Atom Probe Tomography Analysis Research Articles

Microstructural and mechanical properties of precipitation-strengthened Al-Mg-Zr-Sc-Er-Y-Si alloys

We study the precipitation behavior of two L12-strengthened alloys with Mg and Y additions: ultralow-Sc, Si-free, Al-1Mg-0.09Zr-0.007Sc-0.006Er-0.02Y-0.01Si, and low-Sc, Si-added, Al-1Mg-0.09Zr-0.013Sc-0.006Er-0.02Y-0.08Si (at.%), and their ambient temperature strength and high-temperature creep resistance. Scanning transmission electron microscopy analyses reveal that β-Mg2Si precipitates or their precursors (β’, β’’) form in the Si-added alloy at ∼ 200°C, which act as preferential nucleation sites for the L12-nanoprecipitates and cause partial depletion of Si solute atoms, which decelerates L12 precipitation-kinetics, resulting in a microhardness peak at the same isochronal temperature, 475°C, in both alloys. Atom-probe tomography analyses reveal that the L12-nanoprecipitates in both alloys exhibit Sc/Er/Y-rich cores and Zr-rich shells, as well as Mg segregation at the interfacial regions. The L12-nanoprecipitates in the Si-added alloy has a smaller Sc/Er/Y and higher Zr and Si concentrations. The Si-free alloy exhibits superior creep properties at 300°C, due to a larger lattice parameter mismatch of the L12-nanoprecipitates with the Al(f.c.c.) matrix provided by higher Sc/Er/Y concentrations. Both alloys exhibit slower L12 nanoprecipitation-kinetics, extremely high coarsening resistance, and similar high-temperature creep resistance compared to a recently developed Mg/Y-free low-Sc, Al-0.08Zr-0.014Sc-0.008Er-0.09Si (at.%) alloy. The extremely high coarsening resistance of the Si-added alloy is attributed to the smaller Si concentration in the Al(f.c.c.) matrix. Silicon is scavenged by the β-Mg2Si precipitates, thermodynamically stable at < 475°C, which is then not available in the matrix to accelerate the coarsening of the L12-nanoprecipitates.

CALPHAD-assisted composition and processing design of high-strength and high-conductivity copper alloy

In this work, a Cu–Cr–Ag–Mg–Sn alloy was developed assisted by a CALPHAD technique, by which a suitable chemical composition (Cu-0.5Cr-0.1Ag-0.05Mg-0.05Sn) and processing parameters were efficiently designed. The achieved alloy exhibited yield strength and conductivity values of 642 MPa and 74.2% IACS, respectively, which greatly surpassed the commercial C18150 alloy. Transmission electron microscopy observation demonstrated that such a good comprehensive performance was attributed to numerous nano-scale Cr precipitates that possessed a strong coherent relationship with the Cu matrix. Further microstructure characterizations demonstrated that these beneficial Cr precipitates were associated with subgrain boundaries in the alloy introduced by a cold-rolling→short-time secondary solid solution. Atom probe tomography analysis demonstrated that Sn atoms that readily attached to the surfaces of Cr precipitates.

Microstructural Evolutions and Strengthening Mechanism according to the Aging Temperatures of a High Si Cast Aluminum Alloy

A356 cast aluminum alloy contains 7 at.% Si and 0.3 at.% Mg, producing an approximately 50% eutectic microstructure. This high Si content and various casting conditions play a significant role in strengthening A356 alloy, by controlling the eutectic morphology and precipitates of other intermetallic compounds. Understanding how Si-related precipitates and clusters are soluble in the α-matrix is necessary to provide high strength and fatigue resistance to A356 alloys. The aging heat-treatment temperature in the A356 alloy most likely promotes the formation of these precipitates and clusters. The A356 samples were differently aged at temperatures of 110 oC and 130 oC for 2 h, and were labeled 110A, and 130A, respectively. 110A was found to have improved mechanical properties, such as high strength and elongation, compared to 130A, which may be attributed to the formation of secondary phases in the α-phase matrix. Scanning and transmission electron microscopy and atom probe tomography analyses demonstrated Ti2Si precipitation and various-sized cluster formations in 110A. In contrast, 130A had fewer clusters than 110A. Therefore, different aging heat-treatment temperatures relate to a change in the behavior of atoms, affecting the mechanical properties.

The interfaces of Ag-rich and Cu-rich AgCu phases boost oxygen reduction activity

In this study, we demonstrated that the interface between Ag-rich and Cu-rich AgCu phases enhances the oxygen reduction activity of the model alloys. Ag-rich and Cu-rich AgCu alloys (86.8 and 4.5 at. % of Ag, respectively) displayed the homogeneous microstructure representing solid solutions while the AgCu with eutectic composition (59.7 at. % of Ag) showed a typical lamellar structure of Ag-rich and Cu-rich AgCu phases. The eutectic AgCu alloy exhibited better oxygen reaction activity than Ag-rich AgCu alloy despite less phase fraction of Ag-rich AgCu phase. After the activation process, the presence of a CuO layer at the interface of the Ag-rich and Cu-rich AgCu phases was discovered by atom probe tomography analysis. Density functional theory calculations further revealed that the preferential adsorption of OH near the interface between CuO and Ag-rich AgCu lowers the binding strength of OH in the Ag-rich AgCu phase. As a result, the oxygen reduction activity was enhanced by facilitating the most sluggish OH desorption step. These findings provide new insights into the role of phase boundaries in catalytic activity and guidelines for the design of novel electrocatalysts.

Effect of dislocation density on competitive segregation of solute atoms to dislocations

We report on both experimental and modeling of carbon and nitrogen segregation to dislocations in C–Mn ferritic steels. A model based on McLean theory is established to describe the competitive segregation of carbon and nitrogen in the Cottrell atmospheres around dislocations, considering the effect of dislocation density. It is expected from the model that a high dislocation density strongly depletes interstitial atoms from the matrix, resulting in a low excess quantity of atoms on dislocations. Atom probe tomography analyses performed on two different C–Mn weld steels are in accordance with the model: carbon excess is tenfold higher in comparison to nitrogen excess in the deposited metal. However, after tensile test up to failure, this trend is no longer valid and nitrogen segregates preferentially than carbon. This suggests that plastic strain can modify the material properties by changing the nature and quantity of segregated atoms around dislocations.

Understanding the nanoscale chemistry of as-received and fast neutron irradiated Nb3Sn RRP® wires using atom probe tomography

Atom probe tomography (APT) has been used to study the effect of fast neutron irradiation on the local chemistry of Nb3Sn samples. Two RRP® wires doped with 2 at% Ti were analysed, one in the as-received condition and the other irradiated to a neutron fluence (E > 0.1 MeV) of 2.82 × 1022m−2 in the TRIGA-II reactor. The irradiated sample had a reduced T c, an increase in F p, a shift in the peak of the F p curve suggesting the introduction of secondary point pinning, and an increase in the estimated scaling field B*. APT analysis has shown that polycrystalline Nb3Sn has three distinct regions of composition, near stoichiometry Nb3Sn (low Nb), regions with a higher Nb content than expected in equilibrium Nb3Sn (high Nb) and grain boundaries. The summed composition of these three regions lies within the Nb3Sn phase for both the as-received and irradiated samples. The distinct regions of high Nb Nb3Sn demonstrate incomplete diffusion in the as-received sample, and the reduction in volume of these high Nb regions after irradiation implies significant radiation induced diffusion has occurred. The presence of other features in the atomic-scale chemistry, such as the extent of Cu segregation at grain boundaries, three types of dislocation array, and unreacted Nb nanoparticles, are compared between samples.

Hard Magnetic Properties and the Features of Nanostructure of High-Temperature Sm-Co-Fe-Cu-Zr Magnet with Abnormal Temperature Dependence of Coercivity

This paper presents methods and approaches that can be used for production of Sm-Co-Fe-Cu-Zr permanent magnets with working temperatures of up to 550 °C. It is shown that the content of Sm, Cu, and Fe significantly affects the coercivity (Hc) value at high operating temperatures. A decrease in the content of Fe, which replaces Co, and an increase in the content of Sm in Sm-Co-Fe-Cu-Zr alloys lead to a decrease in Hc value at room temperature, but significantly increase Hc at temperatures of about 500 °C. Increasing the Cu concentration enhances the Hc values at all operating temperatures. From analysis of the dependence of temperature coefficients of the coercivity on the concentrations of various constituent elements in this alloy, the optimum chemical composition that qualifies for high-temperature permanent magnet (HTPM) application were determined. 3D atom probe tomography analysis shows that the nanostructure of the HTPM is characterized by the formation of Sm2(Co,Fe)17 (2:17) cells relatively smaller in size along with the slightly thickened Sm(Co,Cu)5 (1:5) boundary phase compared to those of the high-energy permanent magnet compositions. An inhomogeneous distribution of Cu was also noticed in the 1:5 phase. At the boundary between 1:5 and 2:17 phases, an interface with lowered anisotropy constants has developed, which could be the reason for the observed high coercivity values.

Role of Ag segregation on microscale strengthening and slip transmission in an asymmetric Σ5 copper grain boundary

Micropillar compression was used to investigate whether Ag segregation to an asymmetric Σ5[001] grain boundary will lead to measurable strength differences compared to the pure copper bicrystal. Ag segregation was accomplished by deposition and subsequent annealing of an Ag thin-film applied on the surface of the Cu bicrystal. Atom probe tomography analysis indicated Ag segregation at the grain boundary with a peak concentration of 2.3 at.%. While the pristine Σ5 grain boundary shows a yield strength of 288 ± 18 MPa when compressing 1 µm diameter pillars along 〈001〉, micropillars containing an Ag-segregated Σ5 grain boundary demonstrated an increased yield strength of 318 ± 17 MPa. In addition, post-deformation electron microscopy was carried out to examine the active slip systems and slip transmission across Ag-free and Ag-containing bicrystals. The results are compared to reference measurements of the adjacent single crystal grains. The 1 µm pillar diameter promoted deformation governed by dislocation-grain boundary interactions for the bicrystalline pillars. This is the first time that changes in flow stress associated with grain boundary segregation have been quantified locally without interference from other mechanisms such as solid solution strengthening, formation of precipitates or changes in stacking fault energy. The results clearly indicate that purely geometrical models for slip transmission are not sufficient as the local atomic structure and composition influence dislocation transmission through grain boundaries.

On the role of Ag additions on the initial solute hardening and competitive precipitation of Al-Cu-Mg alloys

The initial solute clustering is known to significantly affect the subsequent precipitation of various strengthening precipitates that determine the mechanical properties of Al-Cu-Mg-Ag alloys. In this work, the influence of Ag variations on the initial cluster hardening and microstructure evolution of Al-Cu-Mg-Ag alloys is investigated by quantitative transmission electron microscopy (TEM), electron backscatter diffraction (EBSD) and atom probe tomography (APT) analysis. Our statistical data and calculation results indicate that the overwhelming Mg-Ag co-clustering by increasing Ag from 0.41 to 0.97 indeed enables a higher nucleation rate of Ω phase and contributes to a denser Ω precipitation generating a stronger age-hardening response at 180 ℃. Analysis of the contribution of individual precipitates to the yield strength highlights the positive role of increasing Ag on improving the cluster hardening capacity of Mg-Ag co-clusters by increasing its order hardening level. Compared with Cu-Mg co-clusters and GP zones, Mg-Ag co-clusters exhibit a greatest strengthening contribution to yield strength of the material as the bulk Ag content increases. Except for GP zones, Mg-Ag and Cu-Mg co-clusters prefer to exhibit a strong order hardening rather than modulus hardening in initially aged alloys. In addition, the average grain sizes, as well as the proportions of low angle grain boundaries (LAGBs) and high angle grain boundaries (HAGBs), reveal no distinct differences in both alloys, suggesting the independence of grain structure on the bulk Ag content.

Defect microstructure of a Ti64 alloy under TI2+ ion irradiation. Influence of dose and temperature

The present paper focuses on the microstructural changes in Ti64 alloy after irradiation. The titanium alloys allow a significant decrease in activation and mass with a similar mechanical strength and a similar corrosion resistance than 304 stainless steels, widely used for core internal structures of Pressurized Water Reactors. The use of titanium alloys could be therefore a significant improvement for reactor exploitation and decommissioning. The nature, size and density of the radiation-induced defects are likely to be important parameters governing the in-reactor behavior of the material. The aim of the study was to devise new achievements in thin foil preparation and Transmission Electron Microscopy (TEM) imaging in order to accurately identify and quantify the Ti64 irradiation defects, at temperatures representative of irradiation temperatures in a reactor. The method to image and count the precipitates was developed with samples irradiated by ions at the temperature of 600 °C. Indeed, the microstructure of the sample irradiated at 600 °C was characterized by a low density of tangled dislocations, and big precipitates, that were easily evidenced. Then the process was applied in samples irradiated with ions at 430 °C and 300 °C characterized by a microstructure of tangled dislocations, loops and small precipitates. In 430 °C irradiation condition, the measurement in dark field with a B∼[11–23]α orientation and with a high magnification micrograph provided similar results than Atom Probe Tomography (APT) analyses. In irradiation condition at 300 °C, the precipitates were imaged by TEM as soon as the dose of 1 dpa. However they were not observed at the dose of 0.4 dpa. The precipitates were so small and their density was so high that the measurement by TEM was very imprecise and APT analyses were needed. The results of this microstructural analysis allowed a discussion on the nucleation mechanism of the irradiation defects in Ti64 alloy.

Testing the Influence of Laser Pulse Energy and Rate in the Atom Probe Tomography Analysis of Minerals.

The use of atom probe tomography (APT) for mineral analysis is contributing to fundamental studies in Earth Sciences. Meanwhile, the need for standardization of this technique is becoming evident. Pending the use of mineral standards, the optimization of analysis parameters is needed to facilitate the study of different mineral groups in terms of data collection and quality. The laser pulse rate and energy are variables that highly affect the atom evaporation process occurring during APT analysis, and their testing is important to forecast mineral behavior and obtain the best possible data. In this study, five minerals representative of major groups (albite, As-pyrite, barite, olivine, and monazite) were analyzed over a range of laser pulse energies (10-50 pJ) and rates (100-250 kHz) to assess output parameter quality and evaluate compositional estimate stoichiometry. Among the studied minerals, As-pyrite, with the higher thermal conductivity and lower band gap, was the most affected by the laser pulse variation. Chemical composition estimates equal or close to the general chemical formula were achieved for monazite and As-pyrite. The analysis of multihit events has proved to be the best strategy to verify the efficacy of the evaporation process and to evaluate the best laser pulse setting for minerals.

Scanning electron microscopy and atom probe tomography characterization of laser powder bed fusion precipitation strengthening nickel-based superalloy

Atom probe tomography (APT) was utilized to supplement scanning electron microscopy (SEM) characterization of a precipitation strengthening nickel-based superalloy, Alloy 247LC, processed by laser powder bed fusion (L-PBF). It was observed that the material in the as-built condition had a relatively high strength. Using both SEM and APT, it was concluded that the high strength was not attributed to the typical precipitation strengthening effect of γ’. In the absence of γ’ it could be reasonably inferred that the numerous black dots observed in the cells/grains with SEM were dislocations and as such should be contributing significantly to the strengthening. Thus, the current investigation demonstrated that relatively high strengthening can be attained in L-PBF even in the absence of precipitated γ’. Even though γ’ was not precipitated, the APT analysis displayed a nanometer scale partitioning of Cr that could be contributing to the strengthening. After heat-treatment, γ’ was precipitated and it demonstrated the expected high strengthening behavior. Al, Ta and Ti partitioned to γ’. The strong partitioning of Ta in γ’ is indicative that the element, together with Al and Ti, was contributing to the strain-age cracking occurring during heat-treatment. Cr, Mo and Co partitioned to the matrix γ phase. Hf, Ta, Ti and W were found in the carbides corroborating previous reports that they are MC.

Focused Ion Beam induced hydride formation does not affect Fe, Ni, Cr-clusters in irradiated Zircaloy-2

Room temperature focused ion beam (FIB) milling is known to potentially promote the formation of hydrides in zirconium and its alloys. We used atom probe tomography (APT) to determine the composition of irradiated and as-produced Zircaloy-2 fuel cladding. We consistently found ∼ 50 at% hydrogen in all room temperature FIB-milled specimens run in voltage pulsing APT measurements. Crystallographic analysis of APT data however showed slightly better agreement with δ-hydride (ZrH2, FCC, ∼ 60–66.7 at% H) than γ-hydride (ZrH, FCT, ∼ 50 at% H). Electron energy loos spectroscopy (EELS) measurements prior to APT analyses confirmed the presence of δ-hydride. Hence, APT gives a systematic underestimation of hydrogen for Zr-hydride. Milling at cryogenic temperatures was found to not cause such hydride formation. However, we did not find significant differences in the clusters formed by segregation of the alloying elements Fe, Cr and Ni to irradiation induced a-loops whether the material was identified as α-Zr or hydride. Therefore, analyzing irradiation-induced redistribution of alloying elements in Zr fuel cladding using APT does not rely on FIB preparation at cryogenic temperatures. However, in conjunction with voltage pulsing APT cryo-FIB can be worthwhile if one aims at investigating hydrogen distribution or hydrides.

In Situ Sputtering From the Micromanipulator to Enable Cryogenic Preparation of Specimens for Atom Probe Tomography by Focused-Ion Beam.

Workflows have been developed in the past decade to enable atom probe tomography analysis at cryogenic temperatures. The inability to control the local deposition of the metallic precursor from the gas-injection system (GIS) at cryogenic temperatures makes the preparation of site-specific specimens by using lift-out extremely challenging in the focused-ion beam. Schreiber et al. exploited redeposition to weld the lifted-out sample to a support. Here, we build on their approach to attach the region-of-interest and additionally strengthen the interface with locally sputtered metal from the micromanipulator. Following standard focused-ion beam annular milling, we demonstrate atom probe analysis of Si in both laser pulsing and voltage mode, with comparable analytical performance as a presharpened microtip coupon. Our welding approach is versatile, as various metals could be used for sputtering, and allows similar flexibility as the GIS in principle.

Enhanced helium ion irradiation tolerance in a Fe-Co-Ni-Cr-Al-Ti high-entropy alloy with L12 nanoparticles

• Helium bubbles and large stacking faulted loops are observed as the dominant structural damage in the He ion irradiated HEA reinforced by L1 2 nanoparticles. • The L1 2 nanoparticles provide numerous interfaces for He entrapment and damage elimination, which suppresses He bubble growth. • A correlative TEM/APT characterization reveals that the RIS around He bubbles is dominated by the inverse Kirkendall mechanism. • Irradiation-induced dissolution and re-precipitation of the L1 2 nanoparticles can retain the main microstructure of the L1 2 -strengthened HEA and provide a sustainable irradiation resistance. L1 2 -strengthened high entropy alloys (HEAs) with excellent room and high-temperature mechanical properties have been proposed as promising candidates as structural materials for advanced nuclear systems. However, knowledge about their radiation response is fairly limited. In the present work, a novel HEA with a high density of L1 2 nanoparticles was irradiated with He ion at 500°C. Transmission electron microscope (TEM) and atom probe tomography (APT) were employed to study the evolution of microstructural stability and radiation-induced segregation. Similar to the single-phase FeCoNiCr HEA, the main microstructural features were numerous large faulted dislocation loops and helium bubbles. While the irradiation resistance of the present L1 2 -strengthened HEA is much improved in terms of reduced bubble size, which could be attributed to the considerable He trapping efficiency of the coherent precipitate/matrix interface and the enhanced capability of the interface for damage elimination when the matrix channel width is narrow. APT analysis revealed that an inverse Kirkendall mechanism-dominated radiation-induced segregation (RIS) occurs around bubbles, where a significant Co enrichment and Ni depletion can be clearly observed. In addition, the competing dynamics of ballistic mixing and elemental clustering that raised from the irradiation-enhanced diffusion in a highly supersaturated matrix, along with the low precipitation nucleation barrier due to the small lattice misfit, lead to a dynamical precipitation dissolution and re-precipitation appears under irradiation. Such a promising phenomenon is expected to promote a potential self-healing effect and could in turn provide a sustainable irradiation tolerance over the operational lifetime of a reactor.

Sc/Zr ratio-dependent mechanisms of strength evolution and microstructural thermal stability of multi-scale hetero-structured Al–Mg–Sc–Zr alloys

• A tensile strength of ∼605 MPa and ∼10% elongation were obtained in Al–7Mg–0.3Sc–0.1Zr. • The strength-ductility synergy owes to a multi-scale heterogeneous microstructure. • While Al–7Mg–0.1Sc–0.3Zr had higher thermal stability at ≥ 400 °C than high-Sc/Zr ratio alloys. • High-content Zr atoms are helpful in enhancing precipitation and coarsening resistance of Al 3 (Sc,Zr). Microstructure and its thermal stability are critical in the development of high-performance Al–Mg alloys. Here, we attempt to tailor Al 3 (Sc,Zr) precipitates and thus microstructure characteristics to manipulate mechanical properties and microstructural stability of Al–7Mg alloys fabricated by hot extrusion combined with two-pass hard-plate rolling via changing Sc/Zr ratio. Increasing Sc/Zr ratio leads to improved strength without any loss of ductility. A strength-ductility synergy, i.e. yield strength of ∼548 MPa and ultimate tensile strength of ∼605 MPa with an impressive ductility of ∼10% elongation was achieved in the Al–7Mg–0.3Sc–0.1Zr alloy. The good strength-ductility synergy is ascribed to the multi-scale heterogeneous microstructure promoted by the high Sc/Zr ratio, i.e. a bimodal grain structure, profuse low angle grain boundaries, dispersed nano-sized Al 3 (Sc,Zr) precipitates coexisting with intragranular Mg-Zr co-clusters segregated at dislocations. Upon thermal exposure, the Al–7Mg–0.3Sc–0.1Zr alloy maintained higher hardness at below 250 °C, whereas Al–7Mg–0.2Sc–0.2Zr and Al–7Mg–0.1Sc–0.3Zr alloys exhibited higher hardness in moderate- and high-temperature range of 250-350 °C and ≥ 400 °C, respectively. Atom-probe tomography analysis illustrates that slow-diffusing Zr atoms enhance Al 3 (Sc,Zr) coarsening resistance through forming a higher-content Zr-enriched protective shell around a Sc-enriched core in Al–7Mg–0.1Sc–0.3Zr. Meanwhile, the high Zr content promotes concurrent Al 3 (Sc,Zr) precipitation during thermal exposure at high temperatures. The improved microstructural thermal stability in Al–7Mg–0.1Sc–0.3Zr alloy is further discussed in terms of the recrystallization resistance and grain growth behavior. The present study reveals the feasibility for designing high-strength and thermally stable hetero-structured Al–Mg–Sc–Zr alloys via tailoring Sc/Zr ratios for different application temperature ranges.

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).

Effect of selective atomic clustering, surface texture and grain boundary constitution on the corrosion behaviour of electrodeposited Sn-xBi coatings

Sn-xBi (x = 4, 7, 10, 14, and 17 wt%) coatings were electrodeposited onto mild steel to elucidate their corrosion behaviour. The B14 (Sn-14 wt% Bi) and B7 (Sn-7 wt% Bi) coating exhibited the highest (Polarization resistance (Rp) = 12,810.2 Ω·cm2) and least corrosion resistance (Rp = 2843.2 Ω·cm2), respectively. B14 coating exhibited the lower energy near (301) plane orientation against B7 coating which showed higher energy (321) and (111) plane orientation along the exposed surface. The galvanic coupling effect alleviated in B14 coating due to Sn nanoclusters into Bi grains as revealed by the atom probe tomography analysis.

Revealing complex precipitation behavior of multicomponent Al83Zn5Cu5Mg5Li2 alloy

We investigated the microstructural evolution and precipitation behavior of a multicomponent Al83Zn5Cu5Mg5Li2 alloy using transmission electron microscopy (TEM) and atom-probe tomography (APT) during solution treatment, cooling, and natural aging. The as-cast alloy consisted of high volume fractions of V-Al5Cu6Mg2 and η-Mg(Zn,Cu,Al) phases, which were partially dissolved during solution treatment at 450 °C. Slow furnace cooling (FC) after solution treatment resulted in the formation of polygonal V-phase, elongated η-phase, and platelet Y-phase precipitates. The Y-phase precipitates consisted of one Mg-rich central layer and three adjacent Zn/Cu-rich layers, both lying on the {111}Al planes. Instead of Y-phase precipitates, air cooling (AC) induced the formation of S- and Zn-phase precipitates with well-developed orientation relationships. No precipitation occurred during water quenching (WQ). The microhardness increased significantly during the natural aging of the AC and WQ alloys, whereas no hardening was observed in the FC alloy. TEM and APT analyses demonstrated that the natural aging induced the formation of fine solute clusters with a chemical composition of 52.3Al–21.3Li–18.3Zn–4.9Mg–3.1Cu (at%), contributing to the natural age-hardening of AC and FC alloys. The solute clusters were fully coherent with the Al matrix and exhibited an interfacial region rich in Zn and two central regions rich in Li and Zn.

Core-shell structure and stability of nanocrystalline precipitates in Fe83.3Si4B8P4Cu0.7 soft magnetic amorphous alloy

The core-shell nanostructure of α-Fe precipitates was investigated in a recently developed high-performance Fe83.3Si4B8P4Cu0.7 nanocrystalline soft magnetic alloy utilizing small-angle neutron scattering (SANS) and atom probe tomography (APT). SANS measurements under a saturating magnetic field provided evidence of core-shell structures at the nanoscale. Moreover, the core-shell structure was directly visualized from APT analysis, where P-enriched shells were found around Fe-enriched cores due to the large negative heat of mixing between Fe and P elements and the slow diffusion of P atoms. The core-shell nanostructures induced by nanoscale partitioning are effective in stabilizing the residual amorphous matrix and suppressing the crystal growth. Our findings provide insight into the microstructure of Fe-based amorphous alloys and have direct implications in alloy design for the improvement of the soft magnetic properties.