On the nature of eutectic carbides in Cr-Ni white cast irons

The mechanical and tribological properties of white cast irons are strongly dependent on whether they contain M₇C₃ or M₃C carbides (M = Fe, Cr,etc.). In an effort to improve the wear resistance of such materials, the United States Bureau of Mines has studied the effects of adding 0.3 to 2.3 wt pct (throughout) Si to hypoeutectic irons containing approximately 8.5 pct Cr and 6.0 pct Ni. The eutectic carbides formed were identified by electron microprobe analysis, X-ray diffraction, and scanning electron (SEM) and optical microscopies. In addition, differential thermal analysis (DTA) was used to study the process of solidification. At Si contents of 0.3 and 1.2 pct, the eutectic carbides exhibited a duplex structure, consisting of cores of M₇C₃ surrounded by shells of M₃C. Additionally, the microstructure contained ledeburite (M₃C + γFe (austenite)). At the higher Si content of 1.6 pct, the eutectic carbides consisted entirely of M₇C₃, and some ledeburite remained. Last, when the Si content was raised to 2.3 pct, the eutectic carbides again consisted entirely of M₇C₃, but ledeburite was no longer formed. These observations can be explained in terms of the effects of Si and, to a lesser extent, of Ni on the shape of the liquidus surface of the metastable Fe-Cr-C phase diagram. The addition of Si reduces the roles played by the four-phase class IIp reactionL + M₇C₃ → M₃C + γFe and the ledeburitic eutectic reactionL → M₃C + γFe in the overall process of solidification. N.H. Macmillan, for-merly with the Albany Research Center.     

Microstructure and phase identification in type 304 stainless steel-zirconium alloys

Stainless steel-zirconium alloys have been developed at Argonne National Laboratory to contain radioactive metal isotopes isolated from spent nuclear fuel. This article discusses the various phases that are formed in as-cast alloys of type 304 stainless steel and zirconium that contain up to 92 wt pct Zr. Microstructural characterization was performed by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS), and crystal structure information was obtained by X-ray diffraction. Type 304SS-Zr alloys with 5 and 10 wt pct Zr have a three-phase microstructure—austenite, ferrite, and the Laves intermetallic, Zr(Fe,Cr,Ni)_2+x. whereas alloys with 15, 20, and 30 wt pct Zr contain only two phases—ferrite and Zr(Fe,Cr,Ni)_2+x. Alloys with 45 to 67 wt pct Zr contain a mixture of Zr(Fe,Cr,Ni)_2+x and Zr₂(Ni,Fe), whereas alloys with 83 and 92 wt pct Zr contain three phases—α-Zr, Zr₂(Ni,Fe), and Zr(Fe,Cr,Ni)_2+x. Fe₃Zr-type and Zr₃Fe-type phases were not observed in the type 304SS-Zr alloys. The changes in alloy microstructure with zirconium content have been correlated to the Fe-Zr binary phase diagram.     

A melting and solidification study of alloy 625

The melting and solidification behavior of Alloy 625 has been investigated with differential thermal analysis (DTA) and electron microscopy. A two-level full-factorial set of chemistries involving the elements Nb, C, and Si was studied. DTA results revealed that all alloying additions decreased the liquidus and solidus temperatures and also increased the melting temperature range. Terminal solidification reactions were observed in the Nb-bearing alloys. Solidification microstructures in gastungsten-arc welds were characterized with transmission electron microscopy (TEM) techniques. All alloys solidified to an austenitic (γ) matrix. The Nb-bearing alloys terminated solidification by forming various combinations of γ/MC(NbC), γ/Laves, and γ/M₆C eutectic-like constituents. Carbon additions (0.035 wt pct) promoted the formation of the γ/MC(NbC) constituent at the expense of the γ/Laves constituent. Silicon (0.4 wt pct) increased the formation of the yJLaves constituent and promoted formation of the γ/M₆C carbide constituent at low levels (<0.01 wt pct) of carbon. When both Si (0.4 wt pct) and C (0.035 wt pct) were present, the γ/MC(NbC) and γ/Laves constituents were observed. Regression analysis was used to develop equations for the liquidus and solidus temperatures as functions of alloy composition. Partial derivatives of these equations taken with respect to the alloying variables (Nb, C, Si) yielded the liquidus and solidus slopes t(m _L , m _S ) for these elements in the multicomponent system. Ratios of these liquidus to solidus slopes gave estimates of the distribution coefficients (k) for these same elements in Alloy 625.     

The Au-In-Pb system:The AuIn2-In-Pb portion

The Au-In-Pb system was investigated in the AuIn-In-Pb portion of the system. Pb-AuIn and Pb-AuIn₂ quasibinary systems were found:the first, consisting of a eutectic and a monotectic reaction and the second, a eutectic with an inflection in the liquidus. The chief source of information was differential thermal analyses and verification by metallography, X-ray diffraction and microprobe analyses. Four isothermal reactions were found as follows: 1) L₁ ⇌ L₂ + AuIn₂ + AuIn at 433°Celsius 2) L₁ ⇌ Pb + AuIn₂ + AuIn at 316°Celsius 3) Pb + L ⇌ α + AuIn₂ at 172°Celsius 4) α₁ + L ⇌ AuIn₂ + In at 159°Celsius. Verification of these events and the isopleth from 50-50 wt pct Pb-In, a typical solder, to ≈ 47 wt pct Au are given. This work was supported by the United States Energy Research and Development Administration.     

The ternary phase diagram,Fe-Ni-P

In order to provide the necessary phase equilibria data for understanding the development of the Widmanstatten pattern in iron meteorites, we have redetermined the Fe-Ni-P phase diagram from 0 to 100 pct Ni, 0 to 16.5 wt pct P, in the temperature range 1100° to 550°C. Long term heat treatments and 130 selected alloys were used. The electron microprobe was employed to measure the composition of the coexisting phases directly. We found that the fourphase reaction isotherm, where α+ liq ⇌ γ+ Ph, occurs at 1000° ± 5°C. Above this temperature the ternary fields α+ Ph + liq and α+ γ+ liq are stable and below 1000°C, the ternary fields ⇌+ γ + Ph and γ + Ph + liq are stable. Below 875°C a eutectic reaction, liq → γ + Ph, occurs at the Ni-P edge of the diagram. Altogether nineteen isotherms were determined in this study. The phase boundary compositions of the two-and three-phase fields are listed and are compared with the three binary diagrams. The α + γ + Ph field expands in area in each isotherm as the temperature decreases from 1000°C. Below 800°C the nickel content in all three phases increases with decreasing temperature. The phosphorus solubility in α and γ decreases from 2.7 and 1.4 wt pct at 1000°C to 0.25 and 0.08 wt pct at 550°C. The addition of phosphorus to binary Fe-Ni greatly affects the α/α + γ and γ/α + γ boundaries below 900°C. It stabilizes the α phase by increasing the solubility of nickel (α/α +γ boundary) and above 700°C, it decreases the stability field of the γ phase by decreasing the solubility of nickel(@#@ γ/α + γ boundary). However below 700°C, phosphorus reverses its role in γ and acts as a γ stabilizer, increasing the nickel solubility range. The addition of phosphorus to Fe-Ni caused significant changes in the nucleation and growth processes. Phosphorus contents of 0.1 wt pct or more allow the direct precipitation ofa from the parent γ phase by the reaction γ ⇌ α + γ. The growth rate of the α phase is substantially higher than that predicted from the binary diffusion coefficients. Formerly at Planetology Branch, Goddard Space Flight Center     

Magnetic investigation of martensite ⇌ austenite transformations in Fe-Ni-Co alloys

The martensite ⇌ austenite transformations were investigated in Fe-Ni-Co alloys containing about 65 wt pct Fe and up to 15 wt pct Co. A change in morphology of martensite from plate-like to lath-type occurred with increasing cobalt content; this change in morphology correlates with the disappearance of the Invar anomaly in the austenite. The martensite-to-austenite reverse transformation differed depending on martensite morphology. Reversion of plate-like martensite was found to occur by simple disintegration of the martensite platelets. Reverse austenite formed from lath-type martensite was not retained when quenched from much aboveA _s, with microcracks forming during theM→γ→M transformation.     

Solidification of Nb-bearing superalloys: Part II. Pseudoternary solidification surfaces

Equilibrium distribution coefficients and pseudoternary solidification surfaces for experimental superalloys containing systematic variations in Fe, Nb, Si, and C were determined using quenching experiments and microstructural characterization techniques. In agreement with previous results, the distribution coefficient, k, for Nb and Si was less than unity, while the “solvent” elements (Fe, Ni, and Cr) exhibited little tendency for segregation (k ≈ 1). The current data were combined with previous results to show that an interactive effect between k _Nb and nominal Fe content exists, where the value of k _Nb decreases from 0.54 to 0.25 as the Fe content is increased from ≈2 wt pct to ≈47 wt pct. This behavior is the major factor contributing to formation of relatively high amounts of eutectic-type constituents observed in Fe-rich alloys. Pseudoternary γ-Nb-C solidification surfaces, modeled after the liquidus projection in the Ni-Nb-C ternary system, were proposed. The Nb compositions, which partially define the diagrams, were verified by comparison of calculated amounts of eutectic-type constituents (via the Scheil equation) and those measured experimentally, and good agreement was found. The corresponding C contents needed to fully define the diagrams were estimated from knowledge of the primary solidification path and k values for Nb and C.     

Alloying mechanism of beta stabilizers in a TiAl alloy

The effects of beta stabilizers such as Fe, Cr, V, and Nb on the microstructures and phase constituents of Ti₅₂Al₄₈-xM (x=0, 1.0, 2.0, 4.0, or 6.0 at. pct and M=Fe, Cr, V, and Nb) alloys were studied. The dependence of the tensile properties and creep resistance of TiAl on the alloying elements, especially the formation of B2 phase, was investigated. Fe is the strongest B2 stabilizer, Cr is second, V is an intermediate stabilizer, and Nb is the weakest stabilizer. The composition partitioning of Fe, Cr, V, and Nb in the γ phase is affected by the formation of B2 phase. The peaks of the tensile strengths and creep rupture life of Ti₅₂Al₄₈-xM generally occur at the maximum solid solution of these elements in the γ phase, which is just before the formation of B2 phase. Ti₅₂Al₄₈-0.5Fe shows an attractive elongation of 2.5 pct at room temperature, and the Ti₅₂Al₄₈-1V, Ti₅₂Al₄₈-Cr, and Ti₅₂Al₄₈-2Nb alloys have about 1.1 to 1.3 pct elongation at room temperature. The increase of tensile strengths and creep resistance with increasing Fe, Cr, V, and Nb contents is chiefly attributed to the solid-solution strengthening of these elements in the γ phase. The appearance of B2 phase deteriorates the creep resistance, room-temperature strengths, and ductility. With respect to the maximum solid-solution strengthening, an empirical equation of the Cr equivalent [Cr] is suggested as follows: [Cr]=Cr+Mn+3/5V+3/8Nb+3/2 (W+Mo)+3Fe=1.5 to 3.0. The solid-solution strengthening mechanism of Fe, Cr, V, and Nb at room temperature arises from the increase of the Ti 3s and Al 2s binding energies in Ti-Ti and Al-Al bonds, and the retention of the strength and creep resistance at elevated temperatures in Ti₅₂Al₄₈-xM is mainly attributed to the increase of the Ti 3s and Al 2s binding energies in Ti-Al bonds in γ phase. The decrease of the Ti 3p and Al 2p binding energies in Ti-Ti, Ti-Al, and Al-Al bonds benefits the ductility of TiAl.     

Phase relationships in the Fe-Cr-Ni system at solidification temperatures

The phase relationships between the liquid phase and the primary solid phases were investigated in the iron-rich corner of the Fe-Cr-Ni system as part of a larger study of the Fe-Cr-Ni-C system. The investigation consisted of measurements and modeling of tie-lines and the liquidus surfaces of the liquid-delta (bcc) and liquid-gamma (fcc) equilibria and the peritectic surface involving all three phases in the iron-rich corner of the Gibbs triangle bounded by 0 to 25 wt pct Cr and 0 to 25 wt pct Ni (bal Fe). The temperature ranged from the melting point of iron (1811 K) to about 1750 K. Compositions for tie-lines were obtained from liquid-solid equilibrium couples and temperatures for the surfaces were obtained by differential thermal analysis. Parameters for modeling the system were then selected in the subregular solution model to minimize the square of the difference between experimental and calculated tie-lines. With one ternary parameter employed for each phase, calculations by the model are in excellent agreement with the tie-line and liquidus measurements and in fair agreement with the temperatures for the peritectic surfaceL + δ/L + δ + γ. The usefulness of the model is demonstrated by calculation of the solidification paths of selected alloys in the composition field investigated for the limiting cases of (a) complete equilibrium followed by the alloy system, and (b) no solid diffusion (i.e., segregation) with equilibrium maintained at the solidifying front and complete mixing in the liquid phase.     

Constitution of the Ni-Cr-Fe system from 0 to 40 pct fe including some effects of Ti,Al, Si,and Nb

The positions of the γ/γ + α′ solvus for nickel-rich Ni-Cr-Fe alloys containing 0 to 40 pct Fe have been determined from 816° to 1260°C (1500° to 2300°F) to within 2 wt pct Cr. In addition, the individual effects of Ti, Al, Si, and Nb on the position of the γ/γ + α′ solvus have been determined.     

A study of the weldability and weld related microstructure of cabot alloy 214

The weldability and weld metal microstructure of Cabot Alloy 214 have been investigated with a variety of experimental and analytical techniques. These include Varestraint hot crack testing, hot ductility testing, pulsed Nd:YAG laser welding, scanning and analytical electron microscopy, electron microprobe analysis, and X-ray diffraction. A heat of Alloy 214 containing intentionally alloyed B (0.003 wt pct) and Zr (0.07 wt pct) was much more sensitive to both fusion zone hot cracking as quantified by the Varestraint test and to simulated heat-affected-zone (HAZ) cracking as quantified by hot ductility testing than a heat of Alloy 214 containing no intentionally added B (0.0002 wt pct) or Zr (0.02 wt pct). Scanning electron microscopy of the high B and Zr alloy showed the presence of dendritically-shaped, Zr-rich constituents in interdendritic regions in the gas-tungsten-arc (GTA) welds. Electron microprobe analysis of these welds revealed a segregation pattern of Cr, Al, Mn, and Zr enrichment in interdendritic regions and Ni and Fe enrichment in dendrite core regions. Analytical electron microscopy revealed the presence of ZrX (X = B, C, N, O), M₂₃C₆, andγ′ in the fusion zone of GTA weld specimens,γ′ was also found in the as-received base metal and in the GTA weld HAZ. X-ray diffraction analysis of extractions from the high B and Zr GTA weld metal also indicated the presence of a ZrX-type constituent. The results of this study are in qualitative agreement with earlier work performed on alloys such as NIMONIC 90 and INCONEL 718^∗ relative to the detrimental effect of B and Zr additions on fusion zone and HAZ hot cracking susceptibility. Formerly with Sandia National Laboratories, Albuquerque, NM     

Tensile properties of directionally solidified Al-4 wt pct Cu alloys with columnar and equiaxed grains

The tensile properties of directionally solidified Al-4 wt pct Cu-0.15-0.2 wt pct Ti alloys with equiaxed grains were determined and compared with the properties of directionally solidified Al-4 wt pct Cu columnar structures. The tensile properties of the equiaxed structure were isotropic, but varied with the distance from the chill face. The mechanical properties of the equiaxed structure were generally between those of the longitudinal and transverse columnar structures. The 0.2 pct offset yield stress(σ _y, MPa) is represented as a function of the grain size,d (mm), the average concentration, C_o (wt pct), and the local concentration, C (wt pct), by σ_y = [(15.7 + 22.6 C_o) + (1.24 + 1.04 C_o)d ^-1/2] + [15.7 △C], where △C = C - C_o. The equiaxed structure exhibits inverse segregation similar to that in the columnar structure.     

Structural superplasticity at higher strain rates of hypereutectoid Fe-5.5Al-1Sn-1Cr-1.3C steel

A fine-grained ultra-high-carbon steel—UHC steel—containing 1.35 wt pct carbon, 5.5 wt pct aluminum, 1 wt pct tin, and 1 wt pct chromium exhibits fine-structure superplasticity in the temperature regime between 775 °C and 900 °C at higher strain rates up to 10⁻² s⁻¹. Thermomechanical processing was performed in order to achieve a fine-grained equiaxed microstructure consisting of κ-carbides of about 0.7 to 2.5 μm in size finely distributed within the ferritic Fe(Al, Sn, Cr) solid solution matrix with a linear intercept grain size of 3 to 5 μm. Superplasticity occurred in the strain rate regime of 10⁻⁴<- ≤10⁻² s⁻¹ with m values of 0.5 to 0.6 (stress exponent n=1.6 to 2). Tensile elongations of more than 900 pct were recorded. From thermal activation analysis, activation energies of Q=230 to 243 kJ/mole were determined, which clearly reveal a contribution of the alloying elements Al and Sn to the lattice diffusion of iron. The governing deformation mechanism is grain boundary sliding accommodated by dislocation climb controlled by lattice diffusion sustained by chemical diffusion. At very high strain rates of ≳2 · 10⁻² s⁻¹, the strain-rate-sensitivity exponent decreases to about 0.2≤m≤0.27, which indicates class II solid solution behavior of this material.     

Thermodynamics of the system NaF-AlF3. Part III: Activities in liquid mixtures

Measurements have been made of a) the sodium content of aluminum in contact with NaF-AlF₃ melts at 1020° and 1080°C, and in contact with liquid-solid mixtures along the cryolite liquidus, b) the position of the A1F₃ liquidus line, and c) the electromotive force of solid-electrolyte cells with one side in the NaF-Na₃AlF₆ two-phase region and the other on the cryolite liquidus. With previously determined data for the activity coefficient of sodium in aluminum and for ΔH°_298f for cryolite, activities of NaF and A1F₃ are calculated. As a limiting condition the melts conform to a regular solution model withRTlnγ’₁ = ∔12,200(1 − n’₁)² cal where γ’₁=a₁/n’₁ and n’₁ the molar fraction of either NaF or NaAlF₄ calculated with them as species. This model breaks down progressively as the NaAlF₄ composition is approached, the deviations starting earlier at higher temperatures. The most plausible explanation is the disproportionation equilibrium 2A1F-₄ ⇌ A1₂F-₇ + F^-, stoichiometric NaAlF₄ containing about 70 pct AlF^- ₄ at 1020° to 1080°C. The hypothetical undisproportionated NaAlF₄ has a free energy of formation from NaF(I) and AlF₃(s) of ΔG° = •101,235 + 32.085T + 5.929 × 10⁷/T. This equation, together with that above for γ’NaAlF₄, defines α AlF₃ in all regions where the regular solution model holds. Solutions of sodium in aluminum are found to conform to Henry’s law in the range 100 to 2 ppm, contrary to recent suggestions.     

The Al-Cu-Fe phase diagram: 0 to 25 At. pct Fe and 50

Isothermal sections of the Al-Cu-Fe equilibrium phase diagram at temperatures from 680 °C to 800 °C were determined in the region with 50 to 75 at. pct Al and 0 to 25 at. pct Fe using scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS) techniques. This re- gion includes the face-centered icosahedral phase (Ψ-Al₆Cu₂Fe) which has unprecedented struc- tural perfection and no apparent phason strain. The icosahedral phase has equilibrium phase fields with four distinct phases at 700 °C and 720 °C (β-Al(Fe, Cu), λ-Al₁₃Fe₄, ω-Al₇Cu₂Fe, and liquid) and three phases at 680 °C(β, ω, and λ) and 800 °C (β, λ, and liquid). The B2 ordered β phase has considerably greater solubility for Cu than previously reported, extending from AlFe to ∼Al₅₀Fe₅Cu₄₅. The equilibrium range of composition for the icosahedral phase at these temperatures was determined, and a liquidus projection is proposed.     

Microstructure and yield strength of UDIMET 720LI alloyed with Co-16.9 Wt Pct Ti

We proposed a new method for developing Ni-base turbine disc alloy for application at temperatures above 700 °C by mixing a Ni-base superalloy U720LI with a two-phase alloy Co-16.9 wt pct Ti in various contents. The microstructure and phase stability of the alloys were analyzed using an optical microscope, a scanning electron microscope, energy-dispersive spectroscopy, and an X-ray diffractometer. The yield strength was studied by compression tests at temperatures ranging from 25 °C to 1200 °C. The results show that all the alloys had a dendritic structure. Ni₃Ti (η) phase was formed in the interdendritic region in the alloys with the addition of Co-16.9 wt pct Ti, and its volume fraction increased with the increase in the addition of Co-16.9 wt pct Ti. The results of exposure at 750 °C show that the addition of Co-16.9 wt pct Ti to U720LI had a great effect on suppressing the formation of σ phase due to the reduced Cr content in the γ matrix. Compared to U720LI, the alloys with the addition of Co-16.9 wt pct Ti possessed higher yield strength. The solid-solution strengthening of γ and γ′ and higher volume fraction of γ′ were assumed to cause this strength increase.     

Evolution and coarsening of Al2O3 dispersoids at 500 °C to 600 °C in a mechanically alloyed Al-Ti-O based material

The formation and coarsening of Al₂O₃ dispersoids have been investigated at 500 °C, 550 °C, and 600 °C in a mechanically alloyed (MA) extrusion of composition Al-0.35wt pct Li-1wt pct Mg-0.25wt pct C-10vol pct TiO₂ for times up to 1500 hours. In the as-extruded condition, the dispersed phases included Al₃Ti, Al₄C₃, MgO, cubic TiO (C-TiO), monoclinic TiO (M-TiO), TiO₂, and a small amount of Al₂O₃. However, numerous Al₂O₃ dispersoids (various polymorphs: η, γ, α, and δ) with “block-shaped” morphology were formed after heat treatment due to reduction of C-TiO, M-TiO, and TiO₂. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) showed conclusively the transformation of these phases to additional Al₂O₃ and Al₃Ti. High resolution TEM showed that the α-Al₂O₃ dispersoids exhibited some lattice matching with the α-Al matrix. Coalescence of the block-shaped Al₂O₃ dispersoids occurred after heat treatment, and Al₄C₃ also became attached to them. The length and width of the block-shaped Al₂O₃ dispersoids increased by a factor of ∼1.55 between 340 and 1500 hours at 600 °C.     

Influence of aluminum and chromium on the activity of oxygen in nickel based melts at 1600?C

The activity of oxygen in technically important nickel melts containing 15 wt pct cobalt and 5 wt pct molybdenum has been determined at 1600‡C for various concentrations of chromium ranging from 0 to 30 wt pct and aluminum varying from 0 to 15 wt pct. The activity of oxygen was measured by an electrochemical technique using yttria-doped thoria electrolyte cells. The results obtained are analyzed in terms of activity coefficients of oxygen as a function of aluminum and chromium contents in the melts. Clear positive deviations of the experimentally determined from the calculated activity coefficients of oxygen were found when aluminum was added to Ni-Co-Mo melts with or without chromium. From the results obtained in the range between 1 and 10 wt pct aluminum, the following equilibrium constants for the reaction 2 [Al] + 3 [O] ⇋ Al₂O₃ in the nickel based melts at 1600‡C were calculated: loga ₀ =-2/3 log [pct Al] - 3.94 for 0 pct Cr loga ₀ = -2/3 log [pct Al] - 4.21 for 10 pct Cr loga ₀ = -2/3 log [pct Al] - 4.81 for 20 pct Cr loga ₀ = -2/3 log [pct Al] - 5.06 for 30 pct Cr.     

Mechanisms of Hf dopant incorporation during the early stage of chemical vapor deposition aluminide coating growth under continuous doping conditions

A laboratory-scale chemical vapor deposition (CVD) reactor was used to perform “continuous” Hf doping experiments while the surface of a single-crystal Ni alloy was being aluminized to form an aluminide (β-NiAl) coating matrix for 45 minutes at 1150 °C. The continuous doping procedure, in which HfCl₄ and AlCl₃ were simultaneously introduced with H₂, required a high HfCl₄/AlCl₃ ratio (>∼0.6) to cause the precipitation of Hf-rich particles (∼0.1 μm) at grain boundaries of the coating layer, with the overall Hf concentration of ∼0.05 to 0.25 wt pct measured in the coating layer by glow-discharge mass spectroscopy (GDMS). Below this ratio, Hf did not incorporate as a dopant into the growing coating layer from the gas phase, as the coating matrix appeared to be “saturated” with other refractory elements partitioned from the alloy substrate. In comparison, the Hf concentration in the aluminide coating layer formed on pure Ni was in the range of ∼0.1 wt pct, which was close to the solubility of Hf estimated for bulk NiAl. Interestingly, the segregation of Hf and the formation of a thin γ′-Ni₃Al layer (∼0.5 μm) at the coating surface were consistently observed for both the alloy and pure-Ni substrates. The formation of the thin γ′-Ni₃Al layer was attributed to an increase in the elastic strain of the β-NiAl phase, associated with the segregation of Hf as well as other refractory alloying elements at the coating surface. This phenomenon also implied that the coating layer was actually growing at the interface between the γ′-Ni₃Al layer and the β-NiAl coating matrix, not at the gas/coating interface, during the early stage of the coating growth.