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1.
Yaojun Lin Yizhang Zhou Enrique J. Lavernia 《Metallurgical and Materials Transactions A》2004,35(10):3251-3263
A fundamental study of the factors that govern grain size of 5083 Al processed via reactive atomization and deposition (RAD) is reported. Microstructural observation shows that the average grain size in RAD
5083 Al is slightly smaller than that in the material processed via N2 spray deposition (SDN). A numerical approach, together with measurements of the temperature histories inside the deposited
materials, is implemented to analyze the influence of in-situ reactions during RAD process on the evolution of grain size. The numerical results show that RAD 5083 Al possesses a slightly
higher density of nuclei relative to that present in SDN 5083 Al on a per unit volume of deposited material basis at the beginning
of the slow solidification of remaining liquid phase. Furthermore, the RAD material exhibits a slightly lower coarsening extent
during the slow solidification. Grain growth is negligible during the solid-phase cooling. Accordingly, the calculated grain
size in RAD 5083 Al is slightly smaller than that in SDN 5083 Al, consistent with the observed results. 相似文献
2.
T. Koseki T. Matsumiya W. Yamada T. Ogawa 《Metallurgical and Materials Transactions A》1994,25(6):1309-1321
A computational method for the analysis of phase transformation involving solidification was developed with the assumption
of thermodynamic equilibria at interfaces. The region of interest was divided into finite segments, and solute diffusion across
the segments was computed by the use of the direct finite difference method (FDM). Simultaneously, thermodynamic equilibrium
at each interface was updated at every step of the diffusion analysis to determine the location of the interfaces. The temperature
decrease and the increment of fraction solid were calculated based on thermal balance, including a heat extraction condition.
Solid state transformation from δ to γ phase within each FDM segment was modeled by the use of a Clyne-Kurz (C-K) type analysis
with assumptions of complete mixing of solutes in theδ phase and limited back diffusion in theγ phase. The calculation results were compared with welding solidification experiments in the iron-chromium-nickel ternary
system. Good agreement was obtained with respect to solute distribution and residual fraction ofδ phase over different compositions and solidification modes of the alloys used. 相似文献
3.
Abdulhaqq A. Hamid S. C. Jain P. K. Ghosh Subrata Ray 《Metallurgical and Materials Transactions A》2006,37(2):469-480
In-situ particle-reinforced aluminum alloy-based cast composites have been synthesized by solidification of the slurry obtained by
dispersion of externally added titanium dioxide (TiO2) particles in molten aluminum at different processing temperatures. Alumina particles (Al2O3) form in situ through chemical reaction of TiO2 particles with molten aluminum. Simultaneously, the chemical reaction also releases titanium, which dissolves into molten
aluminum and results in the formation of intermetallic phase Ti(Al1−x
,Fe
x
)3 during solidification. Increasing the processing temperature increases (1) the amount of elongated as well as blocky intermetallic
phase Ti(Al1−x
,Fe
x
)3, (2) the proportion of alumina particles in the reinforcing oxides, and (3) the porosity content in the resulting cast in-situ composite. The difference in particle content and porosity between the top and the bottom of the cast ingot increases with
increasing processing temperature. The hardness of the cast in-situ composite is significantly more than that of the matrix alloy due to the presence of reinforcing particles, but the hardness
is greatly impaired by the presence of porosity at the top of the cast ingot. The percent elongation of the cast in-situ composite decreases with increasing processing temperature possibly due to increasing porosity as well as an increasing amount
of elongated intermetallic phase, which affects the percent elongation of the matrix alloy. The tensile and yield stresses
of the cast in-situ composite decreases with increasing processing temperature again due to increasing porosity, which affects the ultimate tensile
stress more than the yield stress. In the cast in-situ composite containing 3.31 ± 0.77 vol pct of porosity, the Brinell hardness is about 6 times its yield stress. The estimated
yield stress of the cast in-situ composite at zero porosity as given by the linear least-squares fit appears to increase with particle content at a significantly
higher rate than that predicted by the shear-lag model. 相似文献
4.
The principle of electromagnetic separation of phases (primary phase) in alloy melt is that the electromagnetic force scarcely
acts on the primary phases due to its low electric conductivity as compared to the melt. As a result, a repulsive force acts
on the primary iron-rich phases to push them to move in the direction opposite to that of the electromagnetic force. The in-situ surface composite and the functionally gradient composite reinforced by primary Si are produced when the hypereutectic Al-Si
alloy solidifies under electromagnetic force induced by static magnetic field and DC current. Similarly, the Al-Si-1.20 pct
Fe-1.60 pct Mn alloy in-situ surface composite reinforced by primary iron-rich phase is produced. Based on this, a new method for production of in-situ multigradient composite with several layers, by electromagnetic separation of phases and directional solidification technique,
is proposed. 相似文献
5.
D. Ma A. D. Stoica K. An L. Yang H. Bei R. A. Mills H. Skorpenske X.-L. Wang 《Metallurgical and Materials Transactions A》2011,42(6):1444-1448
Time-of-flight neutron diffraction was used to study in-situ texture evolution and the α → β phase transformation in cold-drawn titanium upon continuous heating. The texture changes in the α phase at elevated temperatures upon recrystallization are presented. For the first time, a transient β texture was observed during the α → β transformation, as indicated by the initial rise and the final drop of the {110}
β
reflection intensity. This unusual observation is explained in terms of competitive growth between inter- and intragranular
β allotriomorphs. 相似文献
6.
A. Kumar S. Mishra T. Debroy J. W. Elmer 《Metallurgical and Materials Transactions A》2005,36(1):15-22
A nonisothermal Johnson-Mehl-Avarami (JMA) equation with optimized JMA parameters is proposed to represent the kinetics of
transformation of α-ferrite to γ-austenite during heating of 1005 steel. The procedure used to estimate the JMA parameters involved a combination of numerical
heat-transfer and fluid-flow calculations, the JMA equation for nucleation and growth for nonisothermal systems, and a genetic
algorithm (GA) based optimization tool that used a limited volume of experimental kinetic data. The experimental data used
in the calculations consisted of phase fraction of γ-austenite measured at several different monitoring locations in the heat-affected zone (HAZ) of a gas tungsten arc (GTA)
weld in 1005 steel. These data were obtained by an in-situ spatially resolved X-ray diffraction (SRXRD) technique using synchrotron radiation during welding. The thermal cycles necessary
for the calculations were determined for each monitoring location from a well-tested three-dimensional heat-transfer and fluid-flow
model. A parent centric recombination (PCX) based generalized generation gap (G3) GA was used to obtain the optimized values
of the JMA parameters, i.e., the activation energy, pre-exponential factor, and exponent in the nonisothermal JMA equation. The GA based determination
of all three JMA equation parameters resulted in better agreement between the calculated and the experimentally determined
austenite phase fractions than was previously achieved. 相似文献
7.
8.
Georg Ehlen Andreas Ludwig Peter R. Sahm 《Metallurgical and Materials Transactions A》2003,34(12):2947-2961
The shape and depth of the area molten during a welding process is of immense technical importance. This study investigates
how the melt pool shape during laser welding is influenced by Marangoni convection and tries to establish general qualitative
rules of melt pool dynamics. A parameter study shows how different welding powers lead to extremely different pool shapes.
Special attention is paid to transient effects that occur during the melting process as well as after switching off the laser
source. It is shown that the final pool shape can depend strongly on the welding duration. The authors use an axisymmetric
two-dimensional (2-D) control-volume-method (CVM) code based on the volume-averaged two-phase model of alloy solidification
by Ni and Beckermann[1] and the SIMPLER algorithm by Patankar.[2] They calculate the transient distribution of temperatures, phase fractions, flow velocities, pressures, and concentrations
of alloying elements in the melt and two solid phases (peritectic solidification) for a stationary laser welding process.
Marangoni flow is described using a semiempirical model for the temperature-dependent surface tension gradient. The software
was parallelized using the shared memory standard OpenMP. 相似文献
9.
J. W. Elmer S. M. Allen T. W. Eagar 《Metallurgical and Materials Transactions A》1989,20(10):2117-2131
The microstructures that develop during the solidification of stainless steel alloys are related to the solidification conditions
and the specific alloy composition. The solidification conditions are determined by the processing method,i.e., casting, welding, or rapid solidification, and by parametric variations within each of these techniques. One variable that
has been used to characterize the effects of different processing conditions is the cooling rate. This factor and the chemical
composition of the alloy both influence (1) the primary mode of solidification, (2) solute redistribution and second-phase
formation during solidification, and (3) the nucleation and growth behavior of the ferrite-to-austenite phase transformation
during cooling. Consequently, the residual ferrite content and the microstructural morphology depend on the cooling rate and
are governed by the solidification process. This paper investigates the influence of cooling rate on the microstructure of
stainless steel alloys and describes the conditions that lead to the many microstructural morphologies that develop during
solidification. Experiments were performed on a series of seven high-purity Fe-Ni-Cr alloys that spanned the line of twofold
saturation along the 59 wt pct Fe isopleth of the ternary alloy system. High-speed electron-beam surface-glazing was used
to melt and resolidify these alloys at scan speeds up to 5 m/s. The resulting cooling rates were shown to vary from 7°C/s
to 7.5×106°C/s, and the resolidified melts were analyzed by optical metallographic methods. Five primary modes of solidification and
12 microstructural morphologies were characterized in the resolidified alloys, and these features appear to be a complete
“set” of the possible microstructures for 300-series stainless steel alloys. The results of this study were used to create
electron-beam scan speedvs composition diagrams, which can be used to predict the primary mode of solidification and the microstructural morphology
for different processing conditions. Furthermore, changes in the primary solidification mode were observed in alloys that
lie on the chromium-rich side of the line of twofold saturation when they are cooled at high rates. These changes were explained
by the presence of metastable austenite, which grows epitaxially and can dominate the solidification microstructure throughout
the resolidified zone at high cooling rates.
J. W. ELMER, formerly Graduate Student at the Massachusetts Institute of Technology 相似文献
10.
Weld solidification structure of three different types of stainless steel,i.e., 310 austenitic, 309 and 304 semiaustenitic, and 430 ferritic, was investigated. Welds of each material were made without
any quenching, with water quenching, and with liquid-tin quenching during welding. The weld micro-structure obtained was explained
with the help of the pseudobinary phase diagrams for Fe-Cr-Ni and Fe-Cr-C systems. It was found that, due to the postsolidification
5 → γ phase transformation in 309 and 304 stainless steels and the rapid homogenization of microsegregation in 430 stainless
steel, their weld solidification structure could not be observed unless quenched from the solidification range with liquid
tin. Moreover, the formation of acicular austenite, and hence, martensite, at the grain boundaries of 430 stainless steel
welds was eliminated completely when quenched with liquid tin. The weld solidification structure of 310 stainless steel, on
the other hand, was essentially unaffected by quenching. Based upon the observations made, the weld microstructure of these
stainless steels was summarized. The effect of cooling rate on the formation of primary austenite in 309 stainless steel welds
was discussed. Finally, a simple method for determining the relationship between the secondary dendrite arm spacing and the
solidification time, based on welding speeds and weld pool configurations, was suggested. 相似文献
11.
M. J. Perricone J. N. Dupont T. D. Anderson C. V. Robino J. R. Michael 《Metallurgical and Materials Transactions A》2011,42(3):700-716
A series of 31 Mo-bearing stainless steel compositions with Mo contents ranging from 0 to 10 wt pct and exhibiting primary
δ-ferrite solidification were analyzed over a range of laser welding conditions to evaluate the effect of composition and cooling
rate on the solid-state transformation to γ-austenite. Alloys exhibiting this microstructural development sequence are of particular interest to the welding community
because of their reduced susceptibility to solidification cracking and the potential reduction of microsegregation (which
can affect corrosion resistance), all while harnessing the high toughness of γ-austenite. Alloys were created using the arc button melting process, and laser welds were prepared on each alloy at constant
power and travel speeds ranging from 4.2 to 42 mm/s. The cooling rates of these processes were estimated to range from 10 K
(°C)/s for arc buttons to 105 K (°C)/s for the fastest laser welds. No shift in solidification mode from primary δ-ferrite to primary γ-austenite was observed in the range of compositions or welding conditions studied. Metastable microstructural features were
observed in many laser weld fusion zones, as well as a massive transformation from δ-ferrite to γ-austenite. Evidence of epitaxial massive growth without nucleation was also found when intercellular γ-austenite was already present from a solidification reaction. The resulting single-phase γ-austenite in both cases exhibited a homogenous distribution of Mo, Cr, Ni, and Fe at nominal levels. 相似文献
12.
Nbss/Nb3Al in-situ composite with the nominal composition of Nb-16 mol pct Al-1 mol pct B, consisting of bcc niobium solid solution (Nbss) and A15 ordered Nb3Al, was synthesized by arc melting, homogenization annealing, and isothermal forging, and their superplastic deformation behavior
was investigated by tensile tests and microstructure observations. Maximum superplastic elongation over 750 pct was obtained
at 1573 K and at a strain rate of 1.6 × 10−4 s−1 for as-forged specimens. Phase transformation from Nbss to Nb3Al was observed to occur during superplastic deformation. Dynamic phase transformation during superplastic deformation progresses
more quickly than static phase transformation during annealing without applied stress. Dynamic phase transformation is accompanied
by phase-boundary migration, which operates as an accommodation process of grain-boundary sliding. Dislocation creep dominates
deformation and grain-boundary sliding is inhibited at a high strain rate, while grain-boundary sliding and cavity formation
are promoted at a low strain rate because of insufficient accommodation of grain-boundary sliding arising from sluggish dynamic
phase transformation. It is concluded that there exists an optimum strain rate that guarantees the grain-boundary sliding
and the rapid dynamic phase transformation to achieve maximum superplastic elongation. 相似文献
13.
《Canadian Metallurgical Quarterly》2013,52(2):145-152
AbstractElectrospark deposition is a microarc welding process that has been successfully used to deposit an autogenous ultrafine NiCoCrAlY coating. Primary solidification of the β phase shows epitaxial growth where orientation and heat flux coincide. Electron channelling contrast imaging of the deposits shows a homogeneous distribution of non-stoichiometric phases from the rapid solidification of the electrospark deposition process. Electron backscatter diffraction mapping of the deposit indicates that an ultrafine β structure can grow epitaxially where favourable conditions prevail. The equiaxed structure associated with the γ to β solidification shift was not observed and is believed to result from the homogeneous electrode and substrate compositions. Oxidation of the deposit shows advantageous alumina growth stemming from the ultrafine aluminium rich β phase. The transition from θ- to α-alumina was nearly complete in only 6 h at 1000°C in air. 相似文献
14.
Ragnvald H. Mathiesen Lars Arnberg Kjell Ramsøskar Timm Weitkamp Ph.D. Student Christoph Rau Anatoly Snigirev 《Metallurgical and Materials Transactions B》2002,33(4):613-623
Time-resolved direct-beam X-ray imaging, with intense, coherent, and monochromatic third-generation synchrotron radiation,
and a high-resolution fast-readout detector system have been used for in-situ studies of dendritic and eutectic growth processes in Al-Cu alloys. Temporal and spatial resolutions down to 0.25 seconds
and 2.5 μm, respectively, were obtained with a field of view up to 1.4×1.4 mm2. Solid-liquid interfaces and various phase-specific segregates could be observed, and their dynamics could be traced in a
sequence of temporally resolved images formed by phase and amplitude contrast from the sample. This article does not present
any detailed analysis of a specific solidification phenomenon; instead, it presents to the scientific community an innovative
technique for in-situ monitoring of such a phenomenon in real metallic systems. 相似文献
15.
Solidification of an alloy 625 weld overlay 总被引:1,自引:0,他引:1
J. N. DuPont 《Metallurgical and Materials Transactions A》1996,27(11):3612-3620
The solidification behavior (microsegregation, secondary phase formation, and solidification temperature range) of an Alloy
625 weld overlay deposited on 2.25Cr - 1Mo steel by gas metal arc welding was investigated by light and electron optical microscopy,
electron microprobe, and differential thermal analysis techniques. The overlay deposit was found to terminate solidification
at ≈ 1216 °C by aγ/Laves eutectic-type reaction. The Laves phase was highly enriched in Nb, Mo, and Si. The solidification reaction and microsegregation
potential of major alloying elements in the overlay deposit are compared to other Nb-bearing Ni base alloys and found to be
very similar to those for Alloy 718. Solidification cracks observed in the overlay were attributed to the wide solidification
temperature range (≈170 °C) and formation of interdendritic (γ+Laves) constituent. Reasonable agreement is obtained between the calculated and measured volume percent (γ+Laves) constituent with the Scheil equation by treating the overlay system as a simpleγ-Nb “binary” and using an experimentally determinedk
Nb value from electron microprobe data. 相似文献
16.
The phase-transformation temperatures of a nickel-titanium-based shape-memory alloy (SMA) were initially evaluated under stress-free
conditions by the differential scanning calorimetric (DSC) technique. Results show that the phase-transformation temperature
is significantly higher for the transition from detwinned martensite to austenite than for that from twinned martensite (or
R phase) to austenite. To further examine transformation temperatures as a function of initial state, a tensile-test apparatus
with in-situ electrical resistance (ER) measurements was used to evaluate the transformation properties of SMAs at a variety of stress
levels and initial compositions. The results show that stress has a significant influence on the transformation of detwinned
martensite, but a small influence on the R-phase and twinned martensite transformations. The ER changes linearly with strain during the transformations from both kinds
of martensite to austenite. The linearity between the ER and strain during the transformation from detwinned martensite to
austenite is not affected by the stress, facilitating application to control algorithms. A revised phase diagram is drawn
to express these results. 相似文献
17.
Olivire Ludwig Marco Dimichiel Luc Salvo Michel Suéry Peter Falus 《Metallurgical and Materials Transactions A》2005,36(6):1515-1523
This article presents the first results of a new experimental technique developed to investigate the evolution of the morphology
of the solid and liquid phases during the solidification of a metallic alloy. It consists of ultrafast X-ray microtomography
observations of a solidifying aluminum-copper alloy carried out at ESRF. These experiments allow investigating in-situ the formation of the casting microstructure and of the evolution of the morphology of the solid and the liquid phases. It
allows also the in-situ determination of the solidification path, of the variation of the copper content in both the liquid and solid phases, and
of some other characteristic parameters of the microstructure. Provided that some forthcoming technical improvements on the
experimental setup are performed, more quantitative results can be obtained as well as better image quality and resolution. 相似文献
18.
In situ dynamic observation of the progress of a peritectic reaction and transformation of Fe-(0.14 pct C)- and Fe-(0.42 pct C)-peritectic
Fe-C alloys has been successfully made with a combination of a confocal scanning laser microscope and an infrared image furnace.
The peritectic reaction is characterized by the formation of the γ-austenite phase at the junction of the liquid and the grain boundary of δ-ferrite crystals and subsequent propagation of the three-phase point, liquid/γ/δ, along the liquid/δ boundary, whereas the peritectic transformation occurs by the thickening of the intervening γ toward both the liquid side and the δ side. The rates of the peritectic reaction for the two peritectic alloys are found to be much faster than the rate that would
be controlled by carbon diffusion, suggesting that either massive transformation to γ or solidification as γ controls the rate. This is also the case for the Fe-0.14%C transformation in the hypoperitectic alloy. However, the rate
of the peritectic transformation in the Fe-0.42%C alloy is determined by carbon diffusion.
This article is based on a presentation made in the “Geoffrey Belton Memorial Symposium,” held in January 2000, in Sydney,
Australia, under the joint sponsorship of ISS and TMS. 相似文献
19.
V. I. Savran S. E. Offerman J. Sietsma 《Metallurgical and Materials Transactions A》2010,41(3):583-591
Austenite nucleation and growth is studied during continuous heating using three-dimensional X-ray diffraction (3-D XRD) microscopy
at the European Synchrotron Radiation Facility (ESRF) (Grenoble, France). Unique in-situ observations of austenite nucleation and growth kinetics were made for two commercial medium-carbon low-alloy steels (0.21
and 0.35 wt pct carbon with an initial microstructure of ferrite and pearlite). The measured austenite volume fraction as
a function of temperature shows a two-step behavior for both steel grades: it starts with a rather fast pearlite-to-austenite
transformation, which is followed by a more gradual ferrite-to-austenite transformation. The austenite nucleus density exhibits
similar behavior, with a sharp increase during the first stage of the transformation and a more gradual increase in the nucleus
density in the second stage for the 0.21 wt pct carbon alloy. For the 0.35 wt pct carbon alloy, no new nuclei form during
the second stage. Three different types of growth of austenite grains in the ferrite/pearlite matrix were observed. The combination
of detailed separate observations of both nucleation and growth provides unique quantitative information on the phase transformation
kinetics during heating, i.e., austenite formation from ferrite and pearlite. 相似文献
20.
A. A. B. Sugden H. K. D. H. Bhadeshia 《Metallurgical and Materials Transactions A》1988,19(3):669-674
Nonmetallic inclusions in low-alloy steel welds have an important effect on the microstructure and properties of weld deposits.
This work is an attempt at understanding the factors controlling the spatial distribution of such inclusions, with particular
emphasis on the uniformity of the distribution and the effect of solidification mode during manual-metal-arc welding. The
solidification mode has been controlled by using unusual combinations of base plates and experimental electrodes. It is found
that the first phase to solidify (in the form of columnar grains) is delta-ferrite (δ) when a medium carbon electrode is deposited onto a low carbon substrate, but that it is austenite (γ) when a low carbon electrode is deposited onto a high carbon substrate. Relatively large inclusions have been found to position
themselves preferentially, during solidification, to the columnar grain boundaries of the first phase to solidify, whether
this is 8-ferrite or austenite. The results can be understood qualitatively in terms of a surface tension driven Marangoni
effect, or in terms of the pushing of solid inclusions by the solidification front. Both mechanisms drive the larger inclusions
into cusps in the interface while smaller ones are passively trapped. The implications of the observed nonuniform distribution
of inclusions are more severe for solidification with austenite as the primary phase, since the larger inclusions are in that
case located in the weakest region of the weld where they also do not contribute to the intragranular nucleation of acicular
ferrite. 相似文献