Research On Mechanical Properties And Deformation Mechanisms Of Dual-heterostructured Ni-based Alloy

The introduction of heterogeneous structure is of great significance for breaking the strength-ductility trade-off in conventional metal materials.Common heterogeneous microstructures include heterogeneous grain structure and heterogeneous precipitate structure.Therefore,a large number of researchers have studied the heterostructured alloys.Among Face-Centered Cubic metals,Ni-based alloys are widely used because of the excellent deformability ofγmatrix and the excellent strengthening ability of Ni₃（Al,Ti）-typeγ’precipitates with L1₂structure.In this paper,it is the focus of this topic to improve the strength-ductility trade-off and clarify its deformation mechanism by structural design without changing the composition.In this paper,dual-heterostructured sample and single-heterostructured sample are successfully obtained by cold rolling at room temperature and subsequent heat treatment.The microstructure and dislocations are characterized by Scanning Electron Microscope,Transmission Electron Microscope and Energy Dispersive Spectrometer.The mechanical properties are characterized by uniaxial tensile tests.The degree of deformation of the sample is measured by quasi-in-situ Electron Backscattered Diffraction test.The dislocation density is quantitatively characterized by synchrotron radiation X-ray diffraction.Finally,the following conclusions can be drawn.（1）The microstructure of dual-heterostructured sample consists of nanocrystalline region and ultrafine grain region formed during incomplete recrystallization process.The interior of the ultrafine grain is“walnut-like”precipitate,and the interior of the nanocrystalline is spherical precipitate.The composition and structure of the“walnut-like”precipitate in ultrafine grain,the spherical precipitate in nanocrystalline and the spherical particles in coarse grain are consistent,and they are（Ni,Co,Cr）₃（Al,Ti）-typeγ’precipitates with L1₂ super-lattice structure.（2）The single-heterostructured sample eliminates the nanocrystalline region,and the microstructure consists of ultrafine grains and internal“walnut-like”precipitates.This demonstrates the controllability of the heat treatment process and the stability of ultrafine grains with their internal“walnut-like”precipitates.（3）Under the premise of keeping the composition unchanged,the strength-ductility trade-off of both dual-heterostructured sample and single-heterostructured sample is improved.Compared with the coarse-grained sample,the yield strength of the dual-heterostructured sample is increased to 1750 MPa without loss of uniform elongation.Compared with the coarse-grained sample,the yield strength of the single-heterostructured sample is increased by 200MPa,while the uniform elongation is nearly doubled.（4）The work hardening behavior of heterostructured samples is obviously different from that of coarse-grained sample.The improvement of the work hardening rate at the initial stage of deformation is related to the hetero-deformation induced toughening effect caused by heterogeneous grain structure.For the single-heterostructured sample,the effect decreases with the absence of the nanocrystalline.（5）The work hardening rate of the heterostructured sample is obviously higher than that of the coarse-grained sample at the late stage of deformation,which is mainly related to the ability of dislocation storage in the structure units composed of the ultrafine grains and their“walnut-like”precipitates.