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1.
C. Song H. Park H. Seong H. F. López 《Metallurgical and Materials Transactions A》2006,37(11):3197-3204
In this work, CoCr-Mo compacted powders were sintered at 900°C to 1300°C for 1 to 2 hours and conditions for total carbide
dissolution in fcc cobalt were determined. Accordingly, it was found that sintering at temperatures between 900°C to 1100°C
led to removal of the dendritic structure and to carbide precipitation at the grain boundaries (gbs), as well as in the bulk.
Moreover, recrystallization and grain growth were always found to occur during powder sintering. At temperatures above 1100°C,
no carbide precipitation occurred indicating that carbides were not stable at these temperatures. Hence, compact powders were
annealed at 1150°C to promote the development of a single-phase fcc solid solution. This was followed by rapid cooling to
room temperature and then aging at 800°C for 0 to 18 hours. Rapid cooling from 1150°C promoted the development of up to 64
pct athermal ε-martensite through the face-centered cubic (fcc) → hexagonal crystal structure (hcp) martensitic transformation.
The athermal martensite was associated with the development of a network of parallel arrays of fine straight transgranular
markings within the fcc matrix. Moreover, aging at 800°C for 15 hours led to the development of 100 pct isothermal hcp ε-martensite.
From the experimental outcome, it is evident that isothermal ε-martensite is the most stable form of the hcp Co phase. Apparently,
during aging at 800°C, the excess defects expected in athermal martensite are removed by thermally activated processes and
by the development of isothermal ε-martensite, which has the appearance of “pearlite.” 相似文献
2.
A. M. Thakur N. N. Thadhani R. B. Schwarz 《Metallurgical and Materials Transactions A》1997,28(7):1445-1455
Shock-impact generated tensile-stress pulses were used to induce B2-to-monoclinic martensitic transformations in two near-equiatomic
NiTi alloys having different martensite transformation start (M
s
) temperatures. The NiTi-I alloy (M
s
≈+27 °C) impacted at room temperature at 2.0 and 2.7 GPa tensile stress-pulse magnitude, showed acicular martensite morphology.
These martensite needles had a substructure containing microtwins, typical of “stress-assisted” martensite. The NiTi-II alloy
(M
s
≈−45 °C) showed no martensite formation when shocked with tensile-stress pulses of 2 GPa. For tensile stresses of 4.1 GPa,
the alloy showed spall initiation near the region of maximum tensile-stress duration. In addition, monoclinic martensite needles,
with a well-defined dislocation substructure, typical of “strain-induced” martensite, were seen clustering around the spall
region. No stress-assisted martensite was formed in this alloy due to its very low M
s
temperature. The present article documents results of the use of a metallurgical technique for generating large-amplitude
tensile stress pulses of finite duration for studies of phase transformations involving changes from a high density to a low
density state. 相似文献
3.
Separate 200, 020, and 002 X-ray peaks were recorded for 0.0, 0.4, and 0.8 wt pct carbon (18 pct Ni) martensites after tempering
between 25 and 500°C. The carbon bearing martensites studied here have been tempered initially enough to eliminate the “high
tetragonality” 002 peak usually recorded for as-quenched martensite and the present results apply to tempered martensite only.
The peak maximum is taken to determine the lattice parameter and the peak shape is recorded. At all carbon levels and after
all tempering treatments, the “crd parameter is larger than or equal to the “a” or “b”. The relative enlargement is very small
(0.08 pct) for the lowest carbon level and for any carbon level after severe tempering (500°C for 15 min). For the two higher
carbon alloys tempered at temperatures below 400°C (for 15 min) the “c” parameter is significantly larger than the “a” and
“b” and for the 0.4 wt pct C alloy the “b” is significantly smaller than the“a” whereas in the 0.8 pct C alloy the “b” is slightly larger than the “a”. Within experimental error the mean volume of the
unit cell does not change during the tempering studied here and is nearly unaffected by the initial carbon content. This indicates
that little (at most 0.1 wt pct) carbon is dissolved in tempered martensite. In the low carbon alloy the peaks are symmetric
and sharpen symmetrically during tempering. In the higher carbon alloys the peaks are nearly symmetric and sharp after severe
tempering. After less severe tempering the 002 peak is asymmetrically broadened toward lower9 values (higher lattice parameters) whereas the 200 and 020 peaks are asymmetrically broadened toward higher 0 values corresponding
to lower lattice parameters. This collection of results is tentatively interpreted as being due to strains in martensite due
to transformation induced substructure and precipitated carbides. 相似文献
4.
5.
A. de J. Saldívar García A. Maní Medrano A. Salinas Rodríguez 《Metallurgical and Materials Transactions A》1999,30(5):1177-1184
The evolution of the microstructure of a Co-27Cr-5Mo-0.05C alloy was investigated during isothermal aging between 650 °C and
950 °C. The main structural change observed as a result of aging was an fcc (metastable)→hcp isothermal martensitic transformation.
The relationships between transformation, temperature, and time for this phase transition were determined using two different
techniques: (1) room-temperature X-ray diffraction on samples aged after quenching from 1150 °C to 25 °C and (2) high-temperature
in situ X-ray diffraction on samples cooled at 50 °C/min from 1150 °C to the aging temperature. The results show that the intermediate
water quench significantly retards the kinetics of the phase transition by up to one order of magnitude in time. In addition,
it was found that the grain size of the metastable fcc phase prior to aging does not affect the kinetics of the transformation.
Age hardening resulting from this transformation varies linearly with the amount of hcp phase formed during the isothermal
treatment and does not depend on the aging temperature. It is suggested that local plastic deformation, due to thermal and
transformation stresses induced by quenching, reduces the number of hcp martensite embryos formed in the metastable fcc phase.
This effect decreases the number of nucleation sites available for the fcc→hcp transformation during isothermal aging and
leads to the slower transformation rates observed in water-quenched material. 相似文献
6.
Shingo Kurosu Hiroaki Matsumoto Akihiko Chiba 《Metallurgical and Materials Transactions A》2010,41(10):2613-2625
The isothermal phase transformation behavior in a biomedical Co-29Cr-6Mo alloy without carbon or nitrogen was investigated
during aging at temperatures between 973 K and 1273 K (700 °C and 1000 °C) for up to 90 ks. Transformation from the γ to the ε phase did not occur at 1273 K (1000 °C) as the γ phase was more stable than the ε phase, and the σ phase precipitated at the γ grain boundaries. At 1173 K (900 °C), a γ → ε
1 phase transformation occurred by massive precipitation. Prolonged annealing at 1173 K (900 °C) led to a lamellar structure
of ε
2 and σ phases at ε
1/ε
1 boundaries by a discontinuous/cellular reaction, expressed by the reaction equation ε
1 → ε
2 + σ. After decreasing the aging temperature to 973 K (700 °C), transformation from the γ to the ε phase occurred mainly by isothermal martensitic transformation, but a lathlike massive ε
1 phase and ε
2/σ lamellar colonies were also observed at the original γ-grain boundaries. It is likely that not adding carbon results in the promotion of the massive transformation and the precipitation
of the σ phase during isothermal aging in the Co-29Cr-6Mo alloy system, whose composition corresponds to the ASTM F75 standard for
metallic materials for surgical implantation. The resultant isothermal transformation behavior of the present alloy is described
on the basis of thermodynamic calculations using Thermo-Calc. 相似文献
7.
The thermal cycling of an Fe-17 wt pct Mn alloy between 303 and 573 K was performed to investigate the effects of thermal
cycling on the kinetics of the γ → ε martensitic transformation in detail and to explain the previous, contrasting results of the change in the amount of ε martensite at room temperature with thermal cycling. It was observed that the shape of the γ → ε martensitic transformation curve (volume fraction vs temperature) changed gradually from a C to an S curve with an increasing number of thermal cycles. The amount of ε martensite of an Fe-17 wt pct Mn alloy at room temperature increased with thermal cycling, in spite of the decrease in the
martensitic start (M
s) temperature. This is due to the increase in transformation kinetics of ε martensite at numerous nucleation sites introduced in the austenite during thermal cycling. 相似文献
8.
Shiqiang Lu Baozhong Shang Zijian Luo Renhui Wang Fangchang Zeng 《Metallurgical and Materials Transactions A》2000,31(1):5-13
Composite electron-diffraction patterns (EDPs) for any specified fcc matrix orientation containing deformation twin and/or
strain-induced hexagonal close-packed ε martensite (shortened as hcp phase or hcp platelet subsequently) related to the matrix by specific orientation relationships
have been computed and plotted. These results helped us to select the favorable fcc matrix orientation for distinguishing
between deformation twin and hcp phase and to index experimental EDPs obtained by using transmission electron microscopy (TEM).
The identification of structure features in cold-drawn MP159 alloy with 48 pct reduction in cross-sectional area was performed
by using TEM, combined with the results from the computer-simulated composite EDPs. Investigation results demonstrated that
the closely spaced, intersecting network of fine platelets formed during cold drawing is deformation twins, and no reliable
evidence has been found for the presence of hcp phase, which was generally believed to explain the high strength attained
in cold-drawn MP159 alloy in some literature. It follows that the significant increase in the strength due to cold working
in MP159 alloy results predominately from the formation of the intersecting network of thin deformation twin platelets, which
act as “cells” or “subgrains” and provide strong barriers to the movement of dislocations over large distances. In addition,
the relatively high dislocation density makes a certain contribution to strengthening. 相似文献
9.
The substructure of martensite in a 63.4 at. pct Ni-Al alloy was studied by transmission electron microscopy with particular
attention to the “mixed” martensite morphologies observed in Ni-Al alloys, {111} <11•2>fct transformation twinning was the only mode of lattice-invariant shear found and the measured twin thicknesses lead to an inhomogeneous
shear value,m
2, in excellent agreement with that predicted from the crystallographic theory. The internal twins in the martensite plates
forming characteristic mixed morphologies were found to be at definite angles to one another and these angles were carefully
measured to be 96 deg and 125 deg for the “fork” type or “spear” type martensite morphologies. The corresponding angles as
predicted from the Bowles-Mackenzie theory are 94.2 deg and 124.4 deg. The interface plane between the martensite variants
that form the spear type morphology was found to be a {011}β' type plane with the internal twins in each variant twin related to each other across this mirror plane.
Formerly Graduate Student 相似文献
10.
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
11.
Transformation behavior of TRIP steels 总被引:2,自引:0,他引:2
True-stress (σ), true-strain (ε) and volume fraction martensite(f) were measured during both uniform and localized flow as a function of temperature on TRIP steels in both the solution-treated
and warm-rolled conditions. The transformation curves(f vs ε) of materials in both conditions have a sigmoidal shape at temperatures above Ms
σ (maximum temperature at which transformation is induced by elastic stress) but approach initially linear behavior at temperatures
below Ms
σ where the flow is controlled by transformation plasticity. The martensite which forms spontaneously on cooling or by stress-assisted
transformation below Ms
σ exhibits a plate morphology. Additional martensite units produced by strain-induced nucleation at shear-band intersections
become important above Ms
σ. Comparison of σ-ε andf-ε curves indicate that a “rule of mixtures” relation based on the “static” strengthening effect of the transformation product
describes the plastic flow behavior reasonably well above Ms
σ, but there is also a dynamic “transformation softening” contribution which becomes dominant below Ms
σ due to the operation of transformation plasticity as a deformation mechanism. Temperature sensitivity of the transformation
kinetics and associated flow behavior is greatest above Ms
σ. Less temperature-sensitive TRIP steels could be obtained by designing alloys to operate with optimum mechanical properties
below Ms
σ. 相似文献
12.
S. Zhou K. Zhang Y. Wang J. F. Gu Y. H. Rong 《Metallurgical and Materials Transactions A》2012,43(3):1026-1034
The designed steel of Fe-0.25C-1.5Mn-1.2Si-1.5Ni-0.05Nb (wt pct) treated by a novel quenching-partitioning-tempering (Q-P-T)
process demonstrates an excellent product of strength and elongation (PSE) at deformed temperatures from 298 K to 573 K (25 °C
to 300 °C) and shows a maximum value of PSE (over 27,000 MPa pct) at 473 K (200 °C). The results fitted by the exponent decay
law indicate that the retained austenite fraction with strain at a deformed temperature of 473 K (200 °C) decreases slower
than that at 298 K (25 °C); namely, the transformation induced plasticity (TRIP) effect occurs in a larger strain range at
473 K (200 °C) than at 298 K (25 °C), showing better mechanical stability. The work-hardening exponent curves of Q-P-T steel
further indicate that the largest plateau before necking appears at the deformed temperature of 473 K (200 °C), showing the
maximum TRIP effect, which is due to the mechanical stability of considerable retained austenite. The microstructural characterization
reveals that the high strength of Q-P-T steels results from dislocation-type martensite laths and dispersively distributed
fcc NbC or hcp ε-carbides in martensite matrix, while excellent ductility is attributed to the TRIP effect produced by considerable retained
austenite. 相似文献
13.
F. C. Levey M. B. Cortie L. A. Cornish 《Metallurgical and Materials Transactions A》2000,31(8):1917-1923
A gold alloy with 18 wt pct Cu and 6 wt pct Al undergoes a reversible displacive phase transformation between an incompletely
ordered L21 parent phase and a tetragonal product. The characteristics of these transformations were studied using acoustic emission,
dilatometry, X-ray diffraction, and metallography. The morphology of the transformation products, the structure of the parent
phase, and the generation of significant acoustic emission during the transformations indicate that they are at least quasi-martensitic,
if not martensitic, and that this system is an example of a β-phase shape-memory alloy (SMA). The onset temperatures of the transformations depend on the prior thermal history of the
sample. The martensite start (M
s
) temperature is between 30 °C and 20 °C. The system exhibits hysteresis and will revert to the parent phase when reheated,
with an austenite start (A
s
) temperature between 55 °C and 80 °C. However, freshly cast or solution-annealed and quenched samples of the alloy do not
transform to the tetragonal phase. Aging of such material at temperatures between 30 °C and 200 °C is required before they
will manifest the displacive transformation. The “martensite” phase is considerably more resistant to aging-induced stabilization
than that of most other SMAs. 相似文献
14.
The formation and microstructure of the granular product and its effect on the mechanical properties of a high-strength, low
alloy steel containing molybdenum and niobium have been investigated. It was found that the granular product “islands” are
composed of both twinned martensite and dislocated martensite. The effect of the granular “islands” on the strength at room
temperature and at 400 °C has been determined. The results showed that the strength increased and both the impact and fracture
toughness decreased as the volume fraction of granular “islands” was increased.In situ fracture studies indicated that the three stages of the microfracture process of the specimen containing granular “islands”
are the initiation of voids at interfaces between the granular “islands” and the bainitic ferrite matrix, followed by void
growth and finally, coalescence by shear. 相似文献
15.
P. Sahu A. S. Hamada T. Sahu J. Puustinen T. Oittinen L. P. Karjalainen 《Metallurgical and Materials Transactions A》2012,43(1):47-55
A high-Mn austenitic steel was deformed in cold rolling to study the martensitic transformation and microstructure using X-ray
diffraction and electron backscatter diffraction. Despite heavy deformation of 70 pct reduction (1.2 true strain), α′-martensite could not be induced in this alloy, but about 90 pct of the austenite transformed to ε-martensite. However, a small fraction (~4 pct) of α′-martensite could be observed when the same alloy was subjected to low strain compression tests in a Gleeble simulator. The
stability of ε-martensite was attributed to the increase in stacking fault energy of the steel, expected to be more than 20 mJ/m2 because of the increase in temperature during the cold rolling deformation. 相似文献
16.
The relationship between heat-treatment parameters and microstructure in titanium alloys has so far been mainly studied empirically,
using characterization techniques such as microscopy. Calculation and modeling of the kinetics of phase transformation have
not yet been widely used for these alloys. Differential scanning calorimetry (DSC) has been widely used for the study of a
variety of phase transformations. There has been much work done on the calculation and modeling of the kinetics of phase transformations
for different systems based on the results from DSC study. In the present work, the kinetics of the β ⇒ α transformation in a Ti-6Al-4V titanium alloy were studied using DSC, at continuous cooling conditions with constant cooling
rates of 5 °C, 10 °C, 20 °C, 30 °C, 40 °C, and 50 °C/min. The results from calorimetry were then used to trace and model the
transformation kinetics in continuous cooling conditions. Based on suitably interpreted DSC results, continuous cooling-transformation
(CCT) diagrams were calculated with lines of isotransformed fraction. The kinetics of transformation were modeled using the
Johnson-Mehl-Avrami (JMA) theory and by applying the “concept of additivity.” The JMA kinetic parameters were derived. Good
agreement between the calculated and experimental transformed fractions is demonstrated. Using the derived kinetic parameters,
the β ⇒ α transformation in a Ti-6Al-4V alloy can be described for any cooling path and condition. An interpretation of the results
from the point of view of activation energy for nucleation is also presented. 相似文献
17.
With the aid of a dislocation model for the stress-strain relationship of α-Fe, analytical expressions for the strain and
temperature dependence of the exponentn in the relation, σ= K · ε
n, are derived. These account quite accurately for experimental results obtained with several low alloy steels. It is shown
thatn varies continuously with strain but that the theoretical and experimental log σ-log ε curve in most cases can be approximated
by two straight lines in accordance with the well-known “double-n” behavior. The strain, ε1 at which the two lines intersect is equal to the strain at which the theoretical n(ε) curve has an inflection point. With
the model presented it is also possible to account for the temperature dependence ofn(ε) and of ε1 within the temperature range −78° to 500°C. 相似文献
18.
The tempering of Fe 1.5 pct N martensite has been studied at temperatures up to 300°C using X-ray and electron microscope
techniques. Stage 1 decomposition occurs below 270°C by the general precipitation, resembling spinodal morphology, of fine
τa" (Fe16 N2) lamellae on 001 habit planes in both matrix and twin crystals of the partially 112 twinned martensite plates. Yet, gaged
by changes in the X-ray spectrum, the reaction is discontinuous, the tetragonal martensite doublets decaying in intensity
without change in their Bragg positions. The anomaly and the failure to detect by electron microscopy regions exhibiting fractional
stages of the fine scale α’
→ α + α" reaction is attributed to its occurrence at different times in different martensite (or parts of martensite) plates. It is
believed that transformation occurs in this manner because the nucleation of coherent α" plates is controlled by the prevailing internal stress field. Thus the time exponent “n” for the reaction decays from a normal
value between 1 and 0.67 to less than 0.3 as stress relief by recovery dominates the more protracted stages of the reaction.
Above 200°C the more stable nitride γ’ (Fe4N) forms at an increasing rate as plates on 012 habit planes, accompanied by marked softening. 相似文献
19.
The isothermal transformation kinetics of austenite decomposition in Fe-0.4C-2.78Mn-1.81Si was analyzed by an electrical resistivity
technique in the temperature interval 723 K to 418 K (450 °C to 145 °C). The analysis of transformation kinetics of the bainite
transformation was performed using the Johnson–Mehl–Avrami–Kolgomorov (JMAK) and Austin–Rickett (AR) approaches. The kinetic
parameters, the reaction constant n, rate constant k = k(T), and apparent activation energy Q were evaluated for isothermal transformations below and above the martensite-start temperature M
S
= 548 K (275 °C), which was determined experimentally. The formation of strain-induced martensite, which starts to accompany
the bainite transformation at just above M
S
, increases the rate of transformation and decreases the apparent activation energy of austenite decomposition. 相似文献
20.
The hcp martensitic transformation and its reversion in an Fe-16.86Mn-4.50Si-10.30Cr-5.29Ni alloy have been studied. The fine
structure and morphologies of ε-martensite were systematically investigated using transmission electron microscopy. It is found that, in the overlapped region
of the stacking faults, the nucleus of ε-martensite may form by shear and grow along the {111}γ plane, which is not the stacking fault plane. The nucleus may grow into a small, thin plate and may act as the basal structure
unit of ε-martensite. When the stress increases, the thin plates continue to grow into an ε-martensite ribbon along this preferred orientation and, during the subsequent recovery annealing, the reverse transformation
of these basal structure units occurs. As a result, the ε-martensite plates shrink in both length and thickness. 相似文献