Research On The Micro-mechanics Of Void Damage And The Modeling Of Ductile Fracture For Hot Deformation Of The 316ln Steel

Ductile fracture is a critical issue in the design of suitable forming technology and the development of new forming technology and new materials.In essence,the fracture of polycrystalline metals is a multiscale mechanical behavior.The macroscopic fracture forms with the microscopic void evolution.Although the micro-void is tiny with the initial void volume fraction generally smaller than 1%,the nucleation of void induced by the debonding of inclusions,and the growth as well as the coalescence of void will eventually give rise to the macro-fracture.In common engineering materials,the size of initial void is about 10 μm,and the size of grains is from dozens of micron to hundreds of micron range.The voids and grains are in the same length scale,and the grain structure,including grain size and mixed crystals,and their evolution,such as the dynamic recrystallization(DRX)in hot deformation,thus have a nonnegligible effect on the evolution of voids.Via using the physical experiment,finite element simulation and the theory of plastic mechanic,the ductile fracture behavior of an austenite steel 316 LN possessing a mono-austenite microstructure was investigated in this study.The effect of grain size on damage accumulation in room temperature deformation and the influence of DRX on fracture in hot deformation were analyzed from the microscopic and macroscopic perspective.According to the analysis results,the relationship between grain structure,mesoscale void damage,and macroscopic ductile fracture was identified.Models for predicting the occurrence of ductile fracture in hot deformation were established based on the DRX affected void evolution.In the modeling of thermal fracture,mesoscale damage models which can reflect the micro-mechanics of ductile fracture and macroscale phenomenological models which are simple and convenient for application were considered.Main contents of this research are listed in the following:Microscopically,the deformation heterogeneity induced by the grain misorientation in polycrystals will affect the void damage accumulation and the formation of ductile fracture.For materials with different grain sizes,the level of deformation heterogeneity varies,and the rate as well as the mode of damage accumulation thus differs.To determine the influence of grain size on ductile fracture,experiments and corresponding crystal plasticity finite element(CPFE)simulations were conducted on 316 LN to study its deformation characteristic,void evolution and fracture behavior during uniaxial tensile deformation.According to the results,the relationship between grain size,microscopic heterogeneous deformation,void damage,and macroscopic ductile fracture was figured out.In a certain loading condition,the distribution of void size coincides with the distribution of strain.Void grows more obviously in areas with the larger strain localization.As the grain size is increased,the deformation heterogeneity becomes more obvious and the fluctuation range of void size becomes larger.The distribution of void size is quantitatively analyzed and an extended model representing the effect of grain size was proposed for predicting the growth of void.The variation of void growth in different grain size conditions results in different modes of void coalescence,forming different morphologies of the fracture surface.In the coarse grain condition,the fractography is no longer the cup-cone morphology,but of randomly distributed large dimples,which indicates that the fracture forms with the coalescence of large voids.For the material with abnormal coarse grains approximating millimeter,macroscopic deformation concentration occurs during plastic deformation and the ductility decreases severely.With the complex microstructure evolution occurred in hot deformation,such as the phase transformation,DRX,the precipitation and dissolve of inclusions,the ductility of materials at elevated temperature always shows a dependency on temperature and strain rate.A focus of this study is to find out the effect of DRX on ductile fracture.Hot tensile experiments were conducted at different temperatures and strain rates,and the extent of DRX and the morphology of voids in the longitudinal section of the fracture surface were carefully examined.An in-depth analysis was put on the influential mode of DRX on void evolution and ductility.In the condition of a low degree of DRX,voids in the fracture surface have roughly elliptical irregular shapes.While in the perfect DRX condition,an obvious elongation of voids is observed.The variation of void morphology in different DRX scenarios indicates that the void behavior has a close relation with the occurrence of DRX.The driving force of void evolution is the local stress concentration in materials.When DRX takes place,the stress concentration is greatly relieved and the growth of void is thus retarded.In the partial DRX condition,the growth of void in the interactive regions with DRX grains is inhibited.While in the perfect DRX condition,the void growth was completely impeded.Voids deform with the matrix and become elongated with plastic deformation.The evolution of void and the complicated microstructure in hot deformation were rarely considered in the modeling of ductile fracture.Based on the influence of DRX on the physical process of void evolution,the void parameters in the GTN-Thomason plastic model were revised and a model for predicting the damage accumulation in hot working was developed.On one hand,due to the material softening caused by DRX,the stress concentration in interactive regions of the inclusion and matrix is decreased.Inclusions are thus harder to debond from the matrix,resulting in the increase of void nucleation strain.Therefore,the void nucleation strain is increased with the increase of DRX degree.Meanwhile,as stress concentration in the ligament area between the voids is relived by DRX,the coalescence of void occurs with smaller spacing and the critical size ratio of void coalescence increases.By taking the volume fraction of recrystallized grains as the measure of DRX and introducing it into the void-based GTN-Thomason ductile fracture model,a coupled hot deformation fracture model involving the effect of DRX on void evolution was thus established.The extended model was embedded into FE simulation and the material constants in the model were calibrated according to the experimental results.In addition,validity of the model was corroborated.For the design of reasonable hot forming technology,the accurate prediction of ductile fracture is needed.Out of the consideration of engineering applications,a phenomenological fracture model for hot deformation of metallic materials was established by introducing the relationship between fracture strain and DRX into the stress state dependent cold deformation fracture model.The established hot deformation fracture model is form-simple and convenient for use.Firstly,the fracture strains in different hot working conditions were determined via a combined experiment and simulation approach.By using the Zener–Hollomon(Z)parameter to represent the temperature and strain rate dependent DRX behavior,the relationship between fracture strain and Z parameter was identified.When DRX does not occur in the deformation,the fracture strain presents an independency on Z parameter.When DRX occurs,the fracture strain decreases linearly with the increase of the natural logarithm of Z parameter.Through incorporating Z parameter and the percentage of DRX into a cold deformation fracture model,an extended phenomenological model for thermal fracture prediction was established.The validation results demonstrated that the established model is efficient in capturing the onset and the propagation of fracture in a wide parameter range of stress state and hot deformation.Furthermore,as a case study in industrial application,the developed model was applied to the hot forging of a pressure vessel cap to identify the possibility of fracture initiation.