The effects of a trace addition of silver upon elevated temperature ageing of an Al–Zn–Mg alloy

Microalloying additions of Ag (0.1 at.%) increase the hardening response of Al–Zn–Mg alloys to elevated temperature ageing in the range 100–200°C due to the formation of a high density of very fine η′ precipitate plates. The present study employed transmission electron microscopy (TEM) and three-dimension atom probe (3DAP) to study the early stages of ageing in the alloy Al–1.8Zn–3.4Mg–0.1Ag (at.%) in an attempt to identify the role of Ag in stimulating precipitation hardening. During isothermal ageing at 90°C, the hardening response is attributed to a high density of Zn–Mg–Ag rich solute clusters and GP zones. During ageing at 150°C, η′ precipitates nucleate at Zn–Mg–Ag rich solute clusters, the former growing as {111} platelets with an average composition of approximately 20 at.% Zn, 20 at.% Mg and 1.4 at.% Ag. The 3DAP data indicates that the co-segregation of Zn and Ag and subsequently Zn and Mg atoms precedes the formation of the Zn–Mg–Ag rich solute clusters. The GP zones and η′ precipitates were observed to possess a Zn:Mg ratio close to 1:1, whereas the equilibrium η precipitates possessed compositions consistent with MgZn₂. Furthermore, partitioning of Ag was observed inside all precipitate phases, viz. G.P. zones, η′ and η.     

Effects of microalloying with Cd and Ag on the precipitation process of Al–4Cu–0.3Mg (wt%) alloy at 200°C

The present work investigates the effects of individual and combined additions of Cd and Ag on precipitation processes in an Al–4Cu–0.3Mg (wt%) alloy. Analytical scanning transmission electron microscopy revealed that microalloying with Cd stimulates nucleation of θ′ phase on {001} planes and that Cd-rich particles form on the rim and broad facets of the θ′ platelets. We interpret these observations to suggest that Cd nucleates heterogeneously at the θ′– interface and that θ′ can also nucleate heterogeneously at the Cd– interface. In the quinary alloy, it was observed that Ag and Cd additions seem to work independently resulting in a fine and uniform dispersion of both Ω and θ′. Furthermore, the hardening effect of the {111} Ω phase appears to be more potent than other precipitates formed in this system since the hardness of the quinary alloy was intermediate between the Al–Cu–Mg–Ag and the Al–Cu–Cd alloys.     

A TEM study of precipitation in Al–Mg–Si alloys

A set of extrusion samples of Al–Si–Mg alloys (0.5 wt%Mg and 0.3–0.8 wt%Si) were, respectively, T1- and T4-heat-treated. Differential scanning calorimetry (DSC) was used to heat the samples to particular temperatures to promote the formation of precipitates for study by transmission electron microscopy (TEM). It was found that, apart from β″, β′, and B′, there were many precipitates showing rectangular lattices when viewed along the long axes of the precipitates. It is considered that the residual stresses (or dislocations) in the extrusion after the T1-treatment facilitated the nucleation and growth of the precipitates during the heating in the DSC.     

Three-dimensional atomic scale analysis of nanostructured materials

The 3-dimensional atom probe (3DAP) has been used to provide atomic-scale microcharacterisation of a number of nanostructured materials. Grain boundary segregation has been investigated in electrodeposited nanocrystalline nickel and Ni-P. In the nanocrystalline nickel, there was no observable grain boundary segregation in the as-deposited condition. After annealing, carbon and sulphur contamination was found at the boundary of an abnormally-grown grain. In the as-deposited Ni-P alloy, only limited grain boundary segregation of P is seen, but annealing produces significant segregation and the formation of Ni₃P precipitates at grain boundaries. The phase chemistry in a melt-spun amorphous Fe-Si-Cu-Nb-B-Al (FINEMET-type) alloy has also been studied, and the hetereogeneous nucleation of Fe-Si nanocrystals at Cu precipitates shown conclusively. It is found that at early stages of crystallisation, there is only limited partitioning of the Si between the nanocrystals and the amorphous matrix. Atom probe studies of thin layered films have historically been limited by specimen preparation problems, but recent advances have now yielded data on metallic multilayer films. This has allowed atomic-scale measurements of interface chemistry in these films for the first time.     

Effect of rare earth on the microstructures and properties of a low expansion superalloy

A new Fe-Ni-Co-Nb-Ti-Si superalloy containing trace additions of selective rare earths and having good combination of very low thermal expansion coefficient, high-resistance to stress accelerated grain boundary oxygen embrittlement and fairly good notch-bar rupture strength has been successfully developed. The resistance to oxidation for long time exposure at high-temperatures and the stress rupture life has been improved significantly with trace yttrium addition. The microstructures of the alloys have been studied by means of analytical electron microscopy, chemical and X-ray analysis techniques. The results reveal that the trace yttrium segregates in the strengthening phase with platelet morphology, and helps in transforming A(3)B type epsilon phase into A(5)B type H. The morphology and crystal structures of the grain boundary phases also change with selective additions of rare earth elements. Compared with those in the conventional alloy, the platelet precipitates in the yttrium-containing alloy densely segregate within the grains and along the grain boundaries with smaller size. The segregation of the platelet precipitates within the grains is helpful in improving the strength of the alloy. In addition, its precipitation along the grain boundaries can improve the resistance to stress accelerated grain boundary oxidation and stress rupture property of the alloy and thereby contribute to its temperature stability.     

Solute segregation to grain boundaries and free surfaces in an Fe---Si multicomponent alloy

Solute segregation was measured at both the {310} symmetrical tilt grain boundary and the (310) free surface of a sample of an Fe-6at%Si alloy containing traces of P, S, N and C at 873 K. Large phosphorus enrichment and silicon depletion characterize the grain boundary segregation in spite of a different bulk concentration of nitrogen. The surface segregation in nitrogen-containing samples is controlled by strong cosegregation of Si and N, resulting in the formation of a stable Si_xN_y 2D surface compound, whereas pronounced surface segregation of sulphur dominates in denitridized samples. The differences of grain boundary and surface segregation are discussed as a kind of “anisotropy of interfacial segregation” on the basis of Guttmann's theory with different values of free energies of segregation to grain boundary and free surface. They also suggest that the measurements of surface segregation cannot be unambiguously used for predicting the grain boundary segregation. In some non-brittle multicomponent systems, a better way of predicting segregation behavior at grain boundaries would be the measurement of grain boundary segregation in a related system with solute concentrations that cause embrittlement. The findings can then be applied to the required alloy composition on the basis of Guttmann's theory.     

Upper bound on all products of R-parity violating couplings λ′ and λ″ from proton decay

We prove that any product of R-parity violating couplings λ′ (L-violating) and λ″ (B-violating) can be strongly restricted by proton decay data. For any pair λ′ and λ″ the decay exists at least at one loop level. For squark masses below 1 TeV we find the conservative bounds |λ′ · λ″| < 10⁻⁹ in absence of squark flavor mixing, and |λ′ · λ″| < 10⁻¹¹ when this mixing is taken into account. We study the dependence of the bounds on the flavor basis in which R-parity breaking couplings are determined.     

On the precipitation of delta phase in ALLVAC® 718Plus

ALLVAC 718Plus is a new commercial superalloy derived from Inconel 718, but possessing a higher temperature capability whilst employing the same philosophy regarding the microstructure. Many articles have been published describing various heat treatments exploiting the precipitation of intermetallic phases at grain boundaries to optimize the mechanical properties over a range of testing conditions. The requirement to further improve the mechanical properties of this alloy drives our interest in the precipitation mechanism of the delta and eta phases found in this alloy. We report the presence of finely layered structures composed of two phases, delta and eta, with distinct structures and chemistries. Possible pathways to explain this precipitation in 718Plus are considered as follows: (i) the sequential formation of the delta from eta phase and (ii) the simultaneous precipitation of both eta and delta facilitated via solute rejection. Both can result in the formation of those small delta layers observed in HRSTEM. We discuss which is most likely by comparing the relative alignment of the phases by image processing and the analysis of the HRSTEM images, and propose formation mechanisms consistent with the distinctive dislocation structures observed at the interface.     

Growth, structural, and magnetic properties of iron nitride thin films deposited by dc magnetron sputtering

FeN thin films were deposited on glass substrates by dc magnetron sputtering at different Ar/N₂ discharges. The composition, structure and the surface morphology of the films were characterized using X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and atomic force microscopy (AFM). Films deposited at different nitrogen pressures exhibited different structures with different nitrogen contents, and the surface roughness depended on the mechanism of the film growth. Saturation magnetization and coercivity of all films were determined using superconducting quantum interference device, which showed that if N₂/(Ar+N₂) flow ratio was equal to or larger than 30% the nonmagnetic single-phase γ″-FeN appeared. If N₂/(Ar+N₂) flow ratio was less than 10%, the films consisted of the mixed phases of FeN_0.056 and γ′-Fe₁₆N₂, whose saturation magnetizations were larger than that of -Fe. If N₂/(Ar+N₂) flow ratio was 10%, the phases of γ′-Fe₄N and -Fe₃N appeared, whose saturation magnetizations were lower than that of -Fe.     

Solid-state synthesis of potassium β- and β″-gallates

Direct synthesis of K-β- and β″-gallates by Ga₂O₃–K₂O solid-state reaction is described. The formation of K-β- or β″-gallates depends on the initial Ga₂O₃ phase. -Ga₂O₃ leads to K-β-gallate; β-Ga₂O₃ leads to K-β″-gallate. K-β″-Gallate is stable <1200°C. The high temperature stability of K-β″-gallate can be enhanced by doping with aliovalent ions.     

稀土对镁合金应力腐蚀影响电子理论研究

建立了镁合金纯净晶界及其析出Mg₁₇Al₁₂相的晶界原子集团，应用实空间的递归方法计算了铝、稀土元素在晶界的偏聚能，晶界处铝、稀土原子间相互作用能和不同体系的费米能级.讨论了铝、稀土在晶界的偏聚行为，铝、稀土原子间的相互作用与有序化的关系及稀土对镁合金晶间应力腐蚀影响的物理本质.研究发现：铝、稀土原子偏聚于晶界；铝原子间相互排斥，在晶界区形成有序相Mg₁₇Al₁₂，稀土原子间互相吸引，形成原子团簇；稀土原子团吸引铝原子，使铝原子渗入稀土团簇中，形成稀土化合物.因此，稀土具有抑制铝在晶界形成导致应力腐蚀的阴极相Mg₁₇Al₁₂的作用，提高镁合金的晶间应力腐蚀抗力. 关键词： 电子理论 镁合金 应力腐蚀 稀土     

Anisotropic dielectric properties of PbYb1/2Ta1/2O3 single crystals

PbYb_1/2Ta_1/2O₃ single crystals were obtained for the first time. They were grown by the flux method. The PbOPbF₂B₂O₃ system was used as a solvent. Dielectric investigations were carried out in 1 0 0_c, 1 1 0_c and 1 1 1_c pseudocubic directions. These studies pointed to anisotropy of dielectric properties. Frequency-independent ε′(T) and ε″(T) maxima related to the antiferroelectric–paraelectric (AFE—PE) phase transition are observed for all directions at 562 K. The frequency-dependent ε′(T) and ε″(T) maxima near 400 K related to the ferroelectric (FE)–AFE phase transition are observed only in 1 1 1_c direction. The hysteresis loops were observed in this direction only. These results point that ferroelectric relaxor properties appear only in 1 1 1_c direction. We propose to consider the ferroelectric phase as ferrielectric one.     

Coexisting antiferromagnetism and ferromagnetism in mechanically alloyed Fe-rich Fe–Ni alloys: implications regarding the Fe–Ni phase diagram below 400°C

Fe–Ni alloys below the Invar region with compositions Fe_100−xNi_x (x=21, 24, and 27 at%) were prepared by high-energy ball milling technique (mechanical alloying). The as-milled samples, characterized by X-ray diffraction and Mössbauer spectroscopy, contain a mixture of (BCC) and γ (FCC) phases, whereas the samples annealed at 650°C for 0.5 h show a single γ (FCC) phase displaying a single line Mössbauer spectrum at room temperature (RT). At low temperature, the Mössbauer spectra of annealed Fe₇₆Ni₂₄ and Fe₇₃Ni₂₇ alloys show the existence of a magnetically split pattern together with a broad singlet, which are ascribed to a high-moment ferromagnetic Ni-rich phase and a low-moment Fe-rich phase, respectively. The Fe-rich phase in annealed Fe₇₆Ni₂₄ alloy, which is paramagnetic at RT, undergoes antiferromagnetic ordering at 40 K, estimated from the dramatic line broadening of its spectrum, giving rise to a small hyperfine field (e.g. 2 T at 6 K). The coexistence of these phases is attributed to phase segregation occurring in these alloys as a result of enhanced atomic diffusion. The stability of these alloys towards martensitic (FCC→BCC) transformation at low temperatures is discussed in connection with the Fe–Ni phase diagram below 400°C.     

Interfacial segregation in Ag-Au,Au-Pd,and Cu-Ni alloys: II. [001] Σ5 twist grain boundaries

Atomistic simulations of segregation to [001] 5 twist boundaries in Cu–Ni, Au–Pd, and Ag–Au alloy systems have been performed for a wide range of temperatures and compositions within the solid solution region of these alloy phase diagrams. In addition to the grain boundary segregation profiles, grain boundary free energies, enthalpies, and entropies were determined. These simulations were performed within the framework of the free energy simulation method, in which an approximate free energy functional is minimized with respect to atomic coordinates and atomic site occupation. For all alloy bulk compositions (0.05 C 0.95) and temperatures (400 T (K) 1,100) examined, Cu and Au segregates to the boundary in the Cu–Ni and Au–Pd alloy systems, respectively; although in the Ag–Au alloys, the majority element segregates to the boundary. The width of the segregation profile is limited to approximately three to four (002) atomic planes. The classical theories for the segregation, and the effects of the relaxation with respect to either the atomic positions or the atomic concentrations, are discussed. The boundary thermodynamic properties depend sensitively on the magnitude of the boundary segregation, and some of them are shown to vary linearly with the magnitude of the grain boundary segregation.     

Theoretical aspects of solute segregation to high-angle grain boundaries

A high-angle grain boundary is modeled as a planar defect characterized by its thickness and atomic density. We successively examine the elastic and electronic contributions to the solute/grain boundary binding energy. We deduce the effect of the grain boundary physical parameters on its propensity for segregation. The thickness of high-angle grain boundaries is not a fundamental parameter for segregation. The atomic density in the grain boundary controls the electronic binding energy. The rate of change of elastic constants with the density is the important factor in the elastic contribution to segregation. We conclude that segregation to boundaries with small excess volumes is not precluded.     

Effects of B segregation on Mo-rich phase precipitation in S31254 super-austenitic stainless steels: Experimental and first-principles study

Precipitation in super-austenitic stainless steels will significantly affect their corrosion resistance and hot workability. The effects of Cr and Mo on precipitation behaviors were mainly achieved by affecting the driving force for precipitation, especially Mo has a more substantial promotion effect on the formation of the σ phase than Cr. In the present study, B addition to the S31254 super-austenitic stainless steels shows an excellent ability to inhibit precipitation. The effect of B on the precipitation behaviors was investigated by microstructure characterization and theoretical calculations. The experimental observation shows that the small addition of B inhibits the formation of the σ phase along grain boundaries and changes from continuous to intermittent distribution. Moreover, the inhibitory effect increased obviously with the increase of B content. The influence of B addition was theoretically analyzed from the atomic level, and the calculation results demonstrate that B can inhibit the formation of σ phase precipitates by suppressing Mo migration to grain boundaries. It is found that B and Mo are inclined to segregate at Σ 5 and Σ 9 grain boundaries, with B showing the most severe grain boundary segregation tendency. While B distribution at the grain boundary before precipitation begins, the segregation of Mo and Cr will be restrained. Additionally, B's occupation will induce a high potential barrier, making it difficult for Mo to diffuse towards grain boundaries.     

Solute distribution in nanocrystalline Ni–W alloys examined through atom probe tomography

Atom probe tomography is used to observe the solute distribution in electrodeposited nanocrystalline Ni–W alloys with three different grain sizes (3, 10 and 20?nm) and the results are compared with atomistic computer simulations. The presence of grain boundary segregation is confirmed by detailed analysis of composition fluctuations in both experimental and simulated structures, and its extent quantified by a frequency distribution analysis. In contrast to other nanocrystalline alloys previously examined by atom probe tomography, such as Ni–P, the present nanocrystalline Ni–W alloys exhibit only a subtle amount of solute segregation to the intergranular regions.     

Diffusion, Solute Segregations and Interfacial Energies in Some Material: An Overview

We have shown a connection among the three important properties of interfaces, namely, the free energy, diffusion and solute segregation through the conjecture that the interface free energy is the difference between those responsible for diffusion in the lattice and the interface itself. The interface energy is known to decrease upon solute additions. We discuss the methodology and the thermodynamical analysis of the diffusion parameters which enable extraction of the interfacial energies and illustrate them by results obtained in a wide variety of materials. Investigations carried out in pure polycrystalline metals have yielded grain boundary energies comparable to those directly measured. Furthermore, we discuss the role of solute segregation at grain boundaries in alloys in altering diffusion. From the perturbations caused, the solute segregation parameters—the enthalpy and the entropy of binding—have been extracted and levels of solute concentrations estimated. It is shown that similar analyses when applied to complex materials, e.g. the Pb–Sn eutectic alloy, several intermetallic compounds, and oxide systems, also result in acceptable values of interface energies and segregation factors. Finally, some ad-hoc guidelines are provided to alter diffusion in interfaces through solute additions in order to achieve some end use engineering objectives.     

Electron energy loss spectroscopy studies of nitrogen adsorption on W(100)

Vibrational excitations of nitrogen on W(100) are investigated over the 100–300 K temperature range using elastic and inelastic electron scattering. New vibrational modes of nitrogen are identified that require different mode assignments from previous work. Experimental evidence for a molecular precursor to the atomic β₂ phase of adsorbed nitrogen is presented. Coverage dependent studies of vibrational modes suggests conversion between two different molecular surface phases and between atomic and molecular phases. A new ordered nitrogen phase characterized by a (4 × 1) LEED pattern is observed. The new phase appears to consist of orthogonal domains of p(4 × 1) symmetry that contain atomic nitrogen at the four fold sites (the β₂ atomic phase) with additional bridge-bonded nitrogen atoms in the unit cell.     

W-In体系溶质晶界偏聚行为的第一性原理计算

基于第一性原理构建了钨基合金体系的溶质偏聚模型,以W-In体系为例研究了不同浓度下溶质的晶界偏聚行为和成键特征,从电子结构层面揭示了W-In体系的键合作用,预测了W-In体系界面稳定性随溶质浓度的变化规律.结合键布居、电荷密度、差分电荷密度和态密度等电子结构分析,发现了W-In体系中溶质原子在偏聚过程中的键性转变特征,阐明了W-In键由晶粒内部的离子键过渡为晶界区域强共价键的微观机理.模型计算首次得到了W-In体系中溶质本征偏聚能随In浓度的非单调变化规律,结合键合作用和能量分析揭示了溶质浓度对本征偏聚能的影响机制.计算预测了W-In体系达到高热稳定性所需的最佳溶质浓度范围和应避开的溶质浓度范围.本研究为具有高温稳定性的钨基合金材料的设计与制备提供了理论基础和定量化指导.