Effects of Alloying Elements on the Volume Fraction of Ordered α2 Phase Precipitated in Ti-Al-Sn-Zr Alloys

An ideal method has been established for calculating the precipitation of α2 ordered phase in near-α titanium alloys based on the theory on the critical electron concentration for the precipitation of α2 ordered phase in near-α titanium alloys. With complete precipitation of α2 phase in near-α titanium alloys, the alloys can be considered to be composed of two parts： （1） the α2 ordered phase with the stoichiometric atomic ratio of Ti3X; （2） the disorder solid solution with the critical composition in which the α2 ordered phase is just unable to precipitate. By using this method, the volume fractions of α2 ordered phase precipitated in Ti-Al, Ti-Sn, Ti-Al-Sn-Zr alloys with various AI, Sn and/or Zr contents have been calculated. The influences of AI and Sn on the precipitation of α2 ordered phase are discussed. The calculating results show substantial agreement with the experimental ones.     

Cluster variation investigation of phase equilibria in Fe–Co system using simulated annealing approach

The cluster variation method (CVM) coupled with the constrained simulated annealing (CSA) algorithm for global free energy minimization is proposed as an efficient approach for the study of alloy phase diagram. The chemical order–disorder transitions in Fe–Co alloy system between 300 and 1100 K are investigated using the proposed CSA–CVM approach using the irregular tetrahedron approximation in a bcc lattice. The calculations have been carried out for a hypothetical paramagnetic Fe–Co alloy, i.e. with chemical interactions only, as well as for Fe–Co alloy with both magnetic and chemical interactions acting simultaneously, i.e., considering the quaternary Fe^↑Fe^↓Co^↑Co^↓ system. The calculated A2/B2 boundary is found to match well with that determined experimentally. The incorporation of magnetic interaction is also found to have a large influence on the order–disorder transitions. The main advantages of the CSA–CVM approach is that it works efficiently with no initial value dependence on optimization parameters, and in contrast to some of the previously used minimization techniques, it is always found to converge to the global minimum. Another new aspect of the present investigation is that the phase diagram is obtained here by plotting long range order versus composition for the most ordered phase (B2 in this study) and not the commonly used grand potential versus chemical potential curves for various phases, thereby saving a lot of labor. Precise predictions for the variation of heat capacity with temperature have also been made.     

A computational study of kinetic phase diagrams for CoPt alloy films during epitaxial growth

We have studied the kinetic processes of the epitaxial growth for CoPt alloy films using the master equation method. The kinetic phase diagrams of CoPt alloy films which show the phase formation conditions during the epitaxial growth are determined. From the kinetic phase diagrams, we find that the [001] ordered structure is much easy to be grown at high temperature while the [100] ordered structure is easy to be grown at low temperature although both the [001] and [100] ordering could be the equilibrium ground states. The atomic deposition, ordering and surface segregation lead to a rich variety of phases in epitaxial growth. The surface segregation is found to enhance the [001] ordering and leads to the formation of the [001] ordered phase at high temperature.     

Controlled release of phenytoin for epilepsy treatment from titania and silica based materials

Template technique was used to obtain well ordered nanostructured materials: mesoporous silica and nanostructured titania tubes. This technique permits the synthesis of solids with controlled mesoporosity, where a large variety of molecules that have therapeutic activity can be hosted and further released to specific sites. In this work phenytoin (PH), a drug used in epilepsy treatment, was loaded in ordered mesoporous silica (SBA 15) and nanostructured titania tubes (TiO₂). The pure materials and those containing PH were characterized by X-ray diffraction, FTIR spectroscopy, transmission electron microscopy (TEM), scanning electron microscopy (SEM) and N₂ adsorption–desorption at 77 K. In order to determine the loading capacity of the antiepileptic drug on these silica- and titania-based materials, the loading and release of PH was investigated using UV–vis spectroscopy. Tubular structures were found for the titania samples, for which the X-ray diffractograms showed to be formed by anatase and rutile phases. On the other hand, an amorphous phase was found in the silica sample. A highly ordered hexagonal structure of 1D cylindrical channels was also observed for this material. Loaded PH showed a good stability inside the used materials as observed by spectroscopy analysis. The adsorption and desorption of PH are faster in nanostructured TiO₂ tubes than in mesoporous silica matrix.     

Overaged metallography of alloy 909, a low coefficient of expansion superalloy

Abstract

The microstructure of alloy 909, a low coefficient of thermal expansion Fe-Co-Ni superalloy was examined in the solution treated and over aged condition, following a previous communication by the authors on the solution treated and commercially aged alloys. In the commercially aged alloy, only the Laves and the γ? phases were observed. With some indication, however, of alignment of the γ? phase along crystallograpic directions in the austenitic matrix. This alignment of the γ? phase has been previously reported as a precursor to the growth of the ? phase from the γ? phase. Analysis by TEM showed that the platelet phase was hexagonal (NiFeCo)₂(TiNb) with some variations in the stoichiometry about 2:1 ratio. Tensile and hardness testing indicated that the presence of the platelet shaped ? phase in the solution treated and overaged condition was not as effective in the strengthening of the alloys as the spherical γ? in the commercially aged and solution treated condition. In addition to the presence of ? phase, blocky A₂B Laves phase was abundant, which prevents excessive grain growth up to ～1040°C     

A study of orthorhombic phases in aluminium-transition metal alloys

In recent decades some complex crystalline phases, as also many rational approximants to quasicrystalline phases with rather large unit cells, have been reported with orthorhombic symmetry in aluminium-transition metal (Al-TM) alloys. Furthermore, quite a few quasicrystalline phases, icosahedral as well as decagonal, forming in Al-TM alloys on normal or rapid solidification have been interpreted during the last decade as multiply-twinned orthorhombic crystals growing as superstructures of an orthorhombic cell that forms through welding in three perpendicular directions in the liquid state of 13-atom icosahedral clusters. Following exemplification of this new approach to quasicrystals based on the analysis of the Debye-Scherrer diffraction data from the comparatively defect-free Al-Cu-Fe icosahedral phase, the three types of orthorhombic phases in aluminium-rich Al-TM alloys, numbering 36 in all, have been examined as icosahedral cluster compounds nucleating from icosahedral atomic clusters present in the molten alloys. A detailed analysis of their lattice parameters supports the postulate that all such phases can be viewed as complex and, occasionally, as very large superstructures of a small basic orthorhombic cell. Dedicated to the dearly cherished memory of Professor Sir William Hume-Rothery F.R.S. (1899–1968) in his birth centenary year.     

M_2M'AlB_4(M = Mn,Fe, Co,M' = Cr,Mo, W): Theoretical predicted ordered MAB phases with Cr_3AlB_4 crystal structure

The nanolaminated MAB phases have attracted great research interests due to their unusual combination of metal-like and ceramic-like properties, which is similar to MAX phases. Recently, ordered quaternary MAX phases have been discovered, which enriches the family of MAX phases, and opens a new window to tailor the properties of MAX phases and to develop new MXenes. In the present work, we explored possible ordered quaternary MAB phases with Cr_3AlB_4 structure(space group: Pmmm) by first-principles calculations. The predictions show that M_2M'AlB_4 phases with M = Mn, Fe, Co and M' = Cr, Mo, W exhibit strong tendency of ordering, where M locates at 2t site(0.5, 0.5, z_(2t)) and M' locates at 1 g site(0, 0.5,0.5). The main driving force of ordering may be the differences in bonding strengths between Al and M elements. Analyses on chemical bonds reveal that bonding strengths increase following the order:Al-Mn Al-Fe Al-Co, which is consistent with the prediction that ordering tendency increases when M changes from Mn to Co, as derived from enthalpy differences. The ordered M_2M'AlB_4 phases with M =Mn or Fe are predicted ferromagnetic and ordered M_2M'AlB_4 phases display lower shear resistance and possibly better ductility in comparison to Cr_3AlB_4.     

Alloying behaviour,thermal stability and phase evolution in quinary AlCoCrFeNi high entropy alloy

An equiatomic quinary AlCoCrFeNi high entropy alloy (HEA) has been synthesized by mechanical alloying. Milled powder after 30?h shows good chemical homogeneity and refined morphology with a mean particle size of ～4?μm. Solid solution phase with BCC crystal structure (a?=?2.89?±?0.02?Å) has been confirmed from XRD and transmission electron microscopy in the as-synthesized high entropy alloy. The milled alloy powder is not thermally stable. Differential scanning calorimetric (DSC) thermogram of 30?h milled powder exhibits the presence of a small peak at ～600?°C (873?K) with a thermal shift near the peak. This thermal shift indicates the diffusive type of phase transformation in this alloy while heating. The analysis of the in-situ heating X-ray diffraction patterns at various temperatures extends support to the diffusive nature of the phase transformation. Upon heat treatment, the disordered BCC solid solution phase partially transforms to Ni₃Al prototype L1₂ phase which precipitates at a lower temperature (350?°C (623?K)) as observed by in-situ XRD experiments. However, at high temperature annealing (575–800?°C (848–1073?K)) the evolution of a partially ordered BCC phase (B2) with lattice parameter (a?=?2.87?±?0.02?Å), and L1₂ phase (a?=?3.58?±?0.05?Å), along with tetragonal σ phase (a?=?8.8?Å and c?=?4.53?Å) are observed. Similar types of phases have also been identified after annealing and microwave sintering at 800?°C (1073?K) & 900?°C (1173?K) respectively. The transformation of ordered BCC phases along with two intermetallics such as L1₂ phase and σ phase suggests that the evolution of the high entropy phase in the milled condition leads to a combination of high entropy and medium entropy phases in the annealed condition.     

Influence of disorder on electronic structure and magnetic properties in Fe-rich Fe–Si alloys

We present the results of ab initio calculations of the spin-polarized electronic structure of disordered bcc and fcc Fe_1 − xSi_x alloys from dilute solid solutions of Si on iron up to FeSi intermetallic compound composition (x=0.01..0.5) and ordered intermetallic phases FeSi in B20 structure and Fe₃Si in DO₃ structure. The self-consistent calculations were performed with the coherent potential approximation within the Korringa–Kohn–Rostoker method (KKR–CPA) for disordered case and by using the tight-binding linear muffin–tin orbital (TB LMTO) method for intermetallic compounds. In the last case the supercell approach has been utilized in order to take into account the structural defects for this ordered phase. In particular, we have performed a series of calculations in the BCC Fe_1 − xSi_x alloys with x=0.01, 0.015, 0.1, 0.15, 0.25.     

Effect of Al Content on Precipitatio of α2 Ordered Phase in Ti—Al—Su—Zr—Mo—Si—Nd Alloys

The precipitation characteristics of the α2 ordered phase in Ti-Al-Sn-Zr-Mo-Si-Nd alloys with various content of Al, under different aging conditions,were investigated.The distribution and size of the α2 ordered phase could precipitate at higher temperature near the critical transformation temperature for each alloy experimented.With the addition of Al content,the critical transformation temperature of α2 ordered phase increased.When the aging temperature was relatively low (650℃),the precedent precipitation of α2 ordered phase took place in primary αphase at the early stage of aging ,in the duplex microstructure (the primary α with the transformedβ) of the alloys with lower Al content.But after certain aging time (50h),the size of α2 particles was almost equal in both the primary α and the transformed β.And no obvious growth of α2 particles could be observed after 50h.     

Electron microscopy of metastable phases in rapidly solidified Al-Co alloys

Four binary Al-Co alloys containing 2·4, 14, 20 and 25 at.% Co and a ternary Al-Co-Si alloy with 20 at.% Co and 5 at.% Si were rapidly solidified from the liquid state by melt spinning. It has been shown that the solid solubility of Co in Al can be increased up to at least 1·7at.% from the negligible value under equilibrium conditions. Two metastable crystalline intermediate phases and a D1 _a -type ordered phase were observed in localized regions in the rapidly solidified Al-2·4at.%Co alloy. A quasicrystalline decagonal phase has been observed in all the other rapidly solidified alloys. This decagonal phase was shown to contain dislocations, dislocation loops and also twin-like defects. The phenomenon of polytypism was also observed in this decagonal quasicrystalline phase. In extremely thin areas of the Al-14at.% Co alloy ribbons on amorphous phase was detected. Transmission electron microscopy was used to extensively characterize the phases in the as-solidified condition and also those produced during and after transformation to the equilibrium constitution. Work carried out when both the authors were in the Department of Metallurgical Engineering, Banaras Hindu University, Varanasi 221 005, India     

Influence of Al content on thermal stability of nanocrystalline AlxCoCrFeNi high entropy alloys at low and intermediate temperatures

Thermal stability of mechanically alloyed nanocrystalline Al_xCoCrFeNi (x = 0, 0.3, 0.6, 1 mol) high entropy alloys (HEAs) has been investigated for the low and intermediate temperature range of 673–1073 K. Single phase FCC structure is observed in the as milled CoCrFeNi. A mixture of FCC and BCC phases is exhibited by × = 0.3, 0.6 and 1, alloys where the volume fraction of BCC increases with increasing Al content. Phase evolution in heat-treated Al_xCoCrFeNi HEAs proceeds via increasing BCC fraction at 673 K, followed by subsequent reduction at elevated temperatures. For each alloy, the major phase observed in as milled condition and it is retained even after prolonged exposure at the 1073 K. Al favors the formation of the BCC phase due to its high affinity to form ordered B2 structures with constituent elements Co, Fe and Ni. Thermal exposure of Al_xCoCrFeNi HEAs also leads to the formation of Cr₇C₃, owing to the higher negative free energy of carbide formation for Cr among other constituents. Transmission electron microscopy (TEM) investigations substantiated that nanostructure of milled powder is maintained even after the heat treatment. Grain growth factor for quinary HEAs is relatively lower than quaternary CoCrFeNi owing to their slower rates of diffusion.     

Long-Range Ordered Amorphous Atomic Chains as Building Blocks of a Superconducting Quasi-One-Dimensional Crystal

Crystalline and amorphous structures are two of the most common solid-state phases. Crystals having orientational and periodic translation symmetries are usually both short-range and long-range ordered, while amorphous materials have no long-range order. Short-range ordered but long-range disordered materials are generally categorized into amorphous phases. In contrast to the extensively studied crystalline and amorphous phases, the combination of short-range disordered and long-range ordered structures at the atomic level is extremely rare and so far has only been reported for solvated fullerenes under compression. Here, a report on the creation and investigation of a superconducting quasi-1D material with long-range ordered amorphous building blocks is presented. Using a diamond anvil cell, monocrystalline (TaSe₄)₂I is compressed and a system is created where the TaSe₄ atomic chains are in amorphous state without breaking the orientational and periodic translation symmetries of the chain lattice. Strikingly, along with the amorphization of the atomic chains, the insulating (TaSe₄)₂I becomes a superconductor. The data provide critical insight into a new phase of solid-state materials. The findings demonstrate a first ever case where superconductivity is hosted by a lattice with periodic but amorphous constituent atomic chains.     

Molecular-scale investigation of tolane disulfide self-assembled monolayers on Au(111) using scanning tunneling microscopy

Molecular-scale surface structures of self-assembled monolayers (SAMs) formed by the spontaneous adsorption of tolane disulfides (TDS) on Au(111) in a 1 mM mixed solution of ethanol/N, N'-dimethylformamide (9:1) were examined using scanning tunneling microscopy (STM). The STM study revealed that TDS SAMs formed after a 24 h immersion at room temperature were composed of two-dimensional (2D) ordered phases with inhomogeneous surface morphologies and no clear domain boundaries. However, after 2 h immersion at 50 degrees C, uniform 2D ordered domains with clear domain boundaries were observed, which could be described as c(3 x square root of 3) structures with centered rectangular unit cell. Interestingly, a unique intermediate ordered phase with a low surface coverage was also observed. After a longer immersion for 24 h at 50 degrees C, only the uniform c(3 x square root of 3) domains were observed with a corrugation that may have reflected surface reconstruction of the Au(111) surfaces. From this study, we found that 2D ordered TDS SAMs with large and uniform domains on Au(111) can be obtained by controlling the solution temperature and immersion time.     

Fabrication and experimental investigation of metal grid structure-reinforced aluminum foams

This paper put forward an innovative approach for fabricating reinforced aluminum foams through melt foaming method, where the metal grid structure (MGS) was used as a structural skeleton. Samples with a density ranging from 0.31 to 0.56?g/cm³ were fabricated and presented uniform bubbles. The major surface of MGS was covered with a layer of alloy matrix. Compression tests revealed that the compressive plateau phase was transformed into three small phases, where two slight peaks as well as three valleys were detected. In addition, the maximum compressive stress and energy absorption capacity were improved dramatically especially in low-density samples.     

Fundamental deformation behavior in high-entropy alloys: An overview

High-entropy alloys (HEAs), as a new class of materials, are nearly equiatomic and multi-element systems, which can crystallize as a single phase or multi-phases. Most of the HEAs described in the literature contain multiple phases (secondary phases, nanoparticles, and so on), rather than a single solid-solution phase. Thus, it is essential to review the typical mechanical properties of both single-phase and multiphase HEAs thoroughly, with emphases on (1) the fundamental physical mechanisms and (2) the difference from conventional alloys. In this paper, mainly based on different mechanical properties, HEAs are classified into four types for the first time, i.e., (a) HEA alloy systems of 3d-transition metals only (Type 1), (b) HEA alloy systems of transition metals with larger atomic-radius elements (Type 2), (c) HEA alloy systems of refractory metals (Type 3), and (4) others (Type 4). Then a number of aspects of mechanical behavior are reviewed and discussed, including the elastic anisotropy, yield strength, high-temperature performance, serration behavior, fracture toughness, and fatigue responses, which may serve as a demonstrative summary for the current progress in the scientific research of HEAs. Several mechanisms that quantitatively explain the mechanical properties of single-phase and multiphase HEAs in terms of basic defects (dislocations, twinning, precipitates, etc.) are discussed. A number of future research activities are suggested, based on the emphasis on developing high-performance structural materials. The review concludes with a brief summary of major mechanical properties and insights into the deformation behavior of single-phase and multiphase HEAs. The comparison and contrast between HEAs and conventional alloys remain the most compelling motivation for future studies. With the integrated experimental and simulation investigations, a clearer picture of the fundamental deformation behavior of single-phase and multiphase HEAs could be explored.     

The effect of Ti addition on phase selection of CoCrCu0.5FeNi high-entropy alloys

With the increasing titanium, the volume fraction of face-centred cubic-structured dendrites decreased, and ordered B2 phase, σ, and Laves phase appeared in CoCrCu_0.5FeNiTi_x alloy when x?>?0.5. With the increase of Ti content, ΔS_mix, ΔH_mix, valence electron concentration (VEC), and Λ (=ΔS_mix/δ²) of CoCrCu_0.5FeNiTi_x alloys decreased, while atomic size difference (δ) and electronegativity difference (Δχ) increased. Topologically close-packed (TCP) structures are favoured when ΔS_mix?>?14.53?J?K^?1?mol^?1. TCP phases can be found when δ?>?5.05 and Δχ?≥?0.124. Simple solid solution phase is favoured when VEC?≥?8.26 but multi-phases including TCP phase coexist when VEC??0.637, solid solution and TCP phases coexist when 0.531?相似文献   

Physical metallurgy of concentrated solid solutions from low-entropy to high-entropy alloys

The alloy world could be divided into low-entropy (LEAs), medium-entropy (MEAs) and high-entropy alloys (HEAs) based on the configurational entropy at the random solution state. In HEAs, four core effects, i.e. high entropy, sluggish diffusion, severe lattice distortion and cocktail effects, are much more significant than low-entropy alloys in affecting phase transformation, microstructure and properties. In fact, the degree of the influence from these core effects more or less increases with increased mixing entropy. The trend is gradual from low-entropy alloys to high-entropy alloys. In this article, physical metallurgy of HEAs is discussed with the bridge connected to that of conventional alloys. As disordered and ordered solid solutions are the main constituent phases of alloys, the understanding of solid solutions is fundamental for the understanding of alloys. In addition, as dilute solid solutions have been well treated in current physical metallurgy, concentrated solid solutions from low-entropy to high-entropy alloys are focused in this article. Physical properties are especially emphasized besides mechanical properties.     

Studies on dechlorination of DDT (1,1,1-trichloro-2,2-bis(4-chlorophenyl)ethane) using magnesium/palladium bimetallic system

The aim of our investigation was to compare the rates of dechlorination of DDT using Mg0/Pd4+ system in two different reaction phases, namely, water-acetone and 0.05% biosurfactant in water. Since palladium is expensive and its toxicity effects are not well known we also examined the reuse efficiency of Pd0 immobilized on alumina for dechlorinating DDT. Studies on the dechlorination of DDT in water-acetone (1:1, v/v) and 0.05% biosurfactant phases revealed that the reaction followed second order kinetics and rate of reaction is dependent upon both initial concentrations of the target compound and Mg0/Pd4+. The presence of acid enhanced the rate of reaction by providing protons and preventing passivation of metal that occurs due to deposition of magnesium hydroxide. GC-MS analyses revealed the formation of completely dechlorinated hydrocarbon skeleton of DDT namely, diphenylethane (DPE), as the end product in both reaction phases (water-acetone and 0.05% biosurfactant in water) thereby implying the removal of all five chlorine atoms (three alkyl and two aryl) of DDT. The optimum ratio of water and acetone to facilitate successful dechlorination reaction was found to be 9:1. Results suggested that salt form (K2PdCl6) of palladium had higher potential to dechlorinate DDT as compared to pellet (Pd0-alumina) form (efficiencies of 95 and 36%, respectively, for 100 ppm initial concentration of DDT). We noted that Pd0-alumina pellets could be reused at least four times for successful dechlorination of DDT provided Mg0 granules are present in sufficient quantity. Technical grade DDT (50 ppm) containing significant amounts of DDD was dechlorinated almost completely by the Mg0/Pd4+ (10mg/0.2mg/ml) within 1h in water-biosurfactant phase. Our studies reveal that Mg/Pd system is a promising option due to its high reactivity and its ability to achieve complete dechlorination of DDT. This bimetallic system may be useful for designing indigenous permeable barriers or reactors for the treatment of DDT contaminated water.