The Mechanical Behavior Of Three Different Inverse Gradient Nanostructure Metals

The plastic deformation of nanostructured metal materials is limited by the characteristic size,so the nanostructured materials generally have high strength and a low plasticity.The "trade-off" in strength and plasticity limited the wide application of nanomaterials.Improving the strength of nanomaterials without sacrificing plasticity is a goal pursued by scientists,which has important academic value and potential application prospect.In order to improve the mechanical properties of nanostructured metal materials,the concept of microstructure design has been proposed.As a new metal structure material,the inverse gradient nanostructure material has excellent mechanical properties.Although some achievements have been achieved in the inverse-gradient nanostructured materials,the mechanical behavior of reverse-gradient nanostructured materials is still not studied,and some phenomena need to be further revealed.In this paper,the mechanical behavior of 316 L stainless s teel,Al Cr Fe Co Ni high entropy alloy and Ni at room temperature was studied.The microstructure evolution of inverse-gradient nanostructures was analyzed by transmission electron microscopy(TEM),and the deformation mechanism of different structures in inv erse-gradient nanostructures was revealed.The main research results are listed as follow:The inverse gradient nanostructure 316 L stainless steel(I-GNS 316 L SS)sample exhibited yield strength ?978 MPa and high uniform elongation ?11.4%.We study deformation behavior of different nanostructure in the center at different tensile stage though the transmission electron microscopy(TEM).During the tensile process,the surface inverse gradient layer deformed preferentially,and providing a dislocation source for the deformation of the center nanostructure.At the same time,under the constraint of the inverse gradient layer on the surface,multiple deformation mechanisms of the center nanostructure can be activated.These multiple mechanisms are activated by the I-GNS layer,which imparts a strong confinement to the nanostructured core.It ensured nanostructures in the center deformed coherently with the coarse grain IGNS layer.So a large plasticity of I-GNS sample was achieved with a low volume fraction of recrystallized grains.Two kinds of gradient structures,normal gradient structure synthesized by surface mechanical grinding treatment and inverse gradient structure by ultra-high frequency electromagnetic induction heating treatment,were fabricated on a n AlCrFeNi high-entropy alloy and its effects on mechanical properties were investigated.Uniaxial tensile results show that the normal gradient structure slightly improved the yield strength but deteriorated the tensile ductility.In contrast,inverse gradient structure exhibits an obviously enhanced tensile ductility without sacrifice the strength.Microstructure analyses show that the improved tensile ductility is attributed to the B2?FCC phase transformation and recrystallization.Multiple slip systems and ample rooms are beneficial to the ductility.In addition,declined dislocation density and residual stress also contribute to the improvement in the mechanical properties.Random recrystallized Ni and inverse gradient nanolamellar Ni were synthesized by annealing and electromagnetic induction heating,respectively.The tensile results show that the plasticity of the two structures increased with the deterioration of materials strength.This is attributed to the lack of plastic deformation capacity of th e nanolamellar structure.Only when the volume fraction of the recrystallization exceeds a certain value,a large plastic network can be formed to coordinate the plastic deformation.However,the uniform elongation of the inverse gradient structure increased significantly when recrystallization occurred at a smaller volume.This is because the surface of the inverse gradient structure sample can preferentially form a large number of recrystallization,and the formed recrystallization layer has high plasticity and can transfer the plastic strain during the tensile process.Therefore,the deformation of the nanometer lamellar structure in the core can be restrained.