Macroscopic Tensile Mechanical Behavior Of Gradient Nanostructured316L Stainless Steel

Gradient nanostructured materials with a spatially varying grain size from nanometer to micrometer have high strength and good plasticity.Gradient nanostructured materials have no obvious interface between different grain sizes,which is different from ordinary mixed structures with different feature sizes.Different grain sizes of gradient nanostructure materials will have a synergistic strengthening effect,which not only improves the strength of the material,but also optimizes the elongation.In this paper,the following work is done to study the tensile behavior and preparation method of gradient nanostructure 316 L stainless steel:(1)The parameter inversion method is used to obtain the constitutive relationship of the gradient nanostructured materials.The microhardness data of the gradient nanostructure 316 L stainless steel obtained by the experiment was function-fitted.Then,the microhardness value at any radial depth under any engineering strain is obtained through interpolation.The gradient nanostructure material is divided into a coarse crystal layer and a gradient layer divided into 14 thin layers or 28 thin layers.The linear relationship is selected as the conversion relationship from microhardness to flow stress.The layer-by-layer extrapolation method and parameter separation method are used to inversely identify the transformation relationship parameters of each layer of the gradient layer.The conversion relationship parameters obtained by the inversion are applied to the hardness conversion of the gradient layer to obtain the stress-strain relationship at any depth of the gradient nanostructure 316 L stainless steel.(2)Finite element tensile simulation of 316 L stainless steel with gradient nanostructure.Tensile models of plate and rod were established.The axial residual stress was applied to the rod tensile model.The change of the stress state and the distribution of strain gradient during the uniaxial tension of gradient nanostructure 316 L stainless steel were studied.The uniaxial tensile process of gradient nanostructured materials can be divided into three stages: the elastic stage,elastic-plastic stage and plastic stage.Due to the incongruous deformation of different layers in the tensile process,the multi axial stress state and strain gradient change distribution appears in the specimen.The application of axial residual stress can reduce the error between numerical simulation and experimental results in the initial yield stage.The axial residual stress will not affect the evolution of multi axial stress state,but will increase the strain gradient.(3)The effect of different rolling devices on the sample axis offset is analyzed by quasi-static indentation finite element simulation.A rolling processing device for shaft parts installed on a lathe is designed.The device can be employed to prepare gradient nanostructured materials.Two kinds of finite element models were established to simulate single pass rolling and multi pass rolling respectively,which were utilized to study the influence of processing parameters on the plastic deformation of the samples.Different model parameters,such as rolling speed,machining head size,rolling path and transverse feed of rolling,are set up to simulate the plastic deformation of the sample.The results show that the equivalent plastic strain of the sample is larger with selection of the higher the rolling speed,the smaller the size of the machining head and the adoption the reciprocating rolling path with small cross feed.the larger the equivalent plastic strain of the specimen corresponds to the faster the rolling speed and the smaller the size of the processing head,and the reciprocating rolling path with a small lateral feed.