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1.
To discuss the removal of B by solidification refining of Si with a Si-Al melt, the segregation of B between solid Si and
the Si-Al melt was investigated by use of the temperature-gradient-zone melting (TGZM) method. The segregation ratio of B
at its infinite dilution was determined to be 0.49 (1473 K), 0.32 (1373 K), and 0.22 (1273 K), respectively. With the obtained
segregation ratio, the activity coefficient of B in solid Si at its infinite dilution relative to pure solid B was determined
by the following equation: Calculated results of directional solidification of the Si-Al alloy revealed the removal fraction of B to be as much as 90
pct. The effective removal of B by a solidification refining process with a Si-Al melt is clarified. 相似文献
2.
To eliminate B effectively from Si for its use in a solar cell, a novel process involving the slag refining of molten Si with Sn addition was investigated. The partition ratio of B between CaO-SiO 2-24 mol pct CaF 2 slag and Si-Sn alloy at 1673 K (1400 °C) was determined by the chemical equilibrium technique. It was found that the partition ratio of B was remarkably increased with the increase in Sn content of alloy, which attributes to the increase in activity coefficient of B as well as the oxygen partial pressure. The partition function was accounted as much as 200 when the alloy composition was Si-82.4 mol pct Sn, which was much higher than the reported values in the range of 1 to 3. The required amounts of slag used for B removal from Si-30, 50, and 70 mol pct Sn melts were only 15.6 pct, 6.5 pct, and 1.2 pct of that used for the removal of B directly from MG-Si without Sn addition in a single slag treatment. 相似文献
3.
The solidification of γ-TiAl alloys with relatively low (<2 at. pct) additions of boron is discussed. Binary Ti-Al alloys
containing 49 to 52 at. pct Al form primary α-(Ti) dendrites from the melt, which are subsequently surrounded by γ segregate
as the system goes through the peritectic reaction L + α → γ. Alloys between 45 and 49 at. pct Al go through a double peritectic cascade, forming primary β-(Ti) surrounded by α-(Ti) and
eventually by γ in the interdendritic spaces. Boron additions to these binary alloys do not change the basic solidifi-cation
sequence of the matrix but introduce the refractory compound TiB 2 in a variety of mor-phologies. The boride develops as highly convoluted flakes in the leaner alloys, but needles, plates,
and equiaxed particles gradually appear as the B content increases above ∼1 at. pct. Increasing the solidification rate initially
promotes the formation of flakes over plates/needles and ultimately gives way to very fine equiaxed TiB 2 particles in the interdendritic spaces of the metallic matrix. Furthermore, the primary phase selection in the 49 to 52 at.
pct Al range changes from α-(Ti) to β-(Ti) at supercoolings of the order of 200 K. The different boride morphologies are fully
characterized, and their evolution is rationalized in terms of differences in their nucleation and growth behavior and their
relationship to the solidification of the inter-metallic matrix.
Formerly Research Assistant, University of California-Santa Barbara (UCSB)
Formerly Professor of Materials and Dean of the College of Engineering at UCSB 相似文献
4.
It has been known experimentally that TiAl 3 acts as a powerful nucleant for the solidification of aluminum from the melt; however, a full microscopic understanding is still lacking. To improve microscopic understanding, hot rolling technique has been performed to the Al–5Ti–1B alloy and the effect of shape and size of the particles on grain refinement has been studied. The effect of hot rolling of Al–5Ti–1B master alloy on its grain refining performance and hot tearing have been studied by OM, XRD, and SEM. Hot rolling improves the grain refining performance of this master alloy, which is required to reduce hot tearing in Al–7Si–3Cu alloy. The improvement in grain refining performance of Al–5Ti–1B master alloy on rolling is due to the fracture of larger TiAl 3 particles into fine particles during rolling. The presented results illustrate that the morphology of TiAl 3 particles alter from the plate-like structure in the as-cast condition Al–5Ti–1B master alloy to the blocky type after rolling due to the fragmentation of plate-like structures. The grain refining response and effect on hot tearing of Al–7Si–3Cu alloy have been studied with as-cast and rolled Al–5Ti–1B master alloys. The results display hot-rolled master alloys revealing enhanced grain refining performance and minimizing hot tear tendency of the alloy at much lower addition level as compared to as-cast master alloys. 相似文献
5.
This study examined the microstructural evolution and castability of Al–Mg–Si ternary alloys with varying Si contents. Al–6Mg–xSi alloys (where x = 0, 1, 3, 5, and 7; all compositions in mass pct) were examined, with Al–6 mass pct Mg as a base alloy. The results showed that in the ternary alloys with Si ≤ 3 pct, the solidification process ended with the formation of eutectic α-Al–Mg2Si phases generated by a univariant reaction. However, in the case of ternary alloys with Si > 3 pct, solidification was completed with the formation of α-Al–Mg2Si–Si ternary eutectic phases generated by a three-phase invariant reaction. In addition to the eutectic Mg2Si phases, the primary Mg2Si phases formed in each of the ternary alloys, and the size of both sets of phases increased with increasing Si content. The two-phase eutectic α-Al–Mg2Si nucleated from the primary Mg2Si phases. The inoculated Al–6Mg–1Si alloy had the smallest grain size. Moreover, the grain-refining efficacy of the Al–5Ti–B master alloy in the ternary alloys decreased with increasing Si content in the alloys. Despite the poisoning effect of Si on the potency of TiB2 compounds in the inoculated Al–6Mg–1Si alloy, the grain size of the alloy was slightly smaller than that of the Al–6Mg binary alloy. This resulted from the increasing growth restriction factor (induced by Si addition) of the Al–6Mg–1Si alloy. In terms of the castability, the examined alloys showed different levels of susceptibility to hot tearing. Among the alloys, the ternary Al–6Mg–5Si alloy exhibited the highest susceptibility to hot tearing, whereas the Al–6Mg–7Si exhibited the lowest. The severity of hot tearing initiated by the unraveling of the bifilm was determined by the freezing range, grain size, and the amount of eutectic phases at the end of the solidification process. 相似文献
6.
A metallographic study of the porosity and fracture behavior in unidirectionally solidified end chill castings of 319.2 aluminum
alloy (Al-6.2 pct Si-3.8 pct Cu-0.5 pct Fe-0.14 pct Mn-0.06 pct Mg-0.073 pct Ti) was carried out using optical microscopy
and scanning electron microscopy (SEM) to determine their relationship with the tensile properties. The parameters varied
in the production of these castings were the hydrogen (∼0.1 and ∼0.37 mL/100 g Al), modifier (0 and 300 ppm Sr), and grain
refiner (0 and 0.02 wt pct Ti) concentrations, as well as the solidification time, which increased with increasing distance
from the end chill bottom of the casting, giving dendrite arm spacings (DASs) ranging from ∼15 to ∼95 /im. Image analysis
and energy dispersive X-ray (EDX) analysis were employed for quantification of porosity/microstructural constituents and fracture
surface analysis (phase identification), respectively. The results showed that the local solidification time (viz. DAS) significantly influences the ductility at low hydrogen levels; at higher levels, however, hydro-gen has a more pronounced
effect (porosity related) on the drop in ductility. Porosity is mainly observed in the form of elongated pores along the grain
boundaries, with Sr increasing the porosity volume percent and grain refining increasing the probability for pore branching.
The beneficial effect of Sr modification, however, improves the alloy ductility. Fracture of the Si, β-Al 5FeSi, α - Al 15(Fe,Mn) 3Si 2, and Al 2Cu phases takes place within the phase particles rather than at the particle/Al matrix interface. Sensitivity of tensile properties
to DAS allows for the use of the latter as an indicator of the expected properties of the alloy. 相似文献
7.
Al-3.7 pct Si-0.18 pct Mg foams strengthened by AlN particle dispersion were prepared by a melt foaming method, and the effect
of foaming temperature on the foaming behavior was investigated. Al-3.7 pct Si-0.18 pct Mg alloy containing AlN particles
was prepared by noncompressive infiltration of Al powder compacts with molten Al alloy in nitrogen atmosphere, and it was
foamed at different foaming temperatures ranging from 1023 to 1173 K. The porosity of prepared foam decreases and the pore
structure becomes homogeneous with increasing foaming temperature. When the foaming temperature is higher than 1123 K, homogeneous
pores are formed in the prepared ingot without using oxide particles and metallic calcium granules, which are usually used
for stabilizing a foaming process. This stabilization of the foaming at high temperatures is possibly caused by Al 3Ti intermetallic compounds formed at high temperature and AlN particles. Compression tests for the prepared foams revealed
that the absorbed energy per unit mass of prepared Al-3.7 pct Si-0.18 pct Mg foam is higher than those of aluminum foams strengthened
by alloying or dispersion of reinforcements. It is remarkable that the oscillation in stress, which usually appears in strengthened
aluminum foams, does not appear in the plateau stress region of the present Al-3.7 pct Si-0.18 pct Mg foam. The homogeneity
in cell walls and pore morphology due to the stabilization of pore formation and growth by AlN and Al 3Ti particles is a possible cause of this smooth plateau stress region. 相似文献
8.
Direct and differential thermal analyses have been used to determine the liquidus temperatures and isothermals in the aluminum-base
corner of the Al-Ti-B system, and with microscopical information and electron microprobe analyses provide results on which
is based a provisional phase diagram. The TiB 2 liquidus rises very steeply from the aluminum corner of the diagram and is bounded on either side by monovariant reactions
which almost coincide with the binary Al-B and Al-Ti liquidus curves. If the Ti:B ratio exceeds 7∶3 wt pct (1∶2 at. pct),
TiB 2 precipitates on cooling until there occurs a monovariant reaction involving Al 3Ti, while at lower Ti:B ratios there occurs a monovariant reaction which involves AlB 2. The grain refining action of ternary alloys is discussed with reference to the form of the phase diagram. 相似文献
9.
The mechanism of heterogeneous grain refining of aluminum by ultrafine elemental boron particles was investigated. In order to facilitate the observation of the boron-aluminum interface, a boron filament was introduced in a melt at 1013 K (740 °C) containing different levels of Ti. The Al/B interface was studied using transmission electron microscopy and different phases were identified using the electron diffraction method. The experimental results showed that boron is dissolved in pure aluminum while its dissolution is inhibited in presence of titanium solute. A thin layer of TiB 2 formed at the surface of boron thickens with residence time in the melt. The mechanisms by which aluminum is crystallized on boron are discussed. 相似文献
10.
The purification of metallurgical-grade silicon (MG-Si) by combined solvent refining processes has been studied. The final high-purity silicon was recovered through Sn-Si refining and Al-Si refining processes in sequence after acid leaching, and the removal mechanism of impurities was explored. Inductively coupled plasma (ICP) chemical analysis revealed the concentrations of main impurities including B and P, and typical metallic impurities except for solvents Sn and Al were reduced to below 1 ppmw. The final removal efficiencies of B and P were 97.7 pct and 99.8 pct, respectively, and those of most metallic impurities were above 99.9 pct. SEM analysis showed that P-containing phases (Al-Ca-Mg-Si-P and Al-Si-P) formed on the surface of refined Si after Sn-Si refining and Al-Si refining, which was confirmed to be the main approach for P removal. It was also found that the formation of binary silicide such as Fe 3Si 7 and Mn 11Si 19 or multicomponent phases such as Ca-Mg-Si phase occurred during the solvent refining process, and they segregated on the grain boundaries in Si or attached to the surface of Si, which led to high removal efficiency of metallic impurities by the solvent refining process. 相似文献
11.
By examining the surface morphologies of undercooled Si-20 at. pct Al alloy during and after the solidification process, it
is determined that the critical undercooling for silicon to grow from lateral mode to intermediary mode Δ T* and that from intermediary mode to continuous mode Δ T** are 131 and 239 K, respectively. A method that predicts the solid-liquid interface energy of binary lateral growth materials
on the basis of Δ T* and Δ T** has been developed. Formulas between the physical parameters and the solid-liquid interface energy have been obtained.
The interface energy between silicon crystal and Si-Al melt predicted from Δ T* is almost equal to that from Δ T**. The present results of the solid-liquid interface energy predicted according to Δ T* and Δ T** obtained in Si-20 at. pct Al alloy are in very good agreement with the reported results of the grain-boundary method and
the critical undercooling method from Δ T* and Δ T** obtained in pure silicon. 相似文献
12.
In the present work, ternary Al?CTi?CB master alloys have been prepared in an induction furnace by the reaction between preheated halide salts (K 2TiF 6 and KBF 4) and liquid molten Al. A number of process parameters such as reaction temperature (800, 900, 1,000?°C), reaction time (45, 60, 75?min.) and compositions (Ti/B ratio: 5/1, 1/3) have been studied. The indigenously prepared master alloys were characterised by chemical analysis, particles size analysis, XRD and SEM/EDX microanalysis. Results of particle size analysis suggest that the sizes of the intermetallic particles [Al 3Ti and TiB 2 in Al?C5Ti?C1B and (Al, Ti)B 2 in Al?C1Ti?C3B] present in various Al?CTi?CB master alloys increases with increase in reaction temperature (800?C1,000?°C) and reaction time (45?C75?min.). The population of the particles decreases with increase in reaction time and temperature. Further, SEM/EDX studies revealed that different morphologies of the intermetallic particles were observed at different reaction temperatures and reaction times. Further, the performances of the above-prepared master alloys were assessed for their grain refining efficiency on Al?C7Si alloy by macroscopy, DAS analysis. Grain refinement studies suggest that, B-rich Al?C1Ti?C3B master alloy shows better grain refinement performance on Al?C7Si alloy when compared to Ti-rich Al?C5Ti?C1B master alloy. 相似文献
13.
In the present work, functionally graded (FG) aluminum alloy matrix in-situ composites (FG-AMCs) with TiB 2 and TiC reinforcements were synthesized using the horizontal centrifugal casting process. A commercial Al-Si alloy (A356) and an Al-Cu alloy were used as matrices in the present study. The material parameters (such as matrix and reinforcement type) and process parameters (such as mold temperature, mold speed, and melt stirring) were found to influence the gradient in the FG-AMCs. Detailed microstructural analysis of the composites in different processing conditions revealed that the gradients in the reinforcement modify the microstructure and hardness of the Al alloy. The segregated in-situ formed TiB 2 and TiC particles change the morphology of Si particles during the solidification of Al-Si alloy. A maximum of 20 vol pct of reinforcement at the surface was achieved by this process in the Al-4Cu-TiB 2 system. The stirring of the melt before pouring causes the reinforcement particles to segregate at the periphery of the casting, while in the absence of such stirring, the particles are segregated at the interior of the casting. 相似文献
14.
The effect of the addition of up to 10 pct molybdenum on several metallurgical properties of Fe-28Al (at. pct) to which 1
pct TiB 2 was added for grain refinement has been studied. It was determined that the addition of molybdenum results in a decrease
in grain size, an increase in the recrystallization temperature, and an increase in the DO 3 to B2 ordering transformation temperature. The solubility limit of molybdenum in the matrix of the base alloy was found to
be about 6 pct. At this concentration, another 1 pct is dissolved in the TiB 2 precipitates. Tensile strengths were increased slightly by adding up to 2 pct Mo, but ductility decreased, even though grain
sizes were reduced. The fracture mode in tension did not change with addition of molybdenum up to 2 pct. 相似文献
15.
Solidification microstructures of arc-melted, near-equiatomic TiAl alloys containing boron additions are analyzed and compared
with those of binary Ti-Al and Ti-B alloys processed in a similar fashion. With the exception of the boride phase, the matrix
of the ternary alloy consists of the same α 2 (DO 19) and γ (Ll 0) intermetallic phases found in the binary Ti-50 at. pct Al alloy. On the other hand, the boride phase, which is TiB (B27)
in the binary Ti-B alloys, changes to TiB 2 (C32) with the addition of Al. The solidification path of the ternary alloys starts with the formation of primary α (A3)
for an alloy lean in boron (∼1 at. pct) and with primary TiB 2 for a higher boron concentration (∼5 at. pct). In both cases, the system follows the liquidus surface down to a monovariant
line, where both α and TiB 2 are solidified concurrently. In the final stage, the α phase gives way to γ, presumably by a peritectic-type reaction similar
to the one in the binary Ti-Al system. Upon cooling, the α dendrites order to α 2 and later decompose to a lath structure consisting of alternating layers of γ and α 2. 相似文献
16.
The present investigation was undertaken to elucidate the microstructural evolution of MoSi 2-SiC in situ composites produced by melt processing. An assessment of the existing liquidus projection was performed by a combination
of thermodynamic modeling, analysis of solidification microstructures, and measurements of the thermal history during solidification.
Results show that the quasibinary MoSi 2-SiC eutectic occurs at ∼2 at. pct C and 2283 K, rather than 8 at. pct C and 2173 K, as previously reported. The ensuing L+MoSi 2+SiC monovariant line runs almost parallel to the SiMoSi 2 binary and terminates at a ternary L ↔ Si+MoSi 2+SiC eutectic calculated at 1.5Mo-0.84C (at. pct) and ∼1670 K. The maximum amount of SiC that may be produced by solidification
along the quasibinary isopleth is ∼37 vol pct, of which ∼35 vol pct grows as primary. Analysis of solidification microstructures
shows SiC grows with the cubic β polytype structure (B3), while MoSi 2 grows with the tetragonal C11
b
structure. Primary SiC may grow as equiaxed particles, platelets, and hopper crystals. Coupled growth with MoSi 2 leads to SiC in the shape of thin ribbons, sheets, and needles. The facets of the SiC crystals were identified to be of the
{111} and {002} type, in agreement with the periodic bond chain analysis. The predominant platelike morphology was shown to
develop due to a re-entrant twin mechanism similar to that observed in Si and Ge. 相似文献
17.
The Al-12 pctSi alloy and aluminum-based composites reinforced with TiB 2 and Al 3Ti intermetallics exhibit good wear resistance, strength-to-weight ratio, and strength-to-cost ratio when compared to equivalent other commercial Al alloys, which make them good candidates as coating materials. In this study, structural AA 6028 alloy is used as the base material. Four different coating materials were used. The first one is Al-Si alloy that has Si content near eutectic composition. The second, third, and fourth ones are Al-6 pctSi-based reinforced with TiB 2 and Al 3Ti nano-particles produced by addition of Al-Ti5-B1 master alloy with different weight percentages (1, 2, and 3 pct). The coating treatment was carried out with the aid of GTAW process. The microstructures of the base and coated materials were investigated using optical microscope and scanning electron microscope equipped with EDX analyzer. Microhardness of the base material and the coated layer were evaluated using a microhardness tester. GTAW process results in almost sound coated layer on 6028 aluminum alloy with the used four coating materials. The coating materials of Al-12 pct Si alloy resulted in very fine dendritic Al-Si eutectic structure. The interface between the coated layer and the base metal was very clean. The coated layer was almost free from porosities or other defects. The coating materials of Al-6 pct Si-based mixed with Al-Ti5-B1 master alloy with different percentages (1, 2, and 3 pct), results in coated layer consisted of matrix of fine dendrite eutectic morphology structure inside α-Al grains. Many fine in situ TiAl 3 and TiB 2 intermetallics were precipitated almost at the grain boundary of α-Al grains. The amounts of these precipitates are increased by increasing the addition of Al-Ti5-B1 master alloy. The surface hardness of the 6028 aluminum alloy base metal was improved with the entire four used surface coating materials. The improvement reached to about 85 pct by the first type of coating material (Al-12 pctSi alloy), while it reached to 77, 83, and 89 pct by the coating materials of Al-6 pct Si-based mixed with Al-Ti5-B1 master alloy with different percentages 1, 2, and 3 pct, respectively. 相似文献
18.
The aluminum corner of the ternary Al-B-Ti diagram was explored. A eutectic: Liq — Al + TiAl 3 + (Al, Ti)B 2 was found at approximately 0.05 wt pct Ti, 0.01 wt pct B; 659.5‡C. TiB 2 and A1B 2 form a continuous series of solid solutions, but no distinct ternary phase was found. The addition of boron to aluminum-titanium
alloys expands the field of primary crystallization of TiAl 3 toward lower titanium contents and steepens the liquidus. In equilibrium conditions, pronounced grain refinement is found
only in alloys in which TiAl 3 is primary and nucleates the aluminum solid solution before any other impurity can act. The peritectic reaction facilitates
this priority but it is not necessary for grain refinement. Because of the low diffusivity of titanium and boron in aluminum,
equilibrium is seldom attained and in commercial practice grain refinement by TiAl 3 is found also outside its equilibrium field of primary crystallization. 相似文献
19.
Cast Al-Ti-C grain refiners were synthesized by reacting up to 2 pct graphite particles of 20 micron average size with stirred
Al-(5 to 10) pct Ti alloy melts, which generated submicron-sized TiC particles within the melts, and their solidified structures
showed preferential segregation of the carbide phase in the grain or cell boundary regions and occasional presence of free
carbon whose amount exceeded equilibrium values. At the usual melt temperatures of below 1273 K, though, TiC formed first,
but was subsequently found to react with the melt forming a sheathing of A1 4C 3 and Ti 3AlC which resulted into poisoning of the TiC particles. However, it was possible to reverse these reactions in order to regain
the virgin TiC particles by superheating the melts in the temperature region where TiC particles are thermodynamically stable.
Grain refining tests using the TiC master alloys produced fine equiaxed grains of cast aluminum whose sizes were comparable
to that obtainable with the standard TiB 2 commercial grain refiner. TiC particles introduced via the master alloys were found to occur in the grain centers, thereby confirming that they nucleated aluminum crystals.
On leave from Regional Research Laboratory (CSIR), Bhopal, is Research Associate. 相似文献
20.
A series of in-situ, deformation-processed metal matrix composites were produced by direct powder extrusion of blended constituents. The resulting
composites are comprised of a metallic Ti-6Al-4V matrix containing dispersed and co-deformed discontinuously reinforced-intermetallic
matrix composite (DR-IMC) reinforcements. The DR-IMCs are comprised of discontinuous TiB 2 particulate within a titanium trialuminide or near- γ Ti-47Al matrix. Thus, an example of a resulting composite would be Ti-6Al-4V+40 vol pct (Al 3Ti+30 vol pct TiB 2) or Ti-6Al-4V+40 vol pct (Ti-47Al+40 vol pct TiB 2), with the DR-IMCs having an aligned, high aspect ratio morphology as a consequence of deformation processing. The degree
to which both constituents deform during extrusion has been examined using systematic variations in the percentage of TiB 2 within the DR-IMC, and by varying the percentage of DR-IMC within the metal matrix. In the former instance, variation of
the TiB 2 percentage effects variations in relative flow behavior; while in the latter, varying the percentage of DR-IMC within the
metallic matrix effects changes in strain distribution among components. The results indicate that successful co-deformation
processing can occur within certain ranges of relative flow stress; however, the extent of commensurate flow will be limited
by the constituents’ inherent capacity to plastically deform. 相似文献
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