Atom Probe Tomography Results Research Articles

Precipitation strengthening behavior of Custom 455 stainless steel during tempering at 420 °C

After solid solution at 850 °C, the Custom 455 was aged at 420 °C for different times. The hardness changes were measured using a Vickers hardness tester, and the microstructure and composition of the second phase were studied using atom probe tomography (APT) and high-resolution transmission electron microscopy (HRTEM). The APT results indicate that in the early stage of aging, Cu atoms first segregate and attract Ti to form CuTi clusters; As the aging time increases, Ti atoms in CuTi clusters attract Ni and form CuTiNi clusters, resulting in a rapid increase in sample hardness; Subsequently, CuNiTi clusters grew and gradually separated into Cu-rich and NiTi-rich particles; As the aging time further increases, some NiTi-rich particles grow into rod-shaped Ni3Ti precipitates, while others have Si atoms and grow into spherical G phase particles. The orientation relationship between Cu-rich phase, Ni3Ti phase, and G phase is: [100]M//[0–11]Cu//[-12–10]Ni3Ti//[100]G, (011)M//(111)Cu//(0004)Ni3Ti//(022)G. The formation of the Cu-rich, Ni3Ti, and G phase resulted in a hardness peak of 538 HV after 16 h of aging. Subsequently, the decrease in hardness caused by the coarsening of the Cu-rich and Ni3Ti particles was offset by the newly formed G phase, resulting in a slow decrease in hardness.

Sn and Cu influence on Mg:Si ratios evolution in clusters based on DFT and its impact on precipitation hardening of pre-aged Al-1.0Mg-0.6Si alloys

Using atom probe tomography (APT), transmission electron microscopy (TEM), and hardness tests, the effect of Sn and Cu on the evolution of Mg:Si ratios in clusters and subsequent precipitation hardening behavior of pre-aged Al-1.0Mg-0.6Si alloys are investigated, and the underlying mechanism is revealed by density functional theory (DFT) calculation of interaction energy. It is shown that, the doping of Sn and/or Cu increases the average Mg:Si ratio in clusters. This increase is attributed to the strong interaction of Sn/Cu with Mg solutes, which enhances the binding of Mg to Si-rich clusters and results in a higher proportion of clusters with Mg:Si ratios near 1 (0.75 ∼ 1.25). Consequently, the enhanced precipitation of β" precipitates during artificial aging is observed in Sn and/or Cu doped alloys, due to the ease of clusters with Mg:Si ratios near 1 transform into β" precipitates, leading to an improved hardening response. Notably, the combined doping of Sn and Cu exhibits the strongest aggregation tendency towards Mg solutes, yielding the most pronounced strengthening effects on precipitation and hardening response during artificial aging. Furthermore, a schematic model detailing the evolution of Mg:Si ratios in clusters is proposed, based on the APT results and DFT calculation of interaction energy.

Towards quantitative analysis of deuterium absorption in ferrite and austenite during electrochemical charging by comparing cyclic voltammetry and cryogenic transfer atom probe tomography

Hydrogen embrittlement mechanisms of steels have been studied for several decades. Understanding hydrogen diffusion behavior in steels is crucial towards both developing predictive models for hydrogen embrittlement and identifying mitigation strategies. However, because hydrogen has a low atomic mass, it is extremely challenging to detect by most analytical methods. In recent years, cryogenic-transfer atom probe tomography (APT) of electrochemically-deuterium-charged steels has provided invaluable qualitative analysis of nanoscale deuterium traps such as carbides, dislocations, grain boundaries and interfaces between ferrite and cementite. Independently, cyclic voltammetry (CV) has provided valuable analysis of bulk hydrogen diffusion in steels. In this work, we use a combination of CV and cryogenic-transfer APT for quantitative analysis of deuterium pickup in electrolytically charged pure Fe (ferrite) and a model austenitic Fe18Cr14Ni alloy without any second phase. The high solubility and low diffusivity of hydrogen in austenite versus ferrite and potential influence of oxide layer are highlighted to result in clear observable signatures in CV and cryogenic-transfer APT results. The remaining challenges and pathway for enabling quantitative analysis of hydrogen pick up in steels is also discussed.

Formation of strain reduced In0.54Al0.34Ga0.12As layer of vertically coupled QDs arrays for O-band telecom single photon sources

We demonstrate the correlative research on the multi-stacked vertically coupled InAs quantum dots (VCQDs) capped by 15 nm combinational capping layer of In(0.21)Al(0.21)Ga(0.58)As and GaAs layer through Atom probe tomography (APT), Transmission Electron Microscopy (TEM), Secondary Ion Mass Spectroscopy (SIMS), High-Resolution X-Ray Diffraction (HRXRD) and Reciprocal Space Mapping (RSM). From the comparative studies, we observed a strain reduction of ∼ 2.3 ± 0.05% in the system, leading to the formation of strain-reduced (SR) In0.54Al0.34Ga0.12As layer with a lattice constant of 5.88 Å, which will work as a shielding layer. We found that the middle portion of the 3rd to 8th layer QDs structure was constant in size, shape, composition, and density, which enhances the carrier confinement in QDs. The computed strain energy values contributed to the overall growth structure of the 30 - layers VCQDs were recorded to be 2.2 meV/atoms. We report the detailed analogy of the in-out flow of In/Ga adatoms in the In0.54Al0.34Ga0.12As layer leading to the evolution of fully developed bigger InAs QDs size. To support the APT results, we also present the outcomes studies of the HRXRD Out-plane and In-Plane symmetric RSM experiment at (002), (004), and (006) reflections to investigate the compressive strain contribution in the SR In0.54Al0.34Ga0.12As shielding layer. This 30-layer VCQDs structure can be employed to fabricate high thermal stability operating at 1.3 µm O-band telecom Single Photon Sources (SPSs).

Combined modeling and experimental characterization of Mn segregation and spinodal decomposition along dislocation lines in Fe–Mn alloys

In the current work, Mn enrichment at dislocations in Fe–Mn alloys due to segregation and spinodal decomposition along the dislocation line is studied via modeling and experimental characterization. To model these phenomena, both finite-deformation microscopic phase-field chemomechanics (MPFCM) and Monte Carlo molecular dynamics (MCMD) are employed. MPFCM calibration is carried out with the same Fe–Mn MEAM-based potential used in MCMD, as well as CALPHAD data. Simulation results for Mn segregation to, and spinodal decomposition along, straight screw and edge dislocations as well as dislocation loops, are compared with characterization results from atom probe tomography (APT) for two Fe–Mn alloy compositions. In contrast to classical Volterra (linear elastic) dislocation theory, both MPFCM and MCMD predict a non-zero hydrostatic stress field in screw cores. Being of much smaller magnitude than the hydrostatic stress in straight edge cores, much less solute segregates to screw than to edge cores. In addition, the segregated amount in screw cores is below the critical concentration of 0.157 for the onset of spinodal decomposition along the line. On the other hand, results from MPFCM-based modeling imply that the concentration dependence of the solute misfit distortion and resulting dependence of the elastic energy density on concentration have the strongest effect. The maximum amount of Mn segregating to straight edge dislocations predicted by MPFCM agrees well with APT results. On the other hand, the current MPFCM model for Fe–Mn predicts little or no variation in Mn concentration along a straight dislocation line, in contrast to the APT results. As shown by the example of a dislocation loop in the current work, a change in the hydrostatic stress along the line due to changing character of dislocation does lead to a corresponding variation in Mn concentration. Such a variation in Mn concentration can also then be expected along a dislocation line with kinks or jogs.

Clustering and Precipitation during Early-Stage Artificial Aging of Al–Si–Mg(–Cu) Foundry Alloys

High-Si aluminum foundry alloys are an important material class for products with complex 3D geometries where casting is the most suitable production method. With Mg and/or Cu additions, these alloys gain strength upon heat treatment due to the formation of nanoprecipitates. These precipitated phases are of the same kind as in the wrought Al–Mg–Si(–Cu) alloys having much lower Si contents, which have been the subject of a high number of studies. Some of these studies indicate that atomic clusters formed during storage at room temperature have a strong effect on the phases that evolve during artificial aging. In this work, foundry alloys containing Si, Mg, and Cu are investigated. Room-temperature storage is found to have a great influence on kinetics during early aging. Cu additions accelerate the formation of hardening precipitates during early aging, but 1 month of room-temperature storage negates the positive effect of Cu. The maximum achievable strength is found to be limited mainly by the solubility limits of Si and Mg at the solution heat treatment temperature. With insights derived from transmission electron microscopy and atom probe tomography results, this study contributes to the understanding of the solute balance and early aging kinetics and how wrought and foundry alloys differ in these respects.

Nucleation-Growth Versus Spinodal Decomposition in Fe-Cr Alloys: An Experimental Verification by Atom Probe Tomography and Small Angle Neutron Scattering.

Identifying the operative mode of phase separation [spinodal decomposition (SD) or nucleation-growth (NG)] remains an extremely important area of research. The present work examines this critically in the Fe-Cr system using atom probe tomography (APT) and small angle neutron scattering (SANS), and establishes the framework to distinguish the two different modes of α' phase separation in thermally aged Fe-35 at% Cr and Fe-20 at% Cr alloys. Independent APT analysis determines the mode of phase separation on the basis of (i) the presence/absence of periodic chemical fluctuation through radial distribution function analysis and (ii) interphase interface characteristics (diffuse/sharp). SANS analysis, in contrast, yields virtually indistinguishable correlation peaks for both the modes, which necessitates further investigation of the several different aspects of SANS profiles in the light of APT results. For the first time, key features of SANS profiles have been identified that can unambiguously distinguish SD from NG in the Fe-Cr system: (i) nature of temporal evolution of FWHM of the correlation peak and (ii) appropriate value of γ for fitting with the dynamic scaling model (γ = 6 for SD, Fe-35 at% Cr alloy; γ = 4 for NG, Fe-20 at% Cr alloy).

Irradiation Effects on Stability of δ-UZr2 phase in U-50 wt% Zr Alloy

U-50wt%Zr is a candidate metallic nuclear fuel with potential application in light water reactors due to its excellent thermal properties and high radiation tolerance. The Zr-rich UZr fuels possess greater swelling resistance and fission gas release characteristics compared with U-rich UZr fuels. In this current study, the δ-phase U-50wt%Zr is proton irradiated at room temperature to 1 displacement per atom (dpa) to provide insights on phase stability under irradiation conditions. High resolution characterization of Transmission Electron Microscopy (TEM) and Atom Probe Tomography (APT) characterization techniques are used to elucidate microstructural changes due to irradiation. TEM and APT results show highly oriented bcc β-Zr-rich platelet precipitates nucleating adjacent to α-U phases inside the UZr2 matrix. Formation of this platelet morphology is characteristic of Widmänstatten structure which can be attributed to a variety of factors such as differences in thermal expansion coefficient between the phases, grain size, alloy composition, and cooling rate. The phases present are distinctly different than those observed through in situ annealing, but irradiation accelerates diffusion and phase separation kinetics. These microstructural changes in U-50wt%Zr are different from those achieved by pure thermodynamic or high temperature heavy ion irradiation experiments. Our work, together with previous ones, highlight the necessity to study the U-Zr phase diagram under non-equilibrium thermodynamics conditions to support this material's deployment as a viable nuclear fuel form.

Deuterium Distribution in Fe/V Multi-Layered Films.

The recent progress of Atom Probe Tomography (APT) has opened up atomic-scale elemental analysis including hydrogen species. For APT measurements, the use of deuterium is highly recommended, due to its low mobility compared to the fast and quantum mechanically tunneling isotope hydrogen. In addition, deuterium can be distinguished from hydrogen originating from the APT analysis chamber. To date, however, APT studies on materials with high D concentrations are scarce. In this study, the D concentration profile in a Fe/V multi-layered film sample was investigated, and spanned a wide concentration range. The mean hydrogen isotope concentration was alternatively quantified by electromotive force (EMF) measurements on a similar Fe/V film, thus verifying the APT results. The reduction found in the D concentration at the Fe/V interface results from local alloying at the Fe/V interfaces which accompanies a change in the available volume in the V lattice. Even at the same Fe concentration, the shape of the observed D depth profile was asymmetric at high D2 pressures. This indicates a stress impact caused by the deposition sequence.

The effect of Bi doping on the thermal conductivity of ZnO and ZnO:Al thin films

The dissipation of heat generation has been one of the largest obstacles in the design of semiconductor devices and reducing the thermal conductivity is vital for improving thermoelectric efficiency. This work focuses on the Bi doping effect on ZnO, and ZnO:Al thin films produced by magnetron sputtering with thickness varying between 500 and 900 nm. The approach introduces Bi ions, a higher mass element, into the ZnO metal-oxide matrix, to hinder phonon-mediated heat conduction and, consequently, reduce thermal conductivity. Atom probe tomography (APT) was employed to survey Bi doping distribution in ZnO:Al:Bi and ZnO:Bi thin films and to study the morphology of the grain boundaries. The thermal properties of the thin films were measured by frequency-domain thermoreflectance. Based on thermal conductivity results, it is concluded that the doping of ZnO films with Al has a significant effect on thermal conductivity, being reduced from 6.0 W m−1 K−1 in its undoped state to 3.3 W m−1 K−1 for ZnO with ∼3 at.% of Al, mainly due to alloy scattering of phonons in the wurtzite cell. Further doping with Bi contributes to a slight reduction in the thermal conductivity of ZnO:Al.Bi films (2.9 W m−1 K−1), due to grain boundary scattering by Bi/Bi2O3 phases. This result is understood as the confluence of two counteracting effects. On the one hand, the thermal conductivity of the film decreases because Bi, unlike Al, is segregated to grain boundaries and does not substitute Zn in the wurtzite crystal lattice, which is unequivocally demonstrated by APT results. On the other hand, the simultaneous presence of Al and Bi triggers a morphological change with the film's microstructure becoming more columnar. This change in microstructure from 3D island growth in ZnO:Al and ZnO:Bi to a more regular columnar structure in ZnO:Al,Bi limits further reduction in the thermal conductivity.

Effect of microstructure on fatigue crack deviation in AA2050-T84

Precipitation hardened AA2050 exhibits an unusual crack deviation phenomenon during fatigue testing: when the intended crack propagation direction is not parallel to macroscopic rolling direction, after initial propagation perpendicular to the loading direction, the crack deviates and further abnormally propagates in other direction which can be parallel to the direction of applied loading. Such a behaviour renders difficult the prediction of cracking in AA2050 and, thus, of AA2050 fatigue lifetime. In the present work using multi-scale microstructure characterization (SEM/EBSD/EDS, transmission electron microscopy (TEM) and atom probe tomography (APT)) of AA2050-T84 after fatigue tests, the effect of different microstructure parameters on crack deviation is carefully investigated keeping constant the macroscopic mechanical conditions (sample geometry, loading direction and amplitude, frequency). The respective influence of grain structure (including that of grain boundaries), intermetallics and precipitate-free zones is evaluated. The grain boundaries with misorientation angle ranging from 40 to 60° are observed to be the origin of the deviation phenomenon: the principal crack deviates at grain boundaries and further propagates along grain boundary. Numerous Fe- and Mn-rich intermetallics found in the volume of AA2050-T84 do not represent the major cause for the crack deviation, although, fostering it (only 25% of cracks were observed to deviate in the regions containing the intermetallics). The cause of failure at grain boundaries was deduced from APT results and TEM observation of precipitate free zones comparing T84 and T34 states (the latter do not reveal the crack deviation). In addition, it was observed that because of unrecrystallized state of AA2050-T84, the grains feature “layered” strained structure favourable to deviation inside the grains when a critical internal misorientation is reached along the crack path.

Effect of Ti/Al Ratio on the Elemental Partitioning in the Face-Centered Cubic-Based γ-γ′ Dual-Phase High Entropy Alloy Studied by Atom Probe Tomography

The effect of Ti/Al ratio on the elemental partitioning in the Ni2CoFeCrTixAly high entropy alloy was studied by atom probe tomography (APT). The APT results indicate that the γ′ nanoparticles have a composition of (NiCoFeCr)3TiAl which is independent of Ti/Al ratio. The effect of Ti/Al ratio on the partitioning behaviors of Fe and Cr are significantly different from that of Co. The decreasing of Ti/Al ratio significantly enhanced the incorporation of Fe and Cr into the γ′ nanoparticles. However, the concentration of Co in the γ′ nanoparticles declined with the decrease of Ti/Al ratio.

Effects of Fe-Ions Irradiation on the Microstructure and Mechanical Properties of FeCrAl-1.5wt.% ZrC Alloys.

Fe-13Cr-3.5Al-2.0Mo-1.5wt.% ZrC alloy was irradiated by 400 keV Fe+ at 400 °C at different doses ranging from 6.35 × 1014 to 1.27 × 1016 ions/cm2 with a corresponding damage of 1.0–20.0 dpa, respectively, to investigate the effects of different radiation doses on the hardness and microstructure of the reinforced FeCrAl alloys in detail by nanoindentation, transmission electron microscopy (TEM), and atom probe tomography (APT). The results show that the hardness at 1.0 dpa increases from 5.68 to 6.81 GPa, which is 19.9% higher than a non-irradiated specimen. With an increase in dose from 1.0 to 20.0 dpa, the hardness increases from 6.81 to 8.01 GPa, which is an increase of only 17.6%, indicating that the hardness has reached saturation. TEM and APT results show that high-density nano-precipitates and low-density dislocation loops forme in the 1.0 dpa region, compared to the non-irradiated region. Compared with 1.0 dpa region, the density and size of nano-precipitates in the 20.0 dpa region have no significant change, while the density of dislocation loops increases. Irradiation results in a decrease of molybdenum and carbon in the strengthening precipitates (Zr, Mo) (C, N), and the proportionate decrease of molybdenum and carbon is more obvious with the increase in damage.

Precipitation strengthening of Cu/NiAl co-precipitates in a martensite-austenite dual-phase steel

Precipitation strengthening of Cu/NiAl co-precipitates and its influence on mechanical properties in the martensite-austenite dual-phase steels under aging treatment are studied through a combination of atom probe tomography (APT), small angle neutron scattering (SANS) and theoretical calculation of strengthening mechanisms. The APT and SANS results show that the co-precipitates of NiAl and Cu nanoparticles are only separated out in the martensite phase upon aging treatment, and then the co-precipitates with high coarsening coefficient (4.17 nm 3 ‧h −1 ) rapidly grow up. In the peak aging state, the co-precipitates with mean radius of 2.1 nm and number density of 1.1 × 10 24 m −3 in the martensite, make a strength-ductility synergy in the dual phase steel that the yield strength increases from 474 ± 10 MPa to 964 ± 21 MPa and the uniform elongation simultaneously increases from 20% to 25%. Beyond peak aging, the mean radius of the co-precipitates increases to ~3.6 nm and its number density decreases to 1.4 × 10 23 m −3 , and the contribution of precipitation strengthening slightly decreases but the uniform elongation can be maintained. The precipitation strengthening in the peak aging is mainly contributed by shearing mechanism, while Orowan looping becomes dominant in beyond peak aging. Uniformly, both of shearing and Orowan looping mechanisms partly contribute to the strength and the ratios are related to the martensitic phase content, owing to a composite effect of precipitation strengthening in the martensite and weakening effect in the austenite. The effect of microstructure and nanoprecipitates on the ductility is also addressed and discussed. • Cu/NiAl co-precipitates make a strength-ductility synergy in the dual phase steel. • High Mn contents increase the coarsening coefficient of the co-precipitates. • Shearing mechanism changes to Orowan looping beyond peak aging. • Precipitation strengthening partly contributes to strength due to a composite effect.

Effect of martensite morphology and volume fraction on the low-temperature impact toughness of dual-phase steels

The influence of martensite morphology and volume fraction on low-temperature impact toughness of dual-phase (DP) steels was investigated by impact tests, nanoindentation tests, atom probe tomography (APT) and microstructural examination. The APT results of medium-C martensite show that many small-sized C clusters are formed at the twin grain boundaries inside the lath, which increase nanohardness and decrease impact toughness. The impact test results indicate that the low-temperature impact toughness of lath martensite is significantly superior to ferrite. Refining the lath martensite substructure can improve the low-temperature impact toughness of DP steels. However, martensite twins obviously reduce the low-temperature impact toughness of DP steels. It is further found that the impact toughness of DP steel is affected by the toughness of ferrite and martensite, and conforms to rule of mixtures. Finally, the low-temperature impact fracture mechanism of DP steels was observed, and it is found that the large-size ferrite grains in DP steels preferentially occur cleavage fracture. As the impact temperature decreases, the fracture mode of DP steel changes from dimple plus quasi-cleavage fracture to brittle cleavage fracture.

Omega versus alpha precipitation mediated by process parameters in additively manufactured high strength Ti–1Al–8V–5Fe alloy and its impact on mechanical properties

The high strength metastable β-Ti alloy, Ti–1Al–8V–5Fe (wt%), also referred to as Ti-185, has been successfully processed using the directed energy deposition (DED) based laser engineered net shaping (LENS) process, obviating the beta fleck problem associated with Fe micro-segregation that has been reported in conventionally processed counterparts. The large solidification range for this alloy resulted in finer scale equiaxed β grains in the as deposited condition for a range of process parameters, unlike the large columnar grains observed in case of AM of other titanium alloys such as Ti–6Al–4V. Furthermore, based on the process parameters, a homogeneous distribution of fine scale ω or α precipitates form within the β grains, which has been rationalized based on quantitative thermo-kinetic modelling of a multi-layered deposition process. Atom probe tomography results indicate early stages of β/ω compositional partitioning, leading to a higher tensile yield strength, close to 1000 MPa, as compared to the solution treated/quenched condition of conventionally processed Ti-185. Homogeneous fine scale α precipitation, with a more pronounced compositional partitioning, resulted in an exceptional yield strength exceeding 1200 MPa in the as-processed condition.

Synergistic alloying effects on nanoscale precipitation and mechanical properties of ultrahigh-strength steels strengthened by Ni3Ti, Mo-enriched, and Cr-rich co-precipitates

The synergistic effects of Mo, Ti, and Cr on nanoscale precipitation and mechanical properties of maraging stainless steels were systematically studied using high-resolution scanning transmission electron microscopy, atom probe tomography (APT), thermodynamic and first-principles calculations, and mechanical tests. Our results reveal a notable precipitation pathway involving the co-precipitation of Ni3Ti, Mo-enriched, and Cr-rich precipitates; their formations are not separated, but rather highly interacted. The APT results indicate that Mo partitions to the Ni3Ti precipitate core in the early stage of precipitation, which doubles the number density of Ni3Ti precipitates. Our calculations indicate that the Mo partitioning not only increases the chemical driving force, but also reduces the strain energy for nucleation, thereby accelerating Ni3Ti precipitation. As the precipitation proceeds, Mo atoms are rejected from the Ni3Ti precipitate core to the interface between the Ni3Ti precipitates and matrix, which leads to the heterogeneous nucleation of Mo-enriched precipitates on the outer surface of the Ni3Ti precipitates. This results in a substantial size refinement of Mo-enriched precipitates. In addition, the formation of Ni3Ti precipitates consumes Ni from the matrix, which substantially inhibits the spinodal decomposition and refines the size of Cr-rich precipitates. The cooperative strengthening of Ni3Ti, Mo-enriched, and Cr-rich co-precipitates leads to the development of new steels with a strength of 1.8 GPa; the contributions of these precipitates to the strengthening were quantitatively evaluated in terms of precipitate shearing and Orowan dislocation looping mechanisms.

Precision Thermal Treatments, Atom Probe Characterization, and Modeling to Describe the Fe-Cr Metastable Miscibility Gap

The Fe-Cr metastable miscibility gap has been studied by high-precision thermal treatments, Vickers micro-hardness (HV) measurements, and atom probe tomography (APT). Thermodynamic modeling further supplements the experimental work. The results obtained show that recent thermodynamic descriptions of the metastable miscibility gap found in literature generally overestimates the consolute temperature. We can show that the source of ambiguity in previous studies is most likely a lack of clear distinction between Cr-Cr clustering and alpha^{prime} formation. This distinction is here made by APT results, and it leads to a determined consolute temperature of 580 ± 1 °C for Fe0.50Cr0.50. The revised thermodynamic modeling of the metastable miscibility gap captures the experimental results and is consistent with the overall picture from the Fe-Cr data in the literature.Graphic

Interactions between cadmium and Al3Li precipitates: A new mechanism of accelerating dissolution and transformation of phases in Al–Li–Cu alloy

Abstract The interactions between Cd and δ′-Al3Li precipitates and their impacts were reported for the first time. Transmission electron microscopy and X-ray diffraction results suggested that a trace (0.06 at. %) addition of Cd significantly altered the microstructural evolution and the hardening behavior of high-lithium aluminum alloys. The Cd addition not only led to a lower peak-hardness and shorter peak-aged time, but also caused a sharp decrease of hardness and the dissolution of δ′ precipitates in over-aged state. Atom probe tomography results and first-principles calculations showed that Cd atoms preferentially partitioned to δ′ precipitates, occupying the Al sublattice. Such configuration was shown to be energetically unfavorable, thus would accelerate the dissolution of δ′ precipitates, leading to the decrease of hardness. The dissolution of δ′ subsequently provided ample Li atoms in the matrix, promoting the formation of T2-Al6CuLi3 phase. Eventually, spherical δ′ precipitates completely dissolved, replaced by T2 phases of larger sizes and irregular shapes.

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 of γ-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 makes γ-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 of γ-γ′ HEA.