Dislocation plays a crucial role in controlling the strength and plasticity of bulk materials.However,determining the densities of geometrically necessary dislocations(GNDs)and statistically stored dislocations(SSDs)i...Dislocation plays a crucial role in controlling the strength and plasticity of bulk materials.However,determining the densities of geometrically necessary dislocations(GNDs)and statistically stored dislocations(SSDs)is one of the classical problems in material research for several decades.Here,we proposed a new approach based on indentation size effect(ISE)and strengthening theories.This approach was performed on a laser powder bed fused(L-PBF)Hastelloy X(HX),and the results were verified by the Hough-based EBSD and modified Williamson-Hall(m-WH)methods.Furthermore,to better understand the new approach and essential mechanisms,an in-depth investigation of the microstructure was conducted.The distribution of dislocations shows a clear grain orientation-dependent:low density in large<101>preferentially orientated grains while high density in fine<001>orientated grains.The increment of strengthening in L-PBF HX is attributed to a huge amount of edge-GNDs.Planar slip is the main operative deformation mechanism during indentation tests,and the slip step patterns depend mostly on grain orientations and stacking fault energy.This study provides quantitative results of GND and SSD density for L-PBF HX,which constructs a firm basis for future quantitative work on other metals with different crystal structures.展开更多
Post-heat treatment is generally adopted in the additive manufacturing field due to its alleviation of high residual stress and modification of rapid-solidified multilevel heterogeneous microstructure,and the related ...Post-heat treatment is generally adopted in the additive manufacturing field due to its alleviation of high residual stress and modification of rapid-solidified multilevel heterogeneous microstructure,and the related performance of the heat-treated counterparts calls for a systemic investigation to build a criterion of the heat treatment procedure.In this work,we focus on the heat treatment effects on the recrystallization of the Hastelloy X alloy produced by the laser powder bed fusion(LPBF)method,and the related surface passivation of the heat-treated counterparts is meticulously assessed as well.Results show that the multilevel heterostructure for LPBF Hastelloy X alloy consists of sub-micro dislocation cell substructures with Cr/Mo elemental segregation,fine columnar grains,and periodically-distributed molten pools.After heat treatment,partially and fully recrystallized structures for LPBF Hastelloy X alloys were achieved at 1100 and 1200℃for 1 h,respectively.Furthermore,the as-built LPBF Hastelloy X alloy shows superior corrosion resistance while the heat-treated one(1100℃)exhibits the worst in the borate buffer solution.The growth of passive film exhibited a highly linear correlation with the nucleation process controlled by diffusion,and high dislocation density and low angle grain boundary decreased the diffusion coefficient of cation vacancies,augmenting the nucleation sites of the passive film and enhancing its growth rate.Moreover,the micro-galvanic effect resulting from the partially recrystallized microstructure actively facilitated the formation of inhomogeneous porous passive films,leading to the worst corrosion resistance.展开更多
基金supported by the Swedish Governmental Agency for Innovation Systems(Vinnova Grant No.2016-05175)the Center for Additive Manufacturing-metal(CAM2)。
文摘Dislocation plays a crucial role in controlling the strength and plasticity of bulk materials.However,determining the densities of geometrically necessary dislocations(GNDs)and statistically stored dislocations(SSDs)is one of the classical problems in material research for several decades.Here,we proposed a new approach based on indentation size effect(ISE)and strengthening theories.This approach was performed on a laser powder bed fused(L-PBF)Hastelloy X(HX),and the results were verified by the Hough-based EBSD and modified Williamson-Hall(m-WH)methods.Furthermore,to better understand the new approach and essential mechanisms,an in-depth investigation of the microstructure was conducted.The distribution of dislocations shows a clear grain orientation-dependent:low density in large<101>preferentially orientated grains while high density in fine<001>orientated grains.The increment of strengthening in L-PBF HX is attributed to a huge amount of edge-GNDs.Planar slip is the main operative deformation mechanism during indentation tests,and the slip step patterns depend mostly on grain orientations and stacking fault energy.This study provides quantitative results of GND and SSD density for L-PBF HX,which constructs a firm basis for future quantitative work on other metals with different crystal structures.
基金the National Science Fund for Distinguished Young Scholars(No.52125102)the China Postdoctoral Science Foundation(Nos.2022TQ0203 and 2022M722047)+1 种基金Fundamental Research Funds for the Central Universities(No.FRF-TP-2021-02C2)Shanghai Technical Barriers to Trade(No.2021TBT004).
文摘Post-heat treatment is generally adopted in the additive manufacturing field due to its alleviation of high residual stress and modification of rapid-solidified multilevel heterogeneous microstructure,and the related performance of the heat-treated counterparts calls for a systemic investigation to build a criterion of the heat treatment procedure.In this work,we focus on the heat treatment effects on the recrystallization of the Hastelloy X alloy produced by the laser powder bed fusion(LPBF)method,and the related surface passivation of the heat-treated counterparts is meticulously assessed as well.Results show that the multilevel heterostructure for LPBF Hastelloy X alloy consists of sub-micro dislocation cell substructures with Cr/Mo elemental segregation,fine columnar grains,and periodically-distributed molten pools.After heat treatment,partially and fully recrystallized structures for LPBF Hastelloy X alloys were achieved at 1100 and 1200℃for 1 h,respectively.Furthermore,the as-built LPBF Hastelloy X alloy shows superior corrosion resistance while the heat-treated one(1100℃)exhibits the worst in the borate buffer solution.The growth of passive film exhibited a highly linear correlation with the nucleation process controlled by diffusion,and high dislocation density and low angle grain boundary decreased the diffusion coefficient of cation vacancies,augmenting the nucleation sites of the passive film and enhancing its growth rate.Moreover,the micro-galvanic effect resulting from the partially recrystallized microstructure actively facilitated the formation of inhomogeneous porous passive films,leading to the worst corrosion resistance.
