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This study is aimed at elucidating the mechanistic impact of structural evolution of bulk and grain boundary precipitates on the strength-ductility balance in prolonged artificial aged Al-Zn-Mg-Cu alloys. Combining aberration-corrected scanning transmission electron microscopy and first-principles calculations, the transition of bulk η-phase precipitates, which originated at Zn-terminated interfaces under tensile lattice strain field, was fundamentally explored. Intriguingly, as ageing progressed, significant partitioning of solute-Cu along the interfaces initiated a stacking transition in η-phase from hexagonal C14 to cubic C15 via di-hexagonal C36 Laves phase structures, leading to a reduction in lattice misfit strengthening. The driving mechanism behind this Laves phase transformation was found to link to interfacial lattice strain and Cu solute atom partitioning. Meanwhile, the aspect ratio of grain boundary S-phase precipitates that sporadically developed from the interconnected clusters present along the grain boundaries was progressively increased with ageing time, contributing to improved mechanical stability of grain boundary precipitates. Prolonged ageing led to a small decrease in tensile strength from 567 MPa to 526 MPa and minor increase in elongation from ∼11 % to ∼13 %. The new knowledge derived from the present study has the potential to transform the futuristic design and processing of next generation of aluminum alloys through tailoring of tensile strength and ductility, where the approach will be different from the conventional ageing process.

The 7000 series (Al–Zn–Mg) Al-alloys are renowned for their exceptional “lightweight-high strength” properties, primarily owing to nano-sized meta-stable η′ precipitate as the main strengthening phase. However, these η′ precipitates are susceptible to transforming into the stable η2 phase at elevated temperatures, resulting in a decline in strength. To mitigate this, Cu-segregation has been long proposed as a promising method for stabilizing these meta-stable precipitates. However, the mechanisms involved have not been fully understood. In this study, we conducted a thorough investigation on the structural and energetic changes between Cu-free and Cu-segregated η' and η2 precipitates. Utilizing DFT calculations and HAADF-STEM characterizations, we uncovered a distinctive alternating Cu-segregation pattern. This pattern induces significant structural modifications within the bulk of η' precipitates and at the precipitate-matrix interface, markedly enhancing their interfacial and bulk energetics. We believe these modifications are the key factor in improving precipitate stability. Based on our findings, the underlying mechanism of solute segregation was elucidated and a novel strategy for optimal design of Al–Zn–Mg alloys was proposed.

The microstructures and mechanical properties in the peak ageing condition, warm forming process, and paint baking process of two Al–Zn–Mg–Cu aluminum alloys, which possessed the same base compositions but different levels of Mn (0 and 0.24 wt%), have been investigated. The results show that Mn addition can hardly refine the grain size (around 20 μm) in AA7075 aluminum alloy in warm forming and paint baking processes. Mn-free and Mn-containing alloys subjected to all three processes possess nano-precipitates of nearly the same size, indicating that Mn addition does not influence the nano-precipitate evolution in the aluminum matrix. Energy-dispersive X-ray spectroscopy (EDS) mapping results of dispersoids show that dispersoids in Mn-bearing specimens are enriched in Mn and depleted in Zn and Mg. The Zn and Mg expelled from dispersoids in Mn-containing specimens into the aluminum matrix presumably formed Zn- and Mg-rich zones around the dispersoids, which improved the precipitate transformation reation from GPII to η′ phase at the boundaries between aluminum matrix and dispersoids. Although the solid solution strengthening effect triggered by Mn addition might be less effective because of the formation of dispersoids, the slight enhancement of ultimate tensile strength (UTS) in Mn-containing specimens rather than in Mn-free specimens in all states can be attributed to the formation of nano-precipitates around the dispersoids.

PVD-deposited Cu3N has been demonstrated for Cu-Cu bonding as a low-cost passivation material. Cu3N exhibits stability at room temperature but undergoes decomposition upon heating, making it an attractive candidate for Cu bonding passivation. XRD analysis reveals that the decomposition of PVD-deposited Cu3N primarily occurs within the temperature range of 200 $^{\circ }\text{C}\sim 250^{\circ }\text{C}$ . Cu3N-Cu3N bonding, relying on the decomposition reaction, is successfully demonstrated, and TEM analysis confirms the presence of Cu grains at the bonding interface. Moreover, Cu bonding using Cu3N passivation has been demonstrated as well. The bonding capability at low temperatures is found to be influenced by the thin film properties of Cu3N. For wafer level bonding, the $N_{2}$ flow rate during Cu3N deposition requires careful adjustment to balance the anti-oxidation capability and the $N_{2}$ outgassing phenomenon. For chip level bonding, the $N_{2}$ outgassing phenomenon poses no significant issue. By demonstrating the potential of Cu3N as a passivation material, this study suggests a promising method for achieving lower temperature requirements in Cu bonding through the adjustment of Cu3N thin film properties.

Sn is a well-known grain boundary segregation element that improves the machinability of steel. Sn has been considered as a replacement for Pb in super-free-machining steels. The effect of Sn on the microstructure of the Fe-0.05C-0.03Si-1.28Mn-0.36S-0.05P base composition containing 0.0002 (without addition), 0.062, 0.12, and 0.18 Sn (wt.%) low-carbon free-machining steels was investigated though thermodynamic calculations, optical microscopy, scanning electron microscopy, transmission electron microscopy, electron probe microanalysis, and high-temperature laser scanning confocal microscopy. The microstructure of the free-machining steels was composed of α-ferrite, pearlite, and manganese sulfide. Sn significantly decreased the pearlite content of the steels. Most of the Sn was dissolved in the matrix, and the remainder was dissolved in manganese sulfide. No FeSn intermetallic compound precipitation was observed through transmission electron microscopy, but a significant strengthening effect was observed in α-ferrite. Sn had little effect on the solidification behavior or sulfide precipitation behavior of the steels when its content was lower than 0.2 wt.%. Similar to Al and Si, Sn is a ferrite-stabilizing element that expands both the δ-ferrite and α-ferrite phase regions, promotes α-ferrite formation, and inhibits carbide precipitation. Sn segregates at the interface, decreasing the interfacial energy and promoting the Widmanstätten ferrite phase transformation. A much lower cooling rate than that of Sn-free free-machining steels should be adopted to restrain the formation of Widmanstätten ferrite after hot rolling. Sn addition greatly improved the machinability of the experimental steels.

Two Al-Cu-Mg(-Ag) aluminium alloys, having the same base composition of Al-5.11Cu-0.96 Mg-0.58Mn-0.12Zr (wt.%) but different Ag contents, 0 and 0.7 Ag (wt.%), and accordingly denoted 0A and 7A samples, were aged at 185 °C. In early-ageing, the formation of disk-like Mg-Cu and Mg-Ag nano(co–)clusters can be quantitatively examined by Small angle X-ray scattering (SAXS), respectively. In the prolonged ageing of the 0A samples, the thickening in the disk-like θ-precipitates leads to a lower aspect ratio. For S-precipitates, the change in the total surface area and the single-particle surface area indicates that the coalescence mechanism is prevalent from peak-ageing to over-ageing, and then after over-ageing, the precipitate coarsening dominates. The coarsening in S and θ precipitates presumably leads to a lower mechanical strength. Alternatively, in the aged 7A sample, the SAXS technique in conjunction with transmission electron microscopy (TEM) quantitatively reveals Ω-precipitates possessing a nearly constant thickness but an increase in diameter with ageing time. Additionally, the stable behaviors of the total surface and size of S-precipitates illuminate that the coalescence/coarsening mechanisms are significantly slowed down. The concurrently-existing precipitation kinetics of Ω and S have provided an in-depth understanding of the higher strength of the aged 7A sample.

Abstract Electron work function (EWF) has demonstrated its great promise in materials analysis and design, particularly for single-phase materials, e.g., solute selection for optimal solid-solution strengthening. Such promise is attributed to the correlation of EWF with the atomic bonding and stability, which largely determines material properties. However, engineering materials generally consist of multiple phases. Whether or not the overall EWF of a complex multi-phase material can reflect its properties is unclear. Through investigation on the relationships among EWF, microstructure, mechanical and electrochemical properties of low-carbon steel samples with two-level microstructural inhomogeneity, we demonstrate that the overall EWF does carry the information on integrated electron behavior and overall properties of multiphase alloys. This study makes it achievable to develop “electronic metallurgy”—an electronic based novel alternative methodology for materials design.

A novel dual-phase PHSS consisting of lath martensite plus allotriomorphic δ ferrite (ALF) with nanoprecipitates was characterized by high-resolution field emission transmission electron microscopy for quenched, solid-solution-treated, and aged stainless steel. The effects of aging at various durations prior to H2O or liquid N2 quenching were investigated. Cu-rich nanoprecipitates evolve from body-centered cubic clusters to 9R Cu under quenching to 3R Cu and subsequently to face-centered cubic ε-Cu at various aging durations. Maximum hardness was observed after aging at 600 °C for 1 h. However, after this aging, both reversed austenite and Cu-rich nanoprecipitates coexisted in the martensite matrix. The segregation and diffusion of austenite-stabilizing elements promoted the nucleation of reversed austenite.

The creep ageing process can have a significant influence on the mechanical properties of aluminum alloys. In the present work, microstructural analysis and mechanical testing were implemented to characterize the age hardening effect and microstructure evolution, and to investigate how the stress applied under creep ageing conditions can affect a material’s microstructure. The curves depicting yield strength in relation to creep ageing time suggested that the stress applied in creep ageing can result in a reduction of the strength of aluminum alloy 7050; the yield strength decreases with increasing applied stress. Microstructural analysis by transmission electron microscopy (TEM) revealed that by applying stress, the growth and coarsening rate of the Guinier-Preston (GP) zones and η′ precipitates can be sped up. Even after pure/creep ageing for 8 h, there are still some GP zones in the aluminum matrix, demonstrating that the GP zones’ nucleation is a continuous process.

In the present study, after solid solution treatment, four different artificial aging treatments (100, 120, 140 and 160 °C) were performed on Al-5.98Zn-2.86Mg-1.61Cu (wt.%) alloy, denoted as 7075-LCu, and Al-5.91Zn-2.83Mg-1.98Cu (wt.%) alloy, denoted as 7075-HCu. Peak aging conditions were determined for each aging temperature at various hold time intervals of up to 24 h. It was found that both alloys possessed the optimal strengths after artificial aging at 120 °C for 24 h. Under this condition, the ultimate tensile strengths (UTSs) were 618 MPa (7075-LCu) and 623 MPa (7075-HCu), respectively. Moreover, a method was used to calculate the average sizes and number density of the major strengthening precipitates, η′, under peak aging conditions in these two alloys from transmission electron microscopy (TEM) images and electron energy loss spectroscopy (EELS). The above results indicated that for the 7075-LCu and 7075-HCu samples with the optimal UTS strengths, the former possessed an average thickness of 2.15 nm, and a number density of 3.27 × 1017 cm−3; the latter, 2.04 nm and 3.52 × 1017 cm−3.

The effects of natural ageing treatment prior to artificial ageing treatment on the microstructures and mechanical properties of AA7075 Al-5.7Zn-2.6Mg-1.5Cu-0.18Cr-0.08Mn-0.05Si-0.17Fe (wt.%) aluminum alloy have been investigated. The hardness of solution-treated samples (91.0 HV) profoundly increased to 146.8 HV after 7 days of natural ageing. The purpose of the present work was to examine the kinetic hardening evolution in subsequent artificial ageing treatments of samples naturally aged for 7 days and their counterparts without natural ageing. The former were labelled as NA-7d samples, and the latter, NA-0d samples. After artificial ageing at 120 °C for 2 h, the hardness of NA-0d samples increased rapidly to 148.2 HV, which was approximately the same as that of the specimens with natural ageing for 7 days, compensating for the prior state of lower hardness without natural ageing. After being treated at 120 °C for 16 h, the ultimate tensile strength (UTS) and yield strength (YS) of NA-7d reached the highest value, respectively, 601 MPa and 539 MPa, followed by a slight decrement of UTS when aged to 24 h. On the other hand, NA-0d specimens aged at 120 °C for 16 and 24 h showed nearly the same UTS (598 MPa); the former possessed YS of 538 MPa, and the latter, 545 MPa. The results presumably reveal that the peak ageing condition for NA-0d samples can be achieved under 24 h ageing at 120 °C. Under the same treatment at 120 °C for 24 h, the size of η’ phase in NA-7d sample (with a length of 4.96 nm) coarsened and grew larger than that in NA-0d sample (with a length of 3.46 nm). In addition, some η’ phase in the NA-7d sample was found to be transformed into the η2 phase. The results indicated that the naturally aged specimens (NA-7d) reached the peak ageing condition earlier, but did not significantly enhance the UTS in AA7075 aluminum alloy, as compared to the samples without prior natural ageing (NA-0d).

Two steels with a base composition of Fe-0.2C-0.8Mn-1.2Cr (wt%) but with different niobium (Nb) contents (0.02 and 0.03 wt%) were employed to study the effect of precipitate evolution on the softening resistance in the austenite region under elevated temperature deformation. The thermomechanical procedure was executed by a deformation-dilatometer and involved double deformation processes with 25% strain at a 0.25 s−1 strain rate at 900, 925, 950, and 1000 °C. The softening ratios, reflecting the competition between recrystallization and precipitation, were evaluated. The results indicated that both steels showed better softening resistance at 900 °C than at other temperatures. However, the softening ratio of 0.03 wt% Nb-containing steel (Steel 3N) rose after 100 s at 900 °C, while 0.02 wt% Nb-containing steel (Steel 2N) maintained a low softening ratio within 300 s at 900 °C, indicating that Steel 3N was relatively non-durable. A microstructural characterization showed that in the Steel 3N sample deformed at 900 °C, recrystallization occurred more strongly than for Steel 2N after a 1000 s holding time. A follow-up analysis then showed that Steel 3N treated at 900 °C revealed a faster coarsening of the carbides than Steel 2N even in the early stage of precipitation, evidencing that Steel 2N exhibited a lower softening resistance at 900 °C.

Cs-corrected high-angle-annular-dark-field scanning-transmission-electron microscopy (Cs-corrected HAADF-STEM) was employed to examine the phases in AlCuLi alloy (AA2050), including GP(T1), GP(θ″) and GPB zones with their subsequent nanometer-sized products, T1 (Al2CuLi), θ′ (Al2Cu), and S (Al2CuMg) precipitates, respectively. Under the peak-aging condition, some solute-atom enriched clusters could still be found, and the newly-formed nucleus of GP(θ″) with a mono-layer {100} plane of Cu atoms occurred at the adjacent area of the joint between θ′ and S precipitates or the edge of an individual S precipitate. The transition of a single Cu-layer GP(θ″) → θ′ was presumed to be transformation via in-situ nucleation. The developing GP(θ″) zones and θ′ precipitates were easily subjected to soft impingement. However, hard impingement between two variants of θ′ presumably occurred, wherein one θ′ variant precipitate was blocked out by the other θ′ variant. As for the creep-ageing forming (CAF) treated sample, some precipitates of T1 and θ′ were found to have the cutting characteristic on specific ledges. Keywords: Cs-corrected HAADF-STEM, AlCuLi (AA2050) aluminium alloy, GP(θ″) zones, T1 precipitates, θ′ Precipitates, S precipitates

Laser welding, which is known for its precision and high welding rate, offers a high-cooling-rate processing environment. In this study, laser welding was applied to interstitial-free steel plates with phosphorous additions of 0.002 and 0.009 wt%. After laser welding was performed and the weldment was cooled by protective gas, massive ferrite was produced. The base metal, heat affected zone, and weld region were observed and compared by optical microscopy (OM). It was found that increasing the phosphorous content led to refinement of the grain size of the massive ferrite. In addition, the allotriomorphic ferrite in the base metal and the massive ferrite in the weld were characterized and analyzed under scanning electron microscopy–electron backscatter diffraction (SEM-EBSD). The substructures of massive ferrite in OM can be resolved to be low-angle sub-boundaries in kernel average misorientation (KAM) analysis conducted on SEM. Furthermore, TEM analysis revealed that the substructures of massive ferrite were associated with the dislocation cell structures; it is presumed that during the growth of massive ferrite, the rapid migration of incoherent boundaries generated a high dislocation density, and subsequent cooling led to auto-tempering.

A nitrogen DC-pulse atmosphere pressure plasma jet (APPJ) is used to convert ferric nitrate (Fe(NO3)3) and chloroplatinic ac (H2PtCl6) mixed liquid precursor films into PtFe nanocompounds on a fluorine-doped tin oxide (FTO) substrate. Scanning transmi sion electron microscopy indicates nanoparticles distributed on a thin continuous layer on the FTO substrate. The APPJ-synthesize PtFe nanocompounds contain a mixture of crystalline and amorphous phases. X-ray photoelectron spectroscopy shows that mo Pt is in the metallic phase and most Fe, in the oxidized phase. A dye-sensitized solar cell (DSSC) with only 5-s APPJ-processe PtFe counter electrode (CE) shows significantly improved efficiency. This suggests the rapid processing capability of the nitroge DC-pulse APPJ. A PtFe prepared with higher H2PtCl6/Fe(NO3)3 volume ratio shows better catalytic performance, as confirme by cyclic voltammetry, electrochemical impedance spectroscopy, and Tafel experiments. The DSSC with APPJ-processed PtFe C shows comparable efficiency to that of 15-min furnace-calcined Pt CE, suggesting that the APPJ processed PtFe requires less Pt. [ABSTRACT FROM AUTHOR]