Rolling contact fatigue performance is among the most important issues for applications of bearing steels.In this work,a recently developed surface modification technique,surface mechanical rolling treatment,was appli...Rolling contact fatigue performance is among the most important issues for applications of bearing steels.In this work,a recently developed surface modification technique,surface mechanical rolling treatment,was applied on a rare-earth addition bearing steel.And rolling contact fatigue behavior of treated samples was compared with that of as-received counterparts at different contacting stresses.The results demonstrated that a 700μm-thick gradient nanostructured surface layer is produced on samples by surface mechanical rolling treatment.The grain size decreases while the microhardness increases gradually with decreasing depth,reaching~23 nm and~10.2 GPa,respectively,at the top surface.Consequently,the rolling contact fatigue property is significantly enhanced.The characteristic life of treated samples is~3.2 times that of untreated counterparts according to Weibull curves at 5.6 GPa.Analyses of fatigue mechanisms demonstrated that the gradient nanostructured surface layer might not only retard material degradation and microcrack formation,but also prolong the steady-state elastic response stage under rolling contact fatigue.展开更多
In the present work, an ultrahigh strength bearing steel（AISI 52100） was subjected to surface mechanical rolling treatment（SMRT） at room temperature. Microstructural observations showed that martensitic laths, twi...In the present work, an ultrahigh strength bearing steel（AISI 52100） was subjected to surface mechanical rolling treatment（SMRT） at room temperature. Microstructural observations showed that martensitic laths, twins and cementite particles in the initial microstructure underwent distinct plastic strains and were gradually refined into nanostructures. Consequently, a gradient nanostructured（GNS） surface layer with a mean grain size of -24 nm at the top surface was obtained on the bearing steel, resulting in an increment of -20% in the surface hardness. Analyses based on microstructural evolution, phase constitution and in-depth hardness distribution revealed a mechanically induced formation mechanism of the GNS surface layer. The multiple surface severe plastic deformation under fine lubrication and cooling during SMRT contributed to the formation of a thick hardened surface layer on the bearing steel.展开更多
Welded joints are usually characterized by microstructural and compositional inhomogeneities, which may significantly degrade their fatigue properties and result in unpredictable failures. The present work demonstrate...Welded joints are usually characterized by microstructural and compositional inhomogeneities, which may significantly degrade their fatigue properties and result in unpredictable failures. The present work demonstrates a novel and simple method to effectively optimize the microstructure in the surface layer and promote the fatigue properties of welded specimens. By a recently developed approach—surface mechanical rolling treatment(SMRT), a gradient nanostructured surface layer is formed on welded S355 J2 W steel specimens. The mean grain size is refined to nanometer scale, and the hardness is significantly enhanced in the SMRT surface layer. Independent of the initially inhomogeneous microstructure and hardness distributions, the microstructure and hardness distributions in the surface layers are comparable on different zones of a welded specimen after SMRT with the same procedure. Consequently, fatigue property of the SMRT specimens is significantly enhanced relative to that of the as-welded specimens within the high cycle fatigue regime.展开更多
Severe plastic deformation(SPD)-induced gradient nanostructured(GNS)metallic materials exhibit superior mechanical performance,especially the high strength and good ductility.In this study,a novel high-speed machining...Severe plastic deformation(SPD)-induced gradient nanostructured(GNS)metallic materials exhibit superior mechanical performance,especially the high strength and good ductility.In this study,a novel high-speed machining SPD technique,namely single point diamond turning(SPDT),was developed to produce effectively the GNS layer on the hexagonal close-packed(HCP)structural Mg alloy.The high-resolution transmission electron microscopy observations and atomistic molecular dynamics simulations were mainly performed to atomic-scale dissect the grain refinement process and corresponding plastic deformation mechanisms of the GNS layer.It was found that the grain refinement process for the formation of the GNS Mg alloy layer consists of elongated coarse grains,lamellar fine grains with deformation-induced-tension twins and contraction twins,ultrafine grains,and nanograins with the grain size of~70 nm along the direction from the inner matrix to surface.Specifically,experiment results and atomistic simulations reveal that these deformation twins are formed by gliding twinning partial dislocations that are dissociated from the lattice dislocations piled up at grain boundaries.The corresponding deformation mechanisms were evidenced to transit from the deformation twinning to dislocation slip when the grain size was below 2.45μm.Moreover,the Hall-Petch relationship plot and the surface equivalent stress along the gradient direction estimated by finite element analysis for the SPDT process were incorporated to quantitatively elucidate the transition of defo rmation mechanisms during the grain refinement process.Our findings have implications for the development of the facile SPD technique to construct high strength-ductility heterogeneous GNS metals,especially for the HCP metals.展开更多
The formation mechanism and wear behavior of a gradient nanostructured(GNS) Inconel 625 alloy were investigated using SEM, TEM and ball-on-disc sliding wear tester. The results show that surface mechanical grinding tr...The formation mechanism and wear behavior of a gradient nanostructured(GNS) Inconel 625 alloy were investigated using SEM, TEM and ball-on-disc sliding wear tester. The results show that surface mechanical grinding treatment(SMGT) induced an approximately 800 μm-deep gradient microstructure, consisting of surface nano-grained,nano-laminated, nano-twined, and severely deformed layers, which resulted in a reduced gradient in micro-hardness from 6.95 GPa(topmost surface) to 2.77 GPa(coarse-grained matrix). The nano-grained layer resulted from the formation of high-density nano-twins and subsequent interaction between nano-twins and dislocations. The width and depth of the wear scar, wear loss volume, and wear rate of the SMGT-treated sample were smaller than those of untreated coarse-grained sample. Moreover, the wear mechanisms for both samples were mainly abrasive wear and adhesive wear, accompanied with mild oxidation wear. The notable wear resistance enhancement of the GNS Inconel 625 alloy was attributed to the high micro-hardness, high residual compressive stress, and high strain capacity of the GNS surface layer.展开更多
Heterogeneous gradient nanostructured metals have been shown to achieve the strength-ductility synergy, thus potentially possessing the enhanced tribological performance in comparison with their homogeneous nanograine...Heterogeneous gradient nanostructured metals have been shown to achieve the strength-ductility synergy, thus potentially possessing the enhanced tribological performance in comparison with their homogeneous nanograined counterparts. In this work, a facile laser surface remelting-based surface treatment technique is developed to fabricate a gradient nanostructured layer on a TiZrHfTaNb refractory highentropy alloy. The characterization of the microstructural evolution along the depth direction from the matrix to the topmost surface layer shows that the average grain size in the ~100 μm-thick gradient nanostructured layer is dramatically refined from the original ~200 μm to only ~8 nm in the top surface layer. The microhardness is therefore gradually increased from ~240 HV in matrix to ~650 HV in the topmost surface layer, approximately 2.7 times. Noticeably, the original coarse-grained single-phase bodycentered-cubic TiZrHfTaNb refractory high-entropy alloy is gradually decomposed into TiNb-rich bodycentered-cubic phase, TaNb-rich body-centered-cubic phase, ZrHf-rich hexagonal-close-packed phase and TiZr Hf-rich face-centered-cubic phase with gradient distribution in grain size along the depth direction during the gradient refinement process. As a result, the novel laser surface treatment-introduced gradient nanostructured TiZrHfTaNb refractory high-entropy alloy demonstrates the significantly improved wear resistance, with the wear rate reducing markedly by an order of magnitude, as compared with the as-cast one. The decomposed multi-phases and gradient nanostructures should account for the enhanced wear resistance. Our findings provide new insights into the refinement mechanisms of the laser-treated refractory high-entropy alloys and broaden their potential applications via heterogeneous gradient nanostructure engineering.展开更多
Nanocrystalline surface layers and gradient nanostructure in 5182 aluminum alloy have been produced through surface mechanical attrition treatment(SMAT). The results indicate that the gradient nanostructure can not on...Nanocrystalline surface layers and gradient nanostructure in 5182 aluminum alloy have been produced through surface mechanical attrition treatment(SMAT). The results indicate that the gradient nanostructure can not only improve the mechanical properties of 5182 Al alloy, but also has a certain effect on the Portevin-Le Chatelier(PLC) effect. The yield and ultimate tensile strength of 5182 Al alloy with SMAT are significantly improved combining with the decrease of fracture elongation compared with the as-received one. The PLC effect of 5182 Al alloy could be effectively postponed by the formation of gradient nanostructure after SMAT. It leads to the increase of critical strain of the PLC effect, more concentrated distribution of serrated strain, and increase of average stress amplitude in special strain range. The influence of grain size and gradient nanostructure on the PLC effect of 5182 Al alloy was also discussed in detail. Grain refinement could sharply increase the density of dislocations and hinder the movement of dislocations, which results in the decrease of moving speed of dislocations and the more concentrated distribution of solute atoms. The solute atoms would aggregate to form nano precipitates and further impede movement of dislocation. The stronger interaction between the dislocations and the nano precipitates is the main mechanism of postponed PLC effect.展开更多
In this paper,a silvered gradient nanostructured(GNS)layer was successfully fabricated on Ti6Al4V surface by means of surface ultrasonic rolling treatment(SURT)combined with silvering process.Surface characteristics,m...In this paper,a silvered gradient nanostructured(GNS)layer was successfully fabricated on Ti6Al4V surface by means of surface ultrasonic rolling treatment(SURT)combined with silvering process.Surface characteristics,mechanical properties,corrosion resistance,antibacterial ability and cytotoxicity of GNS Ag/Ti6Al4V were investigated in comparison with those of coarse-grained(CG)and GNS Ti6Al4V samples.Owing to the greatly enhanced diffusion kinetics of Ag in the GNS layer,surface silvering on GNS Ti6Al4V was achieved at a relatively low temperature(500℃),and the release rate of Ag^(+)was substantially accelerated,which endowed GNS Ag/Ti6Al4V with excellent antibacterial property.Moreover,improved wear and corrosion resistance of GNS Ag/Ti6Al4V can be achieved without cytotoxicity,indicating excellent bioadaptability.展开更多
Reducing grain size(i.e.increasing the fraction of grain boundaries)could effectively strengthen nanograined metals but inevitably sacrifices the ductility and possibly causes a strengthening-softening transition belo...Reducing grain size(i.e.increasing the fraction of grain boundaries)could effectively strengthen nanograined metals but inevitably sacrifices the ductility and possibly causes a strengthening-softening transition below a critical grain size.In this work,a facile laser surface remelting-based technique was employed and optimized to fabricate a∼600μm-thick heterogeneous gradient nanostructured layer on an austenitic Hadfield manganese steel,in which the average grain size is gradually decreased from∼200μm in the matrix to only∼8 nm in the nanocrystalline-amorphous core-shell topmost surface.Atomic-scale microstructural characterizations dissected the gradient refinement processes along the gradient direction,i.e.transiting from the dislocations activities and twinning in sub-region to three kinds of martensitic transformations,and finally a multi-phase nanocrystalline-amorphous core-shell structural surface.Mechanical tests(e.g.nanoindentation,bulk-specimen tensile,and micro-pillar compression)were conducted along the gradient direction.It confirms a tensile strength of∼1055 MPa and ductility of∼10.5%in the laser-processed specimen.Particularly,the core-shell structural surface maintains ultra-strong(tensile strength of∼1.6 GPa,micro-pillar compressive strength of∼4 GPa at a strain of∼8%,and nanoindentation hardness of∼7.7 GPa)to overcome the potential strengthening-softening transition.Such significant strengthening effects are ascribed to the strength-ductility synergetic effects-induced extra work hardening ability in gradient nanostructure and the well-maintained dislocation activities inside extremely refined nanograins in the multi-phase nanocrystalline-amorphous core-shell structural surface,which are evidenced by atomic-scale observations and theoretical analysis.This study provides a unique hetero-nanostructure through a facile laser-related technique for extraordinary mechanical performance.展开更多
The effect of surface gradient nanostructure on the fatigue life of commercial pure(CP)Zr was investigated.Four point bending fatigue tests indicated that the fatigue limit of CP Zr with surface gradient nanostructure...The effect of surface gradient nanostructure on the fatigue life of commercial pure(CP)Zr was investigated.Four point bending fatigue tests indicated that the fatigue limit of CP Zr with surface gradient nanostructure was increased by about 28.3%compared with the original sample(annealed state).The microstructure evolution at different fatigue loading stages was characterized.The high strength of surface gradient nanostructure could increase the crack initiation resistance.Furthermore,electron back scattered diffraction(EBSD)analysis demonstrated that the surface nanocrystals grew and rotated gradually during the fatigue loading,which was beneficial to reducing stress concentration,inhibit fatigue crack initiation,and prolong crack initiation life.The stored distortion energy of CP Zr calculated before and after fatigue indicated that the stored distortion energy decreased dramatically during cyclic loading,which provided the driving force for grain growth.Besides,the growth of nanocrystals consumed the mechanical energy produced by the applied load to a certain extent,thus,slowing down the accumulation of fatigue damage.The coarse grains at the interior could deform plastically and reduce the crack growth rate.In addition,the compressive residual stress caused by USSP treatment reduced the local effective stress and the driving force of crack growth.展开更多
The effects of gradient nanostructures induced by supersonic fine particle bombardment(SFPB)on the surface integrity,microstructural evolution,and mechanical properties of a Ni-W-Co-Ta medium-heavy alloy(MHA)were syst...The effects of gradient nanostructures induced by supersonic fine particle bombardment(SFPB)on the surface integrity,microstructural evolution,and mechanical properties of a Ni-W-Co-Ta medium-heavy alloy(MHA)were systematically investigated.The results show that gradient nanostructures are formed on the surface of Ni-W-Co-Ta MHA after SFPB treatment.At a gas pressure of 1.0 MPa and an impact time of 60 s,the ultimate tensile strength and yield strength of the alloy reached the maximum values of 1236 MPa and 758 MPa,respectively,which are 22.5%and 38.8%higher than those of the solid solution treated alloy,and the elongation(46.3%)is close to that of the solid solution treated alloy,achieving the optimal strength–ductility synergy.However,microcracks appear on the surface with excessive gas pressure and impact time,generating the relaxed residual stress and decreased strength.With the increase of the impact time and gas pressure,the depth of the deformation layer and the surface microhardness gradually increase,reaching the maximum values(29μm and HV 451)at 1.0 MPa and 120 s.The surface grain size is refined to a minimum of 11.67 nm.Notably,SFPB treatment has no obvious effect on elongation,and the fracture mode changes from the ductile fracture before treatment to ductile–brittle mixed fracture after treatment.展开更多
Gradient nanostructured(GNS)metallic materials are commonly achieved by gradient severe plastic de-formation with a gradient of nano-to micro-sized structural units from the surface/boundaries to the center.Certainly,...Gradient nanostructured(GNS)metallic materials are commonly achieved by gradient severe plastic de-formation with a gradient of nano-to micro-sized structural units from the surface/boundaries to the center.Certainly,such GNS can be inversely positioned,which however has not yet been reported.The present work reports a facile method in deformation gradient control to attain inverse gradient nanostructured(iGNS),i.e.,tailoring the cross-section shape,successfully demonstrated in Ti-50.3Ni shape memory alloy(SMA)wire through cold rolling.The microstructure of the rolled wire is characterized by a macroscopic inverse gradient from boundaries to the center—the average sizes of grain and martensite domain evolve from micrometer to nanometer scale.The iGNS leads to a gradient martensitic transforma-tion upon stress,which has been proved to be effectively reversible via in-situ bending scanning electron microscopy(SEM)observations.The iGNS Ti-50.3Ni SMA exhibits quasi-linear superelasticity(SE)in a wide temperature range from 173 to 423 K.Compared to uniform cold rolling,the gradient cold rolling with less overall plasticity further improves SE strain(up to 4.8%)and SE efficiency.In-situ tensiling synchrotron X-ray diffraction(SXRD)analysis reveals the underlying mechanisms of the unique SE in the iGNS SMAs.It provides a new design strategy to realize excellent SE in SMAs and sheds light on the advanced GNS metallic materials.展开更多
High-strength Mg alloys have historically suffered from a challenge in achieving good ductility.Here,we report an asymmetric gradient nanostructure design prepared by ultrasonic severe surface rolling(USSR)at room tem...High-strength Mg alloys have historically suffered from a challenge in achieving good ductility.Here,we report an asymmetric gradient nanostructure design prepared by ultrasonic severe surface rolling(USSR)at room temperature.Unlike conventional gradient-nanostructured materials that employ a hard-soft-hard sandwich structure,this new design incorporates a combined gradient distribution of grain microstructure and nanoprecipitates throughout the entire sample along the thickness direction.The nanoprecipitates are identified as theβ-Mg_(17)Al_(12)phase and are primarily generated through In-situ pre-cipitation promoted by the USSR-induced high-density dislocations and temperature increment.Benefit-ing from this unique microstructure,an outstanding strength-ductility synergy is achieved,with a yield strength of 372.8 MPa,an ultimate tensile strength of 453.3 MPa,and an elongation of 11.5%.The en-hanced strength can be attributed to several mechanisms,including grain boundary strengthening,dislocation strengthening,precipitation strengthening,twin strengthening,and hetero-deformation induced(HDI)strengthening.The HDI hardening and activation of multiple deformation modes also contribute to good ductility.This work provides a promising and effective method for overcoming the longstanding strength-ductility trade-offdilemma in Mg alloys.展开更多
Current Ti-based orthopedic implants often suffer from fatigue damage,therefore shortening their service lifespan.To solve this issue,in this study,mechanically polished Ti-6Al-7Nb(P-Ti6Al7Nb)was subjected to surface ...Current Ti-based orthopedic implants often suffer from fatigue damage,therefore shortening their service lifespan.To solve this issue,in this study,mechanically polished Ti-6Al-7Nb(P-Ti6Al7Nb)was subjected to surface mechanical attrition treatment(SMAT).Effects of various SMAT process parameters,including ball diameter and treatment duration,on the surface integrity of P-Ti6Al7Nb were investigated,specifically in terms of surface quality,surface nanocrystalline layer,and residual stress.Subsequently,the microstructure,in-depth residual stress and microhardness distributions,surface roughness,and fatigue behavior in simulated body fluids of optimally SMATed Ti-6Al-7Nb(S-Ti6Al7Nb)were examined and compared to those of P-Ti6Al7Nb.Results showed that based on the experimental conditions established in the present research,the optimal parameters were determined to be a 3 mm ball diameter and a 15 min treatment duration,which resulted in excellent surface integrity;S-Ti6Al7Nb showed a 300μm-thick gradient nanostructured layer comprising the thickest nanocrystalline layer of about 20μm,a 1000μm-deep residual compressive stress field with the maximum surface residual compressive stress,and a microconcave topography but free of any defects or cracks.The microstructural evolution mechanism was also elucidated,revealing that the combination of multidirectional primary and secondary twins’intersections and twin-dislocation interactions contributed to grain refinement.Compared to P-Ti6Al7Nb,S-Ti6Al7Nb exhibited a 40%improvement in fatigue strength,owing to synergistic effects of the gradient nanostructured layer,surface work hardening,high amplitude of residual compressive stress,and improved surface integrity.These factors effectively prevented the initiation of fatigue crack at the surface and shifted it to the sublayer,and inhibited the subsequent crack propagation.展开更多
The emerging gradient nanostructured metals and alloys containing spatially graded structural components with large variations in length scale and/or mechanical properties exhibit unprecedented mechanical performance....The emerging gradient nanostructured metals and alloys containing spatially graded structural components with large variations in length scale and/or mechanical properties exhibit unprecedented mechanical performance.This perspective delineates the basic structural features of gradient nanostructures,i.e.,structural components and spatial gradients,as well as related synthesis methods,excellent tensile properties,and novel deformation mechanisms.The challenges and prospect for the development of gradient nanostructured materials in the future are also addressed.展开更多
The wear behavior of Ni-based single crystal(NBSC)superalloy SRR99 fabricated by laser-directed energy deposition(LDED)is investigated and compared with that of its cast counterpart.While γ'precipitate size in th...The wear behavior of Ni-based single crystal(NBSC)superalloy SRR99 fabricated by laser-directed energy deposition(LDED)is investigated and compared with that of its cast counterpart.While γ'precipitate size in the latter is>400 nm,that in the former is an order of magnitude lower.Dry sliding wear tests reveal that the wear rate and coefficient of friction of the LDED alloy are 75% and 20%lower than that of its cast counterpart,respectively.Detailed transmission electron microscopy investigation of the wear-tested cast alloy indicates that there is orientation change and formation of nanoscale grains only at the top layer of the worn surface,whereas regions below undergo moderate plastic deformation via dislocation slip.In contrast,the sub-surface of the worn LDED alloy has a graded microstructure,with a composite of NiO/γ-Ni on the top,γ'free nano-grains in the middle,and a highly deformed nanoscale layer at the bottom.The improved wear behavior of the LDED alloy is attributed to its higher dislocation density,finerγ'precipitates,and the formation of this graded microstructure.Finally,a detailed description of mechanisms that lead to the formation of this unique graded microstructure is provided.展开更多
In this study,surface mechanical attrition treatment was employed to sucessfully produce a gradient nanostructured layer on WE43 magnesium alloy.X-ray diffraction,energy dispersive X-ray spectrometer,and high-resoluti...In this study,surface mechanical attrition treatment was employed to sucessfully produce a gradient nanostructured layer on WE43 magnesium alloy.X-ray diffraction,energy dispersive X-ray spectrometer,and high-resolution transmission electron microscope observations were mainly performed to uncover the microstructure evolution responsible for the refinement mechanisms.It reveals that the grain refinement process consists of three transition stages along the depth direction from the core matrix to the topmost surface layer,i.e.,dislocation cells and pile-ups,ultrafine subgrains,and randomly orientated nanograins with the grain size of~40 nm.Noticeably,the original Mg;RE second phase is also experienced refinement and then re-dissolved into the α-Mg matrix phase,forming a supersaturated solid solution nanostructuredα-Mg phase in the gradient refined layer.Due to the cooperative effects of grain refinement hardening,dislocation hardening,and supersaturated solid-solution hardening,the gradient nanostructured WE43 alloy contributes to the ultimate tensile strength of~435 MPa and ductility of~11.0%,showing an extraordinary strain hardening and mechanical properties among the reported severe plastic deformation-processed Mg alloys.This work provides a new strategy for the optimization of mechanical properties of Mg alloys via combining the gradient structure and supersaturated solid solution.展开更多
Mg-Cu alloys are promising antibacterial implant materials.However,their clinical applications have been impeded by their high initial biodegradation rate,which can be alleviated using nanotechnology by for example su...Mg-Cu alloys are promising antibacterial implant materials.However,their clinical applications have been impeded by their high initial biodegradation rate,which can be alleviated using nanotechnology by for example surface nanomodification to obtain a gradient nanostructured surface layer.The present work(i)produced a gradient nanostructured surface layer with a∼500µm thickness on a Mg-0.2 Cu alloy by a surface mechanical grinding treatment(SMGT),and(ii)studied the biodegradation behavior in Hank's solution.The initial biodegradation rate of the SMGTed samples was significantly lower than that of the unSMGTed original counterparts,which was attributed to the surface nanocrystallization,and the fragmentation and re-dissolution of Mg_(2)Cu particles in the surface of the SMGTed Mg-0.2 Cu alloy.Furthermore,the SMGTed Mg-0.2 Cu alloy had good antibacterial efficacy.This work creatively used SMGT technology to produce a high-performance Mg alloy implant material.展开更多
Surface nanocrystallization(SNC) has proved to be an effective approach to improve the overall properties of bulk metallic materials.Recently,a new surface nanocrystallization technique,i.e.,surface mechanical grindin...Surface nanocrystallization(SNC) has proved to be an effective approach to improve the overall properties of bulk metallic materials.Recently,a new surface nanocrystallization technique,i.e.,surface mechanical grinding treatment(SMGT),was developed.In this work,a gradient nano-micro structure was achieved in the surface layer of the AISI 52100 steel by using SMGT.We obtained a minimum grain size of about 7nm in the top surface layer.The total thickness of the deformed layer is over 200 micrometer.Meanwhile the surface roughness is rather low. Ferrite grains were deformed to different extents varying with depth from the top surface.Gradient grain sizes were formed from top surface to deep matrix which offered a great opportunity to study the refinement process of the ferrite grains.It is found that dislocation activities play a dominant role in the process.At the initiate stage, dislocations accumulated and interacted to form dense dislocation walls and cells.Increasing strain and strain rate induced more dislocation walls in cells,forming finer cells.This procedure continued until nanograins formed at the top most surface. The existence of cementite particles in ferrite matrix greatly facilitates the ferrite refinement process.Boundaries between ferrites and cementites offered many dislocation sources which accelerate the propagation of dislocations. Dislocation walls were blocked by cementites which certainly lead to finer dislocation cells.The existence of cementites makes it easier to generate fresh dislocation walls in sub-micron grains.A strain gradient was formed from a cementite particle to surrounding ferrite grains.This strain gradient gives rise to more geometric necessary dislocations. As ferrite grain size decreased less than that of cementite particles,fragmentation occurred in cementites.Hard second phase was usually considered as brittle.In this work,evidences of deformation(traces of dislocation activities) in cementites were distinct.Since the stress concentration in the phase boundary(especially triple junction) excesses the shear modulus of cementite,dislocation emission was triggered.It is found in this work that dislocations tend to slip along parallel planes,possibly on(001),(01 0),(100),(110),(10 1 ) and(011) planes,depending upon as the load directions.展开更多
A gradient nanostructured layer was fabricated on the surface of TA15(Ti-6Al-2Zr-1Mo-1V)alloy(produced by selective laser melting)using severe shot peening(SSP).This study focuses on the evolution of the microstructur...A gradient nanostructured layer was fabricated on the surface of TA15(Ti-6Al-2Zr-1Mo-1V)alloy(produced by selective laser melting)using severe shot peening(SSP).This study focuses on the evolution of the microstructure and the mechanism of grain refinement in TA15 titanium alloy during SSP treatment.Transmission electron microscopyand Rietveld refinement methods were employed.The residual stress and microhardness variations with depth were also characterized.The results show:(1)At the initial stage of deformation,plastic deformation is primarily accommodated through twinning and dislocation slip.(2)As the strain increases,twinning disappears,and dislocations interact to form tangles.Some dislocations annihilate and rearrange into subgrain boundaries,subdividing the original grains into subgrains.(3)With continued dislocation activity,the subgrain size decreases until nanocrystals are formed through the dynamic rotational recrystallization.SSP introduced compressive residual stress(CRS)in the near-surface layer of the material,with the maximum CRS of approximately−1141 MPa observed in the subsurface layer.It also induced work hardening,increasing the surface hardness to approximately 479 HV.However,the surface roughness increases,leading to a slight deterioration in surface quality.展开更多
基金The financial supports by the Chinese Academy of Sciences(Nos.XDC04030300 and XDB0510303)CAS-HK Joint Laboratory of Nanomaterials and MechanicsShenyang National Laboratory for Materials Science are acknowledged.
文摘Rolling contact fatigue performance is among the most important issues for applications of bearing steels.In this work,a recently developed surface modification technique,surface mechanical rolling treatment,was applied on a rare-earth addition bearing steel.And rolling contact fatigue behavior of treated samples was compared with that of as-received counterparts at different contacting stresses.The results demonstrated that a 700μm-thick gradient nanostructured surface layer is produced on samples by surface mechanical rolling treatment.The grain size decreases while the microhardness increases gradually with decreasing depth,reaching~23 nm and~10.2 GPa,respectively,at the top surface.Consequently,the rolling contact fatigue property is significantly enhanced.The characteristic life of treated samples is~3.2 times that of untreated counterparts according to Weibull curves at 5.6 GPa.Analyses of fatigue mechanisms demonstrated that the gradient nanostructured surface layer might not only retard material degradation and microcrack formation,but also prolong the steady-state elastic response stage under rolling contact fatigue.
基金supported financially by the National Key Research and Development Program of China (No.2017YFA0204400)Shenyang National Laboratory for Materials Science(No. 2015RP04)
文摘In the present work, an ultrahigh strength bearing steel（AISI 52100） was subjected to surface mechanical rolling treatment（SMRT） at room temperature. Microstructural observations showed that martensitic laths, twins and cementite particles in the initial microstructure underwent distinct plastic strains and were gradually refined into nanostructures. Consequently, a gradient nanostructured（GNS） surface layer with a mean grain size of -24 nm at the top surface was obtained on the bearing steel, resulting in an increment of -20% in the surface hardness. Analyses based on microstructural evolution, phase constitution and in-depth hardness distribution revealed a mechanically induced formation mechanism of the GNS surface layer. The multiple surface severe plastic deformation under fine lubrication and cooling during SMRT contributed to the formation of a thick hardened surface layer on the bearing steel.
基金the National Key Research and Development Program of China(Nos.2017YFA0204401 and 2017YFA0204403)the Liaoning Revitalization Talents Program(No.XLYC1808008)the Shenyang National Laboratory for Materials Science。
文摘Welded joints are usually characterized by microstructural and compositional inhomogeneities, which may significantly degrade their fatigue properties and result in unpredictable failures. The present work demonstrates a novel and simple method to effectively optimize the microstructure in the surface layer and promote the fatigue properties of welded specimens. By a recently developed approach—surface mechanical rolling treatment(SMRT), a gradient nanostructured surface layer is formed on welded S355 J2 W steel specimens. The mean grain size is refined to nanometer scale, and the hardness is significantly enhanced in the SMRT surface layer. Independent of the initially inhomogeneous microstructure and hardness distributions, the microstructure and hardness distributions in the surface layers are comparable on different zones of a welded specimen after SMRT with the same procedure. Consequently, fatigue property of the SMRT specimens is significantly enhanced relative to that of the as-welded specimens within the high cycle fatigue regime.
基金financially supported by the National Natural Science Foundation of China(Nos.51701171 and 51971187)the Partner State Key Laboratories in Hong Kong from the Innovation and Technology Commission(ITC)of the Government of the Hong Kong Special Administration Region(HKASR),Chinafinancial support from the PolyU Research Office(Project Code:1-BBXA)。
文摘Severe plastic deformation(SPD)-induced gradient nanostructured(GNS)metallic materials exhibit superior mechanical performance,especially the high strength and good ductility.In this study,a novel high-speed machining SPD technique,namely single point diamond turning(SPDT),was developed to produce effectively the GNS layer on the hexagonal close-packed(HCP)structural Mg alloy.The high-resolution transmission electron microscopy observations and atomistic molecular dynamics simulations were mainly performed to atomic-scale dissect the grain refinement process and corresponding plastic deformation mechanisms of the GNS layer.It was found that the grain refinement process for the formation of the GNS Mg alloy layer consists of elongated coarse grains,lamellar fine grains with deformation-induced-tension twins and contraction twins,ultrafine grains,and nanograins with the grain size of~70 nm along the direction from the inner matrix to surface.Specifically,experiment results and atomistic simulations reveal that these deformation twins are formed by gliding twinning partial dislocations that are dissociated from the lattice dislocations piled up at grain boundaries.The corresponding deformation mechanisms were evidenced to transit from the deformation twinning to dislocation slip when the grain size was below 2.45μm.Moreover,the Hall-Petch relationship plot and the surface equivalent stress along the gradient direction estimated by finite element analysis for the SPDT process were incorporated to quantitatively elucidate the transition of defo rmation mechanisms during the grain refinement process.Our findings have implications for the development of the facile SPD technique to construct high strength-ductility heterogeneous GNS metals,especially for the HCP metals.
基金financially supported by the National Key Research and Development Program of China (No. 2017YFA07007003)the National Natural Science Foundation of China (No. 51661019)+4 种基金the Program for Major Projects of Science and Technology in Gansu Province, China (No. 145RTSA004)the Hongliu First-class Discipline Construction Plan of Lanzhou University of Technology, Chinathe Incubation Program of Excellent Doctoral Dissertation, Lanzhou University of Technology, Chinathe Lanzhou University of Technology Excellent Students Studying Abroad Learning Exchange Fundthe State Key Laboratory of Cooperation and Exchange Fund。
文摘The formation mechanism and wear behavior of a gradient nanostructured(GNS) Inconel 625 alloy were investigated using SEM, TEM and ball-on-disc sliding wear tester. The results show that surface mechanical grinding treatment(SMGT) induced an approximately 800 μm-deep gradient microstructure, consisting of surface nano-grained,nano-laminated, nano-twined, and severely deformed layers, which resulted in a reduced gradient in micro-hardness from 6.95 GPa(topmost surface) to 2.77 GPa(coarse-grained matrix). The nano-grained layer resulted from the formation of high-density nano-twins and subsequent interaction between nano-twins and dislocations. The width and depth of the wear scar, wear loss volume, and wear rate of the SMGT-treated sample were smaller than those of untreated coarse-grained sample. Moreover, the wear mechanisms for both samples were mainly abrasive wear and adhesive wear, accompanied with mild oxidation wear. The notable wear resistance enhancement of the GNS Inconel 625 alloy was attributed to the high micro-hardness, high residual compressive stress, and high strain capacity of the GNS surface layer.
基金supported by the joint Ph D project between the Hong Kong Polytechnic University and Southern University of Science and Technologythe grant from the Research Committee of Poly U under student account code RK2N+1 种基金supported by the National Natural Science Foundation of China Projects (Nos. 51701171 and 51971187)the Fundamental Research Program of Shenzhen (Grant No. JCYJ20170412153039309)。
文摘Heterogeneous gradient nanostructured metals have been shown to achieve the strength-ductility synergy, thus potentially possessing the enhanced tribological performance in comparison with their homogeneous nanograined counterparts. In this work, a facile laser surface remelting-based surface treatment technique is developed to fabricate a gradient nanostructured layer on a TiZrHfTaNb refractory highentropy alloy. The characterization of the microstructural evolution along the depth direction from the matrix to the topmost surface layer shows that the average grain size in the ~100 μm-thick gradient nanostructured layer is dramatically refined from the original ~200 μm to only ~8 nm in the top surface layer. The microhardness is therefore gradually increased from ~240 HV in matrix to ~650 HV in the topmost surface layer, approximately 2.7 times. Noticeably, the original coarse-grained single-phase bodycentered-cubic TiZrHfTaNb refractory high-entropy alloy is gradually decomposed into TiNb-rich bodycentered-cubic phase, TaNb-rich body-centered-cubic phase, ZrHf-rich hexagonal-close-packed phase and TiZr Hf-rich face-centered-cubic phase with gradient distribution in grain size along the depth direction during the gradient refinement process. As a result, the novel laser surface treatment-introduced gradient nanostructured TiZrHfTaNb refractory high-entropy alloy demonstrates the significantly improved wear resistance, with the wear rate reducing markedly by an order of magnitude, as compared with the as-cast one. The decomposed multi-phases and gradient nanostructures should account for the enhanced wear resistance. Our findings provide new insights into the refinement mechanisms of the laser-treated refractory high-entropy alloys and broaden their potential applications via heterogeneous gradient nanostructure engineering.
基金financially supported by the National Key Research and Development Plan(No.2017YFB1103700)the Natural Science Foundation of China(Nos.51671101 and51464034)+1 种基金the Natural Science foundation of Jiangxi Province(Nos.20172BCB22002,20171BCD40003,20161ACB21003,20162BCB23013)the Science and Technology Key Research Plan in Jiangxi Educational Department(No.GJJ150010)
文摘Nanocrystalline surface layers and gradient nanostructure in 5182 aluminum alloy have been produced through surface mechanical attrition treatment(SMAT). The results indicate that the gradient nanostructure can not only improve the mechanical properties of 5182 Al alloy, but also has a certain effect on the Portevin-Le Chatelier(PLC) effect. The yield and ultimate tensile strength of 5182 Al alloy with SMAT are significantly improved combining with the decrease of fracture elongation compared with the as-received one. The PLC effect of 5182 Al alloy could be effectively postponed by the formation of gradient nanostructure after SMAT. It leads to the increase of critical strain of the PLC effect, more concentrated distribution of serrated strain, and increase of average stress amplitude in special strain range. The influence of grain size and gradient nanostructure on the PLC effect of 5182 Al alloy was also discussed in detail. Grain refinement could sharply increase the density of dislocations and hinder the movement of dislocations, which results in the decrease of moving speed of dislocations and the more concentrated distribution of solute atoms. The solute atoms would aggregate to form nano precipitates and further impede movement of dislocation. The stronger interaction between the dislocations and the nano precipitates is the main mechanism of postponed PLC effect.
基金This work was financially supported by the National Natural Science Foundation of China(Nos.51801064,51961012 and 52001122)Jiangxi Outstanding Young Talents Program(No.20192BCB23014)Jiangxi Key Research and Development Program(No.20203BBE53050).
文摘In this paper,a silvered gradient nanostructured(GNS)layer was successfully fabricated on Ti6Al4V surface by means of surface ultrasonic rolling treatment(SURT)combined with silvering process.Surface characteristics,mechanical properties,corrosion resistance,antibacterial ability and cytotoxicity of GNS Ag/Ti6Al4V were investigated in comparison with those of coarse-grained(CG)and GNS Ti6Al4V samples.Owing to the greatly enhanced diffusion kinetics of Ag in the GNS layer,surface silvering on GNS Ti6Al4V was achieved at a relatively low temperature(500℃),and the release rate of Ag^(+)was substantially accelerated,which endowed GNS Ag/Ti6Al4V with excellent antibacterial property.Moreover,improved wear and corrosion resistance of GNS Ag/Ti6Al4V can be achieved without cytotoxicity,indicating excellent bioadaptability.
文摘Reducing grain size(i.e.increasing the fraction of grain boundaries)could effectively strengthen nanograined metals but inevitably sacrifices the ductility and possibly causes a strengthening-softening transition below a critical grain size.In this work,a facile laser surface remelting-based technique was employed and optimized to fabricate a∼600μm-thick heterogeneous gradient nanostructured layer on an austenitic Hadfield manganese steel,in which the average grain size is gradually decreased from∼200μm in the matrix to only∼8 nm in the nanocrystalline-amorphous core-shell topmost surface.Atomic-scale microstructural characterizations dissected the gradient refinement processes along the gradient direction,i.e.transiting from the dislocations activities and twinning in sub-region to three kinds of martensitic transformations,and finally a multi-phase nanocrystalline-amorphous core-shell structural surface.Mechanical tests(e.g.nanoindentation,bulk-specimen tensile,and micro-pillar compression)were conducted along the gradient direction.It confirms a tensile strength of∼1055 MPa and ductility of∼10.5%in the laser-processed specimen.Particularly,the core-shell structural surface maintains ultra-strong(tensile strength of∼1.6 GPa,micro-pillar compressive strength of∼4 GPa at a strain of∼8%,and nanoindentation hardness of∼7.7 GPa)to overcome the potential strengthening-softening transition.Such significant strengthening effects are ascribed to the strength-ductility synergetic effects-induced extra work hardening ability in gradient nanostructure and the well-maintained dislocation activities inside extremely refined nanograins in the multi-phase nanocrystalline-amorphous core-shell structural surface,which are evidenced by atomic-scale observations and theoretical analysis.This study provides a unique hetero-nanostructure through a facile laser-related technique for extraordinary mechanical performance.
基金supported by the National Natural Science Foundation of China(Grant No.51674187)the Key Industry Chain(Group)-Industrial Field in Shaanxi Province(Grant No.2020ZDLGY05-03)International Joint Research Center for Value-added Metallurgy and Processing of Non-ferrous Metals(Grant No.2019SD0010)。
文摘The effect of surface gradient nanostructure on the fatigue life of commercial pure(CP)Zr was investigated.Four point bending fatigue tests indicated that the fatigue limit of CP Zr with surface gradient nanostructure was increased by about 28.3%compared with the original sample(annealed state).The microstructure evolution at different fatigue loading stages was characterized.The high strength of surface gradient nanostructure could increase the crack initiation resistance.Furthermore,electron back scattered diffraction(EBSD)analysis demonstrated that the surface nanocrystals grew and rotated gradually during the fatigue loading,which was beneficial to reducing stress concentration,inhibit fatigue crack initiation,and prolong crack initiation life.The stored distortion energy of CP Zr calculated before and after fatigue indicated that the stored distortion energy decreased dramatically during cyclic loading,which provided the driving force for grain growth.Besides,the growth of nanocrystals consumed the mechanical energy produced by the applied load to a certain extent,thus,slowing down the accumulation of fatigue damage.The coarse grains at the interior could deform plastically and reduce the crack growth rate.In addition,the compressive residual stress caused by USSP treatment reduced the local effective stress and the driving force of crack growth.
基金supported by the National key Research and Development Program of China(No.2022YFB3705200)the National Natural Science Foundation of China(Nos.U1804146,51905153,52111530068)+1 种基金the Science and Technology Innovation Team Project of Henan University of Science and Technology,China(No.2015XTD006)the Major Science and Technology Project of Henan Province,China(No.221100230200)。
文摘The effects of gradient nanostructures induced by supersonic fine particle bombardment(SFPB)on the surface integrity,microstructural evolution,and mechanical properties of a Ni-W-Co-Ta medium-heavy alloy(MHA)were systematically investigated.The results show that gradient nanostructures are formed on the surface of Ni-W-Co-Ta MHA after SFPB treatment.At a gas pressure of 1.0 MPa and an impact time of 60 s,the ultimate tensile strength and yield strength of the alloy reached the maximum values of 1236 MPa and 758 MPa,respectively,which are 22.5%and 38.8%higher than those of the solid solution treated alloy,and the elongation(46.3%)is close to that of the solid solution treated alloy,achieving the optimal strength–ductility synergy.However,microcracks appear on the surface with excessive gas pressure and impact time,generating the relaxed residual stress and decreased strength.With the increase of the impact time and gas pressure,the depth of the deformation layer and the surface microhardness gradually increase,reaching the maximum values(29μm and HV 451)at 1.0 MPa and 120 s.The surface grain size is refined to a minimum of 11.67 nm.Notably,SFPB treatment has no obvious effect on elongation,and the fracture mode changes from the ductile fracture before treatment to ductile–brittle mixed fracture after treatment.
基金supported by the National Natural Science Foundation of China(Grant Nos.52171007,52101166,51931004)the 111 Projects 2.0(Grant No.BP0618008).
文摘Gradient nanostructured(GNS)metallic materials are commonly achieved by gradient severe plastic de-formation with a gradient of nano-to micro-sized structural units from the surface/boundaries to the center.Certainly,such GNS can be inversely positioned,which however has not yet been reported.The present work reports a facile method in deformation gradient control to attain inverse gradient nanostructured(iGNS),i.e.,tailoring the cross-section shape,successfully demonstrated in Ti-50.3Ni shape memory alloy(SMA)wire through cold rolling.The microstructure of the rolled wire is characterized by a macroscopic inverse gradient from boundaries to the center—the average sizes of grain and martensite domain evolve from micrometer to nanometer scale.The iGNS leads to a gradient martensitic transforma-tion upon stress,which has been proved to be effectively reversible via in-situ bending scanning electron microscopy(SEM)observations.The iGNS Ti-50.3Ni SMA exhibits quasi-linear superelasticity(SE)in a wide temperature range from 173 to 423 K.Compared to uniform cold rolling,the gradient cold rolling with less overall plasticity further improves SE strain(up to 4.8%)and SE efficiency.In-situ tensiling synchrotron X-ray diffraction(SXRD)analysis reveals the underlying mechanisms of the unique SE in the iGNS SMAs.It provides a new design strategy to realize excellent SE in SMAs and sheds light on the advanced GNS metallic materials.
基金supported by the Fundamental Research Funds for the Central Universities(No.B210202094)the Postgraduate Research&Practice Innovation Program of Jiangsu Province(No.KYCX230661).
文摘High-strength Mg alloys have historically suffered from a challenge in achieving good ductility.Here,we report an asymmetric gradient nanostructure design prepared by ultrasonic severe surface rolling(USSR)at room temperature.Unlike conventional gradient-nanostructured materials that employ a hard-soft-hard sandwich structure,this new design incorporates a combined gradient distribution of grain microstructure and nanoprecipitates throughout the entire sample along the thickness direction.The nanoprecipitates are identified as theβ-Mg_(17)Al_(12)phase and are primarily generated through In-situ pre-cipitation promoted by the USSR-induced high-density dislocations and temperature increment.Benefit-ing from this unique microstructure,an outstanding strength-ductility synergy is achieved,with a yield strength of 372.8 MPa,an ultimate tensile strength of 453.3 MPa,and an elongation of 11.5%.The en-hanced strength can be attributed to several mechanisms,including grain boundary strengthening,dislocation strengthening,precipitation strengthening,twin strengthening,and hetero-deformation induced(HDI)strengthening.The HDI hardening and activation of multiple deformation modes also contribute to good ductility.This work provides a promising and effective method for overcoming the longstanding strength-ductility trade-offdilemma in Mg alloys.
基金supported by the National Natural Science Foundation of China(Grant Nos.51631007 and 51971171).
文摘Current Ti-based orthopedic implants often suffer from fatigue damage,therefore shortening their service lifespan.To solve this issue,in this study,mechanically polished Ti-6Al-7Nb(P-Ti6Al7Nb)was subjected to surface mechanical attrition treatment(SMAT).Effects of various SMAT process parameters,including ball diameter and treatment duration,on the surface integrity of P-Ti6Al7Nb were investigated,specifically in terms of surface quality,surface nanocrystalline layer,and residual stress.Subsequently,the microstructure,in-depth residual stress and microhardness distributions,surface roughness,and fatigue behavior in simulated body fluids of optimally SMATed Ti-6Al-7Nb(S-Ti6Al7Nb)were examined and compared to those of P-Ti6Al7Nb.Results showed that based on the experimental conditions established in the present research,the optimal parameters were determined to be a 3 mm ball diameter and a 15 min treatment duration,which resulted in excellent surface integrity;S-Ti6Al7Nb showed a 300μm-thick gradient nanostructured layer comprising the thickest nanocrystalline layer of about 20μm,a 1000μm-deep residual compressive stress field with the maximum surface residual compressive stress,and a microconcave topography but free of any defects or cracks.The microstructural evolution mechanism was also elucidated,revealing that the combination of multidirectional primary and secondary twins’intersections and twin-dislocation interactions contributed to grain refinement.Compared to P-Ti6Al7Nb,S-Ti6Al7Nb exhibited a 40%improvement in fatigue strength,owing to synergistic effects of the gradient nanostructured layer,surface work hardening,high amplitude of residual compressive stress,and improved surface integrity.These factors effectively prevented the initiation of fatigue crack at the surface and shifted it to the sublayer,and inhibited the subsequent crack propagation.
基金supported by the National Natural Science Foundation of China(51931010,92163202,52122104,and 52071321)the Key Research Program of Frontier Science and International Partnership Program(GJHZ2029)the Youth Innovation Promotion Association,Chinese Academy of Sciences(2019196).
文摘The emerging gradient nanostructured metals and alloys containing spatially graded structural components with large variations in length scale and/or mechanical properties exhibit unprecedented mechanical performance.This perspective delineates the basic structural features of gradient nanostructures,i.e.,structural components and spatial gradients,as well as related synthesis methods,excellent tensile properties,and novel deformation mechanisms.The challenges and prospect for the development of gradient nanostructured materials in the future are also addressed.
基金supported by the National Key R&D Program of China(Grant Nos.2023YFB3712002 and 2021YFB3702503)the National Natural Science Foundation of China(Grant Nos.51927801 and U2032205)the National Science and Technology Major Project(Grant Nos.Y2019-VII-0011-0151 and 2019-VII-0019-0161)。
文摘The wear behavior of Ni-based single crystal(NBSC)superalloy SRR99 fabricated by laser-directed energy deposition(LDED)is investigated and compared with that of its cast counterpart.While γ'precipitate size in the latter is>400 nm,that in the former is an order of magnitude lower.Dry sliding wear tests reveal that the wear rate and coefficient of friction of the LDED alloy are 75% and 20%lower than that of its cast counterpart,respectively.Detailed transmission electron microscopy investigation of the wear-tested cast alloy indicates that there is orientation change and formation of nanoscale grains only at the top layer of the worn surface,whereas regions below undergo moderate plastic deformation via dislocation slip.In contrast,the sub-surface of the worn LDED alloy has a graded microstructure,with a composite of NiO/γ-Ni on the top,γ'free nano-grains in the middle,and a highly deformed nanoscale layer at the bottom.The improved wear behavior of the LDED alloy is attributed to its higher dislocation density,finerγ'precipitates,and the formation of this graded microstructure.Finally,a detailed description of mechanisms that lead to the formation of this unique graded microstructure is provided.
基金supported by National Natural Science Foundation of China(Nos.51701171 and 51971187)China Postdoctoral Science Foundation(No.2019M653599)+1 种基金the financial support from Partner State Key Laboratories in Hong Kong from the Innovation and Technology Commission(ITC)of the Government of the Hong Kong Special Administration Region(HKASR),China and the PolyU Research Office(Project Code:1-BBXA)supported by the grant from the PolyU Research Committee under student account code RK25
文摘In this study,surface mechanical attrition treatment was employed to sucessfully produce a gradient nanostructured layer on WE43 magnesium alloy.X-ray diffraction,energy dispersive X-ray spectrometer,and high-resolution transmission electron microscope observations were mainly performed to uncover the microstructure evolution responsible for the refinement mechanisms.It reveals that the grain refinement process consists of three transition stages along the depth direction from the core matrix to the topmost surface layer,i.e.,dislocation cells and pile-ups,ultrafine subgrains,and randomly orientated nanograins with the grain size of~40 nm.Noticeably,the original Mg;RE second phase is also experienced refinement and then re-dissolved into the α-Mg matrix phase,forming a supersaturated solid solution nanostructuredα-Mg phase in the gradient refined layer.Due to the cooperative effects of grain refinement hardening,dislocation hardening,and supersaturated solid-solution hardening,the gradient nanostructured WE43 alloy contributes to the ultimate tensile strength of~435 MPa and ductility of~11.0%,showing an extraordinary strain hardening and mechanical properties among the reported severe plastic deformation-processed Mg alloys.This work provides a new strategy for the optimization of mechanical properties of Mg alloys via combining the gradient structure and supersaturated solid solution.
基金Financially supported by Natural Science Foundation of China(No.51874368).
文摘Mg-Cu alloys are promising antibacterial implant materials.However,their clinical applications have been impeded by their high initial biodegradation rate,which can be alleviated using nanotechnology by for example surface nanomodification to obtain a gradient nanostructured surface layer.The present work(i)produced a gradient nanostructured surface layer with a∼500µm thickness on a Mg-0.2 Cu alloy by a surface mechanical grinding treatment(SMGT),and(ii)studied the biodegradation behavior in Hank's solution.The initial biodegradation rate of the SMGTed samples was significantly lower than that of the unSMGTed original counterparts,which was attributed to the surface nanocrystallization,and the fragmentation and re-dissolution of Mg_(2)Cu particles in the surface of the SMGTed Mg-0.2 Cu alloy.Furthermore,the SMGTed Mg-0.2 Cu alloy had good antibacterial efficacy.This work creatively used SMGT technology to produce a high-performance Mg alloy implant material.
文摘Surface nanocrystallization(SNC) has proved to be an effective approach to improve the overall properties of bulk metallic materials.Recently,a new surface nanocrystallization technique,i.e.,surface mechanical grinding treatment(SMGT),was developed.In this work,a gradient nano-micro structure was achieved in the surface layer of the AISI 52100 steel by using SMGT.We obtained a minimum grain size of about 7nm in the top surface layer.The total thickness of the deformed layer is over 200 micrometer.Meanwhile the surface roughness is rather low. Ferrite grains were deformed to different extents varying with depth from the top surface.Gradient grain sizes were formed from top surface to deep matrix which offered a great opportunity to study the refinement process of the ferrite grains.It is found that dislocation activities play a dominant role in the process.At the initiate stage, dislocations accumulated and interacted to form dense dislocation walls and cells.Increasing strain and strain rate induced more dislocation walls in cells,forming finer cells.This procedure continued until nanograins formed at the top most surface. The existence of cementite particles in ferrite matrix greatly facilitates the ferrite refinement process.Boundaries between ferrites and cementites offered many dislocation sources which accelerate the propagation of dislocations. Dislocation walls were blocked by cementites which certainly lead to finer dislocation cells.The existence of cementites makes it easier to generate fresh dislocation walls in sub-micron grains.A strain gradient was formed from a cementite particle to surrounding ferrite grains.This strain gradient gives rise to more geometric necessary dislocations. As ferrite grain size decreased less than that of cementite particles,fragmentation occurred in cementites.Hard second phase was usually considered as brittle.In this work,evidences of deformation(traces of dislocation activities) in cementites were distinct.Since the stress concentration in the phase boundary(especially triple junction) excesses the shear modulus of cementite,dislocation emission was triggered.It is found in this work that dislocations tend to slip along parallel planes,possibly on(001),(01 0),(100),(110),(10 1 ) and(011) planes,depending upon as the load directions.
基金financially supported by the National Natural Science Foundation of China(No.12262014).
文摘A gradient nanostructured layer was fabricated on the surface of TA15(Ti-6Al-2Zr-1Mo-1V)alloy(produced by selective laser melting)using severe shot peening(SSP).This study focuses on the evolution of the microstructure and the mechanism of grain refinement in TA15 titanium alloy during SSP treatment.Transmission electron microscopyand Rietveld refinement methods were employed.The residual stress and microhardness variations with depth were also characterized.The results show:(1)At the initial stage of deformation,plastic deformation is primarily accommodated through twinning and dislocation slip.(2)As the strain increases,twinning disappears,and dislocations interact to form tangles.Some dislocations annihilate and rearrange into subgrain boundaries,subdividing the original grains into subgrains.(3)With continued dislocation activity,the subgrain size decreases until nanocrystals are formed through the dynamic rotational recrystallization.SSP introduced compressive residual stress(CRS)in the near-surface layer of the material,with the maximum CRS of approximately−1141 MPa observed in the subsurface layer.It also induced work hardening,increasing the surface hardness to approximately 479 HV.However,the surface roughness increases,leading to a slight deterioration in surface quality.
