Research On Constitutive Modeling For Dynamic Plasticity Of Industrial Metals And Its Composites

Since materials deform in a very short time when subjected to a transient dynamic loading,the mechanical response of materials would be totally different from that under quasi-static loading in the plastic deformation process.Therefore,the plastic constitutive modeling of metallic materials under dynamic loading has always been the focus of research in materials machining and mechanics.To construct a precise dynamic plastic constitutive model,the real macroscopic state variables of the material should be considered in modeling.Further more,the macroscopic state variables should be closely related to the microstructure evolution in material.A pervasive and precise constitutive model is of great significance to the optimization application of metals and its composites in engineering.This paper is based on the theory of thermal activated dislocation in crystal plastic deformation.By considering the different microstructures and deformation characteristic of fcc metals,bcc metals and hcp metals,combined with the different microstructure evolution characteristic during the deformation process,physically contitutive modeling and experimental study were carried out under the large strain rate span.Finally,the constitutive parameters of the material were determined by the multi-parameter nonlinear optimization method,and models were successfully applied to a variety of typical engineering materials and verified.Then,constitutive modeling of two-phase metal matrix composites and metal matrix nanocomposites(MMNCs)was further studied.The main research and results are as follows:1)For fcc metals,the dislocation mechanism is mainly to overcome the barrier caused by the forest dislocation,according to the deformation mechanism of fcc metals and the microstructrue evolution of nano-twinned metals,the constitutive equation is established and it is applied to the fcc structural metal nt-Cu,the model can effectively describe the strength transition regime in nano-twinned metals.2)For bcc metals,the dislocation mechanism is mainly to overcome the obstacle barrier caused by the Peier-Nabarro internal stress.According to the plastic deformation mechanism of bcc metals,the constitutive equation is established and it is applied to the typical bcc structural metal HSLA-65 steel,the model can effectively reflect the quasi-static/dynamic plastic deformation behavior of HSLA-65 and gives a precise prediction of the experimental data.3)Hcp metals have the structural properties of both bcc and fcc metals,the constitutive equation of hcp metals can be obtained by linear superposition of these two constitutive equations.It has been found experimentally that the deformation of AZ31 alloy at extremely high strain rate will occur the upturn phenomenon in the flow stress due to dynamic recrystallization.To describe this phenomenon,the dislocation density evolution function is developed in the constitutive model,and the model is improved to the form of the piecewise function,so that the improved model can predict the thermo-viscoplastic flow behavior in lower and higher strain rate respectively.The model is applied to the AZ31 magnesium alloy,it can well depict the up turn phenomenon in the flow stress of the material at high strain rates.4)Meanwhile,the dynamic and quasi-static compressive tests were carried out in conjunction with the microstructural observations on the texture eli,minated AZ31 samples deformed under different loading conditions,to reveal the relation between the properties and microstructure of the material during plastic deformation.It was found that under quasi-static loading,deformation twinning/untwinning plays a key role in the plastic deformation of AZ31 alloy at medium high temperature,while at high strain rates,grain refinement due to dynamic recrystallization becomes the most important factor.5)The constitutive model for two-phase metal matrix composite 93W-4.9Ni-2.1 Fe is basically oriented for single-phase homogeneous metallic materials.Considering the microstructural evolution of components in the composite,the constitutive model can be obtained by weighted superposition constitutive models of matrix material and the reinforced material.And the volume fraction function which varies with the macroscopic state variables(strain,strain rate and temperature)is introduced into the weight coefficient.By comparison with the experimental data and other models,the new model gives a best fit to experimental data which is better than conventional single-phase homogeneous constitutive models.6)A multi-scale constitutive modeling research on the metal matrix nanocomposites(MMNCs)has been done in this paper.Firstly,strengthening mechanisms caused by reinforcement phase is superimposed to the constitutive equation of matrix material,including Hall-Petch strengthening,Orowan strengthening,coefficient of thermal expansion(CTE)mismatch strengthening and elastic modulus(EM)mismatch strengthening.Then,considering the cluster effect of nanoparticles in MMNCs,the parameter,particle radius,in constitutive model is modified by introducing the probability density function of clustered particle radius.Then the radius of nanoparticles is substituted by an equivalent radius which considered the cluster effect.Finally,the model is applied to the A356/nSiC nanocomposite,and some satisfactory predictions were given.