Phase stability and yield stress of Ni-base superalloys containing high Co and Ti

Ni-base superalloys containing high Co (>20 wt pct) and Ti (>5.5 wt pct) were designed in order to study the effects of Co16.9 wt pct Ti addition on phase stability and mechanical property. These new alloys, though they contained high Ti, mainly consisted of γ and γ′ phases. Ni₃Ti (η) phase was observed along the grain boundaries in some of the alloys. The formation of η phase was mainly related to the Ti/Al ratio, Ti content, and alloy composition. Tensile and compression tests showed that these new alloys exhibited higher yield stress than that of the baseline alloy, TMW-1(U720LI). The possible strengthening mechanisms were discussed in terms of solid-solution and precipitation strengthening effects by the Co16.9 wt pct Ti additions. Preliminary results show promising trends for the development of new superalloys for turbine disc applications.     

Effects of Ru additions on the microstructure and phase stability of Ni-base superalloy, UDIMET 720LI

The effects of Ru additions on the microstructure and phase stability of the Ni-base superalloy U720LI were examined. The results show that the alloys with Ru contents of 0 to 6 wt pct had a dendritic structure, and the volume fraction of the primary γ solid-solution phase in the dendritic core increased with the increase in the Ru content. The Ru additions to U720LI had little effect on suppressing the formation of the σ phase, because the elemental partitioning of Cr, Mo, and Co in the γ solid-solution phase was not influenced by the Ru additions. However, the coarsening of the γ′ phase was restrained by the Ru additions in the U720LI.     

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.     

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.     

Coherent <Emphasis Type="Italic">γ′</Emphasis>-precipitates at singular and rough grain boundaries in a model Ni-base superalloy

In a model Ni-base superalloy of Ni-24Co-4Al-4Ti-5Cr-5Mo (by wt pct), the singular and rough grain boundaries that exist at a temperature range between 1050 °C and 1200 °C can be identified by observing the shapes of coherent γ′-precipitates by transmission electron microscopy (TEM). Some grain boundaries become locally curved with the impinging spherical γ′-precipitates. These grain boundaries must be rough. Some grain boundaries maintain their flat shapes even at the contact areas with the γ′-precipitates and triple junctions. Such flat grain boundary shapes indicate that these grain boundaries are singular. Some grain boundaries have hill-and-valley shapes and some of their segments also show flat shapes with impinging precipitates. These boundary segments must also be singular. The results show that the singular and rough grain boundaries in this alloy can be clearly identified by the shape distortions produced by the impinging γ′-precipitates.     

The influence of cobalt,tantalum, and tungsten on the elevated temperature mechanical properties of single crystal nickel-base superalloys

The influence of composition on the tensile and creep strength of [001] oriented nickel-base superalloy single crystals at temperatures near 1000 °C was investigated. Cobalt, tantalum, and tungsten concentrations were varied according to a matrix of compositions based on the single crystal version of MAR-M247.* For alloys with the baseline refractory metal level of 3 wt pct Ta and 10 wt pct W, decreases in Co level from 10 to 0 wt pct resulted in increased tensile and creep strength. Substitution of 2 wt pct W for 3 wt pct Ta resulted in decreased creep life at high stresses, but improved life at low stresses. Substitution of Ni for Ta caused large reductions in tensile strength and creep resistance, and corresponding increases in ductility. For these alloys with low Ta plus W totals, strength was independent of Co level. The effects of composition on properties were related to the microstructural features of the alloys. In general, high creep strength was associated with high levels ofγ′ volume fraction,γ-γ′ lattice mismatch, and solid solution hardening.     

Effect of Solution Temperature on the Microstructure and Mechanical Properties of a Newly Developed Superalloy TMW-4M3

The influence of solution temperature on the microstructure and mechanical properties of TMW-4M3 superalloy has been investigated. Comparisons of mechanical properties have also been made between the heat-treated TMW-4M3 variants and the commercial U720Li. The key microstructural variables examined were grain size and the volume fraction and size of the strengthening γ′ precipitates that control the mechanical properties of these alloys. By increasing the solution temperature from 1373 K to 1393 K (1100 °C to 1120 °C), the volume fraction of primary gamma prime dropped from 16.9 pct to 14.5 pct, whereas the average grain size increased from 8.7 μm to 10.6 μm. Compared with an alloy with a smaller grain size, the alloy with a larger grain size exhibited superior resistances to creep and fatigue crack growth without the expense of reduced tensile strength and low-cycle fatigue resistance. This suggested that a higher solution temperature may benefit TMW-4M3 in terms of superior overall properties. The greater overall properties of TMW-4M3 variants than that of commercial U720Li were also demonstrated experimentally. The possible explanations for the improvement of mechanical properties were discussed.     

Effects of carbon content and ausaging on γ ↔ α′ transformation behavior and reverse-transformed structure in Fe-Ni-Co-Al-C alloys

The effects of carbon content and ausaging on austenite γ ↔ martensite (α′) transformation behavior and reverse-transformed structure were investigated in Fe-32Ni-12Co-4Al and Fe-(26,28)Ni-12Co-4Al-0.4C (wt pct) alloys. TheM _s temperature, the hardness of γ phase, and the tetragonality of α′ increase with increasing ausaging time, and these values are higher in the carbon-bearing alloys in most cases. The γ → α′ transformation behavior is similar to that of thermoelastic martensite; that is, the width of α′ plate increases with decreasing temperature in all alloys. The αt’ → γ reverse transformation temperature is lower in the carbon-bearing alloys, which means that the shape memory effect is improved by the addition of carbon. The maximum shape recovery of 84 pct is obtained in Fe-28Ni-12Co-4Al-0.4C alloy when the ausaged specimen is deformed at theM _s temperature and heated to 1120 K. There are two types of reverse-transformed austenites in the carbon-bearing alloy. One type is the reversed y containing many dislocations which were formed when the γ/α′ interface moved reversibly. The plane on which dislocations lie is (01 l)_γ if the twin plane is (112)_α′. The other type of reverse-transformed austenite exhibits γ islands nucleated within the α′ plates.     

Design and development of hot corrosion-resistant

A systematic study of the effects of refractory metals Ti, Ta, and Nb on the microstructures and properties was conducted with a hot corrosion-resistant alloy system Ni-16Cr-9Al-4Co-2W-lMo-(0~4)Ti-(0~4)Ta-(0~4)Nb (in atomic percent) which was selected based on thed-electrons alloy design theory and some basic considerations in alloying features of single-crystal nickel-base superalloys. The contour lines of solidification reaction temperatures and eutectic (γ + γ′) volume fraction in the Ti-Ta-Nb compositional triangle were determined by differential thermal analysis (DTA) and imaging analyzer. Compared with the reference alloy IN738LC, in most of the compositional ranges studied, the designed alloys show very low amounts of eutectic (γ + γ′) (⪯0.4 vol pct), narrow solidification ranges (⪯65 °C), and wide “heat-treatment windows” (>100 °C). This indicates that the alloys should have the promising microstructural stability, single-crystal castability, and be easier for complete solution treatment. In a wide compositional range, the designed alloys showed good hot corrosion resistance (weight loss less than 20 mg/cm² after 24 hours kept in molten salt at 900 °C). By summarizing the results, the promising alloy compositional ranges of the alloys with balanced properties were determined for the final step of the alloy design,i.e., to grow single crystal and characterize mechanical properties of the alloys selected from the previously mentioned regions. Formerly with the Institute of Metal Research, Academia Sinica, Shenyang 110015, China     

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     

Selective oxidation and internal nitridation during high-temperature exposure of single-crystalline nickel-base superalloys

The process of internal nitridation of the three commercial single-crystalline nickel-base superalloys CMSX-2, CMSX-6, and SRR99 has been studied in air and oxygen-free nitrogen atmospheres at 800 °C to 1100 °C using thermogravimetric techniques supplemented by extensive microstructural examinations. Non-protective oxide formation, particularly cracking and spalling at edges or curved surfaces, enables nitrogen to penetrate into the alloy leading to the precipitation of stable Ti and Al nitrides. The high-temperature corrosion behavior of the superalloys studied is strongly affected by compositional differences between dendritic and interdendritic areas due to segregation resulting in an inhomogeneous internal precipitation zone. Furthermore, the stability of the strengthening γ′ phase (Ni₃(Al, Ti, Ta)) in front of the growing internal-nitridation zone was observed to depend clearly on the alloy composition. Therefore, the near-surface area of the alloys can be weakened by γ′ depletion and by embrittlement resulting from internal-nitride precipitation. The results obtained on the nickel-base superalloys are discussed, taking into account the results of a computer-based simulation of internal-corrosion processes. Furthermore, results on Ni-base model alloys of the system Ni-Cr-Al-Ti provided information on the role of the alloy composition. It was found that a higher Cr concentration seems to increase the nitrogen solubility and diffusion in Ni-base alloys.     

Effect of phosphorus on the microstructure and stress rupture properties in an Fe-Ni-Cr base superalloy

This article describes the effect of phosphorus on the microstructure and stress rupture property at 650 °C in an Fe-Ni-Cr base superalloy. The results showed that phosphorus markedly improved the intergranular precipitation in the range of 0.0005 to 0.016 wt pct, which facilitated M₂₃C₆ and M₃B₂ precipitation but inhibited the formation of MC carbide. A too high phosphorus addition (0.051 wt pct P) resulted in an excessive precipitation at grain boundaries, while a too low phosphorus content (0.0005 wt pct P) led to many precipitate-free grain boundaries. Phosphorus also enlarged the size of the γ′ particles and lowered its stability, that η-Ni₃Ti preferred to form in the alloy with 0.051 wt pct P. Due to the improvement of the microstructure, appropriate amount of P content significantly prolonged the rupture life of the alloys in the range of 0.0005 to 0.016 wt pct. The peak value was 660 hours at 0.016 wt pct, more than 4 times that of the alloy with 0.0005 wt pct phosphorus, but phosphorus reduced the fracture elongation. The mechanism by which phosphorus influenced the alloy is discussed.     

Fatigue crack propagation in aluminum-base copper alloys

Fatigue crack propagation ratesda/dN in binary Al alloys with 3.6 wt pct Cu and 6.3 wt pct Cu and commercial 2024 aged at 21°C were compared with 99.95⁺ wt pct aluminum. Omitting an anomalous region at lowΔK, the extrapolated rates for “pure” aluminum are more than 100 times greater than those in the three alloys at the same ΔK. The data for the alloys fit into a single scatter band of a factor of three. It was suggested thatda/dN varies inversely with the square of the strength of the alloy but that another parameter related to the fatigue crack propagation energy per unit area is also important. Theda/dN vs ΔK curves were determined for 3.6 wt pct Cu single crystals aged seven days at 21°C which containGP zones and two and seven days at 160°C which contain mixtures ofθ′ andθ′’. No systematic variation of (da/dN _Δ with crystallographic orientation was discerned, but the naturally aged specimen had a strong orientation dependence on crack initiation. At low ΔK 21°C aged specimens gave the lowestda/dN while at high ΔK the warm aged specimens gave the lower values ofda/dN. Measurement ofda/dN vs ΔK curves were conducted on specimens of 3.6 wt pct Cu with 1 mm equiaxed grains aged for various times at 130°C, 160°C, and 190°C. All warm aged specimens experienced brittle intergranular fracture at sufficiently high ΔK. The transition ΔK where intergranular fracture first appears is inversely proportional to the aging temperature. The change of fracture mode from intra to intergranular occurs gradually over a broad range of ΔK which shifts to lower ΔK with increase in aging temperature. This research was supportd by U.S. Air Force Office of Scientific Research, Office of Aerospace REsearch, Grant No. AF-AFOSR-73-2431.     

The effects of composition and γ′/γ lattice parameter mismatch on the critical resolved shear stresses for octahedral and cube slip in NiAlCrX alloys

Prototypical single-crystal NiAlCrX superalloys were studied to examine the effects of the common major alloying elements, Co, Mo, Nb, Ta, Ti, and W, on yielding behavior. The alloys contained about 10 at. pct Cr, 60 vol pct of the γ′ phase, and about 3 at. pct of X in the γ′. The critical resolved shear stresses (CRSSs) for octahedral and primary cube slip were measured at 760 °C, which is about the peak strength temperature. The CRSS_Oct and CRSS_cube are discussed in relation to those of Ni₃ (Al, X) γ′ alloys taken from the literature and the γ′/γ lattice mismatch. The CRSS_Oct of the γ+γ′ alloys reflected a similar compositional dependence to that of both the CRSS_Oct of the γ′ phase and the γ′/γ lattice parameter mismatch. The CRSS_cube of the γ+γ′ alloys also reflected the compositional dependence of the γ′/γ mismatch, but bore no similarity to that of CRSS_cube for γ′ alloys since it is controlled by the γ matrix. The ratio of CRSS_cube/CRSS_Oct was decreased by all alloying elements except Co, which increased the ratio. The decrease in CRSS_cube/CRSS_Oct was related to the degree in which elements partition to the γ′ rather than the γ phase.     

The effects of molybdenum and aluminum on the thermal expansion coefficients of nickel-base alloys

The effects of molybdenum and aluminum on the mean linear thermal expansion coefficients from room temperature to 1050°C were determined for two types of nickel-base alloys. The Solid Solution Alloys were cast and homogenized Ni-Co-Cr-Mo alloys with 0, 312, and 612 nominal wt pct molybdenum concentrations. The Gamma Prime Alloys were wrought and heat-treated Ni-Cr-Mo-Al(Ti) alloys with 0, 2, 5, and 8 nominal wt pct molybdenum in each of four aluminum plus titanium levels (3 pct Al, 412 pct Al, 6 pct Al, or 1 pct Al + 312 pct Ti nominal wt pct). Thermal expansion coefficients were determined on at least two specimens from each alloy. It was found that molybdenum lowers the thermal expansion coefficients of both the cast Ni-Co-Cr solid solutions and the wrought Ni-Cr-Al(Ti) two-phase alloys. Both aluminum and titanium were also observed to decrease expansion coefficients in the two-phase, γ + γ, alloys. Results are discussed in terms of relative melting point effects between solute and solvent elements, and in terms of the volume fraction of the γ phase present.     

The effect of cold work on the precipitation of Ω and θ′ in a ternary Al-Cu-Mg alloy

The effect of plastic deformation on the microstructural evolution of an Al-5.0Cu-0.5Mg (wt pct) ternary alloy was investigated. Hardness measurements and quantitative precipitate analysis were performed on specimens that were water quenched from a solution heat treatment, stretched either 0 or 6 pct and immediately aged at ambient temperature or artificially aged at 200 °C or 250 °C for times up to 3000 hours. Quantitative transmission electron microscopy (TEM) was used to characterize Ω and θ′ number density, diameter, and thickness as a function of preage mechanical stretch and artificial aging condition. Age hardening curves for naturally and artificially aged specimens revealed an increase in hardness corresponding with a preage stretch. Quantitative TEM verified an increase in the number density and a refinement of precipitates for both Ω and θ′ between the 0 and 6 pct stretch condition for those samples artificially aged. When aged at 200 °C, θ′ exhibited superior coarsening resistance relative to the Ω phase. The quantified Ω coarsening kinetics were greater than similar Ag-containing alloys. To investigate the effects of trace Si additions on subsequent microstructural evolution, a series of Al-Cu-Mg-Si quaternary alloys were produced. The addition of 0.1Si (wt pct) was found to suppress Ω precipitation in most Al-4.0Cu-xMg alloys investigated. These initial results indicate that Ω precipitation may be related to the Mg/Si ratio.     

A Comparative Study of the γ/γ′ Eutectic Evolution During the Solidification of Ni-Base Superalloys

The evolution of γ/γ′ eutectic during the solidification of Ni-base superalloys CMSX-10 and CMSX-4 was investigated over a wide range of cooling rates. The microsegregation behavior during solidification was also quantitatively examined to clarify the influence of elemental segregation on the evolution of γ/γ′ eutectic. In the cooling rate ranges investigated (0.9 to 138.4 K/min (0.9 to 138.4 °C/min)), the γ/γ′ eutectic fraction in CMSX-10 was found to be more than 2 times higher than that in CMSX-4 at a given cooling rate. However, the dependence of the γ/γ′ eutectic fraction on the cooling rate in both alloys showed a similar tendency; i.e., the γ/γ′ eutectic fraction increased with increasing the cooling rate and then exhibited a maximum plateau at and above the certain critical cooling rate in both alloys. This critical cooling rate was found to be dependent on the alloy composition and was estimated to be about 12 K/min (12 °C/min) and 25 K/min (25 °C/min) for CMSX-10 and CMSX-4, respectively. The calculated solid compositions based on the modified Scheil model revealed that even a small compositional difference of total γ′ forming elements in the initial composition of the alloy can play a significant role in the as-cast eutectic fraction during the solidification of Ni-base superalloys. The evolution of the γ/γ′ eutectic fraction with respect to the cooling rate could be rationalized by taking into account the effects of back-diffusion in solid and dendrite arm coarsening on decreasing the extent of microsegregation.     

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.     

A fine γ′+α cellular structure in Fe-37.3 wt pct Ni-3.6 wt pct Al-3.3 wt pct Ti-0.2 wt pct C and its influence on high-temperature tensile properties

Fe-37.3 wt pct Ni-3.6 wt pct Al-3.3 wt pct Ti-0.2 wt pct C alloy, which reveals an excellent combination of high strength and good elongation endowed by formation of homogeneously dispersed fine γ′ precipitates in the matrix during aging at 823 K, has been investigated by means of transmission electron and optical microscopies, electron diffractions, and tensile tests. The influence of unique γ′+α cellular products on the mechanical properties has also been studied. Because of low elastic mismatch between the austenitic γ matrix and isomorphic γ′ precipitate phases, the homogeneously distributed precipitate particles, which formed at the early stage of aging, were observed to persist even after long-term aging. After very lengthy aging, the fine γ′ phase particles were changed to coarser γ′ lamellae at the grain boundary reaction front, which were alternately arranged with fine α lamellae that were estimated to have been transformed from the austenite-stabilizing-solute(Ni, C)-depleted γ lamellae. The fine duplex γ′+α cellular product did not affect deleteriously the room-temperature tensile properties of the alloy. However, the cellular structure was observed to cause the grain boundary embrittlement of the aged alloy at elevated temperatures higher than 681 K.     

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