Effect of alternative magnetic field on the diffusion layer growth in Al／Zn couple

The influence of an alternative magnetic field on the growth of the diffusion layer in A1-Zn diffusion couple was studied. The thickness of the diffusion layer was examined. The results show that the alternative magnetic field increases the thickness of the diffusion layer and the effect increases with the intensity and frequency of the alternative magnetic field increasing. The growth of the diffusion layer obeys the parabolic rate law and the growth rate increases with the application of the alternative magnetic field. This growth rate change is manifested through a change in the frequency factor ko and not through a change in the activation energy Q. The frequency factor k0 for the diffusion layer growth with the alternative magnetic field is 5.03cm^2/s and the one without the magnetic field is 3.84cm^2/s.     

Phase equilibria in the Ti–Al binary system

New experimental results on the phase equilibria between the β-Ti (A2), α-Ti (A3), α₂-Ti₃Al (D0₁₉) and the γ-TiAl (L1₀) phases in the Ti–Al system using specimens with low levels of oxygen are presented. The results obtained on the α/γ and the α₂/γ equilibria are in good agreement with the previous experimental and calculated phase boundaries, while the ones obtained on the α/β equilibrium deviate significantly from the previously proposed phase diagram. It is confirmed that the β phase field extends to higher aluminum contents and that the width of the α+β two-phase region is very narrow, less than 1 at.% Al. The presence of the A2/B2 order–disorder transition in the β phase is also confirmed by a combination of differential scanning calorimetric (DSC) analysis and extrapolation of ordering phase boundaries from the Ti–Al–X (X=Cr, Fe) ternary systems. A thermodynamic analysis has been carried out taking into account the ordering configurations in the β-Ti (A2)/β₂-TiAl (B2), f.c.c.-Al (A1)/γ-TiAl (L1₀) and α-Ti (A3)/α₂-Ti₃Al (D0₁₉) equilibria. It is proposed that the anomalous α/β equilibrium is due to the A2/B2 ordering reaction.     

Thermal conductivity mapping of the Ni–Al system and the beta-NiAl phase in the Ni–Al–Cr system

Micron-scale-resolution thermal conductivity mapping on graded compositions created in diffusion-multiple samples can be used to rapidly establish composition-phase-property relationships and to reveal the effects of solid-solutioning, order-disorder transition, compositional point defect, and site preference on thermal conductivity.

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Atomistic modeling of the γ and γ′-phases of the Ni–Al system

A new embedded-atom potential has been developed for Ni₃Al by fitting to experimental and first-principles data. The potential describes lattice properties of Ni₃Al, point defects, planar faults, as well as the γ and γ^′ fields on the Ni–Al phase diagram. The potential is applied to calculate the energies of coherent Ni/Ni₃Al interphase boundaries with three different crystallographic orientations. Depending on the orientation, the interface energy varies between 12 and 46 mJ/m². Coherent γ/γ^′ interfaces existing at high temperatures are shown to be more diffuse and are likely to have a lower energy than Ni/Ni₃Al interfaces.     

Phase equilibria in the Al–Mo–Si system

The ternary Al–Mo–Si phase diagram was investigated by a combination of optical microscopy, powder X-ray diffraction (XRD), differential thermal analysis (DTA), electron probe microanalysis (EPMA) and scanning electron microscopy (SEM). Ternary phase equilibria were investigated within two isothermal sections at 600 °C for the Mo-poor part and 1400 °C for the Mo-rich part of the phase diagram. The solubility ranges of several phases including MoSi₂ (C11_b) as well as Mo(Si,Al)₂ with C40 and C54 structure were determined. The binary high temperature phase Al₄Mo was found to be stabilized at 600 °C by addition of Si. DTA was used to identify 9 invariant reactions and thus constructing a ternary reaction scheme (Scheil diagram) in the whole composition range. A liquidus surface projection was constructed on basis of the reaction scheme in combination with data for primary crystallization from as-cast samples determined by SEM measurements.     

Phase equilibria in the system Al–Co–Si

The Al–Co–Si system was studied for three isothermal sections at 600 °C (equilibria with Si), 800 °C (alloys up to 50 at.% Co) and 900 °C (alloys with more than 50 at.% Co). A total number of seven ternary compounds were characterized in the ternary system and the homogeneity ranges of the various ternary solid solutions of binary Co–Al and Co–Si compounds were studied. X-ray powder diffraction and optical microscopy was used for initial sample characterization and electron probe microanalysis of the annealed samples was used to determine the phase compositions within the ternary system. Lattice parameters have been determined for all ternary compounds and the change of lattice parameters with the composition is given for the solid solution phases.     

Thermodynamic reassessment of Ni–Ga binary system

New experimental measurements of the phase diagram and the mixing enthalpy of liquid phase along with the previous experimental data were used in a reassessment of the Ni-Ga system.A set of self-consistent thermodynamic parameters of the Ni-Ga system is obtained using the calculation of phase diagram(CALPHAD) technique,and the available phase diagram and thermodynamic data are reproduced well within experimental error limits.Some noticeable improvements are obtained in the present work compared with the previous assessment:(1) the calculated Ga-rich liquidus is more reasonable;(2) Ni_3Ga_7 is adopted as the most Ga-rich compound rather than NiGa_4;(3) the Ni_5Ga_3 phase is treated as the nonstoichiometric compound with consideration of its narrow homogeneity range;(4) the phase transformation between B8_(1.5)_Ni_3Ga_2 and Ni_(13)Ga_9 is considered instead of treating them as Ni_3Ga_2 phase simply;(5) the latest experimental data of mixing enthalpy for the liquid phase are adopted.The present study can be used as a basis for the development of a thermodynamic database of the Ni-based semiconductor alloy systems.     

Phase equilibria and thermodynamic calculation of the Co–Ta binary system

Experimental investigation and thermodynamic evaluation of the Co–Ta binary phase diagram was carried out. Equilibrium compositions obtained in two-phase alloys and diffusion couples were measured by electron probe microanalyzer (EPMA). A very narrow λ₃(C36) + λ₂(C15) two-phase region is confirmed to be present around 26.5 at.% Ta at temperatures between 950 °C and 1448 °C. Equilibrium relationships above 1500 °C among the liquid, Laves (λ₁(C14), λ₂ and λ₃, whose stoichiometry is described by Co₂Ta), μ(D8_b) and CoTa₂(C16) phases were investigated by microstructural examination in as-cast Co-(24–60 at.%)Ta alloys. The solvus temperature of the γ′ Co₃Ta (L1₂) phase precipitated in the 5.8 at.%Ta γ(Co) and the peritectoid temperature of the Co₇Ta₂ phase in an 8.5 at.%Ta alloy were determined to be 1013 °C and 1033 °C, respectively, by differential scanning calorimeter (DSC). Fine precipitates of the γ′ phase precipitated in the γ (A1) matrix were observed by transmission electron microscope (TEM). Analyzing the present experimental results synthetically, the γ′ Co₃Ta phase was identified to be a metastable phase, of which the γ/γ′ transition temperature of the stoichiometric Co₃Ta alloy was estimated to be 2000 °C. Thermodynamic assessment of the Co–Ta binary system was carried out based on the present results as well as on experimental data in the literature. Calculated results of not only stable but also metastable equilibria were found to be in good agreement with the revised phase diagram. The evaluated stability of the metastable γ′ Co₃Ta coincides with the enthalpy of formation (ΔH(γ'Co₃Ta) = −23.44 kJ/mol) calculated by the ab initio method.     

Phase equilibria of the long-period stacking ordered phase in the Mg–Ni–Y system

Phase equilibria in the Mg-rich Mg–Ni–Y system at 300, 400 and 500 °C have been experimentally investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), electron probe micro-analyzer (EPMA) and transmission electron microscopy (TEM). The results show that a long-period stacking ordered (LPSO) phase with 14H structure is thermodynamically stable in the Mg–Ni–Y system in a wide temperature range, but it dissolves varying from 492 to 559 °C depending on the alloy composition. The equilibrium 14H phase has a very limited solid solution range, and can be nearly regarded as a ternary stoichiometric compound with a formulae as Mg₉₁Ni₄Y₅. The isothermal sections of the Mg-rich Mg–Ni–Y system at 300, 400 and 500 °C have been finally established, and a eutectic reaction, Liquid ↔ α-Mg + 14H + Mg₂Ni, has been determined occurring at 492 °C with a liquid composition about Mg_84.8Ni_12.0Y_3.2.     

Interdiffusion in pseudobinary sections of Ni3Al–Co ternary system

Interdiffusion in Ni₃Al intermetallic alloyed by 0–19 at.% Co was studied in three pseudo-binary sections: c_Al=22, c_Co=3.5, and c_Co=16 (c in at.%). The experiments were carried out at temperatures between 1173 and 1473 K. An attempt was made to analyze the interdiffusion coefficient into intrinsic diffusivities of individual components. Since no detectable shift of tungsten inert markers was observed, it was concluded that the intrinsic diffusivities of element pairs with varying concentration (Al/Ni and Co/Ni) in respective pseudo-binary sections are approximately equal one to another. The interdiffusion in pseudo-binary alloys Al/Ni (c_Co=const.) runs slightly faster than that in pseudo-binary alloys Ni/Co (c_Al=const.). The relations between the measured interdiffusion coefficients in the Ni_3−xAlCo_x ordered γ′ phase agree well with literature data for the interdiffusion coefficients in both the ordered γ′ and γ′+Co phases and also for the disordered Al–Ni and Co–Ni solid solutions. They are qualitatively explained by mutual interaction between constituents and by their interaction with vacancies.     

Reaction-sintering of intermetallic alloys of the Ni–Al–Mo system

A powder metallurgy route has been used for producing binary and ternary alloys of the Ni–Al–Mo system. Elemental powder mixtures were compacted and, then, sintered in a dilatometer. In this way the dimensional changes involved with thermally induced transformations could be followed during continuous heating runs up to the sintering temperatures. Sintering was assisted by the formation of a liquid phase, promoted by the heat output coming from the intermetallic phase formation reactions. The amount of liquid phase and the efficiency of sintering was highly dependent on the heating rate. A threshold value for optimal densification was identified for some compositions. The effect of other processing parameters, such as pre-sintering compaction pressure and sintering atmosphere has been considered too. The characterisation of the final products was mainly based on X-ray diffraction analyses. The microstructural parameters and the phase composition of the sintered materials were evaluated. On the basis of these results it is possible to draw some conclusions concerning the main phenomena occurring during the sintering process.     

Miscibility gap of B2 phase in NiAl to Cu3Al section of the Cu–Al–Ni system

The phase separation in the bcc phase of the Cu–Al–Ni system at 600–700 °C was investigated mainly by energy dispersion X-ray spectrometry (EDS) and differential scanning calorimetry (DSC). The compositions of the β₁ (A2 or B2: Cu-rich), β₂ (B2: NiAl-rich) and γ (γ-brass type) phases in equilibrium were determined. It was found that there is a β₁+β₂ miscibility gap in the β phase region as previously reported by Alexander. It was confirmed by means of high temperature in situ TEM observation that this miscibility gap consists of the B2+B2 phases but not the A2+B2 phases which is sometimes observed in many other Ni–Al and Co–Al base ternary bcc alloys. Thermodynamic calculation was performed which indicates that this characteristic feature suggests that the β₁ (B2)+β₂ (B2) miscibility gap is a part of a Cu-rich B2+NiAl-rich B2 miscibility gap island formed around the center of the composition triangle of the isothermal section. The phase separation in the β phase region and the stability of the ordered bcc aluminide are presented and discussed.     

Phase equilibria and structural investigations in the system Al–Fe–Si

The Al–Fe–Si system was studied for an isothermal section at 800 °C in the Al-rich part and at 900 °C in the Fe-rich part, and for half a dozen vertical sections at 27, 35, 40, 50 and 60 at.% Fe and 5 at.% Al. Optical microscopy and powder X-ray diffraction (XRD) was used for initial sample characterization, and Electron Probe Microanalysis (EPMA) and Scanning Electron Microscopy (SEM) of the annealed samples was used to determine the exact phase compositions. Thermal reactions were studied by Differential Thermal Analysis (DTA). Our experimental results are generally in good agreement with the most recent phase diagram versions of the system Al–Fe–Si. A new ternary high-temperature phase τ₁₂ (cF96, NiTi₂-type) with the composition Al₄₈Fe₃₆Si₁₆ was discovered and was structurally characterized by means of single-crystal and powder XRD. The variation of the lattice parameters of the triclinic phase τ₁ with the composition Al_2+xFe₃Si_3?x (?0.3 < x < 1.3) was studied in detail. For the binary phase FeSi₂ only small solubility of Al was found in the low-temperature modification LT-FeSi₂ (ζ_β) but significant solubility in the high-temperature modification HT-FeSi₂ (ζ_α) (8.5 at.% Al). It was found that the high-temperature modification of FeSi₂ is stabilized down to much lower temperature in the ternary, confirming earlier literature suggestions on this issue. DTA results in four selected vertical sections were compared with calculated sections based on a recent CALPHAD assessment. The deviations of liquidus values are significant suggesting the need for improvement of the thermodynamic models.     

Phase equilibria in Ti–Ni–Pt ternary system

Phase equilibria in Ti–Ni–Pt ternary system have been experimentally determined through diffusion triple technique combined with alloy samples approach. Assisted with electron probe microanalysis (EPMA) and X-ray diffraction (XRD) techniques, isothermal sections at 1073 and 1173 K of this system were constructed and existence of ternary phase Ti₂(Ni,Pt)₃ was confirmed. In addition, binary compounds Ti₃Pt₅ and TiPt_3– were found to be stable at 1073 and 1173 K, and remarkable ternary solubility in some binary compounds was detected, e.g., solubility of Pt in TiNi can be up to about 36% (molar fraction) at 1073 K and 40% (molar fraction) at 1173 K. Furthermore, a ternary invariant transition reaction TiNi₃+Ti₃Pt₅→Ti₂(Ni,Pt)₃+TiPt₃₊ at a temperature between 1073 and 1173 K was deduced.