Phase transformation in an Fe-10.1Al-28.6Mn-0.46C alloy

The microstructure of an (α + γ) duplex Fe-10.1Al-28.6Mn-0.46C alloy has been investigated by means of optical microscopy and transmission electron microscopy (TEM). In the as-quenched condition, extremely fine D0₃ particles could be observed within the ferrite phase. During the early stage of isothermal aging at 550 °C, the D0₃ particles grew rapidly, especially the D0₃ particles in the vicinity of the α/γ grain boundary. After prolonged aging at 550 °C, coarse K’-phase (Fe, Mn)₃AlC precipitates began to appear at the regions contiguous to the D0₃ particles, and —Mn precipitates occurred on the α/γ and α/α grain boundaries. Subsequently, the grain boundary β-Mn precipitates grew into the adjacent austenite grains accompanied by a γ→ α + β-Mn transition. When the alloy was aged at 650 °C for short times, coarse. K-phase precipitates were formed on the α/γ grain boundary. With increasing the aging time, the α/γ grain boundary migrated into the adjacent austenite grain, owing to the heterogeneous precipitation of the Mn-enrichedK phase on the grain boundary. However, the α/γ grain boundary migrated into the adjacent ferrite grain, even though coarse K-phase precipitates were also formed on the α/γ grain boundary in the specimen aged at 750 °C.     

Phase transformation in an Fe-9.0AI-29.5Mn-1.2Si alloy

The microstructure of an (α + γ) duplex Fe-9.0Al-29.5Mn-l.2Si alloy has been investigated by means of transmission electron microscopy. In the as-quenched condition, extremely fine D0₃ particles were formed within the ferrite matrix by a continuous ordering transition during quenching. After being aged at 550 °C, the extremely fine D0₃ particles existing in the as-quenched specimen grew preferentially along (100) directions. With increasing the aging time at 550 °C, a (Si, Mn)-rich phase (designated as “L phase”) began to appear at the regions contiguous to the D0₃ particles. The L phase has never been observed in various Fe-Al-Mn, Fe-Al-Si, Fe-Mn-Si, and Mn-Al-Si alloy systems before. When the as-quenched specimen was aged at temperatures ranging from 550 °C to 950 °C, the phase transformation sequence occurring within the (α + D0₃) region as the aging temperature increases was found to be (α + D0₃ + L phase) → (α + D0₃ + A13 β-Mn)→ (B2 + D0₃ + A13 β-Mn)→ (B2 + A13β-Mn)→ (α + A13 β-Mn)→ (α +γ)→α.     

Tensile deformation behavior of mechanically stabilized Fe-Mn austenite

The tensile deformation behavior of mechanically-stabilized austenite is investigated in Fe-Mn binary alloys. A 30 pct thickness reduction by rolling at 673 K (above the A_f temperature) largely suppresses the austenite (γ) to hcp epsilon martensite (ε) transformation in 17Mn and 25Mn steels. However, the deformation behavior of the mechanically stabilized austenite in the two alloys differs significantly. In 25Mn steel, the onset of plastic deformation is due to the stress-induced γ→ ε transformation and results in a positive temperature dependence of the yield strength. The uniform elongation is enhanced by the γ → ε transformation during deformation. In 17Mn steel, bccα′ martensite is deformation-induced along with e and a plateau region similar to Lüders band deformation appears at the beginning of the stress-strain curve. The mechanical stabilization of austenite also suppresses the intergranular fracture of 17Mn steel at low temperatures. M. STRUM, formerly Candidate for Ph.D. at the University of California at Berkeley     

Effect of Si addition on the microstructure of an Fe-8.0AI-29.0Mn-0.90C alloy

The microstructures of two alloys with Fe-8.0Al-29.0Mn-0.90C, one without Si and one with 1.5 wt pct Si, have been investigated by means of transmission electron microscopy (TEM). In the as-quenched condition, the alloy without Si is single-phase austenite; however, some discrete particles along the austenite grain boundaries can be observed in the Si-bearing alloy. The discrete particles have a mixture of (α + D0₃) phases, indicating that the Si addition enhances the formation of (α + D0₃) phases. This result is in disagreement with those reported by other research on Fe-Al-Mn-Si-C alloys. Transmission electron microscopy examinations reveal that the (α + D0₃) phases are formed by an ordering transition during quenching. When the quenched specimen is aged at temperatures ranging from 450 °C to 1050 °C, the phase transformation sequence occurring within the (α + D0₃) region as the temperature increases is found to be D0₃→ (D0₃ +K phase) → B2 → α.     

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.     

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.     

The effect of additives on the eutectoid transformation of ductile iron

The eutectoid transformation of austenite in cast iron is known to proceed by both the meta-stable γ → α + Fe₃C reaction common in Fe-C alloys of near eutectoid composition, and by the direct γ → α + Graphite reaction, with the graphite phase functioning as a car-bon sink. In addition, the meta-stable cementite constituent of the pearlite can dissolve near the graphite phase (Fe₃C → α + Graphite), producing free ferrite. Isothermal trans-formation studies on a typical ductile iron (nodular cast iron) confirmed that all of these reaction mechanisms are normally operative. The addition of 1.3 pct Mn was found to substantially retard all stages of the transformation by retarding the onset of the eutectoid transformation, decreasing the diffusivity of carbon in ferrite, and stabilizing the cemen-tite. Minor additions of Sb (0.08 pct) or Sn (0.12 pct) were found to inhibit the γ →α + Graphite reaction path, as well as the Fe₃C → α + Graphite dissolution step, but did not significantly affect the meta-stable γ → α + Fe₃C reaction. Scanning Auger microprobe analysis indicated that Sn and Sb adsorb at the nodule/metal interphase boundaries during solidification. This adsorbed layer acts as a barrier to the carbon flow necessary for the direct γ → α + Graphite and Fe₃C → α + Graphite reactions. With the graphite phase dis-abled as a sink for the excess carbon, the metal transforms like a nongraphitic steel. The effects of Mn, Sn, and Sb on the eutectoid transformation of ductile iron were shown to be consistent with their behavior in malleable iron.     

The Transformation of Carbides during Austenization and Its Effect on the Wear Resistance of High Speed Steel Rolls

The transformation of carbides with austenization time of a high speed steel (HSS) roll material, manufactured by a centrifugal casting method, has been studied. The correlation between wear resistance and the type, morphology, volume fraction, and distribution of the carbides has also been investigated. Microstructural observations, X-ray diffraction (XRD) analysis, hardness measurements, and energy dispersive spectroscopy (EDS) have been used to characterize the carbides. The type and volume fraction of carbides were found to change with austenizing time. During austenization, the transformation of the M₃C carbides can be postulated as M₃C + γ-Fe → M₂C, with much finer nodular and rodlike MC carbides also forming through a solid-state transformation. The M₂C carbide decomposes as M₂C + γ-Fe → MC + M₇C₃ + M₆C. The decomposed carbide substantially maintains a platelike shape until the end of decomposition. The most important finding of this study is that austenization results in changes in the type, morphology, volume fraction, and distribution of carbides and that it can be controlled to produced a homogeneous distribution of hard carbides, resulting in an improvement in the wear resistance of HSS rolls. This finding may be of great use for the industrial production of HSS rolls.     

Structure of phases in the δ-AI2O3 fiber studied by convergent beam electron diffraction

The phases in the δ-Al₂O₃ fibers were investigated using the methods of transmission electron microscopy (TEM): convergent beam electron diffraction (CBED) and high-resolution electron microscopy (HREM). A phaseγ′-Al₂O₃ discovered previously by Vewerly in oxide layers with an fcc structure was found and new atomic positions are proposed. A new structure ofδ-Al₂O₃ was also observed. It has aPmma space group and lattice parameters ofa _δ = 2a _γ′,b _δ = l.5a _γ′, andc _δ ≈a _γ′ The correlation of the observed A1₂O₃ lattices to the spinel lattice is discussed and translation of atom positions during theγ′ →γ →δ transformation is studied. All anions must change their positions by a small amount; one-third of the cation positions inγ′ and more than 90 pct of cation positions inδ experience a large translation during that transformation. This implies that for theγ′ it→γ} →δ transformation, the positions of cations in both lattices are important. The results are discussed in relation to the fiber-matrix interaction under spinel formation during thermal loading ofδ-Al₂O₃-fiber-reinforced aluminum piston alloys.     

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.     

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.     

Driving force for γ→ε martensitic transformation and stacking fault energy of γ in Fe-Mn binary system

A regular solution model for the difference of the chemical free energy between γ and ε phases during γ→ε martensitic transformation in the Fe-Mn binary system has been reexamined and partly modified based on many articles concerning the M _s and A _s temperatures of Fe-Mn alloys. Using the regular solution model, the measured M _s temperatures, and a thermodynamic model for the stacking fault energy (SFE) of austenite (γ), the driving force for γ→ε martensitic transformation, and the SFE of γ have been calculated. The driving force for γ→ε martensitic transformation increases linearly from − 68 to − 120 J/mole with increasing Mn content from 16 to 24 wt pct. The SFE of γ decreases to approximately 13 at. pct Mn and then increases with increasing Mn content, which is in better agreement with Schumann’s result rather than Volosevich et al.’s result.     

Phase transformation and microstructural evolution in Ti-44Al-4Nb-4Zr alloy during heat treatment

Phase transformation and microstructural evolution have been studied in Ti-44Al-4Nb-4Zr-0.2Si-0.1B alloys that were cooled from theα +β phase region with various cooling rates. It has been shown that the cooling rates have different influence on the morphology of the transformation products for the three phase transformations studied,α →α ₂, B2 →ω, andα →γ. Under slow cooling, all three transformations can be fulfilled. Under rapid cooling, B2 →ω is partially detained and a diffuseω phase forms as metastable phase, butα →γ is almost completely suppressed, which supports that theγ lamellae formation is diffusion controlled.     

Mechanism of primary MC carbide decomposition in Ni-base superalloys

Three industrial gas turbine blades made of conventionally cast (CC) IN-738 and GTD-111 and directionally solidified GTD-111 Ni-base superalloys were examined after long-term exposures in service environments. All three blades exhibit similar, service-induced microstructural changes (MCs) including γ′ coarsening and coalescence, excessive secondary M₂₃C₆ precipitation, and primary MC degeneration, regardless of the chemical composition and the grain size. Special attention was paid to the primary MC decomposition. It is shown that the primary MC decomposition occurs by carbon diffusion out of the carbide into the γ + γ′ matrix, resulting in the formation of Cr-rich M₂₃C₆ carbides near the initial carbide/matrix interface. A transition zone is shown to develop between the original MC core and its perimeter, demonstrating the gradual outward diffusion of carbon and a slight inward increase in nickel concentration. The hexagonal Ni₃(TiTa) η-phase was also found in the MC transition zone and on the MC-γ/γ′ interface. The primary MC decomposition can be expressed by the reaction MC + γ/γ′ → M₂₃C₆ + η. Finally, it is shown that the grain-boundary (GB) MC decomposes more rapidly than that in the grain interiors. This is consistent with the more rapid GB diffusion that leads to the acceleration of the MC diffusional decomposition processes.     

Microstructural dependence of Fe-high Mn tensile behavior

The tensile properties of Fe-high Mn (16 to 36 wt pct Mn) binary alloys were examined in detail at temperatures from 77 to 553 K. The Mn content dependence of the deformation and fracture behavior in this alloy system has been clarified by placing special emphasis on the starting microstructure and its change during deformation. In general, the intrusion of hcp epsilon martensite (ε) into austenite (γ) significantly increases the work hardening rate in these alloys by creating strong barriers to further plastic flow. Due to the resulting high work hardening rates, large amounts of e lead to high flow stresses and low ductility. Alloys of 16 to 20 wt pct Mn are of particular interest. While these alloys are thermally stable with respect to bcc α’ martensite formation, 16 to 20 wt pct Mn alloys undergo a deformation induced ε →α’ transformation. The martensitic transformation plays two contrasting roles. The stress-induced ε→ α’ transformation decreases the initial work hardening rate by reducing locally high internal stress. However, the work hardening rate increases as the accumulated α’ laths become obstacles against succeeding plastic flow. These rather complicated microstructural effects result in a stress-strain curve of anomolous shape. Since both the M_s and M_d temperatures for both the ε and α’-martensite transformations are strongly dependent on the Mn content, characteristic relationships between the tensile behavior and the Mn content of each alloy are observed.     

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.     

Influence of thermal aging on the reactivity of duplex stainless steel surfaces

The annealing of large cast pieces in duplex stainless steel (SS) and the different heat cycles resulting from repairs involve significant structural changes characterized by carbide and intermetallic phase precipitation. This yields to lower local corrosion resistance in sea water due to changes in the local content of alloying elements. The precipitation of chromium carbide affects the resistance to the intergranular corrosion and the repassivation behavior. The eutectoidal decomposition of ferritic phase into regenerated austenite and in sigma phase (α → γ _r + σ) results in weakening the resistance to pit nucleation in synthetic sea water. In contrast, such precipitation will not have any significant effect when the treatment temperature is high enough to involve a rapid rehomogenization of depleted zones and ensure a self-healing.     

Precipitation strengthening in K5-series γ-TiAl alloyed with silicon and carbon

Interstitial additions and precipitation hardening in fully lamellar gamma TiAl have been investigated in recent years, with a prime objective of improving the high-temperature creep resistance. As a result of this alloy development effort, the alloy system K5 (Ti46Al-2Cr-3Nb-0.2W) was found to show remarkably improved creep resistance when reinforced with C or C+Si additions and then aged appropriately. Precipitation strengthening is the proposed mechanism accounting for the observed creep strengthening of K5SC alloys, with emphasis being paid on the effect of B2 particles, ζ-type silicides, and H-type carbide precipitates delineating γ/γ interfaces. In this study, the creep-deformed microstructures of fully lamellar K5 (S-C)-type alloys in aged and unaged conditions were characterized using detailed electron microscopy, involving high-resolution imaging techniques and in-situ heating studies. Overall, the presence of these particles and their relative distribution result in strengthening of the lamellar structure. The particular effect of each type of precipitate (silicides vs carbides) on creep has been assessed. New information about the nature of the light-element precipitation processes has been obtained by studying the nucleation and growth of the carbide and silicide precipitates at the expense of dissolving α ₂ laths during aging. This article is based on a presentation made in the symposium entitled “Fundamentals of Structural Intermetallics,” presented at the 2002 TMS Annual Meeting, February 21–27, 2002, in Seattle, Washington, under the auspices of the ASM and TMS Joint Committee on Mechanical Behavior of Materials.     

Alloy design of FeMnSiCrNi shape-memory alloys related to stacking-fault energy

The stacking-fault energy (γ _sf ) of iron-based shape-memory alloys was calculated by the extended dislocation-node method. The results show that Ni and Mn increase the γ _sf of the alloys with an austenite structure, while Cr and Si decrease it. An expression relating the alloying elements of Ni, Cr, Mn, and Si to the γ _sf of the alloys is established. Moreover, in terms of the γ _sf values of the alloys and the Schaeffler diagram, the alloy design of iron-based shape-memory alloys is carried out. It is found that the alloy having the lowest γ _sf has the best shape-memory effect (SME), with a martensite transition temperature (M _s ) being a little lower than ambient temperature. The corresponding composition of the alloy is located in the γ-phase zone near the phase line between the γ and γ+ε phase zone and the triple point of α+γ+ε in the Schaeffler diagram.