Laser additive manufacturing(LAM)technique has unique advantages in producing geometrically complex metallic components.However,the poor low-cycle fatigue property(LCF)of LAM parts restricts its widely used.Here,the m...Laser additive manufacturing(LAM)technique has unique advantages in producing geometrically complex metallic components.However,the poor low-cycle fatigue property(LCF)of LAM parts restricts its widely used.Here,the microstructural features of a Ti-6 Al-4 V alloy manufactured via high power laser directed energy deposition subjected to low-cycle fatigue loading were studied.Before fatigue loading,the microstructure of the as-deposited parts was found to exhibit a non-homogeneous distribution of columnar prior-βgrains(200-4000μm)at various scanning velocities(300-1500 mm/min)and relatively coarseα-laths(1.0-4.5μm).Under cyclic loading,fatigue microcracks typically initiated within the alignedαphases in the preferred orientation(45°to the loading direction)at the surface of the fatigue specimens.Fatigued Ti-6 Al-4 V exhibited a single straight dislocation character at low strain amplitudes(<0.65%)and dislocation dipoles or even tangled dislocations at high strain amplitudes(>1.1%).In addition,dislocation substructure features,such as dislocation walls,stacking faults,and dislocation networks,were also observed.These findings may provide opportunities to understand the fatigue failure mechanism of additive manufactured titanium parts.展开更多
The strength of polycrystalline metals increases with decreasing grain size,following the classical HallPetch relationship.However,this relationship fails when softening occurs at very small grain sizes(typically less...The strength of polycrystalline metals increases with decreasing grain size,following the classical HallPetch relationship.However,this relationship fails when softening occurs at very small grain sizes(typically less than 10 to 20 nm),which limits the development of ultrahigh-strength materials.In this work,using columnar-grained nanocrystalline Cu-Ag‘samples’,molecular dynamics simulations were performed to investigate the softening mechanism and explore the strengthening strategies(e.g.,formation of solid solution or grain boundary(GB)segregation)in extremely fine nanograined metals.Accordingly,the softening of pure metals is induced by atomic sliding in the GB layer,rather than dislocation activities in the grain interior,although both occur during deformation.The solid solution lowers the stacking fault energy and increases the GB energy,which leads to the softening of NC metals.GB segregation stabilizes GB structures,which causes a notable improvement in strength,and this improvement can be further enhanced by optimizing the solute concentration and GB excess.This work deepens the understanding of the softening mechanism due to atomic sliding in the GB layer and the strengthening mechanism arising from tailoring the GB stability of immiscible alloys and provides insights into the design of ultrahighstrength materials.展开更多
基金supported by the National Key Research and Development Plan of China(2016YFB1100104)National Natural Science Foundation of China(Grant No.51971166)。
文摘Laser additive manufacturing(LAM)technique has unique advantages in producing geometrically complex metallic components.However,the poor low-cycle fatigue property(LCF)of LAM parts restricts its widely used.Here,the microstructural features of a Ti-6 Al-4 V alloy manufactured via high power laser directed energy deposition subjected to low-cycle fatigue loading were studied.Before fatigue loading,the microstructure of the as-deposited parts was found to exhibit a non-homogeneous distribution of columnar prior-βgrains(200-4000μm)at various scanning velocities(300-1500 mm/min)and relatively coarseα-laths(1.0-4.5μm).Under cyclic loading,fatigue microcracks typically initiated within the alignedαphases in the preferred orientation(45°to the loading direction)at the surface of the fatigue specimens.Fatigued Ti-6 Al-4 V exhibited a single straight dislocation character at low strain amplitudes(<0.65%)and dislocation dipoles or even tangled dislocations at high strain amplitudes(>1.1%).In addition,dislocation substructure features,such as dislocation walls,stacking faults,and dislocation networks,were also observed.These findings may provide opportunities to understand the fatigue failure mechanism of additive manufactured titanium parts.
基金financially supported by the National Key R&D Program of China(No.2017YFB0703001)the Natural Science Foundation of China(Nos.51971166,51790481 and 52130110)+2 种基金the Fundamental Research Funds for the Central Universities(No.3102017jc01002)the Natural Science Foundation of Shaanxi Province(No.2021JQ-651)supported by High Performance Computation Center of Northwestern Polytechnical University.
文摘The strength of polycrystalline metals increases with decreasing grain size,following the classical HallPetch relationship.However,this relationship fails when softening occurs at very small grain sizes(typically less than 10 to 20 nm),which limits the development of ultrahigh-strength materials.In this work,using columnar-grained nanocrystalline Cu-Ag‘samples’,molecular dynamics simulations were performed to investigate the softening mechanism and explore the strengthening strategies(e.g.,formation of solid solution or grain boundary(GB)segregation)in extremely fine nanograined metals.Accordingly,the softening of pure metals is induced by atomic sliding in the GB layer,rather than dislocation activities in the grain interior,although both occur during deformation.The solid solution lowers the stacking fault energy and increases the GB energy,which leads to the softening of NC metals.GB segregation stabilizes GB structures,which causes a notable improvement in strength,and this improvement can be further enhanced by optimizing the solute concentration and GB excess.This work deepens the understanding of the softening mechanism due to atomic sliding in the GB layer and the strengthening mechanism arising from tailoring the GB stability of immiscible alloys and provides insights into the design of ultrahighstrength materials.