Scanning transmission electron microscopy and atom probe tomography analysis of non-stoichiometry long-period-stacking-ordered structures in Mg-Ni-Y/Sm alloys

The long-period-stacking-ordered(LPSO)structure affects the mechanical,corrosion and hydrolysis properties of Mg alloys.The current work employs high angle annular dark field-scanning transmission electron microscopy(HAADF-STEM)and atom probe tomography(APT)to investigate the structural and local chemical information of LPSO phases formed in Mg-Ni-Y/Sm ternary alloys after extended isothermal annealing.Depending on the alloying elements and their concentrations,Mg-Ni-Y/Sm develops a two-phase LPSO+α-Mg structure in which the LPSO phase contains defects,hybrid LPSO structure,and Mg insertions.HAADF-STEM and APT indicate non-stoichiometric LPSO with incomplete Ni_(6)(Y/Sm)_(8) clusters.In addition,the APT quantitatively determines the local composition of LPSO and confirms the presence of Ni within the Mg bonding layers.These results provide insight into a better understanding of the structure and hydrolysis properties of LPSO-Mg alloys.

Experimental Investigation of the Early Stage of Precipitation on Binary Al-Li, Al-Cu Alloys and Ternary Al-Li-Cu Alloys by Means of Atom Probe Tomography

Aluminum-based alloys play a key role in modern engineering and are widely used in construction components in aircraft, automobiles and other means of transportation due to their light weight and superior mechanical properties. Introduction of different nano-structure features can improve the service and the physical properties of such alloys. An improvement of an Al-based alloy has been performed based on the understanding of the relationships among compositions, processing, microstructural characteristics and properties. Knowledge of the decomposition process of the microstructure during the precipitation reaction is particularly important for future technical developments. The objective of this study is to investigate the nano-scale chemical composition in the Al-Cu, Al-Li and Al-Li-Cu alloys during the early stage of the precipitation sequence and to describe whether this compositional difference correlates with variations in the observed precipitation kinetics. Investigation of the fine scale segregation effects of dilute solutes in aluminum alloys which were experienced different heat treatments by using atom probe tomography has been achieved. The results show that an Al-1.7 at.% Cu alloy requires a long ageing time of approximately 8 h at 160°C to allow the diffusion of Cu atoms into Al matrix. For the Al-8.2 at.% Li alloy, a combination of both the natural ageing condition (48 h at room temperature) and a short artificial ageing condition (5 min at 160°C) induces increasing on the number density of the Li clusters and hence increase number of precipitated particles. Applying this combination of natural ageing and short artificial ageing conditions onto the ternary Al-4 at.% Li-1.7 at.% Cu alloy induces the formation of a Cu-rich phase. Increasing the Li content in the ternary alloy up to 8 at.% and increasing the ageing time to 30 min resulted in the precipitation processes ending with δ' particles. Thus the results contribute to the understanding of Al-alloy design.

Elucidating the evolution of long-period stacking ordered phase and its effect on deformation behavior in the as-cast Mg-6Gd-1Zn-0.6Zr alloy

Herein,the evolution of long-period stacking ordered(LPSO)phases in the as-cast Mg-6Gd-1Zn-0.6Zr(wt.%)alloy are investigated via transmission electron microscopy(TEM)and atom probe tomography(APT).The TEM results reveal that two types of LPSO phase(a bulky interdendritic phase and a plate-like matrix LPSO phase)are formed in the as-cast sample.Most of the LPSO phases are confirmed to be of the 14H type,with a smaller proportion being of the 18R LPSO.Further,the APT results reveal that the composition of the interdendritic LPSO phase is closer to that of the ideal 14H phase compared to the matrix LPSO phase,and both the interdendritic and matrix LPSO phases exhibit a Gd/Zn ratio of 2.5,thereby indicating a deficient Zn content compared to the ideal 14H phase(i.e.,1.3).In addition,the influence of the LPSO phases on the deformation behavior is investigated at different compressive plastic strains using electron backscatter diffraction(EBSD)analysis to reveal twinning and slip behavior during deformation.The results indicate that the LPSO phase induces additional work hardening in the late stage of deformation via the suppression of{1011}compressive twinning and the activation of non-basal slip systems.

Redistribution of C and N Atoms in High Nitrogen Martensitic Stainless Steel During Cryogenic Treatment

The redistribution of C and N atoms during cryogenic treatment is crucial for the microstructure evolution and properties of high nitrogen martensitic steel.Here,the distinct redistribution behavior of C and N atoms in a martensitic stainless steel with 0.3 wt%C and 0.5 wt%N after cryogenic treatment were investigated by the atom probe tomography.Carbon clusters begin to form after cryogenic treatment at-60℃and gradually increase with the decrease of cryogenic treatment temperature.While Mo–N and Cr–N pairs are homogeneously distributed in the matrix even after cryogenic treatment at-120℃,and then form enrichment phenomenon when the cryogenic temperature is deeply lowered to-190℃.It is found that the distinct redistributions of C and N atoms are associated with the different interaction energy between substitutional atoms and them.The stronger interaction between Cr,Mo atoms and N delays the segregation of N during the cryogenic treatment.Finally,the mechanical properties results confirmed that the deep lower cryogenic treatment is a promising method to improve the hardness and strength in the high nitrogen martensitic stainless steel.

Influence mechanism of boron segregation on the microstructure evolution and hot ductility of super austenitic stainless steel S32654

Influence mechanism of B segregation on the microstructure evolution and hot ductility of S32654 at850-1250℃was systematically investigated through experimental research and theoretical calculation.The results demonstrated that the segregation of B at grain boundary(GB)played different roles in the microstructure evolution and hot ductility at various temperatures.At 850℃,B segregation inhibited Mo segregation at the GB and enhanced the GB cohesion.At 900-950℃,B segregation restricted the diffusion and segregation of Mo to the GB,inhibiting the precipitation ofσphase.At 1000-1050℃,B segregation accelerated the dislocation accumulation and limited the GB migration,promoting the nucleation and inhibiting the growth of DRX grains.At 1100-1150℃,B has little effect on the DRX due to sufficient energy supply by higher temperature.Under the above beneficial effects of B,the hot ductility of S32654 was improved to varying degrees at 850-1150℃.However,as the temperature increased to1200-1250℃,B segregation decreased the solidus temperature and enhanced the liquefaction cracking tendency,resulting in a deterioration of the hot ductility.

Characterization of the Trace Phosphorus Segregation and Mechanical Properties of Dual-Phase Steels

The segregation behavior of trace amount of phosphorus(P)and the mechanical properties of dual-phase(DP)steels have been systematically studied.The microstructure of DP steels is mainly composed of martensite,ferrite and nanoscale carbides.For the DP steels with diff erent trace amounts of P(≤0.015 wt%),P has almost no effect on the mechanical properties.Atom probe technology(APT)analyses confirm that P segregation was only found at the precipitate/matrix interface.Moreover,the precipitates of(Ti,Mo)C are widely distributed in the ferrite,martensite and ferrite/martensite interface regions.The special segregation feature of P would not concentrate at specific regions such as ferrite/martensite interface and/or martensite lath interface,which reveals that trace amounts of P(≤0.015 wt%)have almost no effect on the mechanical properties of DP steels.It is proved for the first time that the MC-type carbides of(Ti,Mo)C can reduce or eliminate the damage effect of P on the mechanical properties of steels,which provides a new way for the design of alloys to reduce P damage.This work will promote to increase the P content control standard in DP steels from 0.01 to 0.015 wt%,which will not change the mechanical properties,but greatly reduce the scrap rate and increase the energy e fficiency of the manufacturing process.

Correlation between precipitates evolution and mechanical properties of Al-Sc-Zr alloy with Er additions

Correlation between precipitates evolution and mechanical properties of Al-Sc-Zr alloy with Er additions during isothermal ageing were investigated by microhardness measurements,transmission electron microscopy,atom probe tomography and density functional theory-based simulations.The results demonstrate that the Er additions significantly improve the hardness during elevated temperature ageing,especially at 400℃.This is mainly because Er additions increase the nucleation rate of the Al_(3)(Er,Sc,Zr)precipitates,resulting in a higher density of fine and uniform dispersion of L1_(2)structured nanoparticles.First-principles calculations demonstrate that the second nearest neighboring solute-solute interactions for the species Sc,Zr and Er are energetically favored–a key feature to rationalize the observed precipitate structure and the underlying formation mechanism.The sequential formation of the core/shell precipitates in the Er-free alloy and core/double-shell precipitates in the Er-containing alloy arises due to the different solute-solute and solute-vacancy interaction energies,and the relative diffusivities of the Er,Sc and Zr species in Al.These results shed light on the beneficial effects of Er additions on the agehardening behavior of Al-Sc-Zr alloy and provide guidance for designing the ageing treatments for the Al-Sc-Zr(-Er)alloys.

A correlative multidimensional study ofγ’precipitates with Ta addition in Re-containing Ni-based single crystal superalloys

The microstructural evolution in Re-containing Ni-based single crystal superalloys with different Tantalum(Ta)content(2 Ta,5 Ta and 8 Ta in wt%)was investigated.Ta addition significantly affected theγ’precipitate morphology,γ/γ’lattice misfit and microstructural stability during long-term aging.Results showed that the partitioning behaviors of solutes were enhanced by Ta addition,meanwhile,the reversal partitioning behavior of W was triggered which partitioned fromγ’precipitate to matrix.The elemental concentration redistribution caused variations in lattice misfit from positive to negative,the values of lattice misfit were measured to be 0.16%for 2 Ta alloy,then decreased to-0.07%for 5 Ta alloy and negatively increased to-0.23%for 8 Ta alloy.These variations in the lattice misfit were reflected on the transition ofγ’morphology from round-cornered cuboidal shape to cuboidal with sharp corners,accomplished with increasing shape parameter ratioη.Consequently,the optimalγ’shape could be obtained at lattice misfit of approximately 0.3%.Theγ’coarsening investigation at 900℃(up to 2000 h)indicated that Ta addition was beneficial for improving the microstructural stability by reducing the coarsening rate and interfacial energy,accompanied by the enhanced capability of resistingγ’coalescence.By incorporating the calculated interfacial energy,computational modeling,Thermo-Calc and PrecipiCalc,were employed to elucidate theγ’kinetic pathways,the simulation results agreed with experiments,indicating that the model and parameters were reasonable.Additionally,it was found that there was no overlap betweenγ’nucleation and coarsening when theγ/γ’interfacial energy increased to a critical value.

Quantitative study of dimensional stability mechanism and microstructure evolution during precipitation process of 2024Al alloy

In this paper,we were devoted to quantitatively analyzing the dimensional change of 2024 Al alloy during the aging process.Types of precipitates have been defined and the corresponding volume fraction has been measured with Transmission Electron Microscopy(TEM)and Three-Dimensional Atom Probe Tomography(3 D-APT).Guinier-Preston-Bagaryatsky(GPB)zone(0.86 vol.%)was found after aging for 8 h.GPB zone(0.17 vol.%)and S'phase(1.02 vol.%)was found after aging for 24 h.The reduced dimensional change was attributed to the reduction of matrix lattice parameter.Relative to the initial supersaturated solid solution state,the lattice strain of-2.79×10^(-5)and-5.16×10^(-5)was produced after 8 h and 24 h aging respectively.The difference of the lattice parameters between the precipitates and matrix was also considered.A model was established to quantitatively describe the relationship between the precipitation process and dimensional change of 2024 Al alloy.

Characterization ofα_(2)Precipitates in Ti–6Al and Ti–8Al Binary Alloys:A Comparative Investigation

Transmission electron microscopy(TEM)and atom probe tomography(APT)techniques were used to investigate the nanoscale orderedα_(2)(Ti_(3)Al)precipitates in Ti–Al binary alloys.Ti–6Al and Ti–8Al binary alloys were solution treated and aged to obtain Widmanstatten microstructure and promoteα_(2)precipitates.The TEM results displayed strong short-range ordering ofα_(2)precipitates in Ti–8Al alloy,while no evidence of the superlattice reflections ofα_(2)in Ti–6Al alloy.The results acquired from APT showed theα_(2)clusters and atoms distribution at the interface between the matrix andα_(2)precipitates.The size and morphology ofα_(2)particles in Ti–8Al alloy,respectively,obtained by TEM and APT are closely consistent.Meanwhile,the APT results displayed tiny size clusters in Ti–6Al alloy,which supposed to give evidence of the initial ordering process ofα_(2)precipitates in the absence of correlative results from TEM.

Multidimensional thermally-induced transformation of nest-structured complex Au-Fe nanoalloys towards equilibrium

Bimetallic nanoparticles are often superior candidates for a wide range of technological and biomedical applications owing to their enhanced catalytic,optical,and magnetic properties,which are often better than their monometallic counterparts.Most of their properties strongly depend on their chemical composition,crystallographic structure,and phase distribution.However,little is known of how their crystal structure,on the nanoscale,transforms over time at elevated temperatures,even though this knowledge is highly relevant in case nanoparticles are used in,e.g.,high-temperature catalysis.Au-Fe is a promising bimetallic system where the low-cost and magnetic Fe is combined with catalytically active and plasmonic Au.Here,we report on the in s/fi;temporal evolution of the crystalline ordering in Au-Fe nanoparticles,obtained from a modern laser ablation in liquids synthesis.Our in-depth analysis,complemented by dedicated atomistic simulations,includes a detailed structural characterization by X-ray diffraction and transmission electron microscopy as well as atom probe tomography to reveal elemental distributions down to a single atom resolution.We show that the Au-Fe nanoparticles initially exhibit highly complex internal nested nanostructures with a wide range of compositions,phase distributions,and size-depended microstrains.The elevated temperature induces a diffusion-controlled recrystallization and phase merging,resulting in the formation of a single face-centered-cubic ultrastructure in contact with a body-centered cubic phase,which demonstrates the metastability of these structures.Uncovering these unique nanostructures with nested features could be highly attractive from a fundamental viewpoint as they could give further insights into the nanoparticle formation mechanism under non-equilibrium conditions.Furthermore,the in situ evaluation of the crystal structure changes upon heating is potentially relevant for high-temperature process utilization of bimetallic nanoparticles,e.g.,during catalysis.

He-enhanced heterogeneity of radiation-induced segregation in FeNiCoCr high-entropy alloy

Radiation-induced segregation(RIS) is a typical non-equilibrium process that can dramatically alter the behavior of defect sinks and material properties under irradiation. However, RIS mechanisms have been rarely studied around small He bubbles owing to the technical challenges involved in direct measurements of local chemistry. Here, using state-of-the-art atom probe tomography, we report the RIS behavior near He bubbles in the Fe Ni Co Cr high-entropy alloy that indicates Co segregates most strongly, followed by weaker Ni segregation, whereas Fe and Cr are depleted almost to the same degree. Exceptionally, the magnitude of Co segregation around He bubbles is higher than previously measured values at voids and dislocation loops. Electron energy-loss spectroscopy was used to measure the He density and pressure inside individual bubbles. We demonstrate that He bubbles are over-pressurized at the irradiation temperature that could result in the vacancy bias and the subsequent vacancy-dominated RIS mechanism.First-principles calculations further reveal that there are repulsive interactions between He and Co atoms that may reduce the frequency of Co-vacancy exchange. As a result, He atoms likely retard Co diffusion via the vacancy mechanism and enhance the heterogeneity of RIS in Co-containing multicomponent alloys. These insights could provide the basis for understanding He effects in nuclear materials and open an avenue for tailoring the local chemical order of medium-and high-entropy alloys.

Effect of Pre-deformation on Nanoscale Precipitation and Hardness of a Maraging Stainless Steel

The effect of pre-deformation on nanoscale precipitates and hardness of a maraging stainless steel strengthened by the coprecipitation of Ni_(3)Ti,Mo-enriched and Cr-enriched precipitates was systematically studied using electron back scattered diffraction,transmission electron microscopy and atom probe tomography(APT).Hardness measurements showed that the hardness of specimen with a deformation ratio of 90%peaked at HV 718 aged for 24 h,which is higher than that in the undeformed specimen(HV 603)aged for 72 h at 480℃.APT characterization revealed that pre-deformation could shorten the incubation time of the Mo-enriched and Cr-enriched precipitates.At the early-aged stage,pre-deformation increased the stain energy that inhibited the nucleation of Ni_(3)Ti precipitates,but accelerated the rejection of Mo from Ni-Ti clusters.Besides,the strengthening model indicated that strain hardening(43%)makes a larger contribution to the hardness at the early-aged condition,while precipitation hardening(58%)has most contribution to the hardness at the peak-aged conditition.

Elemental partitioning as a route to design precipitation-hardened high entropy alloys

Precipitation-hardened high entropy alloys(HEAs)with carefully tuned compositions have shown excellent mechanical properties,demonstrating great potential for engineering applications.However,due to the lack of precise multiple phase diagrams,the composition design of multi-principal-component HEAs still inevitably relies on the extremely time-consuming trial-and-error approach.The present study,on the basis of powerful composition quantification ability of atom probe tomography(APT)technology,proposed a framework to guide the quantitative design of precipitation-hardened HEAs.In this framework,the elemental partitioning was used as a crucial route to avoid the thermodynamic challenge of designing precipitation-hardened HEAs.As a case study,the role of Ti/Al ratio in the design ofγ-γ’HEAs was predicted through the proposed framework and then validated by experimental studies.The framework predicted that when the total content of Ti and Al is fixed,a higher Ti/Al ratio makesγ-γ’HEA stronger.APT and mechanical results agreed well with these predictions and validated the feasibility of the framework.These findings provided a new route to design the precipitation-hardened alloys and a deeper insight into the design ofγ-γ’HEA.

Enhanced thermoelectric properties of NbCoSn half-Heuslers through in-situ nanocrystallization of amorphous precursors during the consolidation process

Tailoring nanostructures is a general approach used to obtain enhanced thermoelectric properties for halfHeusler compounds because the wide areas of grain and phase boundaries could be scattering centers that lower lattice thermal conductivity.However,a common fabrication method based on the sintering of crystalline precursors crushed from as-cast alloy ingots has limitations in obtaining a homogeneous microstructure without microsized impurity phases,owing to residual elemental segregation from casting.In this study,we used amorphous NbCoSn alloys as a precursor for the sintered specimen to obtain a homogeneous NbCoSn bulk specimen without microsized impurity phases and segregation,which led to the enhanced Seebeck coefficient due to the high purity of the half-Heusler phase after crystallization.Moreover,superplasticity originating from amorphous features enabled the powders to be largely deformed during the sintering process,even at a low sintering temperature(953 K).This resulted in less oxidation at both,the grain boundary and the interior,as the O diffusion pathway was blocked during the sintering process.As a result,the NbCoSn0.95Sb0.05 specimen using an amorphous precursor exhibited an enhanced zT of 0.7,due to the increase in the power factor and a decrease in lattice thermal conductivity compared to the specimen using a crystalline precursor.