Coarsening kinetics of γ′ precipitates in the commercial nickel base Superalloy René 88 DT

René 88 DT samples were subjected to different cooling rates after a supersolvus treatment, and aged for varying periods of time from 25 to 200 h at 760 °C. Primary and secondary γ′ precipitate sizes were measured after each heat treatment. Coarsening rate constants were calculated and reported based on the measured precipitate sizes. When describing the change in radius (r) as a function of time (t), fits between the experimental results and analysis were investigated for two types of functional relationships, r³ vs. t and r² vs. t. The experimental rate constants derived from this analysis were compared with analytical values deduced from two different models: volume diffusion and bulk diffusion through the interface. The applicability of the two mechanisms for γ′ coarsening is discussed based upon the comparison between the analytically derived and experimentally observed values of these rate constants.     

Interdiffusion of Bi in Liquid Sn

The diffusion coefficients for the interdiffusion of Bi in liquid Sn were determined using the thin layer, long capillary technique. The effect of 0.5, 0.8, and 1.6 mm diameter capillaries on the apparent diffusion coefficient was investigated. For any temperature up to 600 °C all three diameter capillaries yielded similar interdiffusion coefficients. At 700 °C, the 1.6 mm diameter capillaries yielded concentration-penetration profiles that exhibited considerable scatter and abnormally high diffusion coefficients, indicative of the presence of convective mixing. At 800 °C, both the 0.8 and 0.5 mm capillaries yielded considerable scatter in the concentration-penetration profiles and abnormally high diffusion coefficients. Because of agreement of the results for both the 0.5 and 0.8 mm capillaries up to and including 700 °C (and the 1.6 mm capillaries up to 600 °C), it was concluded that the interdiffusion coefficients obtained are accurate for the temperature range 300 to 700 °C with no significant contribution from buoyancy driven convection, capillary wall effects and/or Marangoni convection. The interdiffusion coefficient for the diffusion of Bi in liquid Sn can be represented by: D_\textBi^\textSn = { 3.4 ±0.6 ×10^-8 } { exp [ - ( 13,600 ±1200/RT ) ] } \textm²/\texts. D_{\text{Bi}}^{\text{Sn}} = \left\{ {3.4 \pm 0.6 \times 10^{-8} } \right\}\,\left\{ {\exp \,\left[ { - \left( {13,600 \pm 1200/RT} \right)} \right]} \right\}\,{\text{m}}^{2}/{\text{s}}. The results show that Bi diffuses in Sn at a slower rate than does Sn itself. In addition, it is possible that the interdiffusion coefficient may not be predicated on simple temperature dependence in at least some binary liquid metal systems, and that the diffusion coefficient may be affected by liquid/liquid phase transformations occurring at some temperature within the liquid.     

A single step process to form in-situ an alumina foam/aluminide TBC system for alloys in extreme environments at high temperatures

A new approach to manufacture a complete thermal barrier coating system in a single step is studied in the frame of the European FP7 project PARTICOAT. Spherical μm-Al particles are deposited on different substrate alloys. During the sintering process in air, the μm-Al particles are oxidized and converted into hollow alumina spheres forming a ceramic “foam” (top coat), and simultaneously an Al rich diffusion layer (bond coat) is formed in the subsurface zone of the substrate.The “green” coatings deposited by air brush on IN738, René80 and CM247 nickel based alloys were cured at 300 °C and then isothermally exposed at 800 and 1000 °C in air for up to 1000 h. The oxide formation and the microstructure of the coatings were studied by thermo gravimetrical analysis (TGA), X-ray diffraction (XRD), scanning electron microscopy combined with energy dispersive X-ray spectroscopy (SEM-EDX).The coatings were adherent for all the substrates and temperatures tested. René80 shows the lowest mass gain whereas IN738 and CM247 show higher mass gains at the temperatures studied. Additionally, the use of reactive element oxides in the coating was investigated, demonstrating the flexibility and viability of this low cost coating concept.     

Interdiffusion in β-Ti–Zr binary alloys

The aim of this study is the determination of the interdiffusion characteristics in the β-solid solution of Ti–Zr alloys (from 830 to 1730 °C). The interdiffusion coefficients in this binary system are calculated by the den Broeder method. The interdiffusion coefficients are weakly dependent on the composition: their values lie between approximately 10⁻⁹ and 10⁻⁷ cm²/s. The Hall method is used to determine the diffusion coefficients at the Ti-rich and Zr-rich sides. These coefficients are compared with the impurity diffusion coefficients in the pure metals from the literature, and with coefficients calculated by the Vignes and Birchenhall relation. Whereas the impurity diffusion coefficients in Ti and Zr show the anomalous diffusion already observed in many body-centered-cubic metals and alloys, this behaviour is not observed for the interdiffusion in the Ti–Zr alloys. These results are compared with the results obtained in the binary systems Ti–Hf and Zr–Hf.     

Comparison of experimental and simulated multicomponent Ni-base superalloy diffusion couples

A multicomponent diffusion mobility database for the Ni-rich fcc phase [2002 Cam] is evaluated by comparing diffusion simulations to two experimental multicomponent Ni-base superalloy diffusion couples: Ni/René-88 and IN718/René-88. The diffusion simulations use composition-dependent thermodynamic and diffusion quantities within a finite difference code to simulate single-phase and multiphase planar layers. The multiphase layers consist of a matrix phase and a disperse phase. The calculated composition profiles, interdiffusion coefficients, phase fraction profiles, and location of Kirkendall porosity are compared with experimental results. To treat diffusion in the IN718 alloy, iron and carbon are added to the existing diffusion mobility database using previous assessment work and new assessments of Fe-Al and Fe-Co.     

Influences of chromium and hafnium additions on the microstructures of β-nial coatings on superalloy substrates

In this study, Hf-doped and Cr/Hf-modified NiAl coatings were deposited onto René N5^® substrates via direct current magnetron sputtering. Microstructural analysis using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electron probe microanalysis (EPMA) showed the as-deposited coatings to be single phase with B2 crystal structures. Post-deposition annealing at 1000 °C in an Ar + 5%H₂ atmosphere resulted in the formation of nanometer-sized precipitates along grain boundaries and within grain interiors. TEM analyses showed most of the precipitates to be β′-Ni₂AlHf. Three-dimensional atom probe tomography (3D-APT) also revealed the presence of α-Cr precipitates within the NiAlCrHf coatings after annealing. The results have been analyzed and discussed relative to previous research on sputter deposited NiAl–Hf coatings.     

Interdiffusion in the f.c.c. Phase of Cu-Mn Binary Alloys

Interdiffusion coefficients are calculated in the Cu-Mn binary system from 450 to 925?°C. Starting from the diffusion couples between pure Cu and Mn, the solubility limit of the manganese in copper is determined and the interdiffusion characteristics in the f.c.c. solid solution of Cu-Mn alloys are analysed. The interdiffusion coefficients in this binary system are calculated by the den Broeder method. The interdiffusion coefficients are strongly dependent on the composition: their values lie between approximately 10^?14 and 4?×?10^?9?cm²?s in this temperature range. The Vignes and Birchenall method is used to determine the impurity diffusion coefficients of manganese in pure copper. These coefficients are compared with the tracer diffusion coefficients of Mn⁵⁴ in copper from the literature. Activation energies, which lie from 155 to 180?kJ/mol, are of a vacancy controlled diffusion mechanism.     

A study of the diffusion zone in steel 07Kh16N6 and granulated nickel alloy ÉP741NP formed in the process of hot isostatic pressing

Conclusions  

Ultra-fast sulphur grain boundary segregation during hot deformation of nickel

Sulphur grain boundary segregation during hot-compression of nickel (5.4 wt. ppm S) is monitored using Auger electron spectroscopy and wavelength dispersive X-ray spectroscopy. The deformation conditions (temperature/deformation rate) investigated are: 550 °C/0, 550 °C/3 × 10^?5 s^?1, 550 °C/3 × 10^?4 s^?1 and 450 °C/3 × 10^?5 s^?1. It is shown that plastic deformation accelerates the kinetics of sulphur grain boundary segregation by a factor of ～10³ to a few 10⁵, depending on the deformation conditions. Very high levels of segregation (～0.8 monolayer of sulphur) are obtained after very low deformation (～5%). In addition a linear dependence of the segregation level with time and deformation is demonstrated. The segregation kinetics during plastic deformation is proportional to the deformation rate and almost independent of temperature. Several metallurgical mechanisms are discussed and confronted with the experimental results: dislocations dragging, pipe diffusion, dislocation collection/diffusion and acceleration by excess vacancies. It appears that the models developed in this work on the basis of the two latter mechanisms (dislocation collection/diffusion and acceleration by excess vacancies) predict the experimental data correctly.     

Aluminum Impurity Diffusion in Magnesium

The diffusion of Al in polycrystalline Mg (99.9%) was studied via depth profiling with secondary ion mass spectrometry in the temperature range of 573-673 K, utilizing the thin film method and thin film solution to the diffusion equation. Multiple samples with multiple depth profiles on each sample were obtained to determine statistically confident coefficients with a maximum standard deviation between measurements of 16%. The activation energy and pre-exponential factor of Al impurity diffusion in Mg were determined as 155 kJ/mole and 3.9 × 10⁻³ m²/s, respectively. The Mg substrates have a small grain size (~10 μm) and therefore some contributions from grain boundary diffusion are expected in the measurements. Sputter roughening during depth profiling, which is inherent to the SIMS process, also contributes to the measured diffusion coefficient, especially in samples with smaller grain sizes.     

Formation of multiple stoichiometric phases in binary systems by combined bulk and grain boundary diffusion: Experiments and model

The thermodynamic extremal principle has been used by the authors to treat the evolution of binary and multicomponent systems under the assumption that all phases are nearly stoichiometric. Up to now only bulk diffusion has been taken into account. The concept is now extended to combined bulk and grain boundary diffusion possible in each newly formed phase. The grains are approximated by cylinders allowing interface diffusion along the top and bottom of the grains and grain boundary diffusion along the mantle with different interface/grain boundary diffusion coefficients. A consistent analysis yields an effective diffusion coefficient taking into account the combined interface/grain boundary and bulk diffusion of each individual component. The current concept is applied to the Cu–Sn couple which has been studied by a number of researchers. The results of simulations are compared with experiments at 200 °C on solid systems reported in the literature as well as with our experiments at 250 °C with liquid Sn.     

Crystallographic fatigue crack initiation in nickel-based superalloy René 88DT at elevated temperature

The fatigue behavior of a polycrystalline nickel-based superalloy René 88DT was examined in the lifetime regime of 10⁵–10⁹ cycles at 593 °C in air using an ultrasonic fatigue apparatus operating at frequencies close to 20 kHz. Three experimental techniques were combined to obtain new insights into the crack initiation process: serial sectioning, electron backscatter diffraction and quantitative fractographic analysis. Most fatigue failures initiated from internal microstructural sites comprised of large grains. Large crystallographic facets formed at crack initiation sites due to cyclic strain localization on {1 1 1} slip planes in the region close to Σ3 twin boundaries in large grains having high Schmid factors. The micromechanical mechanism of crystallographic fatigue crack initiation was analyzed in terms of both resolved shear stress and elastic incompatibility stresses in regions close to Σ3 twin boundaries. The influence of critical microstructure features on fatigue crack initiation and fatigue life variability is discussed.     

A new technique for evaluating diffusion mobility parameters

The development and evaluation of a multicomponent diffusion mobility database for Ni-rich alloys are reviewed. In an effort to directly evaluate the mobility parameters in the database, a method for optimizing the mobility parameters as a function of the experimental composition profiles is proposed. This method is demonstrated for the NiCo system at 1100 °C. The optimization of four diffusion mobility parameters improved agreement between the experimental and calculated composition profiles and decreased the error between the experimental and calculated interdiffusion coefficients from 45 to <5%. This method can easily be expanded to multicomponent systems.     

Interdiffusion and reaction between Zr and Al alloys from 425° to 625 °C

Zirconium has recently garnered attention for use as a diffusion barrier between U–Mo nuclear fuels and Al cladding alloys. Interdiffusion and reactions between Zr and Al, Al-2 wt.% Si, Al-5 wt.% Si or AA6061 were investigated using solid-to-solid diffusion couples annealed in the temperature range of 425° to 625 °C. In the binary Al and Zr system, the Al₃Zr and Al₂Zr phases were identified, and the activation energy for the growth of the Al₃Zr phase was determined to be 347 kJ/mol. Negligible diffusional interactions were observed for diffusion couples between Zr vs. Al-2 wt.% Si, Al-5 wt.% Si and AA6061 annealed at or below 475 °C. In diffusion couples with the binary Al–Si alloys at 560 °C, a significant variation in the development of the phase constituents was observed including the thick τ₁ (Al₅SiZr₂) with Si content up to 12 at.%, and thin layers of (Si,Al)₂Zr, (Al,Si)₃Zr, Al₃SiZr₂ and Al₂Zr phases. The use of AA6061 as a terminal alloy resulted in the development of both τ₁ (Al₅SiZr₂) and (Al,Si)₃Zr phases with a very thin layer of (Al,Si)₂Zr. At 560 °C, with increasing Si content in the Al–Si alloy, an increase in the overall rate of diffusional interaction was observed; however, the diffusional interaction of Zr in contact with multicomponent AA6061 with 0.4–0.8 wt.% Si was most rapid.     

Glass coatings on stainless steels for high-temperature oxidation protection: Mechanisms

The oxidation behavior of a martensitic stainless steel with or without glass coating was investigated at 600–800 °C. The glass coating provided effective protection for the stainless steel against high-temperature oxidation. However, it follows different protection mechanisms depending on oxidation temperature. At 800 °C, glass coating acts as a barrier for oxygen diffusion, and oxidation of the glass coated steel follows linear law. At 700 or 600 °C, glass coating induces the formation of a (Cr, Fe)₂O₃/glass composite interlayer, through which the diffusion of Cr³⁺ or Fe³⁺ is dramatically limited. Oxidation follows parabolic law.     

Enhanced thermal stability and wear resistance of TiCN-SiC-TiN-Cr3C2-Co cermet modified by B4C for cutting tool application

In this work, a suitable cermet compositions based on TiCN-SiC-TiN-Cr₃C₂-Co-B₄C is identified for cutting tool purpose. The cermets were sintered using spark plasma sintering with the addition of different weight percentages of B₄C (5%, 10%, 15%) in TiCN - SiC - TiN - Cr₃C₂ - Co cermets. The cermets were subjected to the annealing process at three different temperatures of 600 °C, 800 °C, and 1000 °C for a constant soaking time of 4 h to study their thermal stability. All the cermets' compositions showed good thermal stability up to 800 °C and 55% TiCN – 15% SiC – 5% TiN – 5% Cr₃C₂–10% Co – 10% B₄C cermets showed better thermal stability up to 1000 °C. There was a change in microstructure and formation of the oxide phases in the cermets during the high temperature (1000 °C) annealing process and overall lead to a decrease in hardness. The sintered cermets were also subjected to sliding wear in the pin on disk apparatus. EN31 steel disk was used as a counter disk. The wear testing was done at different loads (20 N, 25 N, 30 N, 35 N) and at different sliding velocity (0.55 m/s, 1 m/s, 1.5 m/s, 2 m/s) with a constant track distance of 1000 m. Wear rate was at its highest value of 9.74 × 10⁻⁷ mm³/Nm for the cermet 65% TiCN - 15% SiC - 5% TiN - 5% Cr₃C₂₋10% Co and the lowest wear rate noted was 1.18 × 10⁻⁷ mm³/Nm for cermet 55% TiCN - 15% SiC - 5% TiN - 5% Cr₃C₂₋10% Co – 10% B₄C and the B₄C addition has improved the wear resistance of the cermets.     

Comparative Isothermal Oxidation Behaviour of New Aluminide Coatings from Slurries Containing Al Particles and Conventional Out-of-Pack Aluminide Coatings

This paper investigates the microstructural differences of an out-of-pack (SVPA) aluminide coating with a TBC/aluminide coating deposited by slurry (PARTICOAT) onto René N5 superalloy material. Their isothermal oxidation behaviour in air at 1,100 °C up to 1,000 h is compared with the support of Thermocalc^® modelling. Whereas the kinetics appears lower in the SVPA than in the slurry for short oxidation term (<100 h), the mass gains are similar after long exposure time (1,000 h). Both coatings develop a NiAl₂O₄/Al₂O₃ duplex scale, the slurry showing a thicker spinel layer than the SVPA. Interdiffusion brings about the transformation of β-NiAl into γ′-Ni₃Al in both systems which extends more in the slurry coatings, thereby inducing coarser precipitation of refractory elements. However, in contrast to the out-of-pack coatings, both the precipitates and the top coat of alumina hollow spheres developed in the slurry coatings significantly limit rumpling of the oxide scale.     

Intrinsic and Interdiffusion in Cu-Sn System

Solid-solid diffusion couples assembled with disks of copper, tin and intermetallics (Cu₃Sn and Cu₆Sn₅) were employed to investigate the Kirkendall effect in the copper-tin system at the temperature of 200 °C. In the Cu(99.9%)/Sn diffusion couple, inert alumina particles used as markers were identified in the Cu₆Sn₅ phase, while microvoids were observed at the Cu/Cu₃Sn interface. The Cu(99.9%)/Sn and Cu(99.9%)/Cu₆Sn₅ diffusion couples annealed at 200 °C for 10 days were analyzed for intrinsic diffusion coefficients of Cu and Sn in the Cu₆Sn₅ and Cu₃Sn phases, respectively with due consideration of changes in molar volume. Interdiffusion, integrated and effective interdiffusion coefficients were also calculated for the intermetallic phases. Diffusion couples annealed at 125-400 °C for various times were analyzed for the kinetic parameters such as growth rate constants and activation energies for the formation of Cu₃Sn and Cu₆Sn₅ phases. Uncertainties in the calculated intrinsic diffusivities of Cu and Sn arise mainly from the non-planar morphologies of the interfaces and the non-planar distribution of the markers. Intrinsic diffusion coefficients based on average locations of the marker plane indicate that Cu is the faster diffusing component than Sn in both the Cu₃Sn and Cu₆Sn₅ phases.     

Carbon Diffusion in Lanthanum Erbium Carbide Stabilized in the Cubic Fluorite Structure

Lanthanum erbium carbide, La_0.5Er_0.5C₂, a salt-like carbide with a cubic fluorite phase structure, has been produced from ¹³C, allowing carbon diffusion rate to be determined using ¹²C. Carbon in salt-like carbides exhibits significant ionicity, and a high carbon diffusion rate would enable a new class of high temperature fuel cells based on carbon-ion transport. The complete lack of carbon diffusion data for salt-like carbides is the motivation for this work. The carbon diffusion rate in La_0.5Er_0.5C₂ has now been determined to be 2.0 ± 0.8 × 10⁻¹³ cm²/s at 850 °C, increasing to 1.8 ± 0.8 × 10⁻¹¹ cm²/s at 1150 °C, with an activation energy of about 95 kJ/mole. These diffusion rates are too low for a carbon-ion fuel cell, but a number of other salt-like carbides exist. Be₂C, in particular, is a salt-like carbide with an antifluorite structure, and should have higher carbon-ion diffusion than cubic La_0.5Er_0.5C₂ due to the unoccupied octahedral sites in the antifluorite structure, but Be₂C presents special difficulties due to the toxic nature of its hydrolysis products.     

Investigation of diffusion and electromigration parameters for Cu–Sn intermetallic compounds in Pb-free solders using simulated annealing

An efficient numerical method was developed to extract the diffusion and electromigration parameters for multi-phase intermetallic compounds (IMC) formed as a result of material reactions between under bump metallization (UBM) and solder joints. This method was based on the simulated annealing (SA) method and applied to the growth of Cu–Sn IMC during thermal aging and under current stressing in Pb-free solder joints with Cu-UBM. At 150 °C, the diffusion coefficients of Cu were found to be 3.67 × 10¹⁷ m² s⁻¹ for Cu₃Sn and 7.04 × 10¹⁶ m² s⁻¹ for Cu₆Sn₅, while the diffusion coefficients of Sn were found to be 2.35 × 10¹⁶ m² s⁻¹ for Cu₃Sn and 6.49 × 10¹⁶ m² s⁻¹ for Cu₆Sn₅. The effective charges of Cu were found to be 26.5 for Cu₃Sn and 26.0 for Cu₆Sn₅, and for Sn, the effective charges were found to be 23.6 for Cu₃Sn and 36.0 for Cu₆Sn₅. The SA approach provided substantially superior efficiency and accuracy over the conventional grid heuristics and is particularly suitable for analyzing many-parameter, multi-phase intermetallic formation for solder systems where quantitative deduction for such parameters has seldom been reported.