Study On Structural Evolution And Mechanical Property Of The NiCoCr-based Multi-priciple Element Alloys Under High Pressure

Multi-principle element alloy（MPEA）is discovered and developed rapidly in recent years.Different from the traditional alloys,which have one or two principal components,MPEAs usually possess a collective effect of multi-component elements,such as high entropy effect,sluggish diffusion,lattice distortion and cocktail effect.MPEAs also perform the novel structures and outstanding properties.As a result,they have attracted spread attention in automotive,aerospace,nuclear energy as well as electronics and other key fields.At present,face-centered cubic（FCC）structural MPEAs are extensively concerned by researchers.In the previous literatures,the microstructural regulation and properties of FCC structural MPEAs are mainly studied.Pressure,as one of the fundamental physical parameters,is one of the main factors affecting the phase composition,microstructure and properties of the MPEAs.Therefore,it is of great theoretical significance and practical application value to study the microstructure and deformation mechanism of the MPEAs under high pressure conditions.Based on FCC structural Ni Co Cr alloy,the effects of pressure on their microstructure and mechanical properties were studied from different aspects.In this paper,different MPEAs system treated as the object,the phase transformation and mechanism under pressure were studied by in situ high-pressure synchrotron X-ray diffraction（XRD）technique.Firstly,pressure-induced phase transformation in the Ni Co Fe,Ni Co Cr and Ni Co V MPEAs with different lattice distortion was studied.It is found that the face-centered cubic to hexagonal close-packed phase（FCC-HCP）transformation occurs in the Ni Co Fe,Ni Co Cr and Ni Co V MPEAs,starting as 15.60,13.84,8.20 GPa,respectively.Results demonstrate that the orderly decreasing initial pressure of FCC-HCP phase transformation in the three MPEAs is attributed to the increasing of lattice distortion because lattice distortion could reduce the strain to nucleate Shockley partials dislocation,which promoting the formation of a stacking fault（SF）stack of three atomic layers with HCP stacking.Besides,pressure-induced phase transformation in the ball-milled powder（BM-MPEA）and as-cast alloy（AC-MPEA）with different dislocations density were studied.BM-MPEA and AC-MPEA both undergo a partial irreversible FCC-HCP phase transition under pressure.In comparison to the AC-MPEA,the BM-MPEA has a large number of defects,especially the high density of dislocations.These defects provide new nucleation sites as well as decrease the nucleation energy barrier of HCP phase,further resulting in a lower phase-transition-pressure of 7.0 GPa.A new series of Ni_1.5Co_1.5CrTi_x（x=0.0,0.2,0.4）MPEAs were fabricated by mechanically alloying and high-pressure sintering（HPS）.The microstructure and mechanical properties of MPEAs with different Ti contents were studied.It is found that the MPEAs with different Ti contents display ultrafine grained FCC matrix and uniformly dispersed ultrafine/nano oxides/carbides.The increasing Ti content leads to the greatly decrease of grain sizes of FCC matrix and precipitates.As a result,the compressive yield strength,nano-hardness and wear-resistance property are on the rise.Simultaneously,high pressure torsion（HPT）was used to refine the grains of Ni_1.5Co_1.5CrTi_0.4MPEA.Besides,the deformation mechanism during HPT has been investigated systematically.The MPEA after HPT presents grains with different size along the radius from center to edge.The deformation mechanisms include single slip at low strains and the combination of cross slips,dislocation cell plus deformation twinning at high strains.The microstructural evolution and mechanical property of Ni_1.5Co_1.5Cr system MPEAs were studied systematically by high pressure aging and ambient pressure aging.Result demonstrares that plastic deformation occurs in the fully recrystallized Ni_1.5Co_1.5Cr and Ni_1.5Co_1.5Cr Al_0.2Ti_0.1MPEAs under high pressure.Pressure inhibits the grain growth and the dynamic recrystallization（DRX）process under high pressure and high temperature（HPHT）produces new fine grains.A comparison on the precipitates morphology of Ni_1.5Co_1.5Cr Al_0.2Ti_0.1MPEA under high pressure（HP800）or ambient pressure（AP800）is systematically studied.Both HP800 and AP800 consist of discontinuous precipitate（DP）regions near grain boundaries and continuous precipitates regions in grain interior.However,the volume fraction of DPs regions in the HP800 is more than twice as many as that in the AP800 because the grain refinement under HPHT promotes the formation of DPs,further strengthening the tensile strength of the Ni_1.5Co_1.5Cr Al_0.2Ti_0.1MPEA.