Growth kinetics of grain boundary ferrite allotriomorphs in Fe-C-X alloys

Parabolic rate constants for the thickening (α) and lengthening (β) kinetics of grain boundary allotriomorphs of proeutectoid ferrite have been measured as a function of isothermal transformation temperature in several Fe-C-X’ alloys whereX = Si, Ni, Mn, and Cr. These constants have been corrected approximately for the growth inhibition produced by facets on the allotriomorphs. The corrected α values are compared with those calculated on the basis of three models: equilibrium at α:γ boundaries with partition ofX, local equilibrium with “pile-up” ofX rather than bulk partition, and paraequilibrium. Values calculated from both the paraequilibrium and the “pile-up” models were in order of magnitude or better agreement with the corrected experimental α’s. Similar levels of agreement were obtained for the equilibrium model in the Si and Cr alloys and also in one Ni alloy at lower reaction temperatures. However, an estimate of the maximum possible diffusion distance of alloying element into austenite during growth supported only the paraequilibrium model under nearly all conditions investigated. Even for this model, however, measured rate constants are significantly less than those calculated for Fe-C-Mn and Fe-C-Cr and greater for Fe-C-Si and the higher Ni, Fe-C-Ni alloy. The Mn and Cr discrepancies seem best explained at present by a solute drag-like effect; an accompanying paper indicates that interphase boundary precipitation of carbides is involved in the Si and Ni alloys, though an inverse solute drag-like effect may also be operative. Formerly graduate student, Department of Metallurgical Engineering, Michigan Technological University. Formerly Professor at Michigan Technological University.     

Discussion on the Alloying Element Partition and Growth Kinetics of Proeutectoid Ferrite in Fe-C-Mn-X Alloys

Experimental data on alloying element partition and growth kinetics of proeutectoid ferrite in quaternary Fe-C-Mn-Si, Ni, and Co alloys were reanalyzed using an approximate method, which permits a quick evaluation of alloy partitioning to be made. The method yielded results in good agreement with DICTRA and is applicable to Fe-C base multicomponent alloys. Differences of the predicted local condition at the α/γ boundary from those previously presented in the alloys are noted.     

Origins of internal structure in massive transformation products

The internal structure in massive phases formed during six massive transformations has been reviewed. A counterpart review has also been made for the proeutectoid ferrite reaction, mainly in alloy steels in which bulk partition of alloying elements between austenite and ferrite has not occurred. Both dislocations and twins comprise this structure unless the stacking fault energy is too high to permit twin formation. Volume and shape changes associated with transformation can explain dislocation loops through stress-induced displacement and multiplication of misfit dislocations into the softer phase by means of either a dissociation reaction followed by Ashby-Johnson prismatic looping or emanation of glide loops from Frank-Reed sources. Following Gleiter et al., the “growth accidents” concept used to explain dislocation and twin formation during grain growth proves equally suitable for explaining formation of the same features during the massive and other diffusional transformations. Climb of interfaces produced by edge-to-edge rather than the usual plane-to-plane matching, introduced by Kelly and Zhang and experimentally supported by Nie and Muddle and by Howe et al. for the α → γ _m transformation in near-TiAl alloys, is proposed as another source of dislocations in the product phase. This paper was prepared following participation of its authors in the symposium “The Mechanisms of the Massive Transformation,” held Oct. 9–11, 2000. during the Fall 2000 TMS/ASM Meeting in St. Louis, MO, under the sponsorship of the ASM INTERNATIONAL Phase Transformations Committee.     

Nucleation kinetics of proeutectoid ferrite at austenite grain boundaries in Fe-C-X alloys

The nucleation kinetics of proeutectoid ferrite allotriomorphs at austenite grain boundaries in Fe-0.5 at. Pct C-3 at. Pct X alloys, where X is successively Mn, Ni, Co, and Si and in an Fe-0.8 at. Pct C-2.5 at. Pct Mo alloy have been measured using previously developed experimental techniques. The results were analyzed in terms of the influence of substitutional alloying elements upon the volume free energy change and upon the energies of austenite grain boundaries and nucleus: matrix boundaries. Classical nucleation theory was employed in conjunction with the pillbox model of the critical nucleus applied during the predecessor study of ferrite nucleation kinetics at grain boundaries in Fe-C alloys. The free energy change associated with nucleation was evaluated from both the Hillert-Staffanson and the Central Atoms Models of interstitial-substitutional solid solutions. The grain boundary concentrations of X determined with a Scanning Auger Microprobe were utilized to calculate the reduction in the austenite grain boundary energy produced by the segregation of alloying elements. Analysis of these data in terms of nucleation theory indicates that much of the influence of X upon ferrite nucleation rate derives from effects upon the volume-free energy change,i.e., upon alterations in the path of theγ/(α + γ) phase boundary. Additional effects arise from reductions in austenite grain boundary energy, with austenite-forming alloying elements being more effective in this regard than ferrite-formers. By difference, the remaining influence of the alloy elements studied evidently results from their ability to diminish the energies of the austenite: ferrite boundaries enclosing the critical nucleus. The role of nucleation kinetics in the formation of a bay in the TTT diagram of Fe-C-Mo alloys is also considered. Formerly Graduate Student, Department of Metallurgical Engineering and Materials Science, Carnegie-Mellon University     

Thickening of grain-boundary α allotriomorphs in a Ti-Cr alloy by multiple sets of ledges

A computer model is developed to simulate the growth of grain-boundary allotriomorphs having more than one set of growth ledges at their interfaces. The growth is controlled by the volume diffusion of solute to or from the riser of a ledge. The time dependence of the growth rate of two orthogonal sets of ledges is found to be somewhat different from that of a single set of ledges. However, the operation of multiple sets of ledges is unlikely to alter significantly the growth kinetics of grain-boundary allotriomorphs from those predicted from the disordered growth theory, except at small ledge spacings or at short reaction times. Faster growth kinetics of proeutectoid α allotriomorphs than those of either planar or ellipsoidal disordered boundaries which have been reported in a Ti-6.6 at. pet Cr alloy are not likely to be accounted for with the heights and spacings of double sets of ledges actually observed on the interfaces of allotriomorphs. Hence, the grain-and interphase-boundary diffusion-assisted growth of precipitates, (rejector plate mechanism, RPM) appears to be operative during the growth of a allotriomorphs, as previously proposed on the basis of growth-rate measurements. This paper is based on a presentation made in the symposium “The Role of Ledges in Phase Transformations” presented as part of the 1989 Fall Meeting of TMS-MSD, October 1–5, 1989, in Indianapolis, IN, under the auspices of the Phase Transformations Committee of the Materials Science Division, ASM INTERNATIONAL.     

The incomplete transformation phenomenon in steel

The incomplete transformation (ICT) phenomenon is defined as the temporary cessation of ferrite formation (in the absence of carbide precipitation at α:γ boundaries) before the fraction of austenite transformed to ferrite predicted by the Lever rule is attained. The ICT phenomenon is central to the “overall reaction kinetics” definition of bainite but plays lesser roles in the quite different groups of phenomena comprising the “surface relief” and “generalized microstructural” definitions. Experimental generalizations that can be made about the ICT are briefly noted. Effects of alloying elements, X, upon various aspects of the nucleation and growth of ferrite are listed in order of apparently increasing strength. The ICT is seen to be one of the stronger effects in the latter spectrum. Theories of the ICT are then critically examined. The currently most promising theories involve (1) the cessation of growth induced by the coupled-solute drag effect (C-SDE), accentuated by the overlap of the carbon diffusion fields associated with adjacent ferrite crystals; and (2) the concepts of item (1) plus local alloying element partition between ferrite and austenite (LE-NP), thereby making any further ferrite growth require volume diffusion of X in austenite and thus to take place exceedingly slowly. Distinguishing between these theories will require use of an Fe-C-X system in which the temperature-carbon concentration paths of the paraequilibrium (PE) Ae₃ and of the “no partition” boundary are well separated. Although the Fe-C-Mo system has proved convenient for studying many aspects of the ICT phenomenon, it does not fulfill this specification. Fe-C-Mn alloys do so and should be particularly useful subjects for future investigations of the ICT phenomenon. This article is based on a presentation made in the “Hillert Symposium on Thermodynamics & Kinetics of Migrating Interfaces in Steels and Other Complex Alloys,” December 2–3, 2004, organized by The Royal Institute of Technology in Stockholm, Sweden.     

The role of ledges in the proeutectoid ferrite and proeutectoid cementite reactions in steel

The influence of interphase boundary ledges on the growth and morphology of proeutectoid ferrite and proeutectoid cementite precipitates in steel is examined. After reviewing current theoretical treatments of growth by the ledge mechanism, investigations that clearly document the presence and motion of ledges with thermionic emission electron microscopy (THEEM) and transmission electron microscopy (TEM) are reviewed. A fundamental distinction is made between two types of ledges: (1) mobile growth ledges whose lateral migration displaces the inter-phase boundary and (2) misfit-compensating structural ledges. Both types of ledges strongly affect the apparent habit plane and aspect ratio of precipitate plates. Agreement between measured growth rates of proeutectoid ferrite and cementite (plates and allotriomorphs) and predicted growth kinetics assuming volume diffusion-controlled migration of ledge-free disordered boundaries is shown to be consistently poor. Physically realistic growth models should incorporate the ledge mechanism. More accurate comparisons of the growth models with experimental data will need to account for observed ledge heights, interledge spacings, and ledge velocities. In this vein, the sluggish growth kinetics of cementite allotriomorphs observed in an Fe-C alloy are shown to be quantitatively consistent with a strong increase in interledge spacing with time. This paper is based on a presentation made in the symposium “The Role of Ledges in Phase Transformations” presented as part of the 1989 Fall Meeting of TMS-MSD, October 1–5, 1989, in Indianapolis, IN, under the auspices of the Phase Transformations Committee of the Materials Science Division, ASM INTERNATIONAL.     

Growth kinetics of solid-liquid Ga interfaces: Part I. Experimental

A technique based on the Seebeck effect was used to determine directly the solid-liquid (S/L) interface supercooling and toin situ monitor the interfacial conditions during growth of high-purity Ga single crystals from a supercooled melt. Using this nonintrusive technique, the growth kinetics of faceted (111) and (001) interfaces were studied as a function of the interface supercooling in the range of 0.2 to 4.6 K, corresponding to bulk supercoolings of about 0.2 to 53 K. In addition, the growth kinetics have been determined as a function of crystal perfection related to the emergence of dislocations at the S/L interface. The results show that at low super-coolings, the faceted interfaces grow with either of the lateral growth mechanisms: two-dimensional nucleation-assisted (2DNG) or screw dislocation-assisted (SDG), depending on the perfection of the interface. At increased interfacial supercoolings, however, both growth rates (2DNG and SDG) become a linear function of the supercooling. Application of the existing growth theories to the experimental results gives only qualitative agreement and fails to predict the observed deviation in the kinetics at high supercoolings. A theoretical treatment of the growth of faceted interfaces will be given in Part II of this series.¹ Formerly Research Assistant with the Department of Materials Science and Engineering, University of Florida. This paper is based on a presentation made in the symposium “The Role of Ledges in Phase Transformations” presented as part of the 1989 Fall Meeting of TMS-MSD, October 1–5, 1989, in Indianapolis, IN, under the auspices of the Phase Transformations Committee of the Materials Science Division, ASM INTERNATIONAL.     

The growth of ferrite in Fe-C-X alloys: The role of thermodynamics, diffusion, and interfacial conditions

The growth of allotriomorphic ferrite from austenite in Fe-C-X alloys is studied. Two systems have been selected: the Fe-C-Ni system, in which the substitutional alloying element is expected to have a weak interaction with both the C and the moving interface, and the Fe-C-Mo system, in which these interactions are expected to be non-negligible. The ferrite growth kinetics was measured using two types of experiments: classical isothermal heat treatments and decarburization experiments. All of the experimental observations can be quantitatively rationalized using a model that describes an evolution in interfacial conditions from paraequilibrium (PE) to local equilibrium with negligible partitioning (LENP) during growth. This article is based on a presentation made in the “Hillert Symposium on Thermodynamics & Kinetics of Migrating Interfaces in Steels and Other Complex Alloys,” December 2–3, 2004, organized by the The Royal Institute of Technology in Stockholm, Sweden.     

Effect of cooling rate and alloying on the

The pearlitic hardenability of a high-purity Fe-0.8 pct C alloy and zone-refined iron binary alloys containing Mn, Ni, Si, Mo, or Co was studied by means of hot-stage microscopy. The binary alloys were carburized in a gradient furnace to produce eutectoid compositions, thus eliminating proeutectoid phases. A special technique based on hot-stage microscopy was used to study the effect of cooling rate (10°F/min to 25,000°F/min) on the transformation of austenite and provided data for the construction of continuous cooling-transformation diagrams. From these diagrams critical cooling rates were obtained for hardenability calculations. It was found that molybdenum is the most effective element, followed by Si, Ni, Co, and Mn, in suppressing the pearlite transformation,i.e., in increasing the hardenability of the alloys studied. The alloying additions were grouped into two classes according to their effect on hardenability: α-stabilizers (Mo and Si) and γ-stabilizers (Ni, Co, Mn), with the α-stabilizers being the more effective in improving hardenability. This paper is based on a presentation made at a symposium on “Hardenability” held at the Cleveland Meeting of The Metallurgical Society of AIME, October 17, 1972, under the sponsorship of the IMD Heat Treatment Committee.     

Lattice transformations related to unique mechanical effects

Athermal and stress-induced martensitic transformations are examined in various alloys of the large family which exhibit the unique “memory” and/or “superelastic” shape memory effects (SME). Such mechanical effects are found to be intimately related to details of the martensitic and premartensitic reaction paths in each system. A common feature of various “uncommon” systems is that the usual phenomenological crystallographic analysis cannot completely describe the martensitic transformation in these systems. Addiional features represented by lattice “shuffles” or low-wavelength lattice waves, and the mechanistic role of transformation dislocations are examined. A common thread in various systems such as TiNi, CuZn, AuCd, In-Tl, and so forth, is viewed in terms of evidence related to alloying (electronic entropy) effects on lattice instability of the parent phase. Instability reflected by premonitory phenomena can be given considerable generality when related to observations in systems which exhibit similar dynamic lattice transitions, such as the second-order “athermal omega” lattice transition in Group IV-base systems. The importance of reversibility in martensitic transformation of SME alloys is emphasized. Comparisons with more common non-SME martensitic alloys are made. This paper is based on a presentation made at a symposium on “Phase Transformations in Less Common Metals: A Dialogue,” held at the Fall Meeting in Cleveland on October 16, 1972, under the sponsorship of the Phase Transformations Activity, Materials Science Division, American Society for Metals.     

Granular product in a high strength,low alloy containing molybdenum and niobium

The formation and microstructure of the granular product and its effect on the mechanical properties of a high-strength, low alloy steel containing molybdenum and niobium have been investigated. It was found that the granular product “islands” are composed of both twinned martensite and dislocated martensite. The effect of the granular “islands” on the strength at room temperature and at 400 °C has been determined. The results showed that the strength increased and both the impact and fracture toughness decreased as the volume fraction of granular “islands” was increased.In situ fracture studies indicated that the three stages of the microfracture process of the specimen containing granular “islands” are the initiation of voids at interfaces between the granular “islands” and the bainitic ferrite matrix, followed by void growth and finally, coalescence by shear.     

The influence of the alloying elements upon the transformation kinetics and morphologies of ferrite plates in alloy steels

A series of Fe-C-X and Fe-C-X₁-X₂ alloys in which X, X₁ and X₂ either raise or depress the activity of C iny were investigated by autodilatometer, optical microscopy, and transmission electron microscopy (TEM) to reveal the relations among the chemical composition, transformation kinetics, and morphology of ferrite plates. The incubation time of austenite decomposition at the nose temperature in the time-temperature-transformation (TTT) diagrams, the concentration of C in y in contact with theα/gg boundary, and the growth rate of ferrite were evaluated to estimate the magnitude of the solute drag-like effect (SDLE) for the different alloying elements used. All the results are consistent qualitatively with the SDLE hypothesis. This article is based on a presentation made at the Pacific Rim Conference on the “Roles of Shear and Diffusion in the Formation of Plate-Shaped Transformation Products,” held December 18-22, 1992, in Kona, Hawaii, under the auspices of ASM INTERNATIONAL’S Phase Transformations Committee.     

Overview of Geometric Effects on Coarsening of Mushy Zones

An overview is presented of the thermodynamic, kinetic, statistical, and geometric factors that govern phase coarsening in dendritic mushy zones. The coarsening behavior of such systems is best quantified through the kinetics of the decay rate of the specific surface area,S _v. The geometry of the complex solid-melt interfaces comprising a mushy zone is described statistically as an areal distribution of local curvature parameters. These parameters capture both the intensive and extensive thermodynamic characteristics of the mushy zone. The effects of local interface shape, negative mean, and Gaussian curvatures and the appearance of inactive lengthscales on the coarsening kinetics of dendritic structures are discussed. The combined contribution of all these geometrical effects yields global coarsening rates for ramified mushy zones that are comparable to those predicted from theory for a collection of spherical particles having the identical volume fraction of solid. This article is based on a presentation made at the “Analysis and Modeling of Solidification” symposium as part of the 1994 Fall meeting of TMS in Rosemont, Illinois, October 2–6, 1994, under the auspices of the TMS Solidification Committee.     

Simulation of ferrite growth in continuously cooled low-carbon iron alloys

The growth of a planar ferrite (α): austenite (γ) boundary in low-carbon iron and Fe-Mn alloys continuously cooled from austenite through the (α+γ) two-phase field and the α single-phase field was simulated by incorporating carbon diffusion in austenite, intrinsic boundary mobility, and the drag of an alloying element. At a very high cooling rate (≥ 10³ °C/s), the width of the carbon diffusion spike in austenite approaches the limit at which spikes are viable, so that the growth of ferrite in which carbon is not partitioned can occur even above the α solvus. In this context, the upper limiting temperature of partitionless growth of ferrite is the T ₀ temperature. In the presence of drag of an alloying element, e.g., Mn, both carbon-partitioned and partitionless growth of ferrite begins to occur at finite undercoolings from the Ae ₃, T ₀, or α-solvus temperature, at which the driving force for transformation exceeds the drag force. The intrinsic mobility of the α:γ boundary may play a significant role at an extremely high cooling rate (≥10⁵ °C/s). This article is based on a presentation made at the symposium entitled “The Mechanisms of the Massive Transformation,” a part of the Fall 2000 TMS Meeting held October 16–19, 2000, in St. Louis, Missouri, under the auspices of the ASM Phase Transformations Committee.     

Growth and overall transformation kinetics above the bay temperature in Fe-C-Mo alloys

The kinetics and morphology of isothermal transformation in the vicinity of the time-temperaturetransformation (TTT) diagram bay have been investigated with optical and transmission electron microscopy (TEM) in 19 Fe-C-Mo alloys at three levels of carbon concentration (approximately 0.15, 0.20, and 0.25 wt pct) and at Mo concentrations from 2.3 to 4.3 wt pct, essentially always at temperatures above or at that of the bay,T _b . Quantitative metallography yielded no evidence for incomplete transformation (stasis) in any of these alloys atT > T _b . Measurements of the thickening kinetics of grain boundary ferrite allotriomorphs (invariably containing either interphase boundary or fibrous Mo₂C) demonstrated four different patterns of behavior. The customary parabolic time law for allotriomorph thickening in Fe-C and in many Fe-C-X systems was obtained only at higher temperatures and in the more dilute Fe-C-Mo alloys studied. With decreasing temperature and increasing solute concentrations, a two-stage and then two successive variants of a three-stage thickening process are found. In the most concentrated alloys and at temperatures nearest the bay, the second stage of the three-stage thickening process corresponds to “growth stasis”—the cessation of allotriomorph thickening. Sufficient prolongation of growth stasis presumably leads to “transformation stasis.” A number of models for growth of the carbide-containing allotriomorphs were investigated during attempts to explain the observed kinetics. It was concluded that their growth is controlled by carbon diffusion in austenite but with a driving force drastically reduced by a very strong solute drag-like effect (SDLE) induced by Mo segregation at disordered-type austenite: ferrite boundaries. Carbide growth in the fibrous structure appears to be fed by diffusion of Mo along austenite: ferrite boundaries, whereas carbides in the interphase boundary structure grow primarily by volume diffusion of Mo through austenite. Formerly Republic Steel Corporation Fellow, Department of Metallurgical Engineering, Michigan Technological University, Houghton, MI, and Visiting Graduate Student, Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University, Pittsburgh, PA. Formerly Professor, Michigan Technological University. This paper is based on a presentation made in the symposium “International Conference on Bainite” presented at the 1988 World Materials Congress in Chicago, IL, on September 26 and 27, 1988, under the auspices of the ASM INTERNATIONAL Phase Transformations Committee and the TMS Ferrous Metallurgy Committee.     

Massive transformation and the formation of the ferromagnetic L10 phase in manganese-aluminum-based alloys

Manganese-aluminum alloys in the vicinity of the equiatomic composition exhibit an attractive combination of magnetic properties for technological applications, including bulk permanent magnets and thin-film devices. The technical magnetic properties derive from the formation of a metastable L1₀ intermetallic phase (τ-MnAl) characterized by a high, uniaxial magnetocrystalline anisotropy with an “easy” c-axis. Carbon is generally added to stabilize the tetragonal τ phase with respect to the stable phases in the system. The magnetic hysteresis behavior of the Mn-Al-C genre of permanent magnet alloys is extremely sensitive to the microstructure and defect structure produced during the formation of the τ phase (L1₀) within the high-temperature ε phase (hcp). In this study, modern metallographic techniques, including high-resolution electron microscopy (HREM), have been applied to elucidate the nature of the phase transformation and the evolution of the unique microstructure and defect structure characterizing the structural state of the ferromagnetic τ phase. It is concluded that the metastable τ phase is the product of a compositionally invariant, diffusional nucleation and growth process or massive transformation. The massive product nucleates preferentially at the grain boundaries of the parent ε phase and is propagated by the migration of incoherent interphase interfaces. The interphase interfaces are revealed to be faceted on various length scales. It is concluded that this faceting is not a feature of the bicrystallography of the parent and product phases. The high density of lattice defects within the τ phase, generated by the phase transformation, is attributed to growth faults produced during atomic attachment at the migrating interfaces. Classical nucleation theory has been applied quantitatively to the grain-boundary nucleation process and was found to be consistent with the observed time-temperature-transformation (TTT) behavior. Analysis of the growth kinetics gives an ΔH _D value of 154 kJ mol⁻¹ for the activation energy of the transboundary diffusional process controlling boundary migration. This article is based on a presentation made at the symposium entitled “The Mechanisms of the Massive Transformation,” a part of the Fall 2000 TMS Meeting held October 16–19, 2000, in St. Louis, Missouri, under the auspices of the ASTM Phase Transformations Committee.     

Intragranular ferrite nucleation in medium-carbon vanadium steels

In this study, the mechanism of intragranular ferrite nucleation is investigated. It is found that “intragranular ferrite idiomorphs” nucleate at vanadium nitrides which precipitate at manganese sulfide particles during cooling in the austenite region. It is observed that intragranular ferrite has the Baker-Nutting orientation relationship with vanadium nitride which precipitated at manganese sulfide. According to classical nucleation theory, the proeutectoid ferrite nucleation rate depends on the following factors: (1) the driving free energy for ferrite nucleation, (2) the diffusivity of carbon atoms in austenite, and (3) the increase in the interfacial energy associated with ferrite nucleation. In the Baker-Nutting orientation relationship, the lattice mismatch across the habit planes is likely to be very small. Depleted zones of solute atoms such as vanadium are assumed to be formed in the austenite matrix around precipitates. The effect of the depleted zones on factors (1) and (2) is estimated thermodynamically and it is proved that those effects are negligibly small. Thus, we conclude that the most important factor in nucleation kinetics of intragranular ferrite is the formation of precipitates which can develop coherent, low energy interfaces with ferrite.     

Alloying Element Partition and Growth Kinetics of Proeutectoid Ferrite in Hot-Deformed Fe-0.1C-3Mn-1.5Si Austenite

Alloying element partition and growth kinetics of proeutectoid ferrite in deformed austenite were studied in an Fe-0.1C-3Mn-1.5Si alloy. Very small ferrite particles, less than several microns in size, were formed within the austenite matrix, presumably at twin boundaries as well as at austenite grain boundaries. Scanning transmission electron microscopy–energy-dispersive X-ray (STEM-EDX) analysis revealed that Mn was depleted and Si was enriched in the particles formed at temperatures higher than 943 K (670 °C). These were compared with the calculation of local equilibrium in quaternary alloys, in which the difference in diffusivity between two substitutional alloying elements was assumed to be small compared to the difference from the carbon diffusivity in austenite. Although the growth kinetics were considerably faster than calculated under volume diffusion control, a fine dispersion of ferrite particles was readily obtained in the partition regime due to sluggish growth engendered by diffusion of Mn and Si.     

Effect of Ternary Alloying Elements Addition on the Order-Disorder Transformation Temperatures of B2-Type Ordered Fe-Al-X Intermetallics

The effect of alloying element additions on B2↔A2 order-disorder phase transformation temperatures of B2-type ordered Fe_0.5(Al_1−n X _n )_0.5 intermetallics (X = Cr, Ni, Mo, Ta, Mn, Ti, and W) that readily form single-phase solid solution for X = 1 at. pct were investigated experimentally. It was shown that the type of the ternary substitutional alloying elements have a profound effect on the variation of order-disorder transition temperature of Fe_0.5(Al_1−n X _n )_0.5 alloys. Based on the magnitude of partial ordering energies of the Al-X and Fe-X atomic pairs, predicted normalized transition temperatures, ∆T/T _o , were verified experimentally. Besides the normalized transition temperature, the relative partial ordering energy (RPOE) parameter, β, was also defined to estimate the extent of variation in B2↔A2 order-disorder phase transformation temperatures upon ternary alloying additions. The RPOE parameter, β, takes into account both the effects of magnitude of partial ordering energies of Al-X and Fe-X atomic pairs and also the lattice site occupation preferences of X element atoms over B2-type ordered Fe-Al sublattices. The alloying elements, which are preferentially distributed Fe sublattice sites, β > 0, and owing to β >> 1, are more effective in increasing order-disorder transformation temperature in Fe-Al (B2) intermetallics. On the contrary, alloying elements having β < 1 tend to decrease the transition temperature slightly relative to the binary FeAl intermetallic. The experimentally determined B2↔A2 order-disorder transition temperatures are in good qualitative or semiquantitative agreement with theoretical predictions for all X ternary alloying elements. Accordingly, the present experimental results confirm the validity of the theoretical model and calculations proposed in our previous study on the B2↔A2 order-disorder transition temperatures of single-phase Fe_0.5(Al_1−n X _n )_0.5 intermetallics.