Ageing and overageing of lead–calcium alloys

Lead–calcium alloys are commonly used for their high mechanical properties, compared with pure lead. These alloys evolve quickly at room temperature and could in a few months, or years, undergo a softening (overageing). During the last decade, much research has been carried out on the subject without any unanimous results, due mainly to the diversity of the alloy's transformations and the difficulties of observation (wide range of kinetics, heterogeneity of transformations, etc.). The use of several in situ techniques developed specifically for lead alloys in our laboratories has enabled us to identify five transformations in lead–calcium alloys. The structural hardening begins by two discontinuous transformations. The first one is complete and the second one of ordering type remains incomplete. The end of the hardening is due to a thin precipitation of Pb₃Ca, which is followed by a coarsening (softening) of macroprecipitates aligned in strings. The last softening stage comes with the formation of lamellar Pb₃Ca precipitates by coalescence of the macroprecipitates.     

Microscopy and microanalysis of complex nanosized strengthening precipitates in new generation commercial Al–Cu–Li alloys

Precipitates (ppts) in new generation aluminum–lithium alloys (AA2099 and AA2199) were characterised using scanning and transmission electron microscopy and atom probe tomography. Results obtained on the following ppts are reported: Guinier–Preston zones, T₁ (Al₂CuLi), β’ (Al₃Zr) and δ’ (Al₃Li). The focus was placed on their composition and the presence of minor elements. X‐ray energy‐dispersive spectrometry in the electron microscopes and mass spectrometry in the atom probe microscope showed that T₁ ppts were enriched in zinc (Zn) and magnesium up to about 1.9 and 3.5 at.%, respectively. A concentration of 2.5 at.% Zn in the δ’ ppts was also measured. Unlike Li and copper, Zn in the T₁ ppts could not be detected using electron energy‐loss spectroscopy in the transmission electron microscope because of its too low concentration and the small sizes of these ppts. Indeed, Monte Carlo simulations of EEL spectra for the Zn L_2,3 edge showed that the signal‐to‐noise ratio was not high enough and that the detection limit was at least 2.5 at.%, depending on the probe current. Also, the simulation of X‐ray spectra confirmed that the detection limit was exceeded for the Zn K_α X‐ray line because the signal‐to‐noise ratio was high enough in that case, which is in agreement with our observations.     

Three-parameter approach for elastic–plastic fracture of the semi-elliptical surface crack under tension

Systematic three-dimensional elastic–plastic finite element analyses are carried out for a semi-elliptical surface crack in plates under tension. Various aspect ratios (a/c) of three-dimensional fields are analyzed near the semi-elliptical surface crack front. It is shown that the developed J–Q annulus can effectively describe the influence of the in-plane stress parameters as the radial distances (r/(J/σ₀)) are relatively small, while the approach can hardly characterize it very well with the increase of r/(J/σ₀) and strain hardening exponent n. In order to characterize the important stress parameters well, such as the equivalent stress σ_e, the hydrostatic stress σ_m and the stress triaxiality R_σ, the three-parameter J–Q_T–T_z approach is proposed based on the numerical analysis as well as a critical discussion on the previous studies. By introducing the out-of-plane stress constraint factor T_z and the Q_T term, which is determined by matching the finite element analysis results, the J–Q_T–T_z solution can predict the corresponding three-dimensional stress state parameters and the equivalent strain effectively in the whole plastic zone. Furthermore, it is exciting to find that the values of J-integral are independent of n under small-scale yielding condition when the stress-free boundary conditions at the side and back surfaces of the plate have negligible effect on the stress state along the crack front, and the normalized J tends to a same value when φ equals about 31.5° for different a/c and n. Finally, the empirical formula of T_z and the stress components are provided to predict the stress state parameters effectively.     

铜银锆合金热处理工艺的优化及析出相

对经不同热处理后铜银锆合金的性能进行了研究,通过透射电镜分析了合金中析出相的种类及形貌。结果表明:合金经920℃×40min固溶水淬+30%冷变形+420℃×3h时效空冷处理后,能获得较好的力学性能与电学性能,硬度为123HB,电导率为88.8%IACS,室温抗拉强度为430MPa,断后伸长率为13.5%,断面收缩率为45.2%;热处理后合金基体上弥散分布着Cu5Zr析出相和粗大的铜银锆饱和固溶体。     

Toward structural/chemical cotailoring of phase‐change Ge–Sb–Te in a transmission electron microscope

Ge₂Sb₂Te₅, as the prototype material for phase‐change memory, can be transformed from amorphous phase into nanoscale rocksalt‐type GeTe provided with an electron irradiation assisted by heating to 520°C in a 1250 kV transmission electron microscope. This sheds a new light into structural and chemical cotailoring of materials through coupling of thermal and electrical fields.     

Influence of Ca content and oxygen partial pressure on microstructural evolution of (Co,Ca)O at elevated temperatures

Ca‐doped (1, 1.7, 5 and 10 mol% CaO) cobalt oxide single‐crystal samples, with an [001] orientation, were annealed at elevated temperatures of 1000–1200 °C for different times and at different oxygen partial pressures. The microstructure was examined by means of transmission light and electron microscopy. High‐temperature X‐ray diffractometry was used, with the aim of determining the temperature of the CoO ? Co₃O₄ transition in these materials. Extensive precipitation of Ca‐free Co₃O₄ spinel crystals was observed with increasing Ca content and oxygen activity. It is suggested that the electrical conductivity changes in this material may be related to this precipitation, because it changes the electronic state of cobalt cations.     

Microstructure of a spray-formed Al/SiC composite

The microstructure of an Al–6Cu–2Mn–0·45 Mg–1(Ag, Ti, V, Zr, Cr) alloy, reinforced with 13 vol.% SiC particles, made by spray deposition has been investigated by transmission electron microscopy, high-resolution electron microscopy and electron diffraction X-ray spectroscopy. Particular attention was focused on the influence of the reinforcement on the precipitation sequence. Instead of the expected precipitation sequence due to the high Cu/Mg ratio, there is an additional σ precipitate which was previously observed in A1 alloys containing silicon. This precipitate becomes predominant at the T6 temper. The new precipitation sequence for this reinforced alloy is therefore The precipitation of σ phase is believed to be due to the presence of SiC particles, and seems to be correlated with the occurrence of large Mn-rich particles. Although expected, no S phase precipitation is found to occur in the matrix grains. At the matrix grain boundaries, small Al₂Cu (θ) and Al₂CuMg (S), as well as Mn-rich precipitates are found. At the SiC particle surfaces, preferentially orientated Ag-rich and Mg-rich intermetallic precipitates are found. They can coexist with amorphous patches containing oxygen enclosed in an irregularly shaped Al₂Cu (θ) phase remaining from large crystalline areas which did not go into solution.     

Structure and morphology of Mg–Al–Fe‐mixed oxides derivedfrom layered double hydroxides

The influences of the nature and the extent of M(III) ion substitution on the structure, morphology and surface properties of layered double hydroxides, LDHs [Mg_1?x M(III)_x(OH)₂](CO₃)_x/n·mH₂O, M(III) being Al or/and Fe and x= M(III)/[(Mg+M(III)], and derived mixed oxides were investigated. Three series: Mg?Al, Mg?Al?Fe and Mg?Fe were synthesized using low supersaturation co‐precipitation method at constant pH, with different Mg : Al : Fe ratio and x in the wide range from 0.15 to 0.7 in order to obtain complex, multi‐phase systems with disordered structure, developed surface area, acid–base and redox properties favourable for catalytic application. The morphology of LDHs and their derived mixed oxides did not change considerably although pronounced changes in structural and surface properties occur by thermal decomposition. The increase in Al amount, as well as the deviation of M(III) content from the optimal range for the single LDH phase synthesis, causes the formation of smaller particles and decrease of mixed oxide crystallite size. The nature and amount of M(III) influence the development of surface area, after thermal treatment, depending mainly on the presence of smaller mesopores, not visible by scanning electron microscope. Although the particle size has no considerable influence on the value of the surface area, it was observed that the samples with smaller particles (Mg–Al and Mg–Al–Fe series) have also higher surface area compared with the samples with larger particles (Mg–Fe series).     

TEM and HREM study of Al3Zr precipitates in an Al-Mg-Si-Zr alloy

The structure of Al₃Zr precipitates in Al‐1.0Mg‐0.6Si‐0.5Zr (in wt.%) alloy was investigated using conventional transmission electron microscopy (TEM) and high‐resolution TEM (HREM). After annealing of the alloy in the temperature range 450–540 °C, spherical precipitates of metastable L1₂‐Al₃Zr phase appeared nearly homogeneously within the matrix, and elongated particles were found at grain boundaries. L1₂‐structured Al₃Zr were about 20–30 nm in diameter and coherent with the matrix. Inside some of them, planar faults parallel to {100} planes were revealed by use of HREM. Most probably, these faults are an indication of the transition stage of transformation to the stable D0₂₃‐type Al₃Zr phase. The elongated precipitates (about 100 nm) were identified as D0₂₂‐type Al₃Zr. Energy‐dispersive X‐ray analysis showed that they contain, apart from Al, mainly Zr with small amounts of Si. The substitution of Al by Si increased the stability of the D0₂₂‐Al₃Zr as compared with D0₂₃‐Al₃Zr.     

Microstructural evolution of Cu-1 at% Ti alloy aged in a hydrogen atmosphere and its relation with the electrical conductivity

Copper alloys with titanium additions between 1 and 6 at% Ti emerge currently as attractive conductive materials for electrical and electronic commercial products, since they exhibit superior mechanical and electrical properties. However, their electrical conductivity is reduced owing to the residual amount of Ti solutes in the Cu solid solution (Cu_ss) phase. Since Cu shows only poor reactivity with hydrogen (H), while Ti exhibits high affinity to it, we were inspired by the idea that hydrogenation of Cu–Ti alloys would influence their microstructure, resulting in a significant change of their properties. In this contribution, the influence of aging under a deuterium (D₂) atmosphere of Cu-1 at% Ti alloys on their microstructure is investigated to explore the effects on the electrical conductivity. The specimens were investigated by means of transmission electron microscopy (TEM), field ion microscopy (FIM), computer-aided field ion image tomography (cFIIT), and atom probe tomography (APT).     

Crystalline phases found in rapidly quenched Ni–Nb–Zr alloys

The properties of metallic alloys can be significantly improved by developing non‐equilibrium phases in the microstructures through rapid solidification techniques, thus the characterisation of these unusual structures is extremely important. In this research, the microstructures of three rapidly quenched alloys, namely Ni_65.2Nb_33.8Zr_1.0, Ni_54.8Nb_31.1Zr_14.1 and Ni_54.8Nb_21.6Zr_23.6 (at. %) were investigated in greater detail in order to determine the structures and compositions of their crystalline phases. These crystalline phases were characterised using a combination of scanning electron microscopy, energy dispersive x‐ray spectroscopy, x‐ray diffraction and transmission electron microscopy techniques. The phases were compared to the crystalline structures reported in the literature. Our results indicate some agreement with the Ni–Nb phase diagram and an isothermal section of the Ni–Nb–Zr phase diagram; however, it is detected zirconium solubility in the Ni₃Nb phase, as well as, the absence of expected crystalline phases.     

Use of EBSD to identify phases in interdendrite region of a cast Zn–Al-based alloy (ZA27)

Electron backscattered Kikuchi diffraction methodology was used to identify phases in the interdendrite region of an alloy ZA27. Two Zn‐rich hexagonal close‐packed structure phases η and ɛ phases were distinguished using predetermined lattice parameters of the phases. In relation to studies of scanning electron microscopy and X‐ray diffraction, electron backscattered diffraction results revealed that the Al‐rich precipitates of the α phase were from decomposition of the η′_T, and the four‐phase transformation: α+ɛ→ T′+η, had occurred in the ɛ phase after ageing at 150°C for 8 h.     

Double-diffusive natural convection of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-water nanofluid in an enclosure with partially active side walls using variable properties

Natural convection heat and mass transfer characteristics in a square enclosure using variable thermal conductivity and variable viscosity are numerically studied. The fluid in the enclosure is a water-based nanofluid containing Al₂O₃ nanoparticles. The top and bottom horizontal walls are insulated, while a source (T_h, C_h) and a sink (T_c, C_c) are located at the vertical left and right walls as active parts, respectively, with T_h>T_c and C_h>C_c. The governing equations in the two-dimensional space are discretized using the control volume method. A proper upwinding scheme is employed to obtain stabilized solutions. The study has been carried out for the Rayleigh numbers of 10⁴ to 10⁶, the buoyancy ratios of ?5～5, and different configurations of the source and sink. Results are presented in the form of the streamlines, isotherms and iso-concentraions as well as the average Nusselt and Sherwood numbers. It is observed that average Nusselt number is increased by adding the nanoparticles, while average Sherwood number is reduced. Moreover, both Nusselt and Sherwood number are increased as absolute value of the buoyancy ratio or Rayleigh number is increased.     

Anomalous temperature dependence and tension–compression asymmetry of Fe3Al intermetallics—an internal variable approach

Tension/compression asymmetry in the high-temperature flow and anomalous yield/flow stress of Fe–28 at% Al–5 at% Cr alloy has been investigated. Flow curves were obtained from a series of tensile and compressive load relaxation tests. Constitutive relations at each deformation conditions were formulated using the internal variable theory based on dislocation dynamics. In this study, high-temperature flow stress of selected material was to be safely described as the sum of internal stress and frictional stress. The anomaly and peak temperature of yield strength seem to be controlled by the anomaly of internal stress and the relative portion of internal and frictional stress in total flow stress, respectively. Asymmetric flow stress was observed in analogy with yield strength. In terms of constitutive parameters, critical stress for frictional flow (Σ₀) exhibited considerably higher value in compression, which physically postulates increased critical resolved shear stress (CRSS). Considering the crystallography and microstructure, it is presumably due to the ‘extrinsic’ effect of non-deviatoric stress tensor, as proposed in other studies.     

Fundamental aspects and technological implications of the solubility concept for the prediction of running properties

The first Goodzeit rule, which relates resistance to cold welding and seizure with mutual solubility of metal sliding couples, has been investigated. The sliding properties of Au-Ag and Pd-Ag alloys against SAE 1045 carbon steel were observed in ultrahigh vacuum and in argon. The abrupt change in friction and wear coinciding with the composition of the alloys at which the solubility of iron into the alloy vanishes found by De Gee was not confirmed. Instead a continuous decrease in friction coefficient and wear was observed with increasing silver content. Pure silver showed a very limited tendency to metal transfer, although in some cases the transferred layer was removed spontaneously after 200–300 cycles. Electrical conduction measurements made during sliding revealed a correlation between the momentary friction force and the electrical conductivity which indicated that variations in the friction force are synchronous with variations in the contact area.A model is proposed in which the conditions of friction and metal transfer are described in terms of one system and two material properties, i.e. the interfacial shear strength τ_i, the shear strength τ¹ and the strain hardening exponent n of the junction materials. The friction behaviour is described by a modified adhesion theory: instead of a contaminating layer it is the specific atomic interaction of the contacting metals acting over small regions of atomic contact which controls τ_i. The junction growth mechanism is correlated with strain hardening.It is suggested that the first Goodzeit rule must be interpreted as a result of atomic interaction in the interface between the contacting metals. In this way prediction of the running behaviour of both metals and alloys would become possible.     

高循环稳定性碳化钛/氧化石墨烯复合超级电容

二维材料碳化钛(Ti₃C₂T_x)因具有高导电性和大比表面积的特点,在作为超级电容电极材料时,可以实现较高的能量密度。然而,Ti₃C₂T_x在储能过程中会出现不可逆的氧化失活反应,而且它与基底间的结合力较差,这将导致碳化钛超级电容的循环稳定性欠佳,极大地阻碍了其作为电极材料的广泛应用。将Ti₃C₂T_x作为活性层与氧化石墨烯(GO)分层复合制作成超级电容电极,覆盖在Ti₃C₂T_x薄膜之上的GO层可以削弱氧化失活反应。同时,对电极的热处理可提升Ti₃C₂T_x对基底的附着力。这使得Ti₃C₂T_x/GO复合电极的充放电循环稳定性明显改善,在5 000次循环之后其容量仍高于初始容量。该设计可为制备高循环稳定性超...     

Use of EBSD to study electropulsing induced reverse phase transformations in a Zn-Al alloy (ZA22)

Multi‐phase identification and phase transformations in electropulsing treated Zn–Al based alloy wire specimens were studied using electron back‐scattered diffraction, back‐scattered scanning electron microscopy and X‐ray diffraction techniques. By using electron back‐scattered diffraction, two phases: η′_S and η′_T with a small difference of about 1% in lattice parameters (c₀/a₀) were identified, based on the determined lattice parameters of the phases, and the reverse eutectoid phase transformations: η′_T+?′_T+α′_T→η′_S and ?+α→T′+η were successfully detected. Electron back‐scattered diffraction appeared to be an effective technique for studying complex electropulsing induced phase transformations.     

Analysis of a thermal storage unit containing multiple phase change materials dispersed with high conductivity particles

This paper investigates numerically the combined effect of employing multiple Phase change materials (PCMs) and dispersion of high conductivity particles on the thermal performance of an energy storage unit. The key parameters that govern the performance have been identified following a simple reduction of variables. Among the different parameters, melting temperature of the PCMs, latent heat and melting temperature difference between two consecutive PCMs (ΔT_m) are found to have a significant influence on the performance. A case study considering a 3-PCM unit to study the effect of key parameters during melting has been presented. The results show that for the 3-PCM unit, the performance is better when the second and third PCMs placed have their latent heats larger than the first PCM. It has been recommended to have ΔT_m1 > ΔT_m2 for the 3-PCM unit in order achieve more melting.

     

Continuous precipitation of ceria nanoparticles from a continuous flow micromixer

Cerium oxide nanoparticles were continuously precipitated from a solution of cerium(III) nitrate and ammonium hydroxide using a static microchannel T-mixer. T-mixer synthesis results were compared with synthesis results from batch precipitation. Findings show that the method of mixing is important in the ceria precipitation process. Uniform porous film structures and nanorods were produced when the particle chemistry was synthesized using T-mixing followed by spin coating. Batch mixing, when using higher NH₄OH feed concentrations followed by spin coating, was characterized by the heavy agglomeration of nanoparticles. Similar, high aspect ratio nanorods were produced when feed conditions in both batch mixing and T-mixing were identical demonstrating that the momentum effects of continuous microchannel T-mixing did not impact the synthesis process. In addition, it was found that the micromixing approach reduced the exposure of the Ce(OH)₃ precipitates to oxygen, yielding hydroxide precipitates in place of CeO₂ precipitates. The key advantage of the micro-scale T-mixing approach is higher throughput which is important for the scaling of ceria nanoparticle production.     

Preparation,characterization, electrical conductivity and dielectric studies of Na2SO4 and V2O5 composite solid electrolytes

Composite solid electrolytes (1 − x) Na₂SO₄–(x) V₂O₅ were prepared and characterized by various techniques such as XRD, FT-IR, DTA and SEM. AC impedance spectroscopy revealed that the contribution of grain is strong enough over the grain boundary. Arrhenius plot of the Na₂SO₄ shows a sharp increase in conductivity at 523 K due to the structural phase transition (phase V → I). Composites show the enhanced ionic conductivity than the pristine Na₂SO₄ over the entire temperature range. The maximum conductivity σ = 0.003 S cm⁻¹ at 773 K with the lowest activation energy of 0.28 eV was observed for the x = 0.4 sample. The enhanced value of dielectric constant and dielectric loss in the case of composites was obtained because of increase of conductivity, resulted from the increase of space charge polarization and charge motion.