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1.
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
2.
A. Phillion H. W. Zurob C. R. Hutchinson H. Guo D. V. Malakhov J. Nakano G. R. Purdy 《Metallurgical and Materials Transactions A》2004,35(4):1237-1242
We have evaluated controlled decarburization as a method for probing the effect of alloying elements on ferrite growth from
austenite. The technique permits the exploration of longer-time ferrite layer growth; it minimizes the effects of interface
structure on ferrite growth; and it permits the isolation of the effects of temperature and alloying element concentration
on ferrite/austenite interface motion. The study of the decarburization of initially homogeneous Fe-C-Ni alloys was complemented
by experiments using specimens with a controlled nickel concentration gradient. Although the decarburization method yields
consistent results at longer times, it is found to be less appropriate for the study of initial ferrite growth. Nucleation
in the gas/solid interface region, coupled with uncertainties about the precise time of decarburization, leads to large relative
errors at the earliest times. For these reasons, the method is considered a valuable complement to studies based on precipitation
boundary conditions.
This article is based on a presentation given in the symposium “The Effects 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 TMS-ASM Phase Transformations Committee. 相似文献
3.
The formation mechanism(s), morphology, and crystallography of secondary ferrite sideplates were investigated with transmission
electron microscopy (TEM), scanning electron microscopy (SEM), electron backscatter pattern (EBSP) analysis, and optical microscopy
in a high-purity Fe—0.12 wt pct C—3.3 wt pct Ni alloy isothermally transformed at temperatures of 550 °C, 600 °C, 650 °C,
and 675 °C. The results indicate that two different mechanisms contribute to the formation of these sideplates at austenite
grain boundaries. On the first mechanism, primary sideplates form initially, followed by rapid lateral impingement along their
bases, resulting in a region along the grain boundary which very early in the growth process resembles an allotriomorphic
film. On the second mechanism, sympathetic nucleation of ferrite sideplates occurs atop pre-existing ferrite allotriomorphs,
resulting in ferrite:ferrite grain boundaries and significant crystallographic misorientations between the sideplates and
the allotriomorphs with which they are associated. These results indicate that “secondary sideplates” and the allotriomorphs
from which they evolve arenot composed of monolithic single crystals formed by a morphological instability mechanism but are instead composed of multiple
crystals formed by individual nucleation events. Previous investigations in Ti-Cr alloys and a high chromium stainless steel
suggest that the findings presented here may be applicable to a number of other alloy systems as well.
This article is based on a presentation made during TMS/ASM Materials Week in the symposium entitled “Atomistic Mechanisms
of Nucleation and Growth in Solids,” organized in honor of H.I. Aaronson’s 70th Anniversary and given October 3–5, 1994, in
Rosemont, Illinois. 相似文献
4.
D. L. Davidson K. S. Chan R. C. McClung 《Metallurgical and Materials Transactions A》1996,27(9):2540-2556
The fatigue characteristics of a Cu-bearing high-strength low-alloy (HSLA) steel were investigated in air, relative humidity
≈50 pct, as a function of microstructure, which was altered by heat treatments and welding. Small fatigue cracks (≈30-Μm long)
were naturally initiated from smooth specimens and grown past the transition length (≈200 Μm), where they exhibited the characteristics
of large fatigue cracks. The number of cycles to crack initiation depended on stress magnitude but not on microstructure,
although the site of initiation was microstructurally dependent. Small cracks in all microstructures grew at δK values below the large crack threshold. The as-received (polygonal ferrite) microstructure and one of the lath microstructures
that resulted from heat treatment exhibited the same growth rate correlation as large cracks in the linear (Paris) region,
and could be considered as an extension of the large crack growth region down to the point of initiation. Small cracks grew
at rates faster than expected through one of the heat-treated and the weld microstructures; therefore, the number of cycles
required for growth from initiation to the transition to large crack growth decreased about threefold, which is a potentially
important factor in predicting lifetimes of structures made from this steel. 相似文献
5.
A. O. Kluken Ph.D. Ø. Grong J. Hjelen 《Metallurgical and Materials Transactions A》1991,22(3):657-663
The present investigation is concerned with basic studies of the development of transformation textures in steel weld metals,
using the electron backscattering pattern (EBSP) technique. It is shown that the acicular ferrite (AF) plates exhibit an orientation
relationship with both the austenite and the prior delta ferrite columnar grains in which they grow. The observed orientation
relationship lies within the Bain orientation region and can be described by three texture components,i.e., a 〈100〉 component and two complementary 〈111〉 components. Each of these texture components is orientated approximately parallel
with the original cell/dendrite growth direction. Measurements of the spatial misorientation between neighboring plates confirm
that the morphology of AF in low-alloy steel weld metals bears a close resemblance to upper bainite. 相似文献
7.
M. Nemoto 《Metallurgical and Materials Transactions A》1977,8(3):431-437
The detailed mechanism of the transformation of various mixtures of ferrite and austenite was studied byin situ experiments in a high voltage electron microscope. Various phenomena were observed which seemed to be controlled by the rate
of carbon diffusion when the γ/α interface was curved. The effect of manganese was studied by using commercial steels. In
agreement with theoretical predictions based upon the local equilibrium model, the dissolution of cementite in ferrite can
be more or less prevented by manganese. The effect on the dissolution in austenite is smaller. The reaction from lamellar
pearlite to austenite is more complex, the cementite lamellae providing carbon to the austenite but also acting as barriers.
M. NEMOTO, formerly with Division of Physical Metallurgy, The Royal Institute of Technology, S-100 44 Stockholm 70, Sweden 相似文献
8.
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
9.
Electron channeling contrast and electron backscattering patterns (EBSPs), generated in scanning electron microscopes (SEMs),
were used to study the question of whether tent-shaped surface reliefs associated with ferrite plates in Fe-0.07 pct and Fe-0.50
pct C low-alloy steels are single crystals or back-to-back pairs of plates. Although more than 50 ferrite plates were examined
in detail, any misorientation contained within them must have been <1 deg. Additionally, no sightings of the low-angle grain
boundaries approximately bisecting ferrite plates expected on the back-to-back plates mechanism were ever made. The efforts
of previous investigators to explain tent-shaped (and invariant plane-strain) reliefs on the basis of diffusional ledgewise
growth were further extended.
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. 相似文献
10.
C. Qiu H. S. Zurob D. Panahi Y. J. M. Brechet G. R. Purdy C. R. Hutchinson 《Metallurgical and Materials Transactions A》2013,44(8):3472-3483
The kinetics of ferrite growth in the Fe-C-Co and Fe-C-Si systems has been quantified using controlled decarburization experiments. The Fe-C-Co system is a particularly interesting system since a large range of Co contents can be considered providing a suitable data set for examination of the composition dependence of the solute drag effect. Six Fe-C-Co alloys containing Co from 0.5 to 20 pct have been considered. Three Fe-C-Si alloys have also been considered and each has been transformed at three temperatures proving a suitable data set for examining the temperature dependence of the solute drag effect. This data set, along with ferrite growth data from decarburization experiments on an Fe-C-2Cr alloy has been used to test the ferrite growth model proposed in the companion article by Zurob et al. It is shown that this model for ferrite growth, that includes diffusional dissipation due to interaction between the solute and the migrating boundary, quantitatively captures both the temperature and composition dependence of the deviation of experimental ferrite growth kinetics from the PE and/or LENP models. 相似文献
11.
Melt/substrate contacting experiments, designed to approximate conditions encountered during strip casting, were carried out
to produce as-cast ferritic stainless steel strip. The results show that inoculation of the melt to produce TiN particles,
together with casting onto a smooth substrate, results in the optimum conditions for nucleation and subsequent growth of an
exceedingly high volume fraction of ferrite grains with 〈001〉 oriented within a few degrees of the normal direction (ND) of
the strip surface. It is argued that, during casting, TiN particles either nucleate or deposit onto the substrate with 〈001〉
parallel to the ND, and since these particles exhibit crystallographic features similar to δ-ferrite, subsequent epitaxial growth inherits the initial particle orientation. Such oriented nucleation of ferrite from a smooth substrate results in the optimum heat-transfer conditions for further growth of dendrites with 〈001〉
perpendicular to the substrate, thus producing the intense through-thickness 〈001〉//ND fiber texture in the as-cast strip.
The potential for producing grain-oriented silicon iron by direct strip casting is outlined. 相似文献
12.
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
13.
E. Cotrina A. Iza-Mendia B. López I. Gutiérrez 《Metallurgical and Materials Transactions A》2004,35(1):93-102
The degree of ferrite grain refinement that can be reached in low-carbon microalloyed steels by thermomechanical processing
is limited, to a certain extent, by the grain coarsening which can take place behind the transformation front. The coarsening
of ferrite grains is the result of two different mechanisms: elimination of ferrite grains produced by normal grain growth
after full impingement on the austenite grain boundary plane and/or coalescence between different ferrite grains with close
orientation formed from the same crystallographic variant. The lack of experimental data to support either process is due
to the experimental difficulties encountered when analyzing the phenomenon. Some transmission electron microscope (TEM) observations
reveal that the ferrite grains formed along a prior grain boundary in deformed austenite are separated by a mixture of low
and high angle grain boundaries upon impingement. In the present work, the electron backscattered diffraction (EBSD) technique
has been applied to investigate the microstructural evolution during transformation, with special emphasis placed on the α-α grain boundary character as a means of investigating the contribution of coalescence/grain growth to coarsening. 相似文献
14.
John W. Elmer Joe Wong Thorsten Ressler 《Metallurgical and Materials Transactions A》2001,32(5):1175-1187
Phase transformations that occur in the heat-affected zone (HAZ) of gas tungsten arc welds in AISI 1005 carbon-manganese steel
were investigated using spatially resolved X-ray diffraction (SRXRD) at the Stanford Synchrotron Radiation Laboratory. In situ SRXRD experiments were performed to probe the phases present in the HAZ during welding of cylindrical steel bars. These real-time
observations of the phases present in the HAZ were used to construct a phase transformation map that identifies five principal
phase regions between the liquid weld pool and the unaffected base metal: (1) α-ferrite that is undergoing annealing, recrystallization, and/or grain growth at subcritical temperatures, (2) partially transformed
α-ferrite co-existing with γ-austenite at intercritical temperatures, (3) single-phase γ-austenite at austenitizing temperatures, (4) δ-ferrite at temperatures near the liquidus temperature, and (5) back transformed α-ferrite co-existing with residual austenite at subcritical temperatures behind the weld. The SRXRD experimental results were
combined with a heat flow model of the weld to investigate transformation kinetics under both positive and negative temperature
gradients in the HAZ. Results show that the transformation from ferrite to austenite on heating requires 3 seconds and 158°C
of superheat to attain completion under a heating rate of 102°C/s. The reverse transformation from austenite to ferrite on
cooling was shown to require 3.3 seconds at a cooling rate of 45 °C/s to transform the majority of the austenite back to ferrite;
however, some residual austenite was observed in the microstructure as far as 17 mm behind the weld. 相似文献
15.
H. Guo G. R. Purdy M. Enomoto H. I. Aaronson 《Metallurgical and Materials Transactions A》2006,37(6):1721-1729
A low-carbon balloy steel with relatively high Mn and Si concentrations (0.04 wt pct C-3 wt pct Mn-1.9 wt pct Si) has been
used to explore the effects of alloy chemistry and austenite grain size on ferrite growth. Even at high levels of supersaturation,
the volume fraction of ferrite is found to increase slowly relative to the relaxation time for carbon diffusion. A series
of scanning transmission electron microscopy (STEM) analyses for Mn indicates that initial unpartitioned ferrite growth is
replaced by partitioned growth, accompanied by a dramatic drop in growth rate, and a persistent level of residual supersaturation
in the remaining austenite. The results are interpreted in terms of a transition from an initial paraequilibrium interfacial
condition to partitioned ferrite 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 Royal Institute of Technology in Stockholm, Sweden. 相似文献
16.
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
17.
M. Enomoto 《Metallurgical and Materials Transactions A》2002,33(8):2309-2316
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. 相似文献
18.
A mechanism for the formation of lower bainite 总被引:2,自引:0,他引:2
G. Spanos H. S. Fang H. I. Aaronson 《Metallurgical and Materials Transactions A》1990,21(6):1381-1390
A diffusional mechanism for the formation of lower bainite is proposed based primarily on transmission electron microscopy
(TEM) observations of isothermally reacted specimens of Fe-C-2 pct Mn alloys. The mechanism involves the initial precipitation
of a nearly carbide-free ferrite“spine,” followed by sympathetic nucleation of“secondary (ferrite) plates” which lie at an
angle to the initial“spine.” Carbide precipitation subsequently occurs in austenite at ferrite: austenite boundaries located
in small gaps between the“secondary plates.” An“annealing” process then occurs in which the gaps are filled in by further
growth of ferrite and additional carbide precipitation; the annealing out of ferrite: ferrite boundaries between impinged“secondary
plates” completes this process. This annealing stage contributes to the final appearance of lower bainite sheaves as monolithic
plates containing embedded carbides. The present mechanism accounts for the single variant of carbides oriented at an angle
to the sheaf axis repeatedly reported in lower bainite; it is also consistent with the previous observation of one“rough”
side and one“smooth” side of lower bainite“plates.”
Formerly Graduate Student, Carnegie Mellon University.
Formerly Visiting Professor, Carnegie Mellon 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. 相似文献
19.
Phase-field model prediction of nucleation and coarsening during austenite/ferrite transformation in steels 总被引:1,自引:0,他引:1
A phase-field simulation is performed to study the kinetics of austenite to ferrite (γ → α) transformation in a low-carbon
steel during continuous cooling. Emphasis is placed on the influence of nucleation, along with ferrite grain coarsening behind
the transformation front, on microstructural evolution. Results show that grain coarsening is significant even before all
nucleation has been completed and occurs via two different coarsening mechanisms, grain boundary migration and ferrite grain crystallographic rotation, both of which
can be clearly observed occurring as the simulated microstructure evolves. For some grains, sudden growth jumps are predicted
by the model—a phenomenon that has been observed before by synchrotron X-ray diffraction. This study quantitatively demonstrates
the phenomenon that increasing cooling rate leads to nucleation off initial austenite grain boundaries, which is also verified
by studying the morphology of ferrite grains as predicted using different nucleation mode assumptions. A relationship between
nucleation site distribution and the nucleation rate is demonstrated by computer simulation. 相似文献
20.
Rosalia Rementeria Carlos Capdevila Ricardo Domínguez-Reyes Jonathan D. Poplawsky Wei Guo Esteban Urones-Garrote Carlos Garcia-Mateo Francisca G. Caballero 《Metallurgical and Materials Transactions A》2018,49(11):5277-5287
The bainitic ferrite phase formed at temperatures below 573 K (300 °C) in high-carbon high-silicon steels holds an amount of carbon well above that expected from the thermodynamic paraequilibrium with austenite. Diffraction experiments have shown that the ferrite lattice is sufficiently Zener-ordered to possess a tetragonal symmetry, which allows the structures to be supersaturated in carbon. It could be expected that carbon undergoes ordering beyond that indicated by the Zener-ordering temperature as in the early stages of tempering of Fe-based martensites. This study examines the formation of cluster arrangements of carbon within bainitic ferrite and their relationship to the tetragonal distortion. 相似文献