Study On The Microstructure And Mechanical Properties Of Multi-scale Layered 304ss Sheet

Ultrafine-grained materials (UFG) as advanced structural materials have been an important research field in new century due to advantages of no alloying, high purification and easy to recycle. The strength of the nanocrystalline (NC) and ultrafine grained materials is much higher than that of the coarse grained (CG) counterpart. However, the ductility of NC/UFG materials decreases dramatically with the decrease of grain size, even occurs the transition from the ductile behavior to brittle behavior. Early results show that the ductility deterioration of the NC materials is an intrinsic defect, which is very unfavourable for applications of structural materials. Therefore, it is an important challenge to design a structural material with high strength and larger ductility. Recently, the microstructure and refinement mechanism of the UFG materials fabricated by severe plastic deformation (SPD) have been partly reported, but the evolution of the microstructure after further warm deformation is seldom explored. In order to combine high strength and large ductility, a layered nanostructural (LaNa) steel was designed, which was characterized by a periodic distribution of NC layers and micron-grained (MG) layers with graded evolution. The layered nanostructural sheet was fabricated by using a dual process of surface mechanical attrition treatment (SMAT) and warm co-rolling (WCR). The microstructures, mechanical properties and fracture mechanism of the LaNa 304ss steel were investigated. It was paid more attention to the refinement mechanism,γ/α′transformation and deformation behavior of NC layers in the process of warm co-rolling. Microstructures were characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD). The mechanical properties were tested by tensile test system at room temperature and vickers microhardness testing machine. The main results were summarized as follows:The microstructure of LaNa 304ss was composed ofγaustenite andα′martensite phases with multiscale grain size distribution, characterized by a periodic distribution of nano-/sub-micron and micron grained layers. With the depth increase, the microstructures were transferred gradually from equiaxed NC/UFG grains to sub-micron grains, and to the micron grains with subgrain/dislocation cell/dislocation, indicating a graded evolution. For the LaNa 304ss with an area reduction of 40%, the volume fraction of NC/UFG grained layers and the twin layers was about 33%, respectively. But as to the LaNa steel with an area reduction of 50%, the volume fraction of NC/UFG was high up to 60%, indicating an obvious grain refinement. The refinement mechanism was suggested to be dislocation division, dynamic recovery and recrystallization, and reverseγ/α′martensite transformation.A good combination of high strength and large ductility was achieved in the LaNa 304ss, which exhibited the ultrahigh strength of NC/UFG materials, and obtained a good ductility by using the dislocation accumulation of the micron grains. The results of the tensile test revealed that the yield strength was in the range of 700 MPa~950 MPa, and ultimate strength reached 930 MPa~1000 MPa, moreover, the elongation to fracture ranged from 30% to 50%. The reasons of the strengthening were originated from the grains refinement, dislocation pile-up, and strain-induced martensite transformation of NC/UFG. The high ductility was attributed to the dislocation pile-up in micron grained layers, and transformation-induced plasticity (TRIP), which resulted in obvious work hardening with work hardening exponent, n, high up to 0.47. It was indicated that the layered structure of the dual-phase with multiscale grained size was very effective in improving the ductility of NC/UFG materials. The LaNa steel exhibited a non-localized tensile deformation behavior at room temperature, which could be subdivided into five stages: hardening, sliding, necking propagation, necking, and cracking. The unusual deformation behavior was represented by the occurrence of a sliding band and necking propagation, originated from a special toughening mechanism characterized by repeated crack initiation, propagation and blunting. The joint activation of several strategies controlled the crack propagation path by main crack initiation at interface defects, arresting by compressive residual stress, interface deflection, and blunting by the MG layer. In the process of tensile, the main cracks were initiated from the oxides and defects of the interface, and propagated along with the interface layer, accompanied with the secondary cracks induced from the UFG layer and micron grained layer. While the main cracks were arrested by the compressive residual stress, then deflected and bridged by the interface, they were blunted in situ by the strength mismatch of different grain sizes with graded evolution. This process was repeated until the cracks were distributed along the entire gauge length leading to a multiple cracking in interlaminar. The NC/UFG layers deformed in the form of multiple crakes, secondary sliding to achieve significant plasticity, which further enhanced the toughness and ductility of NC/UFG materials.