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1.
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. 相似文献
2.
G. J. Shiflet J. R. Bradley H. I. Aaronson 《Metallurgical and Materials Transactions A》1978,9(7):999-1008
Three models of the statistical thermodynamics of interstitial solid solutions have been used to reevaluate the thermodynamics
of the proeutectoid ferrite reaction. The models of Kaufman, Radcliffe and Cohen and of Lacher, Fowler and Guggenheim, which
were em-ployed in a previous study of this type, together with the model recently developed by McLellan and Dunn are used
in conjunction with the extensive experimental data of Ban-ya, Elliott and Chipman, of Lobo and Geiger and of Dunn and McLellan
on the activities of carbon in austenite and ferrite. Application of the McLellan and Dunn model and that of Lacher, Fowler
and Guggenheim to carbon in austenite yields activities of carbon which are numerically indistinguishable and activities of
iron which are mathematically identi-cal. However, the new activity data have revealed important differences between the pres-ent
calculations and those of Aaronson, Domian and Pound. An average carbon-carbon repulsion energy in austenite of 1925 cal/mole
(8054 J/mole) was determined from the CO/CO2 data of Ban-yaet al. However, the C-C interaction energy in ferrite was found to be opposite in sign but exhibited erratic variations with temperature
despite the large amount of activity data available. The γ/(α + γ) phase boundary calculated from the new data differs significantly, at lower temperatures, from the best curves reported by
Aaron-sonet al. The calculateda/(α +γ) phase boundary also differs appreciably from the pre-vious results and exhibits only limited agreement with the experimentally
determined phase boundary. Calculation of the free energy change associated with the proeutectoid ferrite reaction andT
0- composition curves differs little from previous results; internal agreement among the new sets of curves, however, is much
improved. 相似文献
4.
C. Capdevila F. G. Caballero C. García de Andrés 《Metallurgical and Materials Transactions A》2001,32(3):661-669
The present article is concerned with the theoretical and experimental study of the growth kinetics of allotriomorphic ferrite
in medium carbon vanadium-titanium microalloyed steel. A theoretical model is presented in this work to calculate the evolution
of austenite-to-allotriomorphic ferrite transformation with time at a very wide temperature range. At temperatures above eutectoid
temperature, where allotriomorphic ferrite is the only austenite transformation product, the soft-impingement effect should be taken into account in the modeling. In that case, the Gilmour et al. analysis reliably predicts the progress of austenite-to-allotriomorphic ferrite transformation in this steel. By contrast,
since pearlite acts as a carbon sink, the carbon enrichment of austenite due to the previous ferrite formation is avoided,
and carbon concentration in austenite far from the α/γ interface remains the same as the overall carbon content of the steel. Hence, the soft-impingement effect should be neglected,
and allotriomorphic ferrite is considered to grow under a parabolic law. Therefore, assumption of a semi-infinite extent austenite
with constant boundary conditions is suitable for the kinetics of the isothermal decomposition of austenite. An excellent
agreement (higher than 93 pct in R
2) has been obtained between the experimental and predicted values of the volume fraction of ferrite in all of the ranges of
temperature studied. 相似文献
5.
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 相似文献
6.
Isothermal transformation from austenite in an Fe-9.14 pct Ni alloy has been studied by optical metallography and examination
by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). In the temperature range 565 °C and 545
°C, massive ferrite (α
q
) forms first at prior austenite grain boundaries, followed by Widmanst?tten ferrite (α
W
) growing from this grain boundary ferrite. Between 495 °C and 535 °C, Widmanst?tten ferrite is thought to grow directly from
the austenite grain boundaries. Both these transformations do not go to completion and reasons for this are discussed. These
composition invariant transformations occur below T
0 in the two-phase field (α+γ). Previous work on the same alloy showed that transformation occurred to α
q
> and α
W
on furnace cooling, while analytical TEM showed an increase of Ni at the massive ferrite grain boundaries, indicating local
partitioning of Ni at the transformation interface. An Fe-3.47 pct Ni alloy transformed to equiaxed ferrite at 707 °C ±5 °C
inside the single-phase field on air cooling. This is in agreement with data from other sources, although equiaxed ferrite
in Fe-C alloys forms in the two-phase region. The application of theories of growth of two types of massive transformation
by Hillert and his colleagues are discussed.
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. 相似文献
7.
G. Spanos 《Metallurgical and Materials Transactions A》1992,23(1):171-181
Carbide precipitation during the eutectoid decomposition of austenite has been studied in an Fe-0.12 pct C-3.28 pct Ni alloy
by transmission electron microscopy (TEM) supplemented by optical microscopy. Nodular bainite which forms during the latter
stages of austenite decomposition at 550 °C exhibits two types of carbide arrangement: (a) banded interphase boundary carbides
with particle diameters of about 20 to 90 nm and mean band spacings between 180 and 390 nm and (b) more randomly distributed
(“nonbanded”) elongated particles exhibiting a wide range of lengths between 33 and 2500 nm, thicknesses of approximately
11 to 50 nm, and mean intercarbide spacings of approximately 140 to 275 nm. Electron diffraction analysis indicated that in
both cases, the carbides are cementite, obeying the Pitsch orientation relationship with respect to the bainitic ferrite.
The intercarbide spacings of both morphologies are significantly larger than those previously reported for similar microstructures
in steels containing alloy carbides other than cementite (e.g., VC, TiC). Both curved and straight cementite bands were observed; in the latter case, the average plane of the interphase
boundary precipitate sheets was near {110}α//{011}c consistent with cementite precipitation on low-energy {110}α//{111}γ ledge terrace planes (where α,β, andc refer to ferrite, austenite, and cementite, respectively). The results also suggest that the first stage in the formation
of the nonbanded form of nodular bainite is often the precipitation of cementite rods, or laths, in austenite at the α:γ interfaces of proeutectoid ferrite secondary sideplates formed earlier. Although these cementite rods frequently resemble
the “fibrous” microstructures observed by previous investigators in carbide-forming alloy steels, they are typically much
shorter than fibrous alloy carbides. The bainitic microstructures observed here are analyzed in terms of a previously developed
model centered about the roles of the relative nucleation and growth rates of the product phases in controlling the evolution
of eutectoid microstructures. 相似文献
8.
M. G. Mecozzi J. Sietsma S. van der Zwaag M. Apel P. Schaffnit I. Steinbach 《Metallurgical and Materials Transactions A》2005,36(9):2327-2340
This article deals with the austenite (γ) decomposition to ferrite (α) during cooling of a 0.10 wt pct C-0.49 wt pct Mn steel. A phase-field model is used to simulate this transformation. The
model provides qualitative information on the microstructure that develops on cooling and quantitative data on both the ferrite
fraction formed and the carbon concentration profile in the remaining austenite. The initial austenitic microstructure and
the ferrite nucleation data, derived by metallographic examination and dilatometry, are set as input data of the model. The
interface mobility is used as a fitting parameter to optimize the agreement between the simulated and experimental ferrite-fraction
curve derived by dilatometry. A good agreement between the simulated α-γ microstructure and the actual α-pearlite microstructure observed after cooling is obtained. The derived carbon distribution in austenite during transformation
provides comprehension of the nature of the transformation with respect to the interface-controlled or diffusion-controlled
mode. It is found that, at the initial stage, the transformation is predominantly interface-controlled, but, gradually, a
shift toward diffusion control takes place to a degree that depends on cooling rate. 相似文献
9.
The formation of austenite from different microstructural conditions has been studied in a series of 1.5 pct Mn steels that
had been heated in and above the intercritical (α+ γ) region of the phase diagram. The influence of variables such as cementite morphology, initial structural state of the ferrite
and the carbon content has been assessed in terms of their respective effects on the kinetics of austenite formation and final
microstructure. Austenite was found to form preferentially on ferrite-ferrite grain boundaries for all initial structures.
The results of this study have shown that the 1.5 pct Mn has lowered both the AC3 and AC1, lines causing large amounts of austenite to form in low carbon steel. The kinetics of austenite formation at 725 °C were
not only very slow but also were approximately independent of the amount formed. Austenite appeared to form slightly more
rapidly from cold rolled ferrite than from recrystallized ferrite or ferrite-pearlite structures. 相似文献
10.
H. I. Aaronson W. T. Reynolds Jr. G. R. Purdy 《Metallurgical and Materials Transactions A》2006,37(6):1731-1745
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. 相似文献
11.
Phase transformations in an Fe-7.8Al-29.5Mn-l.5Si-1.05C alloy have been investigated by means of optical microscopy and transmission
electron microscopy. In the as-quenched condition, a high density of fine (Fe,Mn)3AlC carbides could be observed within the austenite matrix. When the as-quenched alloy was aged at temperatures ranging from
550 °C to 825 °C, aγ → coarse (Fe,Mn)3AlC carbide + DO3 reaction occurred by a cellular precipitation on theγ/γ grain boundaries and twin boundaries. Both of the observations are quite different from those observed by other workers in
Fe-Al-Mn-C alloys. In their studies, it was found that the as-quenched microstructure was austenite phase(γ), and (Fe,Mn)3AlC carbides could only be observed within the austenite matrix in the aged alloys. In addition, aγ →α (ferrite) + coarse (Fe,Mn)3AlC carbide reaction or aγ →α + coarse (Fe,Mn)3AlC carbide +β-Mn reaction was found to occur on theγ/γ grain boundary in the aged Fe-Al-Mn-C alloys. 相似文献
12.
Spatially resolved X-ray diffraction (SRXRD) experiments have been performed during gas tungstenarc (GTA) welding of AISI
1045 C-Mn steel at input powers ranging from 1000 to 3750 W. In-situ diffraction patterns taken at discreet locations across the width of the heat-affected zone (HAZ) near the peak of the heating
cycle in each weld show regions containing austenite (γ), ferrite and austenite (α+γ), and ferrite (α). Changes in input power have a demonstrated effect on the resulting sizes of these regions. The largest effect is on the
γ phase region, which nearly triples in width with increasing input power, while the width of the surrounding two-phase α+γ region remains relatively constant. An analysis of the diffraction patterns obtained across this range of locations allows
the formation of austenite from the base-metal microstructure to be monitored. After the completion of the α → γ transformation, a splitting of the austenite peaks is observed at temperatures between approximately 860 °C and 1290 °C.
This splitting in the austenite peaks results from the dissolution of cementite laths originally present in the base-metal
pearlite, which remain after the completion of the α → γ transformation, and represents the formation of a second more highly alloyed austenite constituent. With increasing temperatures,
carbon, originally present in the cementite laths, diffuses from the second newly formed austenite constituent to the original
austenite constituent. Eventually, a homogeneous austenitic microstructure is produced at temperatures of approximately 1300
°C and above, depending on the weld input power. 相似文献
13.
E. S. Humphreys H. A. Fletcher J. D. Hutchins A. J. Garratt-Reed W. T. ReynoldsJr. H. I. Aaronson G. R. Purdy G. D. W. Smith 《Metallurgical and Materials Transactions A》2004,35(4):1223-1235
A scanning transmission electron microscope (STEM) technique was used to measure Mo concentrations at ferrite:austenite (α:γ) interfaces in an Fe-0.24 pct C-0.93 pct Mo alloy partially transformed at 650°C, 630°C, and 610°C. These concentrations
were quite small at 650°C, which is just below the bay temperature of the time-temperature-transformation (TTT) curve for
the initiation of ferrite formation. There were larger concentrations at 630°C, a temperature at which transformation stasis
(incomplete transformation) occurred. Concentrations at 610°C were intermediate between the values observed at 650°C and 630°C.
The average accumulation at the latter temperatures increased appreciably as a function of transformation time. After each
heat treatment, there was considerable variation in Mo accumulation from one α:γ interface to another and, to a lesser extent, from one region to another along the same interface. These higher Mo concentrations
were deduced to have developed largely through volume diffusion of Mo, mainly through ferrite, to interfaces whose ledgewise
growth had been interrupted by growth stasis. (Mo2C precipitation at α:γ boundaries occurred only at the end of growth stasis.) It appears that only a very small amount of Mo segregation is needed,
probably at specific interfacial sites, in order to produce growth cessation. Growth kinetics anomalies of this kind continue
to provide the best evidence available for the operation of a coupled-solute drag effect.
This article is based on a presentation given in the symposium “The Effect of Alloying Elements on the Gamma to Alpha Transformation
in Steels,” October 6, 2002, at the TMS Fall Meeting in Columbus, Ohio, under the auspices of the McMaster Centre for Steel
Research and the ASM-TMS Phase Transformations Committee. 相似文献
14.
15.
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. 相似文献
16.
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. 相似文献
17.
In this work, a hot compression test was carried out at 1173 K to 1473 K (900°C to 1200 °C), with a strain rate of 0.01 to
1 s−1 up to ~50 pct height reduction on functionally graded steel (FGS) specimens comprised of ferritic, bainitic, austenitic,
and martensitic layers (αβγMγ). The stress-strain curves are strongly dependent on temperature and strain rate. Compressive flow stress varied from 40
to 105 MPa depending on the applied temperature and strain rates. Variation in steady-state flow stress with temperature and
strain rates was studied. The strain-rate-sensitivity exponent (m) and deformation activation energy (Q) for the αβγMγ composite under studied condition were 0.106 and 354.8 KJ mol−1, respectively, which are within the values of boundary layers of ferrite (304.9 KJ mol−1) and austenite (454.8 KJ mol−1) layers. Given the alternative microstructure of the αβγMγ FGS, a range of deformation mechanisms from dynamic recovery to dynamic recrystallization maybe prevails, where the intensity
of each mechanism depends on temperature and strain rates. In accordance with the experimental results, an empirical power-law
equation was developed over the range of temperatures and strain rates investigated. The equation accurately describes temperature
and strain-rate dependence of the flow stress. 相似文献
18.
Zhang G. H. Takeuchi T. Enomoto M. Adachi Y. 《Metallurgical and Materials Transactions A》2011,42(6):1597-1608
The nucleation of ferrite precipitates at austenite grain faces, edges (triple lines), and corners (quadruple points) was
studied in a Co-15Fe alloy in which the matrix phase was retained upon cooling to room temperature by serial sectioning coupled
with electron backscatter diffraction analysis. Nearly half of the edges and corners were vacant at an undercooling of 60 K
from the γ/(α + γ) boundary where the precipitation occurred significantly at grain faces. A significant proportion of precipitates had Kurdjumov–Sachs
(K–S) and to a lesser extent Nishiyama–Wassermann (N–W) orientation relationships with more than one grain at all boundary
sites. Vacant edges and corners were readily observed, of which the misorientations of matrix grain boundaries would permit
a precipitate to have a specific orientation relationship with multiple grains. Small differences in the nucleation activation
energy among the grain faces, edges, and corners may lend support to a view proposed from experiments of nucleation in Fe-C
base alloys that ferrite nuclei are more or less surrounded by low-energy facets of α/γ phase boundary. 相似文献
19.
Development of high toughness in austempered type ductile cast iron and evaluation of its properties
In order to increase the toughness of austempered ductile cast irons, we attempted to strengthen the fracture initiation sites
such as graphite-matrix interfaces and eutectic cell boundaries in a way of the microsegregation of alloying elements. For
instance, the retained austenite which is stable under external stresses may be introduced preferentially into these sites
by the addition of Ni, which segregates to a graphite periphery and of Mn, which partitions mainly to eutectic cell boundaries.
Following this concept, the effects of various austempering processes on toughness are also in-vestigated. The cast iron alloying
with Ni and Mn shows the best fracture toughness when it is heat-treated by either QB' or B' process; here, the QB' means
the oil-quenching from an austenite γ phase range followed by austempering from a ferrite α plus γ range and the B' means
austempering from a (α + γ) range. In the newly developed iron, there is a mixed microstructure composed of the ferrite, bainitic
ferrite, and austenite. Abnormal elongation due to the TRIP effect in the austenite phase is found to have occurred at about
198 K. Moreover, it is shown that this TRIP effect may be caused by the formation of deformation twins. 相似文献
20.
On the growth kinetics of grain boundary ferrite allotriomorphs 总被引:1,自引:0,他引:1
C. Atkinson H. B. Aaron K. R. Kinsman H. I. Aaronson 《Metallurgical and Materials Transactions B》1973,4(3):783-792
Previous work has shown that the thickening kinetics of proeutectoid ferrite allotriomorphs in an Fe-0.11 pct C alloy are
often more rapid than the kinetics calculated for volume diffusion-control from the Dube-Zener equation for the migration
of a planar boundary of infinite extent, assuming the diffusivity of carbon in austenite,D, to be constant at that of the carbon content of the Ae3. Recalculating the thickening kinetics, using a numerical analysis
of the infinite planar boundary problem previously developed by Atkinson in which the variation ofD with composition is taken fully into account, was found to increase this discrepancy. Measurements were then made of the
lengthening as well as the thickening kinetics of grain boundary allotriomorphs in the same alloy. Application to these data
of Atkinson’s numerical analysis of the growth kinetics of an oblate ellipsoid, in which the composition-dependence ofD is similarly considered, produced an acceptable accounting for nearly all of the data. It was concluded that the growth of
ferrite allotriomorphs is primarily controlled by the volume diffusion of carbon in austenite; the presence of a small proportion
of dislocation facets along one of the broad faces of the allotriomorphs, however, usually results in growth kinetics which
are somewhat slower. An alternate treatment of the lengthening and thickening data upon the basis of the theory of interfacial
diffusion-aided growth of allotriomorphs indicated that, in the temperature range investigated (735° to 810°C),the diffusivities
of carbon along γ:γ and γ:α boundaries required for this mechanism to make a significant contribution to growth are too high
to be physically plausible.
Formerly with Scientific Research Staff
Formerly with Scientific Research Staff, Ford Motor Company 相似文献