共查询到20条相似文献,搜索用时 15 毫秒
1.
J. R. Bradley H. I. Aaronson K. C. Russell W. C. Johnson 《Metallurgical and Materials Transactions A》1977,8(12):1955-1961
Austenitizing an Fe-0.23 pct C alloy at 1300°C and further at 900°C prior to isothermal transformation was found to increase
the growth kinetics of grain boundary ferrite allotriomorphs while decreasing their rate of nucleation. A scanning Auger microprobe
was used to establish that sulfur segregates to the austenite grain boundaries and does so increasingly with decreasing austenitizing
temperature. A binding free energy of sulfur to these boundaries of approximately 13 kcal/mole (54.4 kj/mole) was calculated
from theMcLean adsorption isotherm. The kinetic results were explained in terms of preferential reduction of the austenite
grain boundary energy decreasig nucleation kinetics, and adsorption of sulfur at α:γ boundaries increasing the carbon concentration
gradient in austenite driving growth. 相似文献
2.
The crystallography and three-dimensional (3-D) morphology of Widmanstätten proeutectoid ferrite precipitates are examined in an Fe-0.12 wt pct C-3.28 wt pct Ni steel isothermally reacted at 650 °C, 600 °C, and 550 °C. This article integrates new orientation mapping (OM) results with the findings of a companion article to this one on the 3-D morphology of proeutectoid ferrit[1] and an earlier transmission electron microscopy (TEM) study which is reanalyzed here in light of the new OM and 3-D results. All of these studies were performed for the same alloy and heat treatments. The 3-D morphologies and distributions of proeutectoid ferrite precipitates are now known to often be quite different from those deduced by conventional two-dimensional (2-D) microscopy techniques. The present crystallographic studies indicate that “primary” ferrite (nucleated directly on prior austenite grain boundaries) forms monolithic single crystals and can be approximated as elongated triangular pyramids. “Secondary” ferrite morphologies can be described as laths and plates branching into the austenite from a thick and/or broad allotriomorphic ferrite base. These secondary Widmanstätten branches are composed of many misoriented crystals with ferrite: ferrite boundaries between them and appear to approach a common orientation as they extend into the austenite grain. Implications of the current findings on existing growth and crystallography models are discussed, and a preliminary hypothesis or mechanism of ferrite formation has been proposed to account for the present observations. 相似文献
3.
J. R. Bradley J. M. Rigsbee H. I. Aaronson 《Metallurgical and Materials Transactions A》1977,8(2):323-333
The kinetics of lengthening and thickening of grain boundary allotriomorphs of proeutectoid ferrite were measured at several
temperatures in high purity Fe−C alloys containing 0.11 pct, 0.23 pct and 0.42 pct C. These measurements were conducted by
measuring the length of the longest and the width of the widest allotriomorph in each specimen. All specimens were austenitized
so as to make the grain boundaries perpendicular to the plane of polish. This measurement technique appreciably reduced the
scatter in the parabolic rate constant data previously encountered in thermionic emission microscopy measurements. Parabolic
rate constants for lengthening and thickening were calculated, using the experimental aspect ratio, by means of the Atkinson
analysis for oblate ellipsoids. The ratio of the measured to the calculated constants was in all cases less than unity. The
previously made suggestion that these slow growth kinetics are due to faceting was supported through the observation of facets
on allotriomorphs by means of scanning and transmission electron microscopy. The aspect ratio of ferrite allotriomorphs was
shown to beca 1/3, independent of reaction time temperature and carbon content. The dihedral angle of the allotriomorphs was found to be
100±5 deg, as compared with a published angle for recrystallized and equilibrated specimens ofca 115 deg. Several possible explanations for the aspect ratio and dihedral angle findings are considered.
M. RIGSBEE, formerly Postdoctoral Research Associate with Michigan Technological University 相似文献
4.
Grain boundary ferrite allotriomorphs in an Fe-0.12 pct C-0.11 pct V alloy reacted in the temperature range 800‡ to 870 ‡C
develop in two morphologies: conventional allotriomorphs, with an aspect ratio of about one-half, and “snakes≓, whose aspect
ratio approaches zero. Interphase boundary vanadium carbides (VC) precipitate densely in association with conventional allotriomorphs
but not with “snakes≓. Thickening kinetics of “snakes≓ are about an order of magnitude slower than predicted by the paraequilibrium
model. Conventional allotriomorphs, on the other hand, thicken with the kinetics predicted by paraequilibrium. These observations
are considered in terms of the solute drag-like effect and of “wetting≓ of austenite grain boundaries by ferrite in the presence
of V. Absence of a distinct change in the thickening kinetics of conventional allotriomorphs in the vicinity of 845 ‡C, the
approximate upper temperature limit of interphase boundary carbide precipitation, strongly supports the view that such carbides
can precipitate only on immobile,i.e., partially coherent areas of austenite : ferrite boundaries.
Formerly Visiting Research Associate, Department of Metallurgical Engineering, Michigan Technological University, Houghton,
MI 49931
Formerly Republic Steel Corporation Fellow, Michigan Technological University, Houghton, MI 49931
Formerly Professor, Department of Metallurgical Engineering, Michigan Technological University, Houghton, MI 49931 相似文献
5.
E. Sarath Kumar Menon H. I. Aaronson 《Metallurgical and Materials Transactions A》1986,17(10):1703-1715
The nucleation, growth, and overall transformation kinetics of grain boundary α allotriomorphs were measured in Ti-3.2 at.
pct Co and Ti-6.6 at. pct Cr alloys with optical microscopy. Nucleation kinetics were interpreted with classical heterogeneous
nucleation theory, using the pillbox model of the critical nucleus. Growth kinetics of allotriomorphs were significantly accelerated
by the rejector plate mechanism, but much less so than allotriomorphs formed in fcc substitutional matrices at comparable
homologous temperatures. Overall transformation kinetics were accounted for with a modified version of Cahn’s theory of grain
boundary nucleated reactions.
Formerly Graduate Student, Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University, Pittsburgh,
PA 15213 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.
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
8.
Ferrite recrystallization and austenite formation in cold-rolled intercritically annealed steel 总被引:1,自引:0,他引:1
D. Z. Yang E. L. Brown D. K. Matlock G. Krauss 《Metallurgical and Materials Transactions A》1985,16(8):1385-1392
The recrystallization of ferrite and austenite formation during intercritical annealing were studied in a 0.08C-1.45Mn-0.21Si
steel by light and transmission electron microscopy. Normalized specimens were cold rolled 25 and 50 pct and annealed between
650 °C and 760 °C. Recrystallization of the 50 pct deformed ferrite was complete within 30 seconds at 760 °C. Austenite formation
initiated concurrently with the ferrite recrystallization and continued beyond complete recrystallization of the ferrite matrix.
The recrystallization of the deformed ferrite and the spheroidization of the cementite in the deformed pearlite strongly influence
the formation and distribution of austenite produced by intercritical annealing. Austenite forms first at the grain boundaries
of unrecrystallized and elongated ferrite grains and the spheroidized cementite colonies associated with ferrite grain boundaries.
Spheroidized cementite particles dispersed within recrystallized ferrite grains by deformation and annealing phenomena were
the sites for later austenite formation. 相似文献
9.
Structure and properties of a β solution treated,quenched, and aged si-bearing near-α titanium alloy
The microstructure, tensile properties, and fractographic features of a near-α titanium alloy, IMI 829(Ti-6.1 wt pct Al-3.2
wt pct Zr-3.3 wt pct Sn-1 wt pct Nb-05 wt pct Mo-0.32 wt pct Si) have been studied after aging over a temperature range of
550°C to 950°C for 24 hours following solution treatment in the β phase field at 1050°C and water quenching. Transmission
electron microscopy studies revealed that aging at 625°C and above produced discrete silicides at α′ interplatelet boundaries.
However, aging at 900°C and above has also resulted in the precipitation of β phase along the lath boundaries of martensite.
The silicides have been found to have a hexagonal structure withc=0.36 nm anda=0.70 nm (designated as S2 by earlier workers). There is a significant improvement in yield and ultimate tensile strength after aging at 625°C, but
there is less improvement at higher aging temperatures. The tensile ductility is found to be drastically reduced. While the
fracture surface of the unaged specimen shows elongated dimples, the aged samples show a mixed mode of fracture, consisting
of facets, featureless parallel bands, and extremely fine dimples. 相似文献
10.
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. 相似文献
11.
Sung Bo Lee Duk Yong Yoon Michael F. Henry 《Metallurgical and Materials Transactions A》2003,34(7):1433-1437
In a model Ni-base superalloy of Ni-24Co-4Al-4Ti-5Cr-5Mo (by wt pct), the singular and rough grain boundaries that exist at
a temperature range between 1050 °C and 1200 °C can be identified by observing the shapes of coherent γ′-precipitates by transmission electron microscopy (TEM). Some grain boundaries become locally curved with the impinging spherical
γ′-precipitates. These grain boundaries must be rough. Some grain boundaries maintain their flat shapes even at the contact
areas with the γ′-precipitates and triple junctions. Such flat grain boundary shapes indicate that these grain boundaries are singular. Some
grain boundaries have hill-and-valley shapes and some of their segments also show flat shapes with impinging precipitates.
These boundary segments must also be singular. The results show that the singular and rough grain boundaries in this alloy
can be clearly identified by the shape distortions produced by the impinging γ′-precipitates. 相似文献
12.
J. Zhang D. B. Williams J. I. Goldstein 《Metallurgical and Materials Transactions A》1994,25(8):1627-1637
The low-temperature (<500 °C) decomposition of Fe-Ni martensite was studied by aging martensitic Fe-Ni alloys at temperatures
between 300 °C and 450 °C and by measuring the composition of the matrix and precipitate phases using the analytical electron
microscope (AEM). For aging treatments between 300 °C and 450 °C, lath martensite in 15 and 25 wt pct Ni alloys decomposed
with γ [face-centered cubic (fcc)] precipitates forming intergranularly, and plate martensite in 30 wt pct Ni alloys decomposed
with γ (fcc) precipitates forming intragranularly. The habit plane for the intragranular precipitates is {111}fcc parallel to one of the {110}bcc planes in the martensite. The compositions of the γ intergranular and intragranular precipitates lie between 48 and 58 wt
pct Ni and generally increase in Ni content with decreasing aging temperature. Diffusion gradients are observed in the matrix
α [body-centered cubic (bcc)] with decreasing Ni contents close to the martensite grain boundaries and matrix/precipitate
boundaries. The Ni composition of the matrix α phase in decomposed martensite is significantly higher than the equilibrium
value of 4 to 5 wt pct Ni, suggesting that precipitate growth in Fe-Ni martensite is partially interface reaction controlled
at low temperatures (<500 °C). The results of the experimental studies modify the γ/α + γ phase boundary in the present low-temperature
Fe-Ni phase diagram and establish the eutectoid reaction in the temperature range between 400 °C and 450 °C.
Formerly Research Assistant, Department of Materials Science and Engineering, Lehigh University 相似文献
13.
G. Spanos 《Metallurgical and Materials Transactions A》1994,25(9):1967-1980
Isothermal transformation of austenite to lower bainite was studied by optical microscopy and transmission electron microscopy
(TEM), in two high-purity Fe-C-4 wt pct Mn-2 wt pct Si alloys containing 0.4 and 0.6 wt pct carbon, in order to elucidate
the fine structure and formation mechanism of lower bainite. The present results support a mechanism for lower bainite formation
presented previously in which lower bainite sheaves result from the formation of an aggregate of fine ferrite crystals with
thin austenite “gaps” between them; carbide precipitation occurs within these austenite gaps. This mechanism accounts for
the carbides oriented at an angle to the sheaf axis repeatedly observed in lower bainite but is inconsistent with models based
on the precipitation of a high-volume fraction of carbides within highly supersaturated ferrite formed by high-velocity shear.
Observations by a number of other researchers are reviewed and shown to include morphological features consistent with the
present mechanism. Finally, the orientation relationships typically observed among ferrite, austenite, and carbides in lower
bainite are reviewed and shown in most (but not all) cases to be consistent with the view that carbides precipitate in austenite
at ferrite-austenite boundaries, also in agreement with the present model.
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
14.
Widmanstätten ferrite plate formation in low-carbon steels 总被引:1,自引:0,他引:1
The mechanism by which Widmanstätten ferrite plates nucleate and grow in low-carbon steels has been studied. In-situ laser scanning confocal microscopy (LSCM) observations, optical microscopy, and electron backscattered diffraction (EBSD) techniques have been used to characterize the relationship between grain boundary allotriomorphs and Widmanstätten ferrite plates. The issue of where Widmanstätten ferrite plates nucleate is one of some debate, with theories including morphological instability and sympathetic nucleation. Evidence has been found that supports the theory of a sympathetic nucleation mechanism being responsible for the formation of Widmanstätten ferrite plates. The EBSD measurements have shown that low-angle misorientations of between 5 and 10 deg exist between ferrite allotriomorphs and Widmanstätten ferrite plates. 相似文献
15.
S. W. Thompson D. J. Colvin G. Krauss 《Metallurgical and Materials Transactions A》1996,27(6):1557-1571
Decomposition of fine-grained austenite (10-μm grain size) during continuous cooling of an HSLA-80 plate steel (containing
0.05C, 0.50Mn, 1.12Cu, 0.88Ni, 0.71Cr, and 0.20Mo) was evaluated by dilatometric measurements, light microscopy, scanning
electron microscopy, transmission electron microscopy, and microhardness testing. Between 750 °C and 600 °C, austenite transforms
primarily to polygonal ferrite over a wide range of cooling rates, and Widmanst?tten ferrite sideplates frequently evolve
from these crystals. Carbon-enriched islands of austenite transform to a complex mixture of granular ferrite, acicular ferrite,
and martensite (all with some degree of retained austenite) at cooling rates greater than approximately 5 °C/s. Granular and
acicular ferrite form at temperatures slightly below those at which polygonal and Widmanst?tten ferrite form. At cooling rates
less than approximately 5 °C/s, regions of carbon-enriched austenite transform to a complex mixture of upper bainite, lower
bainite, and martensite (plus retained austenite) at temperatures which are over 100 °C lower than those at which polygonal
and Widmanst?tten ferrite form. Interphase precipitates of copper form only in association with polygonal and Widmanst?tten
ferrite. Kinetic and microstruc-tural differences between Widmanst?tten ferrite, acicular ferrite, and bainite (both upper
and lower) suggest different origins and/or mechanisms of formation for these morphologically similar austenite transformation
products.
Formerly Graduate Student, Department of Metallurgical and Materials Engineering, Colorado School of Mines.
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
16.
P. J. Hurley B. C. Muddle P. D. Hodgson 《Metallurgical and Materials Transactions A》2001,32(6):1507-1517
An austenitic Ni-30 wt pct Fe alloy, with a stacking-fault energy and deformation characteristics similar to those of austenitic
low-carbon steel at elevated temperatures, has been used to examine the defect substructure within austenite deformed by single-pass
strip rolling and to identify those features most likely to provide sites for intragranular nucleation of ultrafine ferrite
in steels. Samples of this alloy and a 0.095 wt pct C-1.58Mn-0.22Si-0.27Mo steel have been hot rolled and cooled under similar
conditions, and the resulting microstructures were compared using transmission electron microscopy (TEM), electron diffraction,
and X-ray diffraction. Following a single rolling pass of ∼40 pct reduction of a 2mm strip at 800 °C, three microstructural
zones were identified throughout its thickness. The surface zone (of 0.1 to 0.4 mm in depth) within the steel comprised a
uniform microstructure of ultrafine ferrite, while the equivalent zone of a Ni-30Fe alloy contained a network of dislocation
cells, with an average diameter of 0.5 to 1.0 μm. The scale and distribution and, thus, nucleation density of the ferrite grains formed in the steel were consistent with
the formation of individual ferrite nuclei on cell boundaries within the austenite. In the transition zone, 0.3 to 0.5 mm
below the surface of the steel strip, discrete polygonal ferrite grains were observed to form in parallel, and closely spaced
“rafts” traversing individual grains of austenite. Based on observations of the equivalent zone of the rolled Ni-30Fe alloy,
the ferrite distribution could be correlated with planar defects in the form of intragranular microshear bands formed within
the deformed austenite during rolling. Within the central zone of the steel strip, a bainitic microstructure, typical of that
observed after conventional hot rolling of this steel, was observed following air cooling. In this region of the rolled Ni-30Fe
alloy, a network of microbands was observed, typical of material deformed under plane-strain conditions. 相似文献
17.
Ph. Maitrepierre D. Thivellier R. Tricot 《Metallurgical and Materials Transactions A》1975,6(1):287-301
The influence of boron on the isothermal decomposition of Fe-Ni6-C0.12 (wt pct) steels has been investigated. The isothermal γ→ pro-eutectoid ferrite reaction was studied by quantitative metallography and dilatometry. It was clearly shown that boron
slows down considerably the nucleation rate of ferrite on γ-grain boundaries. End-quench experiments performed on C0.18-Cr-Mn industrial steels emphasized the changes in hardenability with thermal history.
Particular attention was devoted to the study of the state and location of boron in the microstructure of the steels studied.
Ion microscopy, alphagraphy and transmission electron microscopy were used to this effect. It was confirmed that boron segregates
easily to γ-grain boundaries during cooling, which results in the precipitation of iron boro-carbides. This precipitation
was shown to occur both in stable and metastable austenite, prior to the γ → pro-eutectoid ferrite reaction. The precipitates
were identified as Fe23(B, C)6 (FCC structure with a ≈ 10.6?). The grain boundary Fe23(B, C)6 were shown to have a parallel cube-cube orientation relationship with one of the neighboring grains. The role of the Fe23(B, C)6 precipitates with respect to the γ → proeutectoid ferrite reaction is discussed. 相似文献
18.
The α + γ two-phase fields of the Fe-Ni and Fe-Ni (P saturated) phase diagrams have been determined in the composition range
0 to 60 wt pet Ni and in the temperature range 700 to 300 °C. The solubility of Ni in (FeNi)3P was measured in the same temperature range. Homogeneous alloys were austenitized and quenched to form α2, martensite, then heat treated to formα (ferrite) + γ (austenite). The compositions of the α and γ phases were determined
with electron microprobe and scanning transmission electron microscope techniques. Retrograde solubility for the α/(α + γ)
solvus line was demonstrated exper-imentally. P was shown to significantly decrease the size of the α + γ two-phase field.
The maximum solubility of Ni in α is 6.1 ± 0.5 wt pct at 475 °C and 7.8± 0.5 wt pct at 450 °C in the Fe-Ni and Fe-Ni (P saturated)
phase diagrams, respectively. The solubility of Ni in α is 4.2 ± 0.5 wt pct Ni and 4.9 ± 0.5 wt pct Ni at 300 °C in the Fe-Ni
and Fe-Ni (P saturated) phase diagrams. Ternary Fe-Ni-P isothermal sections were constructed between 700 and 300 °C.
Formerly Research Assistant in Department of Metallurgy & Materials Engineering, Lehigh University, Bethlehem, PA. 相似文献
19.
Young Kyu Cho Duk Yong Yoon Michael F. Henry 《Metallurgical and Materials Transactions A》2001,32(12):3077-3090
When an extruded strain-free RENé 88 Ni-base superalloy about 1 to 2 μm in grain diameter is heattreated at 1150°C, abnormal grain growth (AGG) begins after 50 hours. When heat-treated at 1200
°C, AGG occurs at 15 minutes. Some of the grain boundaries are faceted with hill-and-valley structures when observed in transmission
electron microscopy (TEM), and the occurrence of AGG is consistent with the boundary step and dislocation mechanism for the
migration of singular boundaries with faceted shapes, as observed and proposed in other pure metals and alloys. The dissolution
of abundant γ′ precipitates (with a solvus temperature of 1107 °C), which are coherent with the matrix and hence strongly pin the grain
boundaries, does not cause AGG during early stages of heat-treatment at 1150 °C. Small deformations drastically alter the
AGG behavior. When deformed to 4 pct, AGG begins after heat-treating for 10 minutes at 1150 °C, compared to the apparent incubation
time of 50 hours for an undeformed specimen, and very large abnormal grains are produced. With increasing deformation to 6
and 9.2 pct, the abnormal grain size decreases. These results are qualitatively similar to those observed in Cu. This deformation
effect on AGG is attributed to the absorption of lattice dislocations in the grain boundaries, which produces nonequilibrium
structures that, in turn, can apparently cause rapid boundary migration. When heat-treated at 1200 °C, the largest abnormal
grains are found in the specimens deformed to 2 pct. When the initial grain size is increased to about 14 μm by heattreating the extruded alloy at 1150 °C for 30 minutes, similar low deformation effects on AGG are observed. When
these specimens are deformed to 10, 13, and 15.2 pct, primary recrystallization occurs during the heat-treatment at 1150 °C,
and large abnormal grains are again produced because of the small recrystallized grain size. Therefore, there are two peaks
in the grain size vs deformation curve after heat-treating at 1150 °C for 1 hour. A pre-heat-treatment of this alloy at 1050 °C below the solvus
temperature of the γ′ phase greatly reduces the size of the abnormal grains obtained in the specimen deformed to 4 pct after the heat-treatment
at 1150 °C, probably because some recovery of the dislocations takes place at grain boundaries during the pre-heat-treatment.
The deformation effect on AGG observed in this alloy is qualitatively similar to that previously observed in Cu and appears
to be consistent with the boundary step and dislocation mechanism for AGG.
An erratum to this article is available at . 相似文献
20.
Ryo Saito Nobuo Nakada Shouhei Yabu Kohtaro Hayashi 《Metallurgical and Materials Transactions A》2018,49(12):6001-6009
Austenite nucleation sites were investigated in near-eutectoid 0.8 mass pct C steel and hypoeutectoid 0.4 mass pct C steel samples with full pearlite and ferrite–pearlite initial structures, respectively. In particular, the prior austenite grain size had been coarsened to compare grain boundaries in the hierarchical pearlite structure, i.e., the low-angle pearlite colony and high-angle block boundaries with ferrite/pearlite interfaces in the austenite nucleation ability. When the full pearlite in 0.8 mass pct C steel underwent reversion at a relatively low temperature, austenite grains preferentially formed at pearlite block boundaries. Consequently, when the full pearlite with the coarse block structure underwent reversion just above the eutectoid temperature, the reversion took a long time due to the low nucleation density. However, austenite grains densely formed at the pearlite colony boundaries as well, as the reversion temperature became sufficiently high. On the other hand, when ferrite–pearlite in the 0.4 mass pct C steel underwent reversion to austenite, the ferrite/pearlite interface acted as a more preferential austenite nucleation site than the pearlite block boundary even in the case of low-temperature reversion. From these results, it can be concluded that the preferential austenite nucleation site in carbon steels is in the following order: ferrite/pearlite interface?>?pearlite block?>?colony boundaries. In addition, orientation analysis results revealed that ferrite restricts the austenite nucleation more strongly than pearlitic ferrite does, which contributes to the preferential nucleation at ferrite/pearlite interfaces. This suggests that austenite grains formed at a ferrite/pearlite interface tend to have an identical orientation even under high-temperature reversion. Therefore, it is thought that the activation of austenite nucleation within pearlite by increasing the reversion temperature may be effective for rapid austenitization and the grain refinement of austenite structure after the completion of reversion in carbon steels. 相似文献