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1.
L. Zhang 《Metallurgical and Materials Transactions B》2013,44(2):390-395
Pulsed magnetic field (PMF) processing has been employed for refining the microstructure of eutectic (Al-12.4Si) Al-Si alloy in the current study. The effect of PMF on microstructure and mechanical properties of eutectic Al-Si alloy was studied. The results show that the morphology of primary α-Al was refined from coarse columnar dendrites to fine equiaxed dendrites by PMF treatment. Fine short rod-like or rounded particle-like eutectic silicon was formed during solidification of eutectic Al-Si alloy treated by PMF. PMF treatment reduced the size of eutectic silicon from 49 to 2.3 μm in length, and the width from 3.1 to 0.6 μm. The aspect ratio of eutectic silicon was also reduced by PMF treatment from slightly less than 16 to slightly less than 4. The ultimate tensile strength and elongation of eutectic Al-Si alloy with PMF treatment at room temperature were about 201 MPa and 8.8 pct, respectively, which were increased by 47 and 73 pct, respectively, compared with the eutectic Al-Si alloy without PMF treatment. 相似文献
2.
This study examined the relationship between solidification structure and graphitization characteristics of white cast iron
strips produced by strip casting. Experimental results showed that there was an unusual distribution of temper graphite particles
along the through-thickness direction of the graphitized strips in comparison with gravity-cast chill plate. In particular,
the graphite-free zones appeared in the vicinity of the strip surface after the completion of graphitization, especially in
the strip with low carbon and silicon content. There were abnormally straight interfaces between matrix and eutectic cementite
with a strong preferred [001]c growth direction caused by the effect of directional solidification found in the near-surface regions of the strips. The
interfaces did not form a site for the graphite to nucleate and gave rise to the graphite-free zones close to the strip surface.
An increase in carbon and silicon content could significantly increase the number of temper graphite particles and shorten
the time for the completion of graphitization, but an inhomogeneous distribution feature of graphite particles was still observed
in strips with a higher carbon equivalent value (CE). Furthermore, variations in carbon and silicon content resulted in transitions
in carbide morphology and composition, which had a tremendous effect on the graphitization characteristics of the cast iron
strips. 相似文献
3.
Chromium white cast irons are widely used in environments where severe abrasion resistance is a dominant requirement. To improve
the wear resistance of these commercially important irons, the United States Bureau of Mines and CSIRO Australia are studying
their solidification and solid-state transformation kinetics. A ternary Fe-Cr-C iron with 17.8 wt pct (pct) Cr and 3.0 pct
C was compared with commercially available irons of similar Cr and C contents with Si contents between 1.6 and 2.2 pct. The
irons were solidified and cooled at rates of 0.03 and 0.17 K · s-1 to 873 K. Differential thermal analysis (DTA) showed that Si depresses the eutectic reaction temperature and suggests that
is has no effect upon the volume of eutectic carbides formed during solidification. Microprobe analysis revealed that austenite
dendrites within the Si alloyed irons cooled at 0.03 and 0.17 K·s-1 had C and Cr contents that were lower than those of dendrites within the ternary alloy cooled at the same cooling rate and
a Si alloyed iron that was water quenched from the eutectic temperature. These lower values were shown by image analysis to
be the result of both solid-state growth (coarsening) of the eutectic carbides and some secondary carbide formation. Hardness
measurements in the as-cast condition and after soaking in liquid nitrogen suggest an increase in the martensite start temperature
as the Si content was increased. It is concluded that Si’s effect on increasing the size and volume fraction of eutectic carbides
and increasing the matrix hardness should lead to improved wear resistance over regular high-chromium white cast irons. 相似文献
4.
The density of liquid and solid hypoeutectic aluminum-silicon alloys has been measured with high accuracy in the temperature
range 400 °C to 800 °C by using the indirect Archimedian method. A eutectic mixture of KCl and LiCl salts was used as reference
liquid. This method allowed measurements of density for both liquid and solid in the same experiment and thus reduced the
systematic error in estimating the solidification shrinkage. The results show that the density of liquid aluminum-silicon
alloy increases with increasing silicon content, while silicon reduces the density in the solid state. Silicon content reduces
the solidification shrinkage from 6.6 pct for pure aluminum to 4.4 pct for Al-11.6 pct Si. 相似文献
5.
The formation and intergrowth of granular eutectic in austenitic steel matrix composite has been studied by directional solidification
technology. The results indicate that the modifying element Si enhances the dendritic segregation of C and Mn. The surface
active elements, such as Y and Ca, concentrate highly ahead of the solid-liquid (S-L) interface of the composite due to the
nonequilibrium solidification. As a result, the S-L interface of the composite is unstable during solidification. The spatiotemporal
condition of the formation and the growth of the granular eutectic is the formation of granular eutectic between austenitic
dendrite arms at the end of solidification and its growth restricted by the austenitic dendrites. By the simulating eutectic
growth of the granular eutectic by Fe−C−Mn alloy, the Si, Ca, and Y adsorb and enrich on the growing surface of the eutectic
during crystallization, which makes the crystallization model of the eutectic turn from facet/nonfacet to nonfacet/nonfacet.
The intergrowth of the eutectic can be explained by (1) the influence of the modifying elements on the crystallization of
the eutectic, (2) the coarse solidification growth interface of the eutectic and the same growth rate for austenite and cementite
((Fe, Mn)3C), and (3) the austenite and cementite ((Fe, Mn)3C) have not lateral branch during eutectic growth. 相似文献
6.
R. P. Liu D. M. Herlach M. Vandyoussefi A. L. Greer 《Metallurgical and Materials Transactions A》2004,35(2):607-612
Electromagnetic levitation is applied to achieve containerless solidification of 10-mm-diameter droplets of Al-50 at. pct
Si. A maximum undercooling of 320 K is obtained. Phase morphologies on the droplet surfaces and on the deeply etched sections
of the samples solidified at different undercoolings are examined by scanning electron microscopy. The primary silicon shows
well-developed faceted dendrites at a small undercooling, but a fine granular form at a large undercooling. Stratified deposits
of aluminum are found within the primary silicon plates, arising from solute pileup during growth. The microstructural refinement
at a large undercooling has its origins in solute restriction of crystal growth and in fragmentation of the primary silicon
dendrites. The form of the Al-Si eutectic is also found to be changed into an anomalous form at a large undercooling. 相似文献
7.
J. A. Taylor G. B. Schaffer D. H. StJohn 《Metallurgical and Materials Transactions A》1999,30(6):1651-1655
The mechanism by which iron causes casting defects in the AA309 (Al-5 pct Si-1.2 pct Cu-0.5 pct Mg) may be related to the
solidification sequence of the alloy. Superimposing calculated segregation lines on the liquidus projection of the ternary
Al-Si-Fe phase diagram suggests that porosity is minimized at a critical iron content when solidification proceeds directly
from the primary field to the ternary Al-Si-βAl5FeSi eutectic point. Solidification via the binary Al-βAl5FeSi eutectic is detrimental to casting integrity. This hypothesis was tested by comparing the critical iron content observed
in the standard AA309 alloy to that of a high-silicon (10 pct Si) variant of this alloy. 相似文献
8.
P. K. Rohatgi R. C. Sharma K. V. Prabhakar 《Metallurgical and Materials Transactions A》1975,6(2):569-575
Al-10.98 pct Si-4.9 pct Ni ternary eutectic alloy was unidirectionally solidified at growth rates from 1.39μm/sec to 6.95μm/sec.
Binary Al-Ni and Al-Si eutectics prepared from the same purity metals were also solidified under similar conditions to characterize
the growth conditions under the conditions of present study. NiAl3 phase appeared as fibers in the binary Al-Ni eutectic and silicon appeared as irregular plates in the binary Al-Si eutectic.
However, in the ternary Al-Si-Ni eutectic alloy both NiAl3 and silicon phases appeared as irregular plates dispersed in α-Al phase, without any regular repctitive arrangement. The
size and spacing of NiAl3 and Si platelets in cone shaped colonies decreased with an increase in the growth rate of the ternary eutectic. Examination
of specimen quenched during unidirectional solidification indicated that the ternary eutectic grows with a non-planar interface
with both Si and NiAl3 phases protruding into the liquid. It is concluded that it will be difficult to grow regular ternary eutectic structures
even if only one phase has a high entropy of melting. The tensile strength and modulus of unidirectionally solidified Al-Si-Ni
eutectic was lower than the chill cast alloys of the same composition, and decreased with a decrease in growth rate. Tensile
modulus and strength of ternary Al-Si-Ni eutectic alloys was greater than binary Al-Si eutectic alloy under similar growth
conditions, both in the chill cast and in unidirectionally solidified conditions. 相似文献
9.
P. K. Rohatgi R. C. Sharma K. V. Prabhakar 《Metallurgical and Materials Transactions A》1975,6(3):569-575
Al-10.98 pct Si-4.9 pct Ni ternary eutectic alloy was unidirectionally solidified at growth rates from 1.39μm/sec to 6.95μm/sec. Binary Al-Ni and Al-Si eutectics prepared from the same purity metals were also solidified under similar conditions to characterize the growth conditions under the conditions of present study. NiAl3 phase appeared as fibers in the binary Al-Ni eutectic and silicon appeared as irregular plates in the binary Al-Si eutectic. However, in the ternary Al-Si-Ni eutectic alloy both NiAl3 and silicon phases appeared as irregular plates dispersed in α-Al phase, without any regular repctitive arrangement. The size and spacing of NiAl3 and Si platelets in cone shaped colonies decreased with an increase in the growth rate of the ternary eutectic. Examination of specimen quenched during unidirectional solidification indicated that the ternary eutectic grows with a non-planar interface with both Si and NiAl3 phases protruding into the liquid. It is concluded that it will be difficult to grow regular ternary eutectic structures even if only one phase has a high entropy of melting. The tensile strength and modulus of unidirectionally solidified Al-Si-Ni eutectic was lower than the chill cast alloys of the same composition, and decreased with a decrease in growth rate. Tensile modulus and strength of ternary Al-Si-Ni eutectic alloys was greater than binary Al-Si eutectic alloy under similar growth conditions, both in the chill cast and in unidirectionally solidified conditions. 相似文献
10.
Q. G. Wang C. H. Caceres J. R. Griffiths 《Metallurgical and Materials Transactions A》2003,34(12):2901-2912
The strain dependence of particle cracking in aluminum alloys A356/357 in the T6 temper has been studied in a range of microstructures
produced by varying solidification rate and Mg content, and by chemical (Sr) modification of the eutectic silicon. The damage
accumulates linearly with the applied strain for all microstructures, but the rate depends on the secondary dendrite arm spacing
and modification state. Large and elongated eutectic silicon particles in the unmodified alloys and large π-phase (Al9FeMg3Si5) particles in alloy A357 show the greatest tendency to cracking. In alloy A356, cracking of eutectic silicon particles dominates
the accumulation of damage while cracking of Fe-rich particles is relatively unimportant. However, in alloy A357, especially
with Sr modification, cracking of the large π-phase intermetallics accounts for the majority of damage at low and intermediate strains but becomes comparable with silicon
particle cracking at large strains. Fracture occurs when the volume fraction of cracked particles (eutectic silicon and Fe-rich
intermetallics combined) approximates 45 pct of the total particle volume fraction or when the number fraction of cracked
particles is about 20 pct. The results are discussed in terms of Weibull statistics and existing models for dispersion hardening. 相似文献
11.
J. J. Coronado A. Sinatora E. Albertin 《Metallurgical and Materials Transactions B》2014,45(3):811-814
Two hypereutectic cast irons (5.01 pct Cr and 5.19 pct V) were cast and the polished surfaces of test pieces were deep-etched and analyzed via scanning electron microscopy. The results show that graphite lamellae intersect the cementite and a thin austenite film nucleates and grows on the cementite plates. For both compositions, graphite and cementite can coexist as equilibrium phases, with the former always nucleating and growing first. The eutectic carbides grow from the austenite dendrites in a direction perpendicular to the primary plates. 相似文献
12.
Y. Wu T. J. Piccone Y. Shiohara M. C. Flemings 《Metallurgical and Materials Transactions A》1988,19(4):1109-1119
Studies were made of structure and solute distribution in undercooled droplets of nickel-25 wt pct tin alloy and the eutectic
nickel-32.5 wt pct tin alloy. Structures of levitation melted droplets of the Ni-25 wt pct Sn alloy showed a gradual and continuous
transition from dendritic to fine-grained spherical with increasing initial undercooling up to about 180 K. Results suggest
that all samples solidified dendritically and that the final structures obtained were largely the result of ripening. Experimental
data on minimum solute composition in the samples produced are bounded by two calculated curves, both of which assume equilibrium
at all liquid-solid interfaces during recalescence and subsequent cooling. One assumes complete diffusion in the solid during
recalescence; the other assumes limited diffusion, but partial remelting to avoid superheating of the solid. Several observations
support the view that the eutectic alloy solidifies dendritically, much as the hypoeutectic alloy does. Surface dendrites
were seen in regions of surface shrinkage cavities and a coarse “dendritic” structure can be discerned on polished sections,
which seems to correspond to the large surface “dendrites” seen by high-speed photographs of the hypoeutectic alloy. The structure
of highly undercooled eutectic samples is composed fully of an anomalous eutectic. Samples solidified with intermediate amounts
of undercooling possess some lamellar eutectic which, it is believed, solidified after recalescence was complete. 相似文献
13.
《Journal of Light Metals》2001,1(4):219-228
The eutectic solidification mechanisms in an A356.0 (Al–7%Si–Mg) alloy modified by barium, calcium, yttrium and ytterbium have been determined. The crystallographic orientations of aluminium in the eutectic and the surrounding aluminium dendrites were measured by electron backscattering diffraction mapping, and samples were also quenched at different stages during the eutectic arrest and examined by optical microscopy. The combination of these two techniques shows that each of the elements added promote heterogeneous nucleation of eutectic grains in the interdendritic liquid, while the aluminium in the unmodified alloy grows epitaxially from the dendrites. Furthermore, calcium and yttrium result in a strong dependency of eutectic solidification on the thermal gradient, i.e. the eutectic evolves from the walls towards the centre of the sample on a macro-scale. These differences in eutectic solidification mode show a correlation with some thermal characteristics of the eutectic arrest. 相似文献
14.
A. Bedolla-Jacuinde M. W. Rainforth I. Mejía 《Metallurgical and Materials Transactions A》2013,44(2):856-872
This work analyzes the effect of different additions of silicon (0 to 5.0 pct) on the structure of a high-Chromium white cast iron, with chromium content of 16.8 pct and carbon 2.56 pct. The alloys were analyzed in both as-cast and heat-treated conditions. Casting was undertaken in metallic molds that yielded solidification rates faster than in commercial processes. Nevertheless, there was some degree of segregation of silicon; this segregation resulted in a refinement in the microstructure of the alloy. Silicon also generated a greater influence on the structure by destabilizing the austenitic matrix, and promoted greater precipitation of eutectic carbides. Above 3 pct silicon, pearlite formation occurred in preference to martensite. After the destabilization heat treatment, the matrix structure of the irons up to 3 pct Si consisted of secondary carbides in a martensitic matrix with some retained austenite; higher Si additions produced a ferritic matrix. The different as-cast and heat-treated microstructures were correlated with selected mechanical properties such as hardness, matrix microhardness, and fracture toughness. Silicon additions increased matrix microhardness in the as-cast conditions, but the opposite phenomenon occurred in the heat-treated conditions. Microhardness decreased as silicon content was increased. Bulk hardness showed the same behavior. Fracture toughness was observed to increase up to 2 pct Si, and then decreased for higher silicon contents. These results are discussed in terms of the effect of eutectic carbides’ size and the resulting matrix due to the silicon additions. 相似文献
15.
Hypereutectic Al-18 wt pct Si alloy is widely used in automotive industry as a wear-resistant alloy for engine components. However, in the last few years, this traditional composition is being considered for processing by different rapid solidification methods. Positive points include its low thermal expansion and uniform distribution of surface oxides. Nevertheless, the microstructural aspects of Al-Si powders of 18 wt pct Si are still need to be addressed, such as, the eutectic Si morphology, size, and distribution generated by different process conditions during rapid solidification. Based on a detailed quantitative analysis of the microstructures of rapid solidified Al-18 wt pct Si in this work, solidification conditions that yield specific Si morphologies, Si spacing, and thermal cooling conditions are outlined. The focus is determining the solidification conditions that will yield a specified shape of eutectic Si. It is shown that Si morphology is dependent on a combination of growth velocity (based on modified JH model) and temperature gradient. Furthermore, the highest hardness is achieved with globular morphologies of Si. The processing conditions required to achieve these properties are outlined. 相似文献
16.
An approximate method for calculating the macrosegregation in a multicomponent aluminum alloy is proposed. This method is
based on the use of a predefined solidification path (i.e., relation between the solute concentration in the liquid phase and the solid fraction) instead of addressing the fully coupled
micro-macrosegregation problem. In determining the solidification path, it is assumed that the total solute concentration
is constant, and that the solidification history is the same everywhere in the casting. In this manner it becomes quite easy
to take into account how the macrosegregation development is affected by the solute diffusion in the dendrites and the precipitation
of secondary phases, provided that such effects are accounted for in the model used for determining the solidification path.
In order to demonstrate the approximate method, the inverse segregation formation at a chill surface of an Al-4 pct Mg-0.2
pct Fe-0.15 pct Si-0.3 pct Mn (AA5182) alloy is calculated. In this case study, the solidification path is determined prior
to the macrosegregation computation by a microsegregation model discussed elsewhere, and the solid and liquid densities are
related to the concentrations of the different alloying elements by a simple mixture law without distinguishing between the
different solid phases that are formed. The accuracy of the approximate method is discussed by considering a binary alloy.
It turns out that the macrosegregation formation at a chill surface of an Al-4 pct Mg alloy is fairly close to that resulting
from a modeling in which the variation of the total solute concentration is taken into account. Furthermore, the mixture law
is compared to a more elaborate treatment of the densities involving both primary and eutectic solid phases. This comparison
is carried out for an Al-4.5 pct Cu alloy for which literature data exist. The mixture law is found to give a reasonable accuracy
in the calculated macrosegregation. 相似文献
17.
Silicon is an essential element in the Zn-55Al-1.6Si coating. It is added to promote the formation of an adherent coating
and prevent the excessive growth of an intermetallic alloy layer at the steel/coating interface. The addition of silicon also
results in the formation of a silicon phase distributed in the interdendritic region of the overlay, having a flowery pattern
on the surface, and appearing needlelike when observed inside the overlay. The behavior of silicon during the solidification
process of the Zn-55Al-1.6Si coating is examined in the current study. It is found that the coating solidification proceeds
in three stages. At stage I, primary α-Al dendrites develop at about 566 °C to 520 °C, forming the framework of the coating structure. This is followed by stage
II at about 520 °C to 381 °C, where the binary Al-Si eutectic reaction takes place, with the majority of the silicon phase
forming at about 520 °C to 480 °C. At stage III the remaining molten phase undergoes a ternary Al-Zn-Si eutectic reaction
forming the interdendritic zinc-rich network. The ternary Al-Zn-Si eutectic reaction is essentially equivalent to the binary
Al-Zn eutectic reaction because of the very low level of silicon at the Al-Zn-Si eutectic point. 相似文献
18.
Alloys of the eutectic NiAl-34 at. pct Cr with additions of Mo, V, W, and Fe were directionally solidified to determine the effect of the addition element and solidification rate on structure. The directionally solidified NiAl?Cr consists of 〈100〉-oriented columnar grains with long chromium rods of round cross section. The addition of 0.6 at. pct Mo or 0.7 at. pct W or 1.3 at. pct V causes a transition from the round rod grains to 〈111〉-oriented grains with faceted rods and plates of Cr(Mo), Cr(W), or Cr(V). At these compositions, the lattice parameters of the chromium-rich phase and the NiAl matrix phase are nearly equal. Additions of iron produced only the round rod structure as the difference of the lattice parameters was increased. High temperature anneals changed the nonequilibrium faceted rods to rods with regular hexagonal cross sections. Coherency strains near zero mismatch may influence the morphology and may be responsible for the effect of mismatch on structure. Further additions of molybdenum, tungsten, and vanadium resulted in a gradual reduction of the number of faceted rods and finally, the formation of cells with radial plates. Higher solidification rates formed cells and dendrites. A model relating cell and dendrite formation to freezing range of the eutectic alloy was derived. 相似文献
19.
A. K. Dahle K. Nogita S. D. McDonald J. W. Zindel L. M. Hogan 《Metallurgical and Materials Transactions A》2001,32(4):949-960
The effects of different levels of strontium on nucleation and growth of the eutectic in a commercial hypoeutectic Al-Si foundry
alloy have been investigated by optical microscopy and electron backscattering diffraction (EBSD) mapping by scanning electron
microscopy (SEM). The microstructural evolution of each specimen during solidification was studied by a quenching technique
at different temperatures and Sr contents. By comparing the orientation of the aluminum in the eutectic to that of the surrounding
primary aluminum dendrites by EBSD, the eutectic formation mechanism could be determined. The results of these studies show
that the eutectic nucleation mode, and subsequent growth mode, is strongly dependent on Sr level. Three distinctly different
eutectic growth modes were found, in isolation or sometimes together, but different for each Sr content. At very low Sr contents,
the eutectic nucleated and grew from the primary phase. Increasing the Sr level to between 70 and 110 ppm resulted in nucleation
of independent eutectic grains with no relation to the primary dendrites. At a Sr level of 500 ppm, the eutectic again nucleated
on and grew from the primary phase while a well-modified eutectic structure was still present. A slight dependency of eutectic
growth radially from the mold wall opposite the thermal gradient was observed in all specimens in the early stages of eutectic
solidification. 相似文献
20.
A. Munitz V. Y. Zenou C. Cotler M. Talyanker 《Metallurgical and Materials Transactions A》1997,28(4):1035-1046
The impact of cooling rates on the microstructure of Al-U alloys was studied by optical, scanning electron, and transmission
electron microscopy. A variety of solidification techniques were employed to obtain cooling rates ranging between 3 × 10−2 and 106 K/s. High-purity uranium (99.9 pct) and high-purity aluminum (99.99 pct), or “commercially pure” type Al-1050 aluminum alloys
were used to prepare Al-U alloys with U concentration ranging between 3 and 22 wt pct. The U concentration at which a coupled
eutectic growth was observed depends on the cooling rates imposed during solidification and ranged from 13.8 wt pct for the
slower cooling rates to more than 22 wt pct for the fastest cooling rates. The eutectic morphology and its distribution depends
on the type of aluminum used in preparing the alloys and on the cooling rates during solidification. The eutectic in alloys
prepared from pure aluminum was evenly distributed, while for those prepared from Al-1050, the eutectic was unevenly distributed,
with eutectic colonies of up to 3 mm in diameter. Two lamellar eutectic structures were observed in alloys prepared from pure
aluminum containing more than 18 wt pct U, which solidified by cooling rates of about 10 K/s. One structure consisted of the
stable eutectic between UAl4 and Al lamella. The other structure consisted of a metastable eutectic between UAl3 and Al lamella. At least three different eutectic morphologies were observed in alloys prepared from Al-1050. 相似文献