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1.
The oxidation resistance of ZrSi2-4 at.% Al prepared by reaction sintering was evaluated in air at 700°C. Two different stages were distinguished. In the first regime the oxidation rate remains high whereas the second one is characterised by a marked decrease on the oxidation rate. Passage from the first to the second stage, associated with a transition stage, is determined by the formation of a continuous protective alumina-rich layer at the scale/alloy interface. An oxidation mechanism is proposed to explain the different stages occurring through the oxidation. Grain boundaries contribute to the fast inward oxygen transport into the alloy, leading to internal oxidation of non-reacted silicon and aluminium during the reactive sintering stage of the processing. Oxidation resistance could be improved by increasing the grain size of the alloy and by using an appropriated processing method addressed to minimise or suppress the presence of silicon and aluminium particles at ZrSi2 grain boundaries. 相似文献
2.
A. Littner M. Francois B. Malaman J. Steinmetz M. Vilasi E. Elkaim 《Intermetallics》2003,11(11-12):1223
Efforts to improve the high temperature behavior of MoSi2 in oxidizing environments led to the investigation of the Mo–Ru–Si phase diagram. The isothermal section at 1673 K was determined by X-ray diffraction, optical and scanning electron microscopies and EPMA. Five new silicides were identified and their crystallographic structure was characterized using conventional and synchrotron X-ray as well as neutron powder diffraction. Mo15Ru35Si50, denoted α-phase, is of FeSi-type structure, space group P213, a=4.7535 (5) Å, Dx=7.90 g. cm−3, Bragg R=7.13. Mo60Ru30Si10 is the ordered extension of the Mo70Ru30 σ-phase with space group P42/mnm, a=9.45940(8) Å, c=4.94273(5) Å, Dx=6.14 g. cm−3, Bragg R=5.75. 相似文献
3.
The effect of Mo on the structure–property relationships of arc-melted cast in situ composites of Nb solid solution (Nbss) and -(Nb,Mo)5Si3, with hypereutectic Nb–19.1Si–5.2Mo (A) and Nb–17.9Si–26.3Mo (B), and hypoeutectic Nb–12.8Si–4.1Mo (C) and Nb–12.3Si–14.8Mo (D) compositions has been studied. The influence of Mo concentration on lattice constants and microhardness of the constituent phases has been analyzed. The morphology, volume fraction and size distribution of the primary phase and the eutectic have been critically examined and related to the elastic modulus, hardness and fracture toughness of the composites. Fracture toughness values, determined through three-point bend and indentation tests, increase with higher amount of coarse eutectic but decrease with increasing Mo content. Indentation cracking exhibits R-curve type behaviour, promoted through bridging or arrest of cracks by the coarse Nbss in the eutectic. The study suggests promise for near-eutectic compositions of Nb–Si–Mo ternary system with low concentration of Mo for structural applications. 相似文献
4.
Microstructure and mechanical properties were investigated in a directionally solidified (DS) Ni–21.7Al–7.5Cr–6.5Ti (at.%) alloy. The dendrites of the as-grown alloy were composed of β(B2)-matrix (NiAl), coarse γ′(L12)-particles (Ni3Al), fine γ′-needles and spherical α(A2)-precipitates (Cr-based solid solution). The majority of fine γ′-precipitates was found to be twinned. The interdendritic region contained γ(A1)-matrix (Ni-based solid solution) separating ordered domains of γ′-phase and fine lath-shaped α-precipitates. Ageing in the temperature range 973–1373 K decreased the volume fraction of dendrites from about 50 vol.% measured in the as-grown material to about 38 vol.% in the material aged at 1373 K for 300 h. During ageing in the temperature range 973–1273 K the γ-phase transformed to the γ′-phase in the interdendritic region. This transformation was connected with precipitation of lath-shaped α-precipitates. Ageing at higher temperatures of 1373 and 1473 K resulted in stabilisation of the γ-phase and precipitation of spherical γ′-particles in the interdendritic region. Ageing at 973 K significantly increased the microhardness, hardness and decreased room-temperature tensile ductility. Neither ageing nor finer dendritic microstructure were found to be effective in increasing the ductility of the alloy. The measured tensile ductility up to 1.1% can be attributed to the effect of extrinsic toughening mechanisms operating in the β-phase such as blunting and bridging of cracks by the α- and γ′-precipitates. 相似文献
5.
A statistical-thermodynamic model for binary nonstoichiometric A5B3 with D81-structure has been established on the basis of the grand canonical ensemble. In the model the average chemical potentials according to the composition and a new extra equation as a boundary condition are proposed for more accuracy and free experimental data calculations. The model can be used in any A5B3 type compounds with D81 structure. The establishment method of the statistical-thermodynamic model can be extended to other complicated structures. We apply the statistical-thermodynamic model to α-Nb5Si3 and the parameters used in the model are obtained from the first-principles' calculations. From these parameters the expression for the point defect concentrations as a function of compositions and temperatures is obtained by numeral calculations. The constitutional defects and thermal defects in α-Nb5Si3 are discussed. 相似文献
6.
We have confirmed polymorphism of PrIr2Si2 and performed the in situ high-temperature X-ray powder diffraction (XRPD) experiments focused on the dynamics of the crystallographic phase transition from the high-temperature CaBe2Ge2-type phase to the low-temperature ThCr2Si2 phase above 250 °C. A double-exponential time evolution of this phase transformation has been observed at 325 °C. We have also performed density functional calculations to analyze why the low-temperature crystallographic modification becomes the ground state crystal structure. The important role of the c/a ratio in this process can be argued from results of calculations. 相似文献
7.
P. Saravanan M. Manivel Raja R. Gopalan N.V. Rama Rao K. Suresh D.V. Sridhara Rao V. Chandrasekaran 《Intermetallics》2008,16(5):636-641
The SmCo5/α-Fe nanocomposite powders were prepared by high energy ball milling and the inter-diffusion reaction between the SmCo5 and α-Fe magnetic phases were investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), vibrating sample magnetometry (VSM) and 57Fe Mössbauer spectroscopy. While structural and magnetic measurements could reveal only the presence of SmCo5 and α-Fe phases, Mössbauer studies could clearly specify the extent of alloying between Fe and Co atoms in terms of evolution of α-Fe(Co) phase as a function of milling time. It has been found that the fractional volume of α-Fe(Co) solid solution tends to increase at the expense of the initial α-Fe phase upon progressive milling. 相似文献
8.
The effects of Fe and Ti on the microstructure and hardness of the as cast and heat treated Nb–24Ti–18Si–5Fe–5Sn (NV8) and Nb–45Ti–15Si–5Fe–5Sn (NV4) alloys were studied. The microstructure of NV8-AC consisted of (Nb,Ti)ss, (Nb,Ti)3Sn, (Nb,Ti)5Si3, (Nb,Ti)3Si, FeNb4Si, and Fe2Nb3 and a Ti rich oxide. The microstructure of NV8-HT consisted of (Nb,Ti)3Si, (Nb,Ti)3Sn and the Ti rich oxide. In NV8 the formation of Nb5Si3 was destabilised, the stability of Nb3Si was enhanced and the eutectic between Nb5Si3 and the solid solution was suppressed. The microstructure of NV4-AC contained Ti rich and Nb rich solid solutions, 3-1 and 5-3 silicides. The FeNb4Si and Fe2Nb3 phases and the Ti rich oxide observed in NV8-AC were not formed in NV4-AC. The microstructure of NV4-HT consisted of (Ti,Nb)3Sn, β(Ti,Nb)ss, (Ti,Nb)3Si and (Ti,Nb)5Si3 phases. The solubility of Fe in the Ti-based 3-1 silicide was significantly lower than in the Nb-based 3-1 silicide. The β(Ti,Nb)ss + (Ti,Nb)5Si3 → (Ti,Nb)3Si transformation was enhanced in NV4. The effects of Fe and Ti on the hardness of Nb–18Si–5Sn-based alloys, and of alloying elements on the hardness of Nb3Sn, Ti3Sn, and Nb3Si, Ti3Si, and Ti and Nb base 5-3 silicides are discussed. 相似文献
9.
In order to improve the thermoelectric properties via efficient phonon scattering Didymium (DD), a mixture of Pr and Nd, was used as a new filler in ternary skutterudites (Fe1−xCox)4Sb12 and (Fe1−xNix)4Sb12. DD-filling levels have been determined from combined data of X-ray powder diffraction and electron microprobe analyses (EMPA). Thermoelectric properties have been characterized by measurements of electrical resistivity, thermopower and thermal conductivity in the temperature range from 4.3 to 800 K. The effect of nanostructuring in DD0.4Fe2Co2Sb12 was elucidated from a comparison of both micro-powder (ground in a WC-mortar, 10 μm) and nano-powder (ball-milled, 150 nm), both hot pressed under identical conditions. The figure of merit ZT depends on the Fe/Co and Ni/Co-contents, respectively, reaching ZT > 1. At low temperatures the nanostructured material exhibits a higher thermoelectric figure of merit. The Vickers hardness was measured for all samples being higher for the nanostructured material. 相似文献
10.
A new polymorph of NiSb2 was obtained under high-pressure/temperature condition of 6 GPa and 550–650°C. The crystal structure is orthorhombic with the space group Pbca (No. 61), isostructural with the low-temperature form of NiAs2 (pararammelsbergite). The crystallographic parameters were refined by the Rietveld analysis of the powder X-ray diffraction data (a=0.62866(2) nm, b=0.63643(2) nm, c=1.23670(3) nm, Z=8). The structure can be regarded as an intermediate structure between the marcasite-type and the pyrite-type. The pararammelsbergite-type NiSb2 is a high-pressure form of NiSb2 and the density is slightly higher than that of ambient pressure form with the marcasite-type structure. 相似文献
11.
The Re–Si system is assessed thermodynamically, using the CALPHAD method. The calculated phase diagram and thermodynamic properties are in good agreement with available experimental data. Calculated enthalpies and entropies of fusion are compared with available data for other transition metal silicides, against melting points, showing good agreement with the general trends. This is a useful approach for thermodynamic assessment of alloy systems, where experimentally measured thermodynamic data are limited. The stability of the amorphous phase in this system has also been discussed. 相似文献
12.
The effects of Sn and Ti on the microstructure and hardness of the as cast and heat treated Nb–18Si–5Sn (NV9) and Nb–24Ti–18Si–5Sn (NV6) alloys were studied. In both alloys the phases present in the as cast and heat treated microstructures were Nbss, Nb3Sn and Nb5Si3. In NV9, Sn suppressed the formation of Nb3Si, partitioned in Nbss stronger than in Nb5Si3 and did not affect significantly the solubility of Si in the Nbss. In NV6, the solubility of Ti in (Nb,Ti)ss increased in the presence of Sn, the concentration of Ti in Nb5Si3 was sensitive to cooling rate and the solubility of Sn in Nb5Si3 decreased as the concentration of Ti increased. The Ti controlled the partitioning of Si between (Nb,Ti)ss and Nb3Sn and was considered responsible for the macrosegregation of Si in the as cast ingot. The transformation of β to Nb5Si3 was enhanced by the synergy of Sn and Ti. The addition of Ti did not destabilise the Nb3Sn. Silicon increased the hardness of Nb3Sn significantly, Sn did not affect the hardness of Nb5Si3 and Ti reduced the hardness of Nb3Sn and Nb5Si3 significantly. The hardness of NV9 and NV6 decreased and increased, respectively, by heat treatment. The reduction of the hardness of NV6-AC compared to NV9-AC is attributed to the strong effect of Ti on the hardness of Nb3Sn and Nb5Si3. 相似文献
13.
L. Sojka M. Manyako R. erný M. Ivanyk B. Belan R. Gladyshevskii Ya Kalychak 《Intermetallics》2008,16(5):625-628
The Nd11Pd4In9 compound was prepared by arc melting of pure metals under an argon atmosphere. Crystal structure was refined from X-ray single crystal diffractometer data (space group Cmmm, a = 14.843(3), b = 22.284(3), c = 3.7857(6) Å, Z = 2, RI = 0.0584, 653 F2 values). It has own structure type and together with Mn2AlB2, Cr3AlB4, Mo2FeB2 and Lu5Ni2In4 structure types belongs to homological series based on AlB2 and CsCl structure types with common formula Rm+nM2nXm. 相似文献
14.
15.
Marta Lipiska-Chwaek Sergiy Balanetskyy Carsten Thomas Stefan Roitsch Michael Feuerbacher 《Intermetallics》2007,15(12):1678-1685
The development of a single-crystal growth route for the complex metallic alloy phase β-Al3Mg2 is presented. After initial probing of the phase diagram in the vicinity of the existence range of the β-phase, we performed single-crystal growth experiments employing various techniques. The Czochralski and self-flux growth turned out to be the most suitable for this phase, and with both we reproducibly achieved single crystals of several cubic centimeters in volume. While the Czochralski technique allows for the production of deliberately oriented single crystals, the self-flux technique is capable of producing very large but unoriented single grains. 相似文献
16.
Y.C. Chen J. Xu X.H. Fan X.F. Zhang L. Han D.Y. Lin Q.H. Li C. Uher 《Intermetallics》2009,17(11):920-926
The as yet unresolved microstructure of the periodic layers formed in the reactive diffusion system Mg/SiO2 was clarified by using high-resolution field-emission SEM. The periodic layered structure actually consists of the single-phase layer of Mg2Si and the two-phase layer of (Mg2Si + MgO) alternated within the reaction zone. According to the experimental observations and in line with the diffusion-induced stresses model, the mechanism controlling this phenomenon could be attributed to the stresses induced by the difference in interface growth rates of Mg2Si and MgO phases within the layer. When the elastic deformation of the slow-growing aggregated-MgO phase reaches its elastic maximum, it will be split off from the reaction front by the neighboring Mg2Si phase and a new periodic layer forms. The computer simulation results are coinciding well with the experimental data. 相似文献
17.
J.P. Lei X.L. Dong X.G. Zhu M.K. Lei H. Huang X.F. Zhang B. Lu W.J. Park H.S. Chung 《Intermetallics》2007,15(12):1589-1594
Intermetallic Fe–Sn nanoparticles were prepared by a DC arc discharge method and investigated using X-ray diffraction, high-resolution transmission electron microscopy (HRTEM) and differential scanning calorimetry (DSC) analysis. The synthesized nanoparticles have a shell/core structure with an SnO2 shell of 5–10 nm in thickness and a core of polycrystalline intermetallic compounds. It was found that the intermetallic compounds FeSn2 and Fe3Sn2 were generated and coexist with the Sn phase as a single nanoparticle. The DSC analysis showed that Fe–Sn nanoparticles have high oxidation resistance in air. A mechanism of the nanoparticle formation was also interpreted on the basis of experimental results and thermodynamic principles. 相似文献
18.
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 Ti3PdAl2Ox and Ti3PtAl2Ox with a filled Ti2Ni-type are observed in both quaternary systems. 相似文献
19.
《Intermetallics》2015
Four Nb–Si based ultrahigh temperature alloys with compositions of Nb-22Ti-16Si-5Cr-3Al-mHf-nB ((m, n) = (0, 0), (0, 2), (4, 0) and (4, 2), respectively) (at.%) were prepared by vacuum non-consumable arc melting and then heat-treated at 1450 °C for 50 h. The effects of B and Hf additions on the phase selection, phase stability, microstructure and microhardness of these alloys under both as-cast and heat-treated conditions have been investigated. The results show that the microstructures of all the four alloys are composed of primary silicide blocks, Nbss dendrites together with one or two types of eutectic colonies around. However, the crystal structures of silicides, types of eutectic and amounts of constituent phases have obviously varied with B or Hf addition. Moreover, a low melting point three-phase eutectic is also observed in Hf-containing alloys. After 1450 °C/50 h heat-treatment, the microstructural uniformity of the alloys has been significantly ameliorated as well as their equilibrium phases have been basically obtained, and also some phase transformation reactions have occurred. The microhardness of the constituent phases present in the alloys is dependent on their types or crystal structures. 相似文献
20.
Multiphase Al65Cu20Ti15 intermetallic alloy matrix composite, dispersed with 10 wt.% of TiO2 nanoparticles, has been processed by mechanical alloying, followed by spark plasma sintering under pressure in the temperature range of 623–873 K. Differential scanning calorimetry and X-ray diffraction suggest that equilibrium crystalline phases evolve from the amorphous or intermediate crystalline phases. Transmission electron microscopy shows that the composite sintered at 873 K has partially amorphous microstructure, with dispersion of equilibrium, crystalline, intermetallic precipitates of Al5CuTi2, Al3Ti, and Al2Cu of 25–50 nm size, besides the TiO2. The composite sintered at 873 K exhibits little porosity, hardness of 5.6 GPa, indentation fracture toughness in the range of 3.1–4.2 MPa√m, and compressive strength of 1.1 GPa. Indentation crack deflection by TiO2 particle aggregates causes increase in fracture resistance with crack length, and suggests R-curve type behaviour. The study provides guidelines for processing high strength amorphous–nanocrystalline intermetallic composites based on the Al–Cu–Ti ternary system. 相似文献