Modelling of grain refinement around highly reactive interfaces in processing of nanocrystallised multilayered metallic materials

Duplex techniques are attempted to be developed combining nanocrystallisation processes with a subsequent thermomechanical processing in order to produce multilayered bulk structures with improved yield and ultimate tensile... [ view full abstract ]

Duplex techniques are attempted to be developed combining nanocrystallisation processes with a subsequent thermomechanical processing in order to produce multilayered bulk structures with improved yield and ultimate tensile strengths, while conserving an acceptable elongation to failure. However, the impurities deposited on the surface of the materials cause bonding imperfections due to interfacial oxidation during the duplex process. The interfacial oxidation can lead to formation of the discontinuous oxides or continuous oxide layer at the interfaces. The interface oxidation occurring during duplex processes can influence the microstructure development around the interfaces depending on whether the oxide scale is a continuous layer or a layer of discontinuous oxide clusters with heterogeneous thicknesses. Effectively the oxide scale becomes a part of the microstructure development of such nano-crystallised multilayered structures. The behaviour of the highly reactive interfaces during the processing of nanocrystallised multi-layered materials has been investigated numerically using the developed multi-level thermomechanically coupled finite element based model. The macro level part of the model representing the multilayered nanocrystallised metallic material has been linked to the meso- level part representing the oxidised interface within the material. The results of the analysis supported the possibility of strain localisations formed around the oxide islets at the interface between nanocrystallised fcc 316L austenitic stainless steel plates during the hot rolling stage of the duplex processing technique. In this work, the meso- level model has been expanded into evolution of the microstructure using frontal 3D Cellular Automata (FCA) numerical approach. The modelling data reflecting the strain localisations around the oxide clusters allowed for calculation of the grain refinement. The simulations of the grain refinement and changes of the disorientation angle are presented in the paper. The evolution of the initial microstructure with randomly distributed not oriented grains around the scale clusters is simulated focusing on the distribution of the boundaries disorientation angle during rolling taking into consideration different rotation rates among other things. An appearance of new boundaries is accompanied with an increase of the number of low-angle boundaries. The obtained results are in good agreement with the available experimental data derived from the relevant microstructural investigation.