Study Of Mechanical Properties And Deformation Mechanism Of Nanocrystalline Nickel Under Cryogenic Temperature

Compared with coarse-grained materials,nanomaterials have compelling properties,such as yield strength and fracture strength,superior wear resistance and low-temperature superplastic formability,so they have a wide range of potential applications in many fields.However,the low uniform plasticity of nanocrystalline materials restricts their application seriously.It is worth noting that the extremely low temperature has a significant impact on the further improvement of the strength and plasticity of nanostructured materials,so revealing the behavior and mechanism of plastic deformation at low temperature may have a guiding significance for improving the intrinsic plasticity of nanomaterialsBased on the above reasons,the low temperature deformation behavior and mechanism of nanocrystalline nickel deposited by electrochemical deposition were systematically studied by using advanced experimental techniques such as INSTON9580 tensile machine,Vickers microhardness tester and molecular dynamics simulation.The main conclusions are as follows:The thickness of bulk nanocrystalline nickel prepared by Watts bath is about 250 um.The Vickers microhardness tester was used to carry out the hardness test at room temperature.The load was 300 gf and the load retention time was 10 s.The results show that the microhardness distribution of nanocrystalline nickel prepared by electrodeposition is very uniform,and the average microhardness is as high as 5.9 GPa.By comparing the microhardness of coarse-grained nickel,ultrafine nickel and nanocrystalline nickel and combining with the Hall-Petch relationship,the average grain diameter of the prepared nanocrystalline nickel is about 30 nm.Molecular dynamics simulation of uniaxial tension of nanocrystalline nickel at different deformation temperatures was carried out.The results show that the plastic deformation of nanocrystalline nickel with an average grain diameter of 8 nm is mainly dependent on grain boundary sliding and rotation,and there are also a small amount of deformation twins formed by dislocation slip and extended dislocation movement.With the increase of tensile strain,the stress of nanocrystalline polycrystalline nickel first gradually increases and reaches the peak value,and then the stress presents an oscillating decreasing trend,and the decreasing trend gradually increases with the decrease of deformation temperature.With the decrease of deformation temperature,the slip initiation of nano-polycrystalline nickel becomes difficult,which leads to the increase of yield strength with the decrease of deformation temperature.When the temperature decreases from 300 K to 1 K,the yield strength increases from 2.9 GPa to5.1 GPa.The elastic modulus and tensile strength also show a similar pattern.Using molecular statics and molecular dynamics simulation,the generalized stacking fault energy of nickel at finite temperature was calculated.The results show that there is no stable stacking fault energy in the [110] slip direction.In the direction of full dislocation decomposition [112],there is a stable stacking fault energy.According to the ratio of stable and unstable stacking fault energy,the mechanism of incomplete dislocation slip in nanocrystalline nickel was analyzed and obtained.With the decrease of temperature,the stacking fault energy increases,the slip tendency weakens,and the ability to produce deformation twins increases.Finally,the nucleation and evolution of the upper dislocation on the symmetric inclined grain boundary and the symmetric torsional grain boundary with different orientation difference angles were studied.The results show that,with the increase of the orientation difference,the critical tensile stress of the grain boundary dislocation nucleation decreases with the increase of the orientation difference Angle.The tensile stress of grain boundary dislocation nucleation increases linearly with the increase of deformation temperature.The full dislocation decomposes into a partial dislocation circle,and the separate dislocation slides inwards,while the other partial dislocation is absorbed by the grain boundary.