the local strain distribution and strain transfer near grain b oundary[31], leaving open questions, i.e., whether the grain boundary character such as misorientation and boundary plane structure would regulate (i) the deformation twinning nucleation mechanisms associated with a grain boundary and (ii) dislocation dynamics and the deformation twining precursor structure near/at the grain boundary
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