3D analysis of grain refinement around highly reactive interfaces in processing of nanocrystallised multilayered metallic materials by duplex technique

Abstract : Refining the grain size through severe plastic deformation (SPD) is known to provide the processed material with novel properties. The potential application of such advanced materials as biomedical implants has been evaluated by investigating the effect of different grain sizes on a variety of cell–substrate interactions [1]. The refined to the nano-level grain surfaces are considered to have potential to alter absorption of proteins that mediate cell adhesion, control and enhance subsequent cell functions and tissue growth [2]. Normally, these techniques allow for modification only of the surface layer. The affected depth of the nanocrystallised surface layer in such materials is limited to several tens of micrometres [3, 4]. Such limited volume fraction of the nano-grains is not sufficient for improvement of their mechanical properties and often it is the reason for rapid saturation of the yield strength. This problem can be solved by increasing the volume fraction of the ultrafine grains within the material structure. 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 [5]. 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 [6]. 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 around oxidised interfaces using frontal 3D Cellular Automata (FSA) 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.