I have read literature where the twinning deformation phenomena helps in increasing the ductility and strength of a material. But the explaination as to why this occurs is not explained clearly. Please can someone clearly tell me the phenomena behind this.
When the deformation twins and the twin boundaries occur, the room for dislocation nucleation and dislocation pile-ups becomes smaller, because the twin boundaries serve as obstacles to the dislocation motion. This leads to the increase of the strength and ductility of crystal. For the mechanical model explaining this influence of the deformation twinning on the mechanical behavior of crystal see
https://www.researchgate.net/publication/233802377_A_continuum_model_for_initiation_and_evolution_of_deformation_twinning?ev=prf_pub
Article A continuum model for initiation and evolution of deformation twinning
When the deformation twins and the twin boundaries occur, the room for dislocation nucleation and dislocation pile-ups becomes smaller, because the twin boundaries serve as obstacles to the dislocation motion. This leads to the increase of the strength and ductility of crystal. For the mechanical model explaining this influence of the deformation twinning on the mechanical behavior of crystal see
https://www.researchgate.net/publication/233802377_A_continuum_model_for_initiation_and_evolution_of_deformation_twinning?ev=prf_pub
Article A continuum model for initiation and evolution of deformation twinning
I'm still having some doubts regarding this. From what I have understood is that when dislocations movement is hindered, wouldn't the ductility be lesser?? is there some sort of accommodation of dislocations happening in the twin boundaries?
In addition to twin boundaries resulting in a dynamic Hall-Petch effect, twinning is essentially due to dislocations. In TWIP steels (fcc metals too), the twinning is due to a/6 Shockley partial dislocations created through dissociation of a/2 perfect dislocations. Thus you may interpret it as a special kind of rearrangement of the partial dislocations, which will reduce the dislocation hardening due to perfect dislocations (Taylor model) and necking is delayed. Hope it explains.
For a ductile material, one can consider that the ductility of material would be lesser, only if both the necking (Localization) and failure occurs at early strains. So ductility can be increased by delaying necking. Twin boundaries acts as an obstacles and delays the motion of dislocations. Even in some of interactions, dislocations interaction with twin boundary leads to increase/decrease in twin width which affects ductility.
Ductility increase- Plastic deformation due to twinning is less compared to slip as it is a localized deformation mechanism. But during twinning some of the crystallographic planes get oriented in such a way that the probability for slip in those planes increases and hence slip increases. In this way the ductility may increase.
Strength Increase- Twin boundary acts as an obstacle for dislocation movement. They became impermeable for the dislocation to pass through. Eventually dislocation pile up happens around twin boundary and a larger stress is required for dislocation pile ups to pass through . So, strength may increase.
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