Phase Field Study On The Dislocation-based Plastic Deformation Mechanisms For Metals

In general,the technological advance of societies has often linked to their ability to exploit and engineer new materials and their properties.The metal materials'properties,such as plasticity,strength,toughness,anti fatigue,etc.,have a connection with their underlying microstructure.In recent years,based on the concept of the material design,a series of material calculation methods have emerged.The researchers go deep into the micro or nano scale to clarify the relations between the microstructures and macroscopic mechanical properties of materials using these computation methods.These materials computation methods have become more and more important to help us to understand the material microscopic world,to promote the micromechanics and to study plastic deformation of metals.The plastic deformation of metals are governed by the various dislocation-based mechanisms,such as,dislocation slip,dislocation dissociation,nucleation of twinning dislocation,dislocation cross-slip,interaction between dislocation and other defects and so on.Based on elasticity and thermodynamics,we developed some phase field models to study the evolution of dislocations and twins in various metals.This paper tries to link the microstructure evolution under stress and the macroscopic plastic deformation of metals.The main achievements are summarized as follows:1.The simulations of the dislocation evolutions,including dislocation dissociation,nucleation and recombination,under applied stress are presented using a phase field dislocation dynamics model that incorporates the y surface of various fcc metals.As expected,the separation of the leading and trailing partials,termed the equilibrium stacking fault width,is governed by the details of the y surface and the external loading conditions.Two important critical stresses,defined as the singular stress and the nucleation stress,are found to determine the stress-dependent evolution mechanism.As a general rule,the stacking fault width increases with the applied stress and diverges when the applied stress exceeds the singular stress.A spontaneous nucleation of partial dislocation loops within the stacking fault occurs when the applied stress exceeds the nucleation stress.In particular,a new stress-size-dependent nucleation mechanism is observed in the simulations in the case where the singular stress is greater than the nucleation stress for a FCC metal:the nesting loop or nesting dipole can remain in the metastable state without any nucleation even the applied stress is twice as large as the nucleation stress.2.The thickening mechanism of the deformation twinning has been frequently studied in numerous researches and the transverse propagation of that is beginning to trigger the attention of scholars.Recently,some researchers report that the twin front of {1012} mode of Magnesium is composed of a conjugate twin plane and prismatic/basal(PB)planes,and the combined mobility of these planes rule the overall kinetics of twin propagation.Focusing on that,a continuum phase field model is proposed to investigate the equilibrium shape of tensile twins and the kinetics of the twin front.A new form of surface free energy is introduced in this model for the purpose of describing the orientation-dependent properties of twin boundaries.The simulations well reproduce the PB interfaces and the results indicate that the anisotropic surface energy plays a dominant role in forming the irregular facets on the twin front.A generalized energy-momentum tensor is derived and analyzed for shear loading in order to investigate the equilibrium and mobility of twin boundaries,and the simulations show that the configuration forces distributed on the PB interfaces are smaller than that on the other twin planes,which implies that the growth of twin is beneficial for the formation of PB interfaces.The simulations also indicate that the anisotropic twin boundary energy is not responsible for the large aspect ratio nature of twins,which may be governed by the difference between thickening mechanism and transverse propagation mechanism of the deformation twinning.3.The a0/2<111>screw dislocation glides through the nucleation and propagation of the kink-pair which dominates the plastic deformation of the BCC iron.A continuum model and the corresponding numerical methods are developed to investigate the kink mechanism on an arbitrary shape Peierls potential and subject to an external stress field.This model gives a link between the Landau theory of phase transitions and the line tension theory of string models.The order parameter is associated with the screw dislocation in BCC iron for describing the relative slip between adjacent Peierls valley.The kink configurations on the different Peierls potentials,such as the sinusoidal,Eshelby,anti-parabolic and camel-hump potential,are derived.By considering the motion of the screw dislocation on a 2-D Peierls potential surface,the 3-D saddle-point configuration of a non-planar kink-pair is obtained.The configuration is directly related to the details of the 2-D potential surface and it changes along with the applied stress tensor.A parameterized constitutive equation is derived for describing the temperature dependence of the flow stress which is compared with the experimental data from literature.The twinning/anti-twinning asymmetry and the tension-compression asymmetry are reproduced in the model.The results rule out the possibility that the non-Schmid plasticity of the BCC iron is ascribed to split configuration.