Structures and superparamagnetic properties of overaged Fe–Al–Mn–C alloys

Microstructures and superparamagnetic properties in aged-hardened Fe–9%Al–30%Mn– (x)C,Si alloys, resulting from overaging at a temperature of 823 K for 48 h to 313 days, have been investigated by transmission electron microscopy (TEM), X-ray diffraction patterns, and vibrating sample magnetometry (VSM). The results reveal that the precipitate κ-phase [(Fe,Mn)₃AlC] decomposition in this alloy, overaged at 823 K for one week, resulted from two separate mechanisms: (1) wetting of the antiphase boundary segment (APBs) of D0₃ [(Fe/Mn)₃Al] domains by the B2 [(Fe/Mn)Al] phase; and (2) precipitation of the B2 [(Fe/Mn)Al] phase within the domain. A superparamagnetic behaviour was discovered when the alloy was overaged at 823 K for ≈120–313 days. The super-soft magnetic property was mainly attributable to the ferromagnetic spinel-ordered (B2 [(Fe/Mn)Al]+D0₃ [(Fe/Mn)₃Al]) phases and ordered B2 with monoclinic ′Mn structures.     

Structural chemistry and phase relations in intermetallic systems Ti–{Pd,Pt}–Al

Phase relations in the ternary systems Ti–{Pd,Pt}–Al have been experimentally established for the partial isothermal sections at 950°C in the Pd/Pt-poor region (<25 at.% Pd/Pt). The investigation is based on X-ray powder diffraction, metallography, SEM and EMPA techniques on about 45 alloys, which were prepared by various methods employing arc melting, levitation melting under argon or by powder reaction sintering in closed crucibles. Three ternary compounds were observed at 950°C in the Ti–Pd–Al system: τ₃-(Ti,Pd)(Ti,Pd,Al)₂ with Laves-MgZn₂-type, τ₂-(Ti,Al)₆(Ti,Pd,Al)₂₃₊₁ with a filled Th₆Mn₂₃₊₁-type and τ₁-(Ti,Pd,Al)(Ti,Pd,Al)₃ with AuCu₃-type. Due to the wide extension of the Laves phase field, there is no compatibility among γTiAl and τ₂-(Ti,Al)₆(Ti,Pd,Al)₂₃₊₁. The Ti–Pt–Al system at 950°C contains three ternary compounds: τ₃-(Ti,Al)(Ti,Pt,Al)₂ with Laves-MgZn₂-type, τ₂-(Ti,Al)₆(Ti,Pt,Al)₂₃₊₁ with the filled Th₆Mn₂₃₊₁-type and τ₁-(Ti,Pt,Al) with Cu-type. Compatibility exists for Al-rich γTiAl and τ₂-(Ti,Al)₆(Ti,Pt,Al)₂₃₊₁. The typical feature for both alloy systems studied is the three-phase equilibrium: ₂Ti₃Al+γTiAl+τ₃-(Ti,Pd/Al)(Ti,Pd/Pt,Al)₂. The solid solubility of palladium and platinum in the binary titanium aluminides, as observed from EMPA and X-ray data, is rather small and at 950°C accounts to about 2.5 at.% Pd and 2.0 at.% Pt. Two new oxide compounds Ti₃PdAl₂O_x and Ti₃PtAl₂O_x with a filled Ti₂Ni-type are observed in both quaternary systems.     

Processing and mechanical properties of multiphase composites based on Mo–Si–Al–C system

Mo–Si–Al–C-based multiphase compounds and their composites reinforced by micro-SiC and TiC particulates were manufactured by means of reactive hot-pressed sintering method. Their microstructure and room temperature mechanical properties were studied. The results showed that Al addition and the ratio of Si/Al exerted a remarkable effect on the reaction products in the Mo–Si–Al–C systems. For the stoichiometric Mo₅(Si,Al)₃C mixed powders with a molar ratio of Mo:Si:Al:C as 5:1.5:1.5:1, the sintered body contained Mo₃Si, Mo₃Al₂C, and Mo₅Si₃C as the major reaction products whereas and the minor phases consisted of MoSi₂, Mo₂C, and Mo(Si,Al)₂ compounds. When the starting powder mixture was off-stoichiometric with a small amount of excess Si, only Mo₂C accounted for the minor product. Moreover, the relative contents of the former three major phases were affected by the changed Si/Al ratio, where the amounts of Mo₃Al₂C and Mo₅Si₃C compounds decreased and increased, respectively with increasing Si/Al ratio. The two multiphase alloys showed poor mechanical properties, due to the existence of residual porosity. In contrast, the composites exhibited superiority in both flexural strength and fracture toughness at room temperature to the Mo–Si–Al–C-based multiphase compounds. MSAC1/20 wt.%SiC and MSAC1/20 wt.%TiC composites had a respective flexural strength and fracture toughness of 454 and 438 MPa, 4.93 and 4.85 MPa.     

Determination of the Nb–Cr–Si phase diagram using diffusion multiples

A high-efficiency diffusion-multiple approach was employed to determine the phase diagram of the Nb–Cr–Si ternary system which is critical for the design of niobium silicide-based in situ composites. These composites have high potential as a replacement for Ni-base superalloys for jet engine applications. The formation of the Nb(Cr,Si)₂ Laves phase is beneficial to the high oxidation resistance of the composites and the Nb–Cr–Si system serves as the base for understanding the Laves phase formation. The results clearly demonstrate the applicability of the diffusion-multiple approach in determining such complex phase diagrams as Nb–Cr–Si which contains 14 phases. Two isothermal sections at 1000 and 1150 °C were constructed from the results obtained from diffusion multiples using scanning electron microscopy (SEM), electron probe microanalysis (EPMA), and electron backscatter diffraction (EBSD). Three ternary compounds, CrNbSi, (Cr,Nb)₆Si₅ and (Cr,Nb)₁₁Si₈, were observed at both temperatures, and the C14 Laves phase of the Cr–Nb binary system was stabilized by Si to lower temperatures.     

Ce–Zr mixed oxides prepared in molten nitrates

Dispersed Ce_0.75Zr_0.25O₂ and Zr_0.84Ce_0.16O₂ cerium(IV)–zirconium(IV) mixed oxides were prepared by the flux method, from the reaction of hydrated Ce(NO₃)₃ and ZrOCl₂ in molten NaNO₃ at 450–500 °C in the presence of ammonium fluoride. The temperature and the stoichiometry of reactions were determined by mass spectrometry. Powder X-ray diffraction, specific surface area measurements (BET), and scanning electron microscopy were used to study the morphology and particle size distribution of the solid products. Preparation conditions were optimized to obtain pure oxides. Reaction at 550 °C in the presence of 1% wt. of ammonium fluoride gave the best result.     

In-situ P/M Al/(ZrB2+Al2O3) MMCs: processing, microstructure and mechanical characterization

In-situ metal matrix composites (MMCs) offer significant advantages over conventional MMCs from both a technical and an economic standpoint. In this paper, an in-situ MMC, i.e. Al/(10 vol.% ZrB₂+9.2 vol.% Al₂O₃), is produced starting from Al+ZrO₂+B by reactive sintering and subsequently densified by hot-pressing. The formation mechanism of ZrB₂ and Al₂O₃ in Al matrix is studied by XRD, thermal analysis and microstructural characterization. Reaction kinetics are also investigated based on the results of the reaction mechanism. The properties are evaluated in terms of microstructural characterization, Young’s modulus and bending tests. The in-situ processing involves four intermediate steps and the transitional phases are AlB₂, Zr(O, B)₂ and (Zr, Al)(B, O)₂. Regarding the reaction kinetics, conversion fraction vs time relationships have been established for the last three intermediate steps.     

Phase diagram of the Nb–Al–Si ternary system

A high-efficiency diffusion-multiple approach was employed to map the phase diagram of the Nb–Al–Si ternary system which is very valuable for the design of niobium silicide-based composites. These composites have high potential as a replacement for Ni-base superalloys for jet engine applications. Aluminum is an alloying element for these composites, thus the Nb–Al–Si phase diagram, especially solubility of Al in Nb₅Si₃, is important information for the composite design. An isothermal section at 1000 °C was constructed from the results obtained from a diffusion multiple using scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). A ternary phase Nb₃Si₅Al₂ was observed. The solubility data of Al in Nb₅Si₃ and NbSi₂ as well as Si solubility in Nb₃Al, Nb₂Al and NbAl₃ were obtained. The new isothermal section helps to judge the reliability of the existing literature results and to add new data to the Nb–Al–Si phase equilibria.     

Physical properties of Mo6−xRuxTe8 and Mo6Te8−xSx

A series of the Chevrel phases, Mo_6−xRu_xTe₈ and Mo₆Te_8−xS_x (x=0, 1, 2), has been prepared and the various physical properties, such as the elastic modulus, Debye temperature, and electrical resistivity, have been evaluated. The relationships between several properties of the compounds have also been studied. Young’s modulus and Debye temperature of Mo_6−xRu_xTe₈ and Mo₆Te_8−xS_x increase with increasing x value. The relationship between the Vickers hardness and Young’s modulus shows ceramic characteristics for Mo_6−xRu_xTe₈, while they show glass-like characteristics for Mo₆Te_8−xS_x. The electrical resistivities of Mo_6−xRu_xTe₈ and Mo₆Te_8−xS_x increase with increasing x value.     

A comparative study of precipitation behavior of Heusler phase (Ni2TiAl) from B2-TiNi in Ni–Ti–Al and Ni–Ti–Al–X (X=Hf, Pd, Pt, Zr) alloys

In support of the design of high strength TiNi-based shape-memory alloys, the precipitation of L2₁–Ni₂TiAl phase from a supersaturated B2–TiNi matrix at 600 and 800 °C is studied using transmission and analytical electron microscopy (TEM/AEM), and 3D atom-probe microscopy (3DAP) in Ni–Ti–Al and Ni–Ti–Al–X (X=Hf, Pd, Pt, Zr) alloys. A B2/L2₁ fully coherent two-phase microstructure is confirmed to be analogous to the classical γ/γ′ system in terms of precipitate shape, spatial distribution and a minimum distance of separation between L2₁ precipitates as dictated by the interplay between strain and interfacial energies. The effects are also confirmed to disappear with loss of coherency. These results lend further support, at least qualitatively, to the theoretical predictions of microstructural dynamics of coherent aggregates. Selected cohesive properties of stable and virtual B2 compounds are calculated by an ab initio method, showing good agreement with measured site occupancy and lattice parameters. A simple analysis of the L2₁ precipitate size evolution suggests that in the case of alloys with Al, Zr or Hf substitution for Ti, the precipitates follow coarsening kinetics at 600 °C and growth kinetics at 800 °C, while for alloys with Pd or Pt substitution for Ni, precipitates follow one kinetic behavior at both temperatures. The temperature-dependent partitioning behaviors of Hf, Pd, Pt and Zr are established by quantitative microanalysis using AEM and nanoscale analysis using 3DAP. Both Hf and Zr prefer to partition to the B2 phase at 800 °C while they exhibit reverse behavior at 600°C. Pt also partitions to B2 at 800 °C, while Pd partitions to the L2₁ phase at both 600 and 800 °C. To describe the composition dependence of the lattice parameter of multicomponent B2 and L2₁ phases, the atomic volumes of Al, Hf, Ni, Ti and Zr in B2 and L2₁ phases are determined, providing a model for the control of interphase misfit in alloy design.     

Er-doped SiO2–Al2O3 thin films prepared by the sol–gel route

The synthesis of homogeneous and pure silica–alumina binary glasses doped with rare-earth (RE) ions such as Er³⁺ is currently a key challenge for the development of integrated optics devices such as lasers, optical amplifiers or waveguides. In this study Er³⁺-doped SiO₂–Al₂O₃ films were prepared by the sol–gel route. Aluminium sec-butoxide, Al(O-sec-C₄H₉)₃ (ASB), and tetraethoxysilane, Si(OC₂H₅)₄ (TEOS), were used as glass oxide precursors, whereas erbium was introduced as Er(NO₃)₃. The alumina content in the silica matrix was 10 at.%, while erbium doping ranged between 200 and 5000 ppm. The preparation of the starting sol–gel solution and the layer deposition by a dip-coating procedure were performed in dry-box under nitrogen atmosphere. The obtained films were subsequently annealed in air between 300 and 1000 °C. After treatment at 500 °C, layers 200 nm thick were obtained. The composition, microstructure and surface morphology of the films were investigated by X-ray photoelectron spectroscopy (XPS), secondary-ion mass spectrometry (SIMS), glancing incidence X-ray diffraction (GIXRD) and atomic force microscopy (AFM). Crack-free, transparent, high purity films were obtained, characterised by compositional and microstructural homogeneity.     

The zirconium–platinum phase diagram

Phase relationships were studied in Pt-rich, near-equiatomic Zr–Pt alloys. The composition range of the previously unreported rhombohedral compound Zr₃Pt₄, isomorphous with Zr₃Pd₄ above room temperature, extends to the Zr-rich composition Zr₉Pt₁₁ by the formation of lattice vacancies on certain Pt sites. A metastable tetragonal Zr₉Pt₁₁ compound is formed, however, when vacancy formation is inhibited. The rhombohedral structure undergoes a displacement transformation on cooling between 90 and 140 °C to a low-temperature structure that is presumably triclinic. The orthorhombic compound ZrPt is stable from room temperature to 1590 °C where it transforms to a cubic B2-type structure. Structural data are given for the compounds ZrPt, Zr₉Pt₁₁, Zr₃Pt₄ and Zr₇Pt₁₀, and a complete Zr–Pt phase diagram is presented.     

Laser cladding of stainless steel with Ni–Cr3C2 and Ni–WC for improving erosive–corrosive wear performance

The microstructure and the erosive–corrosive wear (ECW) performance of laser-clad Ni–Cr₃C₂ and Ni–WC coatings with overlapping clad tracks (OCT) on a 0.2% C martensitic stainless steel were investigated by scanning electron microscopy (SEM), XRD, EDX techniques and ECW testing. The coating produced by completely dissolving Cr₃C₂ particles in laser melted pool is composed of austenite (γ) dendrites surrounded by a γ-M₇C₃ eutectic, whereas another one is of granular solidifying structure in which contains the incompletely dissolved WC particles. The microhardness of Ni–WC coating is higher than that of Ni–Cr₃C₂, about 300 HV average. The main reason of microhardness difference is that two coatings have different solidified structure. The comparison of ECW tests found that the reduction of ECW rate dose not occur with the increase of hardness. The Ni–Cr₃C₂ coating with lower hardness has a lower ECW rate with respect to the Ni–WC one. Both average ECW rate decreased by approximately 30% and 60% as compared to that of stainless steel substrate, and both coatings had different ECW mechanism. The increase of ECW resistance is closely related to structure state, kind and amount of carbides, microhardness and toughening ability of the clad layer.     

The ternary system Al–Ni–Ti Part I: Isothermal section at 900°C; Experimental investigation and thermodynamic calculation

Phase relations in the ternary system Al–Ni–Ti have been experimentally established for the isothermal section at 900°C for concentrations 0.1x_Al0.7. The investigation is based on X-ray powder diffraction, metallography, SEM and EMPA-techniques on about 40 ternary alloys, prepared by argon-arc or vacuum-electron beam melting of proper elemental powder blends. The existence of four ternary compounds, τ₁ to τ₄, is confirmed, however, in contrast to earlier investigations at significantly different compositions and with different shape of the homogeneity regions. This is particularly true for the phase regions of τ₃-Al₃NiTi₂ with the MgZn₂-type structure ranging from Al₃₀Ni₂₈Ti₄₂ (composition lowest in Al) to Al₅₀Ni₁₆Ti₃₄ (composition richest in Al) and for τ₂-Al₂NiTi. The complex atom site substitution mechanism in τ₃ changing from Ti/Al exchange at Al-poor compositions towards Ni/Al replacement for the Al-rich part was monitored in detail by quantitative X-ray powder diffraction techniques (Rietveld analyses). In contrast to earlier reports, claiming a two-phase region Ni{Al_xTi_1-x}₂+τ₃, we observed two closely adjoining three-phase equilibria: ₂-AlTi₃+Ni{Al_xTi_1-x}₂+ τ₄-AlNi₂Ti and ₂-AlTi₃+τ₃-Al₂NiTi₂+τ₄-AlNi₂Ti. The earlier reported “homogeneous phase at Al₂₃Ni₂₆Ti₅₁′” was shown by high resolution microprobe and X-ray diffraction measurements to be an extremely fine-grained eutectic. The experimental results are in fine agreement with the thermodynamic calculation.     

Microstructural characterization and abrasive wear performance of HVOF sprayed Cr3C2–NiCr coating

Cr₃C₂–NiCr coatings were deposited by high velocity oxy-fuel (HVOF) process with different spray parameters to examine dominant microstructural factors in abrasive wear of the coatings. The microstructure of the HVOF sprayed Cr₃C₂–NiCr coatings was characterized by scanning electron microscopy and transmission electron microscopy (TEM). The apparent average size and volume fraction of carbide particles in the coatings were estimated through a quantitative imaging analysis. The formation of carbide phases in the coating was discussed based on the TEM observation results. The abrasive wear behavior of the coating was evaluated by the dry rubber wheel abrasive wear test and the wear mechanisms were elucidated. Influences of apparent size and volume fraction of carbide particles on the abrasive wear weight loss were examined through correlating the proposed relation with the experimental results. Results showed that Cr₃C₂ particle size was significantly reduced after the spraying and Cr₇C₃ carbide was present around Cr₃C₂ particles, and Cr₂₃C₆ carbide was dispersed in NiCr alloy matrix with a nano-crystalline structure. The three carbides were formed in the coating through different mechanisms. The removal of carbide particles in the coating was mainly responsible for the abrasive wear of the coating. The content and particle size of the Cr₃C₂ carbides were the two key factors controlling the abrasive wear of the HVOF sprayed Cr₃C₂–NiCr coatings.     

HRTEM study of crystallization of Zr–Cu–Ni metallic glass

Employing differential scanning calorimetry (DSC) and high-resolution transmission electron microscopy (HRTEM), the micromechanism for crystallization of Zr₇₀Cu₂₀Ni₁₀ metallic glass under isothermal annealing conditions has been investigated. It is found that the relationship between the annealing temperature and the peak position, incubation time and ending time in the isothermal annealing DSC traces of Zr₇₀Cu₂₀Ni₁₀ metallic glass obeys a first-order exponential function. However, the time–temperature transformation curves of Zr₇₀Cu₂₀Ni₁₀ metallic glass at different crystallized volume fractions can be well fitted by a second-order exponential function. It is observed that at the initial crystallization stage some ordered atomic clusters precipitate first, acting as nucleation sites and facilitating the subsequent crystallization process, and the crystal growth process mainly proceeds through the atomic depositing on the previously formed crystals. This behavior confirms that the new micromechanism for crystallization of amorphous alloys proposed by Lu and Wang can also be applied to the new series of zirconium based amorphous alloys.     

Induced crystallization and physical properties of Li2O–CaF2–P2O5:TiO2 glass system: Part I. Characterization, spectroscopic and elastic properties

Li₂O–CaF₂–P₂O₅ glasses mixed with different concentrations of TiO₂ (ranging from 0 to 0.8 mol%) were crystallized at 500 °C. The samples are characterized by X-ray diffraction, scanning electron microscopy and differential thermal analysis techniques. Spectroscopic properties (IR and Raman) and elastic properties (viz., Young's modulus E, shear modulus G and micro-hardness H) at room temperature are studied. The X-ray diffraction and the scanning electron microscopic studies revealed the presence of lithium phosphate, lithium titanium phosphate and titanium phosphate crystal phases. The differential thermal analysis traces of these samples exhibit three crystalline temperatures. The IR and Raman spectra of these samples have exhibited bands due to TiO₄ and TiO₆ structural units in addition to the conventional bands due to various phosphate structural groups. The analysis of these results indicated that the sample crystallized with 0.6 mol% of TiO₂ possesses the highest density, high mechanical strength and more compact network.     

Spark plasma sintering and in vitro study of ultra-fine HA and ZrO2–HA powders

Spark plasma sintering (SPS) was employed to sinter RF suspension plasma sprayed HA ultra-fine powders and ZrO₂–HA nano-composite powders. The powders were sintered at 1000 °C for 5 min at 11.1 MPa and 1100 °C for 5 min at 11.1 MPa, respectively. After sintering, the samples were ground and polished for subsequent indentation and microscopy studies. The as-sintered compacts of the HA–CaP powders were studied in vitro by immersion in simulated body fluid (SBF). The in vitro studies indicated that a bio-active apatitic layer was formed as early as 1 week after immersion. Optical microscopy and SEM investigation revealed negligible porosity and dense microstructure suggesting liquid phase sintering to have taken place. Phase composition was calculated with the aid of XRD and the Rietveld method. The results indicated that the mechanical properties of the as-sintered compacts were improved in the presence of nano-ZrO₂. The Young’s modulus increased to 130 MPa and the fracture toughness was 1.6 MPa m^1/2 for ZrO₂ loading lower than 3 vol.% indicating greater enhancement of properties than that suggested by the rule of mixtures.     

Internal friction and Young’s modulus of V–Ti–Cr alloy before, during and after proton irradiation

Using a composite oscillator, the Young’s modulus and the internal friction of annealed, plastically bent and bent–straightened V–4Ti–4Cr alloy specimens have been measured, in a wide amplitude range, before, during and after proton irradiation. The proton energy and flux were 8 MeV and 10¹² p cm⁻² s⁻¹. The in situ dependencies on the proton beam current and temperature have been obtained. Softening and overheating (which looks like softening–strenghtening of the alloy) effects, which arise due to radiation damage, have been revealed. The influence of beam heating and defect accumulation–annihilation on the acoustic properties of the annealed and pre-strained samples, during and after proton irradiation, are briefly discussed.     

The lead-zirconium system: binary phases and a series of interstitial compounds of the host Zr5Pb3

The Pb-Zr system contains the phases Zr_5.8Pb (Cr₃Si-type), Zr₅Pb₃ (Mn₅Si₃) and Zr₅Pb₄ (Ti₅Ga₄). Zr₅Pb₄ has a substoichiometric region above approximately 800°C, extending to about Zr₅Pb_3.65 at 1000°C. Reactive powder sintering in sealed Ta containers at 1000–1350°C is the most effective route for the synthesis of pure phases of both the binaries and the interstitial derivatives Zr₅Pb₃Z. Twenty examples of the latter were obtained with Z — Al, Si, P, S, Fe, Co, Ni, Cu, Zn, Ga, Gc, As, Sc, Ag, Cd, In, Sn, Sb, Te, (Pb), (second period Z were not investigated). Single crystals for Z — Al, Cd, Zb, Pb_0.87, Pb_0.94 were obtained by metal flux or vapor phase transport reactions, and the last three were quantified by X-ray crystallography. Volume trends as a function of group and period follow metal/covalent radii trends for Z fairly well.     

Phase equilibria in the Fe–Al–Mo system – Part I: Stability of the Laves phase Fe2Mo and isothermal section at 800 °C

Phase equilibria in the Fe–Al–Mo system were experimentally determined at 800 °C. From metallography, X-ray diffraction and electron probe microanalysis on equilibrated alloys and diffusion couples a complete isothermal section has been established. It is shown that the Laves phase Fe₂Mo is a stable phase. The phase Al₄Mo, which only becomes stable above 942 °C in the binary system, is the only ternary compound found at 800 °C. For all binary phases the solid solubility ranges for the third component have been established. The D0₃/B2 and B2/A2 transition temperatures have been determined for a selected alloy by differential thermal analysis and transmission electron microscopy. The results confirm that the D0₃/B2 transition temperature substantially increases by the addition of Mo, while the B2/A2 transition temperature is about that for a binary alloy with the same Al content.