Theoretical Characterization On Temperature Dependent And Size Dependent Yield Strength Of Metallic Materials

With the continuous development of science and technology,there is a growing requirement of expanding service conditions.High-strength steels,superalloys and metallic glasses have been widely used at different temperatures such as aeroengine,micro-nano electromechanical system,industrial gas turbine,deep space exploration,nuclear reactor and other fields.The requirement of their mechanical properties for engineering design is also getting higher and higher.In engineering applications,metallic materials are generally required to work safely in their elastic stage,so yield strength has become one of the most important mechanical properties in engineering designation.The mechanical properties of metallic materials at high temperature are obviously different from those at room temperature.Nanomaterials will have different plastic deformation and failure modes under the influence of surface stress.These make the research on temperature dependent and size dependent yield strength of metallic materials become one of the most active fields of high and new technology.The theoretical characterization of temperature-dependent yield strength of steel,superalloy,metallic glass and other metallic materials has been carried out.Theoretical characterization of size-dependent mechanical and physical properties of nano-metallic materials has also been studied.The main work is as follows.(1)A method of considering the effect of temperature on materials has been proposed by professor Li,which could be titled as a force-heat equivalence energy density principle.Based on Mises yield criterion and this principle,considering the contribution of thermal energy stored at different temperatures to yield,and considering that there is an equivalent relationship between thermal energy and elastic deformation energy,a temperature-dependent yield criterion for metallic materials is proposed.And a temperature-dependent yield strength model of metallic materials without any fitting parameters is established.From the point of view of kinetic energy and potential energy of lattice vibration,considering the equivalent relationship between these two energies and strain energy,a temperature-dependent yield strength model is proposed for the alloys with specific heat capacity hard to determine.On the basis of the above yield strength models,the corresponding yield strength reduction coefficient models are deduced.Based on this model and Orowan's strengthening mechanism,a temperature dependent yield strength model considering the second phase strengthening mechanism is established.Furthermore,on the basis of the temperature-dependent yield strength model and the Cowper-Symonds model which considers the strain rate effect,a yield strength model considering both temperature and strain rate is established.(2)For metallic glasses,by studying the macroscopic fracture behaviors and unique atomic structure of bulk metallic glasses,a novel strength criterion of metallic glasses suitable for the whole stress state range is proposed on the viewpoint of energy.This criterion could characterize the tension-compression anisotropy of yield behavior of metallic glass very well.Based on the force-heat energy density equivalence principle,a critical yield energy density criterion for shear transition zone instability of metallic glass materials is proposed.And a temperature dependent elastic modulus theoretical model of metallic glasses without any fitting parameters is established.And the model are well verified by experimental results.Based on the proposed strength criterion and temperature dependent shear modulus model,a temperature dependent compressive strength model of metallic glasses is established.The strength criterion can also reasonably explain the temperature dependence of tensile fracture angle.(3)Based on the proposed force-heat equivalence energy density principle and considering the surface effect of materials,a theoretical characterization model of size-dependent Young's modulus of metallic nanomaterials is established.Furthermore,considering the change of material surface area caused by Poisson effect,a theoretical characterization model of size-dependent yield strength of metallic nanomaterials is established,and the model is validated by molecular dynamics simulation results.Based on the force-heat energy density equivalence principle,the size dependent critical melting energy density criterion of nano-metallic materials is proposed,which is based on the idea that surface energy is equivalent to internal lattice energy.Based on the size dependent critical melting energy density criterion,a theoretical characterization model of shape and size dependent melting point without any fitting parameters is established.The model develops the intrinsic relationship between the shape,size,atomic diameter of nanomaterials,melting point of corresponding bulk materials and melting point of nanomaterials.The shape and size dependent melting point model has also been extended to characterize the size-dependent physical quantities such as Debye temperature,melting entropy,melting enthalpy of metallic nanomaterials and glass transition temperature of polymer materials.