| The melting and solidification of powders or wires are carried out under extremely fast and non-equilibrium conditions during the metal additive manufacturing process,the layer-by-layer thermal expansion and contraction stress leads to the periodic plastic deformation of materials,further resulting in a special substructure in metals and alloys,such as dislocation cell structures decorated with elemental segregation at the boundaries.Lots of literature has shown that this cellular structure widely exists in additively manufactured alloys,such as stainless steels,nickel-based alloys,high-entropy alloys and aluminum alloys,which has a vital impact on the service properties of materials.Therefore,this thesis mainly focuses on this special element-enriched dislocation cell structure and takes selective laser melting(SLM)316L stainless steel as the model material.The main purpose is to:(?)establish the relationship between the dislocation cell structure and strengthening and toughening of 316L stainless steels,including the interaction between the dislocation cell boundaries and dislocation movement,and the twinning evolution characteristics;(?)establish the relationship between dislocation cell structure and passivation behavior and hydrogen embrittlement mechanism of 316L stainless steels,including the passivation film environmental stability,semiconductor characteristics,hydrogen diffusion and distribution;(?)based on the stacking fault energy design,the dislocation cell structure is modified by alloying,to improve the strength,toughness and corrosion resistance of the material,which can be used as a guide for the design of high-performance additively manufactured alloys in the future.The main conclusions are as follows:During deformation,dislocations can either sprout from or accumulate at the dislocation cell boundaries and the strengthening capacity of cellular dislocation structure(strengthening coefficient is about 180 MPa ?m1/2)is not as strong as the traditional grain boundary(about 250 MPa ?m1/2),but it still satisfies the Hall-Petch strengthening relationship.The heterogeneity of dislocation cell structure(cell wall and intracellular)provides the SLM 316L stainless steel with additional heterogeneous deformation stress during deformation.In addition,there are nonuniformity of dislocation and twinning behaviors due to the non-uniform size of cellular structure,resulting in asymmetric cracking behavior around the oxides.The Cr/Mo enriched dislocation cell structure increases the current density decreasing rate and the SLM 316L stainless steels exhibit a low and stable current density.Under the same film-forming conditions,the passivation film on the SLM 316L stainless steel is thicker than that of traditionally manufactured counterparts.The main reasons are:(?)high dislocation density promotes the rapid formation of passivation film at the cell boundaries;(?)local micro-galvanic couples also accelerate the formation of the passive layer at the intracellular.After heat treatment,the cellular dislocation structure gradually disappears and the corrosion resistance gradually decreases.Under the solution heat treatment at 1200?,the dislocation cells disappear completely,and the oxide and sulfide inclusions precipitate.As a result,the passivation and pitting ability of the SLM 316Lstainelss steels decrease sharply.The reversible hydrogen trap is located at the dislocation cell boundaries(the activation energy is about 19.1 kJ/mol),which makes the hydrogen diffuse slowly in the material(the hydrogen diffusion coefficient is about one order of magnitude lower than that of forged stainless steel at room temperature)and the distribution is relatively uniform.At the same time,the dislocation cell structure is closely connected with the grain boundary,which can reduce the sensitivity of hydrogeninduced intergranular cracking due to a competitive trapping effect.For traditionally manufactured stainless steels,hydrogen-induced cracks all occur on the slip planes(including twin boundary)and propagate along the slip direction with the highest Schmidt factor.Hydrogen-ingress-induced stress can lead to plastic deformation,promotes the transportation and accumulation of hydrogen caused by dislocation movement at the slip bands,and causes crack propagation therein.However,SLM 316L stainless steels exhibit high austenitic stability,and no martensite is formed after the slow strain tensile tests under hydrogen charging conditions,showing a superior hydrogen embrittlement resistance.The additions of Si and Mo improve the strength of SLM 316L stainless steels,and the Mo addition effect is stronger,mainly because there are more Mo elements enriched at the dislocation cell boundaries,which hinders the dislocation slip.The addition of Mo reduces the fracture elongation of SLM 316L stainless steel,which is mainly due to the increase of stacking fault energy,therefore,a reduced deformation coordination capability with an enhanced dislocation slip and decreased deformation twinning ability.However,the addition of Si improves the plasticity of SLM 316L stainless steel.The stacking fault and twinning occur easier with the reduced stacking fault energy for the SLM 316+Si stainless steels,and the triggered twinning plastic mechanism and dynamic Hall-Petch effects improve the work hardening ability thereof.Besides,the addition of Mo improves the pitting corrosion resistance of SLM 316L+Mo stainless steel,especially the pitting repassivation ability,because the local micro-galvanic couple accelerates the formation of the passive film.While the decreased re-passivation ability of SLM 316L+Si stainless steel is mainly due to more oxide inclusions therein.In addition,the pitting corrosion resistance for the SLM 316L stainless steel with Mo/Si addition is stronger than that of traditionally manufactured counterparts. |
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