Research On Theoretical Hardening Models Of Metallic Materials Under Ion Irradiation

Irradiation hardening is a typical form of nuclear reactor materials performance degradation.Therefore,the study of irradiation hardening behavior and physical mechanisms on metallic materials is critically important for the development of nuclear structural materials.Although the theoretical models of ion irradiated metallic materials have been received much attention for a long time.Moreover,the existing theoretical hardening models can well analyze and characterize the irradiation hardening mechanisms of single crystal and polycrystalline materials without obvious grain size effect,it still needs to be developed and perfected as a whole.Especially when analyzing the hardening mechanisms of polycrystals with obvious grain size effect,nanocrystalline,nanostructured multilayers/substrate systems and some metallic materials whose plastic zone expansion rate changes after irradiation.How to measure the effect of these hardening mechanisms such as the grain size effect of polycrystals,the grain boundary effect of nanocrystalline,the interface effect and substrate effect of nanostructured multilayers/substrate systems,the plastic zone expansion rate decreases due to radiation defect in some special cases,a series of difficulties need to be solved.This dissertation conducts a systematical research for the irradiation hardening of metallic materials,and aims at developing some exploratory work on the theoretical models of irradiation hardening.Four key scientific problems are further studied,including the grain size effect of polycrystals,the grain boundary effect of nanocrystalline,the interface effect and substrate effect of nanostructured multilayers/substrate systems,and the effect of decreasing plastic zone expansion rate in irradiated layer on irradiation hardening of metallic materials under some special cases.We hope this dissertation can establish unified hardness theoretical models for these metallic materials to fully consider the effects of different hardening mechanisms during the whole nanoindentation process,help understand and analyze the irradiation hardening mechanisms,and provide guidance for the research and design of irradiation resistant mateials.Based on this idea,the work carried out and completed in this paper is as follows:(1)A theoretical model is proposed for the depth-dependent hardnesss of ionirradiated polycrystals,which can simultaneously consider the effect of irradiation and grain size.Dominant hardening mechanisms are addressed in the model,including the contribution of ion irradaiton-induced defects,indentation size effect(ISE),grain size effect and the unirradiated substrate.First,based on the contribution of dislocation density,radiation defect and grain size effect to the critical shear stress(CRSS),the expression of CRSS for ion-irradiated polycrystalline materials varying with indentation depth is derived.Two general mechanisms,i.e.the linear and square superposition mechanisms are considered.Then,the hardness models of ion-irradiated polycrystals can be deduced by taking into account the Mises flow rule and Tabor’s factor.And a succinct parameter calibration method is modified to parametrize the models based on the experimental data.Finally,the rationality of these two models is analyzed by comparing the theoretical results with two sets of experimental data.In addition,the new model seperates the grain size effect from the dislocation hardening contribution,which makes the physical meaning of fitted parameters more rational when compared with existing hardening models.(2)A theoretical model is proposed for the depth-dependent hardnesss of ionirradiated nanocrystalline metals,which can simultaneously consider the effects of ion irradiation and grain boundary.Dominant hardening mechanisms are addressed in the model,including the contribution of ion irradaiton induced defects,the ISE,the Hall-Petch effect and the strength of grain boundary.Nanocrystalline metals with small grain size and comparatively large volume of grain boundary,and the grain can be divided into two parts,i.e.grain interior and grain boundary.Considering the effect of grain boundary on dislocation and radiation defect,the CRSS of grain interior and grain boundary with or without ion irradiation can be deduced,respectively.Furthermore,the CRSS of nanocrystalline metals varying with indentation depth is deduced,which by respectively considering the volume fraction of grain interior and grain boundary.Then,the hardness model of nanocrystalline metals can be derived by furthering considering the Mises flow rule and Tabor’s factor.Finally,taking nano-grained Ni-Mo-Cr alloy as an example,the effects of different hardening mechanisms on nanocrystalline hardening behavior are analyzed in detail,and compared with the single crystal.Moreover,the effect of grain boundary on the hardening behavior is investigated,and the validity of the proposed model is verified.(3)A theoretical model is proposed for modeling the depth-dependent hardness of ion-irradiated nanostructured multilayers/substrate systems.Four dominant hardening mechanisms during the whole nanoindentation process are systematically analyzed,which include the ISE induced by geometrically necessary dislocations(GNDs),irradiation hardening determined by irradiation-induced defects,substrate effect and the confined layer slip existing within the multilayers.By considering the interface effect on both the geometrical shape and expansion ability of the plastic zone,the hardness-depth relation of ion-irradiated nanostructured multilayers/substrate systems can then be divided into four stages,including the interface-effect-free stage,the dislocation pile-up stage,the dislocation transmission stage and the stable dislocation transmission stage.Further,based on the evolution of average density of dislocations and defects within the plastic zone,the hardness models of four stages are deduced in details.Moreover,on the basis of this model,the evolution of related microstructures in the film and substrate can be quantitatively analyzed,which shows that the interface influences the evolution of the density of GNDs and irradiation-induced defects by influencing the plastic zone,and finally influence the system hardness.In addition,the new model separates the substrate effect from the whole system,which offers a promising way to analyze other mechanical properties of ion-irradiated nanostructured multilayers/substrate systems.(4)A theoretical model is proposed for ion-irradiated metallic materials with different expansion ability of plastic zone in irradiated layer and unirradiated layer,which can effectively measure the effects of different hardening mehanisms of two different layers.According to the location of the plastic zone,the whole indentation process can be divided into two stages,including the stage where the plastic zone does not cross the irradiated layer and the stage in which the plastic zone ectends to the unirradiated layer,and the hardness model of two stages are deduced in details based on respective plastic zone volume fraction of irradiated layer and the unirradiated layer.Moreover,on the basis of this model,the evolution of related microstructures in two layers can be quantitatively analyzed,which shows that irradiation-induced defects influence the hardening mechanisms of two layers by influencing their respective plastic zone expansion rate,and finally influences the materials hardness.At last,the rationality of this model is validated by comparing the theoretical results with two sets of experimental data.In addition,the different proportional coefficients of two layers fitted by this model are more consistent with the actual situation.