Formation Mechanism And Mechanical Property Of Extremely Fine Nano-grained Nickel Fabricated By Plastic Deformation

The size effect has a significant impact on the properties of metallic materials,which has attracted the attention of material researchers.For example,the excellent strength of nanostructured and ultra-fine grained metallic materials makes them attractive in high specific strength,energy-saving and other aspects.Plastic deformation is widely used in the preparation of nanostructured materials because of its high efficiency and convenience in introducing high-density dislocations and interfaces to refine grains.The experimental results show that the effect of grain refinement induced by plastic deformation is closely related to the material properties and deformation conditions.Taking Ni with a high stacking fault energy as an example,dislocation slip dominates the plastic deformation process during the conventional plastic deformation methods,such as accumulative roll-bonding,equal channel angular pressing,cold rolling,etc.When grains are refined to about 100-200 nm,they will not decrease with an increase of plastic strain,reaching a saturated grain size as a result of dynamic equilibrium of dislocations multiplication and annihilation.Surface mechanical grinding treatment(SMGT)is a strong shear deformation method providing high strain rates and high strain gradients which can refine Ni into nanolamellar(NL)structure with size of?20 nm.However,with the decrease of grain size,the instability of grain boundary increases.The activation of grain boundary(GB)mediated deformation mechanism,such as grain boundary migration,grain boundary sliding and/or grain rotation,makes it a challenge to further refine grains to very small size(<10 nm).The latest research results of our group showed that plastic deformation can relax grain boundaries to a stable state when grains were refined below a critical size.The deformation mechanism transformed from full dislocation slip to formation of twins and stacking faults,obtaining excellent thermal and mechanical stability.These results provided us a new ideas for further grain refinement.Meanwhile,large numbers of theoretical studies and simulation results show that pressure will affect the movement of dislocations and GB-mediated deformation mechanism,which may affect the structure and properties of materials.On the basis of previous work,we break through the grain refinement limit of Ni by reducing temperature(at LNT)and optimizing deformation parameters of SMGT.In addition,high-pressure(HP)deformation is carried out to refine grains to a new limit in Ni by two-step plastic deformation(SMGT+HP).Scanning electron microscopy(SEM),transmission electron microscopy(TEM),aberration corrected high resolution scanning transmission electron microscopy(HAADF-STEM),X-ray diffraction(XRD),precession electron diffraction(PED)and other structural characterization methods were used to study the microstructure evolution,deformation mechanisms and grain refinement mechanism of the deformed structures.The mechanical properties of the extremely-fine nano-grained Ni were evaluated by using microhardness tests and nanoindentation tests,and the thermal stability of them was studied by using SEM and TEM.The main results are as follows:LNT-SMGT method breaks through the grain refinement limit of Ni by traditional plastic deformation methods.The gradient nanostructure with an average grain size of 8 nm in the topmost surface layer is prepared.With an increase of strain and strain gradient,the deformation gradient structure of LNT-SMGT can be divided into dislocation cells structure(DS),ultra-fine lamellar structure(UFL),nano lamellar structure(NL)and equiaxed extremely-fine nanograins(NG).The deformation mechanism of Ni varies with grain size:dislocation slip dominates the plastic deformation process in micrometer-scale dislocation cells,ultra-fine lamellar structure and nano lamellar structure with grain size larger than 60 nm.As the grain size decreases below 60 nm,proportion of grains containing twins increases gradually,peaking at about 20 nm,which means that twinning dominates the plastic deformation for the grains in the range of 60 nm to 20 nm.When the grain size is below 20 nm,the dislocation slip and twinning are inhibited.The plastic deformation is dominated by emitting partial dislocations from the grain boundaries to form stacking faults.Experimental results show that the refinement mechanism from the-20 nm lamellar structure to the equiaxed extremely-fine nano-grains is as follows:partial dislocations slip along the twin boundary of the through-lamellae twins,and the displacement accumulation refines the NL into equiaxed extremely-fine nano-grains along the twin boundary.Many grain boundaries of the extremely-fine nano-grains are(111),(100)low index planes.The Ni grains with average size of 8 nm prepared by LNT-SMGT has both a high hardness and a high thermal stability.The Vickers hardness is 8.5 ± 0.4 GPa,which is 9 times higher than that of coarse-grained connterpart.It is the highest hardness value of pure nanocrystalline Ni prepared using plastic deformation.The deformation mechanism of extremely fine nano-grains is dominated by partial dislocations and hardening is achieved with continuous grain refinement down to few nanometers.The coarsening temperature of the extremely-fine nano-grains Ni with an average grain size of about 8 nm is about 870?,which is much higher than the thermal stability of the UFL and NL structures.It may be due to the existence of a large number of low index grain boundaries and the relaxation of grain boundaries through the emission of partial dislocations.Nickel is a stable metal with FCC structure,and there is no phase transition at high temperatures and high pressures.However,in the process of low temperature plastic deformation,some FCC Ni nano-grains with grain size smaller than 17 nm transform into HCP grains.The formation of HCP Ni may be related to the plastic deformation mechanism dominated by partial dislocations.High stress is helpful to form wide stacking faults in nanocrystalline Ni,which not only increases the excess energy of the system,but also provides more favorable nucleation sites for FCC-HCP structure transition.It is found that high pressure inhibits the migration of nano-grain boundary,and the deformation mechanism changes from dislocation slip to the partial dislocation emission from grain boundaries.The formation of a large number of partial dislocations and the interaction with grain boundaries relax the grain boundaries and inhibit grain boundary migration.The formation of a large number of extended dislocations in Ni may be due to the reduction of stacking fault energy in Ni under high pressure.At the same time,the extremely high stress during the high pressure deformation may also affect the generation and movement of stacking faults.Grain sizes of pure Ni can be refined to?4 nm by using SMGT+HP two-step plastic deformation method.