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     

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.     

Precipitation in a Cu-30 Pct Ni-1 Pct Nb alloy

Decomposition of a Cu-30 pct Ni-1 pct Nb alloy on aging in the range of 866 K (600°C) to 1073 K (800°C) was investigated. The initial decomposition, concomitant with age hardening, occurred through the precipitation of body centered tetragonal metastable Ni₃Nb-γ” precipitates on the 100 matrix planes. Equilibrium orthorhombicβ phase formed either through a grain boundary cellular reaction at low temperature (≤973 K (700°C)) or as Widmanstaettenplatelets on the 1ll planes at higher temperatures (≥1073 K (800°C)) with the following crystallographic relationship: (0l0)_β//111_γ [100]β//[1•11]_γ. Based on the observations, a schematic transformation sequence is presented.     

The influence of coherency strain on the elevated temperature tensile behavior of Ni-15Cr-AI-Ti-Mo alloys

The effect of coherency strain on elevated temperature tensile strength was examined in a model, two-phase y’-strengthened Ni-15Cr-Al-Ti-Mo alloy series. The temperature dependence of coherency strain as represented by the γ-γ’ mismatch was determined over the temperature range 25 to 800 °C. The flow stress incrementAσ _γ, due to precipitation of γ’, was found to correlate well to the magnitude of the γ-γ’ mismatch over the same temperature interval. The correlation was strongest for high misfit alloys regardless of the Antiphase Boundary Energy (APBE). The predominance of by-pass type dislocation-particle interactions in high coherency alloys confirms that strengthening is primarily due to coherency strains. Conversely, alloys with low misfit exhibit two distinct particle shear mechanisms believed to be dependent upon the relative APBE and matrix stacking fault energy of the alloy.     

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     

Precipitation of heusler phase (Ni<Subscript>2</Subscript>TiAl) from B2-TiNi in Ni-Ti-Al and Ni-Ti-Al-X (X=Hf,Zr) alloys

The precipitation of Heusler phase (L2₁: Ni₂TiAl) from a supersaturated B2 (TiNi-based) matrix at 600°C and 800°C is studied using transmission electron microscopy (TEM), analytical electron microscopy (AEM), and three-dimensional atom-probe (3DAP) microscopy in Ni-Ti-Al and Ni-Ti-Al-X (X=Hf and Zr) alloys. The B2/L2₁ two-phase system, with ordered structures based on the bcc lattice, is chosen for its microstructural analogy to the classical γ/γ′ system with an fcc lattice. Knowledge of the temperature-dependent partitioning of alloying elements and their atomic volumes in the B2-TiNi and L2₁ phases is desired to support design of high-performance shape-memory alloys (SMAs) with controlled misfit strain and transformation temperatures. After aging at 600°C for up to 2000 hours, the L2₁ precipitates remain fully coherent at a particle diameter of ∼20 nm. The observed effects of a misfit strain of −1.9 pct on the microstructure of the B2/L2₁ system are similar to those theoretically predicted and experimentally observed for the γ/γ′ system. The similarities are demonstrated in terms of the precipitate shape, spatial distribution, and minimum distance of separation between L2₁ precipitates. However, all these effects disappear after aging the alloys at 800°C for 1000 hours, when the L2₁ precipitates become semicoherent at particle diameters above ∼400 nm. A simple analysis of the size evolution of L2₁ precipitates after an isochronal aging (1000 hours) experiment suggests that they follow coarsening kinetics at 600°C and growth kinetics at 800°C, consistent with the Langer-Schwartz theory of precipitation kinetics, which predicts that a high supersaturation suppresses the growth regime. Microanalysis using AEM and 3DAP microscopy define the TiNi-Ni₂TiAl phase boundaries at 800°C and 600°C. At 800°C, Hf and Zr partition to the B2-TiNi, while at 600°C, they partition slightly to the L2₁ phase, reducing the lattice misfit to −1.7 and −0.011 pct, respectively, and partition strongly to the metastable phase Ti₂Ni₃. To describe the composition dependence of the lattice parameter of multicomponent B2 and L2₁ phases, the atomic volumes of Al, Hf, Ni, Ti, and Zr in the B2-TiNi and L2₁ phases are determined. A simple model is proposed to predict the lattice parameters of these phases in multicomponent systems.     

The compctition between the alpha and omega phases in aged Ti-Nb alloys

The compctition between the stable α phase and metastable ω phase to precipitate in a metastableβ phase matrix was investigated in the present study. Four binary Ti-Ni alloys with compositions between 20 and 35 at. pct Nb were air cooled to room temperature from 1000 °C and then aged at temperatures between 300 and 500 °C. For aging temperatures of 400 °C and lower it was found that the quench before aging enabled ω phase precipitates to grow to the exclusion of α precipitates. When specimens were directly aged at 400 °C only α precipitates were observed. Precipitates which could not be identified using SAD were observed in specimens of the 30 and 35 at. pct Nb alloys. All precipitation reactions became more sluggish as the niobium content of an alloy was increased. The results reported here form the second part of a study of the stable and metastable equilibria of the Ti-Nb alloy system. Formerly a Graduate Student in the Materials Science Program at the University of Wisconsin-Madison     

The influence of molybdenum on the γ/’phase in experimental nickel-base superalloys

The influence of 1, 3, and 5 at. pct Mo on the γ’precipitate has been studied in experimental wrought nickel-base superalloys containing about 14 at. pct Cr and 6-1/2, 9, or 12 at. pct Al, or 2 at. pct Al plus 4 at. pct Ti. Concentrations of all other elements were quite low to limit the observed effects to those of molybdenum alone. Molybdenum markedly increases the γ’ solvus temperature, as determined by the sensitive and relatively simple technique of differential thermal analysis; correspondingly, the weight fraction of γ’ increases with molybdenum additions for a given aging treatment. Molybdenum dissolves extensively in the γ’of the titanium-free alloys, but it dissolves to a considerably smaller extent in the γ’of the titanium-bearing alloys. Molybdenum substitutes for chromium in y’, but does not alter the aluminum or titanium contents of this phase. Lattice parameters of both the matrix and the γ’are increased markedly by molybdenum, in proportion to the molybdenum contents of these phases. The resulting effects on lattice-parameter mismatch correlate rather well with observed γ’morphology, which tends to change from spheroidal to cuboidal in titanium-free alloys, and from cuboidal to spheroidal in 2 at. pct Al-4 at. pct Ti alloys, as molybdenum is added to these alloys. J. W. Freeman was formerly Professor of Metallurgical Engineering, University of Michigan, Ann Arbor, Mich. (Deceased, November, 1970). This paper is based upon a thesis submitted by W. T. Loomis in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Horace H. Rackham School of Graduate Studies, The University of Michigan.     

Low temperature mechanical behavior of Ni-15Cr-Ai-Ti-Mo alloys

A series of Ni-15 Cr-Al-Ti-Mo alloys with varying γy-γ’ mismatch, Ti/Al atomic ratio, and weight fraction of γ’ were tension tested at ambient temperature after aging to peak yield strength at 760°C. After subtracting the alloy yield stress in the solution treated condition, σss, the increment in yield stress due to precipitation of γ’, Δσy, was found to be principally influenced by the weight fraction of γ’ and the measured γ-γ’ mismatch. Only a small contribution of APBE to alloy strength was observed between two compositions of similar mismatch and differing measured APBE. The work hardening behavior of the alloys was similarly influenced by weight fraction of γ’ and γy-γ’ mismatch. Alloys with low mismatch exhibited sheared γ’ precipitates following tensile deformation. A model for shear of ordered γ’ precipitates by residual dislocation loops in low γ-γ’ mismatch alloys is proposed to account for the low work hardening rates observed.     

The growth of gamma prime precipitates in aged Ni−Ti alloys

The kinetics of growth of the γ′ precipitate in a Ni-8.74 wt pct Ti alloy were studied by magnetic analysis and transmission electron microscopy. The variation of the titanium content of the nickel-rich matrix as a function of aging time was studied by measuring the ferromagnetic Curie temperature of alloys aged at 692°, 593°, and 525°C. The kinetics of this process accurately obeyed the predictions of the Lifshitz-Wagner theory of diffusion controlled coarsening after relatively short aging times at all aging temperatures. Dark-field transmission electron microscopy was used to measure the particle-size distributions and the average particle sizes of samples aged for various times at 692°C. The kinetics of particle growth also obeyed the time law predicted by the Lifshitz-Wagner theory within the limits of experimental error. Additional analysis of the data provided a value of approximately 21 erg per sq cm for the interfacial free energy of the γ′-matrix interface, and a value for the diffusion coefficient of titanium in nickel which is in very good agreement with an independently determined value. The distribution of γ′ particle sizes was found to be significantly broader than the theoretical distribution of the Lifshitz-Wagner theory. It is suggested that this is due to the relatively large lattice parameter mismatch between γ′ and the Ni−Ti matrix. The results and conclusions of this study are critically compared with those of other investigations. A. J. ARDELL, formerly Assistant Professor, California Institute of Technology     

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.     

An ω phase in the Cu-16.5 at. pct Sn alloy

In many zirconium- and titanium-base alloys a precipitate of an ω phase is found after quenching from the high temperature β phase region. In a Cu-16.5 at. pct Sn alloy the same phase is present after quenching from the γ-phase region to room temperature. The quenched Cu−Sn alloy shows electron diffraction effects which are very similar to those of the zirconium- and titanium-base alloys,i.e., diffuse scattering and shift of the ω reflexions. In order to explain this shift a model, based on the presence of certain faults in the ω structure, is presented. After aging at 100° and 130°C the ω phase is replaced by a phase which closely resembles the stable Cu−Sn σ phase. Nuclei of this phase are thought to be at least partially responsible for the diffuse scattering observed. W. VANDERMEULENFORMERLY formaerly Assistant, Katholieke Universiteit Leuven, Leuven, Belgium     

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.     

Thermal stability of the nickel-base superalloy B-1900 + Hf with tantalum variations

The microstructure of the solutionized and aged nickel-base superalloy B-1900 + Hf was examined after additional aging at 982 °C for 72, 250, and 1000 hours. Alloy compositions that were examined contained the normal 1.34 at. pct (4.3 wt pct). Ta as well as 0.67 at. pct and zero Ta levels. The γ phase agglomerated, became plate-like in morphology, and decreased in volume fraction for all three alloys throughout the aging treatments. Changes which occurred in the γ and γ' phase compositions were nearly complete after 72 hours of aging while changes in the MC carbide composition continued throughout the aging. Blocky M₆C carbides precipitated along the grain boundaries of all three alloys in the first 72 hours of aging. In addition, an acicular form of this Mo/Cr/Ni-rich carbide developed in the intragranular regions of the Ta-containing alloys. Formerly an Undergraduate Student, Department of Metallurgical Engineering, Michigan Technological University.     

The Gd-Yb and Lu-Yb phase systems

The Gd-Yb and Lu-Yb phase systems were established by thermal analysis, X-ray diffrac-tion, metallography, electron microprobe and chemical analyses. The solubility of Yb in α-Gd ranges from 6.5 at. pct at 500°C to 19.0 at. pct at 1161°C. The addition of Yb to Gd lowers theβ (bec) to α (hcp) transformation temperature to an inverse peritectic reaction at 20.0 at. pct Yb and 1161°C. The addition of Yb to Gd lowers the melting point of Gd to a monotectic horizontal at 1183°C which extends from 21.0 to 71.0 at. pct Yb. The monotec-tic composition is 49.0 at. pct Yb. The solid solubility of Gd in Yb ranges from 0.2 at. pct at 500°C to 2.3 at. pct at 819°C. The melting point of Yb is raised from 816°C to 819°C by the addition of Gd while the γ (bee) toβ (fee) transformation temperature of Yb is lowered from 796°C to 780°C by the addition of Gd. The solubility of Yb in solid Lu ranges from 6.0 at. pct at 800°C to 15 at. pct at 1530°C. The addition of Yb to Lu lowers the melting point of Lu to a monotectic horizontal at 1530°C which extends from 15 to 90 at. pct Yb. The monotectic composition is approximately 30 at. pct Yb. The solid solubility of Lu in Yb ranges from less than 0.1 at. pct at 500°C to 0.3 at. pct at 817°C. The addition of Lu raises the melting point of Yb to 817°C and also raises theβ (fee) to y (bec) transformation temperature to 798°C.     

Isothermal section of the ternary Sn-Cu-Ni system and interfacial reactions in the Sn-Cu/Ni couples at 800 °C

The isothermal section of the Sn-Cu-Ni system at 800 °C has been experimentally determined. There is no ternary compound. A solid solution with a very wide compositional range, the γ phase is formed between the Ni₃Sn(H) phase and Cu₄Sn(H) phase; however, both of these two binary phases are not stable at 800 °C. The binary Ni₃Sn₂ phase also has extensive ternary solubility. The homogeneity ranges of both the γ and Ni₃Sn₂ phases are very large in parallel to the Cu-Ni side, but relatively narrow along the Sn direction. This phenomenon indicates that Cu and Ni are exchangeable in both phases. Three kinds of reaction couples, Sn-55 at. pct Cu/Ni, Sn-65 at. pct Cu/Ni, and Sn-75 at. pct Cu/Ni, were prepared and reacted at 800 °C for 5 to 20 minutes. The reaction paths are liquid/Ni₃Sn₂/γ/Ni₃Sn(L)/Ni for the Sn-55 at. pct Cu/Ni and Sn-65 at. pct Cu/Ni couples, and the reaction path is liquid/γ/Ni₃Sn(L)/Ni for the Sn-75 at. pct Ni couples.     

Thermodynamics of carbon in nickel,iron-nickel and iron-chromium-nickel alloys

Iron-nickel alloys with 8 and 16 wt pct nickel and iron-chromium-nickel alloys with 8 pct nickel and chromium contents in the range of 2 to 22 pct were equilibrated with iron and nickel in flowing CH₄-H₂ gas mixtures and in sealed capsules under partial vacuum at temperatures between 700 and 1060°C. Carbon activities in these alloys were established from the carbon concentrations in the nickel by applying Henry’s law to the solubility of carbon in nickel that was determined in the temperature range of 500 to 1000°C. First-order free-energy interaction parameters were used to relate the carbon activities to composition and temperature in the single-phase austenitic Fe-Ni and Fe-Cr-Ni alloys. An expression was also developed to evaluate carbon activities in Fe-Cr-Ni alloys in the region of higher chromium contents (〉4 wt pct) that result in a two-phase austenite plus carbide mixture at these temperatures.     

Effect of B on the microstructure and mechanical properties of mechanically milled TiAl alloys

The present study is concerned with γ-(Ti₅₂Al₄₈)_100−x B _x (x=0, 0.5, 2, 5) alloys produced by mechanical milling/vacuum hot pressing (VHPing) using melt-extracted powders. Microstructure of the as-vacuum hot pressed (VHPed) alloys exhibits a duplex equiaxed microstructure of α₂ and γ with a mean grain size of 200 nm. Besides α₂ and γ phases, binary and 0.5 pct B alloys contain Ti₂AlN and Al₂O₃ phases located along the grain boundaries and show appreciable coarsening in grain and dispersoid sizes during annealing treatment at 1300 °C for 5 hours. On the other hand, 2 pct B and 5 pct B alloys contain fine boride particles within the γ grains and show minimal coarsening during annealing. Room-temperature compressing tests of the as-VHPed alloys show low ductility, but very high yield strength >2100 MPa. After annealing treatment, mechanically milled alloys show much higher yield strength than conventional powder metallurgy and ingot metallurgy processed alloys, with equivalent ductility to ingot metallurgy processed alloys. The 5 pct B alloy with the smallest grain size shows higher yield strength than binary alloy up to the test temperature of 700 °C. At 850 °C, 5 pct B alloy shows much lower strength than the binary alloy, indicating that the deformation of fine 5 pct B alloy is dominated by the grain boundary sliding mechanism. This article is based on a presentation made in the symposium “Mechanical Behavior of Bulk Nanocrystalline Solids,” presented at the 1997 Fall TMS Meeting and Materials Week, September 14–18, 1997, in Indianapolis, Indiana, under the auspices of the Mechanical Metallurgy (SMD), Powder Materials (MDMD), and Chemistry and Physics of Materials (EMPMD/SMD) Committees.     

Molybdenum in Nb-C alloys

The effects of molybdenum alloying additions to niobium on the carbide phases and their precipitation behavior were investigated. The experimental alloys included Nb-0.1C, Nb-15Mo-0.1C, and Nb-30Mo-0.1C. After selected heat treatments the microstructural changes were determined by metallography and the carbide phases were extracted and identified by X-ray diffraction and chemical analysis. The results are essentially in agreement with recent phase diagram determinations. Additions of 30 wt pct Mo appears to slightly increase the solubility of carbon in niobium at temperatures around 1650°C. The solubility of molybdenum in Nb₂C is very small. Discontinuous precipitation of β-Nb₂C was found to occur in the Nb-30Mo-0.1C alloy during annealing at 1200°C. The important, overall effect of molybdenum in Nb-C alloys is to decrease the rate of niobium carbide precipitation so that appreciable carbon supersaturation can be achieved even after comparatively slow furnace cooling.