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1.
The effects of carbon content and ausaging on austenite γ ↔ martensite (α′) transformation behavior and reverse-transformed
structure were investigated in Fe-32Ni-12Co-4Al and Fe-(26,28)Ni-12Co-4Al-0.4C (wt pct) alloys. TheM
s
temperature, the hardness of γ phase, and the tetragonality of α′ increase with increasing ausaging time, and these values
are higher in the carbon-bearing alloys in most cases. The γ → α′ transformation behavior is similar to that of thermoelastic
martensite; that is, the width of α′ plate increases with decreasing temperature in all alloys. The αt’ → γ reverse transformation
temperature is lower in the carbon-bearing alloys, which means that the shape memory effect is improved by the addition of
carbon. The maximum shape recovery of 84 pct is obtained in Fe-28Ni-12Co-4Al-0.4C alloy when the ausaged specimen is deformed
at theM
s
temperature and heated to 1120 K. There are two types of reverse-transformed austenites in the carbon-bearing alloy. One
type is the reversed y containing many dislocations which were formed when the γ/α′ interface moved reversibly. The plane
on which dislocations lie is (01 l)γ if the twin plane is (112)α′. The other type of reverse-transformed austenite exhibits γ islands nucleated within the α′ plates. 相似文献
2.
Bikas C. Maji Madangopal Krishnan V. V. Rama Rao 《Metallurgical and Materials Transactions A》2003,34(5):1029-1042
The microstructure and phase stability of the Fe-15Mn-7Si-9Cr-5Ni stainless steel shape memory alloy in the temperature range
of 600 °C to 1200 °C was investigated using optical and transmission electron microscopy, X-ray diffractometry (XRD), differential
scanning calorimetry (DSC), and chemical analysis techniques. The microstructural studies show that an austenite single-phase
field exists in the temperature range of 1000 °C to 1100 °C, above 1100 °C, there exists a three-phase field consisting of
austenite, δ-ferrite, and the (Fe,Mn)3Si intermetallic phase; within the temperature range of 700 °C to 1000 °C, a two-phase field consisting of austenite and the
Fe5Ni3Si2 type intermetallic phase exists; and below 700 °C, there exists a single austenite phase field. Apart from these equilibrium
phases, the austenite grains show the presence of athermal ɛ martensite. The athermal α′ martensite has also been observed for the first time in these stainless steel shape memory alloys and is produced through
the γ-ɛ-α′ transformation sequence. 相似文献
3.
A. A. Hussein L. I. El-Menawati H. J. Klaar 《Metallurgical and Materials Transactions A》1978,9(12):1783-1788
The temper hardening of quench-induced Cu-Al martensite was investigated via trans-mission electron microscopy, differential
thermal analysis, microhardness and yield strength measurements. The results obtained are also viewed in conjunction with
inde-pendent set of experimental data dealing with anneal hardening of the α-phase without prior martensitic transformation.
A substantial low temperature hardening of a 10 pct Al martensite is indicated and attributed to the formation of small ordered
clusters, en-hanced by stacking faults already existing in martensite. In the 11 pct Al martensite hardening is also linked
with ordering though less pronounced due to the larger domain size reached. On the other hand the tempering of 11.8 pct Al
martensite is associated with phase separation leading to the(α + γ2) state via continuous and discontinuous pre-cipitation. 相似文献
4.
The martensite substructure after ausforming has been studied for two different martensite morphologies: partially twinned,
lenticular martensite (Fe-33 pct Ni, Ms =-105‡C) and completely twinned “thin plate” martensite (Fe-31 pct Ni-0.23 pct C, Ms = -170‡C), and in both cases ausforming produces a dislocation cell structure in the austenite which is inherited, without
modification, by the martensite. In the Fe-Ni alloy, the dislocation cell structure is found in both the twinned (near the
midrib) and untwinned (near the interface) regions, the latter also containing a regular dislocation network generated by
the transformation itself and which is unaltered by the austenite dislocation cell structure. Similarly, in the Fe-Ni-C alloy,
the transformation twins are unimpeded by the prior cell structure. These observations show that carbide precipitation during
ausforming is not necessarily required to pin the austenite cell structure and that the martensite-austenite interface, backed
by either twins or dislocations, does not exhibit a ”sweeping” effect. Although the martensite transformation twins are not
inhibited by the ausforming cell structure, they do undergo a refinement with increased ausforming, and it is indicated that
the transformation twin width in martensite depends on the austenite hardness. However, the relative twin widths remain unchanged,
as expected from the crystallographic theory.
T. MAKI, Formerly with the University of Illinois 相似文献
5.
Bikas C. Maji Madangopal Krishnan Gouthama R. K. Ray 《Metallurgical and Materials Transactions A》2011,42(8):2153-2165
The effect of Si addition on the microstructure and shape recovery of FeMnSiCrNi shape memory alloys has been studied. The
microstructural observations revealed that in these alloys the microstructure remains single-phase austenite (γ) up to 6 pct Si and, beyond that, becomes two-phase γ + δ ferrite. The Fe5Ni3Si2 type intermetallic phase starts appearing in the microstructure after 7 pct Si and makes these alloys brittle. Silicon addition
does not affect the transformation temperature and mechanical properties of the γ phase until 6 pct, though the amount of shape recovery is observed to increase monotonically. Alloys having more than 6 pct
Si show poor recovery due to the formation of δ-ferrite. The shape memory effect (SME) in these alloys is essentially due to the γ to stress-induced ε martensite transformation, and the extent of recovery is proportional to the amount of stress-induced ε martensite. Alloys containing less than 4 pct and more than 6 pct Si exhibit poor recovery due to the formation of stress-induced
α′ martensite through γ-ε-α′ transformation and the large volume fraction of δ-ferrite, respectively. Silicon addition decreases the stacking fault energy (SFE) and the shear modulus of these alloys and
results in easy nucleation of stress-induced ε martensite; consequently, the amount of shape recovery is enhanced. The amount of athermal ε martensite formed during cooling is also observed to decrease with the increase in Si. 相似文献
6.
Francois Abrassart 《Metallurgical and Materials Transactions B》1973,4(9):2205-2216
The influence of the temperature θαof a prestraining of austenite above Mdon the subsequent stress-induced γ→ α’ transformation in the(M
s, Md) range is examined in two carbon stainless steels. It is shown that the yield stress, which is controlled by the transformation,
increases with θαat given testing temperature and amount of prestraining. This behavior is related to the influence of θαon
the nature and arrangement of the defects present in austenite after the prestraining: planar defects(i.e., stacking faults, twins, e platelets) predominate if θαis close to Mdwhereas undissociated dislocation cells are only to be observed if θif higher. This is consistent with the strong increase
of the intrinsic stacking fault energy of the austenite, as inferred from measurements using the node method on a hot stage
microscope. In addition, the ability of plane defects to propagate under stress is shown to be lower after a prestraining
at higher θα, which is attributed to a segregation of impurity atoms on dislocations. It is concluded that the nucleation
stress of the γ→ α’ transformation is the stress necessary to allow planar defects to propagate in the prestrained austenite.
This work is part of a thesis prepared at the Centre des Matériaux de l’Ecole des Mines, Corbeil, France, and submitted at
the University of Nancy, June 1972. 相似文献
7.
S. Mahajan M. R. Pinnel J. E. Bennet 《Metallurgical and Materials Transactions B》1974,5(6):1263-1272
The microstructural changes in an Fe-Co-V alloy (composition by wt pct: 2.97 V, 48.70 Co, 47.34 Fe and balance impurities,
such as C, P and Ni) resulting from different heat treatments have been evaluated by optical metallography and transmission
electron microscopy. Results indicate that, on air cooling or quenching into iced-brine from the high temperature single-phase
γ (fcc) field, vanadium can be retained in a supersaturated solid solution (α2) which has bcc structure. For the range of cooling rates employed, a portion of the material appears to undergo the γ-α2 transformation massively and the remainder martensitically. Also antiphase boundaries are observed in the air-cooled samples.
On annealing in the two-phase α1 + γ field, α2 decomposes into vanadium-rich subgrains (γ) and vanadium-poor subgrains (γ1), and only the former undergo the γ → α2 transformation during air cooling or iced-brine quenching. The α1t subgrains in a sample, slowly quenched in quartz, show superlattice dislocations and antiphase boundaries, whereas both
the transformed and untransformed areas exhibit (100) superlattice reflections. There is, however, no evidence of long-range
order in the specimens quenched into iced-brine. The two-phase annealing sequence followed by a 2 h anneal at 600°C and air
cooling results in precipitation within the vanadium-rich, transformed subgrains. Also there is evidence of long-range order
in both types of subgrains. 相似文献
8.
The tensile deformation behavior of mechanically-stabilized austenite is investigated in Fe-Mn binary alloys. A 30 pct thickness
reduction by rolling at 673 K (above the Af temperature) largely suppresses the austenite (γ) to hcp epsilon martensite (ε) transformation in 17Mn and 25Mn steels. However,
the deformation behavior of the mechanically stabilized austenite in the two alloys differs significantly. In 25Mn steel,
the onset of plastic deformation is due to the stress-induced γ→ ε transformation and results in a positive temperature dependence of the yield strength. The uniform elongation is enhanced
by the γ → ε transformation during deformation. In 17Mn steel, bccα′ martensite is deformation-induced along with e and a plateau region similar to Lüders band deformation appears at the beginning
of the stress-strain curve. The mechanical stabilization of austenite also suppresses the intergranular fracture of 17Mn steel
at low temperatures.
M. STRUM, formerly Candidate for Ph.D. at the University of California at Berkeley 相似文献
9.
Naresh C. Goel Sandeep Sangal Kris Tangri 《Metallurgical and Materials Transactions A》1985,16(11):2013-2021
A semi-mechanistic model for predicting the flow behavior of a typical commercial dual-phase steel containing 20 vol pct of
‘as quenched’ martensite and varying amounts of retained austenite has been developed in this paper. Assuming that up to 20
vol pct of austenite with different degrees of mechanical stability can be retained as a result of certain thermomechanical
treatments in a steel of appropriate low carbon low alloy chemistry, expressions for composite flow stress and strain have
been derived. The model takes into account the work hardening of the individual microconstituents(viz., ferrite-@#@ α, retained austenite- γ
r, and martensite -α′) and the extra hardening of ferrite caused by accommodation dislocations surrounding the ‘as quenched’
as well as the strain-induced(γ
r→ α′) martensite. Load transfer between the phases has been accounted for using an intermediate law of mixtures which also
considers the relative hardness of the soft and the hard phases. From the derived expressions, the flow behavior of dual phase
steels can be predicted if the properties of the individual microconstituents are known. Versatility of the model for application
to other commercial steels containing a metastable phase is discussed. 相似文献
10.
Yung-Fu Hsu Wen-Hsing Wang C. M. Wayman 《Metallurgical and Materials Transactions A》1991,22(7):1473-1478
The effects of thermal cycling between the parent and martensite phases of two-phase α/β CuZn alloys have been studied by
electrical resistance-temperature measurements, optical microscopy, and transmission electron microscopy (TEM). The martensite
start (Ms) temperature is dominated primarily by the composition of the β phase but increases substantially between the first and second
cycles because of deformation of the α particles and a resultant change in the internal strain fields of the system. With
increasing thermal cycling, the Ms temperature increases slightly and eventually becomes constant. However, the transformation hysteresis becomes smaller, and
more perfect thermoelastic behavior is found. The number of vestigial deformation markings in the β phase is increased by
thermal cycling and becomes more distinct; the dislocation density in the β phase is also increased and features a more crystallographic
arrangement. The vestigial deformation of the β phase is instrumental in subsequent martensite nucleation and in creating
a martensite microstructural memory. 相似文献
11.
《Acta Metallurgica Materialia》1994,42(5):1583-1594
The thermoelastic and mechanical properties of CuAlNiBMn shape memory alloys have been studied as a function of manganese concentration and of heat treatment. Below a limiting value of manganese content, the loss of thermoelastic and pseudoelastic properties has been observed, in particular in the quenched specimens. The partial transformation and its degradation during thermal cycling observed in the low manganese content alloy has been attributed to the lower degree of B2 order achieved during the quench, leading to slower kinetics of DO3 ordering. The accomodation of strains between martensite variants and between the martensite/austenite phases appear to need dislocation accumulation at their interfaces. The presence of dislocations observed during the reverse transformation seem responsible for the degradation of the transformation and the loss of pseudoelastic properties of this alloy. 相似文献
12.
It has been demonstrated that the theory of martensite crystallography is capable of accounting successfully for the form
and crystallography of a range of plate- or lath-shaped transformation products, even when the formation of the product phase
involves significant substitutional diffusion. These transformations include the precipitation of metastable hexagonal γ’
(Ag2Al) plates in disordered face-centered cubic (fcc) solid-solution Al-Ag alloys, the formation of ordered AuCu II plates from
disordered fcc solid solution in equiatomic Au-Cu alloys, and the formation of metastable9R α
1, plates in ordered(B2) Cu-Zn and Ag-Cd alloys. The application of the theory to these transformations is reviewed critically and the features common
to them identified. It is confirmed that, in all three transformations, the product phase produces relief at a free surface
consistent with an invariant plane-strain shape change and that the transformations are thus properly described as displacive.
The agreement between experimental observations and theoretical predictions of the transformation crystallography is in all
cases excellent. It is proposed that successful application of the theory implies a growth mechanism in which the coherent
or semicoherent, planar interface between parent and product phases maintains its structural identity during migration and
that growth proceeds atom by atom in a manner consistent with the maintenance of a correspondence of lattice sites. In the
case of the coherent, planar interfaces associated with γ’ precipitate plates in Al-Ag alloys, there is direct experimental
evidence that this is accomplished by the motion of transformation dislocations across the coherent broad faces of the precipitate
plates; the transformation dislocations define steps that are two atom layers in height normal to the habit plane and have
a Burgers vector at least approximately equivalent to an(α/6)(112) Shockley partial dislocation in the parent fcc structure. However, for AuCu II plates, where the product phase is twinned
on a fine scale, and for α1 plates, for which the lattice invariant strain leads to a substructure of finely spaced stacking faults, the structures of
the semicoherent interphase boundaries and thus the details of the transformation mechanism remain less clearly defined.
Formerly with the Department of Materials Engineering, Monash University
Formerly with the Department of Materials Engineering, Monash University
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
13.
The atomic structures of the γ/α2 and γ/γT interfaces in a TiAl alloy were investigated using conventional and high-resolution transmission electron microscopy (TEM)
in order to understand the growth mechanisms and deformation behavior of the two-phase alloy. The results show that the α2 plates grow from the γ phase by the migration of a/6〈112〉 partial dislocation ledges across the faces and that the γ/α2 interface usually contains closely spaced arrays of interfacial dislocations. Deformation twins cut through both γ twin boundaries
and α2 plates during deformation, although slip of twinning c slocations through α2 appears to be a difficult process. Both the γ/α2 and γ/γT interfaces can be imaged and modeled at the atomic level, although slight crystal and/or beam tilt can complicate image interpretation.
G.J. MAHON, formerly Research Associate with the Dep rtment of Metallurgical Engineering and Materials Science, Carnegie dellon
University
This paper is based on a presentation made in the syrr losium “Interfaces and Surfaces of Titanium Materials” presented at
tl; 1988 TMS/AIME fall meeting in Chicago, IL, September 25–29 1988, under the auspices of the TMS Titanium Committee. 相似文献
14.
Microstructural dependence of Fe-high Mn tensile behavior 总被引:1,自引:0,他引:1
The tensile properties of Fe-high Mn (16 to 36 wt pct Mn) binary alloys were examined in detail at temperatures from 77 to
553 K. The Mn content dependence of the deformation and fracture behavior in this alloy system has been clarified by placing
special emphasis on the starting microstructure and its change during deformation. In general, the intrusion of hcp epsilon
martensite (ε) into austenite (γ) significantly increases the work hardening rate in these alloys by creating strong barriers
to further plastic flow. Due to the resulting high work hardening rates, large amounts of e lead to high flow stresses and
low ductility. Alloys of 16 to 20 wt pct Mn are of particular interest. While these alloys are thermally stable with respect
to bcc α’ martensite formation, 16 to 20 wt pct Mn alloys undergo a deformation induced ε →α’ transformation. The martensitic transformation plays two contrasting roles. The stress-induced ε→ α’ transformation decreases the initial work hardening rate by reducing locally high internal stress. However, the work hardening
rate increases as the accumulated α’ laths become obstacles against succeeding plastic flow. These rather complicated microstructural
effects result in a stress-strain curve of anomolous shape. Since both the Ms and Md temperatures for both the ε and α’-martensite transformations are strongly dependent on the Mn content, characteristic relationships
between the tensile behavior and the Mn content of each alloy are observed. 相似文献
15.
Fe-Pt alloys near the composition Fe3Pt transform from fee austenite to bcc martensite at near ambient temperatures. The effect of austenite ordering in depressing
theM
s temperature has been reported previously, but more importantly the present work shows that ordering leads to a reversible
martensitic transformation. The characteristics of this reversible transformation have been investigated by optical metallography,
cinematography, and electrical resistivity measurements. It is concluded that in austenite ordered to an appropriate degree,
the transformation to martensite possesses all of the characteristics of a thermoelastic martensite transformation. This transformation
in ordered Fe~25 at. pct Pt alloys is the first thermoelastic martensite transformation reported for an iron-base alloy. The
present experiments indicate that martensite “nuclei” are not destroyed by the transformation, and are reactivated on each
cooling cycle at approximately the same temperature.
D. P. DUNNE, formerly with the University of Illinois at Urbana-Champaign, Urbana, 111. 61801 相似文献
16.
D. A. Colling 《Metallurgical and Materials Transactions B》1971,2(10):2889-2896
The martensite ⇌ austenite transformations were investigated in Fe-Ni-Co alloys containing about 65 wt pct Fe and up to 15
wt pct Co. A change in morphology of martensite from plate-like to lath-type occurred with increasing cobalt content; this
change in morphology correlates with the disappearance of the Invar anomaly in the austenite. The martensite-to-austenite
reverse transformation differed depending on martensite morphology. Reversion of plate-like martensite was found to occur
by simple disintegration of the martensite platelets. Reverse austenite formed from lath-type martensite was not retained
when quenched from much aboveA
s, with microcracks forming during theM→γ→M transformation. 相似文献
17.
The basis of this work was the investigation of improving the tensile properties of dislocated martensites by dispersion of
precipitates in the austeniteprior to the martensite transformation. Two types of precipitation-hardenable austenitic alloys were used. One is based on Fe-22
Ni-4 Mo-0.28 C where the precipitates are Mo2C and are obtained by ausforming and aging, and the other is Fe-28 Ni-2 Ti where the precipitates are the coherent fccγ’ (Ni3Ti) ordered phase obtained by ausaging. After the austenitic dispersion treatment both alloys were transformed to martensite
by quenching to liquid nitrogen and the properties measured and compared to martensites obtained by conventional heat treatment
(i.e. no precipitates in austenite). The results show that prior dispersions increase the strength of martensite and this is interpreted
as being due to an increase in dislocation density resulting from dislocation multiplication at the particles during the γ
→M
s transformation. In addition, the stabilities of the austenitic alloys are such that upon certain aging treatments, the alloys
transform partially to martensite (due to precipitation) and “composite” materials are obtained whose strength depends on
the volume fraction and yield strengths of the phases present.
Formerly Graduate Student, Department of Materials Science and Engineering, University of California, Berkeley, Calif. 相似文献
18.
Dislocation structure in a single-crystal nickel-base superalloy during low cycle fatigue 总被引:1,自引:0,他引:1
J. H. Zhang Z. Q. Hu Y. B. Xu Z. G. Wang 《Metallurgical and Materials Transactions A》1992,23(4):1253-1258
An investigation of dislocation structure in a single crystal nickel-base superalloy during low cycle fatigue (LCF) at 760
°C has been conducted. Dislocation bands are found to be produced first in the matrix in some defined directions. With an
increase in cycle numbers, there is an increase in dislocation density in the bands and a decrease in the spacing between
the bands, leading to the formation of the dislocation walls or cells. Sometimes, three-dimensional (3-D) networks are formed
also by the interaction between two sets of parallel dislocations. The Burgers vectors of the dislocations in the network
are 1/2 〈110〉. Clustering of dislocations eventually occurs at γ′/γ interfaces because of the obstruction of the γ′-particles
to moving dislocations. Most of the dislocations observed in the γ′-phases are in the form of superdislocations. Dislocation
shearing through theγ′-phase was found occasionally. Reprecipitation of γ′-phase induced by strain was also observed in the
present study. 相似文献
19.
Young-Kook Lee 《Metallurgical and Materials Transactions A》2002,33(7):1913-1917
The reason why thermal cycling decreases the martensite start (M
s
) temperature of an Fe-17 wt pct Mn alloy was quantitatively investigated, based on the nucleation model of ε martensite and a thermodynamic model for a martensitic transformation. The M
s
temperature decreased by about 22 K after nine cycles between 303 and 573 K, due to the increase in shear-strain energy (ΔG
sh
) required to advance the transformation dislocations through dislocation forests formed in austenite during thermal cycling.
The ΔG
sh
value increased from 19.3 to 28.8 MJ/m3 due to the increase in austenite dislocation density from 1.5 × 1012 to 3.8 × 1013/m2 with the number of thermal cycles (in this case, up to nine cycles). The austenite dislocation density increased rapidly
for up to five thermal cycles and then increased gradually with further thermal cycles, showing a good agreement with the
increase in austenite hardness with the number of thermal cycles. 相似文献
20.
The α→γ phase transformation during rapid quenching and subsequent isothermal aging has been investigated in a Ti-48 at pct Al alloy.
The microstructure changes from a completely massively transformed γ-grain structure to a mixed microstructure of the massively transformed γ grains and the untransformed (meaning massively untransformed) fine α
2/γ lamellae with an increase in the cooling rate from the high-temperature α phase field. Fine γ grains are generated from these fine α
2/γ lamellae by subsequent again at 1323 K. The fine γ grains contain many defects, such as dislocations, microtwins (or stacking faults), domain boundaries, and variants, which
are frequently observed in the massive γ grains. This result suggests that the formation mechanism of the fine γ grains during aging is similar to that of the massive γ grains. When the fine γ/γ lamellar sample, which is formed by preliminary aging at a lower temperature (1173 K), is aged at a higher temperature (1323
K), apparent changes in microstructure could not be recognized. This result indicates that the fine γ-grain formation is closely related to the α
2 → γ phase transformation in the fine α
2/γ lamellae.
This article is based on a presentation made in the symposium “Fundamentals of Gamma Titanium Aluminides,” presented at the
TMS Annual Meeting, February 10–12, 1997, Orlando, Florida, under the auspices of the ASM/MSD Flow & Fracture and Phase Transformation
Committees. 相似文献