Fabrication Technology And Mechanical Properties Of Gradient Nanostructured Materials

Nanostructured metals have a high strength,but their uniform tensile elongation is low in most case.Gradient nanostructured materials have successfully overcome the defect of nanomaterials.Due to the gradient distribution of their grain size in space,the interaction between different sizes of grains has produced synergistic reinforcement,which has greatly improved the strength and hardness of materials.At the same time,the uniform tensile elongation of the material has also been greatly improved under the strain coordination between different layers.The excellent strength and plasticity of the gradient nanomaterials have made it a hot spot of research.In this paper,the following work has been done on the manufacture and mechanical properties of the gradient nanostructured materials.(1)The method of preparing gradient nanomaterials by surface rolling(SMRT)was improved,a new method based on pressure control is proposed.A small prototype was designed for simulating the working condition of the experiment,and the motor power of the experiment is checked based on the simulation results of prototype.In addition,the constant pressure feed device and anti flexure device was designed to ensure the stability of the processing.Through the finite element simulation of the surface indentation,the relationship between the input pressure and the indentation depth is obtained.Finally,the equipment of SMRT was built,and 316 L stainless steel with gradient structure was prepared.(2)Based on the evolution equation of dislocation density proposed by Estrin,A corresponding subroutine is established in ABAQUS.The experiment of surface rolling(SMRT)is simulated by using the material parameters of AISI4340.The conditions of single rolling and multiple rolling were simulated respectively.During the simulation,the effects of the rolling pressure and feed speed on the grain refinement were studied by changing these two variables.In addition the comparison between multiple rolling and single rolling shows that roller compaction plays a positive role in the processing.(3)Establish a constitutive model of gradient nanostructured IF steel based on microhardness.The gradient nanostructure layer is divided into 12 pieces of thin layer.The mechanical properties of each thin layer and the coarse grain core are assumed to be characterized by the Hollomon hardening criterion and the GTN damage model.The material parameters of the constitutive model are identified by the inverse analysis based on the experimental data.The developed constitutive model is able to characterize the synergistic strengthening effect and damage evolution of gradient nanostructured IF steel.The constitutive model is used to analyze the evolution of stress and strain in the tensile section of the material under uniaxial tension.In addition,the critical instability strain of different gradient layers is accurately predicted.