Solute clustering in Al–Cu–Mg alloys during the early stages of elevated temperature ageing

The evolution of atomistic-level nanostructure during the early stages of elevated temperature ageing of rapid hardening (RH) Al–Cu–Mg alloys has been characterised by a combination of atom probe tomography (APT), transmission electron microscopy (TEM) and positron annihilation spectroscopy (PAS). APT analysis confirms that significant dispersions of small solute clusters form during ageing for 60 s at 150 °C. No zone-like precipitate structures were observed by TEM and APT examinations. These small clusters are believed to be responsible for the RH effect. Careful quantitative APT analysis reveals that a high density of Cu–Mg clusters with high Mg:Cu ratio gives the most potent strengthening response. Positron annihilation measurements also show that Cu–Mg clusters provide additional sites for vacancy stabilisation.     

Determination of the interface energies of spherical,cuboidal and octahedral face-centered cubic precipitates in Cu–Co,Cu–Co–Fe and Cu–Fe alloys

The coarsening theory of a spherical particle in a ternary alloy developed by Kuehmann and Voorhees (KV) has been generalized to any centro-symmetric particle. A classical thermodynamic analysis reveals that the generalized KV theory enables us to estimate the interface energy of a particle with a fixed shape, even if the shape of the particle is not controlled by minimization of the interface energy. Data on the coarsening of spherical, {0 0 1}-faceted cuboidal and {1 1 1}-faceted octahedral precipitates in a Cu–Co alloy, a Cu–Fe alloy, and three Cu–Co–Fe alloys with different Co and Fe contents during aging at 873–973 K have been collected by transmission electron microscopy and electrical resistivity. By applying the generalized KV theory to the experimental data, the energies of sphere, {0 0 1} and {1 1 1} interfaces have been determined. Their energies increase with increasing the Fe composition in the alloys.     

Strengthening due to coherent Cr precipitates in Fe–Cr alloys: Atomistic simulations and theoretical models

Atomistic simulations were carried out to study the interaction of dislocations with Cr precipitates in body-centered cubic Fe and Fe–10%Cr at 0 K. The results are compared with predictions of theoretical models accounting for precipitate strengthening based on different mechanisms. It is shown that solute hardening (in Fe–Cr matrix) and precipitate hardening can be considered, as a good approximation, to be additive effects.     

Influence of alloy composition and heat treatment on precipitate composition in Al–Zn–Mg–Cu alloys

The composition of precipitates in three alloys of the Al–Zn–Mg–Cu system has been investigated for different heat treatments, including peak-aged and over-aged states as well as near-equilibrium conditions, by combining atom probe tomography and systematic anomalous small-angle X-ray scattering experiments. We show that the concentration of Cu in the precipitates changes during heat treatments and is alloy dependent. At low ageing temperature (120 °C) the Cu content in the precipitates is close to the alloy content. The precipitate Cu content is shown to increase with increasing temperature and Cu alloy content. We show that in near-equilibrium conditions the precipitate compositions are 33 at.% in Mg, about 15 at.% in Al, about 13 at.% in Cu and balance Zn. Our results strongly suggest that the gradual incorporation of Cu in the precipitates during the heat treatment is essentially related to the slower diffusivity of this element in aluminium.     

Atomic-scale compositional characterization of a nanocrystalline AlCrCuFeNiZn high-entropy alloy using atom probe tomography

We have studied a nanocrystalline AlCrCuFeNiZn high-entropy alloy synthesized by ball milling followed by hot compaction at 600 °C for 15 min at 650 MPa. X-ray diffraction reveals that the mechanically alloyed powder consists of a solid-solution body-centered cubic (bcc) matrix containing 12 vol.% face-centered cubic (fcc) phase. After hot compaction, it consists of 60 vol.% bcc and 40 vol.% fcc. Composition analysis by atom probe tomography shows that the material is not a homogeneous fcc–bcc solid solution but instead a composite of bcc structured Ni–Al-, Cr–Fe- and Fe–Cr-based regions and of fcc Cu–Zn-based regions. The Cu–Zn-rich phase has 30 at.% Zn α-brass composition. It segregates predominantly along grain boundaries thereby stabilizing the nanocrystalline microstructure and preventing grain growth. The Cr- and Fe-rich bcc regions were presumably formed by spinodal decomposition of a Cr–Fe phase that was inherited from the hot compacted state. The Ni–Al phase remains stable even after hot compaction and forms the dominant bcc matrix phase. The crystallite sizes are in the range of 20–30 nm as determined by transmission electron microscopy. The hot compacted alloy exhibited very high hardness of 870 ± 10 HV. The results reveal that phase decomposition rather than homogeneous mixing is prevalent in this alloy. Hence, our current observations fail to justify the present high-entropy alloy design concept. Therefore, a strategy guided more by structure and thermodynamics for designing high-entropy alloys is encouraged as a pathway towards exploiting the solid-solution and stability idea inherent in this concept.     

Study on the solid solubility extension of Mo in Cu by mechanical alloying Cu with amorphous Cr(Mo)

This paper presents the extension of the solid solubility of Mo in Cu by a mechanical alloying technique. Two binary systems, Cu–10 wt.% Mo and Cr–50 wt.% Mo, and one ternary system, Cu–20 wt.% Cr(Mo), are investigated. The solid solubility of Mo in Cu has been shown to be less than 4.3 at.% when the Cu–Mo system is mechanically alloyed, whereas when the Cr–Mo system is mechanically alloyed all of Mo dissolves into Cr, forming an amorphous Cr(Mo). Similarly, all of 10 wt.% Mo dissolve into Cu when Cu–20 wt.% amorphous Cr(Mo) is mechanically alloyed. Based on Miedema’s model, the Gibbs free-energy changes in these three alloy systems during the formation of solid solutions are calculated to be positive, which means that thermodynamic barriers exist for the formation of these three alloy systems in solid solution states. The mechanism of solid solubility extension in these mechanical alloyed systems is discussed. The conclusion is that the extension of solid solubility is favoured by adding a third element, such as Cr, to the Cu–Mo system.     

Precipitation in Fe–15Cr–1Nb alloys after oxygenation

The effect of O on the phase relations at 950 °C in Fe–15Cr–1Nb alloys is experimentally investigated. Fe–15Cr–1Nb alloys are oxygenated by subjecting high-purity Fe–15Cr–1Nb to an O atmosphere at 600 °C. Both the high-purity and the oxygenated Fe–15Cr–1Nb alloys are heat treated for up to 500 h at 950 °C, quenched and investigated by scanning electron microscopy, transmission electron microscopy and electron probe microanalysis. The results show that Fe₂Nb is in equilibrium with α (Fe, Cr) with 0.29 at.% Nb in solid solution in the pure Fe–15Cr–1Nb alloy. The presence of a small amount of O induces the precipitation of a Fe₆Nb₆O_x phase with a cubic crystal structure and lattice parameter 1.13 nm, thereby decreasing the Nb in solid solution in α (Fe, Cr) with increasing O content.     

Three-dimensional atom probe microscopy study of interphase precipitation and nanoclusters in thermomechanically treated titanium–molybdenum steels

Atom probe microscopy was used to generate tomographic analyses of solute clustering and precipitation reactions in a Ti–Mo added microalloyed steel under simulated strip-rolling conditions. It was observed that the interphase row spacing of precipitates was reduced with the application of a pre-strain. The atom probe data also revealed the coexistence of nanoclusters and precipitate particles, even after isothermal holding for 3600 s. These microstructural features occurred both within 3-D interphase precipitate sheets, and in randomly selected fields of view. A bimodal distribution of larger (～8–10 nm) precipitates coexisted with smaller nanoclusters (～3 nm) within the interphase sheets/rows. Both the nanoclusters and the precipitates possessed a disc morphology, although nanoclusters with less than ～30 atoms were more irregular in shape. The size of the nanoclusters and the precipitates was expressed as a Guinier radius, and this varied between 0.5 and 8 nm for both strain conditions, with the average size ～1.8 nm. The composition of the nanoclusters varied over a wide range, yet was mostly rich in C. All of the nanoclusters and precipitates consisted of a mixture of Ti, Mo and C and the average precipitate composition was close to that of MC carbide stoichiometry, where M represents a mixture of Ti and Mo. In the majority of cases, the Ti/Mo ratio in the MC carbides was > 1. As the Guinier radius increased above 2.5 nm, the composition range became narrower, towards the MC carbide stoichiometry, with a small amount of Fe (～3–12 at.%).     

Phase equilibria and microstructures in the Fe–Si–Cr–Ti system

Having known that the (A2+D0₃) two-phase microstructure is obtained at below 873 K in the ternary Fe–Si–Cr alloy with 12–15 at.%Si at constant 10 at.%Cr, the effect of Ti additions to the ternary system on the possibility of having Huesler L2₁ phase in addition to D0₃ phase is examined. Microstructures of the quaternary Fe–10 and 12 at.%Si–10 at.%Cr–0.5 and 1 at.%Ti alloys are investigated by transmission electron microscopy through which identification and morphology of the phases precipitating within the A2 matrix are examined. It is found that the Ti addition to the ternary Fe–Si–Cr system provides (A2+L2₁) two-phase and/or (A2+D0₃+L2₁) three-phase regions. Morphology of the L2₁ phase is spherical or cuboidal, while that of D0₃ precipitates is rod-like, a difference which may be explained by the difference in magnitude of the misfit between the precipitate and the A2 matrix caused by the difference in partitioning of Ti to each phase.     

Irradiation-induced selective precipitation in Cu–Nb–W alloys: An approach towards coarsening resistance

A novel approach towards coarsening resistance in the precipitate-strengthened Cu-based alloys is proposed, taking advantage of selective precipitation during low-temperature ion irradiation. In the case of Cu–Nb–W, W precipitates during room temperature irradiation, forming highly ramified clusters. During subsequent thermal annealing of alloys with composition close to Cu₉₀Nb₉W₁, the more mobile Nb atoms precipitate out on the W clusters, creating a core–shell structure and adopting the Bain orientation relationship within the Cu matrix. This structure is extremely resistant to coarsening. Annealing at 650 °C for 1 h results in a precipitates size <4 nm in diameter, and annealing for an additional 9 h causes no additional growth, even though Nb is highly mobile at this temperature and would coarsen in the absence of W. We attribute the remarkable stability of this precipitate structure to the strong immiscibility of W in Cu and to the highly ramified precipitate structure that W acquires during low-temperature irradiation.     

The site occupancies of alloying elements in TiAl and Ti3Al alloys

The site occupancies of V, Cr, Mn, Fe, Ni, Zr, Nb, Mo, Ta, Ga and Sn (1–5 at.%) in TiAl alloys with different compositions, and in Ti₃Al with the compositions of Ti–26 at.%Al–(1–2 at.%)X, were measured by the atom location channelling enhanced microanalysis (ALCHEMI) method. For TiAl alloys, the results show that Zr, Nb and Ta atoms invariably occupy Ti sites, while Fe, Ni, Ga and Sn atoms occupy Al sites, the alloy composition having no significant influence on their site preference. By contrast, the site preference of V, Cr, and Mn changes considerably with alloy composition (the Ti/Al ratio in particular), the probability of these elements substituting for Ti decreasing in the above order. For quaternary Ti–Al–V–Cr alloys, the site occupancies of V and Cr do not show much mutual influence. In general, with increasing atomic number, elements in the same period show increasing tendency to substitute for Al, as is the tendency to substitute for Ti for elements in the same group of the periodic table. For Ti₃Al alloys, Ga and Sn atoms occupy Al sites, while V, Cr, Mn, Zr, Nb, Mo and Ta atoms occupy Ti sites, the site preference of V, Cr, Mn and Mo in TiAl alloys being different from that in Ti₃Al. The experimental results are interpreted in terms of a Bragg–Williams-type model and bond-order data obtained from electronic structure calculation. Qualitative agreement between the model and measurements is reached.     

Structural stability of the Laves phase Cr2Ta in a two-phase Cr–Cr2Ta alloy

Detailed microstructural analysis of a two-phase alloy of composition Cr–9.8 at.% Ta and lying in the Cr–Cr₂Ta region of the Cr–Ta binary system confirmed that the existing phase diagram is inaccurate; in the cast and annealed condition (1273 K/24 h), blocky primary Cr₂Ta precipitates were observed although the phase diagram indicates the eutectic composition to be ∼13 at.% Ta. The eutectic structure is composed of Cr solid solution and the Laves phase Cr₂Ta; the morphology is primarily lamellar although the rod morphology was occasionally observed. The Laves phase eutectic microconstituent exhibits the C14 (2H) hexagonal structure with a low stacking fault (basal faults) density and an average composition corresponding to 28.5 at.% Ta. After a prolonged high-temperature anneal (1573 K/168 h), the morphology breaks down to form discrete particles of Cr₂Ta; the C14, C36 and C15 structures were all recognized in this annealed condition, often more than one form being present in a single precipitate. The C15 structure was not twinned but contained some stacking faults on the {111} planes. Composition measurements confirmed that these structural transformations were accompanied by composition changes, the precipitates becoming more Ta-rich as they transitioned from the C14 via the C36 to the C15 phase. These observations are coupled with the results from earlier studies to present a discussion on factors that influence the stability and C14/C36/C15 transformation kinetics.     

Strengthening mechanisms in a high-strength bulk nanostructured Cu–Zn–Al alloy processed via cryomilling and spark plasma sintering

A bulk nanostructured alloy with the nominal composition Cu–30Zn–0.8Al wt.% (commercial designation brass 260) was fabricated by cryomilling of brass powders and subsequent spark plasma sintering (SPS) of the cryomilled powders, yielding a compressive yield strength of 950 MPa, which is significantly higher than the yield strength of commercial brass 260 alloys (～200–400 MPa). Transmission electron microscopy investigations revealed that cryomilling results in an average grain diameter of 26 nm and a high density of deformation twins. Nearly fully dense bulk samples were obtained after SPS of cryomilled powders, with average grain diameter 110 nm. After SPS, 10 vol.% of twins is retained with average twin thickness 30 nm. Three-dimensional atom-probe tomography studies demonstrate that the distribution of Al is highly inhomogeneous in the sintered bulk samples, and Al-containing precipitates including Al(Cu,Zn)–O–N, Al–O–N and Al–N are distributed in the matrix. The precipitates have an average diameter of 1.7 nm and a volume fraction of 0.39%. Quantitative calculations were performed for different strengthening contributions in the sintered bulk samples, including grain boundary, twin boundary, precipitate, dislocation and solid-solution strengthening. Results from the analyses demonstrate that precipitate and grain boundary strengthening are the dominant strengthening mechanisms, and the calculated overall yield strength is in reasonable agreement with the experimentally determined compressive yield strength.     

On the role of alloy composition and processing parameters in nanocluster formation and dispersion strengthening in nanostuctured ferritic alloys

An extensive experimental study characterizing the sequence of events that lead to the formation of a very high density of Y–Ti–O solute nanoclusters (NC) in mechanically alloyed, hot isostatically pressed ferritic stainless steels is reported. Yttria dissolves in the Fe–14Cr–3W(0.4Ti) powders during mechanical alloying. The dissolved Y and O, and when present Ti, subsequently precipitate during hot consolidation. The number densities and volume fractions of the NC decrease, and their radii increase, with increasing consolidation temperature. The NC form at 850 and 1000 °C in milled alloys containing Y, both with and without Ti additions. The presence of Ti refines the NC, and both Ti and high milling energy are necessary for the formation of NC at the highest consolidation temperatures of 1150 °C. However, the precise structure and composition of the NC are not well understood. Indeed, their character varies, depending on the alloy composition and processing variables.     

Quantum chemical molecular dynamics study of stress corrosion cracking behavior for fcc Fe and Fe–Cr surfaces

Quantum chemical molecular dynamics simulation was applied to study the oxidation of bare Fe (1 1 1) and Fe–Cr (1 1 1) surfaces with strain in high temperature water. Simulation results implied the surface morphologies differ from Fe to Fe–Cr because of strong bond between oxygen and chromium atoms. Oxygen atoms were trapped around chromium atoms at Fe–Cr surface, whereas oxygen penetrated into the lattice of Fe bare surface. As a result, the oxygen diffusivity into the Fe–Cr crystal surface reduced. It indicated that the preferential oxidation of chromium would take place on Fe–Cr clean surface at the beginning of the oxidation process. Diffusion of hydrogen and oxygen significantly increased when strain applied to the defective surface. Hydrogen atoms being in the lattice of metal possessed the highly negative charge which indicated the surface oxidized by this negative charge H. Negative charged oxygen atoms make bond with the metallic atom which breakage ultimate metal–metal bond. These bond breakages indicated the formation of oxide layer on the surface and play a key role in subsequent localized corrosion nucleation like stress corrosion cracking.     

Site preference of transition-metal elements in B2 NiAl: A comprehensive study

First-principles supercell calculations based on density functional theory were performed to study the T = 0 K site preference of 3d (Ti–Cu), 4d (Zr–Ag) and 5d (Hf–Au) transition-metal elements in B2 NiAl. By adopting a statistical-mechanical Wagner–Schottky model within the canonical ensemble, the effects of finite temperature on site preference were further considered. The calculations showed that, at all alloy compositions and temperatures, Co, Tc, Ru, Rh, Re, Os, Ir and Pt have a consistent preference for the Ni sublattice, while Ti, Zr, Nb, Hf and Ta have a consistent preference for the Al sublattice. In contrast, the site preference of V, Cr, Mn, Fe, Cu, Mo, Pd, Ag, W and Au was found to depend on both composition and temperature. The present calculated results compare favorably with existing theoretical and experimental studies in the literature.     

Solute and vacancy segregation to a/4<1 1 1> and a/2<1 0 0> antiphase domain boundaries in Fe3Al

Segregation of solute atoms and vacancies to antiphase domain boundaries (APDBs) in Fe–Al alloys near the stoichiometry Fe₃Al (Fe–22–28 at.% Al) was studied using a phase-field model based on the Bragg–Williams approximation. Local equilibrium vacancy concentration was determined from experimental data for vacancy formation enthalpy and the configurational entropy of vacancies assuming that the formation enthalpy is independent of long-range order and chemical composition. Fe atoms and vacancies segregate to APDB with the phase-shift vector a/2<1 0 0>(D0₃-APDB) in crystals with stoichiometric composition (Fe–25 at.% Al) and with the Fe-rich composition, whereas both of them tend to be depleted in Al-rich crystals. On the other hand, Fe atoms and vacancies both segregate on APDBs with the phase-shift vector a/4<1 1 1>(B2-APDB) in all compositions studied. The effects of vacancy segregation on APDB energy and thickness is negligibly small; however, the vacancy concentration at the center of APDBs can be up to 80% larger than in the bulk, and therefore it is anticipated that the mobility of APDBs can be significantly affected by the segregation of vacancies as well as by that of solute atoms.     

Observation of passive films on Fe–20Cr–xNi (x = 0, 10, 20 wt.%) alloys using TEM and Cs-corrected STEM–EELS

The structure and composition of passive film formed on Fe–20Cr–xNi (x = 0, 10, 20 wt.%) alloys in deaerated pH 8.5 borate buffer solution was examined by transmission electron microscope and Cs-corrected scanning transmission electron microscope-electron energy loss spectroscopy. Thickness of the passive film on each alloy was measured to be 2.5–2.7 nm and the passive film was enriched with Cr. The passive film formed on the alloys exhibited an amorphous structure, as confirmed by the lack of diffraction contrast and by the fast Fourier transform images taken within a region of the passive film on each alloy.     

Magnetron-sputtered cobalt-based protective coatings on ferritic steels for solid oxide fuel cell interconnect applications

Co, Co–Mn (67:33 at.%) and Co–Cu (67:33 at.%) coatings were fabricated using magnetron sputtering on two kinds of ferritic stainless steels (Crofer22APU and F17TNb) in order to form spinel protective coatings on metallic interconnects for solid oxide fuel cells. Despite the thickness unevenness at different regions, dense metallic coatings were successfully applied onto all necessary surfaces of the channelled interconnect substrates. Upon oxidation, spinel oxide coatings with very low Cr content were formed, reducing effectively the Cr release. Among the three protective coatings, Co–Cu coating showed the lowest area specific resistance (<15 mΩ cm² at 800 °C).     

Novel FePBNbCr glassy alloys “SENNTIX” with good soft-magnetic properties for high efficiency commercial inductor cores

According to a recent study, Fe-based glassy alloys are expected good soft-magnetic properties such as high saturation magnetization and lower coercive force. We focused on Fe-based glassy alloys and have succeeded in developing novel glassy Fe_97?x?yP_xB_yNb₂Cr₁ (x = 5–13, y = 7–15) alloys for an inductor material. The glassy alloy series of Fe_97?x?yP_xB_yNb₂Cr₁ (x = 5–13, y = 7–15) have high glass-forming ability with the large critical thickness of 110–150 μm and high B_s of 1.25–1.35 T. The glassy alloy powder with chemical composition Fe₇₇P_10.5B_9.5Nb₂Cr₁ exhibits an excellent spherical particle shape related to the lower melting point and liquid phase point. In addition, Fe–P–B–Nb–Cr powder/resin composite core has much lower core loss of 653–881 kW/m³, which is approximately 1/3 lower than the conventional amorphous Fe–Si–B–Cr powder/resin composite core and 1/4 lower than the conventional crystalline Fe–Si–Cr powder/resin composite core due to the lower coercive force of 2.5–3.1 A/m. Based on above results, the glassy Fe₇₇P_10.5B_9.5Nb₂Cr₁ alloy powder enable to achieve ultra-high efficient and high quality products in a commercial inductor. In fact, the surface mounted inductor using Fe–P–B–Nb–Cr powder/resin exhibits the high efficiency of approximately 2.0% compared with the conventional inductors made of the crystalline Fe–Si–Cr powder/resin composite core.