Investigation On Severe Plastic Deformation Of Face-centered Cubic Metals And The Limits Of Refinement

In the recent 30 years,the severe plastic deformation(SPD)for preparing ultrafine-grained/nanocrystalline(UFG/NC)materials have been developed rapidly,and involves various deformation techniques.The experimental and theoretical studies on the deformation structures and properties of different materials are also gradually improved.In the basic research,understanding the refinement mechanism and quantifying microstructure evolution needs to be further developed,while the application research focuses on the development of techniques for preparation bulk nanomaterial and promotion related applications.In the rolling process,the grain size/boundary spacing will be geometrically required to reduce in proportion to the sample reduction ratio,which is called geometrical refinement.In the SPD process,the main mechanism of refinement is related to dislocation multiplication,annihilation,and the evolution of dislocation configuration,i.e.the dislocation cell structure evolution.Corresponding to the geometrical refinement,the refinement based on dislocation mechanism is called physical refinement.The stacking fault energy(SFE)as an important material intrinsic parameters can effectively affect the deformation mechanisms and the mechanical behavior of metals through its influence on the dislocation mobility.So far,the influence of SFE on the limits of grain refinement by SPD is still in controversy:one side studied the influence of SFE on grain refinement by SPD through adjusting the SFE by adding alloying elements,and concluded that the lower the SFE the smaller the steady-state grain sizes ds;another side studied pure metals with different SFE,and concluded that the ds is not dependent on SFE.The reason for this contradictory conclusion is that the former one ignores the alloying elements itself,even if it does not change the SFE,it will also affect the mobility of dislocations and deformation dislocation structure through the direct interaction of solute atoms and dislocations,that enhancing the refinement effect.For the latter one,the intrinsic influence of SFE on ds is probably covered up,because other factors(e.g.temperature)that could affect the grain refinement are not normalized.In the present work,the influence of un-normalized interference factors will be excluded through the carefully designed temperature normalization experiment and normalized SFE analyses.An attempt is made to solve the controversy about the influence of SFE on the limits of grain refinement by SPD.Generally,for pure metals or dilute alloys,the grain refinement is based on dislocation cell structure evolution(i.e.physical refinement),and the steady-state grain size d,is hardly refined to<100nm.In more severely deformation conditions,nano-laminated(NL)structures can be prepared in pure metals,and this kind of structure showed excellent properties with ultra-hard and ultra-stable,and attracted much attention.However,due to the severely deformation conditions,the sample dimension(surface layer)usually within 100?m,restricts the further systematic investigation on their properties.Therefore,it is important to break the limits of refinement based on the physical refinement in bulk pure metals,and fabricate NL structures.In this work,we studied the microstructures and mechanical properties evolution,and the limits of refinement in three typical f.c.c.metals(copper,nickel and aluminum)processed by SPD.Take nickel as an example,the UFG structure based on the physical refinement by SPD will be further refined to NL structure by using geometrical refinement,the microstructures,properties and the factors limiting refinement will be investigated.The main conclusions can be drawn for the two questions mentioned above:(1)Pure copper,nickel and aluminum were employed to investigate the influence of SFE on ds obtained by SPD.Based on the equal channel angular pressing(ECAP)at homologous temperatures and experimental data analyses by normalized SFE,establish an experimental rule of the influence of SFE on ds.Based on the dislocation model proposed by Mohamed,a model of the relationship between the ds and the SFE and deformation temperature was established.The calculation by the model and experimental results matched well,and revealed the intrinsic relationship between SFE and ds:(ds/b)?(?SFE/Gb)q,in which the theoretical value of q is 0.25,and the experimental value of q is 0.24?0.31.The smaller value of q reflects the influence of SFE on ds is weak.(2)The grain size of pure nickel was firstly refined to submicron scale by ECAP,then the UFG structure was processed by rolling at liquid nitrogen temperature to fabricate NL structure through geometrical refinement.The lamellar boundary spacing of NL structures reached to?40 nm,and the tensile strength of 2N nickel reached to 1.6GPa.In addition,the fraction of high/low angle grain boundaries(HAGBs/LAGBs)in NL structures can be controlled through the selection of deformation route and accumulated strain in ECAP process.(3)Transmission Kikuchi diffraction(TKD)analysis was carried out to characterize the NL structures quantitatively,and combined with the statistical analyses by TEM and EBSD.It is clarified that two factors affecting the limits of refinement in NL structures:in high purity 4N nickel,the grain coarsening based on grain boundary migration is the main reason that restricts the limits of refinement of NL structures.While in the commercial purity 2N nickel the generation of a large number of shear bands,mediates plastic deformation at a mesoscale,and makes the effective deformation leading to the geometrical refinement of NL structures less than the actual external applied strain,which is the main reason for the limits of refinement.(4)The hardness changes during annealing of three typical deformed structures(UFG obtained by ECAP and NL structures with LAGBs/HAGBs)were compared with each other,and combined with stored energies calculation and annealing kinetics analyses.It is found that the high fraction of LAGBs is the main reason for the higher thermal stability.The initiation temperature of recrystallization in NL structure with high fraction(69%)of LAGBs delayed-50? compared with UFG structure and NL structure with low fraction(25%)of LAGBs,and according to the kernel average misorientation(KAM)analyses the evolution of the microstructure is slower during annealing.The following progresses are also achieved in this work:(1)Copper with different sample sizes were deformed by single pass and multi-pass of ECAP.It is found that the sample size has no significant effect on the microstructures and mechanical properties of copper by ECAP,and clarified the feasibility of scaling up of ECAP in industrial application.(2)Based on the volume flow rate rule a new method for calculating average strain rate during ECAP was proposed,and it is suitable for the calculation of strain rate in ECAP process with various type of channel sections.Compared with other calculation methods and FEM results,its validity has been confirmed.(3)Considering SFE,temperature,and related thermodynamic parameters,based on the dislocation density model(ETMB),established a model involving materials parameters and deformation parameters,that can accurately describe and simulate the evolution of cell wall dislocation density,cell interior dislocation density,total dislocation density and cell size/grain sizes during severe plastic deformation process.