Microstructures,Mechanical Properties And Deformation Mechanisms Of Hetero-Structured Pure Nickel

Hetero-structured(HS)materials comprising of mixtures of soft zones and hard zone,showing superior combination of strength and ductility,have recently attracted extensive attention from the materials community.The strengthening effect of HS materials cannot be fully explained by traditional strengthening mechanisms.At present,it is generally believed that the strain partitioning during the plastic deformation leads to strain gradient near the interfaces.Geometrically necessary dislocations(GNDs)are accumulated to accommodate the strain gradient,resulting in additional hetero-deformation induced(HDI)hardening.Therefore,the density of interfaces,strength difference between soft zone and hard zone,the morphology and distribution of soft zones are critical factors for designing heterogeneous structures.Understanding fabrication processes and deformation mechanisms of HS materials is essential for optimizing mechanical properties.In this paper,pure Ni is selected as reference materials,and gradient structured Ni,heterogeneous lamella structured Ni are used as model materials.The correlation between microstructural architecture and mechanical properties is investigated in detail.The deformation mechanisms for strain accommodations and their connections with HDI stresses are discussed.The main conclusions of this paper are as follows:(1)Gradient structured Ni was obtained by rotational accelerated shot peening(RASP)treatment on the coarse-grained Ni surface.In addition to dislocation slip at the topmost surface,deformation twinning and shear band deformation were activated to form nano-twins and shear bands.In the shear band,dynamic recrystallization and recrystallization twinning occurred to form nanocrystalline grains.Since the mechanical incompatibility between the shear bands and the adjacent deformed structures,cracks formed at interfaces.During tensile test of gradient structure,the core region started dislocation slip first to produce plastic strain,while the surface layer remained elastic.With increasing strain,the elastic-plastic interface gradually moved from the core to the surface.When necking occurred at the subsurface layer,generating microcracks are easy to merge with prior cracks at the topmost layer,leading to fracture.The deformation process missed the part that necking-plastic interface moved from the surface to the core.Gradient structures with a high-volume fraction of core region,delayed the movement of the elastic-plastic interface from the core to the surface,showing higher ductility.It reflects the importance of the rigid constraint on soft zones by the hard matrix in HS materials.(2)Heterogeneous lamella structured Ni was processed by plastic deformation and subsequent annealing treatment.The distribution of soft zones was studied by microstructural analysis.The microstructure of Ni processed by cryogenic rolling,is mainly coarse grains containing low-angle grain boundaries(LAGBs)and shear bands.After annealing treatment,the soft zones distributed along the high-angle grain boundaries(HAGBs)and shear bands,forming the first type heterogeneous lamella structure.The microstructure of Ni processed by equal-channel angular pressing(ECAP)and cryogenic rolling,is mainly elongated ultrafine grains along the rolling direction,original grain boundaries and shear bands.After annealing treatment,the soft zones distribute along the original grain boundaries and shear bands,forming the second type of heterogeneous lamella structure.Microstructure analysis shows that LAGBs have a higher thermal stability,and recrystallization nucleation is more frequent at larger strains,such as HAGBs,original grain boundaries,and shear bands.It can be seen that the microstructure of the original material and deformation mode have an important influence on the formation of HS materials.(3)Systematic study was conducted on the microstructures and mechanical properties of nickel samples with two distinct types of heterostructures.The first is featured with coarsegrained lamellae embedded in a matrix consisting of a very high density of dislocation structures.The second is featured with coarse-grained zones embedded in the ultrafine-grained matrix.The second type of heterostructures exhibits better strength and ductility,although it has a smaller average grain size than the first type.The zone boundaries in the second type of heterostructures are less prone to cracking than those in the first type.Intersecting micro-shear bands formed net-like patterns in the second type of heterostructures during tensile deformation.This is the first ever observation of structural micro-shear-bands in a HS material.It supports the claim that heterostructure promotes the formation of dispersive shear bands.In contrast,a macroscopic shear band formed and caused early failure of the sample with the first type of heterostructures.Our results indicate that well-developed ultrafine/nano grained matrix in HS materials are necessary for preventing crack formation and shear band localization.This should be considered as a key factor for optimizing the mechanical properties of HS materials.