Chromizing Behavior Of Gradient Nanolaminated Interstitial-Free Steel

Gradient nanostructure(GNS)refers to a multi-level architected nanostructure in which the microstructural unit of the material,such as grain size or lamellar thickness,that gradually increases from the nanoscale at the surface to the micrometer scale in the core.Studies have shown that the formation of GNS can improve the strength-ductility synergy,fatigue resistance,friction and wear of metallic materials.In addition,GNS can greatly increase the diffusion rate of alloying atoms in the material,reduce the processing temperature and/or duration of surface alloying(e.g.nitriding and chromizing),and improve the corrosion resistance and wear resistance of the material.However,the thermal stability of the nanostructured surface layer is generally poor,and microstructural coarsening is likely to occur during the heat treatment of surface alloying,resulting in the vanishment of the microstructural characteristics of GNS.Recent studies have shown that the formation of nanolaminated structures is expected to significantly improve the thermal stability of nanostructures.In this work,a gradient nanolaminated(GNL)surface layer with a smooth surface was fabricated on an interstitial-free(IF)steel by means of surface mechanical rolling treatment(SMRT),and its microstructure and thermal stability were investigated by scanning electron microscope(SEM)and transmission electron microscope(TEM).The diffusivity of Cr in the GNL layer was measured by using secondary ion mass spectrometry(SIMS),and then the effect of Cr diffusion on its microstructure evolution as well as the underlying mechanism were revealed.Finally,the corrosion and mechanical properties of chromized samples were studied.The main results are as follows:1.A GNS surface layer with surface hardness of?3.4 GPa and surface roughness(Ra)of?0.2 ?m was obtained on the rod-shaped IF steel samples by applying an optimized SMRT parameters:rotation velocity of the sample V1=300 r/min,moving velocity of the ball V2=0.02 mm/r,processing passes N=4,and total penetration depth of 60 ?m.Within the depth of 10 ?m structures are typical GNL structures.The average thickness of lamellae is?69 nm at the topmost layer,and reaches?96 nm at the depth of 10 ?m.In the depth span of 10?100 ?m structures are ultrafine-laminated/ultrafine-grained mixed structures with boundary spacing/grain size ranging from 100-1000 nm and the proportion of laminated structures gradually decrease with increasing depth.In the depth range of 100?400 ?m structures are deformed structures containing high-density dislocation tangles,dislocation cells and other substructures.Below 400 ?m structure is a coarse-grained(CG)matrix free of plastic deformation.2.Compared with equiaxed nanocrystalline,the thermal stability of the GNL structure is significantly improved.When annealed at 500? for 720 min,the surface hardness of the SMRT sample remained stable,and no obvious structure coarsening was observed in the GNL layer.3.The effective diffusivity of Cr in the GNL layer was 4-6 orders of magnitude higher than the lattice diffusivity within the temperature range from 400 to 500?.Compared with the diffusivity of high-angle grain boundary in relaxed CG sample,the effective diffusivity of Cr in the GNL layer is?20 times higher at 400 ?,basically equivalent at 450?,and slightly lower at 500?.The evidently enhanced diffusion kinetics at lower temperatures was discussed to be related with numerous grain boundaries or dislocation structures with a higher energy state in the GNL surface layer,which provides "short-circuit" diffusion paths for Cr atoms.4.Observation of the microstructure demonstrates that B-type diffusion occurs in the GNL surface layer during chromizing at 500?,that is,Cr atoms diffuse rapidly along the grain boundary of the nanolaminated structure while "leaking" from the grain boundary to the inside of the adjacent grains(The bulk diffusion distance is smaller than 1/2 of the grain size).5.The diffusion-accelerated grain growth(DAGG)behavior occurs in the GNL surface layer during chromizing.After annealed at 500? for 720 min,the nanolaminated structure of Cr-plated samples transformed to equiaxed nanograins with a mean grain size of?260 nm;Under the same heat treatment,the surface layer of the non-Cr-plated samples was still characterized by nanolaminated structure with an average thickness of?88 nm.The analysis shows that the rapid diffusion of Cr along the grain boundary in the GNL layer increases the grain boundary energy due to the enrichment of Cr on the grain boundary.On the other hand,it induces grain boundary migration,which leads to the occurrence of DAGG.6.Compared with the original CG sample,the corrosion resistance of the SMRT sample after chromizing at 500? for 720 min(C-SMRT sample)is significantly improved,and the strength and ductility are simultaneously improved.The potentiodynamic polarization tests show that the self-corrosion potential of the C-SMRT sample is-348 mV in 3.5 wt.%NaCl solution,which is higher than the-477 mV of the CG sample;in the 0.5 M Na2SO4 solution system,obvious passivation occurs in the C-SMRT sample,the maintaining passivation current density is?52.2 ?A/cm2,and the original CG sample shows active dissolution.Tensile experiments show that the yield strength of C-SMRT samples increased from?91 MPa of the CG sample to?105 MPa,and the uniform elongation increased from?28%to?37%.