Determination of the Fe-Ni and Fe-Ni-P phase diagrams at low temperatures (700 to 300 °C)

The α + γ two-phase fields of the Fe-Ni and Fe-Ni (P saturated) phase diagrams have been determined in the composition range 0 to 60 wt pet Ni and in the temperature range 700 to 300 °C. The solubility of Ni in (FeNi)₃P was measured in the same temperature range. Homogeneous alloys were austenitized and quenched to form α₂, martensite, then heat treated to formα (ferrite) + γ (austenite). The compositions of the α and γ phases were determined with electron microprobe and scanning transmission electron microscope techniques. Retrograde solubility for the α/(α + γ) solvus line was demonstrated exper-imentally. P was shown to significantly decrease the size of the α + γ two-phase field. The maximum solubility of Ni in α is 6.1 ± 0.5 wt pct at 475 °C and 7.8± 0.5 wt pct at 450 °C in the Fe-Ni and Fe-Ni (P saturated) phase diagrams, respectively. The solubility of Ni in α is 4.2 ± 0.5 wt pct Ni and 4.9 ± 0.5 wt pct Ni at 300 °C in the Fe-Ni and Fe-Ni (P saturated) phase diagrams. Ternary Fe-Ni-P isothermal sections were constructed between 700 and 300 °C. Formerly Research Assistant in Department of Metallurgy & Materials Engineering, Lehigh University, Bethlehem, PA.     

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     

Driving force for discontinuous coarsening in a Ni-Al-Mo base superalloy

The driving force and mechanism for discontinuous coarsening (DC) in a Ni-Al-Mo base superalloy were studied. Samples were solutionized at 1300 °C, cooled, and aged at 1150 °C to produce DC. Both the intragranular and DC-produced microstructure were studied using optical and scanning electron microscopy of metallographically prepared surfaces and scanning transmission electron microscopy (STEM) of thin foils. Resulting microstructural and microchemical information were analyzed to elucidate the DC process. Grain boundary diffusion of Mo appeared to control the DC growth rate. The principal contributor to the DC driving force was determined to be coarsening of semicoherentγ′ with coarsening of the Mo-rich a phase makingα minor contribution. Using the estimated driving force, grain boundary diffusivities were calculated which were within a factor of 3 of published values for stationary boundaries of comparable misorientation.     

Low-temperature extension of the lehrer diagram and the iron-nitrogen phase diagram

Iron layers are nitrided in mixtures of ammonia and hydrogen at low temperatures, using a thin nickel caplayer as a catalyst. In the coordinate field of inverse temperature vs nitriding potential, we determined the boundaries between areas in which the α, γ′, or ε phases are in thermal equilibrium. Using these data, the Fe-N phase diagram is extended from 350 °C to 240 °C and extrapolated down to 200 °C. The α, γ′, and ε phases probably coexist in a triple point in the Lehrer diagram around 214 °C.     

The relative effects of chromium,molybdenum, and tungsten on the occurrence of σ phase in Ni-Co-Cr alloys

The relative effects of chromium, molybdenum, and tungsten on the occurrence of σ phase have been studied in Ni-Co-Cr alloys. These alloys were designed to simulate the γ matrix in commercial nickel-base superalloys that are strengthened primarily by precipitation of the γ phase, based on Ni₃Al. Three alloy series were studied. The first series comprised four alloys varying in chromium content from 34.63 to 43.65 at. pct. The other two series contained separate molybdenum and tungsten additions of 1, 2, 3, and 4 at. pct at constant chromium contents of 37.5 at. pct. In each of the 12 alloys, the atomic percentages of nickel and cobalt were equal. The alloys were aged in both the annealed and cold-rolled conditions at 1400°F (760°C), 1550°F (845°C), and 1700°F (925°C) for times up to 3000 h. The contributions of the chromium-group elements to σ formation were evaluated both by measuring the volume percentage of σ phase and by determining the final composition of the y matrix after σ precipitation. By these two techniques, critical values of the average electron vacancy number, •N _v , for σ formation at 1550°F (845°C) were found to be 2.518 and 2.512, respectively; σ precipitation was most rapid at 1550°F (845°C). Both techniques in-dicated that under conditions approaching equilibrium, molybdenum and tungsten are equiv-alent in inducing σ formation and about 1.5 to 2 times as potent as chromium. The approxi-mate electron vacancy coefficients(N _v ) for molybdenum and tungsten, as derived from volume-fraction measurements of σ phase, are as follows: 7.35 at 1400°F (760°C) and 1550°F (845°C), and 8.7 at 1700°F (925°C). The values derived from final compositions of the γ matrix after σ precipitation are 7.9 at 1550°F (845°C) and 8.6 at 1700°F (925°C). The bulk diffusion of aluminum into alloys that were otherwise not σ-prone at 1700°F (925°C) caused extensive σ precipitation during aging. This was due to copious precipitation of γ-Ni₃Al and β-NiAl, resulting in enrichment of the matrix in elements of the chromium group. This paper is based on a dissertation submitted by GARY N. KIRBY in partial fulfillment of the requirements for the degree of Doctor of Philosophy, Metallurgical Engineering, The University of Michigan, 1971. The study was conducted in the Ann Arbor Research Labora-tory of the Climax Molybdenum Company of Michigan, a subsidiary of American Metal Climax, Inc.     

Continuous cooling transformation diagrams applicable to the heat-affected zone of HSLA-80 and HSLA-100 steels

Continuous cooling transformation (CCT) diagrams for HSLA-80 and HSLA-100 steels pertaining to fusion welding with heat inputs of 10 to 40 kJ/cm, and peak temperatures of 1000 °C to 1400 °C have been developed. The corresponding nonlinear cooling profiles and related γ → α phase transformation start and finish temperatures for various peak temperature conditions have been taken into account. The martensite start (M _s ) temperature for each of the grades and ambient temperature microstructures were considered for mapping the CCT diagrams. The austenite condition and cooling rate are found to influence the phase transformation temperatures, transformation kinetics, and morphology of the transformed products. In the fine-grain heat-affected zone (FGHAZ) of HSLA-80 steel, the transformation during cooling begins at temperatures of 550 °C to 560 °C, and in the HSLA-100 steel at 470 °C to 490 °C. In comparison, the transformation temperature is lower by 120 °C and 30 °C in the coarse-grain heat-affected zone (CGHAZ) of HSLA-80 steel and HSLA-100 steel, respectively. At these temperatures, acicular ferrite (AF) and lath martensite (LM) phases are formed. While the FGHAZ contains a greater proportion of acicular ferrite, the CGHAZ has a higher volume fraction of LM. Cooling profiles from the same peak temperature influence the transformation kinetics with slower cooling rates producing a higher volume fraction of acicular ferrite at the expense of LM. The CCT diagrams produced can predict the microstructure of the entire HAZ and have overcome the limitations of the conventional CCT diagrams, primarily with respect to the CGHAZ.     

Lattice misfits in four binary Ni-Base γ/γ1 alloys at ambient and elevated temperatures

High-temperature X-ray diffractometry was used to determine thein situlattice parameters,a _γ anda _γ′, and lattice misfits, δ = (a _γ′, -a _γ)/a _γ, of the matrix (γ) and dispersed γ′-type (Ni₃X) phases in polycrystalline binary Ni-Al, Ni-Ga, Ni-Ge, and Ni-Si alloys as functions of temperature, up to about 680 °C. Concentrated alloys containing large volume fractions of theγ′ phase (∼0.40 to 0.50) were aged at 700 °C to produce large, elastically unconstrained precipitates. The room-temperature misfits are 0.00474 (Ni-Al), 0.01005 (Ni-Ga), 0.00626 (Ni-Ge), and -0.00226 (Ni-Si), with an estimated error of ± 4 pct. The absolute values of the lattice constants of theγ andγ′ phases, at compositions corresponding to thermodynamic equilibrium at about 700 °C, are in excellent agreement with data from the literature, with the exception of Ni₃Ga, the lattice constant of which is much larger than expected. In Ni-Ge alloys, δ decreases to 0.00612 at 679 °C, and in Ni-Ga alloys, the decrease is to 0.0097. In Ni-Si and Ni-Al alloys, δ exhibits a stronger temperature dependence, changing to-0.00285 at 683 °C (Ni-Si) and to 0.00424 at 680 °C (Ni-Al). Since the times required to complete the high-temperature X-ray diffraction (XRD) scans were relatively short (2.5 hours at most), we believe that the changes in δ observed are attributable to differences between the thermal expansion coefficients of theγ andγ′ phases, because the compositions of the phases in question reflect the equilibrium compositions at 700 δC. Empirical equations are presented that accurately describe the temperature dependences ofa _γ,a _γ′, and δ over the range of temperatures of this investigation.     

The vanadium-platinum constitution diagram

The system V-Pt was investigated over the entire composition range by metallography, X-ray diffraction and electron microprobe studies. There are at least four equilibrium intermediate phases in this system and they are stable to progressively higher temperatures with increasing vanadium concentration. The phases which have been observed are: γ^′, cubic, Cu₃Au type; θ, tetragonal, TiAl₃ type; δ, orthorhombic, MoPt₂ type; ζ, orthorhombic, AuCd type; and β, cubic, Cr₃Si type (A15). The gg^′ phase is possibly metastable. A very stable ribbon-like growth of ζ phase in the fcc platinum terminal solid solution has been observed in alloys containing about 43 at. pct V. The platinum terminal solid solution forms a congruent melting maximum at about 1805°C. A eutectic reaction occurs at 1720° ± 10°C and a peritectic reaction is indicated at 1800° ± 10°C. Vanadium is soluble in the fcc platinum terminal solid solution up to about 57 at. pct at 1720°C. Platinum dissolves only to the extent of about 12 at. pct at 1800°C in bcc α-V.     

The iron fusion curve and γ-δ-l triple point

The pressure-temperature lines for iron γ-δ and δ-liquid equilibria have been determined using improved methods for internal pressure measurement and improved thermocouple pressure error correction data. The experimental results indicate slopes for the γ-δ equilibrium of 6.3 ± 0.1° per kbar and for the δ-liquid equilibrium of 3.5 ± 0.1° per kbar. The γ-δ-liquid triple point was found at ∼1718°C, ∼52 kbar. A 5 kbar section of the γ-liquid line indicated a γ-liquid line slope of 3.85 ± 0.15° per kbar.     

The effect of molybdenum on γ′ coarsening and on elevated-temperature hardness in some experimental nickel-base superalloys

The coarsening of γ′ and the elevated-temperature hardness have been studied as a function of molybdenum content, time, and temperature in experimental wrought nickel-base superalloys. The alloys were selected from a systematic series containing 3, 4 1/2, and 6 wt pct Al and 1 wt pct Al plus 3 1/2 wt pct Ti. Each of the aluminum (plus titanium) series consisted of four alloys containing 0, 2, 5, and 8 wt pct Mo. The alloys were solution-treated plus aged up to 112 h at 1700°F (925°C) and up to 1000 h at 1400°F (760°C). Molybdenum retards the coarsening of γ′ on aging; this retarding effect is most pronounced in alloys containing 6 wt pct Al. The coarsening of γ′ particles follows Ostwald ripening kinetics. Hardness testingin vacuo at temperatures up to 1750°F (955°C) shows that molybdenum also increases the elevated-temperature hardness significantly. The relation of elevated-temperature hardness to the volume fraction of γ′ is considered, and the influence of aluminum and titanium contents is discussed.     

A phenomenological study of the shape memory effect in polycrystalline uranium-niobium alloys

When uranium-niobium alloys containing between 13.9 and 17.9 at. pct Nb are quenched to room temperature from the BCC (γ) phase at elevated temperatures, diffusion-controlled precipitation of the equilibrium phases is prevented and martensitic transformations to transition phases occur instead. Dilatometry was used to detect transformation temperatures and with the help of X-ray diffraction analysis, a metastable phase diagram was established. At room temperature after quenching, alloys containing < 15.2 at. pct Nb were monoclinic (α″) and those with < 16.6 at. pct Nb were tetragonal (γ°). The deformation behavior and shape memory effects (SME) accompanying the reverse martensitic phase transformations in polycristalline specimens were surveyed and characterized phenomelogically. From uniaxial tensile tests at room temperature, macroscopic stress-strain parameters, associated with the reversible deformation modes in the α″ and γ° martensites, were defined and their composition and structural state dependencies delineated. A diffuse maximum manifested in the stress-strain diagrams was identified with the reversible strain limit, which varied inversely and continuously with composition. A concentration-independent value of 693 MPa was found for the plastic yield strength of the alloys. All the alloys exhibited heat-activated shape recovery but the degree depended on structural state and composition. The α″ alloys showed a much larger effect than γ° alloys. Shape recovery occurred in two stages in all alloys. The first stage of recovery accompanied martensite reversion but final reversion to the equilibrium y phase was not accomplished until much higher temperatures were reached. Rapid, low temperature aging reactions were thought to affect the finish of shape recovery and delay it to higher temperatures. Formerly with Metals and Ceramics Division, Oak Ridge National Laboratory     

Activity of carbon and solubility of carbides in the FCC Fe-Mo-C,Fe-Cr-C,and Fe-V-C alloys

The activity of carbon in austenitic Fe-Mo-C, Fe-Cr-C, and Fe-V-C alloys has been studied by equilibration with controlled CH₄-H₂ atmospheres at temperatures in the range 850° to 1200°C. The observations included a number of compositions in the two-phase fields, γ + carbide. Equations are given for the activity coefficient of carbon as a function of temperature and composition in the austenite field and from these the other thermodynamic properties of the solution may be computed as desired. The phase boundaries γ/γ + carbide were determined by breaks in the isoactivity lines. This was supplemented in the case of Fe-Mo-C alloys by metallographic linear analysis of equilibrated samples. The results confirm certain published phase diagrams and discredit others. T. WADA, formerly with Research Staff, Massachusetts Institute of Technology, Cambridge, Mass. 02139 H. WADA, formerly with Research Staff, Massachusetts Institute of Technology     

Effect of cold rolling on the precipitation behavior of δ phase in INCONEL 718

Systematic research has been undertaken on the effect of cold rolling on the precipitation kinetics of δ phase in INCONEL 718. Above 910 °C, cold rolling promotes the precipitation of δ phase. Below 910 °C, the precipitation of δ phase is still preceded by the γ″ precipitation in cold-rolled INCONEL 718. Cold rolling promotes not only the precipitation of γ″ phase but also the γ″ → δ transformation. The relationship between the weight percentage of δ phase and aging time follows the Avrami equation. Below 910 °C, as cold rolling reduction and temperature increase, the time exponent (n) decreases, whereas the rate of δ precipitation increases. The apparent activation energy of δ precipitation varies in the range of 1113 to 577 kJ/mol for 25 to 65% cold-rolled INCONEL 718 and decreases as cold rolling reduction increases. Precipitation-time-temperature (PTT) diagrams have been determined for the four cold-rolled INCONEL 718. The noses of the PTT curves are located at about 910 °C. These curves are shifted significantly to longer times as cold rolling reductions decrease.     

Compositional Variations between Different Generations of <Emphasis Type="Italic">γ</Emphasis>′ Precipitates Forming during Continuous Cooling of a Commercial Nickel-Base Superalloy

The compositional and microstructural evolution of different generations of precipitates of the ordered γ′ phase during the continuous cooling, followed by isothermal aging, of a commercial nickel-base superalloy, Rene 88DT, has been characterized by three-dimensional atom probe (3DAP) tomography coupled with energy-filtered transmission electron microscopy (EFTEM) studies. After solutionizing in the single γ-phase field, during continuous cooling at a relatively slow rate (~24 °C/min), the first-generation primary γ′ precipitates, forming at relatively higher temperatures, exhibit near-equilibrium compositions, while the smaller-scale secondary γ′ precipitates, forming at lower temperatures, exhibit nonequilibrium compositions often containing an excess of Co and Cr while being depleted in Al and Ti content. The compositions of the γ matrix near these precipitates also exhibit similar trends, with the composition being closer to equilibrium near the primary precipitates as compared to the secondary precipitates. Subsequent isothermal aging at 760 °C leads to coarsening of the primary γ′ precipitates without affecting their composition significantly. In contrast, the composition of the secondary γ′ precipitates is driven toward equilibrium during the isothermal aging process.     

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.     

Reaction synthesis of Ni-Al-based particle composite coatings

Electrodeposited metal matrix/metal particle composite (EMMC) coatings were produced with a nickel matrix and aluminum particles. By optimizing the process parameters, coatings were deposited with 20 vol pct aluminum particles. Coating morphology and composition were characterized using light optical microscopy (LOM), scanning electron microscopy (SEM), and electron probe microanalysis (EPMA). Differential thermal analysis (DTA) was employed to study reactive phase formation. The effect of heat treatment on coating phase formation was studied in the temperature range 415 °C to 1000 °C. Long-time exposure at low temperature results in the formation of several intermetallic phases at the Ni matrix/Al particle interfaces and concentrically around the original Al particles. Upon heating to the 500 °C to 600 °C range, the aluminum particles react with the nickel matrix to form NiAl islands within the Ni matrix. When exposed to higher temperatures (600 °C to 1000 °C), diffusional reaction between NiAl and nickel produces (γ′)Ni₃Al. The final equilibrium microstructure consists of blocks of (γ′)Ni₃Al in a γ(Ni) solid solution matrix, with small pores also present. Pore formation is explained based on local density changes during intermetallic phase formation, and microstructural development is discussed with reference to reaction synthesis of bulk nickel aluminides.     

Crystallization behavior of iron-containing intermetallic compounds in 319 aluminum alloy

The crystallization behavior of iron-containing intermetallic compounds in industrial grade 319 aluminum alloy has been investigated by means of thermal analysis and metallography. In the absence of manganese, the iron compound crystallizes in theβ phase, at all cooling rates ranging from 0.1 °C/s to 20 °C/s under normal casting temperatures (750 °C). However, when the melt is superheated to a high temperature (about 200 to 300 degrees above the liquidus temperature), the iron compound crystallizes in the α phase at high cooling rates. This is due to the fact that γ alumina, which forms at low melt temperatures (≤750 °C), acts as a nucleus for crystallization ofβ phase. When the melt is superheated to high temperature (≥85O °C), the γ alumina transforms to a alumina. This is a poor nucleus for the β-phase crystallization, and as a result, a phase forms. The importance of nucleation and growth undercooling for the crystallization of iron compounds is highlighted. In the presence of manganese, the iron compound crystallizes in a phase at low cooling rates and in both the α andβ phases at high cooling rates. This reverse crystallization behavior is explained in terms of phase diagram relationships.     

Decomposition of Fe-Ni martensite: Implications for the low-temperature (≤500 °C) Fe-Ni phase diagram

The low-temperature (<500 °C) decomposition of Fe-Ni martensite was studied by aging martensitic Fe-Ni alloys at temperatures between 300 °C and 450 °C and by measuring the composition of the matrix and precipitate phases using the analytical electron microscope (AEM). For aging treatments between 300 °C and 450 °C, lath martensite in 15 and 25 wt pct Ni alloys decomposed with γ [face-centered cubic (fcc)] precipitates forming intergranularly, and plate martensite in 30 wt pct Ni alloys decomposed with γ (fcc) precipitates forming intragranularly. The habit plane for the intragranular precipitates is {111}_fcc parallel to one of the {110}_bcc planes in the martensite. The compositions of the γ intergranular and intragranular precipitates lie between 48 and 58 wt pct Ni and generally increase in Ni content with decreasing aging temperature. Diffusion gradients are observed in the matrix α [body-centered cubic (bcc)] with decreasing Ni contents close to the martensite grain boundaries and matrix/precipitate boundaries. The Ni composition of the matrix α phase in decomposed martensite is significantly higher than the equilibrium value of 4 to 5 wt pct Ni, suggesting that precipitate growth in Fe-Ni martensite is partially interface reaction controlled at low temperatures (<500 °C). The results of the experimental studies modify the γ/α + γ phase boundary in the present low-temperature Fe-Ni phase diagram and establish the eutectoid reaction in the temperature range between 400 °C and 450 °C. Formerly Research Assistant, Department of Materials Science and Engineering, Lehigh University     

Isothermal Phase Transformation in Biomedical Co-29Cr-6Mo Alloy without Addition of Carbon or Nitrogen

The isothermal phase transformation behavior in a biomedical Co-29Cr-6Mo alloy without carbon or nitrogen was investigated during aging at temperatures between 973 K and 1273 K (700 °C and 1000 °C) for up to 90 ks. Transformation from the γ to the ε phase did not occur at 1273 K (1000 °C) as the γ phase was more stable than the ε phase, and the σ phase precipitated at the γ grain boundaries. At 1173 K (900 °C), a γ → ε ₁ phase transformation occurred by massive precipitation. Prolonged annealing at 1173 K (900 °C) led to a lamellar structure of ε ₂ and σ phases at ε ₁/ε ₁ boundaries by a discontinuous/cellular reaction, expressed by the reaction equation ε ₁ → ε ₂ + σ. After decreasing the aging temperature to 973 K (700 °C), transformation from the γ to the ε phase occurred mainly by isothermal martensitic transformation, but a lathlike massive ε ₁ phase and ε ₂/σ lamellar colonies were also observed at the original γ-grain boundaries. It is likely that not adding carbon results in the promotion of the massive transformation and the precipitation of the σ phase during isothermal aging in the Co-29Cr-6Mo alloy system, whose composition corresponds to the ASTM F75 standard for metallic materials for surgical implantation. The resultant isothermal transformation behavior of the present alloy is described on the basis of thermodynamic calculations using Thermo-Calc.